Dam safety guidance manual : for Oklahoma dam owners.

              Dam Safety
Guidance Manual
Published by the
Oklahoma Water Resources Board
Dam Safety Program 2011
For Oklahoma Dam Owners
FPO
Mark Harrison,
Oklahoma Conservation Commission
Oklahoma Water Resources Board - Dam Safety Program - March 2011 1
Acknowledgement
Dam safety has attracted a great deal of attention in recent years, and in preparing this
manual, information from a number of sources was used. The National Dam Safety
Program, instituted in response to several major dam failures in the early 1970’s, fo-cused
on the problem nationwide. Federal Emergency Management Agency (FEMA)
has taken the lead in providing assistance to states in promoting dam safety. The
National Dam Safety Program Act of 1996 continues to reinforce the commitment by
the Federal Government to dam safety.
Special recognition is given to the Federal Emergency Management Agency (FEMA)
and the Association of State Dam Safety Officials (ASDSO) for their leadership in
developing effective dam safety programs and policies for the furtherance of dam
safety. Their diligence in assisting the U.S. dam safety community was an important
factor in the issuance of the FEMA grant.
The cooperation of the owners of dams within Oklahoma is essential to the success of
the State’s dam safety effort. The ultimate purpose of such a program is the protection
of the lives and property of citizens of Oklahoma.
Cover and other photographs courtesy of Oklahoma Conservation Commission.
This publication was funded by a grant from the Federal Emergency Management Agency
(FEMA), National Dam Safety Program Grant Agreement 2010-RC-50-0006.
Cover Photo: Kadashan No. 2 in Wagoner County
Courtesy Mark Harrison, Oklahoma Conservation Commission
2 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Contents
DAM SAFETY:
AN OWNER’S GUIDANCE MANUAL................................4
Introduction to Dam Safety......................................................4
Hazards, Risk, Failures...............................................................5
Inspection Guidelines................................................................5
Instrumentation and Monitoring Guidelines.........................5
Maintenance Guidelines............................................................6
Emergency Action Plan Guidelines.........................................6
Operation Plan Guidelines........................................................7
Measures to Reduce the
Consequences of Dam Failure..................................................7
Water Retention Ability...........................................................15
Seepage Through a Dam.........................................................16
Seepage Around a Dam...........................................................16
Release of Water........................................................................16
Principal or Mechanical Spillway...........................................17
Drawdown Facility...................................................................17
Emergency (Auxiliary) Spillway.............................................17
CHAPTER 3:
HAZARDS, RISKS, FAILURES.........................................19
General.......................................................................................19
Hazards as Sources of Risks....................................................19
Natural Hazards That Threaten Dams..................................20
Hazards from Human Activity...............................................21
Site-Specific Structural Risk....................................................22
Sources of Dam Failure............................................................23
Three Categories of Structural Failure..................................23
Failures.......................................................................................23
Age and Its Relation to Failure...............................................25
CHAPTER 4:
DEVELOPING A PERSONAL SAFETY PROGRAM..........27
Objectives of a Safety Program...............................................27
Guidelines for Assessing Existing Conditions......................27
Procedural Guidelines – A Source Book...............................29
Documenting the Safety Program..........................................29
CHAPTER 5:
INSPECTION GUIDELINES.............................................31
Introduction..............................................................................31
Organizing for Inspection.......................................................32
Embankment Dams and Structures.......................................33
Upstream Slope.........................................................................34
Downstream Slope...................................................................35
Crest...........................................................................................36
Seepage Areas............................................................................37
Concrete Dams and Structures...............................................38
Spillways....................................................................................40
Procedure for Inspection of the Spillway..............................44
Inlets, Outlets, and Drains......................................................45
Inspecting the outlet system...................................................47
General Areas............................................................................48
DAM SAFETY:
AN OWNER’S GUIDANCE MANUAL
CHAPTER 1:
AN APPROACH TO DAM SAFETY....................................9
General.........................................................................................9
Urgency for Safety......................................................................9
Dam Ownership and Safety......................................................9
Role of the Dam Owner in Dam Safety.................................10
The Role of Consultants in Dam Safety................................11
Role of the Oklahoma Water Resources Board....................12
CHAPTER 2:
INTRODUCTON TO DAMS.............................................13
General.......................................................................................13
The Watershed System.............................................................13
Types of Dams...........................................................................14
Components..............................................................................14
Construction Materials............................................................14
MARK HARRISON, OKLAHOMA CONSERVATION COMMISSION
Oklahoma Water Resources Board - Dam Safety Program - March 2011 3
Contents
CHAPTER 6:
INSTRUMENTATION AND
MONITORING GUIDELINES...........................................71
General.......................................................................................71
Reasons for Instrumentation..................................................71
Instrument Types and Usage..................................................72
Visual Observations.................................................................72
Movements................................................................................72
Pore Pressure and Uplift Pressure..........................................74
Water Level and Flow...............................................................74
Water Quality............................................................................74
Temperature..............................................................................74
Crack and Joint Size.................................................................75
Seismic Activity........................................................................75
Weather......................................................................................75
Stress and Strain.......................................................................75
Automated Data-Acquisition Systems...................................75
Frequency of Monitoring........................................................76
CHAPTER 7:
MAINTENANCE INSPECTION GUIDELINES..................79
General.......................................................................................79
Maintenance Priorities.............................................................79
Immediate Maintenance..........................................................79
Required Maintenance at Earliest Possible Date..................79
Continuing Maintenance........................................................80
Specific Maintenance Items....................................................80
Earthwork Maintenance and Repair......................................80
Riprap Maintenance and Repair.............................................82
Controlling Vegetation............................................................84
Controlling Livestock..............................................................85
Controlling Animal Damage..................................................85
Controlling Damage From Traffic.........................................86
Mechanical Maintenance.........................................................86
Electrical Maintenance............................................................88
Cleaning.....................................................................................88
Concrete Maintenance.............................................................89
Metal Component Maintenance.............................................89
CHAPTER 8:
EMERGENCY ACTION PLAN GUIDELINES....................91
General.......................................................................................91
Contents of Guidelines............................................................92
Description of the Project.......................................................92
Notification Flowchart.............................................................92
Emergency Detection, Evaluation, and Classifications.......93
Responsibilities.........................................................................93
Preparedness.............................................................................94
Inundation Maps......................................................................94
Implementation........................................................................94
CHAPTER 9:
GUIDELINES FOR OPERATIONS....................................97
General.......................................................................................97
Plan Guidelines.........................................................................97
Background Data......................................................................98
Operating Instructions and Records......................................98
Schedule of Routine Tasks.......................................................99
Record Keeping.......................................................................100
CHAPTER 10:
REDUCING THE CONSEQUENCES
OF A DAM FAILURE.....................................................101
Supplements to a Dam Safety Program...............................101
Liability....................................................................................101
Measures to Reduce the
Consequences of Dam Failures............................................103
Insurance.................................................................................104
Government Assistance.........................................................104
DAM SAFETY: AN OWNER’S GUIDANCE MANUAL
apendix a.....................................................................106
iNSPECTION EQUIPMENT AND CHECKLIST.............107
APPENDIX B: REPORT FORM.......................................111
APPENDIX C: GLOSSARY...............................................113
REFERENCES.....................................................................121
Oklahoma Water Resources Board - Dam Safety Program - March 2011 4
DAM SAFETY:
An Owner’s Guidance Manual
Introduction to Dam Safety
The need for dam safety is urgent. Across the United States thousands of dams are now
in place with many more built each year. Dams—essential elements of the national
infrastructure—supply water for households and businesses and cooling water for power
plants, offer opportunities for recreation, and help control floods. Should a dam fail, many lives
and many dollars’ worth of property are at risk. The legal and moral responsibility for dam safety
rests with the dam owner.
Existing dams are aging and new ones are being built in hazardous areas. At the same time, devel-opment
continues in potential inundation zones downstream. More people are at risk from dam
failure than ever, despite better engineering and construction methods, and continued deaths and
property losses from dam failures are to be expected.
Society and individuals alike may profit from dam operations. Dam ownership, however, is neither
justified nor effective if one cannot assure the safety of citizens and property. The costs of dam
safety are small in comparison to the consequences following a dam failure, particularly in today’s
litigious society. Liability due to dam failure can easily offset years of profitability.
You can directly influence the safety of a dam by developing a safety program which includes
inspection, monitoring through instrumentation, maintenance of the structure, and proactive
emergency planning. A high-quality safety program is attuned to the dam structure and its im-mediate
environment and depends on the owner’s knowledge of the dam and how it works.
Lakes in Oklahoma and in other parts of the country may either be human-built or exist because
of geologic activities such as landslides, erosion, or glaciation. The majority of dams are human
structures constructed of earthfill or concrete. It is important that you, as a dam owner, be aware of
the different types of dams, their essential components and their function, as well as the important
physical conditions likely that influence them.
As in the case with buildings, highways, and other works that we construct, dams require an on-going
maintenance program to insure their continued useful life. This fact has not always been
appreciated. Often there is a tendency to neglect them once construction is completed.
As is the case with buildings, highways, and other works that we construct,
dams require an on-going maintenance program to ensure their continued
useful life. This fact has not always been fully appreciated. Often there is a
tendency to neglect them once construction is completed.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 5
National statistics show that dam failure is an all
too common problem. It is imperative that you,
as a dam owner or operator, familiarize yourself
with the risks and hazards of dam ownership.
Risk has greatly increased for existing dams as
developers have been allowed to construct be-low
dams within their inundation zone and new
dams are being built in areas where the geology
may be inappropriate.
Other risks include natural phenomena such
as floods, earthquakes, and landslides. These
hazards threaten dam structures and their
surroundings. Floods that exceed the capacity
of a dam’s spillway and then erode the dam or
abutments are particularly hazardous. Seismic
activity which appears to be on the rise in
Oklahoma may also cause cracking or seepage.
Similarly, debris from landslides may block a
dam’s spillway and cause an overflow event that
erodes the abutments and ultimately weakens
the structure.
Hazards, Risk, Failures
The three major categories of dam failure are
overtopping by flood, foundation defects, and
piping. For earthen dams, the major reason for
failure has been piping or seepage. For concrete
dams, the major reasons for failure have been
associated with foundations. Overtopping has
been a significant cause of dam failure, primar-ily
where a spillway was inadequate.
Inspection Guidelines
An effective inspection program is essential to
identification of problems and for safe main-tenance
of a dam. The program should involve
three types of inspections: (1) periodic technical
inspections; (2) periodic maintenance inspec-tions;
and (3) informal observations by project
personnel as they operate the dam. Technical
inspections involve specialists familiar with the
design and construction of dams and include
assessments of structure safety. Maintenance
inspections are performed more frequently than
technical inspections in order to detect, at an early
stage, any detrimental developments in the dam.
These involve assessment of operational capabil-ity
as well as structural stability. The third type of
inspection is actually a continuing effort by on-site
project personnel (dam tenders, powerhouse
operators, maintenance personnel) performed in
the course of their normal duties. A fact sheet on
Dam Inspection Guidelines is available online at
www.owrb.ok.gov/damsafety.
Instrumentation and
Monitoring Guidelines
A dam’s instrumentation furnishes data for
determining if the structure is functioning as
intended and continuing surveillance to warn
of any unsafe developments. Monitoring physi-cal
phenomena that can lead to a dam failure
may draw on a wide spectrum of instruments
and procedures ranging from very simple to
very complex. Any program of dam- safety
instrumentation must involve proper design
consistent with other project components. The
program must be based on prevailing geotech-nical
conditions at the dam, and must include
consideration of the hydrologic and hydraulic
factors present before and after the project is in
operation. Instrumentation designed for moni-toring
potential deficiencies at existing dams
must take into account the threat to life and
property that the dam presents. Thus, the extent
and nature of the instrumentation depends not
only on the complexity of the dam and the size
of the reservoir, but also on the potential for
deaths and property losses downstream.
An instrumentation program should involve
instruments and evaluation methods that are as
simple and straight forward as the project will
allow. The involvement of qualified personnel in
the design, installation, consistent and regular
monitoring, and evaluation of an instrumenta-tion
system is of prime importance to the suc-cess
of the program.
Specific information that instrumentation can
provide includes:
6 Oklahoma Water Resources Board - Dam Safety Program - March 2011
• warning of a problem, i.e. settlement ,
movement, seepage, stability
• definition and analysis of a problem, such as
locating areas of concern
• proof that behavior of the dam is as expected
• evaluating any remedial actions
Maintenance
Guidelines
A good maintenance pro-gram
will protect a dam
against deterioration and
prolong its life. A poorly
maintained dam will
deteriorate, and may fail.
Nearly all the components
of a dam and the materials
used for its construction
are susceptible to damaging deterioration if not
properly maintained. A good maintenance pro-gram
protects both you and the general public.
The cost of a proper maintenance program is
small compared to the cost of major repairs or
the loss of life and property and resultant litiga-tion.
You should develop a basic maintenance
program based primarily on systematic and
frequent inspections. Inspections, as noted in
Chapter 5, should be carried out monthly and
after major floods or earthquakes. During each
inspection, fill out a checklist of items requiring
maintenance. An Inspection Checklist is avail-able
online at www.owrb.ok.gov/damsafety.
Emergency Action
Plan Guidelines
History has shown that dams sometimes fail and
that often these failures cause loss of life, injuries and
extensive property damage. You should prepare for
this possibility by developing an emergency action
plan which provides a systematic means to:
• identify potential problems that could
threaten a dam
• determine who would be at risk should a
failure occur
• expedite effective response actions to prevent
failure
• develop a notification plan for evacuating
people to reduce loss of life and property
damage should failure occur
You are responsible for preparing a plan cov-ering
these measures and listing actions that
you and operating personnel should take. You
should be familiar with the local government
officials and agencies responsible for warn-ing
and evacuating the public. An Emergency
Action Guide is available online at www.owrb.
ok.gov/damsafety.
It is important that you make full use of others
who are concerned with dam safety. Emergency
plans will be more effective if they integrate the
actions of others who can expedite response.
People and organizations with whom you should
consult in preparing an emergency action plan
include numerous local participants, state and
federal agencies.
An essential part of the emergency action plan
is a list of agencies and persons to be notified in
the event of a potential failure. Possible inclu-sions
for this list should be obtained from and
coordinated with local law enforcement agencies
and local disaster emergency services. These are
key institutions that can activate public warn-ing
and evacuation procedures or that might be
able to assist you, the dam owner, in delaying or
preventing failure.
Certain key elements must be included in every
notification plan. Information about potential
inundation (flooding) areas and travel times
for the breach (flood) wave is essential. Inunda-tion
maps are especially useful in local warning
and evacuation planning, including identifying
evacuation routes.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 7
Operation Plan Guidelines
Establishing an operation procedure or plan
calls for detailed:
• data on the physical characteristics of dam
and reservoir
• descriptions of dam components
• operating instructions for operable
mechanisms
• instructions for inspections
Measures to Reduce the
Consequences of Dam Failure
Liabilities that are determined following a dam failure strongly affect organizations and
individuals, governments and dam owners alike. Establishing liability is the legal means de-veloped
by society to recover damages due to some intentional or negligent wrong (in this
case, a lack of dam safety) and represents another perspective on the dam safety problem.
A thorough understanding of this legal process can help you decide the steps necessary to
reduce liability.
You can directly and indirectly influence the use of
a variety of measures that will serve to reduce the
consequences of dam failure. For example, insurance
against the costs that will accrue after a failure will
save you money by spreading costs to multiple dam
owners. Some land use measures instituted by gov-ernments
represent better means of mitigating future
disasters. Land use measures that restrict living or de-veloping
in inundation zones radically improve safety
and are among the most effective ways to save lives
and preserve property over the long term; however,
such steps are not always acceptable to the local population or government. Thus, increasing
public awareness and governmental planning are vital measures that must be considered as
ways to reduce the consequences of dam failure.
• instrumentation and monitoring guidelines
• guidelines for maintenance
• guidelines for emergency operations
• bibliographic references
Establish a schedule for both day-to-day tasks
and tasks performed less frequently throughout
the year. The schedule should formalize inspec-tion
and maintenance procedures so that even
an inexperienced person can determine when a
task is to be done.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 9
CHAPTER 1:
An Approach to Dam Safety 1
public and private agencies, and private citizens.
Typical reasons for building dams include water
storage for human consumption, agricultural
production, power generation, flood control,
reduction of soil erosion, industrial use, and
recreation. Thus, dam owners serve society by
meeting important state needs and may also
personally profit from dam operations. How-ever,
those are not sufficient reasons for build-ing
or owning a dam if the owner cannot keep
people and property safe in potential inunda-tion
zones.
Both financially and morally, successful dam
ownership and the maintenance of safety
standards go hand in hand. Investment in dam
safety should be accepted as an integral part of
project costs and not viewed as an expendable
item that can be eliminated if a budget becomes
tight (Jansen, 1980). The potential cost and sta-tistical
likelihood of dam failure to both life and
property are simply too high to ignore.
As national needs for water intensify and its
value increases, more dams are being built. At
the same time, many existing dams are reach-ing
or passing their design life spans and, for
various reasons, people continue to settle near
dams. As builders use poorer sites for dams or
as areas below a dam develop, the job of protect-ing
life and property becomes more difficult.
Therefore, as dam construction continues and
the population grows, exposure of the public to
dam failure hazards increases and the overall
safety problem becomes more difficult.
General
This manual is a safety guide for the dam
owner. The continuing need for dam
safety is critical because of the thousands
of dams now in place and the many new ones
being built each year. Although these dams are
essential elements of the national infrastructure,
the risks to the public posed by their possible
failure are great; a large and growing number of
lives and valuable property are at stake. Though
many are concerned about dam safety, the legal
and moral responsibility essentially rests with
the dam owner.
Urgency for Safety
The critical need for dam safety is clear. World and
national statistics on dam failures show an unaccept-able
record of deaths and property losses. The record
for U.S. losses from major dam failures in recent
years, shown in Table 1 is also discouraging. Actual
national losses are much higher than indicated be-cause
the statistics shown exclude small dam failures
and combinations of dam failure with natural flood-ing
events. Two examples are dams that failed near
Hearne, Texas in May 2004 and the Johnstown,
Pennsylvania, disaster of 1889 which is still regarded
as one of the nation’s great catastrophes. The poten-tial
for future similar catastrophes due to dam failure
remains strong. Only a cooperative effort in dam
safety involving owners and communities can lessen
this potential.
Dam Ownership and Safety
This manual can be applied to dams owned and
operated by a wide range of organizations and
people, including state and local governments,
10 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Name & Location
of Dam Date of Failure Number of
Lives Lost Damages
Mohegan Park, CT 3/1963 6 $3 million.
Little Deer Creek, UT 6/1963 1 Summer cabins damaged.
Baldwin Hills, CA 12/1963 5 41 houses destroyed, 986 houses damaged, 100
apartment buildings damaged.
Swift, MT 6/1964 19 Unknown.
Lower Two Medicine, MT 6/1968 9 Unknown.
Lee Lake, MA 3/1968 2 6 houses destroyed, 20 houses damaged, 1
manufacturing plant damaged or destroyed.
Buffalo Creek, WV 2/1972 125 546 houses destroyed, 538 houses damaged.
Lake “O” Hills, AR 4/1972 1 Unknown.
Canyon Lake, SD 6/1972 33 Unable to assess damage because dam failure
accompanied damage caused by natural flooding.
Bear Wallow, NC 2/1976 4 1 house destroyed.
Teton, ID 6/1976 11 771 houses destroyed, 3,002 houses damaged, 246
businesses damaged or destroyed.
Laurel Run, PA 7/1977 40 6 houses destroyed, 19 houses damaged.
Sandy Run & 5 others, PA 7/1977 5 Unknown.
Kelly Barnes, GA 11/1977 39 9 houses, 18 house trailers, & 2 college buildings
destroyed; 6 houses, 5 college buildings damaged.
Lawn Lake, CO 7/1982 3
18 bridges destroyed, 117 businesses & 108 houses
damaged. Campgrounds, fisheries, power plant
damaged.
D.M.A.D., UT 6/1983 1 Unknown.
Nix Lake Dam, TX 3/1989 1 Unknown.
Silver Lake, MI 5/2003 0 $102,000,000.
Big Bay Lake, MS 3/2004 0 98 houses, 2 churches, fire station, bridge, $2.2 million
Kaloko Res., HI 3/2006 7 Unknown.
Source: Graham, 1983, 2004
TABLE 1.1. Loss of Life and Property Dama ge From Notable U.S. Dam Failures , 1963-2006
Governments across the nation have shown
increasing concern for this problem and have
enacted laws, statutes, and regulations that in-crease
the dam owner’s responsibility. In most
states, including Oklahoma, owners are held
strictly liable for losses or damages resulting
from dam failure. Concurrently, liability insur-ance
costs have risen rapidly.
Role of the Dam Owner in
Dam Safety
An owner should be aware of and use both direct
and indirect means of achieving dam safety. The
owner can monitor and work on factors directly
in his control (for example, structural integrity),
which are detailed below. However, the owner
may also seek to influence governmental policy
and work for positive change in statutes and laws
that affect dam safety (example, zoning laws).
Such indirect influence by an owner could con-tribute
significantly to reducing the likelihood
and consequences of dam failure and, thus, to
overall community safety.
Liability, insurance coverage, and the roles of the
state and federal governments should all be well-understood
by an owner. Liability can apply not
only to the individual dam owner, but also to
any company or organization that possesses the
dam, or any person who operates or maintains
it and, potentially, even those who live around
a lake. If an unsafe condition existed prior to a
new dam owner’s term of ownership, the new
owner cannot be relieved of liability should the
Oklahoma Water Resources Board - Dam Safety Program - March 2011 11
dam fail during this term. Thus, the potential
owner must carefully inspect the structural
integrity of any dam prior to purchase and then
inspect, maintain, and repair it thereafter.
Legally, the dam owner must do what is neces-sary
to avoid injuring persons or property which
usually applies to circumstances and situations
which a reasonable person could anticipate. In
order to meet your responsibility to maintain
the dam in a reasonable and safe condition, you,
the owner, should conduct regular inspections of
the dam and maintain or repair deficient items.
Regular inspections by qualified professionals are
necessary to identify and correct any problems.
A dam owner should have a thorough understand-ing
of the dam’s physical and social environment.
This would include: knowledge of natural and
technological hazards that threaten it, an under-standing
of the developing human settlement pat-terns
around the dam, and an under-standing of
any events that can lead to structural failure.
It is a good idea for every owner of a dam to
pause and consider what lies below their dam.
Several questions need to be asked.
• What is the nature of the land use
downstream: wooded or agricultural land,
scattered homes, roads, villages, urban?
• How many structures are located within a
half mile, a mile or several miles of the dam?
• How are downstream structures located
with regard to the watercourse or floodplain,
with respect to both distances from the
watercourse or river and elevation above it?
• What is the first-floor elevation of homes
located downstream. Are they only a few
feet above the level of the water surface, or
are they on bluffs high above it and out of
danger?
• Is the valley below the dam characterized
by steep hills, or is there a broad floodplain?
This is an important consideration, as it
determines whether water released in a
dam failure or during flooding would soon
spread out and lose its force or whether a
destructive wall of water would travel a long
distance downstream.
Owning a dam brings many different concerns and
possible rewards, but in the end success will largely
be measured by a continuing record of safety.
Owners can also influence the safety of dams
in more direct ways. They can and should de-velop
their own safety programs, which should
include such important elements as inspection,
monitoring through instrumentation, mainte-nance,
emergency action planning, and proper
operation. Such programs are directly related
to a specific dam’s structure and its immediate
environment and depend on the owner’s knowl-edge
of the dam and how it works.
The Role of Consultants in
Dam Safety
A dam is a special kind of structure, simple
in concept but with many complicated com-ponents.
There is no such thing as a standard
dam design; furthermore, each dam site is
unique. The existence of a dam necessitates
the involvement of many specialists to analyze,
design, build, inspect, and repair it. This wide
variety of consultants will include civil, geo-technical,
mechanical, and electrical engineers,
geologists and hydrologists.
As owner, you should know more about your
dam than anyone else. A consultant can advise
you on such important items as:
• the design and construction of a new dam
• the overall stability of the dam under normal
and flood conditions
• any repairs or maintenance needed by the
dam and appurtenant works
• the severity of any problems and indicate in
what order to repair them
• cost estimates for repair work
• adequacy of the spillway to pass the design flood
12 Oklahoma Water Resources Board - Dam Safety Program - March 2011
• an assessment of downstream hazards
• the dam owner’s preparation and procedures to deal with emergency conditions
Hazardous conditions at the dam should be reported verbally and in writing to the dam owner
and the OWRB. A written report from the owner’s consultant is essential for every inspection. It is
uncommon that a dam owner has all of the technical skills needed to monitor the condition of the
dam. Thus, the role of the consulting engineer is critical in dam safety
Role of the Oklahoma Water Resources Board
The Oklahoma Water Resources Board is responsible for administrating state dam safety laws. The
staff of the OWRB has four primary areas of activity in the dam safety program: (1) review and
approval of plans and specifications of new dams, (2) review of plans and specifications for repairs,
modification, or rehabilitation work, (3) periodic inspections of construction work on new and
existing dams, and (4) review of inspection reports and approval of emergency action plans.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 13
CHAPTER 2:
Introduction to Dams 2 General
The purpose of a dam is to impound
(store) water for any of several reasons,
e.g., flood control, water supply for hu-mans
or livestock, irrigation, energy generation,
recreation, or pollution control. This manual
primarily concentrates on earthen dams, which
constitute the majority of structures in place
and under development in Oklahoma.
The Watershed System
Water from rainfall or snowmelt naturally
runs downhill into a stream valley and then
into larger streams or other bodies of water.
The “watershed system” refers to the drainage
process through which rainfall or snowmelt is
collected into a particular stream valley during
natural runoff (directed by gravity). Dams con-structed
across such a valley then impound the
runoff water and release it at a controlled rate.
During periods of high runoff, water stored in
the reservoir typically increases, and overflow
through a spillway may occur. During periods
of low water flow is normally controlled. Hence,
with the insertion of a dam into a watershed
very high runoffs (floods) and very low runoffs
(drought periods) are generally avoided.
Mark Harrison,
Oklahoma Conservation Commission
14 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Types of Dams
Dams may either be human-built or result from
natural phenomena, such as landslides or glacial
deposition. The majority of dams are human struc-tures
normally constructed of earthfill or concrete.
Naturally occurring lakes may also be modified by
adding a spillway to allow for safe, efficient release of
excess water from the resulting reservoir.
Dam owners should be aware of the different
types of dam’s essential components of a dam how
the components function, and important physical
conditions likely to affect a dam. Human-built
dams may be classified according to the type of
construction materials used, the methods used in
construction, their slope or cross-section, the way
they resist the forces of the water pressure behind
them, the means of controlling seepage, and oc-casionally,
their purpose.
Components : The components of a typical dam
are illustrated in Figure 2.1. Nearly all dams possess
the features shown or variations of those features.
Definitions of the terms are given in the Glos-sary.
The various dam components are discussed in
greater detail later on.
Construction Materials : The materials used
for construction of dams include earth, rock, tailings
from mining or milling, concrete, masonry, steel,
and any combination of those materials.
1. Embankment Dams: Embankment
dams, the most common type in use
today, have the general shape shown in
Figure 2.1. Their side slopes typically
have a grade of two to one (horizontal
to vertical) or flatter. Their capacity
for water retention is due to the low
permeability of the entire mass (in the
case of a homogeneous embankment) or
of a zone of low-permeability material (in
the case of a zoned embankment dam).
Material used for embankment dams
include natural soil or rock obtained from
borrow areas or nearby quarries, or waste
materials obtained from mining or milling
operations. If the natural material has a
high permeability, then a zone of very low
permeability material must be included in
the dam to retain water.
2. Concrete Dams: Concrete dams may be
categorized into gravity and arch dams
according to the designs used to resist the
stress due to reservoir water pressure. A
concrete gravity dam (shown in Figure
2.2) is the most common form of concrete
dam. In it, the mass weight of the concrete
and friction resist the reservoir water
pressure. A buttress dam is a specific type
of gravity dam in which the large mass
of concrete is reduced, and the forces are
diverted to the dam foundation through
vertical or sloping buttresses. Gravity
Figure 2.1. Embankment dam (U.S. Army Corps of Enginers).
Oklahoma Water Resources Board - Dam Safety Program - March 2011 15
dams are constructed of non-reinforced
vertical blocks of concrete with flexible
seals in the joints between the blocks.
Concrete arch dams are typically rather thin
in cross-section. The reservoir water forces
acting on an arch dam are carried laterally
into the abutments. The shape of the arch
may resemble a segment of a circle or an
ellipse, and the arch may be curved in the
vertical plane as well. Such dams are usually
built from a series of thin vertical blocks that
are keyed together, with water stops between
the blocks. Variations of arch dams include
multi-arch dams, in which more than one
curved section is used, and arch gravity
dams, which combine some features of the
two types.
A recently developed method for
constructing concrete gravity dams
involves the use of a relatively weak
concrete mix which is placed and
compacted in a manner similar to that
used for earthfill dams. Roller-compacted
concrete has the advantages of decreased
cost and time. In addition, there are no
joints where seepage could occur.
3. Other Types: Various construction
techniques could be used in a single dam.
For example, a dam could include an
earthen or rock fill embankment as well
as a portion made of concrete. In such a
case, the concrete section would normally
contain the spillway or other outlet works.
A recent design for low-head dams (with
a minimal height of water behind the dam
uses inflatable rubber or plastic materials
anchored at the bottom by a concrete slab.
Some dams are constructed for special
purposes, such as diversion of water, or
permit construction of other facilities
in river valleys. These dams are called
diversion dams and cofferdams,
respectively.
Water Retention Ability
Because the purpose of a dam is to retain water
effectively and safely, its water-retention ability is
of prime importance. Water may pass from the
reservoir to the downstream side of a dam by:
• seeping through the dam
• seeping through the abutments
• seeping under the dam
• overtopping the dam
• passing through the outlet works
• passing through or over a primary spillway
• passing over an emergency spillway
The first three ways water pass from a reser-voir
are considered undesirable, particularly
if the seepage is not limited in area or volume.
Overtopping of an embankment dam is also
very undesirable because the embankment mate-rial
may be eroded away. Additionally, only a few
concrete dams have been designed to be overtopped.
Water normally leaves a dam by passing through an
outlet works or spillway. Water should pass over an
emergency spillway only during periods of very high
reservoir levels and high water inflow.
Figure 2.2. CONCRETE GRAVITY dam (U.S. Army
Corps of Engineers ).
16 Oklahoma Water Resources Board - Dam Safety Program - March 2011
tion, resulting in even greater erosion and probable
dam failure.
Obviously, large, unrestricted seepage is undesirable.
To minimize this possibility, dams are constructed
with internal impermeable barriers and internal
drainage facilities such as drainpipes or filter sys-tems,
or other drainage systems such as toe, blanket,
or chimney drains.
Flow through a dam foundation may be diminished
by grouting known or suspected highly permeable
material, constructing a cutoff wall or trench below
a dam, or constructing an upstream impermeable
blanket. Figure 2.3 illustrates a cutoff trench.
In summary, the overall water retention ability of
a dam depends on its permeability, the abutments,
the foundation, and the efforts made to reduce that
permeability or restrict the flow of water through
these components. Should high permeability oc-cur,
seepage can lead to piping, which will likely
result in failure.
Release of Water
Intentional release of water, as stated earlier,
is confined to water releases through a service
spillway or outlet works or over emergency
spillways.
Principal or Mechanical Spilway : The
principal or mechanical spillway maintains the
normal water level in the reservoir. Its function is to
pass expected flood flows past the dam safely and
without erosion. It may consist of a pipe through
the dam or a system of gates that discharge water
into a concrete spillway. Either method uses the
Seepage Through a Dam: All embankment
dams and most concrete dams allow some seep-age.
The earth or other material used to construct
embankment dams has some permeability, and
water under pressure from the reservoir will
eventually seep through. However, it is impor-tant
to control the quantity of seepage by using
low permeability materials in construction and
by channeling and restricting the flow so that
embankment materials do not erode.
Seepage through a concrete dam is usually minimal
and is almost always through joints between blocks,
or through cracks or deteriorated concrete which
may have developed. Maintenance of these joints
and cracks is therefore essential. The seepage water
should be collected and channelized, so that its
quantity can be measured and erosion minimized.
Seepage Around a Dam: Seepage around the
ends of a dam through the abutment materials or
under a dam, through the dam foundation material,
may become a serious problem if the flow is large or
of sufficient velocity to cause erosion. Seepage under
a dam also creates high hydrostatic uplift (pore-water)
pressure, which has the effect of diminishing
the weight of the dam, making it less stable.
Seepage through abutments or foundations can
dissolve the constituents of certain rocks such as
limestone, dolomite, or gypsum so that any cracks
or joints in the rock become progressively larger
and in turn allow more seepage.
Abutment or foundation seepage may also result in
“piping” internal erosion, in which the flow of water
is fast enough to erode away small particles of soil.
This erosion progresses from the water exit point
backward to the entrance point. When the entrance
point is reached, water may then flow without restric-
Figure 2.3. Embankment dam WITH
A CUT-OFF TRENCH (U.S. Army Corps
of Enginers ).
Oklahoma Water Resources Board - Dam Safety Program - March 2011 17
overflow principle. When the reservoir reaches a
certain level, water flows into a standpipe or riser
pipe (Figure 2.4) or over a gate. Intake structures
for spillways must have systems that prevent clog-ging
by trash or debris.
Drawdown Facility : All dams should have
some type of drawdown facility which can:
• quickly lower the water level if failure of the
dam is imminent.
• serve the operational purposes of the
reservoir.
• lower the water level for dam repairs.
• periodically raise and lower the pool level to
kill weeds and mosquitoes.
The valve regulating the drawdown facility
should be on the upstream end of the conduit
to minimize the risk to the dam posed by a pos-sible
internal rupture of the pipe.
Emergency (Auxiliary ) Spilway : As the
name implies an emergency spillway functions
during emergency conditions to prevent over-topping
of a dam. A typical emergency spillway
is an excavated channel in earth (Figure 2.5) or
rock near one abutment of a dam. An emergency
spillway should always discharge away from the
toe of a dam to avoid its erosion. Furthermore,
the spillway should be constructed in such a
manner that the spillway itself will not seriously
erode when it is in use. Obviously erosional fail-ure
of the spillway could be as catastrophic as
failure of the dam itself. An emergency spillway
should be sized to convey the so-called “design
flood”, the rare, large-magnitude flood used to
establish design criteria. The spillways of many
existing dams are now considered undersized
because standards for the design flood have
increased over the years.
Figure 2.4.
Principal Spilway
Figure 2.5.
Emergency
Spilway . Mark Harrison,
Oklahoma Conservation Commission
Oklahoma Water Resources Board - Dam Safety Program - March 2011 19
CHAPTER 3:
Hazards, Risks, and Failures 3
General
Dam failures are severe threats to life and
property and are now being recorded
and documented much more thoroughly
than in the past. Recorded losses have been high.
Statistics on losses of life and property fully justify
the need for dam owners to better understand the
risks to the public posed by dams, the kinds of haz-ards
that promote those risks and owner liabilities
associated with them, and, generally, the reasons
that dams fail. Improving a dam owner’s under-standing
of realistic risks and possible reasons for
failure is an essential first step in any overall effort
to improve dam safety and preserve the benefits of
dam ownership.
Hazards as Sources of Risks
The dam structure itself can be a source of risk
due to possible construction flaws and weaknesses
that develop because of aging. The site immedi-ately
surrounding the structure may also increase
the structural risk if the dam is not positioned or
anchored properly or if excessive reservoir seep-age
erodes the foundation or abutments.
The physical hazards that can cause dam fail-ure
are translated into high risks when people
or properties are threatened. These high risks
are exacerbated by a number of important fac-tors.
For instance, in Oklahoma and most other
states, people are often allowed to build within
a dam’s inundation zone, thereby greatly com-pounding
the associated risk.
Natural hazards such as floods, earthquakes,
and landslides are also important contributors
to risk. These have now become even greater
hazards because development has placed people
and property in their way. Failure to adjust to
these events has been costly both to dam owners
and to the public in general.
Human behavior is another element of dam fail-ure
risk; simple mistakes, operational misman-agement,
negligence, unnecessary oversights,
or destructive intent can interact with other
hazards to compound the possibility of failure.
Figure 3.1 Embankment Dam failure in Cleveland county
20 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Floding from high precipitation : Of
the natural events that can impact dams, floods
are the most significant. A floodplain map of the
U.S. (Figure 3.2) gives the estimated percentage
of land resting within a floodplain. Floods are the
most frequent and costly natural events that lead
to disaster in the U.S. Therefore, flood potentials
must be included in risk analyses for dam failure.
Flash floods can happen anywhere in Oklahoma,
even on small drainages. A common safety factor
for dam design is to construct them to withstand
a “probable maximum flood” (PMF) assumed to
occur on the upstream watershed. A PMF is the
flood that may be expected from the most severe
combination of critical meteorologic and hydro-logic
conditions that are reasonably possible in
the region. However, dams are often built in areas
where estimates of the PMF are based on short
precipitation and runoff records. As a result, spill-way
capacity may often be underestimated.
Flooding from dam failure : When a dam
fails as a result of a flood, more people and prop-erty
are generally placed in jeopardy than during
Thus, a broad range of natural and human
hazards, taken separately or in combination, in-crease
the probability of dam failure and injury
to people and property.
The following discussion of some of the most
significant hazards that lead to public risk illus-trates
the interrelationships among events that
can lead to dam failure.
Natural Hazards That
Threaten Dams
The most important natural hazards threaten-ing
dams include:
• flooding from high precipitation
• flooding from dam failure
• earthquakes
• landslides
Figure 3.2 Floodplain map of the U.S.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 21
natural floods. The Rapid City, South Dakota,
flood of 1970, which killed 242 people, caused a
dam failure which added significantly to the loss
of life. When a natural flood occurs near a dam,
the probability of failure and loss of life almost
always increases.
The sudden surge of water generated by a dam
failure usually far exceeds that expected from a
natural 100-year floodplain estimate; therefore,
residences and businesses that would escape
natural flooding may still be at extreme risk
from flooding due to dam failure. Hence, it is
important to inform residents and business per-sonnel
of the full risk to which they are exposed
so that they can respond accordingly.
To compound the risk even further, when one
dam fails, the sudden surge of water may well
be powerful enough to destroy another dam
downstream. Upstream dams may seem too far
away to be a real threat, but inundation zones
and surge crests can extend many miles down-stream,
especially if the reservoir behind the
collapsed dam held a large quantity of water.
Earthquakes : Pose significant threats to
dam safety. While very common in Oklahoma,
it is rare that earthquakes here are substantial
enough to harm a dam. Nevertheless, dam own-ers
should be aware of the history of seismic
activity in their locality and develop their emer-gency
procedures accordingly.
Both earthen and concrete dams can be dam-aged
by ground motions caused by seismic
activity. Cracks or seepage can develop, leading
to immediate or delayed failure. Dams, such
as those in California, located near relatively
young, active faults are of particular concern,
but dams (especially older concrete and earthen
structures) located where relatively low-scale
seismic events may occur are also at risk. Recent
detailed seismic analyses have indicated a much
broader area of seismic activity sufficient to
damage dams than previously considered; the
seismic risk is essentially nationwide.
Landslides: Rock slides and landslides may
affect dams directly by blocking a spillway or by
eroding and weakening abutments. Indirectly,
a large landslide into a reservoir behind a dam
can cause an overflow wave that will exceed the
capacity of the spillway and lead to failure. A
landslide (or mudslide) can form a natural dam
across a stream which can then be overtopped
and fail. In turn, failure of such a natural dam
could then cause the overtopping of a down-stream
dam or by itself cause damage equivalent
to the failure of a human built dam. In addition,
large increases in sediment caused by such
events can materially reduce storage capacity
in reservoirs and thus increase a downstream
dam’s vulnerability to flooding. Sedimentation
can also restrict the operation of low level gates
and water outlets; damage to gates and outlets
can lead to failure.
Hazards from Human Activity
Human activity must also be considered when
analyzing the risks posed by dams. The “high
hazard” designation does not imply structural
weakness or an unsafe dam. In Oklahoma, the
hazard classification of dams is based on the
potential for loss of life and economic loss in the
area downstream of the dam, not on its structural
safety (Table 3.1). Thus, dams that may be of very
sound construction are labeled “high hazard” if
failure could result in catastrophic loss of life.
Risk may well increase through time because few
governmental entities have found the means to
limit settlement below dams. The hazard level
of more dams is rising to “high” or “significant”
as development occurs in potential inundation
zones below dams previously rated “low hazard.”
Because of short-term revenue needs or other
pressures, governments often permit develop-ment
in hazardous areas despite long-term dan-ger
and the risk of high future disaster costs.
Diversion of development away from potential
inundation zones is a sure means of reducing
22 Oklahoma Water Resources Board - Dam Safety Program - March 2011
risk, but is not always a policy suitable to the
immediate needs of local government. Perhaps
the ultimate irony for a dam owner is to have de-veloped
and implemented a safety program only
to have development permitted in the potential
inundation zone so that the hazard rating and
owner’s liability increase.
All sorts of other human behavior should be in-cluded
in risk analyses; vandalism, for example,
cannot be excluded and is in fact a problem
faced by many dam owners. Vegetated surfaces
of a dam embankment, mechanical equipment,
manhole covers and rock riprap are particularly
susceptible to damage by people. Every precau-tion
should be taken to limit access to a dam by
unauthorized persons and vehicles. Dirt bikes
(motorcycles) and off-road vehicles, in par-ticular,
can severely degrade the vegetation on
embankments. Worn areas lead to erosion and
more serious problems.
Mechanical equipment and associated control
mechanisms should be protected from tamper-ing,
whether purposeful or inadvertent. Buildings
housing mechanical equipment should be sturdy,
have protected windows, and heavy-duty doors,
and be secured with padlocks. Detachable controls,
such as handles and wheels, should be removed
when not in use and stored inside the padlocked
building. Other controls should be secured with
locks and heavy chains where possible. Manhole
covers are often removed and sometimes thrown
into reservoirs or spillways by vandals.
Rock used as riprap around dams is sometimes
thrown into the reservoirs, spillways, stilling
basins, pipe-spillway risers, and elsewhere.
Riprap is often displaced by fishermen to form
benches. The best way to prevent this abuse is
to use rock too large and heavy to move easily,
or to slush-grout the riprap. Otherwise, the rock
must be regularly replenished and other dam-ages
repaired. Regular visual inspection can
easily detect such human impacts.
Owners should be aware of their responsibility for
the safety of people using their facility even though
their entry may not be authorized. “No Trespassing”
signs should be posted, and fences and warning signs
erected around dangerous areas. As discussed in
Chapter 10, liability insurance can be purchased for
protection in the event of accidents.
Site-Specific Structural Risk
Developing site-specific risk analyses involves
consideration of a number of hazards. Such
analyses are helpful in stimulating better aware-ness,
planning, and design. In some cases dam
structure analyses are quantitative. Hence, pre-cise
conclusions about engineering and design
can be made. Probabilistic analyses can also be
important and useful; however, exact quantita-tive
and probabilistic tools are not yet applicable
in many situations and do not fully supplement
or replace qualitative analyses such as informed
perception and judgment of the risks. Judgment
and engineering experience should play an im-portant
role in reaching useful conclusions in
any site-specific analysis of structural risk.
TABLE 3.1. Table of Hazard-Potential Clasification
Hazard -Potential Classification Description
Low Dams assigned the low hazard-potential classification are those where failure would
result in no probable loss of human life and low economic losses.
Significant
Dams assigned the significant hazard-potential classification are those dams where
failure would result in no probable loss of human life but can cause economic loss or
disruption of lifeline facilities.
High Dams assigned the high hazard-potential classification are those where failure will
probably cause loss of human life.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 23
As mentioned in Chapter 2, structural risks tend
to result from design and construction prob-lems
related to the dam materials, construction
practice, and hydrology. The complexity of the
hazard is such that structural design and causes
of dam failure are significant areas of research
in engineering. Indeed, better design criteria
have been developed and safer dams are being
built, but there is no basis for complacency.
Dams continue to age, people continue to move
into inundation zones, and enough hazards exist
that the net risk to the public will remain high
despite design improvements.
Sources of Dam Failure
There are many complex reasons, both struc-tural
and non-structural, for dam failure. Many
sources of failure can be traced to decisions
made during the design and construction
process and to inadequate maintenance or op-erational
mismanagement. Failures have also
resulted from the natural hazards previously
mentioned. However, from your perspective
as owner, the structure of a dam is the starting
point for thorough understanding of the poten-tials
for failure.
Thre Categories of Structural Failure :
Three categories of structural failure alluded to
in Chapter 2 are:
• overtopping by flood
• foundation defects
• piping and seepage
Overtopping may develop from many sources,
but often evolves from inadequate spillway
design. Alternatively, even an adequate spillway
may become clogged with debris. In either situ-ation,
water pours over other parts of the dam,
such as abutments or the toe, and erosion and
failure follow.
Concrete dams are more susceptible to founda-tion
failure than overtopping, whereas earthen
dams suffer from seepage and piping.
Overall, these three events have about the same
incidence. A more specific analysis of the po-tential
sources of failure has to take into account
types of dams. Similarly, the characteristics of
the type of dam being monitored will point to
problems requiring more careful attention by
the owner when developing a safety program.
Failures :
Embankment or Earthen Dams: The major reason
for failure of fill or embankment dams is piping or
seepage. Other hydrologic failures are significant
as well, including overtopping and erosion from
water flows. All earthen dams exhibit some seep-age;
however, as discussed earlier, this seepage can
and must be controlled in velocity and amount.
Seepage occurs through the structure and, if
uncontrolled, can erode material from the down-stream
slope or foundation backward toward the
upstream slope. This “piping” phenomenon can
lead to a complete failure of the structure. Piping
action can be recognized by an increased seepage
flow rate, the discharge of muddy or discolored
water below the dam, sinkholes on or near the
embankment, and a whirlpool in the reservoir
(see Inset 3.1).
Hydrologic failures of earthen dams result from
the uncontrolled flow of water over the dam,
around it, adjacent to it, or from the erosive action
of water on the dam’s foundation. Earthen dams
are particularly susceptible to hydrologic failure
since most sediment erodes at relatively low water
flow velocities. Once erosion has begun during
overtopping, it is almost impossible to stop. In a
very special case, a well-vegetated earthen em-bankment
may withstand limited overtopping
if water flows over the top and down the face as
evenly distributed sheet and does not become
concentrated in a single channel.
Concrete Dams: Failure of concrete dams (see
Inset 3.2) is primarily associated with foundation
problems. Overtopping is also a significant cause
again primarily when spillways are built with in-adequate
capacity. Other causes include failure to
let concrete set properly and earthquakes.
24 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Age and Its Relation to Failure
Foundation failures occur relatively early in the
life of a dam, whereas other causes generally take
much longer to materialize. Thus, it is not surpris-ing
that a very large percentage of all dam failures
occur during initial filling, since that is when
design or construction flaws, or latent site defects,
appear. As dams age, maintenance becomes more
critical. Lack of maintenance will result in dete-rioration
and eventually, failure. Oklahoma dams
are aging as shown in Table 3.2, and problems as
described above are slowly becoming apparent.
INset 3.1.
Examples of Earthen-Dam Failures
Southfork, Pensylvania
The famous Johnstown disaster, caused by the
failure of the South Fork Dam in 1889, in which
2,209 people were killed, is an example of the
overtopping of an earthen dam. Heavy rainfall
in the upper drainage basin of the dam filled
the reservoir and caused overtopping. It was
later calculated that, if a spillway had been built
according to specifications and if the original
outlet pipes had been available for full capacity
discharge, there would have been no overtop-ping.
Teton Dam, Idaho
The Teton Dam failure in 1976 was attributed to
(1) internal erosion (piping) of the core of the
dam deep in the right foundation key trench,
with the eroded soil particles finding exits
through channels in and along the interface
of the dam with the highly pervious abutment
rock and talus to points at the right groin of
the dam; (2) destruction of the exit avenues
and their removal by the outrush of reservoir
water, (3) the existence of openings through
inadequately sealed rock joints which may have
developed through cracks in the core zone in
the key trench; (4) the development of piping
through the main body of the dam that quickly
led to complete failure; and (5) the design of the
dam did not adequately take into account the
foundation conditions and the characteristics of
the soil used for filling the key trench.
Baldwin Hills and St. Francis Dams ,
California
The Baldwin Hills Dam failed in 1963 follow-ing
displacement of its foundation. Foundation
problems were ultimately traced to seismic
activity along nearby faults. The failure of the
large St. Francis Dam (part of the water supply
system for Los Angeles) in 1928 was also attrib-uted
to a variety of problems related to founda-tion
pressures, seepage around the foundation,
and faulty operation.
Source: Jansen, 1980.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 25
Inset 3.2.
Examples of Concrete-Dam Failures
Austin, Pensylvania
An example of a foundation problem can be found in the failure of the Austin, Pennsylvania Dam
in September, 1911. Evidently, the reservoir was filled before the concrete had set sufficiently.
Eventual failure near the base occurred because of weakness in the foundation or in the bond
between the foundation and the concrete. .
Walnut Grove , Arizona
In 1890, the Walnut Grove dam on the Hassayompa River failed due to overtopping, killing about
150 people. The failure was blamed on inadequate capacity of the spillway and poor construction
and workmanship. A spillway 6 x 26 feet had been blasted out of rock on one abutment, but, with
a drainage area above the dam site of about 500 square miles, the spillway did not have nearly
enough discharge capacity.
Source: Jansen, 1980.
Dates Percent of Dams Constructed
Prior to 1950 10.7
1950 – 1959 15.9
1960 – 1969 44.9
1970 – 1979 21.0
1980 – 1989 5.2
1990 – 1999 1.8
2000 – Present 0.6
TABLE 3.2. Ages of dams in Oklahoma
Oklahoma Water Resources Board - Dam Safety Program - March 2011 27
Objectives of a Safety Program
The pressing issue of dam failure points
up the need for a safety program. You,
the owner, should base your program
on an evaluation of your dam’s structural and
operational safety. Your program should iden-tify
problems and recommend remedial repairs,
operational restrictions and modifications, or
further analyses and studies to determine solu-tions.
Components of a safety program that
address the spectrum of possible actions to be
taken over the short and long-term include:
• assessing the condition of the dam and its
components
• conducting preliminary and detailed
inspections
• identifying repairs and continuing
maintenance needs
• establishing periodic and continuous
monitoring capabilities over the long-term
• establishing an emergency action plan to
help minimize adverse impacts should the
dam fail
• establishing operations procedures which
recognize dam failure hazards and risks
• documenting the safety program so that the
information established is available at times
of need and can be readily updated
Develop your safety program in phases, begin-
CHAPTER 4:
Developing a Personal Safety Program4
ning with collection and review of existing
information, proceeding to detailed inspections
and analyses, and culminating with formal
documentation. You can accomplish much of
the preliminary work personally, with the as-sistance
of state and local agencies. However,
depending upon the number and seriousness
of problems identified by the initial assessment,
you may require the professional assistance of
qualified engineers and contractors.
Guidelines for Assessing
Existing Conditions
The guidelines for assessing existing conditions
involve a sequence of steps that will enable you,
the owner, to secure the information you will
need to determine whether subsequent detailed
investigations, repairs, and maintenance are
required. The steps include:
• reviewing existing data
• visiting the site
• inspecting the dam
• assessing significance of observed conditions
• deciding what to do next
Reviewing Existing Data : First and foremost,
collect and review available information on the
dam such as plans of its design, construction,
28 Oklahoma Water Resources Board - Dam Safety Program - March 2011
and operation. Maps of the site, watershed,
and the downstream channel reaches are also
valuable. Review the design of the dam and its
appurtenant structures to assess its actual per-formance
compared to that intended.
Review engineering records originating during
construction to verify that structures were con-structed
as designed. Collect records of subse-quent
construction modifications, as well as op-eration
records that document the performance
of the dam and reservoir. Review any previous
emergency action plan to determine if it is
up-to-date and workable. Incorporate all these
records into a notebook or file; they are most
important in establishing a safety program and
serve as the basis for its supporting documenta-tion.
(For help with the development of such
documentation, refer to Chapters 5 through
10.) If no records exist, a detailed examination
of the structure is appropriate.
Visiting the Dam Site: Undoubtedly you
know it well and have visited it many times, but
in this visit there are particular things for you to
look for. Take a fresh look at the dam structure
and its surroundings from the view of their
potential hazard.
Inspecting the Dam: Also, take a detailed and
systematic look at all components of the dam
and reservoir system. The description of the
site’s components in Chapter 2 should aid this
inspection. (The descriptions are general, so
bear in mind that dams and their components
come in various shapes and sizes and differ
greatly in detail).
Features to inspect include :
• access roads and highways
• upstream slope
• crest
• downstream slope
• left and right abutments
• spillways
• outlets
• drains
• reservoir area (exposed and submerged) area
immediately downstream of the dam
• downstream areas for change in hazard
classification
What to look for :
• obvious deterioration
• cracks and slumps
• boiling seepage
• less than obvious internal corrosion
• weathering
• settlement
• foundation-rock deterioration
• dissolution
A dam can look stable and still be susceptible to
failure from gradual deterioration of its internal
structure. Regular and very detailed inspections
(Chapter 5) and follow-up monitoring (Chapter
6) and maintenance (Chapter 7) are needed to
ensure maximum safety.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 29
Assesing Significance of Observed
Conditions: Chapter 5 presents detailed
information on conducting inspections and
assessing the significance of conditions you
observe. Typically, eroded areas, seepage, slides,
and outflow draw the most attention.
Deciding What to Do Next: Your dam safety
program is now off to a good start. Available infor-mation
on design and construction of the dam and
later structural modifications provides perspective
on its existing condition relative to that intended.
If no documentation exists, then development of
equivalent details should be a first priority. Inspec-tion
and documentation assistance is available from
several sources, including the Oklahoma Water Re-sources
Board, the state agency responsible for dam
safety. Professional engineering consultants can also
perform detailed inspections, testing, and analyses,
and create documentation (Chapter 10).
Procedural Guidelines –
A Source Book
This chapter provides an overview of how to
establish a safety program. Subsequent chapters
detail technical and procedural steps of the pro-gram
components. They include:
• detailed inspection guidelines (Chapter 5)
• monitoring and instrumentation guidelines
(Chapter 6)
• maintenance guidelines (Chapter 7)
• emergency action guidelines (Chapter 8)
• operations guidelines (Chapter 9)
These program components can be visualized
as a sequence of initial and continuing activities
to insure dam safety. The flowchart illustrates
the cyclical nature of the program and the need
for continuing vigilance. Emergency action can,
it is hoped, be avoided, but a well thought out
plan of action (Chapter 8) in case of imminent
or actual failure can greatly reduce damage and
loss of life.
Figure 4.1
Flow Chart of Dam Safety
Pro gram Components
Documenting the
Safety Program
It is important to document a safety program in
order to make the best use of reliable information
about the dam. The procedural guidelines that
follow can serve as an outline or table of contents
for a safety program report. The operations plan
(Chapter 9) presents a detailed outline of the infor-mation
that should be included in the documen-tation.
The chapters that follow suggest forms for
inspections, monitoring, etc., which can be used
to record information. It is helpful to maintain
all the material in a single notebook or file that is
easily accessible so that it can be updated and is
available when needed. Store a duplicate copy of
the report at a different location.
INSPECTION
OPERATIONS MONITORING
rEPAIRS &
mAINTENANCE
EMERGENCY
ACTION
Oklahoma Water Resources Board - Dam Safety Program - March 2011 31
CHAPTER 5:
Inspection Guidelines 5 Introduction
An effective inspection program is essen-tial
for identifying problems and pro-viding
safe maintenance of a dam. An
inspection program should involve four types
of inspections: (1) periodic technical inspec-tions;
(2) periodic maintenance inspections;
(3) downstream development inspections; (4)
informal observations by project personnel as
they operate the dam.
Technical inspections must be performed by
professional engineers familiar with the design
and construction of dams and should include
assessments of structure safety.
Maintenance inspections are performed more
frequently than technical inspections in order to
detect, at an early stage, any developments that may
be detrimental to the dam. They involve assessing
operational capability as well as structural stability.
Maintenance inspections are included as part of
the more comprehensive technical inspection.
Downstream hazard verification inspections
are performed by the dam owner to determine
if there has been any construction of homes,
buildings, or other structures downstream
of their dam which could affect the hazard
classification of their dam. This is a particular
problem for low hazard-potential dams. If
a house, other inhabited structure, or other
construction is built downstream of a dam this
could result in the need to reclassify to a higher
hazard-potential class. This has important
implications for the dam owner as it could
result in a change how often the dam must be
inspected and require structural changes to
the dam. Structural changes could include the
amount of freeboard that must be maintained
and the amount of water the spillway must be
able to pass.
Informal inspections are actually a continuing effort
by the dam owner’s on-site project personnel (dam
tenders, powerhouse operators, maintenance workers)
performed in the course of their normal duties.
The continued effectiveness of these inspections
requires education of new personnel. Regular
visual inspections are among the most economical
means you, the owner, can use to ensure the safety
and long life of your dam and its immediate
environment. Visual inspection is a straightforward
procedure that can be used by any properly trained
person to make a reasonably accurate assessment
of a dam’s condition.
Technical and maintenance inspections involve
careful examination of the surface and all parts of
the structure, including its adjacent environment,
by a professional engineer. The equipment required
is not expensive, and the inspection usually can be
completed in less than one day (see Appendix A).
Hazard verification and informal inspections can
be performed by the dam owner or their operator.
A dam owner, by applying the maximum prudent
effort, can identify any changes in previously noted
conditions that may indicate a safety problem.
Quick, corrective action to conditions requiring
attention will promote the safety and extend the
useful life of your dam, while possibly preventing
costly future repairs.
32 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Recording Inspection Observations : An
accurate and detailed description of conditions
during each inspection will enable meaningful
comparison of conditions observed at different
times. The inspector should record all measure-ments
and observed details required for an ac-curate
picture of a dam’s current condition and
possible problems. Using the forms discussed
in Chapter 9, and given in the appendices, will
help record the details. This information has
three elements:
Location : Accurately describe the location
of any questionable area or condition so that
it can be evaluated for changes over time or
re-examined by experts. Photographs should
be taken of the upstream and downstream
embankments, outlet and conduit structures,
emergency spillway, and toe of the dam, as well
as photographs of any specific problem areas.
Record the location along the dam, as well as
the distance above the toe or below the crest.
Similarly, document the location of problems in
the outlet or spillway.
Extent or area: The length, width, and
depth (or height) of any suspected problem area
should be determined.
Organizing for Inspection
The following discussion is concerned primarily with technical inspections. All inspections
should be organized and systematic. Inspectors should use equipment appropriate for the
task, record observations accurately, and survey the structure and site comprehensively. It
is essential that documentation be developed and maintained in order to ensure adequate
follow-up and repair (Appendix A). Chapter 9 further discusses what form this documenta-tion
should take.
Technical inspections are to be conducted annually for high hazard-potential dams and
once every three years for significant hazard-potential dams. Inspection reports are to be
submitted to the Oklahoma Water Resources Board, Dam Safety Program. Owners of low
hazard-potential dams are only required to submit a downstream hazard-potential verifica-tion
and maintenance inspection once every five years.
Descriptive detail : Give a brief yet detailed
description of any anomalous condition. Some
items to include are:
• quantity of drain outflows
• quantity of seepage from point and area
sources
• color or quantity of sediment in water
• depth of deterioration in concrete
• length, displacement, and depth of cracks
• extent of moist, wet, or saturated areas
• adequacy of protective cover
• adequacy of surface drainage
• steepness or configuration of slopes
• apparent deterioration rate
• changes in conditions
Coverage : An inspection is conducted by
walking along and over a dam as many times as
is required to observe the entire structure. From
any given location, a person can usually gain a
detailed view for 10 to 30 feet in each direction,
depending upon the smoothness of the surface
Oklahoma Water Resources Board - Dam Safety Program - March 2011 33
or the type of material (grass, concrete, riprap,
brush) on the surface. On the downstream
slope, a zigzag inspection path will ensure that
any cracking is detected.
Sequence: The following inspection sequence
ensures that systematic coverage of an entire
site is obtained:
• upstream slope
• crest
• downstream slope
• seepage areas
Following a consistent sequence lessens the
chance of an important condition being
overlooked. Reporting inspection results in
the same sequence is recommended to ensure
consistent records. Inspection forms are
included in Appendix A. The forms should be
supplemented with additional details specific to
a given dam.
Record keping: The inspector should fill out
a dated report for each inspection, which should
be filed along with any photographs taken (which
should also be dated). In addition to inspection
observations, monitoring measurements and
weather conditions (especially recent rains,
extended dry spells, and snow cover) should
also be systematically included in the inspec-tion
record. A sketch of the dam with problem
areas noted is helpful. Immediately following an
inspection, observations should be compared
with previous records to see if there are any
trends that may indicate developing problems.
If a questionable change or trend is noted, and
failure is not imminent, you, the owner, should
consult a professional engineer experienced in
dam safety. Reacting quickly to questionable
conditions will ensure the safety and long life
of a dam and possibly prevent costly repairs or
expensive litigation.
Crucial inspection times: There are at least
six special times when an inspection is recom-mended
regardless of the regular schedule:
(1) Prior to a predicted major rainstorm:
check spillway, outlet channel, and riprap.
(2) During or after a severe rainstorm: check
spillway, outlet channel, and riprap.
(3) During or after a severe windstorm: check
riprap performance during the storm (if
possible) and again after the storm has
subsided.
(4) Following an earthquake in the area: make
a complete inspection immediately after the
event and weekly inspections for the next
several months to detect any delayed effects.
(5) During construction, repairs, or
modification of the dam.
(6) During and immediately after the first
reservoir filling: schedule a regular
program of frequent complete inspections
during the period a reservoir is first
being filled to ensure that design and site
conditions are as predicted. An inspection
and filing schedule are frequently
prescribed by the design engineer.
Embankment Dams and Structures
Embankment dams constitute the majority of
structures in place in the U.S. The major features
include:
• upstream slope
• downstream slope
• crest
• seepage areas
• spillway
Many of the principles and
guidelines presented in that
section are also aplicable to
concrete structures . !
• inlet
• outlet
• spillway
34 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Upstream Slope : Typically, major problems
encountered on an upstream slope are:
• cracks
• slides
• cave-ins or sinkholes
• severe erosion
The first three conditions may indicate serious
problems within the embankment. Severe
erosion obviously can weaken the structure. An
upstream slope should receive a close inspection
because riprap, vegetative cover, and high water
levels can hide problems. (When walking on
riprap, take caution to avoid personal injury.)
Slope protection is designed to prevent erosion
of the embankment slopes, crest, and groin areas.
Inadequate slope protection usually results in
deterioration of the embankment from erosion,
and in the worst cases, can lead to dam failure.
The inspector should look for inadequate slope
protection, including eroded vegetative cover
and displaced riprap.
The two primary types of slope protection
used on embankment dams include vegetative
cover (grass) and riprap (rock). Grass cover is
usually used on most embankment surfaces,
while riprap is commonly used on the shoreline
of the upstream slope. Soil, cement, concrete,
asphalt, articulated concrete blocks, and other
types of slope protection also may be used.
The type of slope protection selected depends
upon economics, how the dam is used, and the
prevailing conditions found at the site. A good
growth of grass on an embankment provides
excellent protection against erosion caused
by rainfall and runoff. Deep rooted grass that
can tolerate repeated wetting and drying cycles
should be used on embankments.
A lack of vegetative cover or insufficient vegetative
cover will result in rapid deterioration of the
embankment by erosion. A lack of riprap, or
improperly designed riprap along the shoreline
can result in erosion of the shoreline soils if riprap
is needed to protect the soil against wave action. It
should be noted that not all dams will require riprap
shoreline protection.
A crisscross path should be used when inspecting the
slope so that cracks and slides can be easily identified.
In many instances, sighting along the waterline
alignment will indicate a change in the uniformity
Figure 5.1. Drawin g of typical Embankment Dam Features .
Oklahoma Water Resources Board - Dam Safety Program - March 2011 35
of the slope; an inspector should stand at one end
of the dam and sight along the waterline, checking
for straightness and uniformity. If a crack is seen,
the crest and downstream slope in its immediate
area should be carefully inspected. Cracks indicate
possible foundation movement, embankment failure,
or a surface slide. Locating them can be difficult.
Cracks more than one foot deep usually are not
produced by drying and are likely cause for concern.
A line of recently dislodged riprap on an upstream
slope could indicate a crack below the riprap.
Slides can be almost as difficult to detect as
cracks. When a dam is constructed, the slopes
may not be uniformly graded. Familiarity
with the slope configuration at the end of
construction can help identify subsequent slope
movements. Moreover, the appearance of slides
may be subtle; for example, they may produce
only about two feet of settlement or bulging in a
distance of 100 feet or more, yet that would still
be a significant amount of settlement.
Dated photographs are particularly helpful in
detecting such changes. Sinkholes or cave-ins result
from internal erosion of the dam—a very serious
condition for earthen embankments. The internal
erosion, or piping, may be reflected by turbid
seepage water on exit. Surface soil may be eroded
by wave action, rain runoff, and animal burrowing.
Such erosion, if allowed to continue, can lessen
the thickness of the embankment and weaken the
structure.
Animal burrows on the upstream slope can also
indicate a serious problem on smaller dams. Beavers,
nutria, and other burrowing animals can create
pathways for seepage. See Chapter 7. To ensure
adequate inspection, prevent potential seepage paths,
and keep the upstream slope free from obscuring
weeds, brush, or trees.
Downstream Slope : The downstream slope
should be inspected carefully because it is the
area where evidence of developing problems
appears most frequently. To ensure adequate
Figure 5.2. figure of various problems with an embankment dam .
no trash rack:
principal spillway
intake structure
partially blocked
with debris
THE PROBLEM DAM
ruts from traffic
with gullies
forming
Settlement Surface crack
Trees
and
brush
Animal
burrows
Emergency
spillway too
small
Emergency
spillway
partially
blocked
Downstream
channel
partially
blocked
Sloughed
area
springs and
excessive
seepage
36 Oklahoma Water Resources Board - Dam Safety Program - March 2011
inspection, keep this area free from obscuring
weeds, brush, or trees. On the downstream
slope, some of the more threatening conditions
that could be identified are:
• cracks
• slides
• seepage
Notify the designated dam-safety authorities
immediately if any of these conditions (Fig. 5.3)
are noted on the downstream slope.
Cracks can indicate settlement, drying and
shrinkage, or the development of a slide. Whatever
the cause, cracks should be monitored and changes
in length and width noted. Drying cracks may
appear and disappear seasonally and normally will
not show vertical displacement as will settlement
cracks or slide cracks. Slides require immediate
detailed evaluation. Early warning signs include
a bulge in the embankment near the toe of a dam
or vertical displacement in the upper portion of an
embankment. Seepage is discussed separately. If a
downstream slope is covered with heavy brush or
vegetation, a more concentrated search must be
made and may require cleaning off the vegetation.
In addition, the downstream slope should be
inspected for animal burrows, excessive vegetative
cover, and for erosion, especially at the contacts with
the abutments. Figures 5.1 & 5.2 show potential
problems with the downstream slope, causes,
possible consequences, and recommended action.
Crest: A dam’s crest usually provides the pri-mary
access for inspection and maintenance.
Because surface water will pond on a crest
unless that surface is well maintained, this part
of a dam usually requires periodic re-grading.
However, problems found on the crest should
not be simply graded over or covered up.
On the crest, some of the more threatening
conditions that may be identified are:
• longitudinal cracking
• transverse cracking
• misalignment
• sinkholes
Longitudinal cracking (Figure 5.3) can indicate
localized instability, differential settlement,
movement between adjacent sections of the
embankment, or any combination of the three.
Longitudinal cracking is typically characterized by
a single crack or a close, parallel system of cracks
along the crest, more or less parallel to the axis of the
dam. These cracks, which are usually continuous
over their length and usually greater than one foot
deep, can be differentiated from drying cracks,
which are usually intermittent, erratic in pattern,
shallow, very narrow, and numerous.
Longitudinal cracking may precede vertical
displacement as a dam attempts to adjust to a
position of greater stability. Frequently, longitudinal
cracking occurs at the edge of the crest with either
slope. Vertical displacements on the crest are
usually accompanied by displacements on the
upstream or downstream face of a dam.
Vertical
Displacement
Figure 5.3. Longitudinal cracks
Oklahoma Water Resources Board - Dam Safety Program - March 2011 37
Transverse cracking (Figure 5.4) can indicate
differential settlement or movement between
adjacent segments of a dam. Transverse cracking
usually manifests as a single crack or a close, parallel
system of cracks that extend across the crest more or
less perpendicular to the length of the dam. This type
of cracking is usually greater than one foot in depth.
If this condition is seen or suspected, notify the
Oklahoma Dam Safety Program office immediately.
Transverse cracking poses a definite threat to the
safety and integrity of a dam. If a crack should
progress to a point below the reservoir water-surface
elevation, seepage could progress along
the crack and through the embankment, causing
severe erosion and—if not corrected—leading
to failure of the dam. Misalignment can indicate
relative movement between adjacent portions of a
dam—generally perpendicular to its axis. Excessive
settlement of dam material, the foundation, or
both can also cause misalignment. Most problems
are usually detectable during close inspection.
Misalignment may, however, only be detectable
by viewing a dam from either abutment. If on
close inspection the crest appears to be straight
for the length of the structure, alignment can be
further checked by standing away from the dam
on either abutment and then sighting along the
upstream and downstream edges of the crest.
On curved dams, alignment can be checked
by standing at either end of a short segment of
the dam and sighting along the crest’s upstream
and downstream edges, noting any curvature
or misalignment in that section. Leaning utility
poles or poles used for highway barriers can
also indicate movement.
Sinkholes can indicate internal collapse, piping,
or the presence of animal dens. The formation or
progression of a sinkhole is dangerous because it
poses a threat to inspectors or vehicles traversing
the crest. A sinkhole collapse can also lead to a
flow path through a dam, which can create an
uncontrolled breach.
The crest should be inspected for animal burrows,
low areas, vegetative cover, erosion, sloping of the
crest, narrowing of the crest, and traffic ruts.
Seepage Areas: As discussed previously, al-though
all dams have some seepage, seepage in
any area on or near a dam can be dangerous,
and all seepage should be treated as a potential
problem. Wet areas downstream from dams are
not usually natural springs, but seepage areas
(Figure 5.5). Seepage must be controlled in
both velocity and quantity. High-velocity flows
through a dam can cause progressive erosion
and, ultimately, failure. Saturated areas of an
embankment or abutment can move in massive
slides and thus also lead to failure.
Seepage can emerge anywhere on the
downstream face of a dam, beyond the toe, or
on the downstream abutments at elevations
below normal reservoir levels. A potentially
dangerous condition exists when seepage
appears on the downstream face above the toe
of a dam (Figure 5.6). If seepage is found on the
top half of the downstream slope, the problem
should be immediately corrected. Seepage
Figure 5.4. transverse cracks .
Initial Transverse
Cracking
Progression of
Transverse Crack
to a Point Below
the Waterlines
38 Oklahoma Water Resources Board - Dam Safety Program - March 2011
on the downstream slope can cause a slide or
failure of the dam by internal erosion (piping).
Evidence of seepage may vary from a soft, wet
area to a flowing spring and may appear initially
as only an area where vegetation is lush and dark
green in color. Cattails, reeds, mosses, and other
marsh vegetation often become established in
seepage areas.
Downstream abutment areas should always be
inspected closely for signs of seepage, as should
the area of contact between an embankment and
a conduit spillway, drain, or other appurtenant
structures and outlets. Slides in the embankment
or an abutment may be the result of seepage
causing soil saturation and high pore pressures.
Since seepage can be present but not readily
visible, an intensive search should be made of all
downstream areas where seepage water might
emerge. Even in short grass cover, seepage may
not be visible and must be walked on to be
found. Ideally, an inspection for seepage should
be made when a reservoir is full.
Concrete Dams and Structures
From a safety standpoint, the principal
advantage of concrete over earthen dams is
their relative freedom from failure by erosion
during overtopping as well as from embankment
slides and piping failures. Although concrete
dams comprise a minority of all dams, they
are commonly of greater height and storage
capacity than earthen structures. Thus, they
often represent a potentially greater hazard to
life and property. It is important that concrete-dam
owners be aware of the principal modes
of failure of such dams and that they be able to
discern between conditions which threaten the
safety of the dam and those that merely indicate
a need for maintenance.
Concrete dams fail for reasons that are
significantly different from earth dams. These
include:
• structural cracks
• foundation and abutment weakness
• deterioration due to alkali-aggregate
reaction
If any of these conditions are discovered during
inspection, an owner should immediately address
the problem with his/her engineer. Structural
cracks occur when portions of the dam are
overstressed; they result from inadequate design,
poor construction, foundation settlement, or
faulty materials. Structural cracks are often
irregular, may run at an angle to the major axes
of the dam and may exhibit abrupt changes in
direction. These cracks can also be noticeably
displaced, radially, transversely, or vertically.
Concrete dams transfer a substantial load to the
abutments and foundation. Although the concrete
of a dam may endure, the natural abutments or
foundation may crack, crumble, or move in a
massive slide. If that occurs, support for the
dam is lost and it fails. Impending failure of the
foundation or abutments may be difficult to detect
because initial movements are often very small.
Figure 5.5. area of sepa ge near toe of dam .
Oklahoma Water Resources Board - Dam Safety Program - March 2011 39
Severe deterioration can result from a chemical
reaction between alkali present in cements
and certain forms of silica present in some
aggregates. This chemical reaction produces by-products
of silica gels, which cause expansion
and loss of strength within concrete. An alkali
reaction is characterized by certain observable
conditions such as cracking (usually a random
pattern on a fairly large scale), and by excessive
internal and overall expansion.
Additional indications include the presence of
a gelatinous exudation or whitish amorphous
deposits on the surface and a chalky appearance
in freshly fractured concrete. The alkali-aggregate
reaction takes place in the presence
of water. Surfaces exposed to the elements or
dampened by seepage will deteriorate most
rapidly. Once suspected, the condition can be
confirmed by a series of tests performed on
core samples drilled from a dam. Although the
deterioration is gradual, an alkali-aggregate
reaction cannot be economically corrected by
any means now known. Continued deterioration
may require total replacement of a structure.
Inspection of a concrete dam is similar to that
of an earthen dam. However, the following
additional items should be considered:
• access and safety
• monitoring
• outlet system
• cracks at construction and expansion joints
• shrinkage cracks
• deterioration due to spalling
• minor leakage
Access and safety are important because the faces
of concrete dams are often nearly vertical, and
sites are commonly steep-walled rock canyons.
Access to the downstream face, toe area, and
abutments of such dams may be difficult and
require special safety equipment, such as safety
ropes or a boatswain’s chair. Concrete dams pose
a special problem for the dam owner because
of the difficulty in gaining close access to the
steep surfaces. Regular inspection with a pair of
powerful binoculars can initially identify areas
where change is occurring. When changes are
noted, a detailed, close-up inspection should be
conducted. Close inspection of the upstream face
may also require a boatswain’s chair or a boat.
Figure 5.6. sepa ge throu gh embankment dam .
Uncontrolled
Seepage Through and
Embankment
Reservoir
water
surface
sepage
surface
foundation sepage
sepage
40 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Monitoring helps detect structural problems
in concrete dams such as cracks in the dam,
abutments, or foundation. Cracks may develop
slowly at first, making it difficult to determine
if they are widening or otherwise changing over
time. If a structural crack is discovered, it should
be monitored for changes in width, length, and
offset, and a network of monitoring instruments
should be installed and read regularly. Outlet-system
deterioration is a problem for all dams,
but the frequency of such damage may be
higher in concrete dams because of their greater
average hydraulic pressure. Thus, outlet-system
inspection should be emphasized for large
concrete dams.
Cracks at construction joints exist because
concrete dams are built in segments, while
expansion joints—referred to as “designed”
cracks—are built into dams to accommodate
volumetric changes which occur in the
structures after concrete placement. These
joints are typically constructed so that no bond
or reinforcing, except non-bonded water stops
and dowels, extend across the joints. Shrinkage
cracks often occur when, during original
construction, irregularities or pockets in the
abutment contact are filled with concrete and
not allowed to cure fully prior to placement
of adjacent portions of the dam. Subsequent
shrinkage of the concrete may lead to irregular
cracking at or near the abutment.
Shrinkage cracks are also caused by temperature
variation. During winter months, the upper
portion of a dam may become significantly
colder than those portions in direct contact with
reservoir water. This temperature differential
can result in cracks which extend from the crest
for some distance down each face of the dam.
These cracks will probably occur at construction
or expansion joints, if any. Shrinkage cracks can
be a sign that certain portions of the dam are not
carrying the design load. In such cases, the total
compression load must be carried by a smaller
proportion of the structure. It may be necessary
to restore load-carrying capability by grouting
affected areas. This work requires the assistance
of an engineer.
Spalling is the process by which concrete
chips and breaks away as a result of freezing
and thawing, corrosion of the reinforcement,
or movement. Almost every concrete dam in
colder climates experiences continued minor
deterioration due to spalling. Because it usually
affects only the surface of a structure, it is not
ordinarily considered dangerous. However,
if allowed to continue, spalling can result in
structural damage, particularly if a dam is thin
in cross-section.
Repair is also necessary when reinforcing steel
becomes exposed. The method of repairing
spalled areas depends upon the depth of the
deterioration. In severe situations, engineering
assistance is required. Minor leakage through
concrete dams, although unsightly, is not usually
dangerous unless accompanied by structural
cracking. The effect may be to promote
deterioration due to freezing and thawing.
However, increases in seepage could indicate
that, through chemical action, materials are
being leached from the dam and carried away
by the flowing water.
Dam owners should note that decreases in
seepage can also occur as mineral deposits are
formed in portions of the seepage channel.
In either case, the condition is not inherently
dangerous and detailed study is required to
determine if repair is necessary for other than
cosmetic reasons.
Spillways
As detailed in Chapter 2, the main function
of a spillway is a safe exit for excess water in
a reservoir. If a spillway is too small, a dam
could be overtopped and fail. Similarly, defects
in a spillway can cause failure by rapid erosion.
A spillway should always be kept free of
obstructions, have the ability to resist erosion,
and be protected from deterioration. Because
dams represent a substantial investment and
spillways make up a major part of dam costs,
Oklahoma Water Resources Board - Dam Safety Program - March 2011 41
a conscientious annual maintenance program
should be pursued not only to protect the public
but to minimize costs as well.
The primary problems encountered with
spillways include:
• inadequate capacity
• obstructions
• erosion
• deterioration
• cracks
• open joints
• undermining of the spillway outlet
• deterioration of spillway gates
Inadequate capacity is determined by several
factors, such as the drainage area served, the
magnitude or intensity of storms in the watershed,
the storage capacity of the reservoir, and the speed
with which rainwater flows into and fills the
reservoir. An inadequate spillway can cause the
water in a reservoir to overtop the dam.
Obstruction of a spillway is commonly due
to excessive growth of grass and weeds, thick
brush, trees, debris, fences across channels to
prevent migration of fish, or landslide deposits.
An obstructed spillway can have a substantially
reduced discharge capacity which can lead to
overtopping of the dam. Grass is usually not
considered an obstruction; however, tall weeds,
brush, and young trees should periodically
be cleared from spillways. Similarly, any
substantial amount of soil deposited in a
spillway—whether from sloughing, landslide
or sediment transport—should be immediately
removed. Timely removal of large rocks is
especially important, since they can obstruct
flow and encourage erosion.
Erosion of a spillway may occur during a large
storm when large amounts of water flow for
many hours. Severe damage of a spillway or
complete washout can result if the spillway
cannot resist erosion. If a spillway is excavated
out of a rock formation or lined with concrete,
erosion is usually not a problem. However, if a
spillway is excavated in sandy soil, deteriorated
granite, clay, or silt deposits, protection from
erosion is very important.
Deterioration of a spillway can greatly affect
its performance. Generally, resistance to
deterioration can be increased if a spillway
channel has a mild slope, or if it is covered
with a layer of grass or riprap with bedding
material. Examples of spillway deterioration
may include collapse of side slopes, cracking
or undermining of concrete lining, erosion of
the approach section, chute channel, stilling
basin, and discharge channel. These problems
can cause water to flow under and around the
protective material and lead to severe erosion.
Remedial action must be taken as soon as any
sign of deterioration has been detected.
Cracks in an earthen spillway channel are usually
not regarded as a functional problem. However,
missing rocks in a riprap lining can be considered
a crack in the protective cover, and must be
repaired at once. Cracks in concrete lining of a
spillway are commonly encountered. These cracks
may be caused by uneven foundation settlement,
shrinkage, slab displacement, or excessive earth
or water pressure. Large cracks will allow water
to wash out fine material below or behind the
concrete slab, causing erosion, more cracks, and
even severe displacement of the slab. The slab may
even be dislodged and washed away by the flow.
A severely cracked concrete spillway should be
examined by and repaired under the supervision
of an engineer.
Open or displaced joints can occur from excessive
and uneven settlement of the foundation or
the sliding of a concrete slab. In some cases, a
construction joint is too wide or has been left
unsealed. Sealants deteriorate and wash away.
Water can flow through the joints, undermining
the slabs, which in turn could result in collapse of
the spillway slabs. Pressures resulting from water
flowing over the open slabs could also result in
lifting and displacement of slabs. Hence, all joints
need to be sealed and kept sealed.
42 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Undermining of the spillway outlet is the
erosion of foundation material and may weaken
support and cause further cracks. Pressure
induced by water flowing over displaced joints
may wash away part of a wall or slab, or cause
extensive undermining. Undermining of a
spillway causes erosion at a spillway outlet,
whether it is a pipe or overflow spillway, and
is one of the most common spillway problems.
Severe undermining of the outlet can displace
sections of pipe, cause slides in the downstream
embankment of the dam, and eventually lead to
complete failure of a dam.
Water must be conveyed safely from the
reservoir to a point downstream of the dam
without endangering the spillway itself or
the embankment. Often the spillway outlet is
adequately protected for normal flow conditions,
but not for extreme turbulent flows. It is easy
to misestimate the energy and force of flowing
water and the resistance of outlet material
(earth, rock, concrete, etc). The required level
of protection is difficult to establish by visual
inspection but can usually be determined
by hydraulic calculations performed by a
professional engineer.
Structures that completely control erosion at
a spillway outlet are usually expensive, but
often necessary. Less expensive protection
can also be effective, but require extensive
periodic maintenance as areas of erosion and
deterioration develop.
The following four factors, often interrelated,
contribute to erosion at the spillway outlet:
1. Flows emerging from the outlet are above
the stream channel. If outlet flows emerge
at the correct elevation, tailwater in the
stream channel can absorb a substantial
amount of the high velocity. The flow and
the hydraulic energy will be contained in
the stilling basin.
2. Flows emerging from the spillway are
generally free of sediment and therefore
have substantial sediment-carrying
capacity. In taking on sediment, moving
water will scour soil material from the
channel and leave eroded areas. Such
erosion is difficult to design for and
requires protection of the outlet for a safe
distance downstream from the dam.
3. Flows leaving the outlet at high velocity
can create negative pressures that can
cause material to come loose and separate
from the floor and walls of the outlet
channel. This process is called cavitation
when it occurs on concrete or metal
surfaces. Venting can sometimes be used
to relieve negative pressures.
4. Water leaking through pipe joints or flowing
along a pipe from the reservoir may weaken
the soil structure around the pipe.
Inadequate compaction adjacent to such
structures during construction can compound
this problem. Deterioration of spillway gates
can result in an inability of the gates to function
during storm events. Causes of structural
deterioration include, but are not limited to:
1. Corrosion can seriously weaken a structure or
impair its operation. The effect of corrosion
on the strength, stability, and serviceability of
gates must be evaluated. A loss of cross section
in a member causes a reduction in strength
and stiffness that leads to increased stress
levels and deformation without any change
in the imposed loading. Flexure, shear, and
buckling strength may be affected. A buildup
of corrosion products can be damaging at
connection details. For example, corrosion
buildup in a tainter gate trunnion can lead
to extremely high hoist loads. Localized
pitting corrosion can form notches that may
serve as fracture initiation sites, which could
significantly reduce the member’s fatigue life.
2. Fracture usually initiates at a discontinuity
that serves as a local stress raiser. Structural
connections that are welded, bolted, or
riveted are sources of discontinuities and
stress concentrations.
Oklahoma Water Resources Board - Dam Safety Program - March 2011 43
3. Fatigue is the process of cumulative
damage caused by repeated cyclic loading.
Fatigue damage generally occurs at stress-concentrated
regions where the localized
stress exceeds the yield stress of the material.
Fatigue is particularly a concern with spillway
gates with vibration problems.
4. Proper operation and maintenance of spillway
gates are necessary to prevent structural
deterioration. The following items are possible
causes of structural deterioration.
a. Weld repairs are often sources of
future cracking or fracture problems,
particularly if the existing steel had
poor weldability.
b. If moving connections are not lubricated
properly, the bushings will wear and result
in misalignment of the gate, resulting in
wear of other parts and unforeseen loads.
c. Malfunctioning limit switches could
result in detrimental loads and wear.
d. A coating system or cathodic protection
that is not maintained can result
in detrimental corrosion of metal
components.
5. Unforeseen loading of a gate can result in
deformed members or fracture. When structural
members become deformed or buckled, they
may have significantly reduced strength or
otherwise impair the performance of the gate.
Dynamic loading may be caused by hydraulic
flow at the seals. Other unusual loadings may
occur from malfunctioning limit switches or
debris trapped at interfaces between moving
parts. Unusual loads may also develop on gates
supported by walls that are settling or moving.
These unusual loads can cause overstressing
and lead to deterioration.
Mark Harrison , Oklahoma Conservation Comission
44 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Procedure for Inspection of the Spillway
Spillway inspection is an important part of a dam safety program. Its basic objective is
to detect any sign of obstruction, erosion, deterioration, misalignment, or cracking. An
inspection of an earth spillway should determine whether side slopes have sloughed
and whether there is excessive vegetation in the channel, and should look for signs of
erosion and rodent activity. The inspector should also use a probe to determine the
hardness and moisture content of the soil, note the location of particularly wet or soft
spots, and see if the stilling basin or drop structure is properly protected with rocks
or riprap. Because some erosion is unavoidable during spilling, an owner should also
determine whether such erosion might endanger the embankment itself.
If the spillway is installed with a sill or wall, a
dam owner should also determine if there are
any cracks or misalignment in the sill or wall
and check for erosion beneath the sill or wall
or downstream from it. Hairline cracks are usu-ally
harmless. Large cracks should be carefully
inspected and their location, width, length, and
orientation noted. Deterioration should be de-termined.
The concrete should be examined for
exposure of reinforcing bars.
Spillway surfaces exposed to freeze-thaw cycles
often suffer from surface spalling. Chemical ac-tion,
corrosion of the reinforcement, movement,
contamination, and unsound aggregates can also
cause spalling. If spalling is extensive, the spalled
area should be sketched or photographed, show-ing
its length, width, and depth. The problem
should be examined closely to see if the remaining
concrete has deteriorated or if reinforcing bars
are exposed. The concrete should be tapped with
a tapping device or rock hammer to determine if
voids exist below the surface.
Shallow spalling should be examined from time to
time to determine if it is becoming worse.
Deep spalling should be repaired as soon as
possible by an experienced contractor.
Walls of spillways are usually equipped with weep
(or drain) holes. Occasionally spillway chute slabs
are also equipped with weep holes. If all such holes
are dry, the soil behind the wall or below the slab
is probably dry as well. If some holes are draining
while others are dry, the dry holes may be plugged
by mud or mineral deposits. Plugged weep holes
increase the chances for failure of retaining walls
or chute slabs. The plugged holes should be
probed to determine causes of blockage, and soil
or deposits cleaned out to restore drainage. If that
work is not successful, rehabilitate the drain sys-tem
as soon as possible under the supervision of a
professional engineer.
Spillway retaining walls and chute slabs are
normally constructed in sections. Between ad-joining
sections, gaps or joints must be tightly
sealed with flexible materials such as tar, epox-ies,
or other chemical compounds. Sometimes
rubber or plastic diaphragm materials or cop-per
foil are used to obtain water tightness. Dur-ing
inspection, one should note the location,
Oklahoma Water Resources Board - Dam Safety Program - March 2011 45
length, and depth of any missing sealant, and
probe open gaps to determine if soil behind the
wall or below the slab has been undermined.
Misalignment of spillway retaining walls or chute
slabs may be caused by foundation settlement
or earth or water pressure. The inspector should
carefully look at the upstream or downstream end
of a spillway near the wall to determine if it has
been tipped inward or outward. Relative displace-ment
or offset between neighboring sections can
be readily identified at joints. The horizontal as
well as vertical displacement should be measured.
A fence on top of the retaining wall is usually
erected in a straight line at the time of construc-tion;
thus any curve or distortion of the fence line
may indicate wall deformation.
At the time of construction, the entire spillway chute
should form a smooth surface. Thus, measurement
of relative movement between neighboring chute
slabs at joints will give a good indication of slab
displacement. Misalignment or displacement of
walls or the slab is often accompanied by cracks.
A clear description of crack patterns should be
recorded or photos taken to help in understanding
the nature of the displacement.
The folowing areas should be in-spected
on al gates in spilways :
• main framing members and lifting and
support assemblies
• locations susceptible to fracture or weld-related
cracking
• corrosion-susceptible areas—normal
waterline, abrasion areas, crevices, areas
where water could stand
• lifting connections and chains or cables
• trunnions
• intersecting welds
• previous cracks repaired by welding
• locations of previous repairs or where
damage has been reported
• seal plates
Inlets, Outlets, and Drains
A dam’s inlet and outlet works, including
internal drains, are essential to its operation.
Items for inspection and special attention include:
• reservoir pool levels
• lake drains and internal drains
• corrosion
• trash racks on pipe spillways
• cavitation
• areas on gates and spillways as listed
immediately above
Reservoir pool level drawdown should not
exceed about 1 foot per week for slopes
composed of clay or silt materials except in an
emergency. Very flat slopes or slopes with free-draining
upstream soils can, however, withstand
more rapid drawdown rates. Pool levels can
be controlled by spillway gates, drain-and-release
structures, or flashboards. Flashboards,
sometimes used to permanently or temporarily
raise the pool level of water supply reservoirs,
should not be installed or allowed unless there
is sufficient freeboard remaining to safely
accommodate a design flood
Conditions causing or requiring temporary or
permanent adjustment of the pool level include:
• A problem that requires lowering of the pool.
Drawdown is a temporary solution until the
problem is solved.
• Release of water downstream to supplement
stream flow during dry conditions.
• Fluctuations in the service area’s demand for
water.
• Repair of boat docks in the winter and growth
of aquatic vegetation along the shoreline.
• Requirements for recreation, hydropower, or
waterfowl and fish management.
!
46 Oklahoma Water Resources Board - Dam Safety Program - March 2011
Lake drains : A lake drain should always be
operable so that the pool level can be drawn
down in case of an emergency or for necessary
repair. Lake-drain valves or gates that have not
been operated for a long time can present a
special problem for owners. If the valve cannot
be closed after it is opened, the impoundment
could be completely drained. An uncontrolled
and rapid drawdown could also cause more
serious problems such as slides along the
saturated upstream slope of the embankment or
downstream flooding. Therefore, when a valve
or gate is operated, it should be inspected and
all appropriate parts lubricated and repaired. It
is also prudent to advise downstream residents
of large or prolonged discharges.
Testing a valve or gate without risking complete
drainage entails physically blocking the drain
inlet upstream from the valve. Some drains have
been designed with this capability and have dual
valves or gates, or slots for stop logs (sometimes
called bulkheads) upstream from the valve.
Otherwise, divers can be hired to inspect the
drain inlet and may be able to construct a
temporary block at the inlet. Since that could be
dangerous, safety precautions are needed. Other
problems may be encountered when operating
a lake drain. Sediment can build up and block
the drain inlet, or debris can enter the valve
chamber, hindering its function. The likelihood
of these problems is greatly decreased if the
valve or gate is operated and maintained on a
schedule prepared by a professional engineer.
Corrosion is a common problem of pipe spillways
and other conduits made of metal. Exposure to
moisture, acid conditions, or salt will accelerate
corrosion. In particular, acid runoff from strip mine
areas will cause rapid corrosion of steel pipes. In such
areas, pipes made of noncorrosive materials such as
concrete or plastic should be used.
Metal pipes which have been coated to resist
accelerated corrosion are also available. The
coating can be of epoxy, aluminum, zinc
(galvanization), asbestos or mortar. Coatings
applied to pipes in service are generally not very
effective because of the difficulty of establishing
a bond. Similarly, bituminous coating cannot be
expected to last more than one to two years on
flow ways. Of course, corrosion of metal parts of
operating mechanisms can be effectively treated
and prevented by keeping those parts greased
and/or painted.
Corrosion can also be controlled or arrested
by installing cathodic protection. A sacrificial
metallic anode made out of a material such as
magnesium is buried in the soil and is connected
to the metal pipe by wire. An electric potential
is established which causes the magnesium to
corrode and not the pipe.
Trash on pipe spilways : Many dams have
pipe and riser spillways. As with concrete
spillways, pipe inlets that become plugged
with debris or trash reduce spillway capacity.
As a result, the potential for overtopping is
greatly increased, particularly if there is only
one outlet. A plugged principal spillway will
cause more frequent and greater than normal
flow in the emergency spillway which is de-signed
for infrequent flows of short duration
Figure 5.7. Keeping the trash rack fre of debris
reduces the chance of overtoppin g the dam .
Oklahoma Water Resources Board - Dam Safety Program - March 2011 47
and thus result in serious and unnecessary
damage. For these reasons trash collectors or
trash racks should be installed at the inlets to
pipe spillways and lake drains (Figure 5.7).
A well-designed trash rack will stop large
debris that could plug a pipe but allow
unrestricted passage of water and smaller
debris. Some of the most effective racks have
submerged openings which allow water to
pass beneath the trash into the riser inlet as
the pool level rises.
Openings that are too small will stop small
debris such as twigs and leaves, which in turn
will cause a progression of larger items to
build up, eventually completely blocking the
inlet. Trash rack openings should be at least
6 inches across, regardless of the pipe size.
The larger the principal spillway conduit, the
larger the trash rack opening should be. The
largest possible openings should be used, up
to a maximum of about 2 feet.
A trash rack should be properly attached to
the riser inlet and strong enough to withstand
the forces of fast-flowing debris, heavy
debris, and ice. It is a common occurrence for
vandals to throw riprap stone into the riser.
The size of the trash rack openings should
not be decreased to prevent this. Instead,
use riprap that is larger than the trash rack
openings or too large to handle. Maintenance
should include periodic checking of the trash
rack for rusted and broken sections and
repair as needed. The rack should be checked
frequently during and after storms to ensure
that it is functioning properly and to remove
accumulated debris.
Cavitation : When water flows through an out-let
system and passes restrictions (e.g., valves),
the pressure may drop. If localized water pres-sures
drop below the vapor pressure of water, a
partial vacuum is created and the water actually
boils, causing shock-waves which can damage
the outlet pipes and control valves. This process
can be a serious problem for large dams where
discharge velocities are high.
Testing the Outlet System
All valves should be fully opened and closed at
least once a year. This not only limits corrosion
buildup on control stems and gate guides,
but also provides an opportunity to check for
smooth operation of the system. Jerky or erratic
operation could signal problems, and indicate
the need for more detailed inspection.
The full range of gate settings should be
checked. The person performing the inspection
should slowly open the valve, checking for noise
and vibration. Certain valve settings may result
in greater turbulence. The inspector should
also listen for noise like gravel being rapidly
transported through the system. This sound
would indicate some cavitation and henceforth,
those gate settings should be avoided. The
operation of all mechanical and electrical
systems, backup electric motors, power
generators, power and lighting wiring associated
with the outlet should all be checked.
Inspecting the Outlet System
Accessible portions of the outlet, such as the
outfall structure and control, can be inspected
easily and regularly. However, severe problems
are commonly associated with deterioration or
failure of portions of the system either buried in
the dam or normally under water.
• Outlet pipes 30 inches or greater in diameter
can be inspected internally, provided the
system has an upstream valve allowing the
pipe to be emptied. Tapping the conduit
interior with a hammer can help locate voids
behind the pipe. This type of inspection
should be performed at least once a year.
• Small-diameter outlet pipes can be inspected
by remote TV camera if necessary. The
camera is channeled through the conduit and
transmits a picture back to an equipment
48 Oklahoma Water Resources Board - Dam Safety Program - March 2011
truck. This type of inspection is expensive
and usually requires the services of an
engineer. However, if no other method of
inspection is possible, inspection by TV is
recommended at least once every five years.
• Outlet intake structures, wet wells, and
outlet pipes with only downstream valves
are the most difficult dam appurtenances
to inspect because they are usually under
water. These should be inspected whenever
the reservoir is drawn down or at five-year
intervals. If a definite problem is suspected,
or if the reservoir remains full over extended
periods, divers should be hired to perform an
underwater inspection.
General Areas
Other areas requiring inspection include:
• mechanical and electrical systems
• the reservoir surface and shoreline
• the upstream watershed
• downstream floodplains
Mechanical equipment includes spillway gates,
sluice gates or valves for lake drains or water
supply pipes, stop logs, sump pumps, flashboards,
relief wells, emergency power sources, siphons,
and other devices. All mechanical and ass