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Edison Pond Dam may have resulted in a larger overtopping of the downstream dam and hence possible failure.
View of breach across crest with masonry wall on upstream face
View of breach immediately below masonry wall
View of breach through earthen embankment
Views of Edison Pond Dam Failure
The masonry wall along the upstream face of the Edison Pond Dam was approximately 3 feet wide. This, in combination with the narrow breach width downstream and a low hydraulic head on the wall probably prevented the total failure of the wall. But, had this dam overtopped for an extended period of time, a more significant portion of the earthen dam may have been eroded making a wider area of exposed masonry wall. This wall may not have been able to withstand the water pressures and may have experienced a total structural failure. With no earth pressures on the downstream side of the wall, a near instantaneous failure could have been expected, resulting in a large release of stored water.
Additional Issues and Research Needs Additional issues and research needs that were identified by State engineers in the Northeast Region as part of the survey are: • Refinement of breach parameters for dams with core walls or vertical concrete or masonry walls on the upstream face. • Research into and refinement if necessary of breach parameters for small dams. • NWS DAMBRK model has problems with large lateral inflows being added downstream of a dam • State engineers unaware of the latest on the new FLDWAV model. Little or no training available. • How do models handle debris flow in the flood wave? Currently engineers concerned with this issue are using high 'n' values in the overbank areas. 6
• Forensic Team. The Research Subcommittee of ICODS recommended to FEMA the development of a Forensic Team. The intent is that this team would be dispatched to the location of dam failures to gather data with respect to the breach and the impacts of the failure. State dam safety staffs are spread thin, and when failures occur, particularly in a wide spread area similar to the many failures that occurred along the east coast as a result of Hurricane Floyd, state engineers have little or no time to gather pertinent information with respect to the breach parameters and resultant damages. The data gathered by the Forensic Team would be useful for future research on dam safety analysis.
7
B-9
Issues, Resolutions, and Research Needs Related to Dam Failure Analysis Workshop Oklahoma City, Oklahoma June 26 -28, 2001 Embankment Dam Failure Analysis by
Francis E. Fiegle II, P. E.
Georgia Safe Dams Program
The Kelly-Barnes Dam failed on November 6, 1977 near Toccoa, Georgia and killed 39 people that fateful Saturday night. That incident led to the passage of the Georgia Safe Dams Act and the formation of the Georgia Safe Dams Program. Since that date, there have been over 300 dam failures recorded in Georgia. Some of these have been catastrophic and two of these have resulted in loss of life. The Kelly Barnes Dam failed in 1977 and unnamed farm pond dam failed between Plains, Georgia and Americus which resulted in three deaths on July 5, 1994.
Kelly Barnes Dam Failure Toccoa Georgia A few of these dam failures have had good investigative follow-up where the size of the breach, initial conditions, and the resulting flood wave depths were measured. For instance, the Kelly Barnes Dam failure was thoroughly detailed by a Federal Investigative Board. The following breach parameters were detailed in the report for the Kelly Barnes Dam which was 38 feet tall: • • • •
Breach side slopes - right 0.5 H to 1.0V - left 1.0 H to 1.0 V Base width of breach - 40 ft Sudden failure Estimated peak flow - 24,000 cfs
-1
However, most of the dams that fail in Georgia have not had these detailed measurements. Every year in Georgia there are usually three or four dam failures of unregulated dams that are reported to our office. The majority of the dam failures have occurred during major rainfall events such as Tropical Storm Alberto in 1994, the 100-year flood in middle Georgia in 1998, and Tropical Storm Allison in 2001.
Lake Collins Dam Sumter County Tropical Storm Alberto July 1994
Clayton County Waste Water Pond Dam 1982
-2
Pritchard's Lake Dam Morgan County March 2001
Unknown Dam Putnam County Tropical Storm Allison June 2001 Over the years, our office has noted that most of the dam breaches have had the following general parameters: -
Side slopes 1.0 H to 1.0 V Base width of breach equal to height of the dam
-3
The side slopes are sometimes steeper in more clayey soils and flatter in sandy soils. The breach width maybe wider if there is a large impoundment (>than 25 acres). The breach parameters recommended in the Georgia Safe Dams Program's Engineering Guidelines mirror the Breach Parameters recommended by FERC and have been modified by our field observations of numerous dam failures. Table I - Breach Parameters
Type of Dam
Breach Width BR (Feet)
Breach Side Slope Z
Time to Failure Hours
Arch
W
Vertical or Slope of Valley Walls
0.1
Masonry; Gravity
Monolith Width
Vertical
0.1 to 0.3
Rockfill
HD
Timber Crib
HD
Vertical
0.1 to 0.3
Slag; Refuse
80% of W
1.0 – 2.0
0.1 to 0.3
Earthen – non-engineered
HD
1.0
0.1
Earthenengineered
HD
1.0
0.5
-4
Table 2 – Breach Parameters Definitions
• • • • • •
HD Z BR TFH W
- Height of Dam - Horizontal Component of Side - Slope of Breach - Base Width of Breach - Time to Fully Form the Breach - Crest Length
Typical Sketch of Breach of Earth Embankment Our office uses the Boss Dambreak software, which is based on the NWS Dambreak, developed by Dr. Danny Fread, P. E. Our office assumes that wedge erosion occurs (see following sketch). Furthermore, the time to failure is conservative for hazard classification of dams. We use a 6-minute time failure for earth fill dams that are not engineered fills or that we have no construction/design information for and 30 minutes for failure of engineered dams.
-5
In Georgia, we use dambreak modeling for the following purposes: • • • •
Hazard classification Flood inundation mapping Emergency action planning Incremental spillway capacity design
In closing, over the years our office has used or has seen dambreak modeling and routings use the following methods: • • • •
NRCS TR66 (1978 to 1982) NWS Dambreak HECI Dambreak in conjunction with HECII or HECRAS stream routing Boss Dambreak (currently used by our office)
In preparation for this workshop, I surveyed the states east of the Mississippi River. I received responses from Virginia, North Carolina, West Virginia, South Carolina, Kentucky, Tennessee, Florida, Ohio, Georgia, and Indiana. The following questions were asked and the responses are detailed. 1. Who does the dambreak routings? • Owner of dam - SC, NC, VA, KY, WV, for new dams - FL, TN • State - SC, OH, GA, NC; TN for existing dams 2. Breach Parameters: • Breach width varies from height to twice the height of the dam • Side slopes varies from 0.5 H to 1.0 H to 1V • Time to failure varies from 6 minutes to 60 minutes 3. What type of dams are routed? • High hazard - SC, WV, VA, KY, TN, OH, NC, GA • Significant Hazard - SC, WV, VA • Low hazard - none • In Florida and Indiana - various hazards are routed 4. Definition of High Hazard Dams: • Floods a building that is occupied • One foot above finished floor • Well-traveled roadways 6 inches deep • Use BurRec Guidelines • Loss of life likely to probably • Any dam over 60 feet in height or stores more than 5000 acre-feet (Ohio) • Application of damage index 5. Type of analyses used • NWS Dambreak (variation of) - TN, SC, KY, OH, WV, NC, GA • HEC1 Dambreak/HECII and HECRAS - TN, SC, OH, WV, NC, VA • Visual Observation - NC
-6
6. Reinventory of Dams Timeframe: • Annually - high hazard only -SC • Two Years - high hazard only - NC • Three Years significant hazard - NC, SC
low hazard - SC
• Five Years all hazards - OH, GA
low hazard - NC
• Six Years - all hazards - VA • Kentucky only reinventories if hazard is noticed • West Virginia reinventories during routine inspections • Florida is locally determined (Water Management Districts) • Tennessee when doing a safety inspections As a result of this survey, there were several issues identified that need attention. It is clear that states need to take the following actions: • • • •
Regularly reinventory dams of all hazard classifications Have consistent hazard classification guidelines Adopt Quality Assurance/Quality Control Procedures Improve technical expertise
The states have requested the following guidance from this workshop based on the assembled expertise: • • • • •
Time to failure guidelines Is there time for emergency response to make a difference? When to use which model or sets of models (field conditions, etc)? What is the level of accuracy for each model? Advantages/disadvantages for each model(s)
As a result of the survey, the following immediate dam safety needs were identified by the states: • • •
Combine HECI and HECII or HECHMS and HECRAS into integrated model(s) Finish Floodwave Model Provide in depth, hands on training in the use of all models
Finally, the states identified the following Research Needs: • • • • • •
Input parameters for breach development for earth and rockfill dams Depth of overtopping that causes failure How does the crest protection influence overtopping failure development? How does the embankment protection influence overtopping failure development? Forensic investigation of breach failures including the condition of the dam Influence of the size of the drainage basin on a "storm in progress" failure
-7
In closing, I wonder if we in the dam safety community are meeting the public's expectations in regulating dams, or better yet, are we classifying dams for regulation to meet our perception of the public's expectation or some variation there of? If we are using our paradigms without adequate explanation to the public and feedback from the "at risk" population, then likely we are imposing additional risk to the "at risk" populations that is not justified.
-8
B-10
ADJUSTING REALITY
TO FIT THE MODEL
MATTHEW LINDON, P.E.
�DEPT OF NATURAL RESOURCES �STATE ENGINEERS OFFICE �DAM SAFETY HYDROLOGIST
CREDENTIALS
AMATEUR ACADEMIC MODELER
�Prep School - Math, Science, Computers, Statistics �Engineering - Calculus, Physics, Thermo, Fluids �Grad Courses - Modeling, Meteorology, Hydrology �Computers - Punch Cards, Batch Files, XT, AT, PC, Math Chip, 286, 386, 486, Pentium I, II….
MODELING
EXPERIENCE
DAM SAFETY HYDROLOGY 20 YEARS �ACOE - HEC I, II, HMS, RAS �NWS - DAMBRK, BREACH, SMPDBK, DWOPER �DHM, FLO2D, TR20, PIPE NETWORK �STORM, SPIPE, FLDRTE, BACKWAT �SIDECHAN, SPILLWAY, STABLE, QUAKE….
Awakening
From the Hypothetical to the Real World �HEC I, HEC II courses and experience �NWS - DAMBRK/BREACH Course - Exercise �Quail Creek dam failure - Calibration opportunity �Necessity is the mother of invention �Measure, survey, interview, history of event
CALIBRATION
CORRELATION TO REALITY � NO correlation of BREACH model with reality - Piping channel start sensitivity - Can’t model actual breach shape and timing � NO correlation of DAMBRK model with reality - Manning Roughness Coefficients unreal - 0.1-0.25 - sensitive to breach size and timing - Can’t model trapezoidal migration - Can’t converge with Manning increase with depth � Sensitive to Black Box Variables - Mannings - Friction, bulking, debris, turbulence, eddys � Limited by time steps and reach lengths - converge? � Supercritical to subcritical hydraulic jumps
Doubt and Disillusionment
Pity the man who doubts what he’s sure of � HEC I - Sensitive to Time Step, Reach Length, Basins - Black box for infiltration, lag, runoff, melt…. - Hydrological routing - No Attenuation - Designed for flat farms not wild mountains � HEC II - Manning Black Box, - 1 dimension limits, boundary conditions - Designed for labs and canals - not rivers � Old equations on new high speed computers � Developed by mathematicians, statisticians and Computer Geeks
Basis of Uncertainty
Close counts in horseshoes, hand grenades & hydrology
�Sensitivity analysis of input variables �Probabilistic approach �Monte Carlo combinations of all variables - Most probable answer - Not best answer - Not worst case
�Fuzziness of results
Apparent Veracity
Computers lie and liars use computers. �Easy inout, user interface, GUI, ACAD, GIS �Garbage in garbage out �Slick output, graphics, color �Windows, WYSIWYG, 3D, Iso views �Computer Credibility - must be FACT � Models using old theories and methods �Lagging physically based, spatial and temporal �Computers effect modeling like writing styles
New age modelers
Post-modern hydrology - form before function.
�Ease of operation encourages the unqualified or unscrupulous to take advantage �Not familiar with theory and methods �Have not done calculations in head or by hand �Don’t understand complex non linear nature of these multidimensional problems. �Know exactly what the models does or don’t use it
Problem Solutions
Good math and science don’t always make good models � Better Models - Eliminate Balck Boxes - 2D, 3D - less assumptions - incorporate new theory and methods - Use spatial and temporal improvements � Qualified modelers - Better modelers for better models - educate, train, help screens, documentation - Use models for intended purpose, scope and scale � Calibrate, Correlate, Calculate - Sensitivity analysis on input variables - Interpolate rather than extrapolate � Express degree of uncertainty of output - Probabilities, confidence, fuzziness, chaos
Get out of the box.
To think outside the box you must get outside. � Natural phenomena are fantastically complex systems - Understand little - Describe less - Model and reproduce even less � Math and Science just our best guess - They are tools like slide rules, computers, hammers. - “Ology” is the study of, not the perfect understanding. - Use a large grain of salt � Observe present and past - Paleohydrology - Walk up and downstream - What does nature want to do � Connect the model with reality
B-11
A Simple Procedure for Estimating
Loss of Life from Dam Failure
FEMA/USDA Workshop
Issues, Resolutions, and Research
Needs Related to Embankment Dam Failure Analysis
26-28 June 2001
Wayne J. Graham, P.E.
INTRODUCTION
Evaluating the consequences resulting from a dam failure is
an important and integral part of any dam safety study or
risk analysis. The failure of some dams would cause only
minimal impacts to the dam owner and others, while large
dams immediately upstream from large cities are capable of
causing catastrophic losses. Dam failure can cause loss of
life, property damage, cultural and historic losses,
environmental losses as well as social impacts. This paper
focuses on the loss of life resulting from dam failure.
Included is a procedure for estimating the loss of life that
would result from dam failure. No currently available
procedure is capable of predicting the exact number of
fatalities that would result from dam failure.
PROCEDURE FOR ESTIMATING LOSS OF LIFE
The steps for estimating loss of life resulting from dam
failure are as follows:
Step 1: Determine dam failure scenarios to evaluate.
Step 2: Determine time categories for which loss of life
estimates are needed.
Step 3: Determine area flooded for each dam failure
scenario.
Step 4: Estimate the number of people at risk for each
failure scenario and time category.
Step 5: Determine when dam failure warnings would be
initiated.
Step 6: Select appropriate fatality rate.
Step 7: Evaluate uncertainty.
The details of each step are as follows:
Step 1: Determine Dam Failure Scenarios to Evaluate
A determination needs to be made regarding the failure
scenarios to evaluate. For example, loss of life estimates
may be needed for two scenarios - failure of the dam with a
full reservoir during normal weather conditions and failure
of the dam during a large flood that overtops the dam.
Step 2: Determine Time Categories For Which Loss of Life
Estimates Are Needed
The number of people at risk downstream from some dams is
influenced by seasonality or day of week factors. For
instance, some tourist areas may be unused for much of the
year. The number of time categories (season, day of week,
etc.) selected for evaluation should accommodate the varying
usage and occupancy of the floodplain. Since time of day
can influence both when a warning is initiated as well as
the number of people at risk, each study should include a
day category and a night category for each dam failure
scenario evaluated.
Step 3: Determine Area Flooded for Each Dam Failure Scenario
In order to estimate the number of people at risk, a map or
some other description of the flooded area must be available
for each dam failure scenario. In some cases, existing dam-
break studies and maps may provide information for the
scenarios being evaluated. Sometimes new studies and maps
will need to be developed.
Step 4: Estimate the Number of People at Risk for Each
Failure Scenario and Time Category
For each failure scenario and time category, determine the
number of people at risk. Population at risk (PAR) is
defined as the number of people occupying the dam failure
floodplain prior to the issuance of any warning. A general
guideline is to: "Take a snapshot and count the people."
The number of people at risk varies during a 24-hour period.
The number of people at risk will likely vary depending upon
the time of year, day of week and time of day during which
the failure occurs. Utilize census data, field trips,
aerial photographs, telephone interviews, topographic maps
and any other sources that would provide a realistic
estimate of floodplain occupancy and usage.
Step 5: Determine When Dam Failure Warnings Would be
Initiated
Determining when dam failure warnings would be initiated is
probably the most important part of estimating the loss of
life that would result from dam failure. Table 1, "Guidance
for Estimating When Dam Failure Warnings Would be
Initiated," was prepared using data from U.S. dam failures
occurring since 1960 as well as other events such as Vajont
Dam in Italy, Malpasset Dam in France and Saint Francis Dam
in California. An evaluation of these dam failure data
indicated that timely dam failure warnings were more likely
when the dam failure occurred during daylight, in the
presence of a dam tender or others and where the drainage
area above the dam was large or the reservoir flood storage
space. Timely dam failure warnings were less likely when
failure occurred at night or outside the presence of a dam
tender or casual observers. Dam failure warnings were also
less likely where the drainage area was small or the
reservoir had little or no flood storage space, i.e, when
the reservoir was able to quickly fill and overtop the dam.
Although empirical data are limited, it appears that timely
warning is less likely for the failure of a concrete dam.
Although dam failure warnings are frequently initiated
before dam failure for earthfill dams, this is not the case
for the failure of concrete dams.
Table 1 provides a means for deriving an initial estimate of
when a dam failure warning would be initiated for the
failure of an earthfill dam. The availability of emergency
action plans, upstream or dam-site instrumentation, or the
requirement for on-site monitoring during threatening events
influences when a dam failure warning would be initiated.
Assumptions regarding when a warning is initiated should
take these factors into account.
Dam Type
Earthfill
Drainage area at dam less than 100 mi2 (260 km2)
Drainage area at dam less than 100 mi2 (260 km2)
Special Considerations
Night
Day
Night
Day
Night
Day
Time of Failure
0.25 hr. after dam failure
0.5 hr. after dam failure
1 hr. before dam failure
1 to 2 hr. before dam failure
2 hrs. before dam failure
0.25 hrs. after dam failure
0.25 hrs. before dam failure
Many Observers at Dam
1.0 hrs. after fw reaches populated area
0.25 hr. after fw reaches populated area
1.0 hr. after fw reaches populated area
0.25 hrs. after fw reaches populated area
0 to 1 hr. before dam failure
1 hr. before dam failure
1.0 hrs. after fw reaches populated area
0.25 hrs. after fw reaches populated area
No Observers at Dam
Table 1
Guidance for Estimating When Dam Failure Warnings Would be Initiated (Earthfill Dam)
Cause of Failure
Drainage area at dam more than 100 mi2 (260 km2)
Day
0.50 hr. after dam failure
0.5 hrs. before fw reaches populated area
Seismic
Delayed Failure
When Would Dam Failure Warning be Initiated?
Overtopping
Drainage area at dam more than 100 mi2 (260 km2)
Night
2 hrs. before dam failure
0.5 hrs. before fw reaches populated area
Piping (full reservoir, normal weather)
Day
2 hrs. before dam failure
Immediate Failure
Night
Notes: "Many Observers at Dam" means that a dam tender lives on high ground and within site of the dam or the dam is visible from the homes of many people or
the dam crest serves as a heavily used roadway. These dams are typically in urban areas. "No Observers at Dam" means that there is no dam tender at the dam,
the dam is out of site of nearly all homes and there is no roadway on the dam crest. These dams are usually in remote areas. The abbreviation "fw" stands
for floodwater.
Step 6: Select Appropriate Fatality Rate
Fatality rates used for estimating life loss should be
obtained from Table 2. The table was developed using data
obtained from approximately 40 floods, many of which were
caused by dam failure. The 40 floods include nearly all
U.S. dam failures causing 50 or more fatalities as well as other flood events that were selected in an attempt to cover
a full range of flood severity and warning combinations.
Events occurring outside of the U.S. were included in the
data set. The following paragraphs describe the terms and
categories that form the basis for this methodology.
Flood Severity along with warning time determines, to a
large extent, the fatality rate that would likely occur.
The flood severity categories are as follows:
1) Low severity occurs when no buildings their foundations. Use the low severity structures would be exposed to depths of (3.3 m) or if DV, defined below, is less m2/s).
are washed off
category if most
less than 10 ft
than 50 ft2/s (4.6
2) Medium severity occurs when homes are destroyed but trees
or mangled homes remain for people to seek refuge in or on.
Use medium flood severity if most structures would be
exposed to depths of more than 10 ft (3.3 m) or if DV is
more than 50 ft2/s (4.6 m2/s).
3) High severity occurs when the flood sweeps the area clean
and nothing remains. High flood severity should be used
only for locations flooded by the near instantaneous failure
of a concrete dam, or an earthfill dam that turns into
"jello" and washes out in seconds rather than minutes or
hours. In addition, the flooding caused by the dam failure
should sweep the area clean and little or no evidence of the
prior human habitation remains after the floodwater recedes.
Although rare, this type of flooding occurred below St.
Francis Dam in California and Vajont Dam in Italy. The
flood severity will usually change to medium and then low as
the floodwater travels farther downstream.
The parameter DV may be used to separate areas anticipated
to receive low severity flooding from areas anticipated to
receive medium severity flooding. DV is computed as follows:
DV =
Qdf -Q2.33
-----------
Wdf
where:
Qdf is the peak discharge at a particular site caused by dam
failure.
Q2.33 is the mean annual discharge at the same site. This
discharge can be easily estimated and it is an indicator of
the safe channel capacity.
Wdf is the maximum width of flooding caused by dam failure
at the same site.
Warning Time influences the fatality rate. categories are as follows:
The warning time
1) No warning means that no warning is issued by the media
or official sources in the particular area prior to the
flood water arrival; only the possible sight or sound of the
approaching flooding serves as a warning.
2) Some warning means officials or the media begin warning
in the particular area 15 to 60 minutes before flood water
arrival. Some people will learn of the flooding indirectly
when contacted by friends, neighbors or relatives.
3) Adequate warning means officials or the media begin
warning in the particular area more than 60 minutes before
the flood water arrives. Some people will learn of the
flooding indirectly when contacted by friends, neighbors or
relatives.
The warning time for a particular area downstream from a dam
should be based on when a dam failure warning is initiated
and the flood travel time. For instance, assume a dam with
a campground immediately downstream and a town where
flooding begins 4 hours after the initiation of dam failure.
If a dam failure warning is initiated 1 hour after dam
failure, the warning time at the campground is zero and the
warning time at the town is 3 hours.
The fatality rate in areas with medium severity flooding
should drop below that recommended in Table 2 as the warning
time increases well beyond one hour. Repeated dam failure
warnings, confirmed by visual images on television showing
massive destruction in upstream areas, should provide
convincing evidence to people that a truly dangerous
situation exists and of their need to evacuate. This should
result in higher evacuation rates in downstream areas and in
a lowering of the fatality rate.
Flood Severity Understanding also has an impact on the
fatality rate. A warning is comprised of two elements: 1)
alerting people to danger and 2) requesting that people at
risk take some action. Sometimes those issuing a flood
warning or dam failure warning may not issue a clear and
forceful message because either 1) they do not understand
the severity of the impending flooding or 2) they do not
believe that dam failure is really going to occur and hence
do not want to unnecessarily inconvenience people. People
exposed to dam failure flooding are less likely to take
protective action if they receive a poorly worded or timidly
issued warning. Warnings are likely to become more accurate
after a dam has failed as those issuing a warning learn of
the actual failure and the magnitude of the resultant
flooding. Precise warnings are therefore more probable in
downstream areas. This factor will be used only when there
is some or adequate warning time.
The flood severity understanding categories are as follows:
1) Vague Understanding of Flood Severity means that the
warning issuers have not yet seen an actual dam failure or
do not comprehend the true magnitude of the flooding.
2) Precise Understanding of Flood Severity means that the
warning issuers have an excellent understanding of the
flooding due to observations of the flooding made by
themselves or others.
15 to 60
no warning
more than 60
15 to 60
no warning
more than 60
15 to 60
no warning
Warning Time (minutes)
Suggested
0.30 to 1.00
Suggested Range
Fatality Rate (Fraction of people at risk expected to die)
0.75
vague
precise
vague
not applicable
precise
vague
precise
vague
not applicable
0.0002
0.0003
0.002
0.007
0.01
0.01
0.03
0.02
0.04
0.15
0.0 to 0.0004
0.0 to 0.0006
0.0 to 0.004
0.0 to 0.015
0.0 to 0.02
0.002 to 0.02
0.005 to 0.06
0.005 to 0.04
0.01 to 0.08
0.03 to 0.35
Use the values shown above and apply to the number of people who remain in the dam failure floodplain after warnings are issued. No guidance is provided on how many people will remain in the floodplain.
precise
precise
vague
precise
vague
not applicable
Flood Severity Understanding
Table 2
Recommended Fatality Rates for Estimating Loss of Life Resulting from Dam Failure
Flood Severity
HIGH
MEDIUM
LOW more than 60
Step 7: Evaluate Uncertainty
Various types of uncertainty can influence loss of life
estimates. Quantifying uncertainty is difficult and may
require significant time to achieve.
Step 1 of this procedure suggests that separate loss of life
estimates be developed for each dam failure scenario.
Various causes of dam failure will result in differences in
downstream flooding and therefore result in differences in
the number of people at risk as well as flood severity.
Step 2 suggests that the dam failure be assumed to occur at
various times of the day or week. It is recognized that the
time of failure impacts both when a dam failure warning
would be initiated as well as the number of people who would
be at risk.
Dam failure modeling serves as the basis for step 3. Dam
failure modeling requires the estimation of: 1) the time for
the breach to form, 2) breach shape and width and 3)
downstream hydraulic parameters. Variations in these
parameters will result in changes in the flood depth, flood
width and flood wave travel time. This will lead to
uncertainty in the: 1) population at risk, 2) warning time
and 3) flood severity.
Estimating the number of people at risk, step 4, may be
difficult, especially for areas that receive temporary
usage. A range of reasonable estimates could be used.
Step 5 focuses on when a dam failure warning would be
initiated. This warning initiation time could be varied to
determine sensitivity to this assumption.
The last type of uncertainty is associated with the
inability to precisely determine the fatality rate, step 6.
There was uncertainty associated with categorizing some of
the flood events that were used in developing Table 2.
Similarly, some of the factors that contribute to life loss
are not captured in the categories shown in Table 2. This
type of uncertainty can introduce significant, but unknown,
errors into the loss of life estimates. Some possible ways
of handling this uncertainty would be to 1) use the range of
fatality rates shown in Table 2, 2) when the flooding at a
particular area falls between two categories (it is unclear
if the flood severity would be medium or low, for example)
the loss of life estimates can be developed using the
fatality rate and range of rates from all categories touched
by the event and 3) historical events can be evaluated to
see if there are any that closely match the situation at the
site under study.
B-12
Workshop on Issues, Resolutions, and Research Needs Related to Dam Failure Analysis Current Practice Natural Resources Conservation Service by Bill Irwin ¹ Introduction The Natural Resources Conservation Service (NRCS) formerly Soil Conservation Service (SCS) is the engineer-of-record on over 26,000 of the roughly 77,000 dams currently identified in the National Inventory of Dams (NID). NRCS has also engineered over 3,000,000 dams and ponds that are smaller than the minimum size dam included in the National Inventory. Typical dams in the NRCS portfolio are relatively small embankment dams built over 30 years ago. Data on NRCS dams in the NID is shown in Figure 1. NID size dams 25ft+ high 45ft+ high 65ft+ high 100ft+ high
26000 15000 2000 400 40
50AF+ storage 500AF+ storage 5000AF+ storage 15000AF+ storage
26000 23000 7000 600 100
30yrs+ old 40yrs+ old 50yrs+ old 60yrs+ old
26000 15000 5000 1000 400
Figure 1 – NRCS Dam Portfolio Current Criteria The NRCS has developed a significant set of design criteria over the years to accomplish this work. The SCS established three levels of hazard classification over as far back as anyone can remember and defined the high hazard classification almost fifty years ago as structures “…where failure may result in loss of life, damage to homes, industrial and commercial buildings, important public facilities, railroads and highways.” ² This classification and subsequent design criteria approach inherently requires evaluation of dam failure parameters. The NRCS has provided increasing degrees of criteria and guidance on selection of such parameters as techniques for analyzing the consequences of dam failures have advanced. Current NRCS failure analysis guidance was initially published the late 1970’s as Technical Release Number 66 (TR-66), “Simplified Dam Breach Routing Procedure”. This procedure is a combined hydrologic-hydraulic method. The hydraulic portion is a simplified version of a simultaneous storage and kinematic routing method which accepts a breach hydrograph at the upstream end of the reach and routes the flood wave downstream, continuously in time and space. The hydrologic portion develops the breach hydrograph based on estimated downstream flow characteristics, total volume of flow from dam pool, and expected maximum breach discharge (Qmax). The Qmax parameter was estimated from a curve fit of the peak discharges from historic dam failures available at the time.
¹National Design Engineer, USDA/NRCS, Washington, DC email:
[email protected] phone: (202)720-5858 ²SCS Engineering Memo No. 3, July 16, 1956
1
The procedure was intended to provide a practical “hand-worked” method appropriate for typical NRCS dam work. One published report ³ compared the TR-66 procedure with three other methods available at the time including the National Weather Service (NWS) and Hydraulic Engineering Center (HEC) models. For a 36ft high embankment dam subjected to a PMP event, the four methods produced comparable breach profile depths, while the TR-66 method computed the lowest peak flow at the dam. Computed peak discharges were 71,355cfs by TR-66, 76,000cfs by Keulegan, 85,950cfs by NWS, and 87,000cfs by HEC-1. Current NRCS breach peak discharge criteria was initially published in the late 1980”s in Technical Release Number 60 (TR-60), “Earth Dams and Reservoirs”. The criteria specifies the peak breach discharge (Qmax) to be used to delineate the potential dam failure inundation area below the dam and subsequently to determine the dam hazard classification. The criteria does not specify downstream breach routing or other hydraulic methodologies to be used. Regardless of the stream routing techniques to be used, the minimum peak discharge is as follows: 1. For depth of water at dam (Hw) at time of failure � 103 feet, Qmax = 65 Hw 1.85 2. For depth of water at dam (Hw) at time of failure < 103 feet, Qmax = 1000 Br 1.35 but not less than 3.2 Hw 2.5 nor more than 65 Hw 1.85, where, Br = Vs Hw / A and, Br = breach factor, acres
Vs = reservoir storage at failure, acre-feet
A = cross-sectional area of the embankment, square feet
3. When actual dam crest length(L) is less than theoretical breach width (T) such that, L < T = (65 H 0.35) / 0.416 use, Qmax = 0.416 L H 1.5 in lieu of 65 Hw 1.85 in category 1 or 2 above, where, H = height of dam at centerline, from bottom of breach to top of dam, feet This suite of expressions for Qmax was derived from a data set of 39 dam failures available in the profession or collected from NRCS sources at the time. Figure 2 taken from the original work shows the relationship between the peak breach discharge from the 39 sites and the peak break discharge predicted by the Qmax criteria.
³ Safety of Existing Dams, National Research Council, National Academy Press, 1983.
2
Figure 2 – Comparison of Predicted vs. Reported Qmax for 39 site data set Failure Experiences NRCS has experience a relatively small number of dam failures considering the magnitude of its portfolio. However, information from some dramatic NRCS dam failures provides insight into NRCS experienced failure modes.
Figure 3 – Obion Creek #36 – looking upstream into reservoir Obion Creek #36 is a typical NRCS flood control dam from the 1960’s. It was built in 1963 and failed a year later during the first reservoir filling storm. An Engineering Investigation concluded that dispersive soils were a major factor in the failure. Note that the dam was constructed with anti-seep collars along the principal spillway pipe as was typical at the time. 3
Although this failure occurred several years ago, it is still representative of similar dams that were built around the same or earlier time periods before needed treatments of dispersive soils or needed filter diaphragms around pipe penetrations were recognized. NRCS has had several similar piping type failures and does have many similarly designed flood control dams that have not yet experienced a significant first filling.
Figure 4 – Coon Creek #41 – note remaining embankment in upper right Coon Creek #41 is also a typical NRCS flood control dam from the 1960’s. It was built in 1962 and failed in 1978 during the first significant reservoir filling. An Engineering Investigation concluded that stress relief fractured rock in the steep abutment was the major factor in the failure. This site was constructed with minimal foundation investigation and foundation treatments as was typical at the time. Although this site failure occurred several years ago, it is still representative of similar dams that were built in similar geologic settings around the same or earlier time periods before such foundation hazards were widely recognized or routinely investigated. NRCS has similar flood control dams which have not yet experienced a significant first filling. Most recently, Bad Axe #24, a similar site in a similar setting built in 1963, failed in a similar fashion last year.
Figure 4 – Ascalmore #11 – looking upstream, note pipe outlet on left 4
Figure 5 – Ascalmore #11 – looking upstream Ascalmore #11 was built in 1959 and failed last year after trash blocked the pipe spillway and a storm quickly filled the flood reservoir. An Engineering Investigation concluded that dispersive soils and animal burrow damage in the upper portions of the embankment were major factors in the failure. The rough appearance of the embankment surface is due to removal of extensive woody vegetation after the breach occurred and before the picture was taken. It is interesting to note that the embankment breached in two separate locations and the energy of the stored water was not sufficient to erode either breach to the base of the dam. Practical Criteria Needs The principal NRCS need related to embankment failure analysis continues to be determination of the breach inundation area below the dam for purposes of determining population at risk, hazard classification, and emergency action planning. The “hand-worked” hydraulic portion of the old TR-66 method is adequate for only very basic hazard class screening on typical rural NRCS dams and ponds. Current software for breach flow profile analysis supported with modern computer capabilities is the professional norm for developing breach inundation maps and eventually emergency action plans. New hydraulic routing software currently being developed in the profession will further advance this aspect of dam failure consequence analysis. The hydrograph portion of the old TR-66 method and subsequently the Qmax equations approach of the current TR-60 criteria are still important. This approach can still provide adequate dam failure analysis criteria for typical NRCS dams since the embankments are small, the area at risk is close to the dam, and agency experience has been a wide variety of failure modes. The NRCS workload involving dams requires that a large number of existing and potential dams be evaluated without significant topographic or soil site data. Such workload without much physical data does not justify a complex analysis. The current 5
Qmax equation needs to be updated considering newer dam failure data available in the profession. Another NRCS need related more to embankment non-failure analysis is allowable overtopping. Agency experience has repeatedly shown that well vegetated dams built with well compacted cohesive materials can sustain substantial overtopping flow with minimal damage. As NRCS begins a rehabilitation program to rehabilitate aging watershed dams, a major issue is increasing the height or spillway capacity of the existing dams to accommodate larger required design storms. Research that can increase the confidence level of the dam safety profession to accept limited overtopping flow in upgrading these dams could eliminate the need for expensive structural upgrades on many existing agency dams. A last NRCS need related to dam failure analysis is a better tool for risk assessment. Recent new authority for NRCS to provide rehabilitation assistance came with the requirement to give priority consideration to those existing dams that are the greatest threat to public health and safety. NRCS has adopted a risk index system based on the common approach that total risk is the product of the probability of loading, the probability of adverse response to that loading, and the probability of consequence due to adverse response. Dam failure research could provide better tools to define the probability of adverse response.
6
B-13
Dam Failure Analyses Workshop Oklahoma City, OK June 26-28, 2001 James Evans FERC Division of Dam Safety and Inspections
Important Areas to Consider in the Investigation and Evaluation of Proposed and Existing Dams • The Embankment Must be Safe Against Excessive Overtopping by Wave Action Especially During Pre-Inflow Design Flood • The Slopes Must be Stable During all Conditions of Reservoir Operations, Including Rapid Drawdown, if Applicable • Seepage Flow Through the Embankment, Foundation, and Abutments Must be Controlled so That no Internal Erosion (Piping) Takes Place and There is no Sloughing in Areas Where Seepage Emerges
•1
Important Areas to Consider in the Investigation and Evaluation of Proposed and Existing Dams (Continued) • The Embankment Must Not Overstress the Foundation • Embankment Slopes Must be Acceptably Protected Against Erosion by Wave Action from Gullying and Scour From Surface Runoff • The Embankment, Foundation, Abutments and Reservoir Rim Must be Stable and Must Not Develop Unacceptable Deformations Under Earthquake Conditions
Design Factors of Safety for Embankment Dams • End of Construction----------------------------------------------------------FS > 1.3 • Sudden Draw Down From Maximum Pool------------------------------FS > 1.1 • Sudden Draw Down From Spillway Crest or Top of Gates----------FS > 1.2 • Steady Seepage with Maximum Storage Pool---------------------------FS > 1.5 • Steady Seepage With Surcharge Pool-------------------------------------FS > 1.4 • Seismic Loading Condition Factor of Safety----------------------------FS > 1.0 • For Zones with Seismic Coefficients of 0.1 or Less – Pseudostatic Analysis is Acceptable if Liquefaction Does not Trigger. • Deformation Analysis are Required if pga 0.15g For Newmark Procedures, Deformation Should be 2.0 feet.
•2
Stability Analyses Programs to Determine the Factors of Safety for the Various Loading Conditions • Computer Programs Such as UTEXAS3 are Used to Determine the Factors of Safety for the Various Loading Conditions Previously Discussed • COE Hand Calculation Method From EM 110-2-1902 are Used to Confirm a Computer Program Critical Failure Surface for Important Projects
Lake Blackshear Dam
• Reasons to Prevent Overtopping of an Embankment Even When Covered With a Good Growth of Grass
•3
•4
Milner Dam
• Control of Seepage Flow Through the Embankment, Foundation, And Abutments to Prevent Internal Erosion (Piping) • The Slopes Must be Stable During all Conditions of Reservoir Operations • Do Not Permit Unacceptable Deformations Under Earthquake Conditions
•5
•6
•7
•8
Santee Cooper East Dam
• The Embankment, Foundation, Abutments, and Reservoir Rim Must be Stable and Must Not Develop Unacceptable Deformations Due to Earthquake Loadings • Use of Hand Calculations to Confirm Computer Calculations
•9
•10
Dayton Dam Canal Embankment Failure & Repair Michael S. Davis - Lead Engineer Chicago Regional Office Federal Energy Regulatory Commission
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Plan View
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1
Pertinent Data •
Licensed: 1923
•
Built: 1925
•
Hazard Potential: Low
•
Flood of Record: 47,100 cfs (11/10/55)
•
Canal Dike Height: 28 feet
•
Canal Dike Length: 725 feet
•
A substation, owned by a separate entity, is located adjacent to the powerhouse.
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Overflow Spillway
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Canal Embankment – before failure
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Intake
Dayton Dam Canal Embankment Failure
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Canal Embankment – before failure
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Events Leading Up to the Breach • On July 17, 1996, 16.91 inches of rain over an 18-hour period was recorded at the Aurora precipitation gage, which is located about 40 miles upstream of the dam. As a result, the reservoir rose throughout the following day. • At 4 p.m., on July 18th, the water level was at about 2 feet below the crest of the headgate structure, and rising about 1 foot per hour. The tailwater was also still rising and was beginning to encroach on the substation. • At that time, the substation owner ordered the plant be taken off line so that the power could be cut to the substation. • As a result, the level in the canal rose about 3 feet at the powerhouse, and began to overtop the canal embankment around both sides of the powerhouse.
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4
The Breach • With the reservoir still rising and the flow now overtopping the canal embankment, the canal embankment breached at about 7 p.m. The breach was initially measured to be about 50 feet wide when it reached the foundation. Several secondary breaches formed on the embankment as well. • The resulting breach lowered the canal level and caused a differential pressure on the raised headgates. As a result, the chains holding the gates failed and all four gates slammed into the closed position and were severely damaged. • The flood peaked at about 55,000 cfs later that night (setting a new flood of record) with the reservoir at about elevation 507.8 feet, which is 8.9 feet over the crest of the spillway, and about 3 inches over the crest of the headgate structure. The tailrace reached a peak elevation of 488.9 feet at the powerhouse.
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Cross-section of intake
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5
Graph
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July 20, 1996 Inspection • The next four photographs show the condition of the project structures two days after the breach. • The inspection was unannounced and was done on a Saturday. Access to the site was limited to the left bank opposite the side of the canal. • The tailrace had receded about 15 feet since the breach.
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6
Breach – 7/20/96
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Canal Embankment 7/20/96
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Downstream Side at Secondary Breach 7/20/96
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Downstream Right Side of PH 7/20/96
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8
July 22, 1996 Inspection • The next nine photographs show the condition of the project structures four days after the breach. • The tailrace had receded about another 6 to 8 feet since the July 20 inspection.
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Headgate Structure 7/22/96
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9
Upstream side 7/22/96
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Upstream Side of Canal 7/22/96
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Secondary Breach Location 7/22/96
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Erosion at House 7/22/96
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Erosion at House 7/22/96
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Breach – 7/22/96
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Horses on dunes 7/22/96
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September 30, 1996 Inspection • On July 25, 1996, the licensee began to place large trap rock upstream of the headgate structure to cut off the flow. • The cofferdam was completed on July 30, 1996. • The next four photographs show the condition of the substantially dewatered canal.
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13
Headgate Structure 9/30/96
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Upstream side 9/30/96
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Breach 9/30/96
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Breach 9/30/96
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15
Investigation and Evaluation • Primary breach was located from Station 6+40 to Station 7+25, which is 85 feet at the crest. • Secondary breaches were located at Stations 1+50, 3+00, and 4+00. • Nine borings were taken of the existing embankment and foundation. The material was found to be heterogeneous, varying from clays, to silt and silty sand, to poorly graded sand. Blow counts ranged from 6 to 18. • A loose-to-medium-dense silty sand layer about 2 feet thick was encountered on the bedrock from the centerline of the embankment to its toe. • The foundation rock is a fine-grained, hard-jointed sandstone with numerous horizontal joints and fractures.
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Survey Exhibit
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16
Reconstruction of the Canal Embankment • Construction began on July 31, 1997. • The canal embankment was completed in late November 1997. • Repairs to the headgate structure were completed in April 1998. • Generation resumed on May 11, 1998. • Cost of repairs was about $1,600,000.
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Exhibit
Dayton Dam Canal Embankment Failure
17
Exhibit
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August 27, 1997 Inspection • The following two photographs show the progress of the reconstruction work. • At the time of this inspection, the contractor was reconstructing the canal invert.
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18
Filling in Canal Bottom 8/27/97
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Filling in Breach 8/27/97
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19
September 30, 1997 Inspection
• The next four photographs show the canal embankment substantially completed.
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Canal Embankment
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Impervious Core in breach 9/30/97
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Cut-outs in canal Embankment 9/30/97
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Canal Embankment 9/30/97
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Final Inspection – January 15, 1998. • Embankment work was completed. • New gates were being installed in the headgate structure.
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Canal Embankment 1/15/98
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23
B-14
Presentation for FEMA / USDA Workshop – 27 June 2001:
Workshop on Issues, Resolutions, and Research Needs Related to Dam Failure Analysis
BC HYDRO INUNDATION CONSEQUENCES PROGRAM Derek Sakamoto, P.Eng. BC Hydro
Introduction BC Hydro is currently working on a program to define a methodology for assessing the consequences resulting from a potential dam breach. This Inundation Consequence (IC) Program was initiated in February 2000, with the goal of defining guidelines for performing the consequence investigation, which is to be followed by the completion of consequences investigations on all BC Hydro’s dam facilities. Included in this overview of the IC Program is a brief discussion of BC Hydro and its assets, a summary of the legislation and guidelines defining the requirements of the program, followed by a discussion highlighting the key components of the IC Program. Primary Rationale & Objectives The key focus of this program is to provide an improved investigative tool for safety management planning. In the case of dam-breach emergency planning, this program will provide decision-makers with realistic characterizations of the various situations to which they may have to respond. Investigation into the effect of parameters such as dam breach scenarios and temporal variation related to the flood wave propagation can be performed. Additionally, severe “non-dam-breach” scenarios, such as the passing of extreme floods, can be investigated. A valuable product from these investigations will also be in providing powerful communication tools. This will benefit decision-makers by ensuring they are well informed of the magnitude of potential impacts related to dam breaches, thus enabling emergency precautions that are proportionate to the consequences and uncertainties. Additionally, meeting regulatory approvals and due diligence are key factors. BC Hydro British Columbia (BC) is the western most of Canada’s provinces, located on the Pacific Ocean along the West Coast. BC Hydro itself is a crown corporation, meaning it is a corporation that is owned by the province. The corporation, however, is run like a business without subsidies from the government; and like other commercial businesses its dividends are provided to its owner which, in this case, is the Province of BC. BC Hydro owns 61 dams located within 6 operating areas: • Columbia River Basin – encompassing the upper region of the Columbia River and draining into Washington State, this area produces approximately 50% of BCH power. • Peace River Basin – the second largest power generating area, the Peace River joins with the Athabasca River in Alberta. • Coastal Region – Several smaller dam facilities located along the BC coast. • Lower Mainland (Vancouver Region) – housing facilities located near the City of Vancouver. • Fraser River Basin – 4 dam facilities draining into the Fraser River. • Vancouver Island – a number of dam facilities located on Vancouver Island (southwestern corner of BC), taking advantage of the high precipitation of the Pacific West Coast. BC Hydro’s assets range from the extremely large Mica Dam to the smaller Salmon River Diversion Dam. Mica Dam, a 243 metre high earth-fill dam, is located at the headwaters of the Columbia River; the Salmon River Dam is a 5.5 metre earth-fill diversion dam located on the Campbell River system on Vancouver Island.
BC Dam Regulations and Guidelines Legislation for dam safety has been recently updated. Managed on a provincial level, the Province of BC passed its Dam Safety Regulations in late 1999. The need for defined safety regulations arose from such recent incidents as the failure of a private dam in May 1995. Although the breach of this small (6 metre high) dam did not result in any loss of life, over $500,000 in damage to property and infrastructure along with massive sediment loading into a local river resulted. The Province of BC has established regulations which define hazard classifications (Very High, High, Low & Very Low) for dams based on their consequence. Based on these hazard classifications, frequency of inspection to ensure the safe operations of the dam facilities are outlined as follows: Item
Very High Consequence
High Consequence
Low Consequence
Very Low Consequence
Site Surveillance [a] Formal Inspection [b]
WEEKLY SEMI-ANNUALLY
MONTHLY ANNUALLY
QUARTERLY ANNUALLY
Instrumentation
AS PER OMS * MANUAL ANNUALLY
WEEKLY SEMI-ANNUALLY or ANNUALLY AS PER OMS * MANUAL ANNUALLY
AS PER OMS * MANUAL ANNUALLY
N/A
UPDATE COMMUNICATIONS DIRECTORY SEMI ANNUALLY REVIEW EVERY 7 - 10 YEARS EVERY 7-10 YEARS [d]
UPDATE COMMUNICATIONS DIRECTORY SEMI ANNUALLY REVIEW EVERY 10 YEARS EVERY 10 YEARS [d]
UPDATE COMMUNICATIONS DIRECTORY ANNUALLY REVIEW EVERY 10 YEARS [d]
N/A
Test Operation of Outlet Facilities, Spillway gates and other Mechanical Components Emergency Preparedness Plan
Operation, Maintenance & Surveillance Plan Dam Safety Review [c]
ANNUALLY
REVIEW EVERY 10 YEARS [d]
Further information regarding the BC regulations can be found at the web site: http://www.elp.gov.bc.ca/wat/dams/reg_final.html In addition to the BC regulations a consortium of dam owners in Canada called the Canadian Dam Association (CDA) has established guidelines defining key design parameters for dam construction. As with the inspection requirements of the provincial regulations, the level of the design requirements are based on the consequence classification: Consequence Category
Earthquake Criteria Maximum Design Earthquake (MDE)
Very High
Maximum Credible Earthquake (MCE) 50% to 100% MCE -
High Low
1/10,000 1/1000 to 1/10,000 1/100 to 1/1000
Inflow Design Flood (IDF) Criteria Probable Maximum Flood (PMF) 1/1000 to PMF 1/100 to 1/1000
Further information regarding the Canadian Dam Association can be found at web site: http://www.cda.ca Inundation Consequence Program Building upon the legislative, design and safety requirements, the IC program is focused on defining the consequences associated with potential dam breaches. In doing this assessment, four key tasks have been defined: • Hydraulic modeling
• • •
Life Safety Model Environmental / Cultural Impact Assessment Economic / Social Impact Assessment
Hydraulic Modeling Previous breach assessment work done by BC Hydro was done during the 1980’s. This analysis provided inundation mapping for assumed dam breach scenarios, which were completed using the NWS DAMBRK model. In looking to update what was “state of the art” of its time, BC Hydro has opted to update these breach studies using the 2-dimensional hydraulic model TELEMAC-2D. The decision to use the 2 dimensional model is driven by two aspects. The 2-D model offers the ability to simulate complex flow patterns, which will be valuable tool in simulating the spread of flood waves over wide areas, or circulation and backwater of flows. Additionally, the 2-D output is an integral part of the Life Safety Model discussed later. There is, however, a need to identify the data requirements in selecting the correct computer model. In cases of “low consequence” dams, it may not be necessary to go to the expense and level of effort required of the 2-D model when a 1-D model can provide the same, or sufficient results. Two levels of assessment in the IC Program may be performed, with 1-D or coarse 2-D models being used on “low consequence” dams, and the more detailed 2-D models being used on the “high consequence” facilities. Life Safety Model BC Hydro is developing the Life Safety Model (LSM), a 2-D computer model which will be used to estimate Population at Risk (PAR) and Loss of Life (LOL) in the event of a dam breach. The power of the LSM is in the ability to simulate the movement of people over real space as they becoming aware of the dam breach, and models how people will escape from a flood. Using national census data, the PAR can be distributed over areas being assessed. Various scenarios are prepared distributing the PAR based on time of day, day of week, or season. This dynamic model will use the flood wave hydrograph produced by TELEMAC-2D as input, simulating the movement of the PAR in real time as the flood-wave propagates. The LSM model then simulates how the people react to the flood-wave, and their means of escape. A key aspect of this modeling is in providing a valuable tool in defining evacuation routes, potential “bottle-necks” in the evacuation plans, and highlighting problems associated with high risk areas such as hospitals or schools. Environmental / Cultural Impact Assessment The dam breach could result in a number of environmental and cultural impacts in areas both upstream and downstream of the dam. Three Consequence Types were identified (Physical, Biological, and Human Interaction) with 18 resulting Consequence Categories: Physical terrain stability, river channel changes, soil loss / deposition, mobilization of debris, & water quality Biological vegetation, fish, fish incubating, wildlife, productivity of reservoir, & productivity of receiving systems Human Interaction forest, agricultural resources, mineral resources, biological resources, settlement, recreation, & heritage Evaluation of these individual consequence categories is based on the net impact the potential dam breach could have on them. For each category, a series of “linkage diagrams” have been established. Each link defines the resulting effect that the breach can have on the specific category in varying degrees of severity. The more severe the impact, the higher up the linkage diagram. Economic / Social Impact Assessment The initial and key challenge in the economic assessment model was in the identification of all structures (residential, institutional, businesses, industries etc.) at risk. Geographic Information Systems (GIS) is being utilized to link the various available databases, the location of areas at risk, and the magnitude of the impending hazard. Databases that were used to identify areas at risk include:
• hydraulic model inundation polygon provided in UTM coordinates (TELEMAC-2D output); • BC Hydro customer database (providing building location with UTM coordinates & address); • BC Assessment Authority database (providing property values, property improvement value, construction material, structure use, age, number of floors, etc.) GIS also provides a valuable assessment tool in yielding a powerful graphical representation of properties at risk. It also yields an easily queried database to assess economic impact based on various scenarios. Future work will entail linking the economic losses with respect to social impact on communities in the inundation zone. IC Program Future A pilot program is currently under way to establish guidelines for completing the Inundation Consequence assessments. A draft of these guidelines is planned for completion during the summer of 2001 and finalized in 2002. Ultimately, inundation consequence assessments will be completed for all the BC Hydro sites.
Presenter:
Contact at:
Derek Sakamoto is an engineer with BC Hydro’s Power Supply Engineering (PSE) group, and works in the Civil Engineering / Water Resources team. Having been with PSE for just over one year, Derek brings to his team over five years in consulting with a focus in design, construction and assessment work in hydraulic/hydrologic related projects. (604) 528-7812 (phone); (604) 528-1946 (fax);
[email protected] (email) BC Hydro - 6911 Southpoint Drive (E13), Burnaby, BC, CANADA, V3N 4X8 (mail)
B-15
Analyzing Flooding Caused by Embankment Dam Breaches: A Consultant’s Perspective By Ellen B. Faulkner, P.E. Mead & Hunt, Inc. Madison/Eau Claire, Wisconsin
Introduction As engineering consultant to owners of dams throughout the United States, Mead & Hunt performs dam safety assessments which must be responsive both to the needs of the dam owner and to the requirements of state and federal regulatory agencies. Frequently, these dam safety studies include the simulation of a hypothetical dam failure for the purpose of hazard classification, emergency action planning, or design flood assessment. Each dam failure study begins with the identification of a critical, but plausible, mode of failure and the selection of specific parameters which define the severity of the failure. These parameters include the ultimate dimensions of the breach, the time required to attain these dimensions, and (in the case of overtopping failures of embankment dams) the depth of overtopping required to initiate a failure. None of these quantities is easily identified. In many cases, the obvious solution is to choose the “path of least resistance” - that is, the parameters which will most easily meet with regulatory acceptance. However, choosing excessively conservative breach parameters may impose significant costs on the dam owner in the form of new design work and remedial actions, additional safety studies, or unnecessarily complex or inefficient emergency action plans. Clearly, the design, construction, and material composition of an earthen embankment significantly affect how a breach will form. As consultants we are aware that analytical approaches exist, based on theory, experiment, and experience with real dam failures, for relating breach size and speed of formation to the characteristics of the embankment. However, these approaches are not yet well-established enough to use in the regulatory settings in which we work. One Mead & Hunt study from northern Wisconsin, now almost ten years old but still fairly representative of the difficulties that may be encountered in this type of study, illustrates how different approaches to simulating an embankment breach can lead to substantially different conclusions with respect to design and safety requirements.
Mead & Hunt, June 23, 2001
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Case Study Setting The Chalk Hill hydroelectric project is located on the Menominee River on the border between northeast Wisconsin and Michigan’s Upper Peninsula. Three miles downstream is the White Rapids project, owned by the same utility. Both dams contain concrete spillway sections and long earth embankments, but Chalk Hill’s embankment is significantly higher (37 feet) than that at White Rapids (25 feet). The river valley below both dams is lightly developed, with a mix of year-round and seasonal residences located near the river and potentially in the dam failure inundation area. The studies described below were performed in 1992. They were the most recent of a series of dam break studies for the dams, which began in 1983 with a HEC-1 storage routing model which indicated that the hazard related to overtopping embankment failure of either dam was minimal. In 1987, failures of the embankments were re-analyzed using the NWS-DAMBRK dynamic routing model. In both the 1983 and 1987 studies, the assumed breach dimensions were consistent with then-current guidelines, which called for a breach bottom width equal to the height of the dam. In 1988, the Federal Energy Regulatory Commission (FERC) issued new guidelines regarding breach assumptions used for emergency action plans, inflow design flood studies, and hazard classifications. In the case of breaches in earth embankments, the assumed average breach width was to be as much as five times the dam height. Reviewing the 1987 reports under these guidelines, FERC requested a re-analysis for Chalk Hill and White Rapids using a wider breach. These analyses were conducted in early 1992. For White Rapids, where the height of the embankment was just 25 feet and downstream development relatively high on the valley walls, the re-analysis still indicated no incremental hazard due to overtopping flows; that is, the existing spillway capacity was adequate. For Chalk Hill, however, the use of a breach width in the high end of the stipulated range led to an IDF determination about twice the existing spillway capacity. Part of the problem at Chalk Hill was an assumed domino failure of White Rapids Dam. Although White Rapids did not pose a downstream hazard by itself, an overtopping failure of White Rapids in conjunction with the peak of the dam failure wave from Chalk Hill would affect residences which were not in the inundation area of White Rapids alone. However, the assumed failure of White Rapids, consistent with FERC’s approach, occurred at the peak overtopping stage after the Chalk Hill failure. This scenario would require that the White Rapids embankment survive about three feet of overtopping before finally failing. If White Rapids could be assumed to fail at some lesser depth of overtopping, the downstream consequences would be less. Mead & Hunt, June 23, 2001
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In most inflow design flood studies, the addition or removal of a few structures to the dam break inundation zone is of little consequence, as long as at least one inhabited structure is affected by the flood. In this case, however, each structure determined to be affected was important because one of the owner’s alternatives was to purchase affected properties. Physical Model Analysis (NWS-BREACH) The inflow design flood determination for Chalk Hill Dam involved a number of separate breach assumptions: the breach formation time and dimensions at Chalk Hill; the formation time and dimensions at White Rapids; and the depth of overtopping which would certainly cause failure at White Rapids. (Another issue, related to the evaluation of alternatives for upgrading the spillway capacity, was whether initially confining Chalk Hill overtopping flows to a low section of the embankment would successfully promote a non-critical failure in that section.) We questioned whether the extreme breach parameters used in the first 1992 study were appropriate, considering two characteristics of the dam. First, the embankments were engineered and well-constructed -- unlike many dams whose actual historical failures formed the database that was apparently the foundation for the guidelines. Second, both reservoirs were small and drawdown would happen quickly. In an attempt to determine whether breach dimensions in the middle or lower end of the FERC’s suggested range would be consistent with the site-specific characteristics of the dams, we used the NWS-BREACH program to assess the breach formation characteristics at both Chalk Hill and White Rapids dams. BREACH is a physically based erosion model for embankment dams, and generates a time sequence of breach dimensions and an outflow hydrograph given an inflow hydrograph, the dam and reservoir capacity, and geometry and material properties for the embankment. The data requirements for BREACH include dike material data such as cohesion, angle of internal friction, void ratio, unit weight, plasticity, and gradation. The availability of almost all of these data through recent boring studies was another factor which made the physical model approach practicable for the Chalk Hill study. Using the NWS-BREACH model was a new approach in our experience, and one not approved by the FERC. To anticipate reviewers’ concerns about the accuracy and conservativeness of the model, we adopted an approach in which we simulated the breach using the most critical lab test values from both borings at each site. We tested one input variable at a time, choosing the single test value from the two borings which gave the most severe breach. When two tests values were not available, we chose the worst-case value of the input variable, Mead & Hunt, June 23, 2001
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based on ranges given in the BRAECH program documentation. The resulting breach description was significantly different from any of those postulated, based on written guidelines, in previous studies. Table 1, below, summarizes the differences between the analyses. Table 1 Comparison of Chalk Hill Embankment Breach Characteristics Using FERC Guidelines and NWS-BREACH Program Breach
Depth of
Breach
Time for
Ultimate
Breach Side
Assumptions
Overtopping
Formation
Breach to
Breach
Slope (H:V)
Required to
Time
Erode to
Bottom
Initiate
Bottom of
Width
Breach (ft)
Dam
Pre-1988
0.5
1 hour
1 hour
Studies Studies
37 feet (1 x
1:1
dam height) 0.5
0.3 hour
0.3 hour
111 feet (3 x
1:1
dam height)
Based on 1988 Guidelines NWS
0.6
> 2 hours
0.1 hour
25 + feet
1:1
BREACH There were two major differences between the BREACH results and earlier assumptions. First, the BREACH program gave a much smaller breach after 2 hours (the time step limit in the program) than we had previously assumed for a one-half-hour formation time. Second, the simulated breach eroded very quickly to the bottom of the dam, at which time the peak reservoir outflow occurred, then continued to slowly widen as the reservoir level dropped. At two hours, the average breach width was about 1.7 times the height of the dam -- within the range given in the FERC Guidelines, but near the lower end. The breach was still widening when the program halted due to time step limits, but the peak outflow had long since passed. We also performed a sensitivity analysis to the individual material properties input to the program. Varying each one by plus or minus 25 percent, we found that the maximum changes in
Mead & Hunt, June 23, 2001
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peak breach outflow were + 10 percent and -28 percent. The most sensitive parameters were cohesive strength and friction angle. We did not vary any of the parameters in combination with others, so did not determine how the various properties interacted in affecting the breach. A separate NWS-BREACH analysis was conducted for the White Rapids project. There, the simulated breach was larger relative to the dam height. The average breach width at White Rapids was more than twice the dam height. The White Rapids embankment materials had a lower unit weight than those at Chalk Hill, were more poorly graded, and were assumed to have zero cohesion (due to a lack of test data). Inflow Design Flood Determination NWS-BREACH develops an outflow hydrograph but does not route it downstream. Therefore, we used the breach parameters predicted by the NWS-BREACH model as input to the NWS DAMBRK model. The resulting Inflow Design Flood was 85,000 cfs -- st ill more than the calculated spillway capacity, but much less than the IDF indicated by the previous study. The NWS-BREACH study never met with regulatory approval, apparently due to the very limited track record of the BREACH model and the startlingly less severe breach it predicted than had been assumed in previous studies. Returning to the more severe guidelines-based breach used in the previous study, however, we still had an unanswered question. This was the determination of risk below White Rapids Dam, which was presumed to fail as a result of the Chalk Hill breach wave. However, for some of the IDF cases considered, White Rapids would be overtopped by several feet before the Chalk Hill failure occurred. The BREACH model -- even if it had been accepted -- was of little help in this question, because it predicted a rapid failure at just 0.5 foot of overtopping. Although that may have been the most likely event, none of the parties involved were comfortable with it as a “worst-case” scenario. Finally, it was agreed, on the basis of professional judgement alone, that the White Rapids embankment need not be assumed to withstand more than two feet of overtopping. Eventually, the owner of the projects addressed the IDF in two ways. One - demonstrating that the best ideas are the simplest -- was to retest the radial opening of the spillway gates. The gates proved to open considerably farther than shown in the design drawings, resulting in a spillway capacity about 20 percent higher than had previously been computed. Secondly, the owner purchased outright or in easements the remaining affected properties, which were relatively few in number. Mead & Hunt, June 23, 2001
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B-16
B-17
Current Dam Safety Research Efforts Presented By: Steven Abt
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