Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
Currently Skimming:
4. Hydrologic and Hydraulic Considerations
4. Hydrologic and Hydraulic Considerations
Pages 71-131
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 71... ...
It should be recognized that hydrologic and hydraulic analyses for dam safety assessments represent a very specialized and complex branch of engineering for dams. For any project where the consequence of dam failure 71
|
From page 72... ...
Bureau of Reclamation, Soil Conservation Service, the state dam safety agency, and other dam owners in the area. Hydrometeorologic characteristics of the watershed of the dam, such as the mean annual precipitation and mean annual flow, should be developed, and past storage fluctuations or reservoir seasonal operation records should be evaluated.
|
From page 73... ...
These approximate procedures should be used with care and under the direction of an experienced hydraulic and/or hydrologic engineer. Comparisons with Historical Peak Discharges Evaluations of the relative magnitude and the credibility of flood peak discharge estimates can be obtained by comparison with known historical peak discharges in other watersheds.
|
From page 75... ...
As discussed in the section Dam Break Analyses, rather complex techniques are available to compute the characteristics of the downstream flood wave following a dam failure. However, a number of simplified methods for making rough estimates of peak downstream flows from dam
|
From page 76... ...
76 o E _ us E ~ E o 8 ,, .- ,' I, , I i A i a 3 1 , .
|
From page 77... ...
on maximum peak flows immediately downstream of dams after their failure. Hagen presents two equations based on experienced dam failures, primarily dam failures in the United States that relate peak flood flows following failure to reservoir levels and storage volumes.
|
From page 78... ...
Analyses have shown that about one-third of recorded dam failures resulted from spillway inadequacy. Also, of approximately 9,000 nonfederal dams in the United States inspected recently, almost 25% were designated as unsafe because of inadequate spillway capacity.
|
From page 79... ...
But the committee noted that an additional and usually overriding requirement for security is to protect downstream interests against hazards that might be caused by the sudden failure of the dam and any ensuing flood wave. The survey found it to be common practice that spillway capacities, in connection with other project features, should be adequate to: · ensure that flood hazards downstream will not be dangerously increased by malfunctioning or failure of the dam during severe floods; · ensure that services of and investment in the project will not be unduly impaired by malfunctioning, serious damage, or failure of the dam during floods; · regulate reservoir levels as needed to avoid unacceptable inundation of properties, highways, railroads, and other properties upstream from the dam during moderate and extreme floods; and · minimize overall project costs insofar as practicable within acceptable limits of safety.
|
From page 80... ...
The prevention of overtopping such dams during extreme floods, including the probable maximum flood, is of such importance as to justify the additional costs for conservatively large spillways, notwithstanding the low probability of overtopping. The policy of deliberately accepting a recognizable major risk in the design of a high dam simply to reduce project cost has been generally discredited from the ethical and public welfare standpoint, if the results of a dam failure would imperil the lives and life savings of the populace of the downstream flood plain.
|
From page 81... ...
· Application of Standard 3 should be limited to dams impounding a few thousand acre-feet or less, so designed as to ensure a relatively slow rate of failure if overtopped and located where hazard to life and property in the event of dam failure would clearly be within acceptable limits. The occurrence of overtopping floods must be relatively infrequent to make Standard 3 acceptable.
|
From page 82... ...
In this case, however, the minimum-sized spillway should safely pass the 100-year flood. As previously noted, in some cases dam owners may not be willing or able to enlarge spillway capacities of existing dams in accordance with the guidelines presented in Table 4-3 because of financial constraints, or a dam may have been in existence for 25 to 50 or more years without any threat of being overtopped, which appears to support the owner's belief that the risk of dam failure and consequent damages is small.
|
From page 83... ...
Any increase in spillway capacity, even though less than indicated by Table 4-3, would decrease the risk of dam failure. However, all parties involved should recognize that such partial steps will not meet the design objectives to protect their interests.
|
From page 84... ...
SOURCE: U.S. Army Corps of Engineers (1982b)
|
From page 85... ...
This basic method has severe limitations in that the data base, i.e., usually annual flood peaks, in the United States is of relatively short duration. Very few records go back 100 years, and the majority are less than 50, so that only limited confidence can be placed in use of an extrapolated curve to predict flood events expected to occur only once in one or two centuries on the average.
|
From page 86... ...
Weather Bureau (now the National Weather Service) embarked on a study to determine probable maximum precipitation (PMP)
|
From page 87... ...
Before using either of these reports the user should consult with the regulatory agency or the National Weather Service on the appropriate report to use.
|
From page 88... ...
FIGURE 4-4 Probable maximum precipitation study regions.
|
From page 89... ...
One procedure that has been used is to consider a number of major floods, less severe than the PMF, that would cause overtopping and failure of the dam and to determine, by analyses of flood waves that would result from the respective failures, the maximum general flood in which the dam failure produces significant additional losses downstream. Such a flood is then adopted as the design flood for spillway improvements.
|
From page 90... ...
Often, hydrologic relationships derived from past experience should be adjusted to reflect the rainfall and runoff conditions visualized in the PMF. The objective of the analyses should be to reflect reasonably the probable maximum flood-producing potential of the basin without illogical pyramiding of improbabilities.
|
From page 91... ...
In some other parts of the country the time distribution pattern suggested in the Standard Project Flood Determinations (U.S. Army Corps of Engineers, 1965a)
|
From page 92... ...
92 o ~o o7 _ oo CO 1 1 =1 CO CO (-U!
|
From page 93... ...
As a general rule, the critical PMP in a small basin results primarily from extremely intense small-area storms, whereas in large basins the critical PMP usually results from a series of less intense large-area storms. A number of National Weather Service studies cover the time sequencing of storms.
|
From page 94... ...
A runoff model translates precipitation excess over a watershed to its resulting flood hydrograph. A number of different types of runoff models have been used,
|
From page 95... ...
Unit hydrographs are usually derived in one of two ways depending on the extent of the data available. In the case of gaged drainage basins, historical flood events are analyzed by separating out the base flow component and calculating the depicted surface runoff volume.
|
From page 96... ...
Base Flow The base flow in a river at the beginning of the main hypothetical flood should be equivalent to the receded flow of any antecedent flood assumed or considered in the study. In the absence of an assured antecedent flood, as may be the case for small drainage basins, a reasonable base flow such as the mean annual flow should be added to the principal flood.
|
From page 97... ...
In view of the uncertainties involved in estimating initial reservoir levels that might reasonably be expected to prevail at the beginning of an SDF, it is common practice, particularly for small and intermediate height dams with a single low-level outlet, to assume that the reservoir is initially filled to the "normal full pool level." This reservoir level should be in accordance with the operational practice for the season of occurrence of the SDF. Use of Gated Spillways Spillway crest gates are frequently used to provide required spillway capacity and to control the release of spillway discharges.
|
From page 98... ...
For existing dams, consideration may be given to taking credit for power releases within certain limitations and to full assurance that the power plant could be operated safely during an SDF. Reservoir Routing The computation by which the interrelated effects of the inflow hydrograph, reservoir storage, and discharge from the reservoir are evaluated is called reservoir routing.
|
From page 99... ...
The results of the routing procedure are a reservoir stage hydrograph and an outflow hydrograph representing the attenuation of the PMF inflow hydrograph by reservoir storage, spillway, and outlet facilities. Freeboard Allowance It is common practice to provide an extra height of dam over the computed maximum reservoir level for the design PMF.
|
From page 100... ...
DAM BREAK ANALYSES Knowledge of the nature and extent of catastrophic flooding and resulting risk to downstream life and property following collapse of a dam is a critical step in assessing and improving the safety of existing dams. Disasters caused by past dam failures and results from the recent Corps of Engineers' dam inspection program, have focused the attention of the public and federal and state officials on finding mitigatory measures for unsafe dams, on emergency action planning and preparedness, and on means for assessing public safety and predicting probable damage in the event of failure of existing dams.
|
From page 101... ...
TR #66 Simplified Dam Breach Model and the Uniform Dam Failure Hydrograph procedure described by Hagen (1982) , while others such as the National Weather Service Dam Break Flood Forecasting Computer Model and the Corps of Engineers HEC-l Flood Hydrograph Computer Package are very complex and require computer analysis.
|
From page 102... ...
Army Corps of Engineers for computing the critical failure hydrograph for all dams regardless of type and condition. This method uses an envelope curve or equation, based on historical dam failure data, to relate a "dam factor," the product of dam height and reservoir storage, to peak discharge from a postulated dam failure.
|
From page 103... ...
· Some Other Methods and Models. There are several other methods available for estimating peak discharge from a postulated dam failure, ranging from simple handworked methods to sophisticated two-dimensional computer models.
|
From page 104... ...
104 _ ~t a' ~ a, Z ~ ~ E C: C: sassol Con ._ ._ Con a)
|
From page 105... ...
Few comparative analyses of different available procedures and models have been made. One study conducted at the University of Tennessee compared peak dam failure discharges for two simple handworked methods (SCS and Classic Equations-Keulegan)
|
From page 106... ...
It is often said that a dam is unsafe because its spillway has inadequate capacity. What is usually meant is that the development must be considered unsafe because the reservoir and spillway acting together are not capable of controlling the adopted design flood to the extent needed to ensure the structural integrity of the dam, thus creating potential for dam failure with uncontrolled release of stored water and serious damages to persons and properties downstream.
|
From page 107... ...
The reasons for this could be lack of funds, lack of a practical method for correction due to space limitation, or a combination of many site-specific circumstances. In arriving at a decision the dam owner must consider the potential for liability from property damage upstream as well as downstream, the potential for loss of life, and the moral obligation to avoid unnecessary hazard to those who might be affected by dam failure.
|
From page 108... ...
Increasing Reservoir Storage Capacities In some cases it may be feasible to alleviate a problem of inadequate spillway capacity at an existing dam by making substantially more reservoir CHUTE AND FUSE PLUG SPILLWAY DESIGN TYPICAL CROSS SECTIONS Crest Width I - - ~ , Original Top of Dam Normal Reservoir Level T _ ~ / Lowered Crest of Fuse Plug ~ _ 18" Reinforced ~ Earth Dam Existing Ground ~ ~= ~—,.<,~ x~ -- ; I've D;_ Crest Detail for Dams Concrete Slab Without Corewalls Wrap Camp Drain Pipe Section. Spillway chute for typical earth dam.
|
From page 109... ...
· Steel sheet piles can be driven in certain types of fill dams to increase height. Diversions Upstream from Reservoir At some projects it may be feasible to direct runoff from the reservoir to mitigate a spillway capacity problem.
|
From page 110... ...
Modification of Project Operations In some instances it may be feasible to modify project operations to substantially increase the reservoir storage space that would be available to regulate the design flood. In considering such a plan the effect of the modified operations on project benefits, the effectiveness of the increased storage in mitigating spillway capacity problems, and the assurance that the storage would, in fact, be available when needed should be appraised.
|
From page 111... ...
(1974) Downstream Hydrograph from Dam Failures, Proceedings, The Evaluation of Dam Safety, Engineering Foundation Conference, November 28-December 3, 1974, published by ASCE, 1977.
|
From page 112... ...
U.~. Army Corps of Engineers (1959)
|
From page 113... ...
"Probable Maximum Precipitation Estimates, United States East of the 105th Meridian," Hydrometeorological Report No. 51, NOAA, National Weather Service, Washington, D.C., June.
|
From page 114... ...
4. Plot the six PMF peak discharges so obtained on logarithmic paper against drainage area, as shown on Figure 4A-7.
|
From page 115... ...
115 Mu o of of .
|
From page 117... ...
117 o o of o o ~ ~ ~ o of o ~ of of o o of o · _ ._ o o - As, ° ._ o o .s x Ct o
|
From page 119... ...
119 h h o ~ ~ ~ ~ r' ~ _ Ct o ant _ o 4Cal ~ .5 o .' o be o .s C: o Cal — lo, C)
|
From page 121... ...
121 Cal Ul '1: — I.U A: Ul of Ct x ·S.~O ONV3nOH '39~HOSlO Ed Coold WnWIXVW 319~90Ud
|
From page 122... ...
SOURCE: U.S. Army Corps of Engineers (1982b)
|
From page 123... ...
SOURCE: U.s. Army Corps of Engineers (1982b)
|
From page 124... ...
( 55 \ ~ 58 ~ 37! ll '~t'59/ 29 ~ I ~ A _ _ _ _ _ _ T 1 FIGURE 4B-5 Observed point rainfalls exceeding 50 % of all-season PMP, west of continental divide for 1,000 square miles and duration between 6 and 72 hours.
|
From page 127... ...
127 CD _ o o o oo oo ~ ~r U)
|
From page 128... ...
MT 48°30 113°38 12 9/3-7/1970 28 — UT 37°38 109°04 6 9/3-711970 29 — AZ 33°49 110°56 6 6/7/1972 30 Bakersfield CA 35°25 119°03 12/9-12/1973 31 — CA 39°45 121°30 48 SOURCE: U.S. Army Corps of Engineers (1982b)
|
From page 129... ...
When using the National Weather Service (NWS) Dam Break Model, it is often necessary to increase the base flow to an exceedingly high value before the computer program can run.
|
From page 130... ...
14. When a lower dam is overtopped from an upstream dam failure, all Dashboards and gates shall be assumed to fail.
|
From page 131... ...
25. To arrive at reasonable dam break mechanics, postulated failures shall be compared with historical dam failures.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.