Hydrologic Issues for Dams - Association of State Dam Safety Officials

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Current Practices.Corps of Engineers Huntington DistrictHydrologic Research Needs for Dam Safety AnalysisByJerry W. Webb 1Huntington District utilizes standard methodologies to develop Probable Maximum Flood(PMF) for individual projects. Out of the 35 dams that have been constructed in theDistrict, nineteen (19) are considered adequate under current criteria, seven (7) werealtered to correct the deficiency, and nine (9) facilities remain that provide 53-79% PMFretention. Historically, the PMF was evaluated using the "then-current" standards andspillway adequacy was assessed. There have been several changes over the years inmethodology and standards, which have impacted the magnitude of the PMF, whichaltered the extent of the deficiency and/or structural fixes required to correct thehydrologic deficiencies. The assessment of hydrologic deficiencies is funded under theOperations & Maintenance program and there is no specific allocation to provide anycontinuity to the program. The Dam Safety Assurance (DSA) requirements at BluestoneDam will exceed $110 million. This single project exceeds the total construction cost ofthe seven altered facilities by a factor of five. There is a significant need to developconsistent, supportable guidelines and methodologies for development of a design stormto be used in Dam Safety analysis that is acceptable over a broad range of scenarios.Proble ms with Existing Methodology.Developin g a Probable Maximum Flood (PMF) for any project continues to rely upon theproper application of current knowledge and the development of individual experience inthe field of hydrology. The final hydrograph for any PMF continues to be subjective andjudgmental due to a large number of factors such as:Orographic Effects.The Huntington District includes a mountainous area that suffers from orographic lifting,whenever the wind has an onslope component. The rate of the lifting is determined bythe magnitude of the onslope component and the degree of slope. The determination ofthe height of the air column to be lifted orographically presents a problem when dealingwith Probable Maximum Rainfall (PMR). Studies suggest that a 3,000-ft mountainbarrier in the path of a strong wind should certainly produces an upward wind componentthat would persist above the 6-km. level. This situation has resulted in the HuntingtonDistrict requesting special studies by the NWS to develop site-specific PMR for theseprojects. The peak discharge has been impacted by 0-7% when compared to standardprocedures.1 Huntington District, U.S. Army Corps of Engineers1Paper 1 - Webb
Lack of Historical Data for Extreme Events.Historical stream flow data provides (when available) a means for calibrating thehydrology relating to the unit hydrograph development and stream routing procedures.The other variables (antecedent condition, initial and constant rainfall lose rate, etc.)cannot be readily determined for a Probable Maximum Flood event. Very few gagingstations provide data representative of extreme events, which suggests that calibration isof limited value in dam safety applications.Variation in Operational Scenarios.Huntington District develops daily forecasts for the purpose of making operationaldecisions. Most of our projects have not been operated for extreme events, and there is aperception that water control plans may be altered on a case-by-case basis during a majorflood event. This consideration complicates the modeling process especially whendealing with the calibration issue. Current assessments are not consistent concerningoperational assumptions during a PMF.Antecedent Rainfall Conditions.The rainfall breakdown for the design storm is not well documented relative toassumptions for antecedent rainfall that precedes the Probable Maximum Rainfall (PMR).The following National Weather Service criteria of incorporating the antecedent eventinto the total storm hydrograph is based on judgment (experience) and not supported byany particular study. The following diagram shows how the antecedent rainfall iscurrently used in the reservoir routing process. It provides a starting pool elevation priorto the main event.30% PMR + 3- Day Dry Period + 100% PMR39% PMR + 5-Day Dry Period + 100% PMRPaper 1 - Webb 2
Page 1 and 2: The National DamSafety ProgramResea
Page 3 and 4: collaboration and expertise, and th
Page 5 and 6: TABLE OF CONTENTSPage #FOREWORD....
Page 7 and 8: NEW DEVELOPMENTS AND NEEDS IN SITE-
Page 9 and 10: FOREWORDThe Federal Emergency Manag
Page 11 and 12: The PMF represents an estimated upp
Page 13: RESEARCH AREASResearch needs were b
Page 17 and 18: should be developed that will allow
Page 19 and 20: • Storms used in HMR 55A and in t
Page 21 and 22: last 20,000 year s in submitting th
Page 23 and 24: d. USBR Guidelines Publication, Dam
Page 25 and 26: was severe. There was a potential,
Page 27 and 28: Thirdly, we must develop better way
Page 29 and 30: For most projects, the first step f
Page 31 and 32: Instead of lumping the loss rates t
Page 33 and 34: adjust the rating curve for when th
Page 35 and 36: In response to this situation, TVA
Page 37 and 38: TVA established a Regional Resource
Page 39 and 40: The Utah Hydrological ExperienceByM
Page 41 and 42: The Utah Hydrological ExperienceByM
Page 43 and 44: Late SpringRain on snow eventModel
Page 45 and 46: State of GeorgiaFEMA Workshop on Hy
Page 47 and 48: a. HEC1• DOS based program - glit
Page 49 and 50: Long te rm hydrologic needs include
Page 51 and 52: Current State and County PracticesA
Page 53 and 54: present standards and, as such, are
Page 55 and 56: Research Needs in Dam Safety Analys
Page 57 and 58: Looking at the four test cases give
Page 59 and 60: Hydrologic Analyses Related to Dam
Page 61 and 62: (iii) Modified Expected Cost Approa
Page 63 and 64: Parameter Estimation Based on Joint
Page 65 and 66:
estimation of lag times and loss ra
Page 67 and 68:
Hydrology for Dam Safety - Private
Page 69 and 70:
Probable Maximum PrecipitationAll o
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The most common synthetic methods a
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the standard would be very conserva
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65Paper 12 - Cecilio
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Page 79 and 80:
Page 81 and 82:
REFERENCES1. U. S. Army Corps of En
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Overview of Flood Estimation Proced
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Extreme FloodsExtreme floods, the t
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Temporal patterns used to distribut
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Preliminary Estimates of Rainfall a
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24 to 72 hours). CRC Research Repor
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CURRENT AND FUTURE HYDROLOGIC RESEA
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ecause existing procedures are inad
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REGULATORY INVOLVEMENT AND COMMUNIC
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Decision BasisThe third feature of
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However, should it be revealed that
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supporting sub-processes. Therefore
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Probability ofFailure P f1Probabili
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Since any rigorous analysis is an i
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well, and usually do include subjec
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Against this background, there is n
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Howson, Colin., and Peter. Urbach.
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Procedures and Analysis Technologie
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Currently transposition limits for
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pro cedures are used to allow the h
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Use of Atmospheric Models in Rainfa
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mountain watersheds in response to
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18. Schaefer MG, Stochastic Modelin
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dams which could be used to save li
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Hydrodynamic modelMIKE 21 BACKGROUN
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A FRAMEWORK FOR CHARACTERIZATION OF
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c) Decision Level Risk Assessment:
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credible estimates of extreme flood
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Fitting a distribution to data sets
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unoff volumes. Examples of this typ
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used to estimate frequency distribu
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Hosking, J.R.M., and J.R. Wallis, 1
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in west central Arizona. The draina
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Trial and error reservoir routings
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Figure 1 Alamo DamAlamo Dam and Spi
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Paper 19 - Chieh and Evelyn 146
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esources are made to mitigate flood
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implementing a risk-assessment meth
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Typically, the elevation of the top
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similar to that of the high-gradien
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and provides a cost-effective frame
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10000E. Colorado
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10000x Peak Discharge95% confidence
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An integrated science approach will
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ReferencesBaker, V.R., Kochel, R.C.
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Partnerships, Proceeding of the 200
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Flood HydrographThe flood hydrograp
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A variety of data including informa
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Improved Flood Frequency Extrapolat
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RESEARCH NEEDS SUMMARYJerry Webb, H
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o Unsteady flow computer models for
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• Upd ate “Generalized Snowmelt
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DISCUSSION1. Genera l - After the p
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participant was asked to pick what
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RISK ANALYSIS100Relative Comparison
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WORKSHOPONHYDROLGIC RESEARCH NEEDSF
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JOE SKUPIENPrincipal Hydraulic Engi
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