Hydrologic Issues for Dams - Association of State Dam Safety Officials

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Multiple Reservoir Systems.A drainage basin with a system of reservoirs cannot be accurately modeled for ProbableMaximum Storm (PMS) spillway deficiency using the single storm centering approach ofHMR51 and 52. Large storm events will drop significant amounts of rainfall over alarger area than the single storm exhibits. A single storm centering will not capture thissituation, when dealing with multiple reservoirs. The potential exists for multiple projectsto receive significant amounts of rainfall during rainfall event frontal movements.New Technologies.Geographical Information System Interface.The determination of basin hydrologic parameters (size, shape, slope, etc.) andhydrologic model development continue to improve with technology and ability to obtaindigital mapping. The future looks very promising from this standpoint and shouldcontinue to improve with onset of ground verified rainfall radar imaging. There is adefinite need to incorporate the PMR distribution into the HEC-HMS modeling software.The drainage area and latitude/longitude of the storm centering could be accuratelyassessed using currently available GIS utilities. Sensitivity to the design stormassumptions could be easily applied to dam safety studies.Risk & Uncertainty Applications.The application of risk and uncertainty principles in the prioritization of dam safetyrequirements appears to have some merit. Developing a portfolio of structures, includingevaluations of risk and uncertainty of potential failure mechanisms and the consequencesof such failures, provides a tool that can be used to justify incremental expenditures forcorrection of some deficiencies while leaving others to a later date. The ongoing R & Dprogram in this arena will develop many tools that will assist dam safety officialsnationwide in making decisions based on highest risk and/or highest consequences offailure. The concept can also be used to prioritize engineering studies necessary to assessdeficiencies.R&D / Policy Needs.Address Current Known Problems & Inconsistencies.The problems discussed above support the need for consistent guidelines that considerorographic effects, large drainage areas subject to frontal movement storms, multiplereservoir systems, lack of historic data for calibration, antecedent rainfall conditions, andvariations in operational scenarios for extreme events. The cost to correct observeddeficiencies is directly related to the magnitude of the design event. Provisions for moreobjective evaluation procedures should be the goal of any new R & D initiatives. Policy3Paper 1 - Webb
should be developed that will allow the issues, raised above, to be addressed in anefficient and economical manner.Extreme Event Volume/Duration/Frequency Criteria.There is significant need to develop a better understanding of the probability/frequencyassociated with the PMF. There is a perception that some agencies have started usingdifferent evaluation criteria for dam safety, which disregards the current design standards.Extreme event frequency projections are absolutely essential to any dam safety analysisprocess.Special Problem/Example.The Huntington District has been involved in developing design storms to determinespillway adequacy for several reservoirs in Nicaragua. HMR51 and HMR52 wereemployed to achieve this assigned task even though they were developed by NWS for themainland USA. Hurricane Mitch rainfall dropped extreme amounts of rainfall over avery large area. PMF storm amounts would not capture this condition when dealing witha large drainage area above an existing project. The outer isohyetal rainfall of the PMFdid not approach the actual hurricane amounts. Special adjustments were made to finalHMR52 rainfall amounts based on air moisture carrying capability and distance from thesea. The diagram demonstrates how the rainfall pattern was placed over one of the studyareas.GUATEMALA(X (X (X(X (X (X#YLegendMictchtrack.shp(X Tropical Depression#Y Tropical Storm"¸ Category 1 Hurricane"¹ Category 2 Hurricane"º Category 3 Hurricane"» Category 4 Hurricane"¼ Category 5 Hurricane#Y #Y #Y#Y"¼ ¼ " " "¼ "¼ ¼ ""¼"¼ "¼"¼» "»"»" "¼"¹"¹"¹ "º "¸"¸"¸"¸"º"º"º #Y#Y #Y #Y#YHONDURAS#Y #Y#Y#CHOLUTECA# CHINANDEGA#LEONApanasBasinNICARAGUAWNSEJWNSIGABRIELH G F E DCJINOTEGAEMANCOTALAPANASTUMAApanasBasin30 0 30 60 KilometersMultiple Storm TracksStorm CenterngPaper 1 - Webb 4
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 and 14: RESEARCH AREASResearch needs were b
Page 15: Lack of Historical Data for Extreme
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
Page 71 and 72:
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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