CHAPTER VIII DETERMINATION OF THE PROBABLE …

1 CHAPTER VIIIDETERMINATION OF THEPROBABLE MAXIMUM FLOODS eptember 2001 September 2001iiChapter VIIID etermination of the PROBABLE Maximum FloodTable of Contents8-1 Background and of PMF Review of Project and Hydrologic and Obtain Preliminary About Upstream Dam, Spillway, Outlet Works, and Power Plant .. Operating Personnel Interviews .. Basin from Previous Streamflow Flow and Volume Hydrometeorological Characteristics of the Drainage Snowpack Water Equivalent and Temperature Data .. on Existing Reservoirs, Spillways, Outlet Works, and Operation Review and Assessment of Data .. Flood Data .. Water-Equivalent Data .. 24 September Temperature on Reservoir Volume, Spillway and O utlet-Works Capacity, and Operation History and Reservoir Spillway and Outlet-Works Capacity.

2 Operation History and Policy .. 268-5 Approach to Tasks for PROBABLE Maximum Flood of Drainage and Ungaged (Sub) Basin(s).. and Identification of Hydrograph for Gaged (Sub) Historical Floods for Calibration and Verification .. DETERMINATION of Basin Average Rainfall .. Cold Season Considerations .. Snowmelt Considerations .. Infiltration Characteristics of Potentially Frozen Soils .. Base-Flow Separation .. of Concentration and Clark's Storage Coefficient for Each Sequence for Recorded Infiltration for Unit-Hydrograph Development .. Unit Where a Single Basin Unit Hydrograph is Sufficient (No Subdivision) .. Unit Hydrographs for Subbasins and Channel Routing .. Unit Hydrographs for Ungaged (Sub)Basin(s) .. Unit Hydrograph Procedures for Each Basin (Subbasin).

3 Regional Analysis .. Data Required .. Rainfall Analysis .. Development of Generalized Regional Relationships .. Empirical Coefficients for Synthetic Unit-Hydrograph Procedures .. Snyder Unit Hydrograph .. Clark Unit Hydrograph .. SCS (NRCS) Dimensionless Unit Hydrograph .. 53 September 2001iv8-8 Loss Rates for Subbasins .. Basin-Averaged Methods .. Distributed Loss Rate Method .. Application of Distributed Loss Rate Method .. Verification and Model Adjustment .. Infiltration Characteristics of Soils Under Frozen Conditions .. 608-9 PROBABLE Maximum Flood Development .. Distribution and Disaggregation of the PROBABLE Maximum Precipitation .. Storm Duration .. Storm Spatial Distribution.

4 Distribution of the PROBABLE Maximum Precipitation .. Antecedent and Coincident Conditions .. Antecedent Conditions .. Coincident Hydrometeorological Conditions .. Snowmelt Estimates .. Reservoir and Channel-Routing Approach .. Base Flow Coincident with PROBABLE Maximum Flood .. Inflow PMF Hydrograph .. and Sensitivity Analysis of Representative PMF Hydrograph .. 708-10 Reservoir Routing to Obtain the Outflow PMF Hydrograph .. Initial Assumptions .. Routing Procedures .. Reporting Requirements .. 728-11 References .. 738-12 Glossary .. 818-13 Appendices ..97 Appendix A Determining the PMF for Civil Works Flow ChartAppendix B PROBABLE Maximum Flood Study Report OutlineAppendix CHEC-1 Data-Analysis Techniques of Infiltration Rate Estimate MethodsAppendix D Loss Rates for Subbasins Distributed Loss Modeling-v-FiguresFigure Covered by PMP -7-Figure Baseflow Simulation in HEC-1.

5 -42-Figure Estimation of Clark Unit-Hydrograph Parameters .. -43-TablesTable Minimum Infiltration Rates for Hydrologic Soil Groups .. -55- CHAPTER VIIID etermination of the PROBABLE Maximum Flood8-1 Background and PurposeThis CHAPTER of the Engineering Guidelines is primarily intended to provide proceduresfor the development of the PROBABLE Maximum Flood (PMF) for use in the evaluation ofproposed and existing dams and other impounding structures. The purpose of theseguidelines is to provide consistency in PMF determinations. The guidelines are not asubstitute for good engineering judgment and experience when available data clearly callfor a departure from recommended procedures. Therefore, the recommended proceduresshould not be rigidly applied in place of other justifiable about the last 50 years, the PMF has received general acceptance as the design floodfor dams in the United States, whose failure would pose a threat to public safety [Myers1967].

6 More recently, the PMF has received acceptance as the design flood for largedams in many other countries as well [ICOLD 1991]. The definition of the PMF contained in these Engineering Guidelines is:..the flood that may be expected from the most severe combination ofcritical meteorological and hydrologic conditions that are reasonablypossible in the drainage basin under PMF is generated by the PROBABLE maximum precipitation (PMP), which is defined as:..theoretically, the greatest depth of precipitation for a given duration thatis physically possible for a given size storm area at a particular geographiclocation at a certain time of a PMF hydrograph for a dam safety evaluation generally involves two steps,which are, respectively, hydrologic and hydraulic in nature:CModeling of runoff through the project drainage basin to produce aninflow PMF for the project of the inflow PMF through the project reservoir and dam outletworks to obtain the outflow PMF and the maximum reservoir elevationat the steps involve considering several coincident or sequential events, each of whichmay have a strong effect on the resulting PMF.

7 This CHAPTER attempts to address the useand estimation of those events to avoid compounding of conservatism and to provide areasonable PMF hydrograph given the limitations of basic hydrologic and important features of a specific project, such as the operation of the reservoir, theoutlet works, etc., which are relevant to routing the PMF through the reservoir and damoutlet works, also need to be addressed. The safety of existing or designed dams is theprimary concern in adopting the PMF as the criterion for safeguarding the public. Thischapter provides guidance on the DETERMINATION of the PMF. Additional guidance ondeveloping inflow design flood's (IDF's) is included in CHAPTER 2 of these is little chance that hydrology will ever become the precise science that designers,owners, and regulators would like to see. So many parameters define the basincharacteristics and hydraulics of runoff that the hydrologic engineer will always need torely on experience and good judgment.

8 This CHAPTER is intended to provide systematicprocedures that will consistently produce a reasonable PMF hydrograph and appropriatereservoir flood levels for evaluation of project keeping the inherent uncertainty of hydrologic calculations in mind, the objectivesof this CHAPTER of the Guidelines are:CTo recommend a preferred method for developing PMF present procedures which, if implemented by two or more qualifiedand experienced hydrologic engineers, would result in reasonably closeor consistent estimates of the make recommendations regarding the assumptions that mustnormally be made in developing a PMF hydrograph for gaged andungaged produce an approach that will minimize the total effort and cost ofrequired studies, while ensuring that the developed hydrograph isreasonable and pertinent for use in the design or safety analysis of provide guidelines for choosing appropriate hydrologic and provide greater consistency nationally for procedures used in PMFdevelopment.

9 While recognizing the wide variety of hydrologicconditions present across the United is the responsibility of owners of dams not under FERC jurisdiction to ensure the safetyof their projects by using the best available technology. The procedures recommended inthis CHAPTER for determining the PMF for FERC jurisdictional projects assume that alldams in the basin upstream of the project will not fail during floods up to the PMF. Therefore, the PMF at the project site will not be a combination of the naturally occurringflood and the flood resulting from a failure of an upstream dam. The PMF at the site isthe result of routing the PMF through upstream dams assuming they remain in place. However, this does not preclude owners of FERC jurisdictional dams from consideringthe failure effect of upstream dams in PMF accepted PMF studies are not required to be reevaluated in accordance withthe new Guidelines, unless it is determined that a re-analysis is warranted.

10 Potentialreasons to re-analyze an existing PMF include, but are not limited to: significant errorsfound in the original study or new data becomes available that may significantly alterprevious study results; significant changes in the conditions of the drainage basin such asbasin development or changes in upstream control structures; changes in the state-of-the-art technology, etc. All new studies should comply with the requirements of PMF determinations are completed using these Guidelines for a project that couldaffect nearby non-FERC-jurisdictional upstream dams, the FERC will advise theappropriate State Dam Safety Office (State) of the PMF study. The State will beinformed that a new PMF study has been done for the FERC-jurisdictional dam,assuming all upstream dams do not fail and that the PMF study is available to the Statefor its review and information at the FERC Regional Office.