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Western Dam Engineering
<br />Technical Note
<br />15
<br />can influence the total watershed outflow hydrograph
<br />(i.e. reservoir inflow hydrograph). The unit hydrograph
<br />methodology does not account for baseflow; therefore,
<br />in some instances the baseflow could have an appreciable
<br />effect on inflow design flood hydrographs and
<br />subsequent spillway adequacy.
<br />Summary
<br />Inflow design flood hydrographs are a key component of
<br />dam safety and are estimated based on the translation of
<br />excess rainfall, or runoff, to a watershed outflow
<br />hydrograph (i.e. reservoir inflow hydrograph). Runoff is
<br />representative of the portion of rainfall that is not lost
<br />during a rainfall event. Rainfall losses are comprised of
<br />several factors and can be estimated using numerous
<br />methodologies.
<br />Once the total volume of runoff is known, it can be
<br />transformed into a hydrograph which models the
<br />variation of runoff discharge from the flood over time.
<br />There are also several methodologies used to transform
<br />runoff to a flow hydrograph, however, the most widely
<br />used and preferred methodology is that of the unit
<br />hydrograph. Reclamation and Clark synthetic unit
<br />hydrographs are the most common approaches where
<br />gaged and watershed calibration data are not available;
<br />however, the USGS approach is also used throughout
<br />Montana.
<br />The purpose of the information presented herein is to
<br />inform the reader of general procedures and
<br />methodologies applicable to the western U.S. and
<br />provide them with references to attain more in-depth
<br />procedure guidance and examples. The information
<br />presented is not comprehensive and readers are
<br />encouraged to further investigate the requirements,
<br />shortcomings, and procedures specific to each
<br />methodology. Readers should also be familiar with
<br />particular methodology preferences specific to the
<br />guidelines and requirements for the state or other
<br />regulating agency with jurisdiction of the dam.
<br />Several state agencies prefer some methodologies over
<br />others, but most do not have specific requirements with
<br />regard to methodology selection. In general, any one of
<br />the numerous methodologies presented could be applied
<br />to aid in the estimation of inflow design floods provided
<br />that the engineer uses appropriate judgment and
<br />justification in methodology selection.
<br />References (links provided where
<br />applicable)
<br />[1]Cudworth, A, 1989.Flood Hydrology Manual. U.S. Bureau of
<br />Reclamation, Water Resources Publication, 1989.
<br />[2]Feldman, A., 2000.Hydrologic Modeling System HEC-HMS - Technical
<br />Reference Manual.U.S. Army Corps of Engineers, Hydrologic
<br />Engineering Center, March, 2000.
<br />[3]Holnbeck, S.R. and Parrett, C., 1996.Procedures for estimating unit
<br />hydrographs for large floods at ungaged sites in Montana.U.S.
<br />Geological Survey (USGS), Water Supply Paper 2420, 1996.
<br />[4]Montana Department of Natural Resources and Conservation (DNRC),
<br />2008.Analysis of Spillway Capacity in Montana. Dam Safety Program,
<br />Technical Note 1. Prepared by Hydrometrics, Inc. on behalf of the
<br />DNRC, October, 2008.
<br />[5]Moody, H., 2004.National Engineering Handbook, Part 630 –
<br />Hydrology, Chapter 9: Hydrologic Soil-Cover Complexes. U.S.
<br />Department of Agriculture, Natural Resources Conservation Service,
<br />July, 2004.
<br />[6]State of Colorado, Division of Water Resources, Office of the State
<br />Engineer (DWR), 2008.Hydrologic Basin Response Parameter
<br />Estimation Guidelines. Prepared by Tierra Grande International, Inc. on
<br />behalf of the DWR, May, 2008.
<br />[7]Viessman, W. and Lewis, G., 2003.Introduction to Hydrology.5th Ed.,
<br />Pearson Education, Inc., 2003.
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