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Western Dam Engineering <br />Technical Note <br />10 <br />Turning Rainfall to Runoff: Estimating <br />Flood Inflows <br />One of the most critical components of a safe dam is the <br />adequacy of a spillway to safely pass the inflow design <br />flood. Inflow design flood requirements are typically <br />based on the potential hazard a dam poses to <br />downstream floodplains and communities (hazard class) <br />and are usually stipulated by the regulating agency for <br />the structure. The inflow design flood itself is a function <br />of rainfall, including distributions and patterns for a <br />particular storm event frequency, and pertinent <br />watershed characteristics. These pertinent watershed <br />characteristics influence the rate and volume at which <br />rainfall is “lost” and the conversion of the excess rainfall <br />to a runoff hydrograph. <br />The purpose of this article is to present an overview of <br />the processes and methodologies available for estimating <br />the effects of watershed characteristics on runoff and the <br />transformation of excess precipitation to an inflow design <br />flood hydrograph. This overview is generally based on <br />small dam watersheds throughout the western U.S. with <br />a focus on Colorado, Montana, Utah and Wyoming. <br />Pertinent reference documents are also presented. <br />Rainfall Event Characteristics <br />Before watershed runoff can be estimated, rainfall event <br />characteristics must be defined. In the last issue of the <br />Western Dam Engineering Technical Note we discussed <br />the guidelines and estimation of precipitation depths and <br />intensity-duration-frequency (IDF) relationships. In <br />addition, each rainfall event must also be characterized <br />by: <br />·Spatial patterns – The physical path of the rainfall <br />event over the watershed. <br />·Temporal distribution – The variation of rainfall <br />with time (i.e. intensity) during the rainfall event. <br />·Aerial reductions – A reduction in rainfall depth as <br />a result of distributing point rainfall depth, as <br />estimated for frequency and Probable Maximum <br />Precipitation (PMP) events, over the watershed. <br />Further reductions may also apply for watersheds <br />located at sufficiently high elevations. <br />·Rainfall weighting – The process of developing <br />incremental rainfall depths, based on equivalent <br />blocks of time (i.e. hyetograph), for the duration of <br />the rainfall event and arranging them such that the <br />peak rainfall depth occurs at a specific percentage <br />of the event duration. <br />These rainfall event characteristics can significantly <br />influence the rate and volume of runoff; however, are <br />outside the scope of this article. The reader is encouraged <br />to further investigate these aspects of rainfall <br />characterization. The primary focus of this article is <br />runoff, so we will begin with basin losses. <br />Basin Loss Parameters and Excess Rainfall <br />Excess rainfall, or runoff, is the portion of rainfall that is <br />not “lost” during the rainfall event. Rainfall losses are not <br />actually “lost,” but are defined as such because they <br />represent the portion of rainfall that does not contribute <br />to runoff and to the subsequent watershed outflow <br />hydrograph (i.e. reservoir inflow hydrograph). The <br />“losses” are instead recycled back to the system through <br />various means. A visual representation of this rainfall- <br />runoff process is presented in Figure 1. <br />Rainfall losses are generally defined by: <br />·Interception - The portion of rainfall that wets and <br />adheres to above ground vegetation and is <br />eventually evaporated. <br />·Depression storage - The portion of rainfall that <br />collects and is retained in surface depressions, <br />which are either impermeable or characterized by <br />infiltration rates less than that of the event rainfall <br />intensity (i.e. excess rainfall is produced, but does <br />not contribute to the total runoff). The retained <br />rainfall is eventually either evaporated or <br />infiltrated. <br />·Evaporation – The portion of rainfall that is directly <br />evaporated based on atmospheric conditions <br />during a given rainfall event. <br />·Infiltration – The portion of rainfall that moves <br />downward through surface soils and eventually <br />recharges aquifers and supports baseflow of the <br />stream.