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<br />, <br /> <br />~ The primary purpose of an elevation-discharge relationship is <br /> <br />for analysis to correlate discharge data with specific elevations to <br /> <br />determine flooded areas, <br /> <br />Procedure, The procedures for establishing rating curves can range <br /> <br />from adjustments of high water marks, as related to known peak discharge <br /> <br />levels, to rigorous analysis by use of backwater computations, The <br /> <br />detailed analytic calculation of water surface profiles, which is done for <br /> <br />survey reports, requires the efforts of the hydraulic engineers, <br /> <br />The process requires the following steps: <br /> <br />1) Assemble the flow data from historic stream gage records; <br /> <br />2) Apply geometric measurements of stream cross sections from survey <br /> <br />data (cross sections are measurements of the physical dimension of a <br /> <br />stream at a given location). The number of necessary cross sections will <br /> <br />increase with changes in topography, stream dimensions and other factors <br /> <br />that contribute to changes in energy loss coefficients, described below, <br /> <br />3) Estimate energy loss coefficients to match observed data, <br /> <br />Changes in the rate of energy loss can occur from the effects of newly <br /> <br />constructed obstructions-- such as bridges, culverts and levees, changes <br /> <br />in sediment and debris load, channel straightening, deepening, or silting, <br /> <br />The observed coefficients can be supplemented or adjusted by judgments of <br /> <br />the effects of changes and the applications of what has been learned from <br /> <br />similar stream reaches, <br /> <br />) <br /> <br />Discharge-Frequency Relationship <br />Definition, A frequency is the number of occurrences that can be <br /> <br />expected out of some possible number. For example, the exceedence <br /> <br />V,ll <br />