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<br />Calculation of Depth and Velocity at Critical Section <br />The third step in hazard classification is the determination of the depth and <br />velocity of flow at selected downstream locations, for the routed peak <br />hydrograph values, QR. The locations, previously defined as critical <br />sections, are where property damage and/or loss of life would be expected to <br /> <br />occur. <br /> <br />The required data for computing the depth and velocity at the critical section <br /> <br /> <br />consists of charmel geometry, slope rouglmess, and QR. The geometry can be <br /> <br /> <br />obtained from topographic maps and field reconnaissance. Manning's equation <br /> <br /> <br />is used to determine depth of flow for the cross-section and the effects on <br /> <br />improvements at the various critical sections. <br /> <br />An example of the ,procedure is provided on Worksheet W-3, entitled "Charmel <br />Cross Sections", for Mason Reservoir (Dam I.D. 120106). In this case, data <br />was obtained from aerial reconnaissance of the drainage and topographic maps. <br />The example uses the generalized form of Manning's Equation because a more <br />complex section exists (Equation 3). However, Equation 2 could have been used <br />without significant loss of accuracy. <br /> <br />Use of judgement is particulary important in selection of the rouglmess <br />coefficient en) and in the use of Manning I s Equation. The equation aSSUIIles a <br />generally uniform charmel at the designated cross section. Restrictions <br />upstream or downstream of the critical section, as well as the effects of dams <br /> <br />formed by debris carried with the flood, significantly reduces velocity and <br /> <br /> <br />increases flooding depth. In general, average flow velocities greater than IS <br /> <br />~ .... _.-._-._-~,,-~ - --p-'", ~ -. - _.~.---.~, <br /> <br />feet per second are suspect of being too high, with the critical flow velocity <br />for the charmel as the ~;~-{~i~:r~]~' ,.-' <br /> <br />17 <br />