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. . . . . . . . . . . '. . . !. . . . . '. . . '. . . i. . . . . . . . . . '. . ,. . . . . . . . Wash Master Plan Phase I: Watershed Floodplain Delineation Shallow concentrated flow was calculated separately from overland flow, The flow length and watercourse slope were determined from distances and contours calculated from GIS mapping, Average velocity was based on Figure 3-2 of TR-55 (SCS, 1986), The watercourse was assumed to most closely resemble a grassed waterway, Total time of concentration is the sum of these two components, Tc=Ttl +Tt2 Because the basin above 1-70 is composed of extremely steep slopes, much steeper than found in most watersheds, a different method was used within this portion of the watershed, A modified Kirpich formula has been used extensively by the Bureau of Reclamation in badlands areas of the west and tends to more accurately represent steeper terrain, For this reason, a modified version of the Kirpich formula was used to calculate times of concentration in the upper basin, The modified Kirpich formula is computed as follows: 11.9L3 t = 60( )0,385 where C H ' tc = time of concentration (min) L = hydraulic length of watershed (mi) H = elevation difference between divide and outlet (ft) Because the SCS hydro graph method uses lag time, it is necessary to transform the computed times of concentration into lag time, It is widely assumed that lag time is approximately equal to 60% of time of concentration, and this relationship was used for this study, Tc calculated by both methods were converted using this approach, A summary of Tc and lag times used in the model is presented in Table 2, ~__~J~~~~~::mmA1J~€ff$~~~~~';.:;;;:f;:-':-":,; . ~ Page 10 @~ ;~