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# Lag time for HEC elements

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Hello,

When modeling a number of diversions (or other elements) with the same geometry at different locations in the watershed, the HEC-HMS generates the same number of basins with zero values such as the Area, Curve number, Impervious %, .... and Lag time (min). What about the Lag time computation for each Diversion, is it right to set it the same for each diversion? Do I have to calculate it manually using the known Lag time equations? If the diversions have the same geometry, the Lag time will be the same, is this correct? Any clarification will be appreciated on setting the Lag time and its importance  and effect on the model calculations.

Best regards

Edited by Ayman98

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There are various ways of computing the lag time for a diversion.  It's up to you to determine the method you want to use to compute the lag time and any losses for the diversion.  Normally you wouldn't have the same lag time for all the diversions in your watershed unless all the diversions have the same length, slope, and geometry.  It would be best to either calculate the lag time manually or to use the Muskingum-Cunge method where you define a diversion length, slope, and the channel geometry.  HMS will compute the lag time if you use this method.  There are not really any good tools for calculating the lag time for a diversion in WMS for an HMS model.  The diversions would have to have the same geometry, length, roughness, and slope for the lag time to be the same.  The impact the lag time has on the model calculations is that the hydrograph downstream of the diversion is routed based on the lag time.  This means if the lag time for a diversion reach is 30 minutes, it takes 30 additional minutes for the hydrograph to reach the downstream end of the diversion and the hydrograph peak is translated by 30 minutes.  There's also a storage attenuation of the hydrograph that occurs along the diversion reach that will decrease the peak flow when the hydrograph is routed using some of the HMS routing methods.

Chris

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