TR-55 Output hydrographs

[sherwood]

Hello-

I am wondering why hydrographs calculated using TR-55 return only 22 hours (1320 minutes) of flow data when we are evaluating a 24 hour rainstorm event. Why are the last two hours omitted?

Thanks,

Sherwood

[Chris Smemoe]

Sherwood,

Thanks for your question; you've made a good observation.  WMS is constrained to the data in the TR-55 manual.  The manual can be downloaded here.  We use the tabular hydrograph method to compute TR-55 hydrographs.  After looking at the TR-55 manual, the maximum hydrograph time for a type IA storm distribution is 22 hours.  For a type I storm distribution, the maximum time is 24 hours.  For types II and III, the maximum time is 26 hours.  I can only speculate as to why the manual defines hydrograph values for only these times, and it probably has something to do with the fact that the most intense rainfall for a type I and IA storm happen closer to the beginning of the 24 hours than with a type II or III storm.  It probably would have been more useful if the developers of the TR-55 method would have included the entire hydrograph instead of cutting the hydrographs off at 24 or even 26 hours, but that's how the model works.

Chris