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Storm duration and hyetograph peaks

Storm duration and hyetograph peaks

Storm duration and hyetograph peaks

(OP)
Hello people,

I am doing calculations for the design of a flood diversion tunnel ( think of a 700 m long culvert). Because it is a small watershed (7.5 km2) there is no flow data available, only precipitation, so I have to build a raunfall-runoff model.

Since I have the Intensity-Duration-Frequency curves, I decided to build design hyetographs using the alternating block method. The time of concentration of the catchment is of about 30 minutes (it is pretty steep, about 12% slope). Furthermore, I am calculating the runoff with the SCS CN number method and using a triangular synthetic unit hydrograph convolution for the outflow hydrograph calculation. The problem i am havinng is the following:

Some sources say that the precipitation corresponding to the time of concentration should be the design duration and cause the "design peak flow". Doing the calculations with the 30 min hyetograph I get a peak flow of about 80m3/s. However, the 60 and 120 minutes hyetographs (calculated with the same method, alternating block) have the same peak and more precipitation before and after the peak, which in turn yield more runoff and also higher peak flows. If i keep calculating with longer durations I keep getting higher peak flows.

Because of the form the alternating block method works, the hyetographs for 30 60, 120, etc min are contained within the longer durations, and all end up having the same peak precipitation. Am i using the method right? (see attached pictures for illustration: https://drive.google.com/drive/folders/0B8avbGM7xi...)

I understand that, for example, when using the rational method one uses the time of concentration as design duration, because one considers only an average rainfall intensity, but what is the consensus for design duration when using a synthetic hyetograph method?

I would appreciate your comments or suggestions on these subjects and thank you in advance for your time and attention.

Cheers,
rasec

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