SSA Rational Method Peak Flow Computation

[qchalmersKACEH]

I am currently testing Rational Method in SSA and comparing it to SBUH. I have a project with a preliminary model where SBUH is working. I made a copy of the model and input the following MRIs to be used to generate the IDF curve for the 100 year storm. These come directly from the jurisdictional design manual.

I have also replaced all the curve numbers assigned to each subbasin for SBUH with the correct runoff coefficients to be used for rational method. In both models I am using the same time of concentration for all subbasins of 5 minutes.

Both models also are being run over a 24 hour period. When I run the rational method model I get very odd results in comparison to SBUH with subbasin flow peaking in the first hour of the 24 hour analysis and then dropping to zero for the rest of the run:

 

The results for the rational method model are exactly the same with all three analysis methods (Steady Flow, Kinematic Wave and Hydrodynamic).

What am I doing wrong here? I know that rational method is a snapshot based model and typically not used over an analysis period but am not sure how to change SSA to model it correctly.

[matt_anderson_pe · Accepted solution]

Without seeing your model, I think it looks appropriate.

Rational Method will be short, intense, and end quickly, especially if your ToC is 5 minutes.

The system's hydraulics (steady, kinematic, or hydrodynamic) that you do not describe would not change the runoff characteristics. 

What is your expectation?

[qchalmersKACEH]

Hi Matt,

Thank you for the response. I've just confirmed you are correct. I was shocked that the peak flow rate is so much higher when calculated using rational method in comparison to SBUH but doing the same calculation by hand produces the same results.

Rational method is giving a peak flow rate for each basin approximately 4 times the SBUH flow rate.