Differences between RAS rain on grid and HMS hydrographs.

[wellman]

I’m working on a project and decided to use the rain-on-grid function in HEC RAS to calculate the water surface elevation in an area. I used HMS to calculate the excess precipitation required in RAS to apply rain-on-grid. Just to check everything out I introduced a line as an internal boundary condition near the exit of the stream and obtined the hydrograph there. I’m a little surprised to see that this hydrograph (the one from RAS) is quite different from the one obtained in HMS. I have think about it but can’t fully explain why, so if anyone can chime in and explain would be great.

[ckatsoulas]

without looking at your HMS setup, it would seem you have not properly considered the stream flow in HMS that is likely causing the faster response time and higher peak in RAS. Also you need to make sure that the mannings roughness used in RAS is a reflection of your HMS catchment setup. I have done many rain-on grid RAS runs and found very close matches to hydrologic packages such as DRAINS that we use here in OZ.

The other thing to check is the volumetric runoff, this will tell you if you have setup HMS properly to match RAS.

[hasansins]

Hard to tell where the problem is without seeing HMS model. But I am guessing your transform method does not reflect actual scenario. Try using kinematic wave transform and enter flow lengths and roughness’s more accurately. I use HMS too and so far its been accurate as long as I my transform inputs reflect reality.

[wellman]

Thank you guys…. You were right.. I ajudsted some parameters and got a very close match hydrograph in both HMS and RAS. Thank you all!

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