Monday, December 8, 2008

Everything's the same...why are my answers different???

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © 2008. All rights reserved.

This seems to happen more often than it should. You have two RAS model runs. Everything in the geometry, flow, and plan files are exactly the same. Yet, the output is slightly different. This can cause a lot of anxiety, particularly if you're doing a FEMA "Duplicate Effective Model" where the two models must agree EXACTLY. Usually the cause of the discrepancy is some obscure computation setting in the plan file, or maybe how the friction slope or conveyance is computed. However, I ran into a case recently where it was simply a bug in RAS.

When RAS runs a multiple profile run, it apparently holds over some of the internal computations of the previous profile to begin comps for the new profile. RAS must guess at the first wsel solution during the backwater computations. It then runs through the computations and compares the result with the guess (standard trial and error, iteration). It continues until it arrives at a solution where the guess and the result are consistent within the prescribed tolerance. The problem here is how RAS establishes that first “guess”. If it is the first profile run, I think RAS simply uses the downstream computed wsel and translates that upstream as the first guess for the next upstream cross section. If it is a second (or third, or fourth, etc) profile run, I think RAS uses the resulting water surface elevation from the previous profile run as it’s first guess.

If I’ve confused you, no matter, it’s really very easy to fix. Just rearrange the flows in your profile table, so that the 100-year profile is listed first (so yours would be 100-yr, 10-yr, 50-yr, 500-yr) in that order. That way RAS computes the 100-year first. Then the answers will compare exactly.

An even easier fix would be to go into the “Set Calculations Tolerances” under the Options menu item in the Steady Flow Analysis window. Change your water surface calculation tolerance to 0.001 for BOTH plans, and you should see the same results for the 100-year profiles, without having to rearrange in the flow file.

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