Friday, October 22, 2010

Some useful debugging tools

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Recently I’ve had some real difficult models to set up and run. Some real beasts. And some of these take hours to run. As you can imagine, when one of these models is crashing, strategies for effectively diagnosing and fixing errors become very important. You don’t want to “chase” problems in models with 2 hour run times. You’ll never get your model stable. You need to be able to diagnose the problem and come up with a confident fix, while minimizing the time spent running the model to “try out” possible fixes.

Here’s a real useful tool that has recently been added to HEC-RAS.

image

In the Runtime Computational Options, you have the option to “disregard” Lateral Structures, Storage Area Connections, Breaches, and Pumps in the computations. Let’s say you turn off lateral structures, rerun your model and it runs fine.

image

Then you know that there is a problem with one or more of your lateral structures. Simple diagnosing tool-but very effective.



Also, I’ve been using the Computation Level Output a lot. By checking this box on the Unsteady Flow Analysis window, you are able to look at some select output parameters at every computational time step interval. These parameters include water surface elevation, flow, and lateral inflow. You can view two types of plots: a spatial plot and a time series plot, by going to the “View” menu item on the main RAS window.

image

The first two are very useful, and at a computation interval level, can show you things that just won’t show up on the detailed output profile plots. However, the ability to monitor lateral inflow, graphically, and at the computation interval level, is a powerful way to determine when and how much discharge is entering a given reach laterally (via lateral structures from other reaches or storage areas).

image This is a common source of errors that can lead to instabilities in complex HEC-RAS models, and aside from this plot, I don’t know of another way to graphically see this.