Thursday, February 26, 2009

Minimum Flow requirements

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © 2009. All rights reserved.

It's common knowlege that HEC-RAS cannot go "dry" at any time during a simulation. However, very low depths can be problematic as well. Very low depths can lead to compounding errors that can cause instabilities, and eventually your model to crash. There are a couple of tricks to get around this problem. Pilot channels and added base flow. None of you inflow hydrographs should start with 0 inflow, unless there is sufficient backwater in you system to prevent the upstream cross section from going dry. The trick with adding base flow is to keep it as small as possible, while providing the necessary stability. Try 1% of the peak of the hydrograph. You definately do not want to add a significant amount of volume to your model before the flood wave arrives-that will limit the extent of hydrograph attenuation you'll get.

To add base flow to an inflow hydrograph, simply add a minimum flow value in the box at the lower left corner of the flow hydrograph editor. Then, for that hydrograph, RAS will use the base flow amount any time the hydrograph flow is less than the minimum flow value.


  1. I do most of my work in very flat areas and have found that I have to generally start with a minimum flow that is around 10% of the peak of the hydrograph. Once I have the model up and running, I reduce the minimum flow until it becomes unstable.

  2. Nice approach to this common problem, Matt. Have you ever tried to figure out at what level of base flow (percentage of peak), you start to influence the results? In class I've always suggested 10% as the highest you'd ever want to go, but I have nothing to back that's just a hunch.

  3. Actually, I haven't tried to figure out the percentage where the peak is influenced, although I probably should have. I always end up reducing the minimum flow from the 10% first guess.

  4. If you ever do...let me know. I'm interested.