Monday, April 20, 2009

SIAM Discharges

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

How are the annual flow discharge and duration data prepared for HEC-RAS SIAM input? First, you have to come up with a representative flow duration curve. Then, you simply discretized the flow duration curve. You have to set up all of the flows as individual steady flow profiles in steady flow RAS first, then when you go to the SIAM flow editor, you simply add a duration and a temperature to each one. The durations are typically entered to represent a full year in SIAM, but they don’t have to be.

Wednesday, April 15, 2009

The HEC-RAS Bloggery Forum is up!

I've had many requests for visiters to this blog for an easy way to ask questions. This blog is set up to receive comments on specfic posts that I made, but there's no real easy way to ask a general HEC-RAS question or to start a discussion. To meet this need, I started the HEC-RAS Bloggery Forum. Please have a look around. It's new, so there's not much up there yet. I will monitor the forum and try to respond as time allows. Thanks for checking in.

Using HEC-HMS for a dam breach simulation

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

Using HMS to perform a dam breach has its advantages, namely that it is easy to set up, the data requirements are minimal, and it is numerically stable. However, routing the breach outflow downstream using HMS is very limited. HMS uses hydrologic streamflow routing which is a simplification of full dynamic routing. Plus, backwater will not be fully accounted for using any of the HMS routing techniques-meaning you can not simulate flow attenuation properly. In short, HMS is okay for routing water from A to B, but if you are interested at all in what is happening between A and B, HMS is not appropriate. Furthermore, there is no direct method for mapping flood inundation from an HMS model.

What I usually suggest is that the breach can be modeled using HMS, but downstream of the dam, RAS should be used. You could go with full unsteady RAS downstream of the dam and use the techniques I described in the dam breach class to get the final inundation mapping. If you are having stability problems due to an overly steep reach, HMS could be used for routing, but the peak flows should be finally run through steady flow RAS to get the flood inundation extents.

Tuesday, April 7, 2009

How do you model Bank Barbs in HEC-RAS?

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

I was recently emailed this question. Here are a few of my thoughts:

You could model the barbs themselves as cross sections with adjusted station elevation points, or as blocked obstructions within cross sections. The blocked obstructions make it easy to compare with and without barbs, however, you are limited to a rectangular (or series of rectangular) shape(s). Make sure you account for ineffective flow between the barbs. The barbs should have their own n values to ensure that the conveyance over the barb is treated independently.

You technically should place the cross sections perpendicular to the anticipated flow paths. However, the “angling” of flow over the barbs will be a very localized effect and you could probably get by with placing the cross section perpendicular across the stream through the middle of the barb. Though it is easy enough to angle the cross sections. Just make sure you understand what you are modeling. If trying to model an extreme flood event, you probably should cut your cross sections straight across the stream.

I would not try to model the barbs as inline weirs in HEC-RAS. Being a 1-D model, defining weir flow on one side of a channel and conveyance on the other doesn't really make sense. However, this does make me wonder if you could try this using the multiple opening analysis in the bridge/culvert editor.

Keep in mind that you are modeling a very 3-dimensional flow pattern (over and around barbs). You should only use HEC-RAS to get a feel for the overall rise in backwater due to the barbs. Do not use RAS to analyze local velocities over and around the barbs, or for sizing of the material (i.e. riprap sizing). If you do want to design the barbs using RAS, make sure you use appropriate safety factors.

Any local analysis around barbs would require a multi-dimension model (at least 2-d, preferably 3-d).