Tuesday, July 28, 2009

RAS Mapper

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

*Update:  This post is outdated now with the release of HEC-RAS Version 5 and higher.  RAS Mapper is now completely different.  

So Riley and I recently gave this a thorough run through. After a few bugs were fixed up, it works quite nicely. The key here is you have to have a terrain model that is spatially and vertically consistent with your RAS geometry. Once you've successfully run your model, and you have a terrain model in the form of a floating point grid format (*.flt), you are ready to map within HEC-RAS (i.e. no ArcGIS required).

On the main HEC-RAS window, go to GIS Tools...Floodplain Mapping. I know, there is no "GIS Tools" menu item on the main HEC-RAS window. In the next release (4.0.1) there will be. This will bring up the RAS Mapper. It looks like a very basic version of ArcGIS. By default you'll see your river and cross sections plotted in plan view. One of the tools allows you to define a projection, but if everything is consistent, you can bypass this and go right to mapping. Here, you'll be able to bring in your terrain model. This is seriously way too easy. Next you simply click on the profiles you want to map and click "Generate Layers." There you'll see all of the floodplains and associated depth layers that you requested. The only thing missing...animation of the RAS Mapper. Maybe if I ask real nice they'll put that in.
Floodplain Map

Depth Map.

Tuesday, July 21, 2009

Other New Features in version 4.0.1

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

Here's a brief summary of some of the other new features that will be debuted in the new 4.0.1 release:

1. Geometry Window...Options...Main Channel Manning's n Value Compositing. I believe this was available elsewhere to a limited degree before. Now you can change the Compositing Slope Criteria or choose to not autmatically composite.

2. Geometry Window...Tables...Contraction/Expansion Coefficients for Unsteady Flow. Not sure what this is about. My understanding is that the C&E losses were built into the unsteady flow equations. I'll have to check into this.
3. Geometry Window...Tables...Minor Losses. Apparently you can add additional losses to a specific cross section. This is set up to take a Minor Loss Coefficient that I'm guessing is muliplied by the velocity head at every time step for the additional loss. Maybe to be used for tunnels or pipe networks (using cross sections with lids, or priessman slot...).
4. Unsteady Flow Analysis...Options...View Runtime messages file. Awesome! Gone are the days where you have to rerun your 10 minute-long simulation just to find out where it crashed because you inadvertantly closed the computation message window. RAS now stores this info in a log file. By selecting this option, you can view that log file. Great add!

5. Unsteady Flow Analysis...Options...Automated Roughness Calibration. This is HUGE if it is what I think it is. I haven't had a chance to try it out, but is appears you set up flow roughness factors and add in some observed stage gage data (unsteady flow editor options), then in this window, you can specify some calibration parameters. RAS will then automatically adjust your Manning's n flow roughness factors to match the gage records. Very cool.

Not sure when this new version will be officially out for the public to download, but hopefully soon. Keep checking back at the HEC website.

Tuesday, July 7, 2009

New Features 4.0.1 Part 3...Modified Puls Hydrologic Routing

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

This will be a very useful feature for those pesky dam breach unsteady flow models (or any very dynamic unsteady flow model) with steep sections of reaches and low depths. By designating a portion of your reach to be solved using the Modified Puls Hydrologic routing technique, you are removing a common problem with unsteady flow solution scheme used-it is very sensitive in steep, shallow reaches, especially during the rising limb of the breach hydrograph.

In the previous version of RAS, we could sometimes model short steep reaches with inline structures, but that always felt a little like cheating to me. Plus, what discharge coefficient do we use? For longer steep reaches, we would have to run a separate HEC-HMS model of the affected reach, then import those results to steady flow RAS to map it. That was kind of painful, but occasionally necessary. Now all of it is built into HEC-RAS, so switching to a different model is no longer required.

First thing you have to do is set up a steady flow run with multiple profiles (flows). These flows should encompass the full range of expected flows in your unsteady flow run, from low initial flows to the peak of the flood event. I found that this feature did not work when it had to extrapolate. Keep in mind that the more steady flows you include, the better storage/outflow relationship can be established.

In the geometry editor, go to Options...Hydrologic Unsteady Routing, and you'll get the following window. Here you simply select regions where you want to apply the modified puls routing and then import the storage-discharge relationship (RC's) from the steady flow profiles for each region. Each cross section within a region will be treated as an individual Modified Puls "Reservoir". Make sure you select the "Use Modified Puls Routing" checkbox, and then you can compute the unsteady flow run.