Problems with bridges and culverts.

Written by Chris Goodell, P.E., D. WRE
Copyright © RASModel.com. 2009. All rights reserved.
(Thanks Eric for the topic). It's very common for HEC-RAS models to show inconsistencies around crossings (bridges and culverts). Usually, 1 of 3 things is going on here: 1. Bad geometry-either incorrectly entered, or poorly defined. 2. Numerical errors. 3. The results are actually correct and can be explained.

For the first case, "bad geometry", here's a technique that can be used to help spot sources of problems. Create and evaluate the hydraulic property plots for the crossing. If you are running unsteady flow, this is done for you during the geometry preprocessing task. If you are running a steady flow model, you can create an unsteady flow plan and just run the geometry pre-processing task (you don’t need to run the computations or the post-processing. Once that’s done, on the main RAS window, go to View…Hydraulic Property Plots. Click Type…Internal Boundaries, and you’ll see the family of rating curves for your crossing. Here you’ll want to examine the curves and look for any abrupt changes, or discontinuities, particularly in the range of flows/depths where you are seeing the discrepancy. Typically you see problem areas where RAS changes equations (i.e. going from low flow to pressure flow, or pressure flow to pressure and weir flow), or when ineffective flow triggers turn off/on. Also, keep in mind that the equations for culverts are very different from those used for briges in HEC-RAS.

Take the following example, in the figure below. First of all, I always like to open up the bridge plot along side its htab plot (make sure the vertical axis is consistent) so that I can graphically explain any discontinuities in the htab curves. This example shows a significant discontinuity at around 10,000 cfs (you can click on the figure above to get a better view). It's very obvious from looking at the plot that this is the range at which the flow transitions from low flow to pressure flow and then on to pressure and weir flow. Also, notice that the ineffective flow triggers turn off in this range. It appears that the creater of this model tried to lessen the impact of the ineffective flow areas instantaneously turning effective by significantly raising up the n-values in the overbank. Not a bad technique, but obviously didn't completely solve the problem.
Things you can tweak that may provide more sensible results and a better set of Htab curves are:

-Coefficients (bridge and culvert coefficients).
-Ineffective flow areas upstream and downstream of the crossing.

-Bridge modeling approach.
-Placement of cross sections. Sometimes if they are too far from the crossing, or to sparsely spaced leading up to the crossing, it can cause these types of problems.
-Consider modeling a bridge as a culvert, particularly if it has a very deep deck and small relative opening. Likewise, consider modeling a culvert as a bridge, particularly if it has a very large opening, relative to the deck thickness (conspan culverts are good examples).

If the problem is only a very small discrepancy in upstream head levels, a refinement of the computation tolerances might yield better results. For example, let's say you are trying to provide a "no-rise" condition, and you feel that there should be no rise (i.e. new bridge opening is bigger than old bridge opening with a higher low chord). However, you're results are showing a 1 to 2 hundreths of a foot of rise for the new bridge. This is most likely numerical issue and a refinement of the computation tolerances might yield better results.

RAS Mapper

Written by Chris Goodell, P.E., D. WRE
Copyright © RASModel.com. 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.

Other New Features in version 4.0.1

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

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

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

New Features 4.0.1 Part 2...Junctions

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
Up until now, unsteady flow HEC-RAS uses a very simplistic method for handling juctions. It simply projected the water surface elevation of the downstream cross section to the two cross sections upstream of the junction. For shallow, deep systems, this worked fine-no problems. However, if you tried to model a steep reach with shallow depths you run into a host of problems at junctions. I've blogged about this problem before and provided examples of ways around to work around it. However, in the new version soon to be released (4.0.1) HEC has taken care of this problem and it works quite nicely.

In the Junction Editor, you'll notice that there is a new option for unsteady flow.

You can either keep the original, simplistic computation mode, or require RAS to perform an energy balance around the junction. I'm assuming HEC left the original "Force Equal WS Elevations" method in as the default so that your results will remain the same when you upgrade to 4.0.1. I don't believe that the new method is computationally burdensome, so I would expect that for new models, you'll want to check the "Energy Balance Method", particularly if you'll be working with steep reaches and/or shallow depths.

New Features 4.0.1

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
I just got a preview of the new release of HEC-RAS 4.0.1. Even though the release id would suggest it's a minor release with only bug fixes, there are (at least) two major additions to the software: Today I'll show you a little bit about the RAS Mapper. This will allow you to take your steady and unsteady flow results and map the floodplains directly within the HEC-RAS environment. In other words, you won't have to export into ArcGIS (at least not initially). The RAS mapper is a lot less complex than ArcGIS, but includes features like adding layers, defining projections, and inundation mapping. Inundation mapping requires a terrain model in floating point grid format (*.flt). These are very easy to create within ArcGIS. To finalize a map and smooth out the boundaries of the floodplain, you'll probably still have to go to ArcGIS. However, now you can just bring in your shapefile created in the RAS Mapper, as opposed to going through the PostRAS procedures in GeoRAS. Unfortunately, for now, GeoRAS is still required to create a geometry file from GIS.

From the look of the new RAS Mapper, it appears that you simply select the profile you want to map, specify your terrain model, give it an output directory and click "Generate Layers".

Now I have to get a floating point grid set up so that I can give this a try...

Energy Grade Line Problems

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
Why do you sometimes see the water surface elevation and the total energy increase in the downstream direction? The energy grade line should always slope downhill. Water surface doesn’t necessarily have to though (i.e. hydraulic jumps, transition from narrow cross section to wide cross section). In this case, I’m not sure what the problem is without seeing the model firsthand, but the EGL indicates localized errors. This usually occurs from either not enough cross sections, or HTAB computation points that are too coarse, or the computation points don’t go low enough in the cross section. Try maxing out the number of HTAB points for each cross section, and decrease the increment as well. Also, use the invert of the channel as the starting computation point (there’s a button to do that automatically). Run the model. If there are still some problems, try interpolating some cross sections in the areas where there is increasing energy. Also, turning on the critical depth variable in your profile plot can help to explain what’s going on.