Latest DSS-Vue Plug In

Posted by Bill McDavitt:


"(DSS-Vue) now allows users to injest 1 hour data (sometimes the gage even has 15-minute data) from the USGS Instantaneous Data Archive using the latest plug-in, just released in November. I've been able to work on some projects that happen to have gages very nearby, which I realize is a bit of a rarity. Nonetheless, with 15 mintue data in a DSS file, making an unsteady flow model using data for a particular storm flow can be done with relative ease and minimal additional time spent.
If a user is comforatable scaling/adjusting the gage data for their non-gaged site, HEC has an Excel tool whereby the flows could be modified in Excel and then repackaged into a DSS file."

2009-A look back in HEC-RAS

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
2009 was a very interesting year for HEC-RAS. A beta version of 4.0.1 came out which included a new Floodplain Mapper (RAS Mapper). This looks like a great start to realizing a fully functional HEC-RAS model with built-in GIS capabilities. Also, Modified Puls Routing for steep streams and a new and improved method for computing at junctions in unsteady flow were added. Sediment Transport continues to improve, and I know that HEC is currently working on a two-dimensional component to HEC-RAS (both in series and in parallel). More water quality functionality was added (though I admit to not having worked with this too much). All great stuff. From what I hear, version 4.1 will be out soon. Keep checking back on the HEC website often.

As for the first full year of the HEC-RAS Bloggery, I was pleased with the response and readership. We talked about the limitations of RAS, failing bridges, Cross section spacing, minimum flow requirements, Htab strategies, cross section interpolation, RAS precision, bridges and culvert strategies, sediment transport issues, and possibly the most popular topic of late...n values in steep streams.

Thanks to all for reading and responding. Please let me know if there are ways to improve the HEC-RAS Bloggery, or any topics you would like me to write/comment about. Also, I want this to be more of a public blog posting site. So if any of you have articles you've written that you would like to post in the HEC-RAS bloggery, feel free to email to me (Chris G.) and I'll post them up. I would like to hear about other's experience with Water Quality in RAS, Sediment in RAS, Channel Modification, more on n values in steep streams, or just something you've done with HEC-RAS that was really cool.

Thanks and all the best in 2010!

n-values in steep streams

Written by Chris Goodell, P.E., D. WRE
Copyright © RASModel.com. 2009. All rights reserved.
What is a good range of n values for a typical river or stream bed? 0.03?0.045? What about a mountain stream? 0.05? 0.07? Jarrett has a very simple formula that serves as a good check on n values in mountain streams. He developed his equation from 75 observations of streams in Colorado. His streams were composed of bed material ranging from cobbles to small boulders. Range of energy slopes were 0.002 ft/ft to 0.09 ft/ft and range of hydraulic radii were 0.5 to 7 ft.

Jarretts equation is: n = 0.39*(S^0.38)*(R^-0.16), where S is the energy slope and R is the hydraulic radius of the stream (in US Customary length units of feet).  (*Note-the original post had mistakenly listed the equation as 0.47*(S^0.38)*(R^-0.16).  That was incorrect.  The correct equation, as published in "DETERMINATION OF ROUGHNESS COEFFICIENTS FOR STREAMS IN COLORADO" by Robert Jarrett is n = 0.39*(S^0.38)*(R^-0.16).  Sorry about the mistake!)

Using his range of energy slopes and hydraulic radii, you could compute n values from 0.032 to 0.21. Yes, that's 0.21!!! I have had discussions with many class participants of mine who indicate that indeed they are finding n values much higher than traditionally what have been used. We're talking as high as 0.12 to 0.15 in some cases. This definately fits within Jarrett's confines. Partly to blame in this underestimation of n values in steep mountain streams would be the very popular table of n values in Chow. Chow lists mountain streams as having n values from 0.03 (gravels, cobbles, and a few boulders) to 0.07 (cobbles with large boulders). Also, another popular n value predictor, Barnes (USGS), lists it's highest n value stream as Rock Creek near Darby Montana, with an n value of 0.075. Rock Creek is composed of boulders with a d50 of about 220 mm. However, this was measured during a flood. It is likely that the n value is much higher at lower discharges where the bed irregularities have a greater impact on the overall roughness.

I like to use Jarrett's equation whenever I'm dealing with steep mountain streams that fall within (or close to) his experimental range. A little secret here: higher n values helps to stabilize unsteady flow models!

I'm curious to know if anyone out there has comment on this topic. I'd like to know what kinds of n values you all are coming up with for steep streams.

Dam Breach class-Portland, ME

I'm heading out to the HEC-RAS Dam Breach class in Portland, Maine tomorrow. Glad to see that enrollment in our classes is making a rebound. Mt last 2 classes were cancelled due to low attendance.

Sediment Mass Plot/Invert Change Issues

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
A very common "head-scratcher" for sediment modeling is the situation where your mass plot is showing a lot of accumulation of sediment at a given cross section (mass plot curve rises with time, indicating deposition), but the channel invert shows no change over the same time period. Usually this is due to the placement of the moveable bed limits.

Keep in mind the spatial plot only shows the minimum elevation node for each cross section. As a result, a depositional zone may not be captured correctly in that plot, if the mobile bed stations are missplaced. This example shows the mobile bed limits placed low in the channel. As the bed aggrades, the low point moves out to the mobile bed limit and its elevation remains constant throughout the simulation. The Sediment Spatial plot will indicate the cross section is stable, when in reality it is depositing a lot of sediment. A check of the mass plot will help to figure this out. One solution would be to move the bed limits out onto the banks, so that the minimum cross section point moves up with the deposition . Also, you could move the bank stations inside of the moveable bed limits, if that is acceptable to your conveyance distribution (remember, the minimum bed elevation is reported inside the bank stations). Or, you could leave it as is, and just recognize that the channel invert plot is misleading.

Initial Reservoir Elevation-Pilot Flow

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
A common question when modeling reservoirs with cross sections (dynamic routing) is how do I get my reservoir elevation to start where I want it to? Using Pilot Flow at your inline structure is a very easy way to guarantee your water surface elevation starts exactly where you want it to. First, you want to make sure that the pilot flow is equal to your initial conditions flow into the reservoir. Now, if you only did this, the reservoir would never fill up, as it would be passing everything that came into the reservoir, right out of it (no storage). To get your reservoir where it should be, go to the unsteady flow editor, and select Options...Internal RS Initial Stages. Pick the cross section just upstream of the inline structure and assign it the starting water surface elevation you desire. Now keep in mind this will only set the initial water surface elevation in the reservoir. For the remainder of the simulation, the reservoir level will move based on inflow, outflow, and storage (as it should). However, pilot flow is constant throughout the simulation. It does not change with reservoir level. Therefore only use this technique if the pilot flow is small compared to the flood discharges you are looking at. A great application is a dam breach flood where the discharge from the dam breach will be orders of magnitude greater than any pilot flow you use.

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.