Monday, December 28, 2009

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!

Tuesday, December 15, 2009

n-values in steep streams

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
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.  (*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.