Issue No. 2
This is the first in a series of CHECK-RAS exercises in which we will examine some of the program's features and problems. This exercise will examine how CHECK-RAS flags erroneous values for hydraulic parameters, and parameters that appear to be outside of the typical range. The parameters that will be examined are the Manning's "n" value and the contraction and expansion coefficients. For those of you who have previously used CHECK-RAS for more complex problems, you should still complete this exercise as we will be using the files and building on the problem in the future.
Before we begin, we will make a copy of the sample data that we will be using. Create a folder under your CHECK-RAS directory called "tutorial 1".
For this tutorial, we will use the set of sample data provided with HEC-RAS called "Bogue Chitto, MS - Example 13" (the file name is bogchit.prj). Run HEC-RAS and open the data set, which should be under your HEC-RAS directory in the folder called "data". Click on File Save project as and save it in the "tutorial1" folder that you created previously. Name it "tutorial1.prj".
We will examine the program's response to "n" values that are too low. We will assign an "n" value for concrete (+/- 0.015) as the channel "n" value. At Station 52.29, this data set also contains a cross section for which the contraction coefficients are 0.3 and 0.5. This cross section is not immediately downstream of the bridge, so CHECK-RAS will flag it.
Follow these steps:
Now we will run CHECK-RAS to find the error:
At the end of the report, a summary of the errors will be given as follows:
ROUGHNESS COEFFICIENT CHECK
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RS: 55.4
NT RC 03 Channel n value is equal to or less than 0.025
The n value of the channel is usually larger than 0.025.
The n value should be reevaluated it if is not representing a
concrete lined channel.
TRANSITION LOSS COEFFICIENT CHECK
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RS: 52.29
NT TL 02 Contraction and expansion loss coefficients are 0.3 and 0.5
respectively. However, this cross section is not at the structure.
They should be equal to 0.1 and 0.3.
Note that the Roughness Coefficient check may produce this message if you are evaluating a channel that is concrete lined on the bottom with grassy slopes and overbanks.
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Last Modified: Friday, 15-Jun-2007 11:57:20 EDT