Is this possible with HEC-RAS? 4

[sedara]

Hi.. Everyone,

I'm little new to the HEC-RAS and have following question,

Two streams connect together and forming one stream. Those two streams starting from two inland water bodies and final stream goes to sea. I know all the cross sections of all streams and starting and ending water surface elevations. I know manings n values, expansion and contraction co-efficients. What I don't know is, flows of each of these streams. I don't know any hydrographs either. My problem is to find the "FLOWS IN EACH OF THESE STREAMS"

Is this possible with HEC-RAS? If so can you please anyone explain me How can I do this?

 

[LHA]

No, it is not possible. HECRAS does not do hydrology at all. You must use other methods to derive the baseline flow AND additional runoff flow.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[sedara]

Thanks Iha,

I'm still learning this HEC-RAS package, I'm little confused with the capabilities of this software although i refered there User manual and Hydrolic reference. What are the things actualy we can do with this? and, if you know please mention what are the other software currently available, which are best for this kind of job?

 

[RWF7437]

There are many programs available to calculate stream flows. The one most often used in Flood studies is a program called NFF (National Flood Frequency). It is free !

Others include HEC-HMS, SMADA, and many commercial,and expensive, programs. Those I've listed are the least expensive and most available. If you have Google, or any decent search engine, search for them. If you don't have a good seach engine, get one ! It will save you thousands of dollars in money wasted on programs that promise more than they can possibly deliver.

Google, by the way, is also FREE.

good luck

 

[sedara]

Thanks RWF7437,

Can anyone please give me the capabilities of HEC-RAS. I'm little confused with the capabilities of this software What are the things actualy we can do with this?

 

[francesca]

HEC-RAS calculates water surface elevations. You input cross-sections (with roughness coefficients) and downstream reach lengths (i.e. distances between the cross-sections) to create a sort-of 3-dimensional physical model of your river. You also add bridges and culverts. Then you enter your flows as a loading, and some boundary conditions (like downstream river slope), pick a flow regime (steady or unsteady; subcritical, mixed or supercritical). When you run the model, the program creates a 1-dimensional version of your river (long section with head losses at each cross-section location) and produces water surface elevations. You can do encroachments, bridge scour calculations or just simple water surface elevations.

I read the Hydraulic Reference Manual and had no trouble understanding the software. I've used it for five 100-year flood elevation studies over the past year.

 

[sedara]

Thank you francesca...

 

[ashhafPE]

Question,
I have a topo that extends 500' left and right of the centerline of a creek. Is it accurate to import a USGS map and add the highest point with that elevation from the USGS map on to the cross-section for HEC-RAS. In other words, if the elevation from the sides of the creek keeps increasing until a distance of 5280 then it starts to drop. If you do not do this then HEC-RAS extends a vertical lines and analyzes based on the info you give it. Which is more accurate?

 

[RWF7437]

ashafPE,

First, I'd suggest you start a new thread. You're much more likely to get an answer that way.

It is usually OK to extend your cross sections laterally but it depends on the range in elevation of the calculated water surface. If the flooded area is very wide, then you may want to consider field surveying the whole flood plain. But, if the flood plain is narrower than 1000' ( 500' on each side of the creek ) then the ground surface outside that doesn't matter since it is not "Wetted" by the flood flow.

Also look at the contour interval of the USGS map. Remember that the probable error in elevation at any point is equal to one half the contour interval. If you have 40' contours, your probable error at any point is plus or minus 20'.

By all means, visit the site and make a judgement about where you need additional topography and where a simple extrapolation would be good enough.

good luck

 

[francesca]

ashrafPE,

Neither option sounds particularly accurate, but it could just be the way you described it not being clear to me.

What are you using the HEC-RAS model for? Can you justify limited accuracy? How long would it take you to correctly enter the cross-sections? Is this extra time worth the accuracy? Is a USGS topographical map the best you can do?

I built some HEC-RAS models based on surveys of the main channel merged with a County Planning Commission-owned DTM created off aerial photographs to an accuracy of 5'. We were defining a 100-year flood zone adjacent to a residential subdivision. Acknowledging the inaccuracy inherant in the model, we allowed an additional 2' in setting the finished floor elevations.

By all means import your USGS topo, but remember that its accuracy is limited to the contour interval reflected on the map. Don't just add the highest point, add points at every change in slope, in order to correctly model the flow area. You'll need to specify roughness coefficients on the overbanks, and I'd wager that these will change horizontally along your cross-sections. You'll need to add a point where your manning's coefficient changes, too.

Good luck.