Two TR-55 questions

[DPAJR]

I did a stormwater runoff calculation on a ravine of about 500 acres in desert country using Hydrocad. FEMA rejected it because they say they are no longer evaluating any commercial software.

That is frustrating because Hydrocad is such an intuitive program loaded with all kinds of helpful features.

The FEMA web site lists quite a few approved pieces of software. I tried TR-55 but it is limited to ten drainages and ten reaches. I have way more than that due to all kinds of smaller ravines running into the bigger one.

I am also looking at HEC-HMS 3.4. It seems to be more capable than TR-55 but also more complicated.

My questions are:

In TR-55 is there a way to calculate the upper half as a separate project and then represent that part as a single entity feeding into the lower half. If so how would you go about that.

The second question is what software would you recommend instead of TR-55? I already shelled out for a 20 node version of Hydrocad so I would prefer one of the freebies on the FEMA site.

Thanks,

 

[sknight701]

Sounds like you are making things way too complicated for such a small watershed. Why not create just one large drainage area for your point of interest, use a weighted average RCN, calculate the time of concentration from the farthest point, and get your flow hydrograph from TR-55?

To directly answer your questions, just combine your smaller ravines into 10 reasonable groups and route them to the point of interest. Regarding the programs, HEC-HMS is a good program, or perhaps you have some published regional regression equations that you could use.

 

[SMIAH]

500 acres = small ?

I like HMS (mainly because it's free).

 

[cvg]

500 acres is not small, however breaking it into more than 10 subareas seems like overkill. generally, more complex models do not result in better results in hydrology - use the kiss" principle ...

 

[DPAJR]

Hi all,

Thanks for the responses. The problem with using TR-55 and making the entire ravine into one drainage area is TR-55 only allows for 100 feet max of sheet flow. For a 7425 foot long drainage 7325 feet of it becomes shallow concentrated flow by default. By breaking it up into areas around each gully I get to count a lot more area as sheet flow which it should actually be. The difference in peak flow is more than double. However, I can't break the upper ravine into ten areas because the lower ravine is already 10 areas. These are large ravines with a whole series of smaller ones flowing in from each side.

I think HEC-HMS is going to be a good choice since the manual says it works well for dendritic systems which these definitely are and so far I haven't found anything in the manuals about limiting the number of subcatchments in HEC-HMS.

Thanks again
DPAJR

 

[tsgrue]

.

The following threads have information that should lead you to a selection...

Various hydraulic/hydrology modeling programs????
thread194-194255

Volume of Runoff Rational Method Formula
thread162-198581

...also, TR-55 is a simplified version of TR-20 and TR-20 is freely available from NRCS. You can find links to download all of these software applications at: "

http://hhwq.blogspot.com".

Hope that helps!

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com