Need HEC-HMS Assistance 6

[Jessica2424]

I am pretty new to HEC-HMS and I seem to be having many problems with it. I am trying to model a watershed of about 1700 acres. There are many basins, ditches and culverts in my watershed. I am having a huge problem modeling the culverts. Can I model culverts in HEC-HMS or should I leave them out of HMS and input them into HEC-RAS? Any ideas? If I should include them in HEC-HMS, then how should I model them?

 

[gbam]

It really depends on what you are looking for. If these culverts will detain flow then I would used stage/storage/discharge rating curves to model a storage basin. If they just direct flow under roads and DO NOT DETAIN flow then there is no need to include them, unless; you may want to know a diversion or ?????

I have used many approaches in my models. It really depends on your point of concentration and what you plan to do with the model results.

Good Luck & Have Fun!

 

[Jessica2424]

Thanks for the help. I really appreciate it. Are you also familiar with HEC-RAS?

 

[francesca]

There have been several recent inquiries regarding HEC-RAS. Do a search, and if none of them answer your questions, just post them in a new thread.

Good luck.

 

[Jessica2424]

When putting in data for subbasins using the SCS Curve Number Method... what is initial loss (in.) or initial abstraction (in.)??? Is it the potential maximum retention (in.) otherwise known as "S" which is =(1000/CN)-10????

 

[sam74]

It's the estimated amount of rainfall that is infiltrated into the ground or gets caught on vegetation or evaporates and is never able to contribute to storm water runoff.

 

[bltseattle]

Initial abstraction is not the same as the storage S. The standard initial abstraction is 0.2, and was introduced by the SCS when developing the CN approach (ie it's empirical). It never changes, whereas S changes with the CN used in the analysis, being a property of soil/cover combination. If you use something different for init-abstract you need to verify whether or not you need another set of CNs that correspond to the non-standard initial abstraction (as discussed in some of the NCRS "white papers").

Reference:

http://www.wcc.nrcs.usda.gov/hydro/hydro-techref-neh-630.html

and

http://www.wcc.nrcs.usda.gov/hydro/hydro-techref-h&h-curve.html

 

[psmart]

The initial absraction actually varies with S. For most studies:
Ia = 0.2 S
S = (1000/CN)-10 [inches]
The Ia/S ratio is sometimes adjusted for specialized situations, such as low-volume events, but should not be changed unless you know exactly what you're doing.

 

[Jessica2424]

So, should I use 0.2 or (0.2)x(S)?

 

[waterguy]

Ia = (0.2)x(S)

 

[psmart]

If the input field is asking for the Ia/S ratio, you would enter just 0.2. If it is expecting the entire equation, it would be 0.2 S

The input prompt should indicate what is expected...

 

[Jessica2424]

Do you need to input both values of CN and IA when modeling a subbasin in HEC-HMS using the SCS Method? Considering the IA equation includes the CN number.

S=(1000/CN)-10

IA= (S)x(0.2)

 

[psmart]

The Ia/S ratio will be the same for all subcatchments, and only needs to be entered once. But you will still need to enter the appropriate CN value for each subcatchment.

 

[Jessica2424]

How do you change from a 10yr storm to a 25yr storm in HEC-HMS? and 25yr. to 50yr. to 100yr. etc?

 

[francesca]

Jessica,

See page 102 of the user manual. A 100-year storm has a 1% exceedence probability annually. It should be set to 10% for your 10-year storm.

 

[Jessica2424]

So for a 25 year storm, should I put 25%.... and for a 50yr, 50%????

 

[RWF7437]

No Jessica.

A "25 year storm" has a probability of being equaled or exceeded of 1 in 25 in any one year. Thus its annual exceedence probability (AEP) is 1/25 = 0.04 = 4%.

If n= 50 the AEP is 1/50 = 0.02 0r 2%
If n= 100 the AEP = 1/100 = .01 = 1%

Good luck

 

[Jessica2424]

Thank you...