Appropriate Runoff Calculation 2

[brianchapman]

I need to know where I can find a list of the appropriate
methods of discharge calculations for different situations.

For example, which runoff equation is most appropriate for
designing small detention ponds for a small basin?

I believe I can use the SCS CN method for most situations... is this accurate?

Oh yah...what is the equation for the SCS CN method?

I have several references and am not sure which equation is most appropriate to determine peak discharge?

Is it the Q=(P-0.2s)/whatever....

Thanks for anyone who can help.

 

[RWF7437]

FEMA recommends the Rational method only for small basins ( less than 1 square mile).

The SCS CN method can be used in many situations. If you have stream gage data that would probably be best but usually we don't. The SCS method seems to work best in rural drainage basins.

For urban areas the Santa Barbara Urban Hydrograph is widely used.

You might also consider the NFF program (National Flood Frequency) which uses regional regression equations.
Depends on the data you have and your confidence in that data. But all of these methods have large "errors" on the order of plus or minus 30 percent.

 

[snsen50]

I am an architect planner pursuing research work on "water sensitive urban planning" I have the following data
1) Daily rainfall amount for the past 30yrs and not the intensity of rainfall for my study area
2) The watershed area is about 162 sq.km, where as the actual town area is only 16 sq. km.( 51% residential)
3) No streamgauge data or slope data
4) Only soil data & bedrock geology data available

Do I use the SCS CN method or Rational Method

 

[cvg]

rational method is generally only accurate for small areas. Standards usually don't allow its use for watersheds over 160 acres. For large offsite watersheds draining into town, you should use unit hydrographs. These can be generated using SCS curve number method or other methods. There are many methods, but I would use the generally accepted procedure for your region. Otherwise, your results will not be accepted by the engineering community.

Daily rainfall amounts will do very little good. You really need to know the time distribution of the rainfall. Example: It rains 1 inch over a period of 24 hours. This is a gentle rain and may generate very little runoff. On the other hand, assume that it rains 1 inch in 20 minutes. This is an intense storm and will produce much more runoff.

You also need good topographic maps to delineate the watersheds and determine the slopes. Water will pond and soak into flat ground, but will runoff steep slopes.

Soil data is also essential however, bedrock has very little influence unless it is exposed.

I would recommend that you retain the services of a qualified civil engineer or hydrologist.

 

[Bayou]

If you do not have documented frequency intensities for your area, then a HEC-FFA analysis (also documented in Technical Bulletin 17B) can be used perform a frequency analysis based on your rainfall available. Of course thirty years of rainfall isn't much, but it is a start.

SCS method is a much better method than Rational. Some say Rational is said to be good for only 200 acres or less, while others say 50 acres.

However for something as large as 162 sq.km. I would suggest a HEC-1 or HEC-HMS analysis. Or any other computer hydrologic model.

 

[gbam]

The hydrologic method may be governed by the an entity over-seeing the project approval. It would be in your best interest to check with them, either Fed, state, county or city etc... Just a thought.

 

[RWF7437]

Soils data is readily available from the National Resources Conservation Service (NRCS) in the US. Rainfall data and flood frequency data is available from the National Weather Service in the US. Topographic maps for the US are available in paper or digital form from several sources.

All of the suggestions submitted by others are good advice but it seems you may not be in the US so these methods may not be common practice wherever you are.

Just as an aside...it helps a great deal to know WHERE your project is. Many people on this website are in Canada, the UK, Europe, the Middle East and elsewhere in the world.

Russ Faust
Salem, OR USA

 

[kknowle2]

If you know your rainfall depths(or have an IDF curve, or equation for your area, to figure the depths with) you can use a simplified Modifed Rational Method for your stormwater pond. Check this web site out:

http://www.kcesco.com/jaxrain.html

http://www.kesco.sphosting.com