Large Drainage Areas 2

[TBCEng07]

New to the forums. I want to find out what everyone's experience is in dealing with large drainage areas. I would define large as greater than 500 acres. My approach is that anything larger than 200 ac or so should not be evaluated with the Rational Method.

What does everyone else think?

 

[cvg]

200 acres for rational is pushing the envelope and probably should be less

 

[francesca]

The only way you'd get a single continuous drainage area (no pipes or detention areas) of 200 acres is in a rural area, which by its rural nature would also have lots of depression storage which the rational method would not take into consideration. Furthermore, the time of concentration would be so long that you'd more than likely get a higher peak flow from a much smaller segment of the basin, say a 5-minute storm in the area of the basin with a 5-minute time of concentration. Certainly you couldn't say with any confidence which duration/area would give your peak flow without a lot of iteration, obviates the point of the rational method (quick-and-dirty). For the rational method, anything over 20 acres makes me jumpy.

My rule of thumb is to use regression equations when I start talking about a drainage basin in terms of square miles rather than acres. I have used the TR-55 method on a basin as large as 2,200 acres, but it was an homogeneous rural densely wooded basin with a good soils map.

 

[tsgrue]

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My opinion...

I have seen documents indicating the Rational Method can be applied to watersheds up to 2000 acres in extent. That is ludicrous, at best. I would put 20 acres as an upper limit for most applications, some possibly higher and some certainly lower. The more you move away from the following, the smaller the watershed should be...

Assumptions/requirements for application of the Rational Peak Runoff Estimation Method include:

- Rainfall occurs uniformly over the entire watershed
- Peak runoff at the watershed definition element (outlet) is a function of the average rainfall intensity during the time of concentration at that location
- Peak discharge frequency is equivalent to the average rainfall intensity frequency
- Time of concentration is span required for surface runoff from the most hydraulically remote point of the watershed to reach the watershed definition element
- The runoff coefficient is dependent upon antecedent conditions being "normal"
- Storm duration must be equal to or greater than the time of concentration of the watershed
- Runoff from low infiltration areas to high infiltration areas is not significant
- Curb/gutter and subsurface conveyances are not significant
- Watershed landuse/landcover is homogeneous or a weighted average runoff coefficient can be determined which adequately represents the watershed landuse/landcover runoff response

(There is some overlap in the above listed assumptions/requirements)

I suggest reading the thread “Volume of Runoff Rational Method Formula” in the Storm/Flood Engineering Forum.

...again, my opinion.

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

http://hhwq.blogspot.com