Runoff Curve Number (RCN) for hydrologic soil group A/D

[login67]

In computing the runoff curve number (RCN) for a soil, say,a soil that falls in both the A and D group, is it appropriate to use an average number between the two or just one? If one, which one?

Thanks.

 

[RWF7437]

You must mean a mixture of soils ranging from Group A to Group D. If you want to be very conservative you would use the curve number corresponding to the Group D soil(s) and the land cover which produces the highest curve number. Most engineers would probably use a weighted average and round off to the next highest whole number. Better yet, make your best guess and, if possible, calibrate your model using known rainfalls and flows.

good luck

 

[login67]

It is one soil and not a mixture of soils. What happens is that the soil is in group D because of high water table that creates a drainage problem. Once this soil is effectively drained, it is place in group A hence named A/D soil.

 

[jgailla]

What specifically are you generating runoff for?
If you are running this to compare pre-development and post-development runoff rates, the use of all type D is unconservative for generating peak flows.
If the soil is type A/D because of high groundwater, I would go ahead and use type D for the pre condition and A for the post condition, if you are sure that the soil will be properly drained after development.
Normally, unless there are specific measures taken to improve the soil drainage, I would use type D for the pre and post developed condition.

 

[LHA]

Aren't there any postdevelopment groundwater recharge requirements? If there are, you will be hard-pressed to claim both that you are adequately draining the high water table (if that is even legal), and that you are meeting groundwater recharge requirements.

I would use D for pre and post, then use the high water table as justification for not providing groundwater recharge.

Engineering is the practice of the art of science - Steve

 

[login67]

The runoffs are being generated for a stream restoration (strem realignment) project. The pre- and post condtion runoff rates are expected to be the same as the existing drainage area is expected to remain the same (residential\agricultural) after the restoration.

 

[TerryScan]

In my local (Coastal Plain, Delaware), we have B/D soils. One directive by regulators was to assume B soils withing 50 ft. of a stream, ditch or pond (drained), and assume D soils anywhere else (unless soil testing proves otherwise).

 

[blueoak]

Normally the cover type has the largest effect on curve number. As for soil, I prefer to run both cases to get a range of values (I figure the estimate is only good to about 20% in most cases anyway). Switching curve numbers in most programs is pretty simple.

I would also read the information on the soil profile itself. I know a lot of engineers only use the lookup tables, but the narrative from the Soil Survey provides a lot of guidance on expected conditions and why exactly a designation like A/D was used. This typically answers most questions I have especially when 2 dissimilar groups have been identified. I have found this helpful for plant selection and also expected drainage when the soil is listed with different properties.

 

[gibfrog]

IMHO, you need to use the CN that the government stormwater reviewer will accept, so ask him. Do not try to use "D" as pre and "A" as post unless you are lowering the seasonal high groundwater at least 3-5 feet below grade. Here in central Florida, normal practice would be to use "D" pre/post, unless special conditions exist.

Since this is a stream realignment issue, do you have historical flow/flood level data on the stream? Is there an existing USGS or FEMA or SWMM model? (Runoff CN is just one component of your model - channel roughness, time of concentration, etc are also very important and require some judgement.) Most importantly, how does your pre model compare to historical data and will the regulators accept your methodology?



Clifford H Laubstein
FL PE 58662