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





RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
good luck
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
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.
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
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
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
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.
RE: Runoff Curve Number (RCN) for hydrologic soil group A/D
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