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# curve numbers

## curve numbers

(OP)
has anyone had experience arriving at curve numbers for gravel or shale areas? Crushed stone? What about for pavers on sand beds? Any experiience creating a parking lot to meet certain CN's. I am dealing with small areas, fractions of an acre, but a highly stringent review process. Also what about dumping water into drywells and having a CN for the overflow or leakage.

### RE: curve numbers

First some background:
1) CN's are used to generate runoff for a given rainfall.
2) S = 1000/CN - 10
3) Runoff = (P-.2S)^2/(P+.8S), with P = rainfall in inches and Runoff = excess rainfall in inches
See http://www.aquarien.com/sptutor/fielda1/

For Shale, Gravel or crushed stone used in parking area.  Let us assume that the soil under the parking area aggregate will eventually compact and become relatively impervious. (If not, then a different approach is used),  The only area that runoff will be stored is in the shale, gravel or crushed stone matrix.  Therefore, if 6" of gravel are used, with a 40% void volume, 2.4" of storage are available.  If you have a 8" rainfall (or P), runoff would be 8"-2.4" or 5.6", then you solve for S and then for CN.  That would be your effective CN for an 8" rainfall with 6" of gravel.  This method may be used to conservatively solve all your questions.

Paver Blocks on may be analyzed the same as the above method.  Let us assume that your paver blocks are 4" deep and 50% open area and set on a 8" clean sand bed under the blocks.  Lets us assume a 30% void volume for your sand and 0% for the paver block.  Your effective storage in the 12" are the sums of the storage in the 4" paver block/sand layer and the 8" sand layer. Once you determine your void volume, you know how much storage is available and you may back calculate your CN for your rainfall.

A dry well may also be treated as the gravel parking lot but it is just deeper.

I hope that this helps.

Clifford H Laubstein
FL Certified PE #58662

### RE: curve numbers

the assumption is not so conservative if the land is sloped...
you will not get the full 2.4 inch storage over the entire area.  For thin soil cover, you may want to assume little if any retention.

What if antecedent moisture conditions are type III?  again, not very conservative.  you might want to assume that the ground is already saturated - with complete runoff and not storage in the gravel/shale layer at all.

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