peak runoff rate

[Ambrug20]

i learn that you ask to find Pick Runoff Rate for different type of soil in different ares sizes, need to find weighted C or SN.; after it calculated, need to perform other calculation to find the result. In this particular problem, CN in found for each area separatly.

does anyone knows why? Look below:

Half of a 3 mi^2 watershed is in row crops on group C soils, and the other half is a subdivision with 1/4 ac lots, equally divided between group A and group B soils. The watershed is located in a geographic region with a unit hydrograph peak of 75 cfs/mi^2-in for a 24 hr Type II rainfall. If the design rainfall for a 24 hr 100-yr storm is 7.5 in, what is most nearly the peak runoff rate?

880 ft3/sec

1100 ft3/sec

1200 ft3/sec

1300 ft3/sec

Solution:

Using the United States Natural Resources Conservation Service (NRCS) methods, a curve number, CN, is identified for each combination of land use and soil type. The maximum water storage can be calculated using the NRCS method as

S=(1000/CN)-10

The water balance equation can be written using an empirical correlation between the initial abstraction and the total storage capacity.

Q=(P-0.2S)^2/P+0.8S

Tabulated values of the curve numbers are given for normal soil moisture conditions, known as Antecedent Runoff Conditions (ARC-II).

Can't copy the table, but they found CN, S, Qr, Qr*A separetly for each case, and then total of A & Qr*A

Pick runoff rate found by: Qp=q*a*Qr

I did the same thing, but with weighted CN, and got wrong anwer of cause.

could anyone explain.

 

[Falco]

I believe the reason is because the CN's are different for connected and un-connected cases. The CN for 1/4 acre subdivision assumes that impervious areas are directly connected. The corn fields are obviously un-connected. I think this requires that two different curve numbers are calculated.

If the question had been written differently, such as half the area being corn field and the other half being farm houses draining to a nearby creek with no pavement or pipes connecting the imprevious surfaces to the creek, then a composite curve number would be used.