Question

22a.        What would be the peak runoff rate by the NRCS Soil-Cover-Complex method for a 100 year return period for a watershed in coastal South Carolina with the following practices? Use the NRCS curve number method to find the time of concentration.

Acres	Land use	Soil quality	Soil Class	Length ft	Slope %
50	Small grain, contoured,	Good	A	1000	4
50	Golf course, with 80% grass cover	Good	C	1500	4

Answer 1

Ans) We know, according to NRCS,

Q = (P - 0.2 S)^2 / (P + 0.8 S) ...............................(1)

where, P = Rainfall intensity

S = Potential maximum retention

First determine Time of concentration using NRCS velocity method,

Tc = L / V

For contoured land use, V = 5.032 S^0.5

=> V = 5.032 (0.04)^0.5

=> V = 1 ft/s

=> Tc = 1000 / 1 = 1000 sec or 16.67 min  

Similarly,

For grass covered land , V = 16.135 S^0.5

=> V = 16.135 (0.04)^0.5

=> V = 3.22 ft/s

= >Tc' = 1500 / 3.22 = 465.8 sec or 7.76 min

Total time of concentration = 16.67 + 7.76 = 24.4 min

Now, according to NOAA Atlas 14 , for coastal south carolina with Tc = 24.4 min and 100 year return period , rainfall (P) = 2.82 in  

Now, since the watershed is divided into differnent type of land cover we have to calculate weighted average Curve number (CN)

Weighted average CN = (CNi Ai) / A

Curve number for small grain,contoured in good condition and soil class A = 61

Curve number for golf coarse in good condition with 80% grass cover  and soil class C = 74

=> CN = [(61 x 50) + (74 x 50)] / 100

=> CN = 67.5 68

Hence, S = (1000/CN) - 10

=> S = (1000/68) - 10 = 4.70 in

Putting values in equation 1,

=> Q = [2.82 - 0.2(4.70)]^2 / [2.82 + 0.8(4.7)]   

=> Q = 0.537 in or 0.04475 ft

Runoff volume = Runoff depth x area

Area = 100 acres or 4356000 sq.ft

=> Runoff volume = 0.04475 x ft x 4356000 ft^2 = 194931 ft^3

=> Runoff rate = 194931 / (24.4 x 60) = 133.15 cfs