In: Civil Engineering
Ans a) Before mining condition :
According to NRCS, TR-55 method,
Curve number for HSG B soil with woods in good condition = 55
Curve number for HSG C soil with woods in good condition = 70
Averge CN = 0.50(55) + 0.50(70) = 62.5 63
Ans b) We know,
Initial abstraction , Ia = 0.20 S
S = Potential maximum retention = (1000/CN) - 10
=> S = (1000/63) - 10 = 5.87 in
=> Ia = 0.20 x 5.87 = 1.175 in
Hence, about 1.175 in of rainfall on this watershed will removed as initial abstraction before mining
Ans c) After mining condition :
According to NRCS, TR-55 method,
Curve number for HSG B soil with cultivated land without treatment = 78
Curve number for HSG C soil with cultivated land without treatment = 85
Average CN for mine = 0.50(78) + 0.50(85) = 81.5 82
Average CN for forested land = 63 (calculated above)
Area weighted CN for whole watershed, CN = CNi Ai / A
=> CN = [(18.6 x 82) + (81.4 x 63)] / 100
=> CN = 66.53 67
Ans d) We know,
Initial abstraction , Ia = 0.20 S
S = Potential maximum retention = (1000/CN) - 10
=> S = (1000/67) - 10 = 4.925 in
=> Ia = 0.20 x 4.925 =0.985 in
Hence, about 0.985 in of rainfall on this watershed will removed as initial abstraction after mining