2 14 hectare watershed is slated for commercial and business

2. (14%) hectare watershed is slated for commercial and business development Currently the slated for commercial A 200 watershed is open space(good condition) with 40% B-soils and 60% D-soils. Determine the increase in runoff depth from a 10 cm rainfall event due to developing the watershed.

Solution

Step 1: Given data:

Area=200 hectares

40% of B soils

60% of D soils

step 2: Calculation of curve number:

These curve numbers are applicable only for open spaces

Curve number for D soils =80 (Good condition)

Curve number for B soils=61 (Good condition)

C.N=0.4X80+0.6X61

C.N=68.6

step 3: Calculate S(Basin retention)

S=1000/C.N-10

S=1000/68.6-10

S=4.57

Step 4: Calculate the volume of runoff:

Pe=[P-0.2S]²/[P+0.8S]

Pe=[10-0.2(4.53)]²/[10+0.8(4.53)]

Pe=6.07 cm

so the increase in the runoff depth will be 6.07 cm due to 10 cm rainfall