Question

A rural catchment area of 5,5 km² has an average slope of 3% is covered by grass with clay soil. The length of the main watercourse is 2 km with an average slope of 2% and overland flow can be ignored. Calculate the peak run-off by means of the Rational method at the time when the catchment received 40mm of rain.

Answer 1

The value of runoff coefficient for given case can be assumed to be 0.2 from the table of runoff coefficients.

Time of concentration is calculated in minutes using Kirpich's equation.

Area of catchment (A) = 5.5 km² s 550 ha. For clayey soil with slope 2-7%, with grass - Runoff coefficient = 0.18-0.22 But for smaller crear with dense soils, moderate to steep slopes and sparse vegetation, highest uzalue of rum off coefficient is assigned Hence, lets assume c = 0.20 Length of main watercours - 2km with Aug slope = 2 %. equation, "Time of concentration te=0.01947 LOTOS-0-385 = 0.01947 (2000) ott (0.027-0-385 = 30.57 minutes = 0.5095 hours using Rainfall in catchment = 40mm = 4cm. Critical Ramfall intensity = Rainfall internsity at time to = 4 = 7.85 am) hor. 0.5095 Now, using Rational Method - Peak discharge - - - - Copco O where, c = Ranoff coefficient Q. = 1 0.2 x 7.85 x 550 pcs Rainfall intensidy to an A = Area of catchment in ha Qp = 23.986 ~ 24 m/s & Ans