Question

A catchment 420 hectares in area is situated in the coastal region near Mossel Bay where the mean annual precipitation (MAP) is 900 mm. The 5 m interval contours falling within the catchment area were measured to be 188 km in length. The main stream draining the catchment is 4.5 km in length. The stream starts at an elevation of 250 m and flows at a fairly uniform slope for 2.5 km as a natural channel to an altitude of 186 m at which point the general slope and nature changes to an excavated channel for the remainder of its length. At the discharge point the elevation of the stream is 130 m. Dense forest with semi-permeable soil covers 142 hectares, cultivated land with very permeable soil covers a further 166 hectares and the remainder of the area is grassland on permeable soil. Calculate the peak discharge from the catchment for a 20 year return period storm.

Answer 1

GRuen Axea of Catchment, A - Li20 ha Area covered by dense teest, Ai = 142ha Covered by cultialed lond, A2= 1Ga ba Axea Avea cosened by qaslasd on gemi- poumeable 501, A3 - ? A3 - A-A--AR 420-144 -IGG 110 A3 z 110ha. Mar. Lergth of «urej L 4SKM Head at lopt 250m dischauge pott, ub 130m Head at Retun period, T 20 cS Scanned with CamScanner

(2 AsSuning uno o the ast avea, 2d Coelicet axea ae C 0.4 C3 -0.3 Noo, Calculale tbe eghted value c or entore aoatershed, A.C A22+A3C3 A-+A2 A3 142x05IGG X0-4+ i0 KO.3 1G6 + 110 142 =l -t GG 4 433 142+1GG+ito 1t0.4 20 C Q--OS cs Scanned with CamScanner

(3) Corsidex the below expoenon, 3 (45000)3 250-130 Kc 84R1-25 Calculate the lîme ol Concertralio. O.1947X (371421.25191 O 194 37495:1 300-31 mo. t 3300.3 GO 121-G bas. the ntentye afau Calaulate O 17 Ct+o506 CS Scanhrerc with CamScanner

.GX C20911 Sb9 SE O.IG Chr Calaulale -e peak dischauqe tor the catcne = CIA 3GO O.405 x IG-3 X4O 3GO 2840.G Peak dischage Pe the Catchmet O 78 sec cs Scanned with CamScanner