# Catchment Area Calculation | Drainage Basin | Topography

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calculation for catchment area
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To Determine Catchment Area or Drainage Area The catchment area of a river is determined by using contour map. The watershed line which indicates the drainage basin of a river passes through the ridges and saddles of the terrain around the river. Thus, it is always perpendicular to the contour lines. The catchment area contained between the watershed line and the river outlet is then measured with a planimeter (Figure 18.4   ). << Back | Next >> Storage capacity of a Reservoir The storage capacity of a reservoir is determined from contour map. The contour line indicating the full reservoir level(F..!) is drawn on the contour map. The area enclosed between successive contours are measured by planimeter (Figure 18.   ). The volume of water between F..! and the river bed is finally estimated by using either Trape#oidal formula or \$rismoidal formula. << Back | Next >>    ExampesEx! #\$  %n a hydro&electric pro'ect, the reservoir provides a storage of . million cubic meter between the lowest draw down and the top water level. The areas contained within the stated contours and the upstream face of the damare as follows  Conto%r &m' *++1 1+18 18+1 1+1- Area & ( ) s* m' 444*8**+1-118%f the .!. of the lowest draw down is 1- m, find the reduced level of water at the full storage capacity of the reservoir. So%tion + The area contained in lowest draw down level i.e. at 1- m is as follows /iven, contour interval 0 mThe area contained between 1- m and 1+ m level is (11 & 8)  1+ ) ,   1+ )  s2 m i.e., For a height of m, difference in area 0   1+ )  s2 m Therefore between 1- m and 1- m, i.e. for a height drift of * m, the area difference0 1.*  1+ )  s2 mThe area contained in 1- m contour 0 (8 3 1.* )  1+ )  s2 m 0 .* 1+ )  s2 mow from given and calculated data and using trape#oidal rule Conto%rArea containe- & ( ) '.o%me containe- /et0een & ( ) '.o%me containe- /y & ( ) ' 1-.*   +.   1+11.+   +.   -.   1 1-.+   .8   +.+   18+*+.+   18.8   1+.   18 *.+   * .   1*.   1+*8.+   4**.8    1 .+   1 4.+   .8   1 .+   *++44.+   *.85o, at full storage capacity, the height of water level lies between 1 m and *++ m. The volume of water beyond 1 m height is ( .  1+ -  & .8  1+ - ) 0 1.**  1+  cu.m!et h be the height of water level above 1 m height. Then area contained in (1 3 h) m contour is 0 4  1+ 4  3 The volume between 1 m and (1 3 h) m contour is or, h *  3 4 h &1*.* 0 + 5olving, we get h 0 +. m Thus the reduced level of water at the full reservoir capacity is (1 3 +. ) 0 1 . m << Back | Next >>  Exercise 18 Ex.18-1 The areas enclosed by contours on the upstream face of dam in a hydro-electric project asContour (m) 800  !0 80 0  0 #0 \$0 %0 &rea (hectares) %1.\$1 ' .\$ '\$.8! ''.'% 1!.% 1.\$ 1'.!1
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