RECTANGULAR channel for a given area, perimeter(P) should be minimum.Calculation:Width of rectangular channel is b and depth of rectangular channel is yThan Area, A = byPerimeter, P = b + 2yP = (A/y) + 2yFor P to be minimum, dP/dy = 0(-A/y2) + 2 = 0A = 2y2by = 2y2b/y = 2Rectangular channel sectionTrapezoidal channel sectionR = y / 2A = 2y2 T = B = 2yP = 4yD = yR = y / 2\(A = \SQRT 3 {y^2}\)\(T = \frac{{4Y}}{{\sqrt 3 }}\)\(P = \frac{{2y}}{{\sqrt 3 }}\) D = 3y / 4