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very very tough needs a genius

a cofferdam has to be constructed in a river which has a stage-discharge relationship given by q = 12.74(z-18.63)^1.15 where q is the flow in m^3/sec and z is the water surface level in metres above datum. the cofferdam is expected to be in place for 2 years and the probability of it being overtopped by a flood must be less than 2%. calculate the necessary height of the top of the cofferdam above datum

here are some formulae and i have attached tables

gumbel reduced variate y = -ln[-ln(1 - 1/T)]

cunnane formula T = n + 0.2/m-0.4