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**Relavent Cofferdam Diameter .... HELP!!!**

hi, i've been having alot of trouble with this question and do not know where to begin, can anyone help me?

Flow occurs in a wide rectangular channel at a depth y. A construction project over this channel requires that cofferdams spaced at distance w on centrelines be placed in the channel. Assuming that the cofferdams create no energy losses between locations 1 and 2, determine the maximum permissible diameter of the cofferdams without creating upstream backwater effects (i.e. without increasing the upstream depth) and the resultant drag force on each cofferdam (assume that CD = 0.15).

Q = 1.5 m2/s; y = 1.8 m; w = 6 m

if anyone can help or point me in the right direction it would be greatly appreciated.