A weir is an obstruction in a channel flow which can be calibrated to measure the flow

Question:

A weir is an obstruction in a channel flow which can be calibrated to measure the flow rate, as in Fig. P5.32
The volume flow Q varies with gravity g, weir width b into the paper, and upstream water height H above the weir crest. If it is known that Q is proportional to b, use the pi theorem to find a unique functional relationship Q(g, b, H).