You have the right information. You just need to make sure you have the maximum discharge over a 100 year period. Recurrence interval tells how often it happens.

cross sectional area (feet) is the opening where the water runs through and velocity is the speed of the water in feet per second.

The Gage height should give you an insight into either the discharge or velocity (discharge is cubic feet per second, velocity is square feet per second.

In my industry in my country we calculate it differently, so that's all the help I can give!