What do you mean by " 150% of the 100-year design storm"? 150% of the peak flow, 150% of the runoff volume ? 150% of the rainfall ??
What method are you using to generate the hydrograph ? What duration ? If you're using the Rational Method, and the drainage area is small enough that this is appropriate, then Francesca's suggestion may work. But if you're using other methods you may find it easier to change the rainfall amount in Event Manager.
