When using the modified rational method, what is the difference between using “Scenario” versus “Modified Rational Catchment” as the “Target Flow/Volume Source” in step 1 of Pondmaker?
Under normal circumstances you should use the “Modified Rational Catchment” as the “Target Flow/Volume Source”, when using the Modified Rational method. When doing this and selecting your modified rational catchment as the target element, Pondmaker uses the predeveloped peak from that catchment as the target peak flow to design against. This is based on the Predevelopment Tc, Predeveloped C and Predevelopment area (if the outflow criteria is set to Pre-Development) or the “Target” flow (if the outflow criteria is set to User Defined) entered in the catchment properties. If you choose “Scenario” as the “Target Flow/Volume Source”, the target peak flow that your design will be based on will come from the computed peak of the hydrograph at the select target element, in the selected scenario. In this case, you would not want to select the same scenario as your design scenario, with the modified rational catchment as the target element. This is because the hydrograph in that scenario, at th
