When using the PondMaker tool to size a pond for multiple return events, how can you simulate different tailwater conditions for each one? For example you may want to take the downstream tailwater from a separate analysis of each return event, in cases where that tailwater may rise above the bottom of the pond and effect outflow.In the outlet design step, only one outlet structure can be selected, which applies to all return events in the worksheet.
Go through the steps to design your pond for the highest return event. When you get to the outlet design step, select Constant Tailwater for the tailwater type in the composite outlet structure, then enter the corresponding tailwater elevation that you'd like to use. In this case, you will likely only be sizing the outlet's weir, but be sure to include the other outlet components (such as orifices and risers) as they will contribute to total flow.
After completing the routing step, create a new pondmaker worksheet using the new button at the top, then repeat step 2 for the next lowest return event that you need to design for (for example 50 year). This time, use the same pond dimensions and outlet from the first pass (with the highest return event), except change the tailwater to correspond to what you want to use for that return event. In this case, you may be designing the size of just the riser.
Repeat the process again for the other return events. You may need to go back and forth between outlet design steps of each pondmaker run (each return event), as a tweak to the outlet could effect higher return events.
Set up the time-elevation table to have a constant elevation, covering the full duration of the simulation.
Switch to the next return event's scenario and repeat, entering the tailwater corresponding to that return event.
Repeat for each return event. If the alternatives are set up correctly, each return event will have a separate tailwater.
Set up and proceed with your PondMaker worksheet, entering all the desired return events. When you reach the outlet design step, select "user defined tailwater" as the tailwater of the composite outlet structure. For the minimum and maximum tailwater elevation, enter the pond bottom and top elevation.
When you compute the outlet design step, each return event should pick up on the corresponding tailwater from the outfall.