Problem

What do all the lines in PondMaker step 4 represent?

Solution

The blue line represents the performance of the outlet structure that you're currently trialing (the rating curve).

The orange line represents the target rating curve you are trying to meet, based on the predevelopment peak flows.

The dashed lines are the estimated maximum water surface elevation for each storm event, from the estimated storage volume and designed pond from the previous step.

In this step of PondMaker the object is to define an outlet structure whose rating curve matches the target rating curve as closely as possible. If your outlet's curve is above and to the left of your target rating curve, it is essentially too small and has restricted flow too much. On the other hand, if your outflow curve is below and to the right of the target rating curve your outlet structure is too large and is letting too much water out.

In the best case you would like to meet your target rating curve exactly, but this is not always possible. For many designs as long as you are within a certain tolerance your design will considered sufficient. You should check with your local governing agency on the policy for your town, city, or state. The "pass" or "fail" designations in the Design worksheet are based on the above/below tolerances set in the Options tab.

Below is a simple example of an outlet structure that is slightly too small/restrictive for the 10 year event and just slightly too large (releasing too much flow) for the 100 year event, but meets the acceptable tolerance design for this project of 10% above/below.

Remember that this step is just an estimate. In the following step, you will actually route flow through the designed pond and outlet and will be presented with a similar graph/diagram. In many cases you will need to make further adjustments to the pond dimensions and outlet to meet the design criteria, especially if the shape of the outflow hydrograph does not match closely to what was used in the storage estimate step.

See Also

Using PondMaker

Pond design start to finish in under five minutes with PondMaker (short video blog post)