Rainwater Management- Exemplary Performance

Michael-
Where did you get this answer from? Is there a interpretation on this?

Colleagues-
I sent an inquiry into a LEED v4 coach to gain further insights on how to achieve EP for this credit. I have copy pasted their response below- hope it helps you.

To earn Exemplary Performance for this credit, project teams must manage on site 100% of the rainwater from the 100th percentile storm event computed in the Historical Data worksheet of the v4 Rainfall Events Calculator.

The "percentile" storm event is a tough concept to grasp at first. As an example, if the project must manage the 95th percentile storm event, it must manage 100% of the rainwater that is generated by the 95th percentile storm event. The same is true if the project only needs to manage the 85th percentile event (say, if it’s a zero-lot-line-project). Similarly, if E.P. is pursued, the project must manage 100% of the 100th percentile event.

Also, for LEED v4, "manage on site" means the project must reduce the volume, which cannot be achieved only through detention (i.e. slowing down the rate of the outflow). A significant change in LEED v4 is that projects no longer get credit for reducing peak flows, and instead must capture a certain volume of runoff and prevent it from leaving the site, such as by infiltrating it back into the groundwater, or reusing the runoff for some other use on the project site. Therefore, in LEED v4, simply detaining the runoff and releasing it slowly is not an acceptable means of complying with the credit.

The gathering of the historical rain data and the computation of the volume required to be managed on site are the first steps. The historical data gets entered into the calculator, and the calculator provides the "Depth" of runoff for each percentile storm event, in inches, at the top of the Historical Data worksheet. This Depth in inches is used by your civil engineer to compute the overall volume of runoff from the project site for that particular storm event (i.e., the 85th, 95th, or 100th, depending on which Option/Path the project is pursuing). That computation is based on a variety of site-specific factors, such as ground slope, land cover, and soil properties to name a few. This volume of runoff then gets entered into the top of the "Strategies" worksheet. Finally, a volume of runoff must be computed for each of the different strategies employed by the project.

It is possible that the percentage of rainwater managed on site can be greater than 100%, and this is acceptable, but will not result in additional points. Therefore, if the project team is pursuing E.P. for this credit, you must first determine the volume of rainwater for the 100th percentile storm event, and then manage an equal or greater volume within the project site.