The "First Flush"
The very first amount of stormwater runoff generated during the first hour or two of a rainstorm is considered the "first flush." During this time, pollutants resting on impervious surfaces (rooftops, compacted lawns and landscapes, sidewalks, streets and parking lots) are picked up and "flushed" down the storm drain. As it rains longer, stormwater runoff gets cleaner.
Couple that with the fact that growing communities change how water is managed through the building process, more impervious surface area equals greater runoff. Even small amounts of rain become runoff, rather than soaking into the ground. To improve and protect water quality in Iowa we must mimic how water was managed prior to development by preventing pollution and soaking up the first flush, which is a 1.25" rain that falls over 24 hours so it doesn't become runoff in the first place. When we do that, we ensure the "first flush" is better, as before.
Pre and Post Development Impact on Stormwater Infiltration