The "First Flush"
As stormwater moves across impervious surfaces it collects pollutants. Some of those include: bacteria from pet and animal wastes, nutrients such as nitrogen and phosphorus from fertilizers, heavy metals from motor vehicle fluids, and floatables such as plastic bags and cups and other litter. Most stormwater systems are not designed to remove these pollutants that are discharged directly to local waterways and impact water quality and aquatic life.
The first amount of stormwater runoff generated during the early part of a rainfall event is called the "first flush." During this time, pollutants that have accumulated on impervious surfaces (rooftops, compacted lawns and landscapes, sidewalks, streets and parking lots) are "flushed" into storm sewers.
The majority or about 90% of rainfall events in Iowa generate 1.25 inches of rainfall or less. So, from a water quality perspective, if the first flush can be captured and treated, a lot of the pollutants in stormwater can be removed. The other concern with the stormwater is the volume of rainfall generated and the velocity at which it makes it way to the local stream. Peak discharges of stormwater can cause severe stream channel and bank erosion that erodes tremendous amounts of sediment into local water bodies. This has a great impact on the aquatic life and stability of the stream.
The best way to address is to try to mimic the historic way that rainfall was infiltrated into the soil by making soils more functional so they can soak up more rainfall and by having best management practices (BMPs) that will capture, filter, and treat the stormwater. This can be done using a more natural approach using such practices as soil quality restoration (improving soil health), rain barrels, rain gardens, bioretention cells, permeable pavers, and other practices. These are referred to in Iowa as Rainscaping practices.