SCA Survey The Powerful Effect of Stormwater Run-off from Impervious Surfaces

The Powerful Effect of Stormwater Run-off from Impervious Surfaces

– Discharge in 17 minutes caused by 0.53 inches of rainfall

3:29 PM Before a rainfall event: almost no discharge is visible.
3:34 PM Within 5 minutes the outfall begins to discharge concentrated surface run-off from impervious surfaces.
3:36 PM Notice that vapor can be seen from the discharge. Sunlight heated the impervious surfaces above this outfall and the precipitation on the surfaces are heated accordingly. High water temperature poses impacts on aquatic animals.
3:39 PM
3:46 PM Now the vapor is clearly visible.   This flashy discharge occurred in just 17 minutes from the start of the precipitation. This discharge is from just one stormwater pipe outfall out of thousands. Cumulatively, the discharges form erosive streamflow downstream.

 

• Light Rainfall Doesn’t Cause Runoff in a Forest

The precipitation at Reagan National Airport at 4:52 PM was 0.53 inches.  In a forest, 0.53 inches of rainfall would cause no stormwater run-off and no direct discharge to a stream.  In fact no surface run-off was observed when these pictures were taken from a forested park.

 

• More Pavement, More Discharge

The area where the discharge was observed is not a densely developed area; Figure 1 (below) shows the location of the outfall in a clearly low-to-moderate density residential area. A more highly paved area would lead to torrential discharges of far greater magnitude.

Figure 1 : The stormwater pipe outfall location is shown in this map.
The outfall is located in a low-moderate density residential area

 

• Downstream Erosion

Discharges from all pipe outfalls form higher volume and velocity which would be erosive to streams. An example of an erosion site is shown below in Figure 2.

Figure 2: An example of erosion downstream from the stormwater pipe outfall, here eating into a private property.
Sand bar formation can be seen on the right, sediment deposited as a result of erosion upstream.

 

• High Temperature Impact on Aquatic Animals

Notice the vapor visible especially in the last picture in the above table.  It is assumed that impervious surfaces such as roads, heated by the sun, caused the water to become heated. Then the heated discharge emits the water vapor at the outfall. The pictures were taken on 7/12/2006. The day’s highest air temperature was 89F and the lowest was 74F at Reagan National Airport. The temperature of impervious surfaces such as roads and rooftops are higher than the air temperature.