Before (top) and after close up of in- stream equipment (in the white PVC tube) being covered by algae. High nutrient levels may cause large seasonal algal blooms to develop. The bottom photo shows a close-up of excess algae covering stream equipment in a developing watershed.
Why does urban stormwater cause problems in streams and the Chesapeake Bay?
In a natural landscape in the Chesapeake Bay watershed, much of the rain that falls evaporates, transpires, or infiltrates into the soil, leaving a relatively small portion of the rainfall to travel to the streams as runoff. However, in an urban landscape, impervious surfaces such as roads, driveways, and roof tops, prevent the rain water from infiltrating into the ground, instead converting it directly to stormwater runoff. Stormwater runoff from urban and suburban areas poses a threat to local and regional waterways because of the increased quantity of runoff that rapidly enters streams, and the pollutants, including nutrients and sediment, it carries from the urban landscape into the streams and eventually to the Chesapeake Bay.
Volume and Speed
The increased quantity of runoff in urban areas may be quickly transported to nearby streams via storm sewer infrastructure, potentially causing large, unnatural flow rates in the streams. The increased stream flow can cause increases in stream bank erosion (mobilizing even more sediment), affect stream vegetation and habitat, and can pose severe risks for areas downstream due to fast moving flood waters. Mitigation of stormwater flow and timing is one goal of the use of stormwater BMP.
Transport of Pollutants
Nitrogen and phosphorus are essential nutrients for plant and animal survival and growth. However, excess nutrients can cause negative environmental conditions including algal blooms that grow quickly and consume dissolved oxygen in the water when they die and decompose. This may result in water with insufficient dissolved oxygen levels required to support living plants and animals. Removal of pollutants, including excess nutrients, from stormwater runoff is another goal of the use of stormwater BMP.
Transport of Sediment
Sediment movement is also essential for healthy stream and downstream ecosystems. However, excess sediment loads in streams, as typically seen in developing landscapes, can deposit on the stream bed and impede flow, and bury in-stream vegetation and organisms. Excess sediment can also cause cloudy water, further impeding plant and animal habitat conditions in local streams and downstream ecosystems. Removal and retention of excess sediment is another goal of the use of stormwater BMP.