Maryland Coastal Bays

Stormwater Management

Impervious surfaces impact environmental quality

Impervious surfaces are any areas that prevent water from penetrating the ground, such as pavement, rooftops, decks, compacted soils, etc. Water runoff volume increases as natural areas are replaced with these hardened surfaces. In a natural system, soil, forests, and wetlands act like sponges, soaking up the rain. Most of the water infiltrates into the soil and is slowly released into streams, rivers, and bays through groundwater. The steady absorption and release of water to streams minimizes flooding, erosion, and drought, as well as thermal, nutrient, and chemical pollution.

The construction of roads, buildings, and other watertight surfaces disrupts this natural hydrology, changing land from a filter to a funnel for stormwater runoff. This change alters stream hydrology, transports pollution downstream, and increases both the volume and velocity of stormwater runoff. Even relatively small amounts of impervious surface can impact waterbodies. Research, from streams to estuaries, finds significant biological degradation whenever impervious cover in a watershed exceeds 10%. Impervious surface area can add up quickly. In an average subdivision, more than 20% of the total land area is impervious. In a typical shopping center, 95% of the land area is impervious.
 

How runoff affects natural streams

The rural lowland of Delmarva is characterized by slow-moving streams, well-drained soils, and low topography. Many natural streams have been modified and ditches have been cut into the landscape to manipulate the water table and provide a means for moving water off agricultural fields and away from roads and residential areas. These changes have provided the region with better living and economic conditions but have also contributed excess nutrients, pesticides, and sediments to local waterways. We must be careful to balance ecological manipulation with human expectations.

The relationship between stream health and the amount of impervious surface in the watershed is described below.

Stable—Vegetated streambanks, large forests, fields, and wetlands protect streams by filtering out pollutants and soaking up stormwater. Stream water flows clear and contains low levels of nutrients. Stable streams support diverse and abundant fish and wildlife populations.

Impacted—Increased runoff volume from impervious surfaces erodes and widens streambanks and transports an increasing amount of pollution to streams. Overall water quality deteriorates as nutrients create algae blooms and sediments cloud the water. Populations of fish and aquatic insects that are sensitive to pollution decline sharply. At this stage, proper stormwater management can help mitigate stream degradation.

Highly degraded—Stream water runs brown with sediments and pollutants from developed areas. Streams in watersheds with more than 25% impervious cover can only support a few fish species that are able to tolerate high levels of pollution. This level of development is a point of no return— even the best stormwater management practices cannot mitigate all the impacts of overwhelming runoff.

Recommendations

• Use pervious pavers, rain barrels, swales, and rain gardens to prevent neighborhood flooding, slow down runoff, and allow the water to penetrate into the soil.

• Consider impervious surfaces effects on aquatic systems in the planning process and avoid exceeding the 10% threshold in a watershed by a ‘safe’ margin (2–3% at least).

• Initiate research projects to define successful measures for preventing streams.

• Determine the feasibility of starting a stormwater utility in each bay watershed to raise funds for drainage improvements to handle water volume and water treatment.

• Consider stormwater infrastructure retrofits, such as catch basin inserts which collect trash and debris and prevent downstream pollution.

• Increase the use of advanced methods for stormwater management, including those that reduce the developed’ footprint.

 
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program
Coastal Bays Program