As undeveloped areas are converted to urban uses, the natural vegetation is removed, land slopes modified, and natural pervious areas covered with impervious surfaces, such as roads, drives and roofs. These changes can significantly affect the quantity and quality of stormwater runoff.

Urbanization impacts surface waters in several ways:

• As an area becomes urbanized, natural pervious areas typically are covered with impervious surfaces, such as pavement and buildings, and less pervious landscaped areas. This reduces rainfall infiltration and increases the quantity of stormwater runoff. In addition, drainage modifications constructed during urbanization decrease the travel time of stormwater runoff. The result is that peak stormwater discharge from the urbanized area is increased, and the dry weather stream flow may be decreased. Therefore, one result of urbanization can be higher stream flows during periods of rainfall and lower stream flows during dry periods.
• A second impact related to the changes discussed above is streambank erosion caused by an increase in stormwater runoff. In order for a stream channel to adapt to the increase in stormwater runoff resulting from urbanization, it may erode a larger channel. This eroded material causes the same impacts as sediment from other sources. The changes to the stream channel also may affect aquatic habitats, water depths and stream-side vegetation.
• The increase in pollutants and its impact on water quality is a third major concern of urbanization. These pollutants can vary widely from runoff event to runoff event and over the course of the year. These pollutants are byproducts of modern urban life and include road salt, fertilizers, pesticides, heavy metals, oils, nutrients, oxygen-demanding substances, and bacteria. There are several mechanisms for depositing pollutants on the urban landscape. These include deposition of atmospheric pollutants; direct application on the land of such materials as road salt and sand, fertilizers and pesticides; and applications that are unintentional results of urban activity, such as oil drippings from motor vehicles. Trace metals can be picked up from metal roofing and flashing, metal culverts, paints, and automobile products. Additional sources of pollutants include pet droppings, vegetative matter, litter and anything else deposited upon the urban landscape and capable of being washed off. These pollutants are picked up by runoff and carried along, ultimately reaching a water body.

Vegetated filter strips are areas of land with natural or planted vegetation designed to receive sheet runoff from up-gradient development. They may be, or resemble, various natural environments such as meadows or riparian forests. Their primary function is to remove soil particles and nutrients from overland sheet flow before it reaches a surface water. The primary removal mechanisms are sedimentation and infiltration as the flow moves through the strip.

Grassed swales are shallow, vegetated, manmade ditches designed so that the bottom elevation is above the ground water table to allow runoff to infiltrate into the ground. The vegetation prevents erosion, filters sediment and provides some nutrient uptake.

Extended detention ponds are structures designed to hold stormwater for up to 24 hours. The extended detention pond is normally dry between storm events, but may have a shallow marsh in the detention area. Unlike dry detention ponds, which only detain runoff long enough to reduce the peak rate of runoff, extended detention ponds detain stormwater runoff for a longer time to allow for settling of particulates.

Wet ponds are designed to have a permanent pool of water, which prevents the resuspension of sediments in the pond from previous storm events. Microorganisms and plants in the permanent pool assist in biological uptake and degradation of pollutants. Additional storage is provided above the permanent pool to detain stormwater. Properly designed wet ponds can achieve both pollutant removal and peak discharge reduction.

Constructed wetlands are engineered systems designed to simulate the water quality improvement functions of natural wetlands to treat and contain surface water runoff pollutants and decrease loadings to surface waters. Constructed urban runoff wetlands differ from artificial wetlands created to comply with mitigation requirements in that they do not replicate all of the ecological functions of natural wetlands.

Infiltration practices are designed to infiltrate surface runoff into the ground. These devices include both infiltration trenches and infiltration ponds. An infiltration trench is a subsurface trench filled with stone to which runoff is either piped directly or flows overland. An infiltration basin is an open area to which the runoff is discharged and allowed to pond while infiltrating through the sides and bottom of the basin.

Water quality inlets are also known as oil and grit separators. They are underground, multi-chambered tanks designed to remove sediments and to a lesser degree floatable solids.

Generally, the less the concentration of runoff, and the less sophisticated the treatment measures, the better the solution. Natural solutions are preferable to constructed solutions, e.g., buffer strips are preferable to water quality inlets. However, it is recognized that certain developments in highly urbanized areas do not have sufficient land area for natural treatment methods.

Proper planning of the development is important, it has been shown that impervious areas directly connected to storm drainage systems are the most critical to the quantity, rate and pollutant concentration of the runoff. Proper planning can allow for landscaping which incorporates areas for shallow ponding and infiltration, overland flow through vegetated areas, and reduces stormwater flowing directly from impervious areas to the storm drainage system. In areas requiring stormwater detention for peak flow reduction, it is frequently best to combine the treatment and detention measure in one device such as an extended detention pond, wet pond, or created wetland.

Additional information is included in the publications "Best Management Practices for Urban Stormwater Runoff" and "Stormwater Management and Erosion and Sediment Control Handbook for Urban and Developing Areas in New Hampshire". Both are available from the N.H. DES PIP Unit (603)-271-2975.