The City of Newark is in the process of developing a stormwater management plan in order to improve the quality of stormwater runoff reaching creeks and tributaries and to comply with state and federal regulations. It is also currently considering implementation of a Stormwater Program Infrastructure Fund in order to provide stormwater services to the City.
The State of Delaware developed a Technical Document to give some guidance into stormwater management and some background information on it.
POST CONSTRUCTION STORMWATER MANAGEMENT BACKGROUND
Introducing impervious surfaces to a landscape can substantially impact receiving streams and water bodies by increasing both stormwater runoff and its associated pollutants. It has been estimated that a site with 35-50% imperviousness has three times the runoff compared to a more natural groundcover. By limiting the ability of the soil to infiltrate, an increase in impervious surfaces leads to reduced groundwater recharge as well as increased stormwater runoff volume, peak rate and duration of flow, all of which tends to increase the potential for flooding. In addition, data indicates a direct relationship between the amount of imperviousness in a given watershed and the degree of degradation. A recent report to Congress by the National Academies of Science places additional emphasis on the habitat impacts from urban stormwater runoff. Highly impervious watersheds tend to be flashier and exhibit lower base flows. The frequency of out-of-bank occurrences also increases, leading to increased bank erosion and sediment deposition. All these factors stress aquatic organisms, potentially shifting the aquatic ecosystem to favor less desirable species.
Sediment has been determined to be the most significant pollutant of concern associated with stormwater runoff. Suspended sediment particles cause turbidity problems in the water treatment process and act as an environmental stressor on aquatic life. In addition, as soil particles wash off the land through the erosion process, their chemically active nature makes them particularly conducive to transporting adsorbed nutrients, metals, toxics, and other contaminants into the receiving waters. Typical loadings for total suspended solids from urban land uses range from less than 100 lb/ac/yr for low density residential development to over 500 lb/ac/yr for urban highways. In addition to these land-based sources, urban stream channel erosion can lead to sediment loads that are an order of magnitude greater in the receiving waters themselves depending on the effective imperviousness of the watershed.
Nitrogen is a nutrient associated with the soluble component of stormwater runoff. Although necessary for plant growth, excess nitrogen in water becomes a pollutant by stimulating the growth of algae and other less desirable plants. Nitrogen enrichment is typically more problematic in estuarine ecosystems. Major sources associated with urban stormwater runoff include fertilizers and atmospheric deposition. Typical loadings for total nitrogen from urban land uses range from 10 to 15 lbs/ac/yr. Phosphorus is a nutrient more associated with the particulate component of stormwater runoff, since it readily adsorbs to sediment. Also necessary for plant growth, excess phosphorous becomes a pollutant typically more problematic in freshwater ecosystems. The major source of phosphorus associated with urban runoff is fertilizer. Some soluble phosphorus can be traced to septic systems; however, the use of low phosphorus detergents has significantly reduced this source. Typical loadings for total phosphorus from urban land uses range from 0.75 to 1.25 lbs/ac/yr. Other pollutants in urban stormwater runoff include bacteria, trace metals and hydrocarbon derivatives.
The traditional approach to stormwater management was one of quantity control to prevent flooding. Later, greater emphasis was placed on managing stormwater runoff from a water quality perspective. A wide selection of best management practices (BMPs), including manmade ponds, filtration systems, and infiltration structures have been successfully used to manage stormwater runoff. Until recently, however, such BMPs have largely sought to control the particulate pollutants found in surface runoff, such as sediment and those pollutants which tend to adsorb to sediment, such as phosphorus. Soluble pollutants, such as nitrogen, can be found in both surface runoff and subsurface flow. BMPs that have a vegetative component designed for nutrient uptake and/or an anaerobic component to induce denitrification, such as constructed wetlands, biofiltration systems, and bioretention structures, can reduce these pollutants. Current urban BMP designs remove 10 to 50 percent of total nitrogen, and 45 to 75 percent of total phosphorous. With the recent recognition of the habitat impacts associated with urban stormwater runoff, stream stability issues must also be addressed. The traditional approach of “collect and convey” using a single BMP is no longer seen as adequate to manage the complex problems associated with urban stormwater runoff. Contemporary designs aim to reduce runoff volume utilizing distributed techniques and “treatment trains” tailored to the problems associated with a specific site. The post-construction stormwater management component adheres to this philosophy.