Stormwater runoff from development, whether it is buildings, parking lots, sidewalks, etc…, is a major source of pollution for our waterbodies. Once easily absorbed into the ground, large amounts of stormwater now flows as runoff across pavement and other impervious surfaces. It gains speed and picks up sediment, fertilizers, oils, debris, trash, etc… as it flows across these surfaces to the storm sewer and to our waterbodies, where we swim, fish and use as a source for drinking water.
The Omaha Stormwater Program goal is to reduce stormwater pollution and eliminate prohibited non-stormwater discharges. Two areas of focus to address stormwater pollution with regards to development include construction and post-construction, read more on each area below.
Polluted stormwater runoff from construction sites often flows to MS4s (Municipal Separate Storm Sewer Systems) and ultimately is discharged untreated into local lakes, rivers, and streams. Sediment is usually the main pollutant of concern. According to the 2000 National Water Quality Inventory, States and Tribes report that sedimentation is one of the most widespread pollutants affecting assessed rivers and streams, second only to pathogens (bacteria). Sediment runoff rates from construction sites, however, are typically 10 to 20 times greater than those of agricultural lands, and 1,000 to 2,000 times greater than those of forest lands. During a short period of time, construction sites can contribute more sediment to streams than can be deposited naturally during several decades. This, along with other pollutants from construction sites, can cause physical, chemical, and biological harm to our waters. For example, excess sediment can quickly fill rivers and lakes, requiring dredging and destroying aquatic habitats.1 The construction site program utilizes a permitting process (grading permit) to implement site controls that are designed to reduce pollution from construction sites.
Post-construction stormwater management in areas undergoing new development or redevelopment is necessary because runoff from these areas has been shown to significantly damage our environment. Many studies indicate that prior planning and design for the minimization of pollutants in post-construction stormwater discharges is the most cost-effective approach to stormwater quality management. There are generally two forms of substantial impacts from stormwater runoff. The first is caused by an increase in the type and quantity of pollutants in storm water runoff. As runoff flows over areas altered by development, it picks up harmful sediment and chemicals such as oil and grease, pesticides, heavy metals, and nutrients (e.g., nitrogen and phosphorus). These pollutants often become suspended in runoff and are carried to receiving waters, such as lakes, ponds, and streams. Once deposited, these pollutants can enter the food chain through small aquatic life, eventually entering the tissues of fish and humans.
The second kind of post-construction runoff impact occurs by increasing the quantity of water delivered to the waterbody during storms. Increased impervious surfaces interrupt the natural cycle of gradual percolation of water through vegetation and soil. Instead, water is collected from surfaces such as asphalt and concrete and routed to drainage systems where large volumes of runoff quickly flow to the nearest receiving water. The effects of this process include streambank scouring and downstream flooding, which often lead to a loss of aquatic life and damage to property.2 The post-construction program utilizes a review and approval process to implement site controls that are designed to provide water quality treatment and control of the first ½ inch of runoff.