What Is Stormwater?

For the purposes of this manual, stormwater is water running off the land in an urban environment. In a natural Piedmont landscape without development, nearly 90% of precipitation is absorbed into the ground, taken up by plants, or returned to the atmosphere through evaporation (Figure 7). This gives needed water to flora and fauna, replenishes surface and groundwater reserves, and delivers water slowly to the stream via groundwater. In a developed urban landscape, more precipitation falls onto impervious surfaces that don’t absorb water, like roads, sidewalks, rooftops, parking lots, or construction sites. As a result, the falling water is swept across these surfaces as runoff. This depletes groundwater and results in rapid transport of stormwater to the stream – causing more frequent and higher magnitude floods.

Figure 7 depicts the changes for a highly developed landscape like an urban core or downtown. The change isn’t as great in less densely developed residential areas, but the general pattern is similar. 

Figure 7 Impact of Urbanization on Stormwater Runoff

How Can Stormwater Runoff Be a Problem?

In a natural system, the soil ecosystem including plant root zones filter pollutants as water infiltrates and moves through the ground. Without infiltration, water flows across the ground as stormwater runoff, causing erosion and picking up pollutants and carrying them into local creeks, rivers and lakes, where they may eventually be transported to the ocean.

The most pervasive pollutant in Carrboro is sediment, not only from construction sites, but from overland runoff that sweeps away and displaces soil, gullys, and streambank erosion.  Other important pollutants include:

●        Pet Waste

●        Automotive Fluids

●        Yard Waste

●        Fertilizer, Herbicides and Pesticides

●        Trash and Litter

These pollutants can affect the water where we swim, play and get our drinking water. They also impact other species that live in and rely on creeks and other bodies of water. Runoff can also cause localized flooding when storm drains take on too much water and degradation of stream channels and reduced replenishment of groundwater

Most runoff from older development is conveyed directly to nearby streams, rivers, or other water bodies without treatment. Runoff from newer developments requires treatment via Stormwater Control Measures (SCMs) before entering streams. SCMs are designed to mimic the services provided by natural infiltration, slowing down the flow and removing pollutants.

By working together, we can reduce runoff and positively impact our creeks and keep them clean for everyone and all species to enjoy.

What Are the Components of the Stormwater Infrastructure in Carrboro? Who is Responsible for Taking Care of It?

Carrboro’s stormwater infrastructure supports two main functions:

Stormwater conveyance refers to the network of above- and below-ground infrastructure that collects and moves runoff from its upstream origins until it reaches a stream (Figure 8). This includes ditches, swales, inlets (in yards, parking lots, along curbs), catch basins, junction boxes and pipes, including culverts. 

Stormwater treatment refers to the management of runoff to reduce stormwater quantity and improve its quality using SCMs. Some SCMs are specifically designed for flood mitigation, for infiltration or filtration, or reuse of rainfall. Others are multifunctional and can also be integrated into a landscape for non-stormwater related benefits (e.g., aesthetics, pollinator support, carbon sequestration, etc.).

Figure 8 Swales, gutters, inlets, and pipes are all part of a Stormwater Conveyance System.