Stormwater

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.

Figure 9 Bioretention Cells and Rain Gardens can remove runoff pollutants.

Carrboro relies on guidance from the State[1] for the approved types and design of SCMs, including: 

• Green Roofs
• Level Spreaders
• Permeable Pavement
• Rain Gardens (aka, “bioretention cells” – see Figure 9.)
• Rainwater Harvesting
• Stormwater Wetlands
• Underground Detention Systems
• Wet Ponds and Dry Ponds

SCMs are most often built in conjunction with new development, but in some circumstances have been “retrofitted” into older developments. Once constructed, stormwater infrastructure requires on-going maintenance to ensure it continues to perform as intended. Maintenance of storage or flood mitigation SCMs typically includes removing accumulated sediment and debris, routine mowing, and minor repairs to the structures managing the inflow and outflow. Management of some SCMs, such as those that provide additional water quality benefits, can be more complex. These may require more intensive vegetation management, maintenance of flow control features, and restoration or replacement of filter media.

Public Stormwater Infrastructure

In Carrboro, almost all public stormwater facilities are the responsibility of the Public Works Department or the NCDOT for state-owned and maintained roads. The system is regularly inspected by both organizations. In Public Works, staff from the Landscaping and Grounds, Solid Waste, Streets, Engineering, and Stormwater Divisions are all involved. 

Most of our public infrastructure is for conveyance. More recently, the Town has been pursuing water quality treatment projects. The Town’s Land Use Ordinance focuses on new development, including treatment requirements for large storms; older development predates the ordinance. 

Private Stormwater Infrastructure

Two principal types of private stormwater infrastructure in Carrboro are residential and commercial. For both, the design and construction of stormwater infrastructure are the responsibility of the developer; Town staff review and approve plans and construction. Long-term operation and maintenance are the responsibility of the property owner, including homeowner’s associations. Private landowners are responsible for maintaining vegetation and clearing debris to ensure ditches and swales are not obstructed.  

What Are Carrboro’s Responsibilities?

The Town holds a National Pollution Discharge Elimination System/Municipal Separate Storm Sewer System (MS4) permit. This requires comprehensive stormwater management to reduce pollutants in runoff to the maximum extent possible. The permit is required by EPA under the Clean Water Act and is administered in North Carolina by the state Department of Environmental Quality. Under the permit, the Town must implement the following minimum measures:

●        Public Education and Outreach

●        Public Involvement and Participation

●        Illicit Discharge Detection and Elimination

●        Construction Site Runoff Controls

●        Post-Construction Site Runoff Controls

●        Pollution Prevention and Good Housekeeping for Municipal Operations

Public Works staff are initiating an inspection program for permitted commercial and residential Stormwater Control Measures under the Post-Construction measure to assure proper maintenance is occurring.