Level spreaders : overview, design, and maintenance

Level Spreaders: Overview, Design, and Maintenance Level spreaders are stormwater structures that can support the filter¬ ing action of riparian buffers if designed and installed properly. Distributed in furtherance of the acts of Congress of May 8and June 30, 1914. North Carolina State Uni¬ versity and North Carolina A&T State University commit themselves to positive action to secure equal opportu¬ nity regardless of race, color, creed, national origin, reli¬ gion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State Univer¬ sity, North Carolina A&T State University, U.S. Department of Agriculture, and local gov¬ ernments cooperating. This publication presents the latest research findings on level spread¬ ers in North Carolina and describes recommended practices for design¬ ing, installing, and maintaining these structures. Since 1998, North Carolina has implemented rules to protect riparian buffers in several major river basins. These rules require that concentrated stormwater runoff be diffused, or spread, prior to discharge into a ripar¬ ian area. To accomplish this, the Divi¬ sion of Water Quality (DWQ) in the N.C. Department of Environment and Natural Resources (NCDENR) recom¬ mended the use of level spreaders and developed initial design standards in October 2001 . An overview of level spreaders and riparian buffers can be found in Urban Stormwater Structural Best Management Practices , AG-5 88- 01, of the Urban Waterways series. This fact sheet provides more detailed information on designing, installing, and maintaining level spreaders. »miivi4X4ii Ul Mill I ММ «МП COO PE RATI VI EXTENSION *■•**»*• DIFFUSE FLOW: WHAT IS IT? Diffuse flow, sometimes called sheet flow , occurs when water spreads out evenly across an area (Figure 1). In contrast, when stormwater collects in a drainage system and flows to a stream via a pipe, swale, or ditch, it does not make enough contact with or bypasses the riparian buffer — a vegetated area along streams, riv¬ ers, and other water bodies that helps to filter runoff and prevent erosion. Riparian buffers can improve water quality in urban environments by re¬ ducing stormwater peak flow, reducing runoff volume through infiltration, and removing nutrients and sediment through physical and biological pro¬ cesses. They are most effective, how¬ ever, when stormwater flows through them at a shallow, uniform depth — the diffuse flow that well-designed level spreaders can provide. LEVEL SPREADER SYSTEMS: AN OVERVIEW Level spreader systems consist of three parts: the forebay, the channel, and the riparian buffer (Figure 2).