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Level Spreader Update: Performance and Research This Publication Presents an Update on the Research Findings on Level Spreaders in North Carolina

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Level Spreader Update: Performance and Research This Publication Presents an Update on the Research Findings on Level Spreaders in North Carolina Level Spreader Update: Performance and Research This publication presents an update on the research findings on level spreaders in North Carolina. Level spreaders are structural stormwater practices that are often employed upslope of riparian buffers (RB) and vegetative filter strips (VFS). Publications that provide an overview practice that may have potential for use on level spreaders include Urban Storm- as a part of LID is the level spreader- water Structural Best Management vegetative filter strip (LS-VFS). Practices, AG-588-01, and Level Spreaders: Overview, Design, and Main- REVIEW OF TERMINOLOGY tenance, AG-588-09W, of the Urban Level spreaders may be located upslope Waterways series. A companion Urban of riparian buffers (LS-RB) or vegeta- Waterways publication to this fact sheet tive filter strips (LS-VFS). Require- reviews the design, construction, and ments for LS-RB or LS-VFS systems maintenance of level spreaders (Level include a flow splitter, an overflow (or Spreader Update: Design, Construction, bypass) conveyance, a forebay, a blind and Maintenance, AG-588-20W). swale, and a level spreader. An ap- Urbanization in North Carolina has propriately sized flow-splitting device led to construction of impervious sur- should be installed at the inlet to the faces such as rooftops, roads, and park- LS-RB or LS-VFS. All runoff generated ing lots. These surfaces cause changes by rainfall above the design intensity in the hydrologic cycle, including (typically 1 inch per hour) must be di- reduced groundwater recharge, limited verted to a bypass swale. The remaining evapotranspiration, and greater storm- runoff enters a forebay, to still flow and water runoff. Low impact development remove sediment from stormwater. The (LID) techniques such as reducing stormwater runoff is conveyed from the impervious surfaces, using clustered de- forebay to the blind swale. When the velopments, building on the site’s least blind swale fills, flow diffuses along the permeable soils, and using structural length of the level spreader (Figure 1). stormwater best management practices Infiltration of stormwater and removal (BMPs) can help mitigate these impacts. of particulate pollutants are expected as Common BMPs used in LID include runoff flows through the riparian buffer bioretention areas, permeable pave- or vegetative filter strip downslope of ment, cisterns, and green roofs. Another the level spreader. Figure 1. Plan view of level spreader–vegetative filter strip adjacent to riparian buffer LEVEL SPREADER – RIPARIAN BUFFERS inclusion of a forebay and a flow bypass structure. (LS-RB) Recent research on level spreaders has furthered un- derstanding of these systems; updates on hydrologic State regulations require that flow be diffused as benefits, water quality benefits, pollutant removal stormwater enters a riparian buffer in the Neuse mechanisms, and buffer topography of LS-VFSs will River basin, Tar-Pamlico River basin, Catawba River be presented. mainstem, Randleman Lake watershed, Goose Creek, Six Mile Creek, Waxhaw Creek, and Jordan Lake LEVEL SPREADER – VEGETATIVE FILTER watershed. These regulations were the reason for STRIPS (LS-VFS) the widespread use of level spreader-riparian buffer systems in North Carolina. Riparian buffers are inher- Vegetative filter strips (VFSs), a type of stormwa- ently disadvantaged for keeping surface flow diffuse: ter BMP, have been used for years for agricultural they cannot be graded, and they naturally contain runoff. They are similar to riparian buffers, except draws and channels. In nutrient-sensitive watersheds, that they are located in upland areas. Stormwater is a total of 50 feet or more of riparian buffer must be routed through the VFS, which removes nutrients and maintained. The 30-foot section of buffer closest to sediment through physical and biological processes. the stream may not be manipulated or disturbed. VFSs may be vegetated with trees and shrubs or Hathaway and Hunt (2008) studied 24 LS-RB with (perhaps frequently mowed) grasses. In North systems in the North Carolina piedmont and found Carolina, the latest regulations require the use of that none were performing as intended, mainly due to grass to receive pollutant removal credit (NCDENR failures at the level spreader, although channelization 2007). There are typically fewer design restrictions was observed in some of the riparian buffers. This re- on LS-VFS systems than on LS-RBs. LS-VFSs allow search led to substantial design changes, including the an engineer to grade the buffer, plant deep-rooted use of a stable material (such as concrete) to construct vegetation, and amend soils with coarse particles the level lip, guidelines for level spreader sizing, and (sand) or nutrient-sorbing minerals. LS-VFSs may 2 be engineered to a much greater extent than LS-RB RESEARCH SITES systems. Further information on the various uses of Previous research on LS-VFSs has been focused level spreaders in North Carolina can be found in the in North Carolina and Virginia. For each of these Urban Waterways fact sheet titled Level Spreader studies, the watershed size, composition, VFS width, Update: Design, Construction, and Maintenance VFS Area, VFS slope, VFS vegetation, and drainage (AG-588-20W). area to filter strip area (DA:FSA) ratio are presented Future research is needed on LS-VFSs to refine in Table 1. For projects with multiple LS-VFSs design guidance and provide designers with tools to (Franklin et al. 1992; Winston et al. 2010), sites are understand relationships between VFS design param- abbreviated as shown in the watershed size column. eters (length, width, slope, vegetation type, soil type) All sites utilized existing in-situ soils, except for the and VFS performance. Hunt et al. (2010) study, which had soils amended OVERVIEW OF RESEARCH with coarse-grained sand. Hydrologic and water quality results for these studies are presented in the Research on LS-VFSs has been concentrated in the following sections. mid-Atlantic states, with a total of nine LS-VFSs studied at six research sites. This section will summa- HYDROLOGIC BENEFITS rize those studies, including the hydrologic and water LS-VFS systems can improve urban hydrology quality benefits of LS-VFS stormwater BMPs. Pol- through infiltration of stormwater in the filter strip. lutant removal mechanisms will be discussed in the This practice reduces the amount of stormwater enter- following section. ing the storm sewer and allows the infiltrated water to be slowly released as groundwater-fed interflow, Table 1. Summary of LS-VFS research sites VFS VFS VFS Watershed Watershed DA:FSA Reference Location Width Area Slope VFS Vegetation Size (ac) Composition ratio (ft) (ft2) (%) 3.5 (G1) Agricultural 100 13000 4 Forested 11.9 Franklin et al. Granville County, NC 1992 3.2 (G2) Agricultural 130 24740 9 Forested 5.7 Yu et al. Urban Parking Charlottesville, VA 9.9 150 40580 6 Grass 10.6 1993 Lot Line and Johnston County, NC 0.86 Urban Highway 56 4700 5.2 Grass 27.9 Hunt 2009 Hunt et al. Urban Charlotte, NC 2.15 150 9700 1.5 Grass 44.8 2010 Residential Urban Parking 0.52 (A1) 25 460 6.2 Grass 49.2 Lot Apex, NC 1st 25 ft: Grass Urban Parking 0.52 (A2) 50 860 7.3 Last 25 ft: For- 26.3 Lot Winston et ested al. 2010 Urban Commer- 0.49 (L1) cial and Parking 25 420 4.9 Grass 45 Lot Louisburg, NC Urban Commer- 1st 25 ft: Grass 0.49 (L2) cial and Parking 50 930 7 Last 25 ft: For- 20.2 Lot ested 3 where it will become baseflow in a nearby stream or Table 3. Summary of research findings on level spreader river. In past NC studies, flow volume reductions var- pollutant concentration reductions ied from 28 percent to 92 percent (Table 2). Peak flow Site Location TN (%) TP (%) TSS (%) rate reduction varied from 23 percent to 89 percent. The best-performing LS-VFSs had low slopes, dense Charlottesville, VA NM1 40 84 vegetation, and small drainage area to filter strip area Johnston County, NC 14 -11 70 ratios. Many factors, including watershed area, wa- tershed imperviousness, filter strip length, watershed Apex, NC (A1) 16 33 65 area to filter strip area ratio, soil type, and slope may Apex, NC (A2) 32 40 72 have an impact upon hydrologic performance for LS- VFSs. Further research is needed to determine how Louisburg, NC (L1) -17 -27 51 each of these design variables affects the hydrologic Louisburg, NC (L2) 18 -2 67 performance of these systems. 1Not Measured Table 2. Summary of research findings on LS-VFS hydrologic benefits a soluble form of phos- Range of Storm Flow Volume Number of Peak Flow Rate phorus that is difficult to Site Location Event Rainfall Reduction Storm Events Reduction (%) remove in filter strips. Depths (in) (%) This is contrasted with Granville County, 29 0.12 – 3.19 28 36 results at Apex, where a NC (G1) majority of influent TP Granville County, concentrations were in 8 0.20 – 1.18 92 89 NC (G2) the particle-bound state and were captured via Johnston County, 13 0.29 - 1.22 49 23 sedimentation. NC Although concentra- Charlotte, NC 23 0.08 – 3.72 85 Not Measured tion reduction is still a commonly used metric Louisburg, NC (L1) 58 0.10 – 2.67 48 61 for stormwater BMPs, many have argued that Louisburg, NC (L2) 58 0.10 – 2.67 41 68 it is a poor indicator of BMP performance. Load reduction, which WATER QUALITY BENEFITS accounts for both concentration and volume reduc- Major pollutants of concern in North Carolina tions, is a better indicator of how a BMP functions to include total nitrogen (TN), total phosphorus (TP), improve water quality. The load reduction metric con- and total suspended solids (TSS). NCDENR gives credit to stormwater BMPs based upon their removal POLUTANT REMOVAL rates for these three pollutants.
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