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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 spreaders are structural storm water practices that are often
employed upslope of riparian buffers (RB) and vegetative filter strips (VFS).
Publications that provide an overview
on level spreaders include Urban Storm¬
water Structural Best Management
Practices, AG-588-01, and Level
Spreaders: Overview, Design, and Main¬
tenance, AG-58 8-09 W, of the Urban
Waterways series. A companion Urban
Waterways publication to this fact sheet
reviews the design, construction, and
maintenance of level spreaders (Level
Spreader Update: Design, Construction,
and Maintenance, AG-588-20W).
Urbanization in North Carolina has
led to construction of impervious sur¬
faces such as rooftops, roads, and park¬
ing lots. These surfaces cause changes
in the hydrologic cycle, including
reduced groundwater recharge, limited
evapotranspiration, and greater storm¬
water runoff. Low impact development
(LID) techniques such as reducing
impervious surfaces, using clustered de¬
velopments, building on the site’s least
permeable soils, and using structural
stormwater best management practices
(BMPs) can help mitigate these impacts.
Common BMPs used in LID include
bioretention areas, penneable pave¬
ment, cisterns, and green roofs. Another
practice that may have potential for use
as a part of LID is the level spreader-
vegetative filter strip (LS-VFS).
REVIEW OF TERMINOLOGY
Level spreaders may be located upslope
of riparian buffers (LS-RB) or vegeta¬
tive filter strips (LS-VFS). Require¬
ments for LS-RB or LS-VFS systems
include a flow splitter, an overflow (or
bypass) conveyance, a forebay, a blind
swale, and a level spreader. An ap¬
propriately sized flow-splitting device
should be installed at the inlet to the
LS-RB or LS-VFS. All runoff generated
by rainfall above the design intensity
(typically 1 inch per hour) must be di¬
verted to a bypass swale. The remaining
runoff enters a forebay, to still flow and
remove sediment from stormwater. The
stonnwater runoff is conveyed from the
forebay to the blind swale. When the
blind swale fills, flow diffuses along the
length of the level spreader (Figure 1).
Infiltration of stonnwater and removal
of particulate pollutants are expected as
runoff flows through the riparian buffer
or vegetative filter strip downslope of
the level spreader.
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