Low Impact Development Manual for Michigan - OSEH - University ...

Q O = perforation discharge rate (cfs.)
N = number of perforations per length of pipe,
provided by manufacturer based on pipe diameter (#/ft)
AND
Q O = C x A x 2gH
Where:
Q O = perforation discharge rate (cfs)
C d = Coefficient of discharge (typically 0.60)
A = Cross sectional area of one perforation (ft 2 )
g = acceleration due to gravity, 32.2 ft./sec 2
H = head, average height of water above perforation
(ft.) (provided by manufacturer)
11. Flows may bypass a level spreader in a variety of
ways, including an overflow structure or upturned
ends of pipe. Cleanouts/overflow structures with
open grates can also be installed along longer
lengths of perforated pipe. Bypass may be used
to protect the level spreader from flows above a
particular design storm.
12. Erosion control matting, compost blanketing, or
riprap on top of filter fabric are recommended
immediately downhill and along the entire length
of the level spreader, particularly in areas that
are unstable or have been recently disturbed by
construction activities. Generally, low flows that
are diffused by a level spreader do not require
additional stabilization on an already stabilized and
vegetated slope.
Stormwater Functions
and Calculations
Volume reduction
In general, level spreaders do not substantially reduce
runoff volume. However, if level spreaders are designed
similarly to infiltration trenches, a volume reduction
can be achieved. Furthermore, for outflow level spreaders,
the amount of volume reduction will depend on the
length of level spreader, the density of receiving vegetation,
the downhill length and slope, the soil type of the
receiving area, and the design runoff. Large areas with
heavy, dense vegetation will absorb most flows, while
barren or compacted areas will absorb limited runoff.
Peak rate mitigation
Level spreaders will not substantially decrease the overall
discharge rate from a site.
Water quality improvement
While level spreaders are low in water quality pollutant
removal, they are often an important BMP used in
concert with other BMPs. For example, level spreaders
can work effectively (and improve performance)
with related BMPs such as filter strips and buffers. In
addition, level spreaders can avoid erosion problems
associated with concentrated discharges.
Construction Guidelines
The condition of the area downhill of a level spreader
must be considered prior to installation. For instance,
the slope, density and condition of vegetation, natural
topography, and length (in the direction of flow) will all
impact the effectiveness of a distributed flow measure.
Areas immediately downhill from a level spreader
may need to be stabilized, especially if they have been
recently disturbed. Erosion control matting, compost
blanketing, and/or riprap are the recommended measures
for temporary and permanent downhill stabilization.
Manufacturer’s specifications should be followed for
the chosen stabilization measure.
Maintenance
Compared with other BMPs, level spreaders require
only minimal maintenance efforts, many of which may
overlap with standard landscaping demands. The following
recommendations represent the minimum routine
inspection maintenance effort for level spreaders:
Once a month and after every heavy rainfall (greater
than two inches):
1. Inspect the diverter box and clean and make
repairs. Look for clogged inlet or outlet pipes
and trash or debris in the box.
2. Inspect the forebay and level spreader. Clean
and make repairs. Look for:
° Sediment in forebay and along level
spreader lip,
° Trash and/or leaf buildup,
° Scour, undercutting of level spreader,
° Settlement of level spreader structure
(no longer level; you see silt downhill below
level spreader),
° Fallen trees on level spreader, and
° Stone from below the level spreader lip
washing downhill.
LID Manual for Michigan – Chapter 7 Page 225