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

The presettlement and post-development hydrographs
for the drainage area should be calculated using the
NRCS’s methodology described in the NRCS National
Engineering Handbook Part 630, Chapter 10. The
NRCS’s method uses a non-dimensional unit hydrograph
and the soil cover complex method to estimate runoff
peak rates. Once the hydrograph has been computed, it
can be routed manually or with a computer-modeling
program.
Peak rate is primarily controlled in detention facilities
through the transient storage above any permanent water
surface. The degree to which peak rate is controlled is
a function of the transient storage volume provided
(i.e., depth and area) and the configuration of the outlet
control structure. (See Chapter 9, LID Stormwater
Calculations and Methodology.)
Water quality improvement
Wet ponds and constructed wetlands rely on physical,
biological, and chemical processes to remove pollutants
from influent stormwater runoff. The primary treatment
mechanism is settling by gravity of particulates and
their associated pollutants while stormwater is retained
in the pond. Another mechanism for the removal of
pollutants, especially nutrients, is uptake by algae and
aquatic vegetation. Table 7.10 summarizes the pollutant
removal efficiencies.
The longer the runoff remains in a wet pond or
constructed wetland, the more settling (and associated
pollutant removal) and other treatment can occur, and
after the particulates reach the bottom the permanent
pool protects them from resuspension when additional
runoff enters.
The long detention or retention time associated with
wet ponds can be problematic in coldwater fisheries due
Table 7.10
Pollutant removal efficiencies by detention facility
to the potential increase in water temperature. In these
situations, detention times should be limited to a maximum
of 12 hours, or other treatment alternatives (e.g.,
infiltration) should be explored.
Underground detention facilities are usually intended for
applications on sites where space is limited and are not
intended to provide significant water quality treatment.
Construction Guidelines
• The following guidelines pertain to dry ponds, wet
ponds, and constructed wetlands. Underground
detention systems should be installed per the
manufacturer’s recommendations.
° Install all temporary erosion and sedimentation
controls.
° Separate pond area from contributing drainage
area:
° All channels/pipes conveying flows to the pond
must be routed away from the pond area until it
is completed and stabilized.
° The area immediately adjacent to the pond must
be stabilized in accordance with the Michigan
DEQ’s Soil Erosion and Sedimentation Control
Program prior to construction of the pond.
° Prepare site for excavation and/or embankment
construction.
° All existing vegetation should remain if
feasible and only be removed if necessary for
construction.
° Care should be taken to prevent compaction of
the basin bottom.
• If excavation is required, clear the area of all
vegetation. Remove all tree roots, rocks, and
Type TSS TP TN Temperature
Dry Pond 40-60% 35% 25% Low
Wet Pond 60-88% 16-41% 39-76% Low/Medium
Constructed Wetland* 60-99% 13-73% 33-90% High
Underground Detention
* Studies have shown that shallow marsh wetlands are more effective (13 to 75 percent TN removal; 33 to 90 percent TP
removal) then constructed wetlands (0 to 30 percent TN; 15 to 70 percent TP).
For more information, see Chapter 9, LID Stormwater Calculations and Methodology, which discusses water quality criteria.
LID Manual for Michigan – Chapter 7 Page 182