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

Hydrology
Additional design considerations for wet ponds
• Wet ponds must be able to receive and retain
enough flow from rain, runoff, and groundwater
to ensure long-term viability. A permanent water
surface in the deeper areas of the wet pond should
be maintained during all but the driest periods. A
relatively stable permanent water surface elevation
will reduce the stress on vegetation in an area
adjacent to the pond. A wet pond should have a
drainage area of at least 10 acres (five acres for
pocket wet ponds) or some means of sustaining
constant inflow. Even with a large drainage area,
a constant source of inflow can improve the
biological health and effectiveness of a wet pond
while discouraging mosquito growth.
Additional considerations for constructed wetlands
• Constructed wetlands must be able to receive
and retain enough flow from rain, runoff, and
groundwater to ensure long-term viability.
Hydrologic calculations (e.g., a water balance)
should be performed to verify this. Shallow
marsh areas can become dry at the surface but
not for greater than one month, even in the most
severe drought. A permanent water surface in the
deeper areas of the constructed wetland should be
maintained during all but the driest periods. The
average target pool depth to maintain emergent
wetland vegetation is six to 12 inches. Maximum
water depths of three to four feet should not be
exceeded for more than 12 hours at a time, for
more than a few days out of the year. The deeper
the water and the longer it sits the greater the
chances that a wetland vegetation monoculture,
such as cattails, will develop. A relatively stable
normal water surface elevation reduces the stress
Table 7.9
Definitions of Wetland Vegetation Zones
Vegetation Zone Description
Open Water Areas between 18-inches and 6-feet deep
Emergent Areas up to 18-inches deep
Low Marsh Portion of the emergent zone between 6- and 18-inches deep
High Marsh Portion of the emergent zone up to 6-inches deep
Ephemeral Storage Areas periodically inundated during runoff events
Buffer Area outside of maximum water surface elevation
on wetland vegetation. A constructed wetland must
have a drainage area of at least 10 acres (five acres
for “pocket” wetlands) or some means of sustaining
constant inflow. Even with a large drainage area,
a constant source of inflow can improve the
biological health and effectiveness of a constructed
wetland. Michigan’s precipitation is generally well
distributed throughout the year and is therefore
suited for constructed wetlands.
Stormwater Functions and
Calculations
Volume reduction
Dry ponds and underground detention systems do not
provide an appreciable amount of volume reduction.
Although not typically considered a volume-reducing
BMP, wet ponds and constructed wetlands can achieve
some volume reduction through infiltration and evapotranspiration,
especially during small storms and high
temperature periods.
According to the International Stormwater BMP
Database, wet ponds have an average annual volume
reduction of seven percent (Strecker et al., 2004).
Hydrologic calculations should be performed to verify
that the wet pond or constructed wetland will have
a viable amount of inflow can also predict the water
surface elevation under varying conditions. The volume
stored between the predicted water level and the lowest
outlet elevation will be removed from the storm that
occurs under those conditions.
Peak rate mitigation
Inflow and discharge hydrographs must be calculated
for each design storm. Hydrographs should be based
on a 24-hour rainfall event. The Natural Resources
Conservation Service’s (NRCS) 24-hour Type II rainfall
distribution should be used.
LID Manual for Michigan – Chapter 7 Page 181