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

Variations
For this manual, detention basins are classified into four
main types:
• Dry ponds,
• Wet ponds,
• Constructed wetlands,
• Underground detention, and
• Bioretention (see Bioretention BMP for more
information).
Additional variations exist within each of the types and
some designs may not fit entirely into one classification.
Some examples of further variations are described below.
Wet ponds
Wet ponds can be designed as either online or offline
facilities. They can also be used effectively in series
with other sediment-reducing BMPs, such as vegetated
filter strips, swales, and filters. Wet ponds may
be a good option for retrofitting existing dry detention
basins. Wet ponds are often organized into the following
three groups:
• Wet ponds primarily accomplish water quality
improvement through displacement of the
permanent pool and are generally only effective for
small inflow volumes (often they are placed offline
to regulate inflow).
• Wet detention ponds are similar to wet ponds but
use extended detention as another mechanism for
water quality and peak rate control. (Discussion
of wet ponds in this BMP section focuses on wet
detention ponds as described above because this
tends to be the most common and effective design.)
• Pocket wet ponds are smaller wet ponds that serve
drainage areas between approximately five and
10 acres and are constructed near the water table
to help maintain the permanent pool. They often
include extended detention.
Constructed wetlands
Constructed wetlands can be designed as either online
(within the stormwater system) or offline facilities.
They can be used effectively in series with other flow/
sediment reducing BMPs that reduce the sediment load
and equalize incoming flows to the constructed wetland.
They are a good option for retrofitting existing detention
basins and are often organized into the following
four groups:
Special Storage
Special detention areas are locations on a site designed
primarily for other uses but can also temporarily
detain stormwater. By detaining and slowly releasing
stormwater, special detention areas can attenuate
peak discharge rates. However, they are not effective
in either improving water quality or reducing runoff
volume. Therefore, special detention areas should be
combined with other BMPs that address water quality,
quantity, and groundwater recharge.
Variations
• Parking lots - In depressed areas or along curbs by
controlling flow at stormwater inlets.
• Rooftops - By restricting flow at scuppers, parapet
wall openings, or roof drains.
• Plazas and athletic fields - Recessed areas can be
designed with detention through the use of flow
control structures and berms.
General design considerations
• Flow control structures should be designed to discharge
stored runoff in a timely manner so that the
primary use of the area can be restored.
• Storage areas should be adequately sloped towards
outlets to ensure complete drainage after storm
events.
• Emergency overflows should be designed to prevent
excessive depths from occurring during extreme
events or if the primary flow control structures
become clogged. Emergency overflows must be designed
to safely and effectively convey flows away
from the special detention area.
25 acre constructed wetland development along M-53 in
Romeo, MI
Source: Hubbell, Roth, & Clark
LID Manual for Michigan – Chapter 7 Page 174