Stormwater Management Standards Manual - Toledo Metropolitan ...

General Applicability
Land Area
Constructed wetlands are a widely applicable stormwater management practice and provide excellent
pollutant removal. However, wetlands consume a large area, which may limit their use on sites where
land values are high or additional land is not available.
Soil Types
Maintaining minimal water levels so that the wetland plants can survive is essential to the proper
function of stormwater wetlands. Therefore, stormwater wetlands are recommended for large drainage
areas (greater than 20 acres). Sites with type A and B soils may have high infiltration losses. These
sites may require clay or geo-textile liners to help maintain water levels.
Nuisance Abatement
Geese, ducks, and insects can become undesirable inhabitants of the wetland if it is not designed
properly. Limiting the amount of deep, open water as well as keeping open grass areas to a minimum
will discourage nuisance species. Careful consideration should be given to the possible adverse effects
of attracting wildlife to the site, prior to choosing this BMP.
Stormwater vs. Natural Wetlands
Stormwater wetlands that are separated from natural wetlands are typically not regulated under
appropriate State and Federal laws. However, State and Federal permit requirements should be
reviewed prior to stormwater wetland design. Required permits and certifications may include Section
401 water quality certification, Section 402 NPDES stormwater construction permit, Section 404
wetlands permit, dam safety permits, and local grading permits. Generally, stormwater wetlands should
not be located within delineated natural wetland areas, nor be used to mitigate the loss of natural
wetlands. Disruption or discharge to natural wetlands will not be allowed without a Section 404 permit
and State certification under Section 401 (See Chapter 9 for additional guidelines on Natural Wetlands
Protection).
Design Guidelines
Treatment Sizing Criteria & Release Rates
Water quality storage: WQ v
Water quality draw down time: 24 hours
Floodwater storage: Q f
Maximum release rate: Pre-development 5-year, 24-hour storm event
Emergency spillway capacity: Minimum 100-year, 24-hour storm event
A minimum of one foot of freeboard is required above the 25-year, 24-hour storm water elevation on all
detention/retention facilities.
Third Edition Stormwater Standards Manual September 2008 Page 88