For the purpose of this chapter, certain terms and words are defined as follows:
(a) Bio-retention Basin (Storm Water Wetland) – A constructed system that is explicitly designed to mitigate the impacts of storm water quality and quantity that occur during the process of urbanization. It does so by temporarily storing storm water runoff in shallow pools that create growing conditions suitable for emergent and riparian wetland plants. The runoff storage, complex microtopography, and emergent plants in the basin together form an ideal matrix for the removal of urban pollutants.
Bio-Retention Basins usually fall into one of the five basic designs, listed below.
(1) Shallow marsh system – The shallow marsh design has a large surface area, and requires a reliable source of baseflow or groundwater supply to maintain the desired water elevations to support emergent wetland plants. Consequently, the shallow marsh system requires a lot of space and a sizeable contributing watershed area (often in excess of 25 acres) to support the shallow permanent pool.
(2) Pond/wetland system – The pond/wetland design utilizes two separate cells for stormwater treatment. The first cell is a wet pond and the second cell is a shallow marsh. The multiple functions of the wet pond are to trap sediments, reduce incoming runoff velocity, and to remove pollutants. The pond/wetland system consumes less space than the shallow marsh, because the bulk of the treatment is provided by the deeper pool rather than the shallow marsh.
(3) Extended detention wetland – In extended detention (ED) wetlands, extra runoff storage is created above the shallow marsh by temporary detention of runoff. The ED feature enables the wetland to consume less space, as temporary vertical storage is partially substituted for shallow marsh storage. A new growing zone is created along the gentle side-slopes of ED wetlands that extend from the normal pool elevation to the maximum ED water surface elevation.
(4) Pocket wetlands – Pocket wetlands are adapted to serve smaller sites from one to ten acres in size. Because of their small drainage areas, pocket wetlands usually do not have a reliable source of baseflow, and therefore exhibit widely fluctuating water levels. In most cases, water levels in the wetland are supported by excavating down to the water table. In drier areas, the pocket wetland is supported only by storm water runoff, and during extended periods of dry weather, will not have a shallow pool at all (only saturated soils). Due to their small size and fluctuating water levels, pocket wetlands often have low plant diversity and poor wildlife habitat value.
(5) Fringe wetlands – Fringe wetlands are formed by shallow aquatic benches installed along the perimeter of the permanent pool of a wet pond. These benches are normally 10 to 15 feet in width on both sides of the normal pool (although they can be extended in width). Fringe wetlands are a very useful design feature in ponds, as they promote a more natural appearance, conceal trash and changes in water levels, reduce safety hazards, and provide some aquatic habitat. While fringe wetlands are a desirable feature in wet points, they provide only a minor increment of additional pollutant removal.
(b) Design Year – The frequency with which a storm of a specific intensity and duration will be experienced over time. Therefore, it signifies the probability of a storm with a specific intensity occurring within one year.
Probability = 1/Design Year
Example: 1/25 year = 4% probability of occurring every year
Typical design year events are the 1, 2, 5, 10, 25, 50, and 100-year storms
(c) Detention Basin – A storm water facility whose purpose is to detain storm water from a specific drainage area prior to discharging to a downstream channel. For the purpose of these requirements, a detention basin will be considered to be "dry" prior to a rain event.
(d) Development – A man-made change to any improved or unimproved real estate not including single-family residential homes (up to 4,000 square feet in floor plan including garages) and associated driveways and sidewalks.
(e) Drainage Outlet – Any location where storm water runoff leaves the proposed development site. The outlet typically is a ditch, stream, storm sewer or approved basin.
(f) Major Flood Path – A system that conveys and temporarily stores runoff from rarer storms, such as the 25-through 100-year events. The major flood path is utilized whenever the capacities of the street gutters, storm sewers, and inlets are exceeded. The major flood path components can consist of the following:
(1) Streets
(2) Swales
(3) Detention/retention basins – multipurpose
(4) Manmade channels – open and closed
(5) Natural creeks, streams and rivers
(g) Retention Basin – A storm water facility whose purpose is to detain storm water from a specific drainage area prior to discharging to a downstream channel. For the purpose of these requirements, a retention basin will be considered to contain water at all times.