PPP Notes - Runoff Control

RUNOFF CONTROL SYSTEMS

 (These notes are compiled from AGS)

NATURAL WETLAND SYSTEMS

 

• Wetlands naturally detain and filter water.
• Scattered throughout the United States, from tropical areas to tundra, they form in depressions in the landscape where the water table is near or at the surface of the soil.
• They may be as small as a tabletop or span tens of thousands of acres.
• There is no single, correct, ecologically sound definition for wetlands, primarily because of their diversity. 
• These systems are an important part of the ecosystem because they produce food and timber, purify drinking water, absorb and store floodwater, suppress storm surges, and help maintain biodiversity.
• Water is supplied to a wetland either by surface sources (i.e., streams or rivers) or by groundwater.
• The sensitivity of wetlands determines appropriate buffer distances between them and developed areas.
• Buffers, which may range from 30 to 300 ft. or more, should respond to the effect runoff may have on the wetland ecosystem.  (Consult a wetlands scientist to formulate buffer distances).
• In general, four wetland sensitivity issues should be taken into account:
• HYDROLOGY: The wetland’s source of water could be altered by development.
• VEGETATION: The plant species in a wetland have different levels of hardiness.
• ECOLOGICAL STATE: More pristine systems are more sensitive to development and runoff pollution.
• ANIMAL SPECIES: Nesting birds, for example, need greater buffer distances than wintering waterfowl.

 

ON-SITE RUNOFF CONTROL MEASURES

 

• Architects can use several on-site measures to control runoff in development projects.
• One of the most commonly used measures is a simple open storage area for runoff.
• The configuration of such open systems varies, depending on the desired level of pollutant treatment.
• Typically called storage ponds, retention basins, or (when made to resemble a natural environment) a constructed stormwater wetland, open systems generally operate more thoroughly with increased retention time.
• Simple storage ponds are typically dry between storms after runoff has evaporated or infiltrated the groundwater.
• Dry ponds sometimes include a wet lower area for additional runoff retention.
• Wet ponds are permanently wet, allowing pollutants to settle to the bottom. 
• Wet ponds that extend runoff retention time with control devices can remove a very high percentage of particulate pollutants.
• Constructed stormwater wetlands (engineered, shallow marshlike areas) retain runoff for long periods), allowing pollutants to settle out of the water column and providing biological, chemical, and physical processes for breaking down pollutants.
• Wetland vegetation slows the velocity of stormwater, reducing erosion and allowing pollutants to settle.
• Many organic and inorganic compounds are removed from wetlands by the chemical processes of absorption, precipitation, and volatilization.
• Constructed stormwater wetlands can also filter excess nutrients such as nitrogen and phosphorous contained in runoff from gardens and septic tanks.
• To correctly size a wetland used for stormwater runoff control, consider the total volume and velocity of water entering and leaving the system.
• Potential advantages of using constructed stormwater wetlands are that they have relatively low capital and operating costs, off consistent compliance with permit requirements, and greatly reduce operational and maintenance costs.

 

STORMWATER WETLANDS

 

• Stormwater wetlands can be defined as constructed systems explicitly designed to mitigate the effects of stormwater quality and quantity on urban development.
• They temporarily store stormwater runoff in shallow pools that create growing conditions suitable for emergent and riparian wetland plants.
• In combination, the runoff storage, complex microtopography, and emergent plants in the constructed wetland form an ideal matrix for the removal of urban pollutants.
• Unlike natural wetlands, which often express the underlying groundwater level, stormwater wetlands are dominated by surface runoff.
• Storm water wetlands can best be described as semitidal, in that they have a hydroperiod characterized by a cyclic pattern of inundation and subsequent drawdown, occurring 12 to 30 times a year, depending on rainfall and the imperviousness of the contributing watershed.
• Storm water wetlands usually fall into one of four basic designs:
• SHALLOW MARSH SYSTEM: The large surface area of a shallow marsh design demands a reliable groundwater supply or base flow to maintain sufficient water elevation to support emergent wetland plants.  Shallow marsh systems take up a lot of space, requiring a sizeable contributing watershed (often more than 25 acres) to support a shallow permanent pool.
• POND/WETLAND SYSTEM:  A pond/wetland design utilizes two separate cells for stormwater treatment., a wet pond and a shallow marsh.  The multiple functions of the latter are to trap sediments, reduce incoming runoff velocity, and remove pollutants.  Pond/wetland systems consume less space than shallow marsh systems because the bulk of the treatment is provided by a deep pool rather than a shallow marsh.
• EXTENDED DETENTION WETLAND: In extended detention wetlands, extra runoff storage is created by temporarily detaining runoff above the shallow marsh.  This extended detention feature enables the wetland to occupy less space, as temporary vertical storage partially substitutes for shallow marsh storage.  A growing zone is created along the gentle side slopes of extended detention wetlands, from the normal pool level to the maximum extended detention water surface.
• POCKET WETLANDS: Pocket wetlands are adapted to serve small sites (from 1 to 10 acres).  Because the drainage area is small, pocket wetlands usually do not have a reliable base flow, creating a widely fluctuating water level.  In most cases, water levels in the wetland are supported by excavating down to the water table.  In drier area, a pocket wetland is supported only by storm water runoff, and during extended periods of dry weather it will have no 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.
• The selection of a particular wetland design usually depends on three factors:
• AVAILABLE SPACE
• CONTRIBUTING WATERSHED AREA
• DESIRED ENVIRONMENTAL FUNCTION
• However, storm water wetlands are not typically located within delineated natural wetland areas, which provide critical habitat and ecosystem services, and are protected under local, state, and federal statutes.
• It’s also important to point out that storm water wetlands should not be confused with constructed wetlands, which are used to mitigate the permitted loss of natural wetlands under wetland protection regulations.
• The primary goal of wetland mitigation is to replicate the species diversity and ecological function of the lost natural wetland, whereas the more limited goal of storm water wetlands is to maximize pollutant removal and create generic wetland habitat.
• Storm water wetlands are also distinguished from natural wetlands that receive storm water runoff as a consequence of upstream development.
• Although not intended for stormwater treatment, wetlands influenced by stormwater are common in urban settings. 
• Storm water runoff that becomes a major component of the water balance of a natural wetland can severely alter the functional and structural qualities of the wetland. 
• The end result is a storm water-influenced natural wetland that is more characteristic of a storm water wetland than a natural one.

 

SHALLOW MARSH SYSTEM

 

• Most of the shallow marsh system is 0 to 18 inches deep, a depth that creates favorable conditions for the growth of emergent wetland plants. 
• A deeper forebay is located at the major inlet, and a deep micropool is situated near the outlet.

 

POND/WETLAND SYSTEM

 

• The pond/wetland system consists of a deep pond that leads to a shallow wetland. 
• The pond removes pollutants and reduces the space required for the system.

 

EXTENDED DETENTION WETLAND

 

• The water level in an extended detention wetland can increase by as much as 3 ft. after a storm, returning to normal levels within 24 hours.
• As much as half the total treatment volume can be provided as extended detention storage, which helps protect downstream channels from erosion and reduces the space needed for the wetland.