Storm Water Design 5 Classes of BMPs
Storm water best management practices (BMP’s) have been devised to help manage, clean and dispose of small amounts of storm water on site. These BMP’s reduce the amount of overland storm water flow and the amount of run off as well as most non-point pollutants that find their way through a centralized storm water collection system.
Storm water BMP’s are designed to slow the movement of storm water so that it may be filtered, infiltrated, evaporated and transpired through living vegetation. Water thus cleaned will re-enter the water cycle and become pure over time as a result of natural processes on, above and below ground.
Storm water management on small urban sites, parcels of land under ten (10) acres in size, are relatively easy to manage with a variety of storm water best management practices. There are many BMP’s that have been defined by the Environmental Protection Agency and many of them use vegetation as part of the water cleaning regime. These micro-storm water practices fall into six broad categories that can be used to help meet water quality (WQv) or ground water recharge volume (Rev) criteria. They are grouped according to detailed performance criteria based upon the design of conveyance, pretreatment, treatment, landscaping and maintenance requirements needed to prolong the function of the facility. To be effective, they must be capable of capturing and treating the required water quality volume (WQv), remove a proscribed amount of TSS (total suspended solids), remove a proscribed percentage of TP, (total phosphorous) and have an acceptable length of function in the field with minimal maintenance. The State of Maryland for instance sets these standards as volume calculated by water basin, 80% of TSS and 40% of TP. Five (5) years is considered a reasonable amount of time for any vegetation based BMP to function without overgrowth that may clog or hamper peak performance.
The five (5) classes of storm water BMPs mentioned in the model code and define below include storm water ponds; storm water wetlands; infiltration practices; open channel systems; non structural or management methods and
Storm water ponds contain a permanent pool of water, extended detention and a shallow designed wetland equivalent to the entire design storm WQv. These are called by various names including detention ponds, wet ponds, pond systems or pocket ponds. These designed systems will capture and hold water either permanently or temporarily based upon the need and design.
The second BMP type is referred to as storm water wetlands. Storm water wetlands are characterized by large amounts of wetland vegetation within designed shallow wetlands but they may also incorporate permanent pools or extended detention storage to achieve full WQv. These also are referred to by many names including shallow wetlands, wetland systems or pocket wetlands. These wetlands may include preserved wetlands, constructed wetlands and even preserved forested areas with wetland soils and vegetation. Since wetlands are very common in the state of Louisiana as a result of very shallow slopes, clay based soils, and limited topography they are commonly found in most cities.
The third type of BMP used in on-site storm water facilities includes infiltration practices. This method includes various designed systems that will allow WQv to filter through vegetation, through top soil into a prepared foundation soil and from there into native sub-soils. Filtering through vegetation and various soil horizons will remove sediments and trap various non-point pollutants and then allow filtered run off to be returned to a conveyance system that will carry it to its point of disposal. There are many names for this type of BMP. They are often called surface sand filters, infiltration trenches, dry wells, organic filters, storm water buffer plantings and bio-retention facilities such as grassed circular depressions, planted rain gardens or storm water groves all of which must be designed to accommodate WQv.
The fourth group of best management practices includes open channel systems. Open channel systems are designed to direct the flow of storm water as well as its volume. By proper design methodologies including the proper slope, length, cross section, side and bottom infiltration rate and landscape plantings this family of BMP’s will allow for a designed flow rate as well as opportunities for evaporation, infiltration, filtering and plant material transpiration. WQv is not only managed, but the volume is reduced if minor check dams, weirs or wetlands, detentions, wet ponds are designed into a meandering planted channel. This type of open channel system is often called swales, grassed swales, wet swales, bio-swales or planted drainage channels. Extending the length of a open channel system, widening its cross section, increasing the amount of planted vegetation and reducing its slope ramps up the ability of this water management system to clean storm water.
The fifth and perhaps the least considered BMP family is called non-structural which means the BMPs are based more on good site planning, education or management methods. This family of storm water practices reduces the generation of storm water run-off from a site by augmenting the sites capability to keep the water on site where it falls and to reduce or eliminate down slope flow. These methods are based upon a site design process that recognizes natural methods of treating storm water and reducing the amount of off site flow. There are several approaches to design that can be used, sometimes together, sometimes independently. They include site landscaping, reducing the use of impervious surface, rooftop or parking lot disconnection, sheet flow to landscape buffers, retention of natural forest floor, wetland habitat preservation, water harvesting for irrigation systems and design using low impact development strategies. Many of these non-structural BMP’s are defined in greater detail below because they all are very appropriate for inclusion into local landscape codes that regulate the creation of site plans.
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