The low-impact development (LID) function was new to SWMM 5.0.019/20/21/22 and SWMM 5.1+ It is integrated within the subcatchment and allows further refinement of the overflows, infiltration flow and evaporation in
rain barrel,
swales,
permeable paving,
green roof,
rain garden,
bioretention and
infiltration trench. The term
low-impact development (Canada/US) is used in Canada and the United States to describe a land planning and engineering design approach to managing stormwater runoff. In recent years many states in the US have adopted LID concepts and standards to enhance their approach to reducing the harmful potential for storm water pollution in new construction projects. LID takes many forms but can generally be thought of as an effort to minimize or prevent concentrated flows of storm water leaving a site. To do this the LID practice suggests that when impervious surfaces (concrete, etc.) are used, they are periodically interrupted by pervious areas which can allow the storm water to infiltrate (soak into the earth) A variety of sub-processes in each LID can be defined in SWMM5 such as: surface, pavement, soil, storage, drainmat and drain. Each type of LID has limitations on the type of sub-process allowed by SWMM 5. It has a good report feature and a LID summary report can be in the rpt file and an external report file in which the surface depth can be seen, soil moisture, storage depth, surface inflow, evaporation, surface infiltration, soil percolation, storage infiltration, surface outflow and the LID continuity error. There can be multiple LID's per subcatchment and no issues have been had because of having many complicated LID sub networks and processes inside the Subcatchments of SWMM 5 or any continuity issues not solvable by a smaller wet hydrology time step. The types of SWMM 5 LID compartments are: storage, underdrain, surface, pavement and soil. a bio-retention cell has storage, underdrain and surface compartments. an infiltration trench lid has storage, underdrain and surface compartments. A porous pavement LID has storage,
underdrain and pavement compartments. A rain barrel has only storage and underdrain compartments and a vegetative swale LID has a single surface compartment. Each type of LID shares different underlying compartment objects in SWMM 5 which are called layers. This set of equations can be solved numerically at each runoff time step to determine how an inflow hydrograph to the LID unit is converted into some combination of runoff hydrograph, sub-surface storage, sub-surface drainage, and infiltration into the surrounding native soil. In addition to Street Planters and Green Roofs, the bio-retention model just described can be used to represent Rain Gardens by eliminating the storage layer and also Porous Pavement systems by replacing the soil layer with a pavement layer. The surface layer of the LID receives both direct rainfall and runon from other areas. It loses water through infiltration into the soil layer below it, by evapotranspiration (ET) of any water stored in depression storage and vegetative capture, and by any surface runoff that might occur. The soil layer contains an amended soil mix that can support vegetative growth. It receives infiltration from the surface layer and loses water through ET and by percolation into the storage layer below it. The storage layer consists of coarse crushed stone or gravel. It receives percolation from the soil zone above it and loses water by either infiltration into the underlying natural soil or by outflow through a perforated pipe underdrain system. New , the EPA's National Stormwater Calculator is a Windows desktop application that estimates the annual amount of rainwater and frequency of runoff from a specific site anywhere in the United States. Estimates are based on local soil conditions, land cover, and historic rainfall records. The Calculator accesses several national databases that provide soil, topography, rainfall, and
evaporation information for the chosen site. The user supplies information about the site's land cover and selects the types of low impact development (LID) controls they would like to use on-site. The LID Control features in SWMM 5.1.013 include the following among types of
Green infrastructure: • StreetPlanter: Bioretention cells are depressions that contain vegetation grown in an engineered soil mixture placed above a gravel drainage bed. They provide storage, infiltration and evaporation of both direct rainfall and runoff captured from surrounding areas. Street planters consist of concrete boxes filled with an engineered soil that supports vegetative growth. Beneath the soil is a gravel bed that provides additional storage. The walls of a planter extend 3 to 12 inches above the soil bed to allow for ponding within the unit. The thickness of the soil growing medium ranges from 6 to 24 inches while gravel beds are 6 to 18 inches in depth. The planter's capture ratio is the ratio of its area to the impervious area whose runoff it captures. • Raingarden: Rain gardens are a type of bio-retention cell consisting of just the engineered soil layer with no gravel bed below it. Rain Gardens are shallow depressions filled with an engineered soil mix that supports vegetative growth. They are usually used on individual home lots to capture roof runoff. Typical soil depths range from 6 to 18 inches. The capture ratio is the ratio of the rain garden's area to the impervious area that drains onto it. • GreenRoof:
Green roofs are another variation of a bio-retention cell that have a soil layer laying atop a special drainage mat material that conveys excess percolated rainfall off of the roof. Green Roofs (also known as Vegetated Roofs) are bio-retention systems placed on roof surfaces that capture and temporarily store rainwater in a soil growing medium. They consist of a layered system of roofing designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface. The thickness used for the growing medium typically ranges from 3 to 6 inches. • InfilTrench:
infiltration trenches are narrow ditches filled with gravel that intercept runoff from upslope impervious areas. They provide storage volume and additional time for captured runoff to infiltrate the native soil below. • PermPave or
permeable pavements: Continuous Permeable Pavement systems are excavated areas filled with gravel and paved over with a
porous concrete or asphalt mix. Continuous Permeable Pavement systems are excavated areas filled with gravel and paved over with a porous concrete or asphalt mix. Modular Block systems are similar except that permeable block pavers are used instead. Normally all rainfall will immediately pass through the pavement into the gravel storage layer below it where it can infiltrate at natural rates into the site's native soil. Pavement layers are usually 4 to 6 inches in height while the gravel storage layer is typically 6 to 18 inches high. The Capture Ratio is the percent of the treated area (street or parking lot) that is replaced with permeable pavement. • Cistern:
Rain barrels (or
cisterns) are containers that collect roof runoff during storm events and can either release or re-use the rainwater during dry periods. Rain harvesting systems collect runoff from rooftops and convey it to a cistern tank where it can be used for non-potable water uses and on-site infiltration. The harvesting system is assumed to consist of a given number of fixed-sized cisterns per 1000 square feet of rooftop area captured. The water from each cistern is withdrawn at a constant rate and is assumed to be consumed or infiltrated entirely on-site. • VegSwale:
Vegetative swales are channels or depressed areas with sloping sides covered with grass and other vegetation. They slow down the conveyance of collected runoff and allow it more time to infiltrate the native soil beneath it.
Infiltration basins are shallow depressions filled with grass or other natural vegetation that capture runoff from adjoining areas and allow it to infiltrate into the soil. •
Wet ponds are frequently used for water quality improvement,
groundwater recharge, flood protection, aesthetic improvement or any combination of these. Sometimes they act as a replacement for the natural absorption of a forest or other natural process that was lost when an area is developed. As such, these structures are designed to blend into neighborhoods and are viewed as an amenity. •
Dry ponds temporarily store water after a storm, but eventually empties out at a controlled rate to a downstream water body. •
Sand filters generally control runoff water quality, providing very limited flow rate control. A typical sand filter system consists of two or three chambers or basins. The first is the sedimentation chamber, which removes floatables and heavy sediments. The second is the filtration chamber, which removes additional pollutants by filtering the runoff through a sand bed. The third is the discharge chamber.
Infiltration trench, is a type of best management practice (BMP) that is used to manage stormwater runoff, prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. It is a shallow excavated trench filled with gravel or crushed stone that is designed to infiltrate stormwater though permeable soils into the groundwater aquifer. • A
Vegatated filter strip is a type of buffer strip that is an area of vegetation, generally narrow and long, that slows the rate of runoff, allowing sediments, organic matter, and other pollutants that are being conveyed by the water to be removed by settling out. Filter strips reduce erosion and the accompanying stream pollution, and can be a best management practice. Other LID like concepts around the world include
sustainable drainage system (SUDS). The idea behind SUDS is to try to replicate natural systems that use cost effective solutions with low environmental impact to drain away dirty and surface water run-off through collection, storage, and cleaning before allowing it to be released slowly back into the environment, such as into watercourses. In addition the following features can also be simulated using the features of SWMM 5 (
storage ponds,
seepage, orifices,
Weirs, seepage and evaporation from natural channels):
constructed wetlands,
wet ponds,
dry ponds, infiltration basin,
non-surface sand filters,
vegetated filterstrips, vegetated filterstrip and infiltration basin. A WetPark would be a combination of wet and dry ponds and LID features. A WetPark is also considered a constructed wetland. == SWMM5 components ==