page_title: Storm Water Management Model
section_title: Low-impact development components
hierarchical_section_title: Storm Water Management Model / Low-impact development components
caption_attribution_description: These grates surround the trees that line Main Street in downtown Miles City, Montana.
context_page_description: The United States Environmental Protection Agency Storm Water Management Model is a dynamic rainfall–runoff–subsurface runoff simulation model used for single-event to long-term simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas. It can simulate the Rainfall- runoff, runoff, evaporation, infiltration and groundwater connection for roots, streets, grassed areas, rain gardens and ditches and pipes, for example. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. Besides, low impact development and best management practice areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators.
context_section_description: 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 as of July 2013, 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
caption: Main Street Tree Planter, Miles City (281991376)
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