Bassett Creek Watershed Management Commission

Appendix B:  Water Quality Definitions ¹

BCWMC:

Bassett Creek Watershed Management Commission
 

Best management practices (BMPs):

the structural, non-structural, and institutional controls used to improve the quality of stormwater runoff. Additional BMPs may be found in Protecting Water Quality in Urban Areas (MPCA, 1989), Minnesota Urban Small Sites BMP Manual (Metropolitan Council, July 2001), State of Minnesota Stormwater Manual, (MPCA, November 2005)
 

Better site design:

the application of non–structural practices at residential and commercial sites to reduce impervious cover, conserve natural areas, and use pervious areas to more effectively treat stormwater runoff.
 

Bioretention: 

a soil- and plant-based stormwater management best management practice (BMP) used to filter runoff
 

Catch basin insert: 

device that attaches to the entrance of a catch basin or mounts inside the catch basin. Catch basins inserts are designed to improve stormwater quality by either preventing debris and pollutants from entering the basin, or by retaining or treating the water in the basin.
 

Check dam: 

a small temporary or permanent dam constructed across a drainage ditch, swale, or channel to lower the speed of concentrated flows for a certain design range of storm events, reducing erosion
 

Commercial, industrial, institutional or public development/redevelopment projects: 

typically result in larger areas of impervious surface, typically in the range of 60 to 80 percent imperviousness. Examples of these developments include shopping malls, stores, schools, hospitals, and warehouses.
 

Commercial, industrial, institutional or public expansion/addition projects: 

additions to existing projects for which approval of the existing project was obtained prior to adoption of this water quality policy (September 14, 1994). Examples of such projects include parking lot expansions/additions and building expansions/additions.
 

Complex projects: 

include projects that are 40 acres or more, controversial, involve more than one property owner, require detailed hydrologic or hydraulic modeling, require vast changes to infrastructure (such as stormwater systems), include many wetland impacts, require extensive environmental review, or involve many different land uses within the same development project 
 

Construction sequencing: 

a specified work schedule that coordinates the timing of land-disturbing activities and the installation of erosion-protection and sedimentation-control measures
 

Critical duration runoff: 

generally accepted reservoir routing procedures using critical duration runoff events refer to the hydrologic methods—usually computer models—used to determine flowrates and flood levels resulting from stormwater runoff events. The event which results in the highest flood level or flowrate is the critical duration event. Examples of such methods include TR-20, Hydrocad, SWMM, HEC-1, and other approved watershed models.
 

Curve number:

an index combining hydrologic soil group, land use factors, treatment, and hydrologic condition. Used in a method developed by the Soil Conservation Service (SCS)/Natural Resources Conservation Service (NRCS)* to determine the approximate amount of runoff from a rainfall event in a particular area.
 

Dead storage:

the permanent storage volume in a pond
 

Detention time:

the theoretical calculated time that a small amount of water is held in a settling basin
 

Disturbed area:

total graded area as part of a commercial industrial, institutional, public, residential, or road project 
 

Emergency spillway: 

a stable channel or other structure used to convey excess flood flows from a treatment device, typically for 100-year or greater flood flows 
 

Erosion control: 

any efforts to prevent the wearing or washing away of the soil or land surface
 

Extended detention:

designed to receive and detain stormwater runoff for a prolonged period of time
 

Filter bed:

a sand- or gravel-bottomed treatment used to filter stormwater
 

Filter strip:

vegetated areas that are intended to treat sheet flow from adjacent impervious areas
 

First flush: 

the majority of pollutants carried in urban runoff are carried in the first ½ inch of runoff from a site.
 

Floodplain:

land adjacent to a water body which is inundated when the discharge exceeds the conveyance capacity of the normal channel. Often described in the regulatory sense as the extent of the 100-year flood.
 

Flood pool:

live storage, or storage above the principal outlet that is used to temporarily store stormwater runoff
 

Flow control: 

controlling the rate and volume of water leaving a site
 

Flow splitter:

device that is used to divert a portion of a flow (from a pipe or channel) to an offline treatment device such as an infiltration basin
 

Flow spreader:

device use distribute water evenly over a surface such as an infiltration basin or a sand filter
 

Forebay:

an extra storage space or small basin located near the inlet to settle out incoming sediments before water moves on into a pond or detention area
 

Grade breaks:

point where the ground slope changes
 

Grit chamber:

tanks designed to slow down the flow so that solids will settle out of the water
 

Ground water mounding:

the localized rise in water table or potentiometric surface caused by the addition of water at an infiltration basin
 

Hydrologic soils groups (HSG):

an NRCS designation given to different soil types to reflect their relative surface permeability and infiltrative capability. Rankings range from high infiltration rates in Group A to very low infiltration rates in Group D.
 

Impervious surface:

a surface in the landscape that impedes the infiltration of rainfall and results in an increased volume of surface runoff
 

Infiltration basin:

stormwater runoff impoundment designed to capture and hold stormwater runoff and infiltrate it into the ground over a period of days. This impoundment does not retain a permanent pool of water.
 

Low impact development (LID):

the application of non-structural practices at residential and commercial sites to reduce impervious cover, conserve natural areas, and use pervious area to more effectively treat stormwater runoff 
 

Media filters:

filtration of stormwater through a variety of different filtering materials whose purpose is to remove pollution from runoff
 

Nondegradation:

results in no increase in pollutant loads from a redevelopment site  
 

Offline practice:

a practice that does not receive all the stormwater flow from a conveyance system such as a pipe or channel, but rather only a portion of the flow as the result of a flow splitter or other diversion device
 

Onsite or regional treatment facility:

a stormwater treatment basin designed to treat the stormwater runoff generated from either the project site (onsite) or an area larger than the project site (regional)
 

Peak flow control:

controlling the timing and magnitude of the largest flow either leaving the site or flowing through the watershed, utilizing stormwater management techniques to avoid flooding or damage downstream
  

Perimeter control:

activities or practices designed to contain sediments on a project site
 

Permanent storage pool:

the volume in a pond or reservoir below the lowest outlet level, designed to settle out particles and nutrients for water quality treatment purposes.
 

Pollutant load:

the product of flow volume times pollutant concentration
 

Proprietary devices:

stormwater treatment devices which are privately developed and owned
 

Rate control:

controlling the rate that stormwater is released from localized holding areas into larger conveyance systems
 

Residential development/redevelopment projects:

typically result in smaller areas of impervious surface, typically in the range of 25 to 60 percent imperviousness. Examples of these projects include single family home construction, townhome construction, and apartment building construction.
 

Retention:

the permanent or temporary storage of stormwater to prevent it from leaving the development site 
 

Retrofit:

the introduction of a new or improved stormwater management element where it either never existed or did not operate effectively
 

Road construction or reconstruction projects:

include any project which results in the complete removal of the road surface, exposing the base, and/or removal of the vegetated surface within the road right-of-way. Examples include road widening projects, ditch work, road replacement and utility installation. Road overlay projects and  road resurfacing projects which do not disturb the road base will not be covered by the requirements of this policy.
 

Runoff or stormwater runoff:

under Minnesota Rule 7077.0105, subpart 41b, stormwater “means precipitation runoff, stormwater runoff, snow melt runoff, and any other surface runoff and drainage.” (according to the Federal Code of Regulations under 40 CFR 122.26 [b][13], “stormwater means stormwater runoff, snow melt runoff and surface runoff and drainage.”).  Stormwater does not include construction site dewatering.
 

Seasonally high water table:

the highest level the water table reaches during a given year or the highest level it has reached in the recent past as indicated by soil mottling or color changes. Methods for determining the seasonal high water table are given in Minnesota Rule part 7037.3300, subpart 5.
 

Sediment control:

The methods employed to prevent sediment from leaving the development site.  Sediment control practices include silt fences, sediment traps, earth dikes, drainage swales, check dams, subsurface drains, pipe slope drains, storm drain inlet protection, other appropriate measures, and temporary or permanent sedimentation basins.
 

Short circuiting:

occurs when an inlet and outlet from a pond or other device are very close to each other and the treatment capacity of the device is reduced 
 

Silt fence:

fence constructed of wood or steel supports and either natural or synthetic fabric stretched across an area of non-concentrated flow during site development to trap and retain on-site sediment due to rainfall runoff
 

Skimmer:

device used to take up or remove floating matter from the water’s surface
 

Soil amendment:

tilling and composting of new lawns and open spaces with a development site to recover soil porosity and bulk density, and reduce runoff 
 

Source water protection area:

an identified area with restricted or modified land use practices designed to protect public drinking water supply from the introduction of contaminants 
 

Stormwater (management) facilities:

include storm sewer pipes, ditches, ponds, infiltration basins, etc.
 

Surface sand filter:

consists of a pretreatment basin, a water storage reservoir, a flow spreader, and underdrain piping that treats stormwater runoff via filtration
 

Temporary protection (measure):

short-term methods employed to prevent erosion. Examples of such protection include straw, mulch, erosion control blankets, wood chips, and erosion netting.
 

Thermal protection:

techniques and practices such as infiltration and shading which act to preserve and protect the ambient temperatures of streams and waterbodies from temperature-raising effects of stormwater runoff
 

Trunk system:

The trunk creek system is the responsibility of the BCWMC and includes the Main Stem of Bassett Creek from Medicine Lake to the box culvert/tunnel; the North Branch from upstream of Co. Rd P to its junction with the Main Stem; the Sweeney Lake Branch from its source in Section 5, T117N, R21W to its junction with the Main Stem downstream of Sweeney Lake; and Plymouth Creek from the point where it intersects with Highway 55 in Section 17, T118N, R33W, to Medicine Lake. 
 

Under drain:

an underground drain or trench with openings through which the water may percolate from the soil or ground above 
 

Water quality pond: 

a collection area with a permanent pool of water for treating incoming stormwater runoff
 

Water quality volume: 

the permanent pool in a water detention pond
 

Wetland: 

defined in Minn. R. 7050.0130, subp. F and includes those areas that are inundated or saturated by surface water or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas. Constructed wetlands designed for wastewater treatment are not waters of the state; to be a wetland the area must meet wetland criteria for soils, vegetation, and hydrology as outlined in the 1987 U.S. Army Corps of Engineers Wetland Delineation Manual.
 

Wet detention basin: 

a collection area with a permanent pool of water for treating incoming stormwater runoff. See water quality pond.
 

Wet vault: 

a vault stormwater management device with a permanent water pool generally 3 to 5 feet deep used to treat stormwater runoff

*Natural Resources Conservation Service (NRCS): division of the United States Department of Natural Resources, formerly known as Soil Conservation Service (CSC) 

Soil Conservation Service (CSC): division of the United States Department of Agriculture, currently known as the Natural Resources Conservation Service (NRCS)