Key stormwater sizing terminology

A review of key stormwater sizing terminology.
Link to this table

Term	Definition
Better Site Design (BSD)	Better site design refers to the application of non-structural practices at new development sites to reduce site impervious cover, conserve natural areas, and use pervious areas to more effectively treat stormwater runoff. Also know as low impact development.
Channel Protection (VcpVolume)	Refers to the recommended runoff storage volume needed to control post-development bankfull flow velocities so they do not increase erosion in downstream channels. typically, detention and/or extended detention of intermediate sized storms (0.5 to 2.0 inches of rainfall) are used for this purpose. The channel protection volume is denoted as Vcp.
Design Storm	An engineering term for a single rainfall event with a defined intensity, duration and statistical recurrence interval commonly ranging from 0.5 to 100 years. These single event storms are based on long-term rainfall data and are used in hydrologic models to predict the peak discharges and runoff volumes associated with each type of storm. Unless otherwise indicated, all design storms discussed in the Manual has a 24-hour duration and a Type II distribution.
Detention Time	Various definitions for detention time exist in hydraulic manuals and in help screens on computer models. For this Manual, a simple method of computing detention time is recommended. Detention time is equal to the length of time starting at basin full (for a specific design storm) and ending either when the basin is dry (filtration or infiltration) or the basin has attained normal water level (stormwater ponds or constructed wetlands).
Extreme Storm Volume (Vp100	The greatest runoff storage volume is used to the peak discharges of infrequent but very large storm events to pre-development levels. The 100-year design storm, which has a statistical recurrence interval of occurring once in one hundred years, is used by most communities. Extreme floods can cause catastrophic damagae and even loss of life. The storage volume needed to store and detain them is denoted as Vp100. Note that stormes more "extreme" than the 100-year event do occur in Minnesota. The extreme term is used relative to other volume terms for perspective.
Hydrologic Soil Group (HSG)	HSG is an USDA-NRCS designation given to different soil types to reflect their relative surface permeability and infiltrative capability. Group A soils have low runoff potential and high infiltration rates. Group B soils have moderate infiltration rates. Group C soils have low infiltration rates. Group D soils have high runoff potential with very low infiltration rates and consist chiefly of clay soils (TR-55, 1986).
Other Sensitive Receiving Waters	In addition to special waters defined in the CGP, there are other receiving waters that merit additional management attention because of their sensitivity, as defined by various state and local entities. Recommended stormwater criteria exist for Lakes (most sensitive and sensitive) Wetlands (susceptible and non-susceptible) Drinking water source areas (ground and surface water) Impaired waters (computable and non-computable.
Overbank Flood Volume (Vp10)	Refers to the runoff storage volume needed to prevent an increased frequency of floods that spill out of the channel and onto the floodplain where they may cause damage to the conveyance systems, property and infrastructure. Overbank flooding is normally controlled by detention of post-development 10-year storm so that pre-development peak discharge rates (as defined by state or local agencies) are maintained, and is denoted by Vp10 (assuming the local review authority requires control of the 10-year storm event).
Permanent Pool Volume (Vpp)	The CGP requires that all wet sedimentation basins contain a permananet pool with a volume of 1,800 cubic feet of storage for each acre that drains to the basin. This equates to 1/2 inch of runoff per acre. The permanent pool must reach a minimum depth of three feet, stay below 10 feet and be configured to minimize scour and resuspension of solids.
Pre-Development Conditions	The term pre-development conditions can be interpreted in many different ways. The MPCA uses land cover conditions immediately preceding the current development project as the CGP pre-development condition, whereas many other local and watershed managers use a more natural definition, such as meadow or woodlands in good shape (stated in a manner to help in CN selection). Obviously the CGP version will usually result in a smaller net runoff increase for most land that has anything but a natural cover. It is recommended that the CGP definition be used for its intended purpose as part of state permit issuance, but that the more natural pre-development condition be used if a stormwater manager wants to assume a more conservative condition.
Recharge Volume (Vre)	Refers to the recommended volume of runoff which should be spread over pervious areas and otherwise infiltrated into the soil to promote groundwater recharge. The recharge volume is denoted as Vre and is normally included as part of the water quality volume.
Special Waters	A list of eight categories of receiving waters are specifically designated as "special waters" in the CGP. Additional BMPs and enhanced runoff controls are required for discharges to the following special waters: Wilderness areas Mississippi River (Lake Itasca through Morrison County) Scenic or recreation river segments lake Superior Lake trout lakes Trout lakes Scientific and natural areas Trout streams
Total Storage Volume (Vts)	For ponds built under the requirements of the CGP, the total storage volume required is the sum of the permanent pool volume (Vpp) plus the water quality volume (Vwq).
Water Quality Volume (Vwq)	Generic term for the storage volume used to capture, treat and remove pollutants in stormwater runoff. It is normally expressed as a volume (watershed-inches or acre-feet) and is denoted by Vwq. For ponds and wetlands, the MPCA CGP defines it as the volume of live storage above the permanent pool (above the dead storage) used for water quality. For non-pond BMPs, MPCA defines the water quality volume in the same manner as the general definition above.