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 enginerring 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 stormwa 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) Example
Permanent Pool Volume (Vpp) Example
Pre-Development Conditions Example
Recharge Volume (Vre) Example
Special Waters Example
Total Storage Volume (Vts) Example
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.

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