Overview of unified stormwater sizing criteria

Overview of unified stormwater sizing criteria
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Target	Current State or Local Sizing Criteria
Recharge (Vre) or Infiltration	Regular Waters: None required, although recharge and infiltration are strongly encouraged through better site design and stormwater credits. CGP Special Waters: Full infiltration of the excess runoff volume from the two year-24 hour storm event is one option contained in the CGP to meet the requirement for temperature control for trout streams. Other Sensitive Receiving Waters: Recharge recommended as a local option. Recharge may be restricted or prohibited in certain settings, as outlined in the Recharge and Infiltration Criterion below.
Water Quality (Vwq)	Regular Waters: Apply MPCA Sizing Rule 1 or 3 in 2003 GCP, depending on BMP used. CGP Special Waters: Apply MPCA Sizing Rule 2 or 4 in 2003 GCP, depending on BMP used. Additional BMPs required under CGP Appendix A (B) for special waters. Other Sensitive Receiving Waters: Recommend expanded sizing criteria for most-sensitive lakes, trout streams (in CGP), susceptible wetlands and impaired waters, as described later in this section.
Channel Protection (Vcp)	Regular Waters: No current state requirement. It is recommended that communities adopt a criterion for either 24-hour extended detention of the 1-year, 24-hour design storm or one-half of the 2-yr, 24-hr pre-development peak flow when revising or adopting local stormwater ordinances for peak flow control (and eliminate two-year peak discharge requirements). CGP Special Waters: One- and two-year design storm peak discharge and volume control required in four special water categories (wilderness, trout lakes, lake trout lakes, and scientific and natural areas). Other Sensitive Receiving Waters: 12-hour detention of water is recommended as the most for discharge to trout streams (to prevent heating), while other sensitive receiving waters should maintain the 24-hour minimum.
Overbank Flood (Vp10)	Current Local Requirement: Varies, but typically involves either 10-year design storm peak discharge control, or a combination of 10- and 25-year design storm peak discharge control. CGP Special Waters and Other Sensitive Receiving Waters: No additional requirements.
Extreme Storm (Vp100)	Current Local Requirement: Varies, but frequently involves 100-year design storm peak discharge control. In other cases, peak discharge control is waived if development is excluded from ultimate 100-year floodplain, or an acceptable downstream hydrologic analysis indicates it is not needed. Stormwater BMPs must be designed to provide safe overflow of the 100 year-peak discharge even if extreme storm control is not required at the site. CGP Special Waters and Other Sensitive Receiving Waters: No additional requirements.