Overview of unified stormwater sizing criteria

Regular Waters: None required, although recharge and infiltration are strongly encouraged through better site design and stormwater credits.

CSW Permit Special Waters: Full infiltration of the excess runoff volume from the two year-24 hour storm event is one option contained in the CSW Permit 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.

Regular Waters: Apply MPCA Sizing Rule 1 or 3 in 2003 CSW Permit, depending on BMP used.

CSW Permit Special Waters: Apply MPCA Sizing Rule 2 or 4 in 2003 CSW Permit, depending on BMP used. Additional BMPs required under CSW Permit Appendix A (B) for special waters.

Other Sensitive Receiving Waters: Recommend expanded sizing criteria for most-sensitive lakes, trout streams (in CSW Permit), susceptible wetlands and impaired waters, as described later in this section.

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).

CSW Permit 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.

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.

CSW Permit Special Waters and Other Sensitive Receiving Waters: No additional requirements.

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.

CSW Permit Special Waters and Other Sensitive Receiving Waters: No additional requirements.