2006-08-22_PWETC_AgendaPacket - Laserfiche WebLink

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The eXistIng stormwater management standards do not impose bounce or inundation period standards for waterbodies except for landlocked lakes. It is recommended that District Rules mandate hydroperiod standards for the 2-year event outlined by wetland susceptibility as defined by the State of MN Stonnwater Advisory Group, "Stormwater and Wetlands Planning and Evaluation Guidelines for Addressing Potential Impacts of Urban Stormwater and Snowmelt RZ/IlojJ on Wetlands ", June 1997. These standards are summarized in the following table. T bl 2 P dW I dM S d d a e rODose et an anaeement tan ar s Hydroperiod Highly Moderately Slightly Least- Susceptible Susceptible Susceptible Susceptible Standard Wetlands Wetlands Wetlands Wetlands Storm Bounce Existing Existing plus 0.5 Existing plus 1.0 No limit feet feet Discharge Rate (out Existing Existing Existing or less Existing or less of wetland) Inundation Period Existing plus I Existing plus 2 Existing plus 7 for Existing 2-year Event day days days Run-out Control o to 1.0 feet o to 4.0 feet Elevation No change No change above existing above existing (free flowing) run out run out Run-out Control Above Above Above Above Elevation delineated delineated delineated delineated (landlocked) wetland wetland wetland wetland Stonnwater management standards could be flexible for utilization of Least and Slightly Susceptible wetlands for water quality treatment and rate control. If allowed, a pre-treatment basin equivalent to NPDES sediment removal standards should be provided, inundation and bounce criteria discussed above should still be met, and the combination of the pre-treatment basin and the wetland should satisfy the District water quality treatment and rate control standards before discharge of runoff from the site. Pre-Development Baseline In an attempt to reduce the frequency and duration of flooding within the District, the design criteria for rate control are proposed to be an assessment between proposed conditions and a base-line land use condition or existing conditions (which ever is stricter). This is recommended since the majority of large-scale development occurring in the District is converting existing agricultural land to low- and medium-density residential subdivisions. Curve Numbers (CNs) published in the Minnesota Hydrologic Guide are higher for row and small grain crops than CNs for residential development up to Y. acre lots. Under this frequent development scenario, using the existing agricultural land CNs, it is contentious enforcing rate control. Proposed base-line land use condition CN values for each major soil type are proposed below: Table 3. Proposed Pre-Development Baseline CNs Emmons & Olivier Resources, Inc. Page 7 of 8