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External Correspondence To: Phil Belfiori District Administrator, RCWD Date: January 26, 2011 Cc: File R105555- 080.003 Tom Schmidt, RCWD Elizabeth Hosch, RCWD Houston Engineering Inc. From: Chris Otterness, P.E. Through: Mark Deutschman, P.E., Ph.D. Subject: Effects on Water Quality and Volume Century Farms North — Permit 02 -0129 HEI Project No. R105555- 080.003 Task 001 RCWD Account No. 80 -03 -4500 Introduction The purpose of this memorandum is to provide the Rice Creek Watershed District (RCWD) with a qualitative assessment of impacts to water quality and runoff volumes resulting from alternatives for restoring function to an infiltration basin (hereafter referred to as "Basin E2 ") in the Century Farms North development. A memorandum entitled Drainage Alternatives for Centzny Fa rnzs and dated January 11, 2011 detailed the history and cause of the non - functioning infiltration basin and outlined two potential alternatives as corrective measures. In a subsequent meeting with City of Lino Lakes staff; a third option of accepting the E2 infiltration basin as-is (i.e. a "no- action" alternative) was discussed and considered. This memorandum is intended to qualitatively address the water quality and volume consequences of a "no- action" alternative compared to the "design" conditions and the other two alternatives capable of correcting the basin's function. Impacts on Peak Flow Rates and Runoff Volumes Per RCWD rule C (effective August 1998 to February 2008), Basin E2 was designed with the intent to provide some infiltration of stormwater runoff and reduction of downstream runoff volumes from its contributing drainage area of 22- acres. Specifically, the design standard under the permit was a requirement to infiltrate runoff from a 0.34 -inch precipitation event for the contributing impervious surface area; i.e., 255 -cubic feet for Basin E -2. Even though borings were submitted and reviewed as part of the permit review process, due to tailwater and elevated ground water levels this basin currently provides little or no infiltration for the design volume. Because of constraints provided by the existing elevations and structures at this location, we are not aware of a feasible alternative to retrofit the basin or the development as a whole to achieve the infiltration required by District rules as they existed at the time of the permit. The two main issues relative to stormwater runoff are maintaining the pre - development peak rate of runoff and achieving the volume control standard. The contributing drainage area to Basin E2 is less than one fifth of the total project area of approximately 122 acres for the entire Century Farms North development and is a small fraction of the overall Century Farms development (which is approximately 280 acres). Because of the considerable storage provided by four open water (i.e., wet) ponds immediately downstream of Basin E2, the storage volume provided by Basin E2 (or lack thereof), has no quantifiable impact on water surface elevations or peak flow rates downstream of the basin. The E2 infiltration basin was intended to intercept