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Item No: 7E Meeting Date: March 24. 2014 'j\ /(off MS ,'� FE T Type of Business: Council Business 1Vl lJl.'111J � v iL �'V Administrator Review: City of Mounds View Staff Report To: Honorable Mayor and City Council From: Nick DeBar, Director of Public Works Item Titie/Subject: Resolution 8230, Authorizing Barr Engineering Company to Perform Phase 1 Sediment Sampling for Silver View Pond Background: As a requirement of the Storm Water Pollution Prevention Program (SWPPP), the City must manage, operate, and maintain the stormwater system in a manner to reduce the discharge of pollutants "to the maximum extent practicable". The SWPPP is a requirement of the Municipal Separate Storm Sewer System (MS4) general permit, which gives owners or operators of municipal separate storm sewer systems approval to discharge stormwater to lakes, rivers, and wetlands in Minnesota. This permit is mandated by the federal regulations under the Clean Water Act and administered locally by the Minnesota Pollution Control Agency (MPCA). Silver View Pond was created by excavation in 1982 to provide stormwater storage for developments proposed within its 146 -acre drainage area. The pond is approximately 9.3 acres in surface area and is located in Silver View Park. The pond was designed and constructed with three distinct arms (or basins) with narrow connections between each to slow the movement of sediment and nutrients toward the outlet (located near Long Lake Road). The permanent pool in the pond provides water quality treatment, while the storage ability above this permanent pool elevation provides rate control for flood protection. The MPCA recommends that stormwater ponds like Silver View Pond be evaluated for sediment removal when the permanent pool volume is reduced to 50% by sediment storage. The timeframe for sediment to build up to this level varies depending on many factors, but a rule of thumb is roughly 30 years. Excess sediment in the pond can decrease its ability to treat stormwater runoff to acceptable water quality levels. The City Council authorized Barr Engineering Company to perform a sediment survey of Silver View Pond in October 2011 (Res. 7838). In their May 2012 assessment report, Barr determined that approximately 38% of the originally excavated pond volume (permanent pool volume), or 45,335 cubic yards, is filled with sediment deposits. These sediment deposits were carried into the pond with stormwater runoff flowing into the pond and precipitated to the pond bottom. Barr's report also detailed some recommendations to correct "short-circuiting" of the basin due to the close proximity of storm inlets to the basin outlet. Discussion: Although there is not an immediate need to remove sediment from Silver View Pond for treatment effectiveness, there will be significant cost when that time comes. Barr estimates the cost to remove and dispose of the sediment to range from $997,363 to $2,040,062 (2012 dollars). This estimate is dependant upon the level of contamination in the sediment that determines howand where the sediment can be disposed of. There are three sediment disposal categories: 1. Level 1: Residential — excavated material may be reused as fill on any site; 2. Level 2: Industrial — excavated material may be reused as fill only on industrial sites; and 3. Level 3: Landfill — excavated material must be disposed of at an appropriate landfill facility. In an effort to budget for the inevitable sediment removal from the pond, a sampling and testing assessment of the sediment is proposed. This process is performed in a two-phase approach to