Agenda - Public Works Committee - 09/15/2015

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The property owner contacted the City again in 2014 again noting his concerns that water was constantly standing in the swale, making it difficult to maintain the swale and rendering that part of his yard unusable. Staff met the resident on site and verified that the swale and standing water was being contained within the drainage and utility easement, and that the storm sewer system was functioning properly and was not plugged. Staff met with the resident again in May of 2015, at which time Staff was asked if the swale could be redirected or removed. The property owner also inquired whether the volume of stormwater runoff being directed through the swale on his lot might have increased due to the new development across the street (Village of Sunfish Lake Subdivision). Staff researched the drainage plans for Village of Sunfish Lake and found that the two stormwater ponds for the Village of Sunfish Lake were constructed as land -locked basins and were sized to contain back to back 100 year storms. One of the ponds is located south of Sunwood Drive and east of 145th Lane, and the second pond is located in the southwest quadrant of Sunwood Drive and Radium Street. Stormwater from the Village of Sunfish Lake will therefore only reach the swale in extreme conditions. Staff also contacted Anoka County per your property owner's request so see if they might have increased stormwater runoff into Sunfish Lake over the last decade or so due to road construction or maintenance projects along Sunfish Lake Boulevard/CR 57. The County has no records of any such projects that would have resulted in a significant quantity of groundwater or surface water being directed into Sunfish Lake for extended periods of time that may have resulted in increased lake levels. The Minnesota Department of Natural Resources (DNR) maintains a database of lake levels, including Sunfish Lake. The records cover the period from 1985 to 2015, a span of 30 years. The Ordinary High Water level (OHW) determined by the DNR is 861.5. The record shows that from 1985 — 1988 the water level was up to 1.5 feet above the OHW. The water level dropped to 857.2 in 1989 before rebounding to 861.6 in 1991. The lake level dropped below 860 from 1994 to 2005, reaching a low of 856.7 in 2000. The lake level has generally been above 860 since 2011. The outlet structure for the swale is an inverted siphon, the inlet and outlet pipes are lower that the invert of the manhole connecting the pipes. The outside pipe ends are at 860.0 while the common connection between the pipes is at 861.3. Water must therefore build up to 861.3 in the swale before it is discharged into Sunfish Lake through the outlet control structure. Staff also obtained rainfall records for the same time period. The lake elevation generally corresponds with the annual rainfall totals, with the lake level rising when precipitation is above the 30 year average for several years in a row. Staff then investigated whether a structural water quality treatment structure could be used in place of the swale. These devices function by circulating the stormwater inside a large diameter manhole. There are baffles inside which direct the stormwater around the edge of the device. The soil particles settle out of the water as it flows around the structure. The rate at which particles settle out is related to the distance the water travels and speed at which the water is flowing. The devices are designed to treat a certain quantity of flow. If the inflow exceeds the designed flow then water flows over a weir and passes out of the structure without treatment (bypass flow). The storm sewer outlet from the street is at elevation 859.6, two (2) feet below the OHW. Staff contacted a supplier of these devices and asked if the device would function properly when the outlet is submerged. The supplier said the device would function in bypass mode and would not provide any treatment. Staff also contacted the St. Anthony Falls Laboratory and asked staff there if there had been research on the effectiveness of structural stormwater treatment devices when the outlet is submerged. They have not done a research project like this; however, they generally agreed with the manufactures representative that the device would function in bypass mode when the outlet is submerged. Bypass mode does not provide any treatment of the storm water passing through the device so this would not be a viable option. City staff also discussed the option of converting the swale to a raingarden feature to enhance aesthetics but the property owner was not interested in this option as it would not result in more useable space in their back yard. The only option that the property owner is therefore interested in pursuing is to construct a settling basin at the storm sewer outlet north of Sunwood Drive to treat the roadway runoff before it is discharged into the wetland. Timeframe: