Review Updated Estimated Costs for Sunfish Lake Sedimentation Basin Improvements

Purpose:
The property owner at 14520 Sunfish Lake Drive NW is requesting that the City reconstruct specific stormwater treatment improvements impacting a portion of their back yard. The requested work includes constructing a settling basin at the end of the storm sewer outlet from Sunwood Drive where it intersects 145^th Lane NW for the purpose of reducing the frequency and volume of stormwater runoff running through the existing drainage swale along their rear lot line, and removing the existing cattails and other types of vegetation commonly found in wetlands from the swale and restoring the swale to a lawn type condition using sod, though seeding could be acceptable if done correctly. Attached Figure 1 shows 2014 aerial photography of the property and the surrounding area including parcel lines, property addresses, and street names, and identifies the locations of the swale and various storm sewer outlets in relation to the property. Attached Figure 2 shows the concept design for the requested improvements.

Background:
The property owner at 14520 Sunfish Lake Drive NW has contacted the City on numerous occasions since the mid-2000’s regarding a drainage swale located within a 20-foot wide drainage and utility easement that runs across the back edge of his lot. The lot backs up to Sunfish Lake. The lake border adjacent to the property is predominantly cattails, and the edge of open water is approximately 100 yards northwest of the property.

This property is located in the Sunfish Lake Estates subdivision which was platted in 1996. The plat dedicated the 20-foot drainage and utility easement parallel to the rear lot line to accommodate a drainage swale that connects the storm sewer outlet from Sunwood Drive to an outlet control structure located near the southwest corner of the lot. Attached is a certificate of survey that shows the lot boundaries and locations of the rear lot drainage easement and berm along the back side of the swale.

The drainage swale and outlet control structure are not shown on the final Grading and Utility plans filed for the plat. The structure and the swale are shown on the plans the City prepared for the extension of Sunwood Drive. A copy of this plan is attached.

City files show that the City constructed Sunwood Drive around the time the plat was filed. This provided access to the site. An EAW was required prior to construction of Sunwood Drive. The Minnesota Department of Natural Resources (DNR) provided a response letter noting that the stormwater from the roadway should not be discharged directly to the wetlands adjacent to the roadway. A copy of this letter is attached with this text highlighted in yellow. The as-built plans for Sunwood Drive show that the storm sewer is directed to the swale on the back side of the lot. This swale provides treatment for the stormwater runoff from Sunwood Drive prior to discharging the runoff into Sunfish Lake via the outlet control structure.

According to the property owner, they were able to mow the swale and outer berm for the first several years after moving in around 1999, and standing water first appeared in the swale in the mid to late-2000’s with the frequency of standing water increasing over time. The property owner contacted the City about the standing water in the late-2000’s, after which Staff visited the site and informed the property owner that the swale was designed for this purpose, and that the drainage and utility easement was sized to accommodate the water.

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 145^th 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.
 
This item was last discussed with the Public Works Committee on September 15, 2015.  At that time, staff was directed to develop a more complete cost estimate for the requested improvements, and to present the updated estimated costs to the Committee for further discussion.  Attached is an updated cost estimate which totals $46,782.75.  This includes all estimated costs required to construct the settling basin shown in attached Figure 4, to construct required storm sewer modifications, to fill in the drainage swale across the rear of the Bergevin property, to remove the storm sewer structure at the end of the swale on the west side of the Bergevin property, to restore all disturbed areas using sod (Bergevin property only) and a wetland seed mix (all other areas), and to vacate the easement the swale was located in.  The estimated costs also include 10% contingency costs for unknowns and miscellaneous pay items not included in the cost estimate, as well as 23% indirect costs for administrative, engineering, finance, and legal fees.  Staff previously estimated construction costs for the proposed settling basin and restoration of the swale would cost between $15,000 and $45,000.
 
A link to this case was sent to the property owner, who indicated they will be present at the meeting Tuesday evening.

Approximately 20 minutes for presentation and discussion.

Staff believes that the water observed in the bottom of the swale in recent years is most likely not from storm water runoff but is rather groundwater rising in conjunction with the lake level rising.

Permits from both the DNR and the Lower Rum River Watershed Management Organization would be required to complete the requested improvements as the settling basin and overflow would be located within wetland 114P (see attached Figure 3) which requires a WMO permit, and the work would be occurring below the Ordinary High Water Level which requires a permit from the DNR.  In addition, wetland mitigation credits would need to be purchased at a 2:1 ratio to replace the impacted wetland due to construction of the settling basin.  This is reflected in the attached estimated costs.

Staff recommends that the property owner be required to pay for the requested work since it solely benefits the use of his property. However, the City could consider contributing an amount commensurate with the estimated long-term savings associated with maintaining a settling basin next to a public street versus maintaining a linear drainage swale along the rear of a private property. Based on a 50 year maintenance schedule, this cost would be approximately $500 assuming the City would dredge and restore the turf in the swale twice, once every 25 years.
 
If the settling basin is constructed Staff also recommends filling in and sodding the swale across the rear yard of the subject property, removing the storm sewer structure on the west end of the swale, and vacating the existing drainage easement that encumbered the swale and drainage structure to prevent the need to maintain this infrastructure since a public purpose is no longer being served.  However, the easement over the sanitary sewer line on the east side of the property would still be retained.

Staff is requesting Committee direction on whether this request should be forwarded to Council for consideration of approval and, if so, whether the Committee has a recommendation on whether the City should contribute anything towards the project.  If the Committee is in agreement with the property owner’s request, Staff requests that the Committee provide a formal recommendation for the City Council since this item would require Council approval since it is not a budgeted project.