Anoka County Ditch 53-62, Branch 1 Lateral 1 Repair Memorandum Date: November 2, 2016 Project: R

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1 Technical Memorandum To: Cc: From: Phil Belfiori, District Administrator, RCWD Tom Schmidt Mark Deutschman Nancy Stowe, P.E. Through: Chris Otterness, P.E. Subject: Anoka County Ditch 53-62, Branch 1 Lateral 1 Repair Memorandum Date: November 2, 2016 Project: R INTRODUCTION On August 24, 2016, the RCWD received a letter from the City of Blaine (City) requesting assistance on resolving a drainage issue affecting the back yards of homes located between 107 th Avenue and Marmon Street, south of I-35W and Eastside Park, which are within the contributing drainage area to Branch 1, Lateral 1 of the Anoka County Ditch public drainage system (hereafter referred to as Lateral 1) (see Figure 1). The City s letter suggested that flooding on these properties may be a result of obstructions within Lateral 1, and specifically related to a weir located as part of the outlet structure of the Home Depot stormwater pond. The purpose of this memorandum is to determine whether the public drainage system is contributing to drainage problem, and if so, present the contributing factors to the drainage efficiency of the public ditch system, repair alternatives which could improve drainage, a qualitative assessment of the benefits and feasibility of each alternative, and an opinion of probable construction cost for each alternative EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 1 OF 7

2 Figure 1: Project Location 6901 EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 2 OF 7

3 ASSESSMENT OF THE DRAINAGE SYSTEM PERFORMANCE Although the letter from the City referred specifically to the weir controlling the outflow from the stormwater pond behind Home Depot, other possible contributor factors related to the drainage efficiency of Lateral 1 have been evaluated. These contributing factors may include elevations of downstream control structures, flow capacity of structures along the public ditch, obstructions to flow, grade of ditch profile, and seasonal rainfall patterns. To investigate these factors, a field visit, detailed survey, review of ditch profile, and review of rainfall records were completed, and the results are presented in the following sections. SURVEY OF CONTROL ELEVATIONS On September 30, 2016, Houston Engineering, Inc. (HEI) conducted a survey of the location experiencing poor drainage between 107 th Avenue and Marmon Street, as well as key hydraulic control points and water surface elevations downstream to Lateral 1 and north along Lateral 1 to the Lochness Lake outlet. The results are summarized in Table 1 (from upstream to downstream). Table 1: Hydraulic Control Elevations Downstream of the 107 th Ave. / Marmon St. Backyard Area Map Location # Survey Location Elevation 1 (feet) 1 Culvert outlet in backyard area Control structure for ponds in East side park (weir crest) 36 RCP culvert crossing I-35W RCP stormsewer crossing Home Depot parking lot Outlet Structure for Home Depot pond (weir) Lochness Lake outlet (24 CMP) Note that the weir controlling the outflow from the pond behind Home Depot is nearly 2.4 feet below the culvert serving as the outlet for the backyards between 107 th Avenue and Marmon Street, and that the outlet of Lochness Lake is nearly 2 feet below this culvert. Therefore, the elevations of structures along the public drainage system are not directly resulting in the drainage issues reported in area between 107 th Avenue and Marmon Street SURVEY OF WATER SURFACE ELEVATIONS HEI also conducted a survey of water surface elevations on September 30, 2016 which are presented in Table 2. 1 All elevations provided herein are based on North American Vertical Datum of 1988 (NAVD 88) EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 3 OF 7

4 Map Location # Table 2: Surveyed Water Surface Elevations on September, Survey Point Location Water Surface Elevation (feet) 1 Back yards along 107 th Ave./Marmon St Ponds in Eastside Park Upstream of 36 RCP crossing I-35W Downstream of 36 RCP crossing I-35W Pond south of 109 th (behind Home Depot) Wetland north of 109th Lochness Lake The elevations indicate that on September 30, water levels in Lateral 1 and upstream to Eastside Park were not being affected by the condition of the public drainage system, but rather by water levels downstream of Lochness Lake (i.e. tailwater). Because of the substantial volume of rainfall occurring during the summer and early fall of 2016, there has been an insufficient duration of dry weather to allow water levels in the Main Trunk of ACD to subside. Note that the water surface elevations are only reflecting the conditions on September 30. Substantially higher water elevations were reported following the large rainfall event occurring on September 21. A homeowner along 107 th Avenue set a stake at the highest water surface elevation observed that day. HEI later surveyed the elevation of the stake at According to the landowner, the water level lowered from the peak elevation to a level marked by discoloration of the grass, surveyed to be , for a few days. Five days after the rainfall event, the water started receding relatively rapidly down to a surveyed elevation September 30. The sustained water level followed by a rapid draining of the backyards can be attributed to conditions at the Home Depot pond outlet and subsequent maintenance activities. The outlet structure to the pond behind Home Depot was designed with a grate covering the opening in the manhole to prevent vegetation and other debris from collecting in the manhole. However, the grate is prone to obstruction from floating debris and vegetation, substantially decreasing flow. On September 26 (the same day the landowner recalled the lowering of water levels in his backyard, the Anoka County Highway Department removed vegetative material from in front of the outlet structure grate as an emergency response to prevent the flooding of 109 th Avenue (County Road 12). PROFILE OF PUBLIC DRAINAGE SYSTEM The Main Trunk of ACD was designed with a flat (0%) gradient from Lexington Avenue to Branch 1 (nearly 2 miles). Flow requires a positive grade,so the standing water depths in the open channel get substantially deeper the farther upstream you walk in the system. More importantly, such a flat gradient in the trunk of the system substantially increases the time it takes the system to subside after a rainfall and return to a 6901 EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 4 OF 7

5 state of relative equilibrium. Frequently recurring rainfalls (as the District experienced this summer) exacerbate this condition (see below). RAINFALL PATTERNS Rainfall amounts have been much higher than average over the past three years, which also contributes to high water levels experienced by landowners draining to Lateral 1. Table 3 compares the summer rainfall totals of the last three years to the 30-year normal rainfall totals. Table 3: Measured Precipitation Monthly Totals (Source: MnDNR, Twin City International Airport) Year Rainfall Totals (inches) Jun Jul Aug Sep 30-yr Normal CONCLUSIONS CURRENT FUNCTION OF DRAINAGE SYSTEM The field investigation documented above enables us to draw the following conclusions regarding the capacity and function of Lateral 1: 1. The primary cause of high water levels upstream of 109 th Avenue is related to maintenance, and specifically the plugging of the grate on the manhole outlet structure of the pond behind Home Depot. This factor could be alleviated through maintenance, modifying the pond outlet to reduce the likelihood of clogging, or realigning the public drainage system around the pond. 2. Other factors which substantially contribute to sustained high water levels in the system include: a. The flat gradient of the public drainage system as it was originally constructed, and b. Recent wet weather patterns We are not aware of any feasibility solutions to either of these factors. 3. The pond outlet behind Home Depot, when functioning as designed (i.e. is not clogged), reduces downstream flows for small rainfall events but has little effect on upstream properties. This is by design, since the pond is intended to provide storage and water quality treatment. Modifying the outlet structure to increase capacity for low flows (for example, removing the v-notch weir) will decrease the water elevation for frequent rainfalls (less than 2-year recurrence) EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 5 OF 7

6 REPAIR ALTERNATIVE Based on the conclusions above, we have conceptualized four alternatives that are designed to reduce the likelihood and magnitude of recurrent flooding resulting from the condition of the public drainage system. ALTERNATIVE #1 MAINTAIN OUTLET STRUCTURE OF POND FREE OF OBSTRUCTION The inlet grate of the pond outlet structure behind Home Depot may continue to be maintained as it becomes obstructed with floating vegetation and debris. This would require frequent monitoring and maintenance, which results in substantial long-term costs. As with roadway and driveway crossings, this structure is not a component of the public drainage system, and as such, the RCWD is not responsible for the maintenance of this structure. However, the condition of the structure affects the public drainage system, so the RCWD is likely to be requested to support maintenance efforts as the structure becomes clogged again in the future. Because of the challenges in identifying and enforcing maintenance responsibility for the structure, this alternative is not considered a feasible option. ALTERNATIVE #2 INSTALL WOODEN BAFFLE ON OUTLET STRUCTURE This alternative consists of installing a wooden baffle mounted to the front of the outlet structure to act as a skimmer structure, inhibiting vegetation from reaching the grate and obstructing flow. This alternative would reduce maintenance requirements for longer periods of time, but would still require periodic removal of debris. Wooden baffle structures are very susceptible to ice damage, and may require periodic replacement. ALTERNATIVE #3 REPLACE POND OUTLET STRUCTURE This alternative consists of removing the entire outlet structure on the pond and replacing it with a new outlet structure that includes a) a low inlet pipe (below the normal water elevation and floating debris); b) an emergency overflow skimmer; and c) a baffle wall at the same elevation as the existing v-notch weir. This alternative would substantially decrease maintenance efforts by keeping vegetation and bog material away from the primary inlet, yet preserve the design and function of the current outlet as it was originally constructed. ALTERNATIVE #4 EXTEND PIPE RUNNING NORTH ALONG EAST SIDE OF HOME DEPOT TO 109 TH AVENUE This alternative consists of extending the stormsewer under the Home Depot parking lot north under 109 th Avenue, bypassing the pond and the outlet structure. This alternative would separate the function of the pond and at the public drainage system, essentially eliminating maintenance responsibilities for the District. Flow capacity would be increased due to the lack of restriction provided by the current pond outlet. However, it would also result in a substantial portion of the Home Depot site which is currently treated by the stormwater pond to pass untreated as it flows north into the Lochness Lake public water. As such, we do not consider this a feasible alternative EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 6 OF 7

7 PRELIMINARY OPINION OF PROBABLE CONSTRUCTION COST A Preliminary Opinion of Probable Construction Cost (POPCC) has been estimated for each alternative and is tabulated in Table 4. Please note that the cost for Alternative 4 does not include wetland mitigation, which may be required for that alternative. The costs in Table 4 include engineering cost and a 20% contingency, but do not include future maintenance costs. Alternative 1 will result in greatest future maintenance cost, with Alternatives 3 and 4 requiring the least.. Table 4: Preliminary Opinion of Probable Construction Cost for Each Alternative Alternative Estimated Cost ($) 1 Maintain outlet structure of pond free of obstruction 0 2 Install wooden baffle on Outlet Structure $15,600 3 Replace pond outlet structure $36,000 4 Extend pipe running north along east side of Home Depot to 109 th Ave. $144,400 RECOMMENDATION Because it requires the least future maintenance of the feasible alternatives, we recommend the District proceed with Alternative 3 Replacing the pond outlet structure. Although this alternative has greater initial cost than a wood baffle wall weir, it will require less frequent future maintenance and replacement, and will decrease the duration of peak flows for frequent (<2 year recurrence) rainfall events. This alternative will not require permitting under the Wetland Conservation Act, but may require a right-of-way permit from Anoka County EAST FISH LAKE RD, STE 140 MAPLE GROVE, MN PAGE 7 OF 7

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