Shingle Creek Watershed Management Commission Wetland 639W Outlet Modifications Summary Feasibility Report

Transcription

1 Shingle Creek Watershed Management Commission Wetland 639W Outlet Modifications Summary Feasibility Report The proposed project is the modification of the outlet of Wetland W to reduce phosphorus export from the wetland to Upper Twin Lake, an Impaired Water that exceeds the state standard for total phosphorus concentration. The proposed modification consists of the construction of a sheet pile weir just upstream of the outlet to increase storage in the wetland, and the construction of an upstream outlet structure and high flow overflow channel. Wetland 639W is shown in Figure 1. Project Background A network of storm sewers and a channel known locally as Twin Creek conveys stormwater from approximately 1,010 acres of fully developed mixed use land in the cities of Crystal, Brooklyn Center, Brooklyn Park, and New Hope to Wetland 639W (Figure 2). In addition, approximately 324 acres of land on the Crystal Airport drains overland to the wetland, which is located on property owned by the Metropolitan Airports Commission (MAC). Drainage is conveyed by sheet flow across the main basin of the wetland, and is discharged from the basin by Twin Creek to Upper Twin Lake. Monitoring performed by the Shingle Creek Watershed Management Commission (SCWMC) in Twin Creek upstream and downstream of the wetland has found that total phosphorus concentration in the water discharged from the wetland is significantly greater than the concentration at the inlet, indicating that the wetland is discharging phosphorus. The phosphorus load discharged from the wetland into Upper Twin Lake is an estimated 40 percent of the total load of phosphorus to Upper Twin Lake, and a Total Maximum Daily Load (TMDL) study prepared for the lake concluded that this phosphorus load contributes significantly to its impairment. Because Upper Twin is connected to Middle Twin by a channel, the nutrient-rich water from Upper Twin flows directly to that lake and contributes to its Impaired Waters status. Reducing phosphorus load exported from Wetland 639W is the highest-priority action in the EPA-approved TMDL and MPCA-approved Twin and Ryan Lakes Nutrient TMDL Implementation Plan. The SCWMC and the cities of Brooklyn Center and Crystal have for a number of years investigated ways to improve the Twin Lake chain. Modeling of the lake s watershed and water quality testing at outfalls into the lake in 1999 suggested that the part of the watershed that outlets through Wetland 639W was exporting more phosphorus to the lake than would be expected for the land uses in the watershed. More detailed monitoring upstream and downstream of the wetland conducted in 2002 confirmed that the wetland was the likely source of the excess phosphorus, but the exact mechanism causing that export was not known. Options Analysis Prior to undertaking this Feasibility Study, the Commission hypothesized that one or more of the following conditions were causing the export of phosphorus from the wetland. 1

2 Figure 1. Wetland 639W. 2

3 Figure 2. Drainage area to wetland 639W. 3

4 1. Stormwater is conveyed through the upper wetland in a channel. At the wetland midpoint, just above the central wetland basin, the channel disappears and the flow diffuses into sheet flow. This sheet flow moves more slowly across the wetland basin, and becomes deoxygenated as vegetation and soils capture and use the dissolved oxygen. Anoxia at the soil-water interface across the wetland breaks electrochemical bonds and releases phosphorus from the mineral soils. 2. The wetland vegetation and soil dries out in summer, becoming friable. Periodic large events mobilize and flush organic material and detached soil particles out of the wetland and into the lake. 3. The wetland soils have reduced ability to bind phosphorus due to the transport of sediment and nutrients to the wetland from historic agricultural and current urban stormwater. In 2008, as the first phase of this Feasibility Study the SCWMC undertook additional monitoring, replicating the 2002 stream monitoring but also tracking groundwater levels in and near the wetland and analyzing soil cores taken from the wetland. A detailed analysis of the monitoring data and observation of conditions suggests that the primary reason phosphorus is released from the wetland is that the central basin dries out significantly during the summer. This drying out has two effects. First, the soil becomes mineralized faster than it would were it saturated, releasing phosphorus. Second, the soil becomes more friable, and stormwater sheet flowing across the wetland then mobilizes these mineralized soil particles and conveys them and the associated phosphorus load downstream to Upper Twin Lake. The groundwater elevation monitoring in 2008 suggests that groundwater movement in the immediate vicinity is generally from all directions toward the central basin of the wetland, which is vegetated with dense cattail growth. Cattails have a high evapotranspiration crop coefficient, meaning they use a lot of water. During periods of lower precipitation and runoff, when there is little to no standing surface water, cattails rely on groundwater for the water necessary for photosynthesis was a very low precipitation year, and piezometers measured a 2-3 foot drop in surficial groundwater over the summer growing season. The main basin of the wetland experienced extended periods of soil dryness, so then when rain events did occur, pulses of phosphorus were discharged from the wetland to the lake. These pulses were high in orthophosphate, which is the form of phosphorus that is most readily available for plant uptake and which fuels lake algal blooms. The Commission and the Technical Advisory Committee (TAC) considered several options to reduce the phosphorus export from the wetland. Some of these were identified but not pursued further as they were considered infeasible for cost, maintenance, or regulatory reasons as described in Table 1. Table 1. Options to reduce phosphorus export from wetland 639W. Option Implementation Considerations 1. Bypass all flow around the wetland Option would eliminate all surface water inputs from the upper watershed, leaving only direct runoff from the surrounding watershed, groundwater inputs, and precipitation. This would significantly affect the wetland s hydroperiod and change the wetland type, likely requiring wetland mitigation. May be difficult to obtain a permit. 2. Bypass high flows allowing low flows Would eliminate the most damaging high flows but allow low flows to to pass continue, reducing the impact to the wetland hydroperiod. 3. Modify wetland outlet to retain water Needs to be carefully designed to avoid creating areas of permanent in the wetland open water that would be attractive to waterfowl and other aviation nuisances. 4

5 Option Implementation Considerations 4. Divert channel flow into storage cells Would create areas of permanent open water that would be attractive to in the wetland to slow rate of flow waterfowl and other aviation nuisances. 5. Soil amendment Does not address the cause of soil mineralization problem, would have to be reapplied periodically. 6. Remove cattails and excavate mineralized soils Expensive, would change wetland type, likely requiring wetland mitigation. Would lower wetland surface elevation creating areas of permanent open water. 7. Chemical treatment of effluent Expensive, an ongoing cost for operations, needs a flocculation basin that would periodically have to be dredged and the material disposed. 8. Chemical treatment within the wetland Alum columns, limestone berms, etc. Would have to be widely dispersed to be effective given the volume of water passing through. 9. Reduce peak rates from upper watershed Very long term, also needs to be done carefully to avoid creating openwater habitat. Preferred Option The most promising options, options 2 and 3, were studied in more detail. The preferred option is a combination of the two approaches. Limiting outflow from the wetland would help the wetland stay as wet as possible, reducing the periods of soil dryness that lead to soil mineralization and export of friable material. This could be accomplished through the construction of a sheet pile weir along the bottom of the wetland. This weir would be set at elevation 856.0, approximately two feet above the ground elevation. Figure 3 shows a schematic drawing of the proposed modified outlet. A new outlet set at elevation would be constructed at a point upstream of the central wetland basin. Three options were considered for this new outlet: a new channel to be constructed at the edge of the upland wooded area adjacent to the wetland; a piped outlet following the chip trail corridor; and a combination pipe and channel to the east into a wetland in Brooklyn Center. Brooklyn Center expressed concern about the unknown impacts of discharging into its wetland, so that scenario was not pursued. The piped outlet as discussed below was estimated to be considerably more expensive than the open channel. The preferred option, the open channel, would outlet through an existing swale downstream of the sheet pile weir. Figure 4 shows the general location of the new channel. Water would be stored in the wetland until the elevation exceeds 855.5, at which point the upper outlet would begin to discharge into the channel. Hydrologic and hydraulic modeling indicate that channel discharge and storage in the wetland between elevations and can accommodate up to a 1.5 inch event. When the basin is full (i.e., at elevation 855.5), events smaller than 1.5 inches will be retained in the wetland. Larger events would be discharged both through the channel and over the top of the sheet pile weir. Modeling is discussed in more detail below. Figure 3. Conceptual cross section of proposed sheet pile weir and new outlet. 5

6 Figure 4. Proposed Wetland 639W outlet modification. Storage and Potential for Open Water Installation of the sheet pile weir will increase runoff storage in the central basin. Because of the proximity to the Crystal Airport, it is of prime importance to prevent the creation of open water that would attract waterfowl. The central basin of the wetland is currently vegetated mainly with dense cattails several feet in height interspersed with higher elevation areas of scrub shrubs and trees. There are some pockets vegetated with grasses and less dense cattail growth. During large events lower elevation areas of the basin may for up to several days contain a foot to 18 inches of standing water, compared to field observations that the wetland currently rarely contains more than six inches of standing water. It is unlikely that any significant new permanent open water will be created in the main basin as the height of the standing cattails in the low areas exceeds the potential standing water depth. However, there is a possibility that small permanent pockets of open water may be created in the lower basin immediately adjacent to the sheet pile weir. This area is a foot lower in elevation from the central basin, and may contain one to two feet of standing water on a continuing basis. This may cause changes in vegetation that might result in patches of open water. However, these low areas are small in size and are surrounded by higher ground that could be expected to remain vegetated with cattails, grasses, and scrub shrubs, limiting the attractiveness to waterfowl. 6

7 Hydrologic and Hydraulic Modeling Hydrologic and hydraulic modeling were completed to assess the impact of various improvement scenarios on flow through the wetland and on wetland elevations. The P8 model prepared for the Twin and Ryan Lakes Nutrient TMDL and the Shingle Creek Watershed Management Commission s watershed SWMM model were used in this analysis. The SWMM model hydrology is well-calibrated to watershed runoff measured at various locations throughout the watershed during the development of the Shingle Creek chloride TMDL. P8 hydrology was calibrated to the SWMM model and the pollutant loading was calibrated to Twin Creek water quality data collected in 2002 and Once the models were calibrated the proposed new sheet pile weir, outlet structure, and outlet channel were added. The models were then used to optimize the design details of the weirs and evaluate the impact of the revised outlets on wetland hydrology. Figures 5 and 6 below show the hydrographs for the 2-year event and the 100-year event respectively assuming the basin is full (i.e., at 855.5) as a starting condition. The hydrographs show the elevation of water in the basin at Node SW-2, which is the downstream sheet pile weir. The 2-year event hydrograph shows that the wetland very slightly overtops the weir, and then the basin is drawn down through the outlet channel to the starting conditions in a few days. The 100- year event hydrograph shows the weir overtopped and flowing for about one day, and then drawing down the second day. A ten year precipitation record was run through the SWMM model. The wettest year in this period was 2002, where the model predicts the sheet pile weir would have been overtopped five times. However, even during that wet year there were periods when the wetland elevation would have fallen below the outlet elevation and there would have been no flow in the channel. The average bounce, or change in wetland elevation due to a precipitation event, is estimated to be less than one foot during this period of record. The lower areas adjacent to the downstream weir might experience a bounce of up to two feet. In 2008, a very dry year, the model predicts that water elevation in the wetland would never have reached the level where the outlet weir would be overtopped. In an average year, the weir would likely be overtopped two to three times. Even in wet years, the model predicts that overflow through the channel would likely be flowing only for short periods. 7

8 Start Day 2 Day 3 Day 4 Day 5 Figure 5. Hydrograph, 2- year event at Node SW-2, the sheet pile weir. Flow just overtops the downstream weir set at

9 Start Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 8 Day 9 Day 10 Figure 6. Hydrograph, 100-year event for Node SW-2, the sheet pile weir. Flow overtops the downstream weir for approximately one day and then is drawn down by the next day. 9

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11 Cost and Performance Previous study and the Twin and Ryan Lakes Nutrient TMDL estimated that wetland 639W exports between pounds of total phosphorus annually, depending on the amount of precipitation received. The goal of this project is to reduce phosphorus export by an average 300 pounds per year. This would accomplish a significant fraction of the approximately 750 pound annual total phosphorus wasteload reduction required to Upper Twin Lake. Because Middle Twin Lake is connected to Upper Twin Lake through a short channel, improving Upper Twin Lake will have a beneficial effect on Middle Twin. Table 2 below is summary of construction cost (excluding engineering and other costs), and an estimated cost per pound of phosphorus removed. Table 3 below shows the detailed performance calculations indicating the preferred option, Option 3, meets the removal goal of 300 pounds per year. Table 2. Estimated costs, load reductions, and cost per pound removed for wetland 639W design options. Estimated Cost Load Reduction (lbs) and Cost per Pound Wet Year Dry Year Average 1. No build $0 $0 $0 $0 2. Sheet pile weir only $325,000 $1,555 $1,633 $1, Bypass + sheet pile weir To east $590,000 $855 $1,616 $1,150 To west (pipe) $890,000 $1,290 $2,438 $1,735 To west (channel) To south (pipe) $410,000 $770,000 $ $1,116 $1, $2,110 $ $1,501 Table 3. Estimated phosphorus export load reductions for each of the three Wetland 639W options. Monitored Avg Annual Load Annual Load Annual Load P8 Volume TP Wetland Year Option Reduction Reduction (af-yr) Concentration Outflow (kg/yr) (lb/yr) (mg/l) (kg/yr) 1. No build 1, Wet Year 43.8 precip 2008 Dry Year 22.7 precip 2. SP weir only Bypass + weir Bypass Flow over weir No build SP weir only Bypass + weir Bypass Flow over weir

12 Year Option P8 Volume (af-yr) Monitored Avg TP Concentration (mg/l) Annual Load Wetland Outflow (kg/yr) Annual Load Reduction (kg/yr) Annual Load Reduction (lb/yr) 1. No build SP weir only Average 3. Bypass + weir Bypass weir Note: Concentrations used for 2002 and 2008 are the monitored summer average concentration; for the average the concentration is the average of the 2002 and 2008 monitoring results. The No build, SP weir only, and flow over weir option concentrations are at the outlet of the wetland. Concentrations used for the bypass option are the summer average at the midpoint of the wetland, near where the bypass weir would be located. Ongoing Operations and Maintenance The City of Crystal has agreed to take on responsibility for ongoing operations and maintenance of the weirs and channel. These activities are expected to include but not be limited to: Routine inspection of weirs, outfalls, and the channel. Removal of debris accumulated on the weirs or blocking the outfalls. Removal of trees, woody debris, sediment deltas, and other blockages in the outlet channel. Maintenance, repair, and replacement as necessary of weirs, outfalls, and the channel. Estimated Cost and Funding The following is a summary of the estimated costs and funding sources for this project. The MPCA Section 319 construction grant extends through August 29, The benefitting cities include Brooklyn Center, Brooklyn Park, Crystal, and New Hope. Those cities have not yet reached an agreement on cost sharing between the partners. Because the project is located mostly in Crystal, Crystal has agreed to be the lead agency and to manage the construction project if approved. Costs Construction $410,000 Contingency 85,000 Final Design and Construction Engineering 50,000 Followup Monitoring and Reporting 25,000 Total $570,000 Funding MPCA Section 319 Grant $300,000 Shingle Creek WMC 142,500 Cities 127,500 Total $570,000 12