Final Design Description
Dimensions: Because of the 2MGD (million gallons per day) effluent discharge and the given area of 1.5 acres (65340 ft2) to work with, the wetland has a rectangular geometry of 440ft by 148ft. This will maximize the acreage utilized without exceeding the design constraint. The outer 5ft strip of land on the wetland’s perimeter within the 440x148 ft dimensions will be space for planting. To make sure water will flow from the inlet to the outlet smoothly, the wetland will need to slope downward; an optimal angle would be 1-2 degrees. The problem statement suggests that the land be excavated to be level with Paint Creek, or approximately 8ft. However, the plants that will be treating the water thrive in a 1-3ft depth zone. To account for all three considerations, the wetland will be excavated to a depth of 4ft at the outlet and 1ft at the inlet, giving the wetland an absorption zone with a 0.39-degree slope. Additionally, the wetland will include two sinusoidally-shaped streams excavated to a depth of 8ft at the outlet and 1ft at the inlet, giving these streams a 0.93-degree slope. This way, the streams are excavated to the required depth while still allowing the rest of the wetland's area to be kept shallow enough for plants. Ideally, the slope of the streams would be slightly larger; however, the outlet depth is bounded by the problem statement, and the inlet needs to be deep enough to accommodate the infill materials. Additionally, Dr. Messer’s course on wetland design recommends a 3:1 berm grade ratio. However, because the wetland is in a public area with a nearby walking trail, the wetland has a less severe slope for public safety and is an 8ft rise with a 6ft run (at its deepest, whereas at the inlet this is a 1ft rise with a 1ft run). In addition to safeguarding the public, this will allow a maintenance plan to be established more easily.
Natural Material Choices: Natural material choices were made based on nutrient removal properties and integration to the existing ecosystem. The infill at the inlet will be a 1ft thick layer of limestone and gravel mixture; limestone is chosen for phosphorous removal. Dr. Messer recommends a 3-5cm thick layer of soil below the infill; the wetland will have a 2" partial peat soil mixture due to the high cost of pure peat. Peat will be included in the soil layer for phosphorous removal. The 5ft perimeter strip will be dedicated to planting button brush for ecosystem integration and to provide a natural barrier that will deter trespassers. To avoid creating a monoculture in the wetland, a mix of plant seeds native to the area will be planted on the berm. The absorption zone, or the area within the berm, will be planted with hard-stemmed and dark green bulrush for ammonia removal, swamp smartweed for nitrogen removal, and broad-leaved cattail for phosphorous removal.
Impermeable Layer: A relatively impermeable layer of material is included in the wetland design to prevent undesirable nutrients in the wetland from seeping into groundwater. A clay pond liner is chosen for this, which is more cost efficient than a polyethylene or polypropylene liner. For more heavy-duty uses, such as above-grade installations, a polypropylene liner would've been necessary despite the cost. (Agriculture Solutions, 2022).
Wastewater Treatment Plant (WWTP) Pipe: The existing pipe outfall to the creek is 48” in diameter (Ridgewater, LLC, 2023). Keeping with this, a 48” corrugated polyethylene pipe will be used to divert the wastewater treatment plant effluent to the wetland. A polyethylene pipe is chosen as opposed to PVC or polypropylene due to its ability to be easily installed if a trenchless installation is used for construction (Pipelife International, 2023). If possible, a trenchless install may be preferred during construction planning to avoid excess environmental disturbance to the area near Paint Creek. In this case, the pipe will be installed underground with gravel insulation. To avoid freezing in the winter, the pipe and insulation will be installed below the frost line, or the depth that groundwater in soil is expected to freeze to. In Ohio, the frost line is 32" (World Population Review, 2023), so the pipe will be installed with the top 44" below ground level, allowing for 12" of gravel insulation surrounding it.
Inlet Flashboard Riser/Partial Diversion: Due to the wetland’s hydrologic loading rate (HLR), the wetland may become overloaded if the full 2MGD is diverted to it. To avoid this, the wetland will only have a portion of the effluent wastewater diverted to it, and the rest will be diverted to Paint Creek. Based on calculations, the wetland would ideally clean approximately 8% of the effluent wastewater. The inlet design assumes 10% diversion, with a concrete flashboard riser installed to control the flow of water into the wetland. A flashboard riser with an adjustable board slot will allow the bottom 10% of the WWTP pipe’s cross-sectional area into the wetland distributor pipe, and the rest of the water will be diverted into Paint Creek using a split corrugated pipe at the entrance of the flashboard riser. Presently, the design is based on the assumption that the WWTP effluent has a constant flow and that the current 48” WWTP outflow pipe is “full” of water when it reaches the riser. Under these assumptions, the cross-sectional area of the WWTP pipe that will have water diverted into the wetland is 181in2. The pipe diameter is used with a variable height to back solve until the desired area was reached using an equation in which the segment height and radius are variables of the cross-sectional area function (Math Open Reference, 2023). The flashboard riser will allow the bottom 7.75in of water in the pipe through, and any water above that level will be diverted into Paint Creek through the split pipe. To accommodate the size of the outfall pipe, the square-shaped riser will have 72" x 60" x 24" dimensions.
Inlet Distributor Pipe: The inlet of the wetland will consist of a standard size HDPE (high density polyethylene) corrugated distributor pipe. Five distribution holes, each with a 6" diameter, will be spaced evenly apart along the inlet of the wetland. This will allow all of the plants to be removing nutrients without overwhelming any one part of the wetland.
Outlet Culvert: The outlet culvert of the wetland will consist of a 48” HDPE corrugated pipe and flashboard riser installed at the corner furthermost from the WWTP by length and closest to Paint Creek by width. A flashboard riser like that at the inlet will control flow out of the wetland; rather than allowing the “bottom” 10% of water into the wetland, it will let the “top” 10% of water out, allowing water to flow from the wetland once it has filled most of the way up. The outlet pipe will be installed with the center 48” below the wetland surface.
Factor of Safety/ Natural Spillway: As a factor of safety, a natural spillway will be excavated and laid with pebbles over the outlet culvert to prevent and mitigate flooding. The spillway will be excavated with cross-sectional dimensions of 12” depth and 60” width extending from the edge of the wetland to the edge of Paint Creek.
Citations:
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