Proposed Plans

When making preliminary design decisions for the Washington Court House wetland, primary considerations were the design constraints and the location of the wetland itself. Because we are only given 1.5 acres (6070 m²) to use for the wetland design, we chose to make a rectangular wetland and use as much of the area contained in the acreage as possible without exceeding the constraint. From Dr. Messer’s course on wetland design, it is recommended that the wetland have a length to width ratio of at least 3:1. With nearly a 3:1 ratio, the initial proposed wetland has a 134m length and a 45m width. Before choosing a berm grade ratio, we decided to dedicate 1.5m on each edge of the wetland’s perimeter to planting shrubs from our plant choices. Doing this will allow for smooth ecosystem integration and provide a natural barrier to the wetland to deter trespassers.

The problem statement requests that 8ft of soil be excavated for the wetland to give it the same elevation as Paint Creek, so the wetland is a depth of 2.5m (8.2ft). Although Dr. Messer’s wetland design course recommends a berm grade ratio of 3:1 (rise to run) be used, we chose to have a more gently sloped wetland. While our proposed berm does follow the recommendation of a higher rise than run, the slope is less steep, since the wetland will be located near neighborhoods and a walking trail. If a kid were to play near the wetland, we wanted to reduce the chances of someone getting hurt by falling in and make it easier to get back out if that happened. This will also make any potential upkeep more accessible.

In the wetland’s cross-sectional view, the inlet pipe is shown to distribute the water throughout the wetland, so that all the wetland space will be used efficiently for nutrient removal and keep any one section from becoming much more overloaded than the others. Right now, the distribution pipe includes three distributors, a preliminary decision that will be evaluated when solidifying design choices.

The choice to use peat as the soil and limestone in the infill are detailed in our material choices milestone; the limestone and peat used for their phosphorous removal properties. A 3-5cm layer of soil was recommended by Dr. Messer. A polyethylene liner was chosen for the impermeable layer, which keeps the nutrients being treated from seeping into groundwater. We chose a polyethylene liner intended for lining ponds rather than a polypropylene liner, which is “recommended for above grade installations” (Agriculture Solutions, 2022).

Some additional design ideas that we have been considering include the use of a flashboard riser at the outlet, which would allow water to be held in the wetland to mitigate flooding. Additionally, from our design calculation milestone, we’ve determined that the area given is not able to support a wetland ecosystem with the current wastewater discharge amount according to the Hydrologic Loading Rate (HLR), q, calculated using equation 1.

A maximum q value of 10cm/day is recommended (Burchell, 2022) to avoid overloading the wetland. However, as shown in the table below, our calculated HLR is much higher than this.

To mitigate this, we’ve proposed that we could choose to divert only part of the discharge to the wetland, a decision that will be thoroughly evaluated when solidifying design decisions. To choose this, we will run a cost-benefit analysis to determine how many pounds of nutrients will be removed with a higher discharge but a lower efficiency from overloading the wetland vs. pounds removed with a wetland operating at 100% efficiency, which we estimate will happen at a maximum discharge diversion of 8%.