Final Report

First Semester Final Report

Problem Statement

Lake Hickman is situated adjacent to Richmond Road near New Circle Road and is one the largest recreational lakes located in Lexington, KY. Aside from its recreational value, the lake is also used as a reservoir for municipal drinking water. Four major companies - Alltech, KY American Water, Lexington Cartage, and Lexington West Hickman Sewer Treatment - are permitted to dispose of their waste into the lake. This waste, coupled with the lake’s location within a highly developed residential area, has led to a degradation of the ecosystem in the area. The HOA which owns the lake has installed several aerators to correct anoxic conditions and is now conducting dredging operations. Stormwater draining from the developed upland areas is not subject to the environmental processes necessary to clean it because its natural pathways have been replaced with impervious channels.

A common way to treat lakes undergoing similar issues as Lake Hickman is point reduction; in this case however, due to the size of the lake and the vastness of how and where the nutrients are spread, these nutrients provide the perfect fuel for growth for algae in the lake. These algal blooms are unsightly, and can be potentially toxic. Having a cleaner lake will make for happier residents- they’ve been pushing for a more aesthetically pleasing, safer, and cleaner lake. By following our proposed design we hope to increase property values, improve water quality, provide support for vulnerable biota, and to encourage recreational activities.

Proposal

Solution:

Budgetary considerations and site hydrology led to the selection of a surface flow constructed wetland. When comparing sub-surface flow wetlands to the proposed surface flow wetland it was found that “sub-surface flow constructed wetlands do not provide the same level of habitat value as free water constructed wetlands because the water isn’t accessible to birds and animals.”(Virginia Department of Environmental Quality) Free water surface constructed wetlands are becoming increasingly popular due to their positive environmental impacts, year-round operability, and low cost compared to other treatment methods. Apart from these umbrella pros, a surface flow constructed wetland is the best course of action for the specific watershed- a breakdown of land use of the watershed can be seen in Table 1.

This wetland would remove a portion of the excess nitrogen and phosphorous from the runoff entering Lake Hickman and serve as an example for other communities to follow.

Hydrological Data:

Using curve numbers calculated through GIS and a 1 year, 24 hour design storm the following hydrological analysis was derived Table 2.

From this data the overall inflow of stormwater can be established. The volumetric flow rate lets us know how fast the water will be entering the wetland.

Channel and Wetland Design:

The suggested wetland design, starts off with slowing the initial velocity of the inflow with a rock bed, the flow then continues to slow through a multi-functional forebay that provides a trench like area that not only decelerates the water, but also, provides as a mosquito dead zone: the forebay is too deep for mosquitos to use to lay their eggs in. Continuing down the wetland, the flow will be filtered through an assorted array of plants-a list and justification for each can be seen in table 3- located in the inundation zone. Once the flow reaches the deep pool it’s held behind the weir where the velocity is controlled and the water is passed along through a vegetation polishing channel, where it will goes through a final treatment process. In the event of a hundred year storm an emergency spillway has been added to the side of the deep pool that leads the water back to the lake.

The land area available for the installation of the constructed wetland is less than 3% of the contributing watershed’s land area, so a single-cell wetland was selected to treat the incoming stormwater. The following figures show the location of the constructed wetland relative to Lake Hickman and the layout of the wetland. Figure 2 also illustrates the use of the existing topography to reduce excavation costs.

The water will be slowed by a cobble channel before entering the forebay. The forebay is important because it prevents the constructed wetland from clogging with sediment over time. The majority of the constructed wetland’s area will consist of a shallow pool for contaminant removal where a design wetland pool depth of approximately 23 inches was calculated.

Vegetation:

The following plants have been selected for implementation in our constructed wetland. These plants were selected based on their nutrient uptake rates, invasiveness characteristics, and overall aesthetic appeal. Additionally, our group is considering adding Iris to our wetland system. These plants thrive in wetland environments and add to the overall appeal of the wetland. However, these plants can be invasive and difficult to eradicate. Finally approval from our client will be obtained before any decision is made on this matter. (Source: Behrends, 2010)

Budget Considerations

The total projected budget for this design will be $960,000. However, the yearly price will be much lower. This is because the excavation, growth media, plants, planting, legal fees, and site investigation will all be one time payments. Following the first year, the wetland will require a yearly budget of around $13,600 for cleaning and basic maintenance. As can be seen from our budget table, having the site excavated will require the majority of our budget.