Linda Benedict, Xu, Yi Jun
Y. Jun Xu
University Lake on the Louisiana State University Baton Rouge campus is an invaluable asset to citizens in the state’s capital city. Beyond serving local residents, this lake is greatly enjoyed by out-of-town visitors. Like many urban lakes in the world, University Lake is faced with the twin problems of siltation and nutrient enrichment from the runoff of the drainage area (Figure 1). The accumulation of soil and organic particles can exacerbate internal release of nutrients, leading to a degradation of water quality and to the impairment of designated uses.
Many urban lakes face similar water quality problems. In the United States, 44 percent of all lakes and 59 percent of human-made lakes have been classified as impaired, and stormwater runoff has been attributed as the major cause of the impairment (Figures 2a and 2b). When the water quality issue becomes imminent, dredging of the lake-bottom sediments is often used for urban lakes. However, dredging can be extremely costly as well as undesirable when a popular lake in a highly urbanized area must be dried out for an extended period of time. University Lake is a prime example of the situation.
University Lake, along with its five sister lakes, was created in the 1930s from a swampy bayou area through damming, timber harvest and earth moving (Figure 3). By the 1970s these lakes were silted up, and the depth of much of the open water areas became less than 2 feet. High nutrient levels, especially phosphorus, were found, and frequent fish kills occurred in the late 1970s. The lakes were then dredged during 1982 and 1983, removing approximately 600,000 cubic yards of lake sediments and increasing the average depth by more than 1 foot. Water quality conditions improved immediately after dredging.
However, the improvement of water quality of the LSU lakes system was short-lived. Studies showed that 10 years after dredging, the lakes’ phosphorus level and turbidity reached the same level of pre-dredging. Water depth measurements conducted in 2009 found that much of University Lake is shallower than 2.5 feet. Water quality monitoring at the lake showed a considerable increase of nitrogen and phosphorus from 2008 to 2014. The concentration of chlorophyll during summer months has increased by three to six times, causing supersaturation of dissolved oxygen in the lake. Based on the current condition, the lake can be classified as hypereutrophic, or very nutrient-rich, with frequent and severe algal blooms and low transparency.
The Baton Rouge Area Foundation recently contracted a group of firms to create a master plan for the restoration of the lake. As a chief solution, dredging of approximately 800,000 cubic yards of the lake bottom sediments is being proposed. However, the cost of such a large-scale dredging project would easily amount to tens of millions of dollars, which no agency or private entity would see as realistic. Apart from the unrealistic cost, dredging does not provide a sustainable solution. The LSU lakes system was created through damming a bayou. The current water quality problem is also caused by the dams that encourage siltation and nutrient enrichment (Figures 4a and 4b).
A proposed engineering solution – sediment and nutrient sluicing – would cost only a fraction of the cost of a dredge project. A sluice gate can regulate lake bottom water flow by lifting the gate through valves, which can flush sediments as well as reduce nutrient-rich water. The height, frequency and duration of sluice gate lifting can be operated in accordance with the weather conditions, lake water conditions and management goals. Replacing the dams at City Park Lake and University Lake with sluice gates would work effectively in the long term in reducing both sediment and nutrient accumulations. In addition to its low cost and effectiveness, sediment and nutrient sluicing at the LSU lakes system would provide educational opportunities for students to learn about urban water quality and conduct research in hydraulics, hydrology and water quality (Table 1).
Y. Jun Xu is a professor in the School of Renewable Natural Resources.
The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages