Waves and Mixing in an Urban Subtropical Lake: Restoration Implications

Urban lakes, valuable centerpieces of communities, serve as wildlife refuges, recreation areas, and storage for rainwater runoff. However, as urban lakes are the final destination for runoff, they are at high risk for eutrophication and are vulnerable to fish kills and poor water quality. Many communities have implemented strategies, such as dredging, to restore the ecological and societal value of the degraded lake systems. These plans typically focus on water quality parameters but do not address how the hydrodynamics of the lakes may change. For example, University Lake is a small (area = 0.79 km²) and shallow (average depth = 0.6 m) eutrophic lake located in Baton Rouge, Louisiana that has been plagued by algae blooms and fish kills. A restoration plan has been suggested, but this plan focuses primarily on water quality concerns and neglects how the hydrodynamics of the lake would change. In particular, wind waves could be highly influential on water column mixing, water quality, and eutrophication processes within the lake, and thus it is important to predict how they will be affected by lake restoration. Using a numerical model (Delft3D) validated using wave measurements, we compared the present lake scenario to possible restoration plans: uniformly increasing water depth, dredging the center of the lake, and increasing the average depth while contouring the lake boundaries. Under the present conditions, measured wave heights reach 17 cm and a median wave height of 7 cm. Each restoration plan led to an increase in wave height and altered the mixing dynamics. While lake deepening has been shown to decrease water quality issues in many urban lakes, we demonstrate that this deepening alters the hydrodynamics in the lake – affecting lakeshore property owners and recreational use of the lake. Using a holistic approach to urban lake restoration, and including not only water quality and ecological concerns, will provide for a better and more sustainable outcome.