Research about 63rd Street Beach

USGS2012

Wave-induced mass transport affects daily Escherichia coli fluctuations in nearshore water

Characterization of diel variability of fecal indicator bacteria concentration in nearshore waters is of particular importance for development of water sampling standards and protection of public health. Significant nighttime increase in Escherichia coli ( E. coli ) concentration in beach water, previously observed at marine sites, has also been identified in summer 2000 from fixed locations in waist- and knee-deep waters at Chicago 63rd Street Beach, an embayed, tideless, freshwater beach with low currents at night (approximately 0.015 m s –1 ). A theoretical model using wave-induced mass transport velocity for advection was developed to assess the contribution of surface waves to the observed nighttime E. coli replenishment in the nearshore water. Using average wave conditions for the summer season of year 2000, the model predicted an amount of E. coli transported from water of intermediate depth, where sediment resuspension occurred intermittently, that would be sufficient to have elevated E. coli concentration in the surf and swash zones as observed. The nighttime replenishment of E. coli in the surf and swash zones revealed here is an important phase in the cycle of diel variations of E. coli concentration in nearshore water. According to previous findings in Ge et al. (Environ. Sci. Technol. 2010, 44, 6731–6737), enhanced current circulation in the embayment during the day tends to displace and deposit material offshore, which partially sets up the system by the early evening for a new period of nighttime onshore movement. This wave-induced mass transport effect, although facilitating a significant base supply of material shoreward, can be perturbed or significantly influenced by high currents (orders of magnitude larger than a typical wave-induced mass transport velocity), current-induced turbulence, and tidal forcing.

Illinois↗

USGS2012

Nearshore hydrodynamics as loading and forcing factors for Escherichia coli contamination at an embayed beach

Numerical simulations of the transport and fate of Escherichia coli were conducted at Chicago's 63rd Street Beach, an embayed beach that had the highest mean E. coli concentration among 23 similar Lake Michigan beaches during summer months of 2000-2005, in order to find the cause for the high bacterial contamination. The numerical model was based on the transport of E. coli by current circulation patterns in the embayment driven by longshore main currents and the loss of E. coli in the water column, taking settling as well as bacterial dark- and solar-related decay into account. Two E. coli loading scenarios were considered: one from the open boundary north of the embayment and the other from the shallow water near the beachfront. Simulations showed that the embayed beach behaves as a sink for E. coli in that it generally receives E. coli more efficiently than it releases them. This is a result of the significantly different hydrodynamic forcing factors between the inside of the embayment and the main coastal flow outside. The settled E. coli inside the embayment can be a potential source of contamination during subsequent sediment resuspension events, suggesting that deposition-resuspension cycles of E. coli have resulted in excessive bacterial contamination of beach water. A further hypothetical case with a breakwater shortened to half its original length, which was anticipated to enhance the current circulation in the embayment, showed a reduction in E. coli concentrations of nearly 20%.

Illinois↗