In the last issue of Breaking Waves, we advocated the importance of beaches to the national economy. For instance, beach related tourism contributes approximately $257 billion to the national economy1. In this issue, we examine the environmental issue that affects our beaches, namely water quality.
According to a recent National Resources Defense Council report on beach water quality, there were over 25,000 closing and health advisory days at beaches within the US, with more than 14,000 due to unknown sources2. The number of closing/advisory days is the highest in the 17 years that the NRDC has been tracking beach closures. The number of closing/advisory days due to storm water was more than 10,000 in 2006, with 79% of these being pre-emptive closures. Local health officials often cannot wait for the results of water quality testing and close beaches based on rainfall.
Woods Hole Group has been active in defining mixing processes in the nearshore zone, applying a variety of techniques to evaluate the influence of discharges in estuaries and along beaches. These projects have covered a variety of topics including defining discharge mixing zones, identifying zones of influence, tracking water column debris such as decayed vegetation (detritus), and evaluating sources of contamination. Lately, we’ve applied these tools to develop solutions to improve water quality and help keep our beaches open.
One example to reduce the number of beach closures is an ongoing project with the Town of Middletown, Rhode Island. Middletown is located on the south coast of Rhode Island with beaches facing the Atlantic Ocean. The two local beaches are Easton’s Beach and Atlantic Beach Club, and there are three known discharges within the nearshore zone. The first discharge is a stream mouth that carries storm flow. The other two discharges are 36 inch and 16 inch storm sewer pipes located along the eastern section of the embayment forming the beach.
The Rhode Island Department of Health (RIDOH) collects water samples between May and September and use enterococci bacteria as an indicator of the risk of illness. The standard for enterococci is 104 colony forming units (cfus) per 100 mls of saltwater. Between May and September 2006, Atlantic Beach Club was closed for 16 days due to water quality samples above the standard. Easton’s Beach was closed for 20 days, including an 8 day stretch just after the popular Fourth of July holiday vacation. High bacteria levels are not only a summer problem. In the Fall/Winter of 2006/2007, a Rhode Island organization titled Clean Ocean Access continued the same routine of water quality sampling as RIDOH to determine beach water quality during the off season. Although the number of visitors to the beach decreases with the temperature, the off season in New England is prime time to don the thick wetsuit and catch some waves, leaving a select group of individuals to remain in contact with beach water of unknown bacterial levels.
Woods Hole Group is helping the Town of Middletown investigate and design measures to reduce beach closures due to the two storm sewer pipes. One alternative being considered is to relocate these pipes to an offshore location where they will not cause beach closures. Moving the pipes offshore also has the potential for water quality impacts, so it’s important to investigate design alternatives to minimize the size of the mixing zone. Initially, a simple analytical dilution model was applied to simulate the proposed relocation of the storm sewer discharges to an offshore underwater location. The results of the modeling indicated there is potential to reduce the number of beach closures; however, questions remained regarding offshore characteristics.
Based on the encouraging preliminary findings , Woods Hole Group has been working to better characterize the discharge water, offshore environment, and mixing zone. Although there has been extensive sampling of the bacterial levels in the water, little has been done to quantify the source flow. The Town of Middletown has done extensive cameraing through the storm sewer discharge pipes to identify erroneous connections; such as waste or sanitary sewer connections. All of these connections have been repaired such that the flow within the storm sewers is solely an artifact of rainfall or groundwater seepage. Due to the nature of rainfall, flow within the pipes is episodic, and these episodes have the potential to carry a large quantity of bacterial contamination to the beach. Such an example is October 20, 2006 when after 0.6 inches of rain, the enterococci level in the water sample from the 36 inch discharge pipe was above 24,192 cfus/100 ml. In order to characterize the flow regime with the pipes, Woods Hole Group installed two sewer flow meters designed to operate in variable flow environments.
Along with characterization of the source water physical characteristics, Woods Hole Group is improving the characterization of the ambient or receiving water body. There is little available data to determine the tidal current within the offshore embayment. Woods Hole Group placed a bottom-mounted, upward-looking Acoustic Doppler Current Profiler (ADCP) for two months in the offshore zone approximately where a combined discharge might be located. The data collected by the ADCP are being used to refine the estimates of the dilution/mixing zone of the discharge, as well as to support necessary environmental impact assessment and permitting.
The goal of the combined data collection, modeling, and environmental investigations is to develop an effective and affordable resolution to a municipal water quality problem, reducing beach closures for residents/visitors and protecting the water quality for the ecosystem.
Brian Caufield, Coastal Engineer
Mr. Caufield is a Coastal Engineer specializing in the areas of hydrodynamic modeling, flushing studies, wave transformation, coastal structure design and sediment transport. He received his M.S.E. in Naval Architecture and Marine Engineering from The University of Michigan, his M.S.C.E. in Civil Engineering and his B.S.C.E. in Civil Engineering both from Purdue University. Mr. Caufield has professional and academic experience with the analysis of nearshore wave conditions and impacts of waves on coastal erosion and in applying finite difference and finite element hydrodynamic models to predict tidal flushing of harbors and lake circulation modeling.
1“The Economic Value of Beaches,” presented by Dr. James Houston, USACE, at Sustainable Beaches Summit, Sandestin, FL, March 30, 2004.
2 Testing the Waters: A Guide to Water Quality at Vacation Beaches 17th Edition, National Resources Defense Council, August 2007.