Using a Simple Dilution Model to Estimate Wastewater Contaminant Concentrations Behind Moving Passenger Vessels

An independent Science Advisory Panel (The Panel) was formed to assist the Alaska Department of Environmental Conservation evaluate the effects of wastewater discharges from cruise ships in Alaskan waters. The Panel evaluated the dilution of wastewater discharges from moving cruise ships. The results of that evaluation were used in turn to evaluate possible effects on water quality. This paper describes the dilution evaluation. The Panel evaluated a number of different studies and concluded that 1) the water that is displaced by a moving ship creates turbulent mixing upon its return astern of the ship; 2) the propellers further enhance the turbulence and mixing; 3) a cruise ship can be represented as a moving cross-sectional area; the larger the cross section and/or the faster the cruise ship moves, the greater the volume of water displaced per unit time leading to increased mixing rates; and 4) the diluted concentrations achieved will be inversely proportional to the rate of discharge. Hence, the mixing will be rapid, substantial and related to the vessel’s size, speed and discharge rate. Based on two theoretical studies and six field studies, The Panel concluded that the cross sectional mixing area behind a large cruise ship rapidly expands to about 4 times the cross-sectional area (beam times draft) of the ship. The dilution factor (df) for a discharge from a large moving cruise ship can be calculated as follows: df = 4 x (width x draft x speed)/(discharge rate) Links to The Panel’s final report, as well as interim reports and other related matters can be found at: http://www.state.ak.us/dec/press/cruise/cruise.htm