Abstract

Many long sea outfalls discharge sewage effluent into the sea surrounding the United Kingdom. To monitor the dispersion and dilution performance of existing or planned outfalls, tracers can be released into the sewage and the resultant plume tracked by boat. In recent years aerial photography has proved to be a useful adjunct to boat surveys as it provides a synoptic view of a plume at one instant, which is something that a boat survey is unable to do as a result of tides, winds and currents. The aim of the survey reported here was to use aerial photography not just to provide a synoptic view for a boat survey but to produce maps of dye tracer plumes, to known levels of accuracy. Calibrated black-and-white aerial photographs were digitized and then manipulated on a digital image processor to enable the correlation of dye concentration in the sea with reflectance (r = 0.93, significant at the 99 per cent confidence level). This correlation was then used as the basis for predictive mapping of dye concentration on other digitized and calibrated black-and-white aerial photographs. As a result, detailed isopleth plots of dye concentration were prepared. The accuracy of these plots, which was determined by reference to boat-survey data, was 77-82 per cent at the 95 per cent confidence level for a seven-class classification increasing to 78-87 per cent at the 95 per cent confidence level for a four-class classification.