Aspects of beach sand movement, at Gibraltar Point, Lincolnshire
In this study fluorescent sand tracer techniques were used to investigate sediment movement on the foreshore at two sites on the south Lincolnshire coast. Working over one tidal cycle, low water to low water tracer release at different points across the beach revealed a complexity of sand movement in this strongly tidal environment. It was found that sand moved both at different rates and in contrasting dominant directions on different parts of the beach over the same tidal cycle. At Gibraltar Point there was confirmation from the tracer experiments of the importance of tidal generated currents on sand movements on the lower foreshore. Tests conducted to study the patterns and rates of movement of different grain sizes produced inconclusive results largely due to low recovery rates for the tracer. However, from the evidence available, it appeared that sorting of sediment was taking place only in the sense that finer material was moved away from the release point at a faster rate than coarse material and not because different sized grains were moved in different directions. In a consideration of models for the prediction of longshore sand transport two of the most commonly used models were tested using data collected during the tracer experiments. The results confirmed the success of the two models, one based on the wave power equation and the other on an energetics approach, and coefficients produced were in close agreement with those obtained by Komar (1969) in a previous study. Finally, field measurements of a series of variables such as wave height, wave period and longshore current velocity were combined with measures of sand movement and direction in a linear multiple regression analysis to study the associations present and to produce a simple predictive model of sand movement. Using both stepwise and combinatorial methods of regression it was found that 80% of the variation of amount of longshore sand movement could be accounted for by the 'best' equation. Wave height alone explained 61.3% of the variation but beach slope, water temperature and longshore current velocity were also of importance. 59% of the variation of average distance moved by sand grains normal to the shore was explained by beach slope, whilst wave period was seen to be the major factor in determining the direction of sand movement onshore or offshore. A set of equations was produced which together with longshore current flow direction, could be used to predict the average position of tracer on the beach face after one tidal cycle. At the same time individual equations could be used to model specific aspects of sand movement.