Welsh Journals

SAND MOVEMENT AT FALL BAY, GOWER. A. T. WILLIAMS Gower is a world renowned beauty area. Its rugged coastline encompasses a variety of bays that make it difficult to be surpassed in natural beauty. One such is Fall Bay. It is located on the south western tip of the Gower Peninsula and is 500 metres in length with a maximum exposed width of 150 metres at low water. Beaches in this area tend to be influenced by prevailing south-west and westerly winds which constitute some 35% of the wind frequency pattern. The average wave frequency of Atlantic swell is just over one per day having a period of 10 seconds. The bay is encircled by craggy Carboniferous limestone (Upper limestone shales) which are composed of crinoidal limestone and pseudo-breccias the latter being the product of recrystalization of muddy limestone. It is situated in a macro- tidal high energy environment and the beach is made up of medium grained sand (0.8/ to 1.890. Throughout 1972 and 1973 experiments were carried out on the beach in order to try to quantify the erosional depth of sediments distributed by wave activity. This is important in bays because the sand layer acts as a 'cushion' in absorbing energy derived from the breaking wave. The thicker the sand layer, the better preserved is the bay floor. If it is too thin, then erosional processes become dominant in a very short time. Holes (sedimentation stations) were dug 20 cm deep into the beach face at low, mid and high tide positions (Fig. 1). These were then filled with sand dyed a bright red (printer's ink and petrol) and stakes were hammered into the beach two metres to the east of each spot, and measured. After a tidal cycle they were re-measured in order to calculate any topographic changes. In addition, the thickness of undyed sand deposited on top of the dyed sand was measured. This would be representative of the amount of erosion and deposition that had taken place over a tidal cycle. Wave heights, periods and beach slope were all measured by rule, stop-watch and clinometer respectively. Ideally these experiments should be carried out under equilibrium beach conditions, i.e. when tidal cycle erosional and depositional rates coincide a rarity in beach work. In order to cut down this error source, measurements obtained when the sand surface changed by more than 2 cm over a full tidal cycle were rejected for statistical analysis. To obtain 16 mid-tidal readings took nearly 2 years. Only 4 high-tide readings fulfilled the above conditions; 2 low-tide ones. Work done by King (1951) on an adjacent beach at Rhossili suggests that no difference with respect to depth of disturbance is obtained between high and low foreshore zones. She attributes this to the many broken waves which cross the bay (up to eight). Raw data results from Fall Bay suggested that there was no change in disturbance depth for high and mid tide position on a spring tide (5.35m), but that there was on a neap tide (3.25m). The maximum number of surf waves recorded at any one time within the bay was three and it is in this zone that the greatest movement of sediment takes place. King (1951, 135) noted that an increase in wave height caused an increase in the depth of disturbance of beach sand, 'the depth of