Case study: Sediment budget in direction of longshore drift on Changi beach, Singapore.
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Coastal Fieldwork a case study of the spit on Changi Beach on how the amount of beach material changes in the direction of longshore drift in relation to gradient and length of the beach face 1. Introduction Changi Beach is one of the longest natural beaches in the north-eastern part of Singapore. The northern part of the beach is extended as a spit, a landform of coastal deposition. Spits are accumulations of beach material which result from longshore drift. Longshore drift is a process which causes beach material to move in a down-drift direction, due to waves approaching the beach at an oblique angle. This causes the swash to carry material in the direction of longshore drift, the process continuing such that material is transported along the beach. In the case of the spit at Changi Beach, longshore drift occurs in the north-west direction (refer to figure 1 and 2). Figure 3 illustrates the oblique angle at which the waves approach the beach. This study aims to examine the development of the beach profile, with focus on the amount of beach material along the spit in the direction of longshore drift. While most studies would consider longshore drift along a beach, this study differs slightly as the beach face on the spit itself, as a landform of deposition, is being investigated.
These two values of gradient and length of the beach face can be used to mathematically calculate the cross-sectional area of each transect. While the resultant values may not be completely accurate, and the base of the cross-section is more or less arbitrary, and follows the mark used for measuring the length, observations can be made based on the differences between the values. Representing the gradient of the slope as and the length of the beach as , the formula for calculating the cross sectional area is as follows (refer to figure 7): The formula gives the vertical height, represented on figure 7 as , and gives the horizontal distance, represented as The values were computed using MS Excel, refer to figure 8 for table. The values are represented on figure 9. It is by looking at the resultant values shown on figure 9 that the effect of longshore drift on the amount of sediment on the spit becomes immediately apparent. The individual values of gradient and length do not show as clearly the amount of sediment, as both depend on the deposition of material at different places on the beach face, i.e. generally a steep gradient is caused by more material further up the beach face, and a longer beach is caused by more material deposited lower down the beach.
It can also be said that the idea that both length and gradient of the beach increases in the direction of longshore drift is true, though to interpret it another way, it would be more of the case that, due to increase of material, the beach face is thus steeper and longer. In conclusion, though the effect of longshore drift on gradient and length is not always clear, but these factors, as well as the calculation of the cross sectional area based on the values; have been proven to be reliable in estimating the differences in amount of beach material. The hypothesis that beach material increase in the direction of longshore drift has been proven to occur on the coastal depositional feature of a spit. The amount of beach material increases along section A and accumulates at the tip of the spit. It is likely that with the continual process of longshore drifting, section A would continue to extend in the future. Further studies are required to fully understand whether the hypothesis of beach material increasing in the direction of longshore drift is in fact exclusive to spits, as highlighted in the introduction on the fact that change to the beach face in the direction of longshore drift is not normally observable. Possibly, this is a unique case where changes in the beach face are observable in the direction of longshore drift. 5.
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