To understand better the effects of strong waves/currents on erosion, we will examine a study that examines the sediment transport processes from these sand-filled islands. Information obtained from this study was used to plan the optimal construction schedule.
Sediment Transport Processes
Waves attack the island, making sand drift away from the outlined fronds and trunk. The greatest loss is at the ends of the fronds because there they encounter a combination of littoral (long-shore) and perpendicular (cross-shore) sand transports, not to mention, how the frond ends are the least protected part of the island at the beginning stages of construction.
Wash-over transport
Wash-over transport occurs when the crest level is lower than the wave run-up level. The waves wash over and reshape the constructed berm. The sand is moved by tidal currents parallel to the shore, down-slope directed density currents, and breaking wave-induced currents. During the winter, when there are severe storms and higher waves, more breaking of waves occur, resulting in greater transport rates and more sand loss. However, since the first winter (2001-2002) had relatively mild climate, there were low transport rates and less sand loss.
Long-Shore Transport
Long-shore Transport occurs when the crest level is higher than the run-up level of waves. Essentially this means that waves are blocked. Transport is parallel to the berm. Three types of formulas were used to measure long-shore transport: CERC, Bijker (1971), and Van Rijn (1993).
According to the table, transports calculated by Van Rijn were 100 fold of those calculated by Bijker and CERC. Bijker and CERC had the “same magnitude” of long-shore transport rates. There are discrepancies between the transports because each formula measures certain variables that may have been neglected or not weighed as much in other formulas. For instance, Van Rijn takes beach slope into account. (Less steep slopes result in lower transport rates.) Bijker and CERC suggest that slope does not really affect long-shore transport. (de Jong, et. al)
Cross-Shore Transport
Cross-shore transport is perpendicular to the coastline. Sand will be moved more down the slope and less upwards, resulting in a less steep slope over time. The crest line will shift toward the shore and sand will deposit outside the boundaries.Cross-shore transports were measured using three formulas: Swart’s model, Durosta, and Unibest-TC. Swart’s model states that sand transport is dependent on wave height, wave period and grain size. Durosta is used more for computing offshore-directed sediment transport of dune profiles during storm conditions – good for computing the erosion process along steep initial slopes of Palm Island. Unibest-TC is good for computing cross-shore sediment transports and the change in profiles along a coastal profile of any shape due to waves, long-shore tidal currents and winds.
Data revealed that small waves did not substantially alter the beach profile. The profile was only affected by higher waves – those greater than 0.5m tall.