THE CUSPATE SAND BAR OF CHEK JAWA, PULAU UBIN
The sand bar of Chek Jawa dominates the eastern end of Pulai Ubin, extending southeastwards from Tg. Balai and northeastwards from Tg. Chek Jawa to meet in an apex about 600m from the coastline (Figure 1). Behind the cuspate sand bar is a low-tide lagoon, which is completely submerged during high tide. It is in this lagoon area of sand and mud that communities of marine plants and animals have thrived.
Fig 1:Cuspate sandbar of Chek Jawa
MORPHOLOGY OF SAND BAR AND LAGOON
The northern arm of the cuspate bar is slightly higher in elevation than the southern arm. The general sequence of submergence is from south to north during rising tides but the central portion of the northern arm is slightly lower. An examination of tide levels during submergence on the 5th of August 2001 suggests that the northern sand bar, consisting of a well defined ridge, is about 35m wide, crests about 1.1-1.4m (Figure 2, below).
The more diffuse and lower southern sand bar is 21m wide and crests at about 0.8m. Shoreward here is a smaller but more pronounced sand bar cresting at about 1.05m (Figure 3, below).
Near the northern proximal end, tidal creeks have dissected the sand bar. The whole cuspate sand bar system becomes completely submerged when the tidal elevation reaches 1.0m (chart datum). This means that there are only a few days in each month, just after the new moon and full moon, when the sand bar is exposed for more than three hours. A triangular-shaped lagoon is clearly recognizable behind the cuspate sand bar during low tide. The lagoon floor slopes seawards from the beach to the outer sand bar; its inner edge is slightly higher than height of the sand bar so that part of the inter-tidal flat is still exposed when the sand bars were completely submerged. The lagoon floor is uneven, with minor deposits of sands, rounded depressions excavated between seaweeds/sea grasses, forming sand pools, and channels cut by tidal scouring. Calmer areas under permanent water are dominated by mud whereas areas subjected to active flowing water are sandy. When the tide level reaches 1.15m, half the inter-tidal area is submerged. It is completely submerged when the tide reaches about 1.8m.
LAGOON AND BAR DYNAMICS
The lagoon area is highly dynamic with sands from the outer sand bar moving shoreward across the surface, burying and then exposing the old surface. The seaweeds and sea grasses are more sensitive to burial and are not able to grow through them resulting in the mobile sand areas being usually devoid of any vegetation cover. The moving sand also provides a different habitat for marine life. The lagoon is drained by numerous tidal creeks, which have cut both shallow and deep channels across the lagoon floor. These creeks are active even during maximum low tide, draining out the water from the lagoon as well as the seepage from land. Part of the lagoon is always submerged and this helps to provide a habitat for marine life sensitive to desiccation. Even during maximum low tide when most of the lagoon is exposed, there remain pools of water providing refuge. The flowing water helps to ventilate the lagoon ecosystem. The tidal creeks merged into bigger tidal systems before dissecting the outer sand bar in several places, breaking them up into discrete bodies.
EVOLUTION OF CHEK JAWA: AN INTERPRETATION
The Holocene sea level history of Singapore, similar to that elsewhere on the Sundaland, suggests a high sea level of about three metres when the sea reached its maximum during the Holocene marine transgression about 5,000 years ago. During this period, the sea stood against the granite coasts of Tg. Chek Ewa and corals formed a fringing reef along the drowned slopes. Behind the coral rim is a lagoon of coral gardens. As the sea receded, the lagoon became shallower and became more sheltered. Finer sediments began to accumulate, smothering and killing the corals on the lagoon floor. Sands supplied from the sea floor and from disintegrating coral reefs began to accumulate around the edge of the coral platform to form the present day cuspate spit. As further sedimentation takes place in the lagoon and as some of the sands migrate shoreward across the lagoon floor, the old coral platform became completely buried by sand and mud, which were colonized by mangroves along the inner edge and by seaweeds and sea grasses elsewhere. The presence of a buried coral platform can only be confirmed by coring although the continual supply of weathered coral fragments suggest its presence. In general, the lagoon continues to become shallower with the sedimentation of mud and as sands migrate across it. However, in other areas, the burrowing activities of bottom dwelling fish and other marine animals help to lower part of the lagoon floor. Some deeper areas are maintained by tidal scouring.The sedimentation history of Chek Jawa can only be revealed by detailed coring on land, on mangroves, and across the lagoon floor to the outer edge of the sand bar. Dating will help reveal the chronology of events. Knowing the evolutionary history of Chek Jawa will mean learning more about Singapore.