Beach changes on a monsoon coast, Peninsular Malaysia

Abstract

The northeast monsoon (November-February) has a greater impact than the southwest monsoon (May-August) on the east coast of Peninsular Malaysia. Beach changes from the southwest monsoon to the northeast monsoon are abrupt, compared to the gradual recovery of the beach from the northeast monsoon to the southwest monsoon. Although its impact varies yearly, the northeast monsoon leaves behind a beach in which the backshore and the upper foreshore are relatively undisturbed by low wave action in the succeeding monsoon until the onset of the next northeast monsoon. The impact of the northeast monsoon on the beaches is essentially erosional; the foreshore and backshore are cut back, ridges are flattened and the beach profile is smoothed out. In contrast, the beaches are accretional during the southwest monsoon; accretion takes place on the foreshore and decreases in intensity landward. Ridges, bars, berms and cuspate features are more common during the southwest monsoon than during the northeast monsoon. Foreshores with sand less than 0.5 mm diameter tend to build a relatively steeper gradient in the southwest monsoon than in the northeast monsoon; foreshores with sand more than 0.5 mm diameter are unable to build a relatively steeper gradient in the southwest monsoon and in fact have a slightly lower gradient in the southwest monsoon. Beach cusps of the northeast monsoon are found at a higher level on the beach and are spaced 24.38-30.48 metres (80-100 feet); they remain in a degraded form and are succeeded in the southwest monsoon by another series at a lower level on the beach with spacings at 15.24-18.29 metres (50-60 feet). The ridge and runnel topography is more common in the southwest monsoon than in the northeast monsoon. For beaches with such topography in both monsoons, the ridges and runnels are more pronounced and of a higher amplitude in the southwest monsoon than in the northeast monsoon. Under constructive wave action, the ridges migrate upbeach and eventually merge with the upper foreshore or the berms. Where there is an abundant supply of fine sand, the ridge topography is further enhanced by aeolian action. Beach vegetation also helps to maintain the height and configuration of the ridges. The alternation of 'cut' during the northeast monsoon and 'fill' during the southwest monsoon in an area of active ridge formation will eventually produce a successive series of parallel ridges. The monsoons also affect the direction and magnitude of the littoral transport. The nearshore topography acts as a reservoir for material removed from the beaches during the northeast monsoon and a source of migratory forms moving landward and upbeach during the southwest monsoon.