Influence of seasonal hydrological variation on swash zone morphological changes

Abstract

Studies on hydrodynamics of sediment transport in recent years stressed on knowledge advancement and practical applications of swash processes. The hydrodynamic processes significantly affect sediment transport mechanisms that control beach face morphology. In Peninsular Malaysia, seasonal variation of dry and wet periods in the months of May to September and November to March respectively, noticeably influenced the evolution of beach profile. This study investigates the influence of seasonal hydrological variation to beach groundwater table and how it affects swash zone morphological changes. Field monitoring was carried out at a section of Desaru beach, Johor for 30-months consisting of data on a beach cross-shore profile, rainfall distribution, groundwater table, tidal variation and swash flow velocity and depth. These were monitored and investigated to check for patterns of interrelationships among the parameters. Results indicated that the seasonal variation of Desaru beach morphology was primarily controlled by the cross-shore exchange of sediment in the swash zone. The beach profiles showed erosive condition due to seaward sediment movement during the wet season, while during the dry season, the beach profile showed accretion condition due to the increase of landward sediment movement. The total sand volume at Desaru beach was found to be highly correlated (R2 = 0.86) with the shoreline position. For detailed beach profile analyses, the Empirical Orthogonal Function (EOF) technique was applied and showed changes in profile configuration as a function of space and time. The analysis confirmed that the swash zone of Desaru beach is the most dynamic region on sandy beach with cyclic processes of erosion and accretion. Most of the variation in the beach profiles can be explained by the first five eigenfunctions (EOF1-EOF5) that has the highest eigenvalues. It was revealed that 99.99% of the data variance was captured by these eigenfunctions. This confirmed that the method provides a means of presenting space and time variability of beach profiles. The study also illustrated that the groundwater table elevation was influenced by rainfall patterns where higher groundwater table during the wet season and lower groundwater table during the dry season was observed. The fluctuating groundwater table is an important mechanism to explain the role of infiltration and exfiltration processes in the morphology of swash zones. The relationship between the average shoreline position with groundwater level monitored at wells BH2 and BH3 indicated moderate correlation with values of R2 equals to 0.75 and 0.67, respectively. The velocity reduction of uprush to backwash flow during the dry period was higher (73.08%) compared to during wet period (46.00%). Similar pattern was found in the swash flow depth analysis where the reduction percentage showed higher reduction during the dry period (52.17%) compared to during the wet period (24.64%). This study quantified seasonal hydrological variation influence on the swash zone morphological changes and provided a better understanding of beach profile evolution, leading to improved beach management practices in Malaysia.