1 EFFECTS OF SEASONAL VARIATIONS ON SANDY BEACH GROUNDWATER TABLE AND SWASH ZONE SEDIMENT TRANSPORT Norasman Othman 1 , Ahmad Khairi Abd Wahab 2 and Mohamad Hidayat Jamal 3 The hydrodynamics in the swash zone significantly affected the sediment transport mechanisms that mostly control beach face morphology especially under different weather conditions. Rainfall distribution patterns during dry and wet seasons in Peninsular Malaysia will influence groundwater table elevation and the beach profile. This study is aimed at investigating the effects of seasonal variations to beach groundwater elevations and surface profile changes. This work was undertaken at the Desaru Beach, Johor. Rainfall depth, groundwater table, tides, beach profiles, swash depth and swash velocity data were monitored and investigated at the study area. The results showed that the groundwater table was affected by rainfall patterns; higher during the wet season and lower during the dry season. The beach profile also showed erosive condition due to increasing of offshore sediment transport during the wet season, whereas in the dry season the beach profile showed accretion condition due to the increasing of onshore sediment transport. Swash properties like swash water depth and velocity were also monitored and analysed during this study in order to get a clear view about the saturation level effect due to seasonal variations into a infiltration processes at the swash zone. Finally the data showed that there is a lag time between rising and falling of groundwater tables and tides due to the lower hydraulic conductivity effect. Keywords: swash zone, rainfall; groundwater; beach profile change; field experiment INTRODUCTION The relationship between beach groundwater dynamics and swash hydrodynamics provides a dominant factor for swash zone sediment transport, which affects the morphology of the beach especially by controlling the movement of offshore or onshore transport. Several authors have successfully shown in their field works, laboratory experiments or numerical simulations that beaches with a low groundwater table are expected to accrete while beaches with a high groundwater table tend to erode (Duncan, 1964; Grant, 1984; Baird and Horn, 1996; Li et al., 2002; Ang et al., 2004; Horn et al., 2007; Bakhtyar et al., 2011). Infiltration and exfiltration are among the significant factors which are suggested by many researchers in order to explain clearly about why beaches with a high water table are likely to erode and low water table tend to accrete. The beach groundwater table position will affect the infiltration and exfiltration processes during the uprush and exfiltration. When the uprush reaches the beach face above the watertable’s exit point, the water may infiltrate into the bed and consequently decrease the uprush volume, depth and velocity. It is totally different during the lower exit point (saturated condition), the backwash flow will increase due to the groundwater seepage (Horn, 2006). However, these two factors need to be understood carefully especially under different types of beach materials. During the uprush flow, the seawater will spread quickly into the upper layers of the beach surface. At the end of the uprush process, the flow will turn to backwash and subsequent reduction in swash depth, there will be a rapid decrease of pore-pressure, producing forces acting vertically upwards just below the beach surface. This condition may lead to rapid groundwater outflow or exfiltration. If the upper layers of sediment become fluidised, then this might considerably increase the sediment transport since the fluidised layer would quickly become entrained by the seaward flow during the backwash. This hypothesis was tested using a model by Baird and Horn (1996), who concluded that fluidisation due to exfiltration may happen, especially in the final stages of the backwash process. Water may infiltrate into the sand at the upper part of the beach during uprush or backwash activities if the beach groundwater table is quite low. In contrast, groundwater ex-filtration may occur across the beach surface with higher water table. Such relations have been confirmed to have a big impact on the swash sediment transport in the past field studies by Duncan (1964) and Grant (1984). Water infiltration under lower water table is found to increase onshore sediment transport, while groundwater exfiltration under higher water table encourages offshore sediment transport. These field observations have theoretically guided or helped the beach dewatering technique to lower beach groundwater table in order to prevent the beach from erosion (Turner and Leatherman, 1997). Although some success has been achieved in 1 Faculty of Civil Engineering & Earth Resources, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300, Gambang, Kuantan, Pahang, Malaysia. 2 Coastal and Offshore Engineering Institute (COEI), Universiti Teknologi Malaysia Kuala Lumpur, Jalan Semarak, 54100 Kuala Lumpur, Malaysia. 3 Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia.