International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 399–410 - 399 - International Journal of Sediment Research 25 (2010) 399-410 Hydrodynamics and suspended sediment transport at tidal inlets of Salut Mengkabong Lagoon, Sabah, Malaysia Md. Azharul HOQUE 1* , Bobita Golam AHAD 1 and Ejria SALEH 1 Abstract Salut-Mengabong Lagoon is located at the west coast of Sabah facing the South China Sea. At the bay side of the main inlet the lagoon splits into Salut and Mengabong Channels. Sediment dynamics at the inlets of the lagoon have recently received considerable attention. But any direct measurement of hydrodynamics and sediment flux are yet to be well documented. This study covers the field measurements of current velocity, water flux, suspended sediment concentration and sediment flux across the three transects (main inlet, Salut entrance and Mengkabong entrance) during typical spring and neap tidal cycles in southwest monsoon and northeast monsoon. Temporal variations and time- averaged values of measured parameters are discussed. The inlets of Salut-Mengkabong Lagoon are found to be ebb-dominated. The time-averaged velocities during spring tidal measurements are found to be higher in the main inlet followed by Mengkabong entrance and Salut entrance. Suspended sediment concentration and sediment fluxes are substantially higher in spring tidal cycles compared to the same in neap tidal cycles. During spring tidal cycles, ebb tidal sediment fluxes are higher than the flood tidal fluxes. The ebb dominated flux across the main inlet led to the large ebb shoal. Key Words: Hydrodynamics, Suspended sediment, Lagoon, Tidal inlet, Salut, Mengkabong 1 Introduction Located in the inter-tidal area of the west coast of Sabah, Malaysia, Salut-Mengkabong Lagoon plays an important role in the coastal ecology as well as the socio-economic development of the entire area. The lagoon ecosystem of Salut-Mengkabong is characterised by mangroves, shallow coastal waters, tidal flushings and surface wter flow from inland areas. Increased developement pressure and anthropogenic activities adjacent to the lagoon raised the importance of a number of issues: erosion, accretion, and morphodynamics behavior along the coastline, water quality and nearshore marine habitats. Due to excess sediment accretion, some areas are becoming shallower and the main inlet is becoming narrower thereby affecting the coastal ecology of the area (Saleh et al., 2005; Hoque et al., 2009). Increasing suspended sediment accumulation affects the valuable marine inhabitants on the seabed and the water column (Saleh et al., 2005). Tidal inlet of a lagoon is the most dynamic area for water circulation and suspended sediment transports (Bruun and Gerritsen, 1960; Bruun, 1969). The water exchange and sediment flux through the inlet govern the spatio-temporal distribution of hydro-geomorphologic characteristics and physico-chemical water properties, which are important for the description of productivity and carrying capacity of a lagoon. The flow of water current creates natural flushing to maintain good water circulation and for physical water properties, such as the salinity, p H , temperatures, and dissolved oxygen. Furthermore, the flow of water current also transports suspended sediments to the specific area, which is important to sea life, as 1 Ph.D., Lecturer, 2 Graduate student, 3 Lecturer, Borneo Marine Research Institute, University Malaysia Sabah Kota Kinabalu 88999, Sabah, Malaysia, *Current address: Water Control System Planning Engineer, Manitoba Water Stewardship Winnipeg, Manitoba, R3J 3W3, Canada, E-mail: azharulhoq@hotmail.com Note: The original manuscript of this paper was received in Sept. 2008. The revised version was received in July 2010. Discussion open until Dec. 2011.