Proceedings of the 2005 International Conference on Simulation & Modeling V. Kachitvichyanukul, U. Purintrapiban, P. Utayopas, eds. ABSTRACT The Tonle Sap Lake in Cambodia is the largest permanent freshwater body in Southeast Asia. Cambodian floodplains, including Tonle Sap floodplain, contain the most extensive wetland habitats in the Mekong system. Mekong Upstream developments, such as the construction of dams may lead to significant trapping of sediments and nutrients and re- duce the productivity of the Tonle Sap system. The 3D EIA hydrodynamic and water quality models have been set up for Tonle Sap for simulating water levels and currents, inundation of the floodplain, suspended sediment transport and sedimentation, and dissolved oxygen to understand the ecosystem processes and the possible changes in it due to the upstream development. The model simulations can be used to assess the impacts of development scenarios and support sustainable resource management in lake and its floodplain. 1 INTRODUCTION The ecosystem of the Tonle Sap Lake is driven by the monsoon floods of the Mekong River, one of the largest rivers in the world. In wet season the lake grows fourfold and the water level rises 6-10 meters. This unique pulsing system makes the Tonle Sap one of the most productive freshwater ecosystem in the world. A dominant feature of the Tonle Sap system is that the sediment flux to the Tonle Sap Lake in the flood season (June-September) is many times larger than the outflow flux in the dry season (October-May). This means that the Tonle Sap Lake and floodplain ecosystem is retaining more than 80 % of the sediments it receives from the Mekong River and tributaries and utilizing this material in the eco- system processes. The Mekong River is responsible for the main part (ca. 70%) of the annual 7 million tons sediment load to Tonle Sap system (WUP-FIN, 2003). It is hypothesized that sediments carried by the Me- kong waters to the Tonle Sap Lake bring in the essential nutrients that feed into the lake’s food webs (WUP-FIN, 2003). The higher the flood the more sediments and nutri- ents imported (van Zalinge et al., 2003). Upstream devel- opments in Mekong such as dam construction have already led to significant trapping of sediments and nutrients (Kummu et al., 2004) and may reduce the fertility of the Tonle Sap system. Significant changes in the flood regime may influence the productivity of ecosystem. The EIA 3D model system has been set up for the lake during the “Modelling of the Flow Regime and Water Quality of the Tonle Sap” (MRCS/WUP-FIN) project to assess and evaluate the impacts of physical and environ- mental changes in the Tonle Sap Lake in relation to the whole Mekong basin, as well as more locally in Cambodia. The aim has been to assist in maintaining sustainable con- ditions of Tonle Sap system. 2 TONLE SAP LAKE The Tonle Sap River connects the Tonle Sap Lake to the Mekong River and joins it at Chaktomuk junction near Phnom Penh, after which the river immediately splits into the smaller Bassac River and the larger Mekong River (Figure 1). The area is globally unique and the lake has an extraordinary hydrological system. In the wet season, the Tonle Sap River changes its direction and flows to the Tonle Sap Lake instead of from the lake because of the flooding of the Mekong River. The lake functions as a natural flood water reservoir for the Mekong system and thus, is very important source of water for the Mekong delta during the dry season. MODELLING SEDIMENT TRANSPORTATION IN TONLE SAP LAKE FOR IMPACT ASSESSMENT Matti KUMMU Water Resources Laboratory, Helsinki University of Technology, Finland matti.kummu@iki.fi Jorma KOPONEN Environmental Impact Assessment Centre of Finland Ltd (EIA Ltd), Finland Juha SARKKULA Finnish Environment Institute (SYKE), Finland