The main objective of the salinity modeling exercise is to investigate the extent of salinity during raw water abstraction at the Bakong Intake facility when drought event coincides with a spring tide scenario, and the impact of sea water level rise on salinity extent. The saline intrusion limits along the river is crucial because the Bakong Intake is the only existing abstraction facility available to supply treated water to the nearby dwellers. Field surveys were carried out to obtain river cross section data, real time water level sampling and salinity sampling data. The collected data were used as input for the hydraulic model development and calibration. 180 mm/yr sea water level rise proposed by IPCC (2007) was adopted in hydraulic modeling to investigate the impact of sea water level rise on saline intrusion and intake abstraction operation. The modeling results indicate that at an abstraction rate of 240 Million Liters per Day (MLD) over a 1 in 50 year drought, the salinity level at the Bakong Intake would have reached 7 mg/L and 11 mg/L without and with sea water level rise respectively, which is well within the acceptable level of 250 mg/L based on the Malaysian National Standard for Drinking Water Quality 2000. Therefore, in a 1 in 50-year drought event, an abstraction rate of 240 MLD at Bakong Intake is sustainable with maximum simulated salinity level below 20 mg/L.

The Batang Baram is the second longest river system in Sarawak, after the Batang Rajang. It encompasses a catchment area of 22,600 sq km (Figure 1). The main tributaries upstream of Marudi are the Batang Tinjar and the Sungai Tutuh. The Sungai Bakong is a relatively minor tributary of the Batang Baram with an estimated catchment area of 1,600 sq km. It flows into the Batang Baram at approximately 75 km upstream of the Batang Baram river mouth with the sea as shown in Figure 2. The gradients of the riverbed of the lower part of Batang Baram have been observed to be very gentle indeed. From local information, tidal influence can be felt as far up as Long Lama along the Batang Baram and Long Teru along the Batang Tinjar.

Local information on the salinity front during the past drought events appear to confirm that the saline front does not advance past the town of Marudi. Along the Sungai Bakong itself, the saline front reportedly reached the Sg. Arang tributary in previous droughts, some 18 kilometers downstream of the existing Bakong Intake plant. Being so close to the Bakong Intake, this salinity study is therefore, considered very essential to ensure that the raw water at the Bakong Intake will not be severely affected by salinity during a design drought with maximum abstraction rate.

In order to study the drought flow (Michael et al., 2009) in the Bakong River together with water quality and salinity parameters of the Batang Baram, a mathematical model has been constructed, calibrated and utilized. The INFOWORKS RS software package as developed by HR Wallingford of the UK has been used in this research. The essential data required for the development, calibration and verification of INFOWORKS RS are surveyed for river cross sections and real time water level and water quality field measurement (Terry, 2001).

Environmental Sciences Journal, Hydrogen Chloride, Hydraulic Modeling, Global Positioning System, Sonarlite Digital Echo Sounder, DID Rain Gauging Stations, Bakong River Gauging Station, Water Quality Measurements, Soil Moisture Accounting, Unitvariate Gradient Method.