Revisiting the salinity condition in the Kuantan Estuary

The downstream region of the Kuantan River has a high potential of being affected by salinity issues according to research conducted nearly 40 years ago in studying the salinity effects on the freshwater intake in Kobat. The resulting results from the study then lead to the construction of a barrage in 1985 to prevent salt intrusion into the Kobat freshwater intake station area during low flow and high tides. However, salt intrusion problems were reported to occur in recent years. Thus, this research aims to: 1) determine salt intrusion extends in the Kuantan Estuary after the construction of the barrage; 2) calibrate and validate the Van der Burgh Coefficient K, and dispersion coefficient D in determining the salt intrusion pattern; and 3) compare the salinity pattern before and after the construction of the barrage. A 1-D analytical salt intrusion model was adopted in this study for the salinity analysis. Field measurements were conducted along the dry season at spring tide when the salinity travels furthest with a minimal amount of freshwater discharge. Salinity data was collected only up to the Kobat barrage by using the moving boat. The total salinity length at HWS obtained for 27th & 29th March 2017, 3rd October 2017, 7th March 2018 and 15th April 2018 are 21 km, 18.82 km, 21.05 km, 16.24 km, and 24 km, respectively. Results obtained from the salt intrusion simulation showed that the Van der Burgh Coefficient K for Kuantan Estuary is 0.45 and the dispersion ranged between 100 and 250 m2/s. Meanwhile, the comparisons of the newly surveyed salinity results with the previous research have indicated that the pattern of longitudinal salinity distribution has significantly changed after the construction of the barrage. The finding from the recent survey demonstrated the scatter distribution pattern compared to the gradually uniform pattern in the previous survey. This condition shows that the natural hydrodynamic process in the Kuantan Estuary has no longer in equilibrium condition and is unstable. The validation results obtained show good agreement between the simulated and observed salinity distribution in the estuary with low RMSE in the range 0.4 ppt-5.0 ppt and high NSE values ranges 0.97 to 1.00. This indicates that the model is reliable and can become an important tool for water managers in conducting salt intrusion studies for this area.