MODELLING OF SALINITY DISTRIBUTION AND WATER AGE IN THE MAHAKAM DELTA, INDONESIA

Pham Van, Chien [UCL] de Brye, Benjamin [UCL] Soares Frazao, Sandra [UCL] Deleersnijder, Eric [UCL] Hoitink, Ton [Wageningen Univerisity] Sassi, Maximiliano [Wageningen Univerisity] Hidayat, Hidayat [Wageningen Univerisity] et al.[show all]

The spatial distribution of salinity and water age in the Mahakam Delta are investigated with the help of numerical simulations obtained by the finite-element SLIM model. The depth or cross-section integrated shallow-water and advection-diffusion equations are solved by a means of the discontinuous Galerkin finite element method on unstructured meshes of domain of interest. A two-dimensional unstructured mesh exhibiting a highly variable resolution covers the Makassar Strait and the various branches of the Mahakam Delta, as well as a series of buffer lakes located over 150 km upstream of the delta. The Mahakam River itself and major tributaries are represented by one-dimensional elements. It is observed that the salinity in all tributaries and tidal channels of the delta decreases significantly from the delta front to the Delta Apex location during a simulation period from September to October 2009 due to the change of tides, the narrow and meandering channels, and the effects of river flow. The salinity is less than 5.0 PSU in the distance of 15 km downstream the Delta Apex even with the very low river flow is imposed at upstream boundary located 300 km upstream from the delta. On the other hand, using the concept of the CART, the water age in both low and high flow conditions is explored. The water age of four main tidal channels in the delta varies from 3.24 days under the high river flow (>4,200 m3/s) to 6.84 days under the low flow (<1,400 m3/s) condition. The water age changes depending on each tidal channel in the delta and the maximum value is about one neap-spring tidal cycle of 16 days in all simulation cases.