THE INFLUENCES OF TIDAL REGIME AND MORPHOLOGY CHANGE ON SALINITY INTRUSION IN HAU RIVER

Salinity intrusion is becoming a more serious phenomenon in the Vietnam Mekong Delta (VMD) under the impact of upstream dams’ development. Due to the significant change of sediment budget, about 85% reduction in 2015 compared to 1992, the erosion in the river and estuary makes the river wider and deeper. Besides, the tidal regime is more complicated with the average amplitude varying from 2.5 to 3.8m. They are the two major causes of severe saltwater intrusion. Two field investigations were carried out separately for measuring firstly 200 cross-sections along the Hau River and Tien River and secondly 300 salinity points along 45km in length and 5 cross-sections in Hau River during high tide and low tide. The salinity distribution vertically and longitudinally along 45km from the river mouth of the two branches in Hau River is very complex. During the high tides, at the cross-section 5km from the river mouth, the difference of salinity concentration between surface and bottom can reach 8.58g/l and varies sharply with the change of riverbed. While during low tides, salinity amplitudes differentiate less than 2g/l and distributed equally among layers. At all cross-sections, the salinity on the left bank is always higher than that on the right bank. During flood tides salinity penetrates 45km and 14km longer than that at low tides, the maximum difference of salinity is about 20.7g/l between the estuary and 45km point. The partial mixing and moderate stratification prevail over 45km of Hau River during the ebb - flood tide, but sometimes the salt wedge appears in the range of 10 to 20km at ebb tide and at lowermost of Hau River at flood tide. The results of this research are the basis for proposing the location of salinity control structures and building the operation rules for the existing sluice gates.