The effects of inlet sedimentation on water exchange in Maha Oya Estuary, Sri Lanka

Abstract: The Maha Oya river mouth, located on the west coast of Sri Lanka, is seasonally closed by a sand bar formed at the river mouth due to little rainfall and thus little river discharge. A new Outlet, located just north of the Maha Oya river mouth, was created by the 2004 tsunami. The Outlet remains open all year because of its location in the lee of an offshore breakwater. When the Maha Oya river mouth is closed, the river discharge flows out to sea through this new Outlet via the connecting Dutch Canal. For this thesis field measurements were undertaken over nine weeks during the dry season in Sri Lanka. The measured data included salinities, water levels and discharges for three cross sections close to the above mentioned tsunami Outlet. The main objective of this thesis was to investigate how the water exchange in estuaries and river mouths is affected by sedimentation in the coastal areas of Sri Lanka, using the Maha Oya as an experimental site. A mathematical model, HEC-RAS, was used to calculate water surface elevations and discharges at given points for the investigated area. The model was calibrated with the measured data. Simulations were then carried out for different openings of the river mouth with varying discharges in Maha Oya in order to quantify the effect on the water exchange through the Outlet. As expected, the result of the study showed that the water exchange in the Outlet was considerably higher for a closed river mouth than for a completely open river mouth due to the decrease of runoff in the Dutch Canal. When the river mouth was just a few meters open and the discharge from Maha Oya was strong the water exchange in the Outlet was large due to increased runoff in the Dutch Canal. Overall, the Outlet created by the tsunami does not seem to have a large impact on the water exchange in Maha Oya. However its existence might facilitate the everyday life for the people living in the area, providing them with a passage to the sea for the periods when the river mouth is closed. The effect on the water levels and the risk of flooding in the area is also diminished during heavy downfall thanks to the Outlet.