A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Urban Water Engineering and Management jointly offered by the Asian Institute of Technology, Thailand and UNESCO-IHE, The Netherlands

Thesis (M.Sc.) - Asian Institute of Technology - UNESCO-IHE, 2017

This research focussed on assessment of salt water intrusion in Lower Chao Phraya River Basin under the influence of future sea level and different river flow regimes, in relation to municipal water supply in Bangkok. One dimension MIKE 11 hydrodynamic and advection dispersion modules, were applied in the investigation of the extent of salt water intrusion under sea level scenarios RCP4.5 and RCP 8.5 provided by downscaled GCM (BCCCSM1.1) and different change in river flow rates in years with the highest monthly sea levels. They included; 2032, 2056 and 2079. The model was set up with two upstream boundaries created from two cases including; average daily discharge (1979-2014) and lowest river flows in 2014. Downstream boundary was provided by downscaled hourly tidal elevations. The model calibration and validation was based on measured data collected from relevant institutions in Thailand. Simulation results showed that salt water intrusion in Chao Phraya River, is more related to the phenomenon of change of river flows and sea level rise. This was demonstrated by river flows derived from average daily discharge and decreased by 10%, resulting to salt intrusion of 76.07km in 2032. Further reduction of river flows by 40%, increased the intrusion length to 99.59km in 2056. On the other hand, simulation results under increased river flows depicted gradual decrease in the extent of salt water intrusion. The study showed that salt intrusion was only 70.27km from the river mouth under 10% increase in flows which reduced further to 61.79km under 40% increase in river flows in all the years considered. Salinity simulations in which upstream boundaries were created using the lowest flows in 2014 showed that salt intrusion could vary between 113.47km to 133.37km with decrease in discharge by 10% and 40% respectively in the year 2056. Further, the extent of salt water intrusion decreased from 103.86km to 94.62km with increase in discharge from 10% to 40% respectively. Modelling provided insight into understanding the effect of future change of river flows and sea level on salt water intrusion and hence formulation of alternative measures. The occurrence frequency of salt water with concentration greater than 0.25g/L has been shown to vary between (8-9) days in 2032, (12-20) days in 2056 and (18-25) days in 2079. Salinity simulations results using the lowest river flows in 2014 showed future salt water concentration greater than 0.25g/L could persist for up to 108 days. Formulation of alternative measures were based on the effect of dilution on salinity, the furthest distance salt water can intrude in order to locate the upstream position where the intake could be safe from the incidences of salt intrusion and effect of increasing upstream discharge through reservoir operation to provide sufficient water to prevent ingress of saline water. The results on economic evaluation showed that the alternative on raw water reservoir for dilution of salt water in the main canal and upstream relocation of the intake facility are not economically feasible in 2032 but viable from 2056 onwards when the occurrence frequency of salinity with concentration greater than 0.25g/L is between (8-25) days. However, based on a retention time of 108 days, the two alternatives are also not economically viable in 2032. The same alternatives become viable in 2056 only if the discount rate is less than 2%. The alternative on increased inflow from main reservoirs is economically feasible in all the years under consideration and can be considered as a current short term measure because it is highly dependent on water levels in the reservoirs which is highly uncertain and variable.

A thesis submitted in partial fulfillment of the requirements for the Degree of Master of Science in Urban Water Engineering and Management jointly offered by the Asian Institute of Technology, Thailand and UNESCO-IHE, The Netherlands