Simulation of Water Quality Process in A Tidal River with Boating Activities and Mangrove Forests / Alifatul Haziqah Abu Hanipah

Submitted in Fulfillment of the requirements for the Degree of Doctor of Philosophy

ABSTRACT
This research is carried out to develop a numerical model for hydrodynamic and water quality simulation of the Brunei River. However, the Brunei River has several specific features that pose challenges to the establishment of water quality modelling. One particular feature is the mangroves that grow along the river bank, which affect water flow and water quality. However, the kinetics of pollutant degradation in mangroves, which is referred to as the degradation coefficient, is not known.
Other features are the human activities conducted on the water that also affect water quality, such as at the water village and water taxis. As opposed to the common point (PS) and non-point source (NPS), the water village is considered in this research as a distributed pollutant source as wastewater is directly discharged and distributed on a large area of the Brunei River water surface. As it is uncommon, a distributed pollutant source has not been studied previously in water quality modelling.
Communities and visitors rely on boats as a means of transport for commuting between the mainland and water village. Besides the natural reaeration of the tidal river, intense boat traffic contributes oxygen through artificial reaeration from the mechanical propellers. The reaeration process, which is interpreted as the reaeration coefficient, for a tidal river with intense boat traffic has not been explored previously. Therefore, this research has been initiated to deal with these specific features in order to establish the numerical models.
Specific investigations were conducted on three main aspects: 1) pollutant degradation coefficient for mangroves, 2) reaeration coefficient caused by intensive boat traffic, and 3) pollutant loading from the distributed source of the water village on the water surface. The kinetics of pollutant degradation in mangroves were obtained from experiments as indicated by the degradation coefficient, Kdegradation, which is adjusted up to 20 ppt of salinity and is suitable for tropical mangrove rivers. The study of reaeration implies the importance of boat activities as an additional source of oxygen in the Brunei River. A predictive equation was derived for the reaeration coefficient caused by boat activities, Kreaeration, boat, as a function of boat traffic intensity (N) and water depth (H). Investigations of the distributed source on the water surface have shown that it is less significant compared to the pollution load from PS and NPS of other sub-catchments. The finding has corrected the existing common belief that the water village is highly impacting the Brunei River water quality.
The coefficients which have been identified from the experimental studies have been incorporated into the ECO Lab model, which also includes the uncommon distributed source through a specific approach in both the hydrodynamic and ECO Lab models, to simulate the water quality process of the Brunei River. Special bathymetric, hydrological, and water quality surveys were conducted for the establishment, calibration, and validation of the model since this was the first time it has been attempted in Brunei.
By adjusting the hydraulic roughness of Manning's 'M' coefficient for the mangroves and the main channel, the hydrodynamic model was successfully calibrated with measured water levels at Limbang Station (R-sq of 96% and MAE of 0.1 m) and Sg. Ukut Station (R-sq of 98% and MAE of 0.01 m). The model was validated with the measured current flow velocity magnitudes at Station 1 and 2, which shows satisfactory comparison during flooding (MAE of 0.08 m/s and 0.07 m/s respectively) but with less.

Thesis ( Doctor of Philosophy )

Includes bibliography references.