Suspended Sediment and Light Attenuation Characteristics in Singapore Waters

Abstract

Light is attenuated by materials present in the water column by scattering and absorption. The knowledge of light attenuation and its contributing factors influencing visibility/turbidity is unknown in Singapore waters. This MSc thesis consists of field measurement, laboratory experiment and numerical modelling in 3 parts; parameterisation of light attenuation coefficient? K?_d, numerical modelling of underwater light field and visibility. The objectives of this thesis are; a) to determine the dynamics of optical properties in Singapore waters, b) to investigate factors contributing to light attenuation in Singapore waters and c) to model underwater light field and visibility using Hydrolight and Delft3D WAQ. This first part addresses the dynamics of light attenuation (in relation to low/high tide), the spectral nature of optically significant constituents (water, CDOM, phytoplankton and sediment) as well as the effects of particle size distribution (PSD) to light scattering. For PSD with a higher slope j in N(D) = ?KD?^(-j), the d50 ranges from 7.11 to 9.90 ?m in Singapore Strait representing sediment dominated environment. Higher TSS during low tide is advection dominated while lower TSS during high tide is suspension dominated. Relative contributions of CDOM, chlorophyll and sediment to light attenuation coefficient? K?_d is 1.4 – 6.5 % (average of 3.3 %), 1.3 – 62 % (average of 24 %) and 31.3 – 95.2 % (average of 70 %) respectively. Four (4) empirical equations to predict ? K?_d was developed depending on its optical and physical characteristics. Underwater light field modelling shows that light attenuation depend mostly on the concentration of optically significant constituents and only weakly dependent on light structure, cloud cover and fluorescence. The modelled ? K?_d from Hydrolight ranges from 1.2 to 2.3 m-1 with lower ? K?_d in chlorophyll dominated waters while the empirical estimations underestimate K_(d(PAR)) compared to the modelled ? K?_d from Hydrolight. The modelled Secchi depth, S_d ranges from 0.6 – 1.4m corresponding to euphotic depth z_eu of 2.0 – 3.8 m with deeper z_eu for chlorophyll dominated waters. The modelled visibility is generally lowest during Northeast (NE) followed by Southwest (SW) monsoon and Inter monsoon (IM) period with evidence of spatial homogeneity for all monsoons. The visibility model reproduces the recorded? S?_d reasonably well except for Johor Strait and sheltered areas in WCP due to the fact that some processes are not unaccounted in the model. The ? K?_d variation coincides with SSC variation with no phase difference; the visibility is lowest during spring low tide and highest during neap high tide. Convective interaction between the diurnal and semidiurnal components is important in offshore locations while the non-linearity of tidal propagation contributes in the shallow water of Johor estuary. The residual turbidity in Singapore waters is due to tides (semi diurnal and diurnal spring neap interactions) and non-tides (monsoonal effect) in approximately equal magnitude during monsoons. The results from this thesis are applicable in coastal engineering, ecological and remote sensing.