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Janggu, Joyce Anak (2024) Fine sediment loads and dynamics in Baleh River, Sarawak. MRes thesis, University of Nottingham.

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Abstract

Dams are well known to cause changes in sediment dynamics in downstream rivers, as a result of sediment trapping by the dam and changes in flow competence. While a major programme of dam building is underway in the tropics, the vast majority of fluvial geomorphic studies of dams globally have taken place in temperate and Mediterranean climate regions. More generally, sediment loads in tropical system are less well known than those in more northerly regions. Hence, there are fundamental and applied knowledge gaps related to fluvial processes in tropical rivers.

This thesis provides baseline data on fine sediment loads in the Baleh River (Sarawak, Malaysia Borneo) prior to damming. Baleh dam is due for completion in 2027. This thesis is part of a larger project designed to feed into the development of a Functional Flow regime for Baleh dam, prior to its operation. It provides insights into fine sediment loads near the catchment outlet over a one-year period, and information on spatial variation in suspended sediment concentration along the 100km length of the river.

A continuously logging sensor was installed near the Baleh confluence with the Rajang. This recorded data on turbidity over a one-year period, which were converted to suspended sediment concentration (SSC) via the instrument’s internal calibration set up. Data were related to discharge obtained from a nearby gauging station. Spatial patterns of SSC were assessed using drone and satellite images. Sensor signals in both these remote-sensing tools were converted to SSC using a combination of field samples and regression-based calibration.

It was estimated that the Baleh conveyed approximately 15 million tonnes annually into the Rajang. SSCs at the monitoring station were typically around 135mg/L (median concentration). Loads and concentrations both varied between months, although strong season patterns (wet versus dry seasons) were less evident. The study year was rather unusual, showing less pronounced seasonal hydrograph than normal for the Baleh.

The satellite images suggested while there are major changes in SSC along the Baleh, these are very complex and locally variable. Marked increases and decreases in SSC occur, as turbid and clean tributaries discharge their water to the mainstem respectively. However, the relative direction of these changes differs over time, with individual tributaries sometimes delivering water that is less turbid and sometimes more turbid than the mainstem. An overall increase in SSC along the 100km length survey was only evident when flows were relatively low.

Drone-based analyses indicated highly variable SSCs in confluence zones. Marked contrast in SSC was evident across spatial distances of only a few meters, with little missing of tributary and mainstem waters until several hundred metres downstream. The higher resolution (cm) of drone images compared to the satellite (10m) meant that they provided much more detail of such local contrasts. Together, the three approaches (time series data, satellite analysis of catchment scale patterns, and drone-based assessment of confluence zones) provided useful insights into fine sediment loads and dynamics in the Baleh prior to damming.

Item Type:	Thesis (University of Nottingham only) (MRes)
Supervisors:	Gibbins, Christopher Neil
Keywords:	sediment loads, sediment dynamics, downstream rivers, flow competence, dam building, tropical rivers
Subjects:	G Geography. Anthropology. Recreation > GE Environmental Sciences
Faculties/Schools:	University of Nottingham, Malaysia > Faculty of Science and Engineering — Science > School of Environmental and Geographical Sciences
Item ID:	78401
Depositing User:	Janggu, Joyce
Date Deposited:	27 Jul 2024 04:40
Last Modified:	27 Jul 2024 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/78401

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