The interactive effects of climate, land cover change and damming on the flow regime and fine sediment dynamics of a tropical river

Lum, Zoe Xin Yi (2023) The interactive effects of climate, land cover change and damming on the flow regime and fine sediment dynamics of a tropical river. MRes thesis, University of Nottingham.

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Tropical river ecosystems are being increasingly modified by the accelerated construction of hydropower dams. Dams are known to have detrimental effects on downstream flows and sediment transport, with long-lasting implications for fluvial processes, habitats and ecosystems. The tropics are also experiencing high rates of deforestation and forest degradation. These types of land use change can alter components of the hydrological cycle through the modification of terrain characteristics, evapotranspiration and fine sediment runoff. Moreover, predictions of increased temperatures and changes in precipitation in tropical regions may further modify the hydrological cycle and, in turn, river flows.

The state of Sarawak, Malaysia, is witnessing the construction of 12 mega-dams as part of the Sarawak Corridor of Renewable Energy. The compounding effects of these dams, land use change and climate change pose a significant threat to the hydrology and overall functioning of rivers in Sarawak, but this threat has received little scientific attention. Baleh river was chosen as the study site because it is a typical Malaysian river surrounded by intact forest and its catchment is a headwater region with clear hydropower potential that will soon be realized. Thus, it is an area which can provide highly valuable new information on how large dams interact with landcover and climate change to alter river dynamics. This study therefore aimed to assess how damming, land cover and climate change interact and influence runoff of water and sediment in the River Baleh, a naturally forested tropical catchment in Sarawak. It incorporates two main components: (i) long-term land cover change assessment, and (ii) hydrological modelling of the impacts of the dam, climate and land cover change on flows and sediment loads in the Baleh. The land cover change assessment involved processing multi-temporal satellite images in Google Earth Engine (GEE) and carrying out supervised classification in ArcGIS. Besides providing empirical data on the magnitude and nature of land cover change across the Baleh catchment, the classified image outputs from this component formed input data for the second component - the catchment hydrological modelling. For the hydrological modelling, the Soil and Water Assessment Tool (SWAT) was used to simulate discharge and fine sediment loads for the whole of the catchment. The model was calibrated and validated using observed flow data. The performance of SWAT on daily and monthly time-steps was good (NSE > 0.62). Baseline conditions in the catchment were established with SWAT before running the future climate, land cover and dam scenarios; baseline models runs used the most recent five-year period. The future scenarios modelled the influence of land cover change with both low and high deforestation rates as well as climate models that involved increased temperature and decreased rainfall. Two operational scenarios were devised for the dam: a non-hydropower regime simply balancing dam outflow and inflows, to maintain lake levels and avoid dam spilling, while a hydropower regime was created which involved the dam operating at specific percentages of its capacity over the course of the year.

Analysis of satellite images indicated that there has been very minimal land cover change in the Baleh catchment over the last two decades (<2% reduction in forest cover) but a significant expansion of logging roads; these roads may promote future deforestation. SWAT models suggested that even high deforestation rates (loss of 5% per year) will not cause major hydrological changes in the Baleh River, but the models indicated dramatic increases in sediment yield from sub-catchments and, in turn, increases in the total amount of sediment exported by the catchment (up to 736% increase by 2050, compared to baseline). Modelling of climate scenarios suggested a counter-acting effect, with predicted lower rainfall and high temperature decreasing flow and sediment loads. SWAT simulations suggest that the dam will have a greater impact on flow and sediment loads in the Baleh catchment than the future land cover and climate change scenarios that were modelled. The presence of the dam has the greatest impact on flow and sediment at the dam site, reducing sediment loads by approximately 95% and reducing discharge variability. Simulations suggest that impacts on flow are still evident almost 100 km downstream, despite tributary inputs. Impacts on sediment loads at the catchment outlet are more difficult to understand, because of how the river might adjust to cumulative alterations in flow, competence and supply over decadal timescales. Preliminary analysis for the first five years of dam operation suggests that sediment yield at the basin outlet may differ from baseline much less than it does at the dam site but further work on this is needed, particularly because of the sensitivity to exact dam operational regimes (which at present are unclear). The high sediment yield from the upper sub-catchments has implications for the operation of the Baleh dam due to siltation, but the large size of the reservoir means that even by 2050 its storage capacity will have been reduced by only around 5%.

Overall, this study demonstrates how models such as SWAT can be used to provide insights into the complex interacting effects of anthropogenic stressors in tropical catchments. It is recommended that a period of 10-30 years is used for SWAT studies of the downstream effects of dams, to capture the sequences of transient states which will evolve in response to altered flow regimes and sediment supply. The traits of tropical rivers such as the Baleh (high discharge and sediment loads relative to catchment area) and the numerous tributaries create the potential for more rapid downstream ‘recovery’ than in other hydroclimatic settings, but this is confounded by ongoing climate and land cover changes which modify boundary conditions. Modelling assessments of the type presented here should be complemented by empirical studies of fluvial adjustment, to fully understand the habitat and ecological changes that follow impoundment.

Item Type: Thesis (University of Nottingham only) (MRes)
Supervisors: Gibbins, Christopher
Batalla, Ramon
Vericat, Damia
Lechner, Alexander
Keywords: hydropower dams, river ecosystem, deforestation, forest degradation, renewable energy
Subjects: Q Science > QH Natural history. Biology
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Science > School of Environmental and Geographical Sciences
Item ID: 69859
Depositing User: LUM, Zoe
Date Deposited: 18 Feb 2023 04:40
Last Modified: 18 Feb 2023 04:40

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