Chong, Xin Yi
(2023)
Developing environmental flows for the Baleh River – hydrological and geomorphological processes in the Baleh catchment prior to damming.
PhD thesis, University of Nottingham Malaysia.
Abstract
Dams have been built in many parts of the world since the earliest human civilizations, but currently there is a significant program of dam building underway in the tropics. Despite general awareness of the likely detrimental impacts of dams, the limited number of empirical tropical studies means that specific effects of dams on these important but imperilled ecosystems are hard to predict. Although the Functional Flows (FFs) concept is now prominent in the scientific and river management literature, it has been applied only to some very specific cases in tropical areas to guide dam operation (e.g. to support periodic flooding of floodplain areas). In these and many other cases, FFs have been applied retrospectively, to design operational flows for existing dams. The work presented in this thesis concerns development of flow recommendations prior to the construction of a large tropical hydropower dam on Baleh River, Sarawak. Specific objectives of the thesis are to: (i) develop a full statistical understanding of the natural hydrological regime of Baleh River, (ii) identify critical habitat forming discharges (those responsible for coarse sediment entrainment and transport) and relations between discharge magnitude and hydraulic habitat heterogeneity, (iii) to assess the impact of the dam and other land use changes on sediment connectivity, and (iv) provide specific recommendations for dam operation to support sediment entrainment and maintenance of habitat heterogeneity, and for management of lake levels to limit changes in connectivity.
The work involved analyses of historical data along with a number of empirical field data collection, modelling and remote sensing approaches. For Objective (i), a 51-year hydrological and hydro-climatological dataset was analysed in order to characterise the natural hydrological regime of the Baleh River. The analyses focussed on understanding the frequency, magnitude, duration, and timing of natural high and low flow events. For Objective (ii), HEC-RAS® models were built for 3 study reaches. These models were used to simulate hydraulic conditions across the range of discharges extending from high to low flows. The output from the hydraulic models was combined with information on critical entrainment thresholds of sediment at the 3 reaches in order to understand which flows are capable of entraining sediment. The focus of these analyses was to understand whether the expected hydropower regime is likely to cause sediment entrainment in the way that occurs currently under the natural regime. The analyses also used HEC-RAS® to evaluate hydraulic habitat heterogeneity at low flow conditions. The HEC-RAS® modelling was integrated with Soil and Water Assessment Tool (SWAT® or ArcSWAT®) to understand flow magnitude in the river under different conditions of hydropower operation and rainfall (which would lead to tributaries downstream from the dam discharging high flows to the mainstem Baleh). In addition, the model coupled with the DEMs was used to look at the extent of exposure of gravel bars during low flow conditions. The SedInConnect model was used to understand patterns of sediment connectivity across the Baleh catchment. This model computes the sediment connectivity index (IC). For the modelling, it was necessary to develop DEMs and collate information on landcover in order to calculate topographic roughness and IC. Information on sediment entrainment was used to make recommendations about flows needed to maintain the geomorphic processes (i.e. bedload transport) post-impoundment that occur currently in the natural river. The low flow analyses were used to understand whether hydraulic conditions became more or less heterogeneous at low flows.
The hydrological analyses identified 5 distinct classes of high flow event in the Baleh (defined as ‘Event Types’). These types have distinct magnitudes and durations. HEC-RAS® modelling indicated that the smallest of these events led to sediment entrainment of D16 and greater at the study sites, and that larger events resulted in increasing areas of the bed across the study reaches experiencing entrainment. The maximum likely hydropower releases are smaller in discharge magnitude than the smallest of these natural events. The modelling indicated that more or less no sediment entrainment would occur during normal hydropower operations. However, integration of the HEC-RAS® and ArcSWAT® models indicated that when the catchment is wet, tributaries are contributing enough flow to result in sediment entrainment at the study sites that equates to that under some of the higher natural event types. Hydraulic habitat heterogeneity was relatively high during low flows and dropped at moderate and high flows. Moreover, at low flows, gravel bars became exposed at some of the study sites adding to overall habitat heterogeneity. According to the IC models, the impact of historical landcover changes on connectivity between the Baleh and the Rajang has not been pronounced. However, the models predicted that future forest clearance and roads will have observable impacts on the connectivity between the Baleh and Rajang. The models indicated that the construction of the dam would result in a significant impact on connectivity, with the effects of impoundment being greater than those of road and landcover changes, leading to a large portion of the Baleh catchment being almost completely disconnected from the Rajang. The operation of the dam will affect IC upstream of the dam due to the changes in connectivity caused by different lake levels.
The HEC-RAS® modelling suggested that over time, in the absence of significant bed entrainment and transport of material, the bed could be expected to become progressively armoured and coarser, as seen in dammed rivers worldwide. However, the modelling also indicated that when the catchment is wet and downstream tributaries are contributing much additional water, flows in the Baleh will greatly exceed turbine releases and appreciable entrainment of bed material can be expected. Recommendations for geomorphic functional flows are made on the basis of these findings. Recommendations are to ensure that hydropower releases using the maximum number of turbines are made during times when the catchment is already wet, to ensure entrainment of bed sediment. Based on analyses of the natural regime, the recommendations include suggestions for timing and duration of such releases. They should last around 20 days, occurring 8 to 15 times per year, predominantly during the wet season. Low flow recommendations were made to occasionally allow dam releases to drop to 1 to 3 turbines to mimic natural low flow events. The recommendation for these events was to allow flow to drop to the natural respective monthly baseflows; these flows should last around 3 days with a frequency of at least 3 times per month during the wet season, and 2 times per month during the dry season. The dam will disconnect a major part of the Baleh catchment from the Rajang. The modelling suggested that the effects of the dam on connectivity will swamp the effects of landcover change and road construction on connectivity. Nevertheless, localised impacts of the road were predicted by the model, with an increase in connectivity on the upslope areas. It is recommended that measures to mitigate this are implemented as part of road construction and management. The model suggested that careful management of lake levels could be used to reduce the impact of the dam on connectivity in the upper part of the basin.
The study of functional flows presented in this thesis is of growing importance for sustainable catchment management, particularly in Southeast Asia, where rapid dam construction and vulnerability to the impacts of climate change are prevalent. It is one of few studies globally that has explicitly considered fluvial processes when developing functional flow recommendations. It is also novel in integrating reach scale HEC-RAS® models with catchment scale SWAT® models to help in the design of functional flow recommendations. As far as the author is aware, it is the only study globally to have used the SedInConnect model to understand the combined and interactive effects of damming, landcover change and road construction on structural connectivity at a catchment scale. These approaches can be applied to large rivers in Southeast Asia and other Global South regions, aiding sustainable catchment and water management. It is one of few functional flow studies to have been undertaken before dam closure, allowing us to implement functional recommendations in advance of construction, rather than retrofitting them to a river that has been regulated for some time. It is recommended that a program of monitoring is implemented in the Baleh in order to understand how the river changes in response to impoundment and the success of flow recommendations for maintaining sediment dynamics and habitat heterogeneity. This monitoring will provide the basis for adaptive management.
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