Lip, Yih Yoong
(2024)
Evaluating the use of sustainable approaches for reducing river flooding and erosion in the Upper Trusan River, Sarawak.
PhD thesis, University of Nottingham.
Abstract
This thesis summarises work undertaken to address flooding and erosion problems in the upper part of the Trusan catchment, Sarawak, Malaysian Borneo (hereafter the ‘Upper Trusan’). The Upper Trusan has been experiencing severe flooding and associated riverbank erosion, both of which affect rice production. This work aimed to identify and evaluate options for reducing the risk and impacts of these phenomena using sustainable ‘green’ approaches. I assessed the suitability of different types of riverbank protection measures (brush walls, brush mattresses, bamboo fencing and geotextile mattresses) as short-term measures for reducing erosion in some of the worst affected areas. In addition, we evaluated opportunities for managed flooding to help reduce flood peaks and attenuate flood waves in the downstream areas. I also assessed the contribution that land use management (especially the conservation of forest cover) could play in minimising flood risk under future land cover scenarios.
Local bank protection measures were partly successful. Brush mattresses placed on graded (sloping) banks provided the most protection, lasting longer than other measures. Because of the upland nature of the Trusan, local measures need to withstand frequent high flow forces. Consequently, non-living measures such as bamboo fencing provided only limited and temporary protection from bankfull flows. The assessments indicate that ensuring structures remain living and have adequate toe protection and regular maintenance is essential if they are to be successful. Even so, in high-energy systems such as the Trusan, local measures should not on their own be seen as a long-term solution.
Flood modelling using HEC-RAS® indicated that managed inundation of floodplain areas would have very little effect on flood peaks in the Upper Trusan. The main reason for this is that, due to the confined nature of the valley, the floodplain area is relatively small – this limits its capacity to store sufficiently large volumes of water. The models suggested that more or less the whole of the floodplain would need to be allowed to flood in order to affect any significant reduction in flood magnitude. This would defeat the objective of the exercise since the whole area that communities want to protect from flooding would need to be allowed to flood. Thus, there is limited scope for corridor scale management using managed flooding.
Satellite-based analysis of land cover in the Upper Trusan suggests that there has been limited forest loss in the last three decades. Catchment scale hydrological modelling using SWAT® indicated that this change has had very little effect on the hydrograph and has not contributed to the reported increased incidence of flooding. However, modelling future land cover change scenarios indicated that significant loss of forest – especially conversion to bare earth – would increase the number of bankfull flood events from 2 to 3 each year. Adoption of Government proposals set out in a land use Master Plan would see up to 35% of the Upper Trusan converted to agriculture. SWAT® modelling suggested that agriculture at this scale would not have a major impact on flooding unless it is accompanied by extensive road systems and built-up areas that would alter hydrological responses to rainfall.
Overall, the work suggests that there is little scope for the adoption of managed flooding in the Upper Trusan and that, in this high-energy system, local green bank protection measures should only be seen as a short-term solution to reducing bank erosion. Conservation of natural forests is key to minimising future hydrological changes. The analysis leads to a number of conclusions and recommendations: (i) Local green measures can play a role in protecting riverbanks from erosion. However, emphasis should be on using live materials and ensuring maintenance, so they do not get washed out. In flashy rivers such as the Upper Trusan, a particular challenge is finding sufficiently long-time windows so they can be built and allowed to stabilise before the next high flows. (ii) The opportunity for using floodplains to store water in upland valleys such as the Upper Trusan is constrained by space. This and the likelihood of future climate and land cover change make it imperative that a wider perspective is taken to find a way for communities to become resilient in the long-term. (iii) What is needed now is a full socioeconomic assessment to fully understand rice's contribution to local community incomes and evaluate options for diversifying income sources. This way, communities can become less reliant on the very precarious floodplain rice production. In turn, this will create opportunities for floodplain retreat and the possibility of giving land back to the river, neither of which are possible in the current livelihood model. Such approaches help to conserve the physical integrity of the channel because of how they will reduce the need for perpetual engineering intervention.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Gibbins, Christopher
Vericat, Damia
Batalla, Ramon
Teo, Fang Yenn |
| Keywords: |
sustainable approaches, natural solutions, green measures, bioengineering, riverbank protection, erosion, flooding, flood mitigation, floodplain storage, floodplain retention, hydrology, SWAT, soil and water, runoff, landcover, landuse |
| 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 |
| Related URLs: |
|
| Item ID: |
77570 |
| Depositing User: |
Lip, Yih
|
| Date Deposited: |
27 Jul 2024 04:40 |
| Last Modified: |
27 Jul 2024 04:40 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/77570 |
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