An environmentally sensitive erosion control technique in the Mekong River Delta: 10 years on

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1 7 th Australian Stream Management Conference - Full Paper An environmentally sensitive erosion control technique in the Mekong River Delta: 10 years on John Tilleard 1, Tony Ladson 2 1 Moroka Pty Ltd, PO Box 67C, Somers, Vic, john.tilleard@moroka.com.au 2 Moroka Pty Ltd. P.O. Box 1245, Fitzroy North, Vic tony.ladson@moroka.com.au Key Points Bank retreat of 6-35 m/yr threatened the stability of the My Thuan bridge site in the Mekong delta Bank revetment was expensive, damaged the land-water interface and was unsuited to dominant channel migration processes. Submerged permeable groynes were proposed as an alternative. Physical modelling was used to optimize the configuration of 12x100 m groynes for bed stability. 3 years of post-construction monitoring confirmed velocity reduction achieved by the groynes. Recent observation, 10 years after construction, suggests the works are performing well. Formal monitoring is now needed to ratify the performance and confirm applicability to other sites. Keywords Mekong, permeable structures, bank erosion, channel migration, pile fields, bridge protection, groynes, erosion control, monitoring, My Thuan, Vietnam Introduction In 2002, to minimize erosion and scour threats to a major new bridge (Figure 1) in the Mekong Delta, AusAID funded works to reduce the rate of channel migration over more than 2 km of river upstream of the bridge. Figure 1. AusAID built the My Thuan Bridge in the Mekong delta in 2002 Instead of locally standard techniques involving bank battering and shaping followed by some form of rock or concrete revetment, highly permeable lines of submerged piles extending 100m out from the bank were used to disturb the formation of secondary currents, reduce water velocities and boundary shear stress and reduce the scour of sediment from the bed and banks. There was minimal interference with the river bank itself, and its existing form and vegetative cover was maintained. Intensive monitoring of velocities and bed and bank profiles for several years after construction provided reassurance that the technique would be successful. 10 years on, during a visit to the area, the bank was observed to have retained its largely natural character but bank migration appears to have been arrested. If this could be confirmed by a new round of monitoring, then the technique would offer an effective approach to managing erosion which is less intrusive and has less impact than standard revetment techniques, even on a large mobile bed river. and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages

2 History of channel migration upstream of the My Thuan bridge In the Mekong Delta channel migration over many kilometres of river is associated with rapid bank retreat (Figure 2). The retreat is the result of the combined effects of natural river and tidal processes, impacts of land use in the catchments, floodplain and riparian zone, use of the river for gravel and sand extraction, navigation, irrigation and hydropower and impacts of climate change. The typical response by local agencies and international aid organisations is to form and shape the river bank and protect it from erosion using concrete revetment (Figure 3). Figure 2. There are many kilometres of eroding bank in the Mekong Delta Figure 3. Concrete revetment is the standard approach to erosion control Channel migration upstream of the My Thuan Bridge threatened long term bridge security (as well as ongoing social dislocation and reduced food production). Figure 4 is a satellite image of the locality with river flow from left to right. The bank where the groyne field annotation is shown exhibited the greatest rate of bank retreat. Continued downstream migration of this bend would eventually impinge on the stability of bridge approaches. There was also a 40m deep scour hole in the river bend associated with the bend. Downstream migration of this would threaten the stability of bridge foundations. Prior to works, rates of channel migration were extreme (Figure 5) with rates of bank retreat over 20 m per year. As well as the new bridge, roads and houses were regularly threatened by river bank erosion (figure 6). Design principles and process Revetment acts to stop erosion by providing a physical barrier to protect bank material from removal by the flowing water. This has profound environmental impact at the land-water interface and does nothing to dissipate flow energy. It treats effect not cause. The use of revetment assumes that the primary mechanism of erosion is removal of material from the face of the bank rather than progressive undermining of the toe of the bank by migration of the channel thalweg which is the implied dominant erosion mode at least at this site. AusAID recognized the limitations of standard revetment treatments and supported an alternative approach using permeable groynes. and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages

3 Groyne Field Scour hole Figure 4. Satellite image showing location of My Thuan bridge, scour hole and groyne field. Flow is left to right. Figure 5. Between 1958 and 1992, historical rates of bank retreat were between 6 and 35 m/year at 5 transects along the left bank upstream of the scour hole Figure 6. Erosion damaging roadway and threatening houses in 1996 Intensive physical modelling was undertaken at the Vietnamese Institute for Water Resources Research (Figure 7). The aim of the modelling was to test the velocity reductions that could be achieved with different groyne configurations and to develop optimum configurations to meet velocity reduction criteria based on the mobility characteristics of bed and bank sediments. and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages

4 Modelled velocities guided optimization of: Groyne locations Groyne length Groyne spacing Groyne orientation Size of piles Permeability Height of groynes above the bed Need for scour protection around groynes The groyne layout was configured to reduce velocities in the near bank zone to below the threshold of motion of the bed material. The eventual design provided for 12 lines of 100 submerged piles extending 100m out from the river bank (Figure 8). 450 mm square concrete piles were driven into the river bed from a floating barge. The piles were spaced at 1125 mm and they were driven so that their tops were up to 15 m below water level. Rock was placed via a chute from a barge to limit bed scour around and through the groynes. Figure 7. Model groynes to test velocity reduction Figure 8. Example of design profile of two groynes (groynes 3 and 4) Figure 8. Example of design profile of two groynes (Groynes 3 and 4) and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages

5 Monitoring Extensive monitoring was undertaken during construction and then for 3 years. Monitoring included bed level surveys using side scan sonar, pile integrity surveys using divers and velocity monitoring throughout the groyne field using an Acoustic Doppler Current Profiler (ADCP). Figure 9 demonstrates the velocity reductions achieved in the groyne field. Figure 9. Measured velocity profile along a vertical 60 m out from river bank after construction of groynes. Flow is left to right. X axis is distance (m) along the river. Approach velocity > 1m/sec is slowed to around 0.5 m/s within the groyne field. Observations 10 years on Monitoring ceased in 2004 and the author is aware of no formal evaluations of the performance of the works since then. However during a visit to the area in May 2013, it was possible to make some brief observations of the location and condition of the bank upstream of the bridge and through the groyne field. No sign of active erosion was observed and the bank appears to have stabilized in its 2003 location. Figure 10 (May 2013) indicates that the bank retreat shown in Figure 6 has stabilized. Figure 10 shows a stable bank in the vicinity of the upstream groynes. No formal measurements or monitoring were undertaken during this visit. Figure 10. May Erosion shown in Figure 6 appears to have stabilised. and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages

6 Conclusions Submerged permeable groynes appear to offer an alternative to bank revetment for managing bank erosion on a large river such as the Mekong in the Mekong delta. Submerged permeable groynes can disturb secondary currents and dissipate energy, reducing the water velocities that cause bank retreat by scour of the river bed at the toe of the river bank. Compared to bank revetment, submerged permeable groynes have limited environmental and visual impact. They need not interfere with shipping or other uses of the river and its environs. A new tranche of monitoring involving ADCP measurements, pile integrity surveys and satellite comparisons could ratify the effectiveness of the submerged permeable groyne approach and confirm its suitability for further development and application. Acknowledgments AusAID are gratefully acknowledged for their willingness to back an innovative approach to protecting their substantial asset. The investigation and design was undertaken by Ian Drummond and Associates Pty Ltd for Maunsell Pty Ltd. Lindsay White and Bob Keller were instrumental in developing the approach. Dr Thai at the Water Resources Research Institute in Hanoi supervised the modelling. Ben de Witt oversaw structural aspects of design, looked after the project during construction and managed the monitoring program. References Maunsell Pty Ltd (1996a). My Thuan Bridge Project Vietnam. Phase 3 Detailed Design and Documentation. Final Report on River Studies. Prepared for Australian Agency for International Development and Ministry of Transport and Communications, Vietnam. October 1996 Maunsell Pty Ltd (1996b). My Thuan Bridge Project Vietnam. Phase 3 Detailed Design and Documentation. Report on River Bank Protection Works. Prepared for Australian Agency for International Development and Ministry of Transport and Communications, Vietnam. November 1996 Maunsell Pty Ltd (1996c). My Thuan Bridge Project Vietnam. Phase 3 Detailed Design and Documentation. Final Bridge Design Report. Prepared for Australian Agency for International Development and Ministry of Transport and Communications, Vietnam. November 1996 and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management Conference. Townsville, Queensland, Pages