Bank Erosion in Mekong Delta and along Red River in Vietnam
Report Mission 23 November - 6 December 2003
Delft, March 2004 ii
Bank Erosion in Mekong Delta and along Red River in Vietnam
Report Mission 23 November - 6 December 2003
Delft, March 2004
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Contents Main Report
Preface Executive summary
1. Introduction 1.1 Background 1.2 Terms of reference 1.3 Composition of team 1.4 Approach 1.5 Set-up of report
2. Background information on the Mekong and the Red Rivers 2.1 Introduction 2.2 General information on Vietnam 2.2 Mekong River and Mekong Delta 2.4 Red River Delta
3 Main observations of the Mission to Mekong Delta and Red River
4. Additional technical observations 4.1 Introduction 4.2 River morphological and bank erosion aspects 4.2.1 Introduction 4.2.2 Morphology of Mekong and Red River, similarities and differences 4.2.3 Recent studies on Mekong bank erosion 4.2.4 Causes of bank erosion 4.2.5 Has bank erosion increased over the years and what are possible causes? 4.3 Prediction of bank erosion 4.3.1 Introduction 4.3.2 Present practice in Vietnam 4.3.3 Methods used elsewhere 4.3.4 Use of mathematical models 4.3.5 Need for additional studies 4.4 Geotechnical aspects 4.4.1 Introduction 4.4.2 Geological structure and effect on soil characteristics and erosion mechanism 4.4.3 Main geotechnical aspects 4.4.4 Modelling and stability prediction 4.4.5 Flow slides in sand 4.3.6 Observations and remarks from meeting and site visits 4.5 Bank protection works 4.5.1 Introduction 4.5.2 Measures to cope with or counter bank erosion and to reduce damages 4.5.3 Observations on bank protections 4.5.4 Bank protection materials and structures 4.5.5 Construction methods 4.5.6 Design manuals and guidelines 4.5.7 Recommendations for further implementation work 4.6 Response of river to bank protection works and consequences for future 4.7 Maintenance 4.8 Need for setting up of data base
5 Additional non-technical observations and capacity building 5.1 Introduction 5.2 Institutional and legal aspects 5.2.1 Introduction 5.2.2 Institutional aspects 5.2.3 Existing legislation in Vietnam 5.2.4 Need for additional legislation 5.3 Socio-economic/environmental aspects of flooding, bank erosion and counter-measures 5.4 Master planning for bank erosion mitigation and river training 5.4.1 Need for a Master Plan and a long-term strategy for bank protection 5.4.2 Master plan as part of Integrated River Basin Planning and Management 5.4.3 Elements of a Master plan 5.4.4 Some details on some components of Master plan for River training and Bank Protection 5.5 Data and information management 5.6 Capacity building 5.6.1 Capacity building and cooperation 5.6.2 Staffing of Dike Department 5.6.3 Need for training of provincial staff 5.6.4 Use of models 5.6.5 Upgrading university curriculae
6 Proposed Action Plan 2004-2007
7 Conclusions and recommendations
References
Appendices I Mission participants II Contacts in Vietnam III Mission program and schedule IV Field visits Mekong River V Meetings Mekong River VI Field visits Red River VII Meetings Red River (including Final meeting) VIII Damage overview (collected data) IX Supplementary informations
Contents Supplement: Short review on bank erosion and bank protection
1. Causes of erosion and failure a 2. Cliff erosion 3. Bank erosion in stable river systems
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4. Bank erosion and planform changes 5. Survey and data collection 6. Types of bank protection 7. Techniques of bank protection 8. References
List of Tables
1 Comparison of river characteristics of the Mekong and the Red Rivers 2 Advantages of including the effect of sedimentary features in the prediction methods for the Jamuna River (Sarker & Khayer, 2002) 3 Comparison of different protection types. 4 Proposed time schedule for identified actions in the fields of capacity building in bank protection and river training in Vietnam
List of Figures
1 Some information on flooding in Vietnam 2 Mekong Delta location, provinces and regions 3 Hydrology of the Mekong Delta 4 Flood map 1984 5 Final meeting at the Dike Department of MARD where the main conclusions from the Mission were discussed with senior MARD/DDMFC staff 6 Faults and their possible impact on the planform of the Lower Mekong River 7 Water levels in the Lower Mekong during the year 1982 8 Flood levels Lower Mekong River 9 Some information on the composition of the Mekong Delta 10 Eroding banks along Mekong River 11 Eroding banks Red River 12 Different types of bank erosion along the Mekong River in relation to planform development 13 River planform and bank protection works along the Red River (taken from provincial map, note that other province is not indicated) 14 Some information on flood peaks of the branches Lower Mekong River in the period 1978- 1998 15 Local bank erosion rates of the Mekong River at Sa Dec versus relative curvature (source Le Manh Hung & Dinh Cong San (2002)) 16 Prediction method for bank erosion along outer bends in the Jamuna River as an example of the methods developed by Sarker & Khayer (2002) 17 Alluvial stratification along the Tien river 18 Procedure for evaluating riverbank stability (US Army, 1981) 19 Example of physical components of bank erosion (US Army, 1981) 20 Example of slope stability calculation for a cross section of a dike with berm and slip circle 21 Schematization of flow slides for sand and clay 22 Plan view of a flow slide 23 Physical principle of liquefaction phenomenon 24 Sa Dec slope protection 25 Serious erosion near Vinh Long
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26 Cracks in dike left bank Red river between K82 and K84 in Hung Yen province 27 Bank cracking along the Red River at location (Ha Tay province) 28 Some examples of groynes in Mekong and Red Rivers 29 Floating factory for production of gabions 30 Upstream morphological development threatening Chandpur town protection, Bangladesh 31 Measuring vessel equipped with echo-sounding, GPS and data storage with pc 32 Decision diagram for corrective and preventive maintenance (Source: van Noortwijk et al, 1996) 33 Possible effect of planform correction measures
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Preface
In 2002 the Department of Dike Management and Flood Control (DDMFC) of the Ministry of Agriculture and Rural Development (MARD) requested the Rijkswaterstaat (Dutch Public Works Dpt.) to review the problem of bank erosion in Vietnam and to assist in the preparation of a plan for tackling this problem. The mission took place in the period 23 November-6 December 2003.
The goal of the mission was to get a more or less reprentative picture of these bank erosion problems and to give recommendations on how in the future to tackle problems, and it was certainly not intended to prepare a professional consultancy report on the bank erosion problems in Vietnam. The mission spent one week in the Mekong Delta and 4 days in the Red River area. Due to the limited time it was not possible to make a full inventory of problems on bank erosion in Vietnam. Therefore, to get a general (and hopely representative) picture of the problem it was decided to visit a number of representative sites in the upper part of Mekong Delta (non-tidal area) and in the lower part of the Red River area. Special attention and time was allocated for discussion with persons working for a long time in the visited areas (praciticians) or doing studies for these areas (researchers). The guides from Dike Deparment (DDMFC) provided useful information on legal, organizational and planning aspects.
The direct findings of the mission members are reported in this report. Apart from common observations, observations are given using the disciplinary expertise of the different Mission members (hydrodynamic, morphology, erosion, geotechnics, bank protection).
The mission likes to stress that such a short visit can not give the full overview of the erosional problems in Vietnam (and hence it cannot replace a more fundamental study on this problem). However, the work of the Mission and this report can be considered as a second opinion on this problem provided by an independent group from abroad. It has provided the opportunity to confront the actual Vietnamese approach to this problem with some foreign approaches/expertise, and has made it possible to draw some conclusions and recommendations in this respect. The mission hopes that their findings will be of use for further improvements of the Vietnamese approach.
The results of this mission, conform to its terms of reference, are presented in this report. It should be stressed that it was a challenging task, which would have been impossible to realize without the close cooperation with Vietnamese experts and counterpart staff. A list of all contacts in Vietnam is given as Appendix 2. We wish to express our deep appreciation for the friendship and dedication of all mentioned persons who were involved in this complex effort.
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Executive summary
This report present the findings of a combined Netherlands-Vietnamese mission to support MARD/DDMFC in formulating an approach to the bank erosion problems, especially concerning the improvement of the organizational and technical measures, and possibly, to obtain an international help for bank erosion problems in Mekong Delta and Red River Delta in Vietnam. In late 2003 the mission spent one week in the Mekong Delta and 4 days in the Red River area and had a number of meetings with the most important organizations active in the field of river training and bank protection in Vietnam.
To obtain an idea of the scale and extend of bank erosion problems in Vietnam the Mission visited a number of representative sites in the upper part of Mekong Delta (non-tidal area) and in the lower part of the Red River area. Special attention and time was allocated for discussion with persons working for a long time in the visited areas (practitioners) or doing studies for these areas (researchers).
The Mission recognizes the large scale of erosion problems in Vietnam, which are associated with many social and economic implications and consequences. The funds available for tackling these problems are extremely limited. It is surprising to see that even under such difficult conditions (under such strict financial and other constraints), the DDMFC, the provinces and the local agencies are able to generate acceptable results. However, the work done can be classified rather as emergency management than as a planned development of the river systems. The approach is not transparent enough (especially not how the urgency of problems is evaluated) and it is far from being optimal. Besides the financial and technical matters, the organizational matters and cooperation (and thus the optimal use op human potential and facilities) needs further improvement.
The limited time allocated for this mission did not allow for making detailed studies of the challenges and problems experienced. This report provides the findings of the mission; these are more factual findings than an in-depth analyse. The information collected or the impressions of individual members with different backgrounds is preserved in this report for possible use in the follow-up studies. The main recommendations concern improvement of prediction and monitoring techniques, setting up of data base, strategic planning and capacity building. In Chapter 3 of the report the main findings of the mission are presented. More detailed observations and comments are given in the next two Chapters, where in Chapter 5 a lot of emphasis is placed on the need for the development of Master plans for river training and a strategic plan for bank protection works both for the Mekong and the Red River capacity building for the Dike Department and the provincial offices active in the design of bank protection works.
In Chapter 6 an Action plan is formulated together with some suggestions for possible funding mechanisms and a tentative time schedule. Two major projects can be visualised, the first one a capacity building project for those at present active in the field of bank protection in Vietnam, and the second one a project in which the curriculum of the Hanoi Water Resources University is extended to include river training and river bank protection possibly in the wider context of integrated water resources planning and management.
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The capacity building project should be aimed at staff of the Dike Department, of the Water Resources Institutes and of the provincial engineering bureau’s, and should be applying a via learning by doing approach. The following activities are proposed under this project: Setting-up data bases (probably 2 types: one national, and another more technical per river?) Master plan and long term strategy for river training Mekong River, Red River and Central Vietnam Rivers, in cooperation with RBO’s Improved monitoring capability Improvement design manuals and standards Improved legislation Improvement cooperation Dike Department and the provincial design bureau’s Publication of yearbooks with progress and revolving yearly planning bank protection works Training (two types: training courses in Vietnam for larger audience and MSc studies of some students at UNESCO-IHE in NL) Training and exposure tours to the Netherlands NL (RWS/Delft Hydraulics/GeoDelft) and to Bangladesh (CEGIS, for remote sensing and set-up data bases) For this capacity building project an international donor should be found.
The second project is a proposal for curriculum development of HWRU (both in Hanoi and in HCMC), to be prepared together with TU Delft and UNESCO-IHE, and it would be an extension of an already existing cooperation between the two universities and UNESCO-IHE, for which possibly Netherlands funding would be available.
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1 Introduction
1.1 Background
The Mekong Delta in Vietnam is the most downstream part of the Lower Mekong River Basin and it is of great importance to the Vietnamese community and economy. It is potentially an area of great productive capacity and its development is of crucial importance to the nation’s economic prosperity and food balance. At the same time the Delta is a difficult area, with both considerable physical resources and environmental constraints: great annual variety in the Mekong’s hydrological regime, large tracts of lands with acid sulphate soils and vulnerable wetlands. It is also an area, which is heavily and frequently affected by flooding and bank erosion resulting in loss of live and high economical damage. The similar problems are noticed in the red River area. The Vietnamese Government has recognized this problem in recent years and it has been decided to undertake the necessary remedial actions.
The Department of Dike Management and Flood Control (DDMFC) of the Ministry of Agriculture and Rural Development (MARD) requested the Rijkswaterstaat (Dutch Public Works Dpt.) to assist in the preparation of a plan for tackling the problem of bank erosion. This mission is the first step in this direction. The results of the mission, conform to its terms of reference, are presented in this report.
1.2 Terms of reference
The mission aims at providing a framework for tackling the problem of bank erosion in the Mekong Delta and along the Red River, with reference to the following elements:
To counteract present problems 1. Assessment through field visits of bank erosion problems in the Mekong Delta and along the Red River, especially during or just after floods 2. General analysis of the bank erosion problems in an attempt to determine mechanisms, causes and other factors which have a crucial impact 3. Assessment for the 6 Mekong problem locations at Thuong Phuoc, Tan Chau, Hong Ngu, Sadec, Long Xuyen and Can Tho, and for some locations in the Red River, whether technical solutions are applicable and what are their limits and implications
To counteract future problems 4. Indication of which banks have a high risk on future bank erosion 5. Analysis of the used prediction method and suggestions for improvement 6. Proposal for a straightforward monitoring and data collection system to enhance insight in bank erosion problems 7. Analysis of existing protection structures and techniques and suggestions for improvement 8. Recommendations for new protection structures and flood fighting strategies, which also involve the local population 9. Definition of a suitable approach to bank erosion problems and assessment of the need for a master plan in which the local population participates and which involves both bank protection and planform stabilization of river branches 1.3 Composition of Mission
The Mission (see Appendix 1 for details about its composition) was composed of both Netherlands and Vietnamese members. Three of the Netherlands members were from Publics Works Department (Hydraulic and Road Engineering Department), whereas two Netherlands Mission members (one from GeoDelft and the other from UNESCO-IHE) were selected because of their specialized knowledge and experience. The Vietnamese members were from the Dike Department of the Ministry of Flood Control and Drainage.
1.4 Approach
To comply with the terms of reference the following approach was taken: 1. Literature study about the general physical aspects as well as bank erosion along the Mekong and the Red River before departure of the Netherlands members of the Mission; 2. Study of the available research data about bank erosion along the Mekong River, which was done during the visit of the Mission to Vietnam; 3. Field visits to a number of provinces to inspect existing revetments and locations with bank erosion along the Mekong and the Red River; 4. Meetings with the authorities of the DDMFC (Department of Dike Management and Flood Control), the WRRI (Water Resources Research Institute), the HWRU (Hanoi Water Resources University), the SARD (Service for Agriculture and Rural Development) and the Southern Planning Institute (PI); 5. Preparation of a draft-report of the Mission by the Netherlands members of the Mission on the basis of the above activities plus formulation of an Action Plan for the years 2004 – 2007 and subsequent discussion of the main findings and the proposed plan with the Vietnamese members; 6. Preparation of the Final Report
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1.5 Set-up of present report
This report presents the findings of the Mission, and as such it is the response to the Terms of Reference of the Mission. The set-up of the report is as follows. It is divided in two parts. The first part is the Main Report, which consists in total of 7 Chapters and 9 appendices. The Main Report also contains an Executive Summary.
The main findings of the Mission are listed in Chapter 3 of this Main Report, but this Chapter is preceded by a Chapter 2 which was prepared in The Netherlands before the Mission left for Vietnam. It is a summary of information available on the Mekong Delta in Vietnam and the Red River, mostly obtained from the Internet. The main findings in Chapter 3 are elaborated in more detail in the Chapters 4 and 5, which present Additional technical observations (in Chapter 4) and Additional non-technical observations and capacity building (in Chapter 5), respectively. Based on these findings Chapter 6 outlines a Proposed Action Plan for the period 2004-2007. Chapter 7 summarizes the main conclusions of the Mission and presents a number of recommendations.
The nine appendices present a.o. the mission program and time and travelling schedule, and short reports of the different field visits and meetings in relation to the Mekong Delta and the Red River bank erosion and bank protection works. Moreover a damage overview is presented on the basis of data collected during the visits to the different provinces.
In a Supplement a number of Technical Chapters are presented, which often are a summary of exisiting literature on the different topics of interest, notably causes of erosion and failure a, Cliff erosion, bank erosion in stable river systems, survey and data collection, Types of bank protection, and techniques of bank protection.
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2 Background information on the Mekong and the Red River
2.1 Introduction
In preparation of the Mission, available information on Vietnam and on the Mekong and the Red River was collected. Mostly information from Internet sites was used. In this Chapter a summary of the collected information is presented, thus providing some background information for the Mission.
2.2 General information on Vietnam
Vietnam is situated in the tropical monsoon area of South East Asia with an average rainfall of 1800 to 2500 mm/year and moreover it is a typhoon-prone country. A large number of people, who are mainly involved in the agricultural and fishery sectors, live on the low lying river floodplains, deltas and coastal margins. The most important ports are located along the coast.
The potential for disaster in these areas is high, as protective river and sea dikes are frequently overtopped or breached, which results in flooding. During floods serious bank erosion occurs in the lower reaches of the Red River in the North of the country and the Mekong in the South. Flooding and bank erosion cause loss of life and damage to agricultural land and infrastructure. Some information on floods in Vietnam is presented in Figure 1.
Figure 1: Some information on flooding in Vietnam (Source: Pilarczyk and Sy Nuoi, 2002)
2.3 Mekong River and Mekong Delta
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The Mekong River is one of the largest rivers of the world and it is approximately 4,220 kilometres long. From its source in the Xizang plateau, the river flows through the Xizang and Yunnan regions of China, and it forms the boundary between Laos and Burma and subsequently between Laos and Thailand. Below Phnom Penh, the river divides into two branches, the Song Han Giang and Song Tien Giang, and continues through Cambodia and the Mekong basin before draining into the South China Sea through nine mouths. In Vietnam these nine mouths are referred to as nine ‘dragons’. The river is carries substantial quantities of silt, and has in Vietnam a gentle slope and a large water depth. It is navigable by seagoing craft of shallow draft as far as Kompong Cham in Cambodia.
In historical times a connection developed near Phnom Phen between the Mekong River and the Tonle Sap Lake, a shallow fresh- water lake that presently acts as a natural reservoir to stabilize the flow of water through the lower Mekong. When the river is in the flood stage, its silted delta outlets are unable to carry off the high volume of water. Floodwaters back up into the Tonle Sap, causing the lake to inundate as much as 10,000 square kilometres. When the flood subsides the flow of water reverses and proceeds from the lake to the sea. This effect significantly reduces the danger of devastating floods in the Mekong Delta, where the river floods the surrounding fields each year to a level of one to two metres. Habitation of the delta remained restricted by these stagnant waters until canals could be constructed, at the end of the 19th century (Mburu, 2001).
The Vietnamese part of the Mekong river has a length of 230 km and is called Cuu Long. It has two main branches, the Tien (North branch) and the Hau (South branch). The Mekong Delta is an important region in Vietnam; with its area of 39,000km2, it covers about 12% of the country’s total area and it provides about 50% of the national agricultural production. About 15 million people live within the delta with 3.5 million in the urban centres.
Figure 2: Mekong Delta location, provinces and regions
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The Mekong Delta is a low-level plain not more than three metres above sea level at any point and criss-crossed by a maze of canals and rivers for transport, irrigation, drainage and flood control. So much sediment is carried by the Mekong's various branches and tributaries that the delta advances sixty to eighty metres into the sea every year. The estimated amount of sediment deposited annually is about 1 billion cubic metres. About 10,000 square kilometres of the delta are under rice cultivation, making the area one of the major rice-growing regions of the world. To prevent detrimental effects to any of the riparian countries all developments on the main stream as well as the tributaries are co-ordinated by an inter-riparian Mekong Committee.
The Mekong River has created a variety of natural landscapes, ranging from tidal flats, sandy ridges and tidal backswamps in the coastal plain, estuaries at river mouths, to river floodplains, broad depressions, peat swamps, alluvial levees and terraces further inland. Wetlands created by seasonal or permanent inundation have an important function in the Delta. They form a buffer between sea and land, trap river borne sediment brought with floods, play an important role in soil conservation and coastal protection, provide a habitat for wildlife, and serve as spawning and nursing grounds for fish. They are extremely fragile and could easily and irreversibly be affected by improper management.
Figure 3: Hydrology of the Mekong Delta
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Water regime and tides
The most determinant features of the natural water regime of the Delta are depicted in the figure 3. Both rainfall and river flow in the Delta have a pronounced seasonal pattern with very high and very low rainfall and discharge values in respectively the wet season and the dry season which each last for roughly half a year. Periods of water excess alternate with periods of water shortage and all the necessary water control measures essentially originate from this regime feature.
Mean annual precipitation ranges from about 2,400mm in the western part of the Delta to 1,300mm in the central part and 1,600mm in the eastern part. The duration of the rainy season is from April to November in the western part and from May to November in the rest of the Delta.
Another important feature of the water regimes of the Delta are the tides of the surrounding seas. The tide of the South China Sea is predominantly semi-diurnal with an amplitude of some 2.5- 3.0m. The tide of the Gulf of Thailand, however, is mostly of the diurnal type, while its amplitude is only some 0.4-1.2m. The tides have a significant influence on the river and connected canals in the coastal zone and also in the area adjoining the main Mekong river branches all the way into Cambodia. On the long-term, also the impact of the sea level rise on the Delta will be considerable, given the extremely flat topography and the tidal influence throughout the Delta (0.3m is often mentioned as the most probable sea level rise for this area).
Flooding and saline intrusion
The average elevation of the Vietnamese part of the Delta equals about 0.8m+MSL. During the period of high discharge, the banks of the Mekong in the north of the Delta, below Kampong Cham and above Can Tho, are overtopped and the land is inundated, up to depths of 4.5m. The inundation usually starts in July/August and ends in November/December. A positive effect of the flooding is the deposition of sediments in the flood plains.
As the capacity of the river system increases downstream, there is a considerable attenuation of the water levels and less flooding. At the coast, the combined action of river deposition and the sea has created a slightly higher coastal belt which further reduces flooding.
The flooding problem in the North is aggravated by high rainfall. In the South, excess rain water also leads to large scale inundation of the land outside the river flooding zone. This occurs especially in the South Western part of the Delta. In the poorly drained depression areas, the inundation may last as long as 6 months.
During the March-May period Mekong discharges are low and for an important part required to prevent deep saline intrusion. Higher rates of abstraction would increase salinity intrusion which is already affecting large areas.
Flood protection The Mekong Delta is largely unprotected and therefore characterized by widespread, uncontrolled and prolonged floods. A system of drainage channels and pumping stations is used to make agriculture possible. Houses are situated on high places such as along the canal banks, roads or sandy ridges but during the high floods they are usually still flooded. Alternatively, the
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houses are built on stilts or raised foundations above the flood level. Boats are used for communication during floods. Flood control usually consists of low embankments along the primary and secondary canals, while the secondary-tertiary canal connections are provided with simple sluices or temporary earth dam closures. These means provide partial protection to agricultural production during the early part of the rainy season. In the deeply flooded areas, embankments are overtopped later in the flood season during a normal flood year but the embankments may give year round protection in a low flood year. In the coastal area some flood protection schemes also prevent salt water intrusion. Quite a few large sluices in primary-secondary connections were built in that area and many more are under construction or planned.
The main canal systems are generally planned, designed, constructed and operated by the national and provincial water resources development organizations (primary and secondary units). The tertiary canals/water control system and on-farm developments, on the other hand, are undertaken at the district/village/farmers group level. Flood control is generally practised at the level of provincial authorities (secondary unit).
Development and flood protection strategies
The need for economic growth and diversification of the economy in an environmentally sound and sustainable manner will govern the scope and pace of the development of the Delta’s resources.
The main thrust of water resources development would be on-farm development and canal improvement to bring more irrigation water to the already irrigated areas and to improve drainage conditions and promote flushing of acid water. The development would also include embankment improvement in the deeply flooded areas, for the time being to prevent flooding till the end of August only and full year round protection in the shallowly flooded, already more developed areas.
On good soils forest cannot compete with crop production or aquaculture in terms of income or employment generation. Its development potential lies in areas with low graded, acid sulphate soils and along the coast. Inland swamps (Melaleuca) and mangrove forests are essential for biodiversity conservation and to save the few natural reserves that have been left. The sustainability of shrimp culture and fisheries depends on their existence. In addition, they provide coastal protection.
Because of the specific situation of the Mekong Delta it is neither economically justified nor environmentally sound to provide complete flood protection. Controlled flooding would still allow for acidity flushing, would maintain the natural fertilizing effect of sediment, and would minimize the disruption of fish migration and spawning. The actual policy recommendations include: 1. Low embankments in the deeply flooded parts to protect against early floods; 2. Full embankments in shallow agricultural areas to protect against 10-year floods; 3. No embankments on land that has potentially serious acid-sulphate problems; 4. Adequate forecasting and warning systems; 5. Adequate evacuation plans and an adequate number of escape/rescue facilities; 6. Maintenance of natural flooding regimes in sanctuaries and swamps and mangrove forests.
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Figure 4: Flood map 1984
Channel migration The high sediment load of the Mekong River system, estimated at 160 million tons per year, results in an inherently dynamic channel system with rapid rates of change. Commonly, such changes are associated with channel migration, whereby deposition along a riverbank is countered by erosion of the opposite bank. Susceptibility to channel migration and the type of mechanism responsible vary according to the location within the deltaic system. The upper delta experiences very rapid rates of channel migration (with banks erosion rates commonly up to 20 m/year), caused by the lateral accretion of point-bars and mid-channel bars / islands, and the downstream migration of mid-channel bars.
Mid- and lower delta channels are more stable (bank erosion 5-10 m/year), and channel change here is mainly caused by the slow accretion of elongated point-bars and mid-channel bars. The slower current velocities and cohesive bank material, as well as the protection afforded by mangroves and nypa palms (Nypa fruticans) in saline reaches, are the principal reasons for the
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relative channel stability here. Near the mouths of the main distributaries, channel changes are common and result from the formation and shifting of distributary-mouth bars.
Another group of channel change involves the abandonment of channel segments, which generally leads to their progressive siltation. At a small scale, channels separating a mid- channel or channel or distributary-mouth bar from the river bank may infill with sediment to eventually result in the coalescence of the bar with the bank.
At a larger scale, individual distributaries may also become abandoned. The progressive sediment accumulation within the Ba Lai sub-branch of the Mekong is a manifestation of this. Also, many of the smaller rach-type channels along the South China Sea coast (i.e. Ca Mau peninsula and the area about the mouths of the Saigon and Vaico rivers) are prone to change in position and abandonment, as the large tidal range along this coast results in the progressive inward transport of sediment from the sea and eventual channel infilling. Mangroves are likely to assist in sediment accumulation within these channels.
Sedimentation and erosion processes in the Mekong Delta are highly seasonal given the large annual fluctuation in both the river discharge and sediment load. Suspended sediment load of the river inflow varies from less than 100 mg/l during the dry season to 600 mg/l during the peak flood season.
During the flood season, most bedload, consisting predominantly of sandy material, is transported and deposited on the channel bed and in bars. The finer suspended load is either deposited on the delta plain through overbank flooding or flushed out into the ocean. During the low-flow period, suspended sediments also get deposited in-channel. In the seaward parts of the channels, this deposition is aided by saline intrusion, which causes sediment flushed to sea during the flood season to be re-imported into the delta. In the larger channels, much of the dry-season deposition is ephemeral, as the fine sediment is reworked during the following flood season.
In the smaller channels, tidal creeks and canals, mud deposition is more likely to be cumulative over successive dry seasons.
Bank erosion
Bank erosion is considered a serious socio-economic problem in the upper delta provinces of An Giang and Dong Thap provinces. Problems are especially severe at Tan Chau on the Mekong branch in An Giang, where erosion rates attain 30 m/year, and approximately 400 households have had to be relocated recently due to destruction of their dwellings through bank collapse. Bank erosion has resulted in major disruptions to local livelihoods, and financial burden on the provincial government by necessitating the relocation of inhabitants and localised bank protection works (e.g. Truong Dang Quang). Losses due to bank erosion appear to have increased in the last decade, probably due to the growing urban population and the resultant concentration of activity and capital along the waterfront (e.g. Truong Dang Quang).
The severity of erosion at Tan Chau is largely attributable to the sharp meander-bend morphology, which focuses the river flow energy onto the concave bank (where the town is situated). The gradual downstream rotation of the point-bar on the opposite bank has resulted in a progressive downstream shift in the zone of erosion; stretches of river bank upstream of Tan
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Chau, which formerly experienced severe erosion are now experiencing bank accretion (e.g. Truong Dang Quang). Other erosion hotspots further downstream within An Giang (e.g. at Long Xuyen) are mostly associated with the downstream migration of mid-channel bars, which creates a shifting zone of erosion downstream and to the sides of the bar, and a zone of accretion to its upstream.
Sand mining Sedimentation on the opposite bank, which accompanies bank erosion, also represents an economic cost in places, through the shoaling of navigation channels, the stranding of wharves, docks and other water transport infrastructure, and the blocking of entrances to canals. However, sedimentation in the main distributary channels is regarded by many as an economic benefit, given the predominantly sandy nature of channel sediments, and the increasing demand for construction sand driven by urban expansion.
Numerous sand dredging operations exist along most of the length of both the Mekong and the Bassac branches. An individual operation may extract volumes in the order of 10.000 m3/year from the bed of the channels (e.g. Ky Quang Vinh). Local over-exploitation of sand is also blamed for the frequent occurrence of bank erosion in the Mekong Delta.
2.4 Red River Delta
The two major rivers systems in the Red River Delta in the north of Vietnam are the Red River system and the Thai Binh River system. The Red River system consists of the confluents Da River, Thao River and Lo River and five branches, these being the Duong River, Luoc River, Tra Ly River, Dao River and Ninh Co River. The Red River is so named because of the high amounts of red sediment it carries. The Red River carries about 200 million tonnes of sediment each year.
In the Red River Delta in, people have built 3000km of river dikes and 1500km of sea and estuary dikes to protect against flooding. Many of these dikes are old and were built by using inadequate manual construction technology and poor materials. Dike foundation conditions and stability have not always been properly evaluated before construction or improvement. River dikes often suffer damage from under-seepage and piping, slides or local collapse during high flood stages. Moreover, the construction of dikes has gradually reduced the areas of the flood plains that are available to accommodate excess flood flows, with the result that river-flood levels have become increasingly higher.
Bank erosion Due to instability of the river channel the Red River is affected by siltation of the river channel below Son Tay as well as a general increase in bank erosion, threatening dikes at numerous locations. The most likely causes for this instability are an increase in slash-and-burn land clearing practices in the highlands of Vietnam and China and the release of high energy, sediment-poor water from reservoirs such as Hoa Binh.
Slash-and-burn land clearing increase both runoff and sediment load, resulting in sedimentation below Son Tay, where the bed gradients are lower and the river can no longer cope with the sediment. This sedimentation may cause widening and meandering of the river, resulting in local bank erosion.
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The release of high energy, sediment-poor water from reservoirs leads to scour and bank erosion for some distance below the dam, and sedimentation beyond this area, when the bed gradient becomes smaller. Furthermore flow regulation by reservoirs implies an increase in the mid-bank to full-bank flow duration and therefore in bank saturation and bank erosion.
Bank erosion and other negative impacts of channel instability are counteracted by structures, which should stabilize the riverbank and channel and protect the dikes. Usually, the following structures are applied: 1. Rock filled wire baskets (gabion mattresses), underlain with geotextile filter cloth to prevent erosion. A concern with these gabions is the disintegration of the wire casings by corrosion, which eventually will lead to flowing away of the relatively small stones and failure of the structure; 2. Rock and concrete blocks and mats; 3. 2 to 3 meter thick bamboo platings, spaced 5 to 10 meters apart at the toe of the dike; 4. Groins or hard points.
The dikes are protected usually by hard revetments, e.g. of the following types: 1. Interlocking rectangular blocks with raised rectangular surface; 2. Six-sided interlocking blocks with a raised triangular surface. 3. Riprap The function of the raised surfaces is the dissipation of wave energy. The revetments are usually underlain with a granular filter, or, in more recent structures, with a geotextile filter cloth to prevent erosion.
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3 Main observations of the Mission to Mekong Delta and Red River
In this Chapter the main conclusions from the Mission are given, whereas in the subsequent Chapters these will be elaborated in more detail. The scope and findings of this Mission can be considered as complementary to the findings of the study on ”Environmental issues and recent infrastructure development in the Mekong Delta” (ARMC, 2001), in the sense that for this Mission the emphasis was on bank ersion and bank protection, whereas the ARMC study was considering infrastructure development in general. Still however many of the findings of the latter study are quite relevant. For this reason the first part of this Chapter consists of some texts taken from the ARMC report, but adjusted to emphasize the aspect of bank protection and morphological development.
As stated in ARMC (2001) for the Mekong Delta alone, the Mekong Delta and the Red River have experienced an unprecedented increase in infrastructure over the last decades. Although this development has contributed significantly to economic growth within both deltas and also nationally, it has resulted also in a number of adverse impacts on the society and on the natural environment. In retrospect many impacts have originated from the failure to recognise and maintain the rivera and the deltas as dynamic biophysical systems, and an emphasis on rapid economic development based often on export commodity production. Another relevant matter is that the flooding risks and the morphological dynamics of both rivers threaten the further development at locations near to the rivers, and a balance has to be found between the maintaining the rivers as dynamic biophysical systems and a further economic development.
This chapter provides an overview of major issues related to flooding, bank erosion and bank protection in both deltas, and sets out various approaches for protection, including institutional, managerial, technical, economic and scientific approaches. The roles played by public awareness and national and international co-operation are also discussed. In addition, it offers advice for aid agencies for priority activities technical and financial assistance for a better integration of bank protection concerns in environmental protection and infrastructural development activities. This is elaborated in more detail in Chapter 5, where a checklist and other pertinent reference materials is attached for environmental impact assessments of projects affecting the river environment.
The main objectives of this report are to provide advice and assistance: • to strengthen national and regional capabilities in bank erosion prediction, prevention and management; • to develop a national bank erosion monitoring and information management network; • to strengthen the ability of country to implement and enforce international environmental and technical codes; • to develop and initiate sustainable financing mechanisms which will support ongoing activities beyond the life of the cooperation project between RWS and MARD.
First and foremost conclusion and recommendation is that future infrastructure development should incorporate a greater appreciation of the system characteristics of the Mekong Delta and the Red River. An activity in one part of one of the rivers might generate impacts in other areas. The analysis of environmental and socio-economic costs of proposed projects will need to be carried out, not only for the project areas, but also in the entirety of both rivers (AMRC, 2001). An improved coordination of activities between the discrete project areas, promoted in the first
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instance by an increased level of inter-provincial cooperation possibly through the new RBO’s, would minimise the generation of cross-project and cumulative impacts.
The prediction and gauging of physical, environmental and social impacts arising from infrastructure development in the Mekong Delta and Red River are often hampered by the lack of adequate baseline and pre- and post-implementation monitoring data. Monitoring of the existing situation and past and ongoing projects will assume increasing importance in providing input for future projects, as the increase in the number of projects within both deltas will bring about a corresponding increase in cumulative impacts. It is not to say that data do not exist; various government agencies and institutions both within and outside the Mekong Delta region and the Red River have carried out a number of studies, but their temporal coverage is often too short to enable trends to be identified. Data collection may not be of benefit initially, but their utility grows with time. There is also an urgent need to improve the coverage of environmental data on the Mekong and Red River catchments. This is especially important in light of uncertainty over the effects of current and future dam construction in the Mekong on the delta. It needs to be a fundamental change in the planning and design of projects, namely a move away from the “defensive” approach that pervades many recent infrastructure projects. Instead of total control, prevention and elimination, emphasis should be placed on partial control, amelioration and, in general, adaptation to the natural environmental conditions. For example, there is more long-term benefit in replacing the current approach of flood-control, involving much investment in hard infrastructure for defending the delta plain from overbank flooding, with strategies which involve re-routing overbank flow and partial protection.
Figure 5 Final meeting at the Dike Department of MARD where the main conclusions from the Mission were discussed with senior MARD/DDMFC staff
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Hereafter the main observations of the Mission are listed:
General observations on bank erosion in Vietnam