This paper summarizes the final mission report prepared as a result of a visit in December 1997 to the Lao People's Democratic Republic (Lao PDR) to formulate a pilot project for flood protection and management, close to the Mekong River in Bolikhamxai Province
Following exceptional flooding in 1995, FAO Special Relief Operations Service (TCOR), at the request of the Government of Lao PDR (GoL), carried out an assessment for emergency relief and proposed eight pilot projects for flood mitigation and management within the Flood Loss Prevention and Management Plan. Investigations for the proposed pilot project for the construction of flood control works in Bolikhamxai Province commenced in mid 1996 with a mission to collect hydrological and field data for the preparation of the Flood Loss Prevention and Management Plan. Field Document 1 was submitted in June 1996, while Field Documents 2 and 4 followed in May and August 1997, respectively, concentrating mainly on flood plain and topographic mapping requirements for the flood control works.
The Terms of Reference (ToR) for this last mission were directed at the flood control works required for the pilot project and comprised the following main tasks:
assess, revise and estimate the overall cost of the proposed flood control works;
verify topographic surveys and subsequent mapping carried out in selected pilot areas;
ascertain areas and crops that can be irrigated from storage and dry season flows in tributaries with flood control structures;
comment on the capacity of the Department of Irrigation (DoI) in the Ministry of Agriculture and Forestry to design and implement the project.
collect data on unit costs; and
assess the environmental impact of the proposed works and any mitigation measures required.
Laos has experienced 24 floods on the Mekong River in the last 32 years (1966 to 1997). A major flood has occurred every year in the last four years with that of 1995 (estimated at 20 750 m3 sec at Vientiane and 26 000 m3/sec at Paksane) being widespread and devastating while that of 1997 affected only Paksane and areas downstream. Vientiane Municipality constructed flood protection works following a severe flood in 1966 (estimated at 26 000 m3/sec) and these have, in the last two years, been raised and extended with funds provided under EU grant aid. Downstream areas of the city and its environs (mainly agricultural land under intensive cultivation) are located on the outside of a meander and have experienced erosion in recent years. Protection of property and counteracting this erosion were given as the main justification for these works. Further works to comprehensively improve mains drainage in the city, funded by JICA, are under way.
Floods generally occur due to a slow and steady backing up of the Mekong as the monsoon season progresses and usually occur in the Vientiane-Paksane area in late August or early September. Lasting between one and two weeks normally, agriculture and settlement patterns have adapted to this regime and, generally, crops (rice, the main wet season crop, once established, can survive about 15 days under such floods) and livestock (villagers move livestock to higher ground once the rains have improved grazing) do not suffer significant losses. Traditionally, such floods are regarded more favourably than droughts as affected farmers are able to grow a dry season crop as the flood recedes and soil fertility benefits from the silt deposited on the land. Occasionally, however, tropical storms coincide with this flooding (as occurred in 1966 and 1995) causing tributaries of the Mekong to flood which, when the Mekong is high, cannot discharge downstream. Such flooding can be disastrous and almost impossible to prevent: flood control structures would be required both on the Mekong and on its major tributaries in Laos.
Options to ameliorate such flooding are limited: either the source of the flooding (the Mekong River) can be controlled through a regulating device such as a dam which would be, through necessity, large and expensive (i.e. the low [67 m high] Pa Mong dam sited near Vientiane and considered in the early 1990's was estimated to cost $2 billion) or the effects of a flood can be reduced and controlled through flood protection and management works. The advantage of the latter is that the works can be carried out on a staged basis, as funds and time permit and works can be “tailored” to allow controlled flooding for recharge of fisheries and for irrigation. This paper summarizes the works proposed under a pilot project (yet to be funded) for Bolikhamxai Province that aims to provide a example for subsequent projects downstream.
Bolikhamxai Province is located in central Laos and has a total population of 163 487 (1996 census) in 455 villages and 26 700 households. Agriculture is dominated by subsistence rice production with paddy rice grown on 11 087 ha out of a total agricultural area of 11 146 ha (1997 Dol field survey estimate). Approximately 4 000 ha is put to irrigation in the dry season with pumping (either electric or diesel powered) from the Mekong or perennial streams. The pilot project area is immediately adjacent to the town of Paksane and comprises areas that have been inundated four times in the last four years. The flood of 1995 was particularly severe with tropical storm Lois depositing up to 500mm of rain on surrounding catchments in three days at the end of August. Some areas were under water for as long as eight weeks - with significant losses to life, property, crops and livestock. The Dol has estimated (from field surveys) that Bolikhamxai province was the worst affected area in Laos with regard to crop production with 9154 ha of paddy flooded and losses estimated at US$4.38 million. The subsequent flood of 1996 inundated 4276 ha and cost US$1.46 million.
Preliminary designs for the flood control works has been based on an estimated 1:5 year return period peak flood, at Paksane, of 24 000 m3/sec at elevation above mean sea level (amsl) of 155.00m.
The pilot project
The project objectives are:
to protect up to 5 000 ha of agricultural land and village communities from flooding through the construction of dikes, flood control gates and structures;
to allow the irrigation of up to 5 000 ha of land in the dry season from related storage;
improve capability within the Dol to design and construct similar schemes elsewhere in the Lao PDR; and,
to improve productivity and diversify crops in the Bolikhamxai province.
Storage and irrigation potential
As a result of the last mission and subsequent detailed examination of the topographic maps available, revisions of potential capacities for storage and irrigation have been made with the following results:
The total storage capacity behind structures on the Pak Peng and Nam Kadan rivers is estimated at 17.9 million m3, sufficient for the annual irrigation of 1 200 ha.
Pak Peng River (at 150.50 m level)
Surface area of storage reservoir : 1 85 ha
Maximum volume of water stored : 3.4 MCM
Potential area that can be irrigated : 230 ha
Nam Kadan River (at 150.50 m level):
Surface area of storage reservoir : 7 89 ha
Maximum volume of water stored : 14.5 ha
Potential area that can be irrigated : 970 ha
The calculations of the irrigable area take into account that land is located below the 153.00 m contour level and that pumping lifts are not exceeding 7.0 m. Water requirements for two rice crops per annum are estimated at 15 000 m3/ha.
Inflows to the reservoirs are not considered - any flow (both rivers generally flow until March and, occasionally, throughout the year) would “top up” the reservoirs and compensate for surface water evaporation losses and dead storage (i.e. below 146.00 m level). Significant inflows later in the dry season would also allow more irrigation to take place but, as these cannot be guaranteed, they have been excluded from the estimates above.
Scope and Project area
The proposed project now comprises three sub-components:
At present, the area is generally unprotected, with only one water control structure existing at Pak Peng where three control gates function largely to store water in the river for irrigation rather than flood control. Until 1996, irrigation in the dry season was solely from the Mekong, with electric powered pumps providing water for about 100 ha of land. With the ongoing installation of additional pumps in the area, irrigation for the 1997–98 dry season may reach 1 000 ha. The total area under irrigation in Bolikhamxai Province is currently 4 000 ha.
The pilot project proposes to construct dikes and control structures for flood protection and irrigation, whereas the drainage outlets would be used to store water as the rainy season ends.
Existing irrigation from the Mekong involves pumping lifts of about 12 m in December-January rising to 20 m by April-May as the river level falls. Irrigation from the proposed storage will have pumping lifts between 3 and 7 m maximum allowing for smaller pumps and reduced power requirements.
Overall cost summary
Full details of each component are provided in the mission report. All costs will have to be revised as details of the project are finalized following further investigations for structures, more detailed survey for storage and irrigation potential and staffing and equipment requirements become better known. Provisionally overall costs are shown in Table 1.
Summary of costs
|Flood Control Works||4 000 000|
|Training, Technical||974 500|
|Assistance. & Equipment|
|Overall Total||4 974 500|
For the pilot project, two important environmental issues exist that require further study:
very little is known of the effects flood protection works have on flows in the Mekong. The EU project report on construction of 45 km of dikes complete with 44 flood control structures and river training works to reduce bank erosion around Vientiane, contains less than one page on environmental matters. It was assumed that the Mekong was so large that any small changes to the banks are quickly absorbed downstream. No mention is made of transfer effects where flood levels increase up or downstream of such works and no investigation was made before, during or after the project was completed.
Nam Kadan basin is a recognized wetlands area, with about 50 km2 of its catchment of 81 km2 seasonally flooded. Six km2 below 150.00 m AMSL are flooded for long periods of the year. In 1975, the village of Ban Phon Ngam constructed a small weir on the stream (for domestic water supply when flows reduce after March). This weir is repaired every year and, although low (1.2 m) this appears to have had no negative environmental effects.
It is likely that the construction of an eight m high structure on the Nam Kadan will benefit the area environmentally with more controlled seasonal flooding and productive use of the water for irrigation. However, this can only be known after further study and when detailed maps of the area are available. Protecting 5 000 ha of agricultural land from uncontrolled flooding will also be advantageous, but the effects downstream or on the other bank are not known. Erosion is not a problem in this stretch of the Mekong but, by increasing velocities and levels through containing floods within the river may lead to problems on the outer edge of the wide meander that exists downstream of Paksane. The outer edge is located on Thai territory.
Conclusions and recommendations
The project requires more investigations before its feasibility can be fully assessed. With reference to the mission report, the main points are:
The proposed structure at Nam Kadan cannot be designed or costed until further investigations into the foundation conditions and the hydrology of the area has been investigated in detail. Alternative sites should also be considered.
A full EIA study should be carried out with emphasis on the effects of the project on the hydrology of the Mekong and Nam Kadan rivers.
MRC Bangkok is to provide up-to-date flood forecasts and other information for the Paksane area of the Mekong River.
The dike alignments require longitudinal surveys completed/updated as required to sufficient detail to allow design and costing of the embankments.
For each proposed structure, a detailed large scale topographic survey should be carried out to permit design and costings to be made.
The project, Vientiane Plain Flood Protection, Urgent Phase, was financed by a grant from the European Commission (EC) of the European Union. The project objective was to implement urgently required flood protection works to reduce the risk of the Mekong River flooding Vientiane and adjacent areas of the Vientiane Plain. The project commenced in April 1994 and was completed in August 1997.
To meet its objective the project rehabilitated existing flood protection dikes and associated structures and constructed new dikes and structures over a total length of river bank of 46.9 km. Some of the rehabilitated dikes had originally been constructed after the 1966 and 1971 Mekong floods by agencies financed by USAID and were in a very poor condition. Later in the project, a 420 m reach of river bank protection works located in front of the Hat Dokkeo Agricultural Research Station on the agriculturally important Casier Sud Section of the Project was also included.
The project was implemented under the authority of the Ministry of Agriculture and Forestry (MAF) through the Department of Irrigation (DoI) in 1994 and from 1 January 1995 to the end of the Project, by the Division of Agriculture and Forestry of the Vientiane Municipality. Representing MAF and the Municipality was the Project Management Unit (PMU) which consisted of a national project director and an expatriate co-director from the consultants, Mott MacDonald of the UK. Other PMU personnel included:
seven full-time engineering staff from MAF and the Municipality;
part-time personnel from the four districts where construction activities were located who were involved with liaison with land owners and land acquisition when necessary;
from Mott MacDonald, an assistant resident engineer for 10 months and short term specialist missions as required, for a total input of a further 10 work-months; and
full-time and part-time national personnel employed by the consultant and by the project. Most significant was a Lao consultant earth and concrete works quality control training engineer.
An office was equipped on the premises of the DoI and two Land Rover Defenders were purchased.
The original Project duration was 36 months and the budget in the financing agreement (No. ALA/92/40), between the Government of Lao (GoL) and the EC was Ecu 2 100 000, including technical assistance. Exchange rates between the Ecu and US Dollar fluctuated throughout the project duration, but at a general rate of Ecu 1 = US$ 1.2, the budget was the equivalent of US$ 2 520 000. Because of various reasons described later, a four month project extension was required as was extra construction requiring an amendment to the financing agreement for an additional Ecu 419 345 or approximately US$ 503 000.
Description of the works
For geographical and physical reasons, the works were divided into the following three sections:
the upstream existing Kaolieo to Chinaimo section, 13.4 km long, located entirely within Vientiane city and urban areas with reaches in densely populated residential areas;
the intermediate Casier Sud section, 25.3 km long. This was an existing dike which protects an important agricultural area, including a number of irrigation systems, which is a major supplier of agricultural products to Vientiane. The dike is also an important rural road and finishes at the LaoStar Earth Satellite Station on the Thadeua Road; and
the downstream Thanaleng to Dong Phosey section, 8.2 km long, which consisted of two entirely new reaches of dike located on either side of the Thai-Lao Friendship Bridge over the Mekong River.
Included in the consultant's terms of reference (ToR) was the review and updating of the designs and tender documents prepared in 1992 by the consultants SETA or Temporary Association of European Consulting Firms. It was found that there were many problems with the designs of the Casier Sud and Thanaleng-Dong Phosey sections, requiring more survey and investigations to solve, and so it was decided to divide the works into the following two contracts:
Contract No. VPFP-01, Kaolieo-Chinaimo Section (dike rehabilitation and dumped rock);
Contract No. VPFP-02, Casier Sud, Thanaleng-Dong Phosey and River Bank Protection Works.
As Contract No. VPFP-01 was located in the city area it was envisaged that the progress would be slow and could start while new designs and contract documents were prepared for the other two sections.
A prequalification process was undertaken and six contractors were prequalified. After tendering procedures, both of the contracts were awarded to contractors from the Lao PDR. Contract No. VPFP-01 was awarded to First May Civil Construction Co. Ltd. in joint venture with Chiang Mai United Engineering Construction Ltd., Part. of Thailand. Contract No. VPFP-02 was awarded to the Irrigation Construction Enterprise, Savannakhet.
Both were unit price contracts. Contract No. VPFP-01 was signed in June 1995 and completed in May 1997. Contract No. VPFP-02 was signed in March 1996 and was completed at the end of July 1997.
River bank erosion
Both contracts have reference to river bank protection works because the Mekong River is an actively meandering river in the Vientiane Plain, eroding the river bank and bed in some reaches and depositing sediment load in others. River bank erosion is a major problem in many reaches of the project area and was the main technical problem encountered. Further reference to this will be made later in this paper.
On the Kaolieo to Chinaimo Section two of the main upstream problem eroding reaches were to be, and since have been, protected by the Road and Bridge Division of the Municipality using funds from the Australian Government (AusAID) and designs prepared with the assistance of the Mekong River Commission. This project did not have the finance to fully protect all other problem reaches, and it was beyond the scope of the proposed urgently required works. Therefore the SETA designs had showed short reaches of dumped rock to protect the toe of the river bank at critical locations. A critical location was defined as one where the dike was immediately adjacent to an eroding river bank, the dike crest had partially collapsed, causing a traffic hazard, and the dike could not be realigned because of adjacent properties. The dump rock theory is not meant to be a long term solution but it provides strengthening for the lowest level of the river bank to prevent the immediate undermining and collapse of the upper sections at a fairly low cost. In the project inception stage this strategy was agreed, as there were no low cost alternatives.
In 1994 and 1995, river bank erosion was particularly severe adjacent to the existing Casier Sud dike upstream and in front of the Hat Dokkeo Agricultural Research Station. For example in November 1995, during draw down of the river level, there was nine metres of bank lost in a few days. It was clear that unless immediate preventative action was taken, the reach of dike would have had to be realigned around the back of the station and that the station would have had to be moved in a few years time. Consequently in late 1995 the GoL requested the EC that river bank protection works at this location be included in the project. This was agreed and in the 1996 dry season the project implemented a 420 m length of protection works under Contract No. VPFP-02.
Although the construction quantities were not large, the Kaolieo-Chinaimo was the most difficult section to complete because:
it was located entirely within the city, near the centre and through urban and 8 km of residential areas. This caused problems with the lack of available land to increase the dike dimensions and also with construction equipment access and small items of plant had to be frequently used by the contractor;
the existing dike was in poor condition, in many locations very marrow, sometimes consisting of the original badly eroded temporary dike which in some places had been made narrower due to housing being built around it and in others by it being directly above an eroding river bank.
Consequently as the works progressed many small modification orders were required to solve problems, minimize problems with land owners and locally improve the design.
A dike crest width of 4.2 m was attempted for vehicular access for operation and maintenance purposes but this was not always possible and in one short upstream reach of about 50 m, the dike cannot be driven along because of the location of houses and the fact that the project could not pay compensation for removing them. The completed dike varies in height between 1 and 2 m high. Side slopes were 1 to 1.5.
An obvious important consideration for the construction of flood dikes is the presence of good quality borrow material located as close as possible to the works so that haul distances and hence costs are minimized. There are borrow areas in the municipality with good quality laterite that compacts to high field densities and is resistant to erosive forces. Therefore the quality of fill material was not a problem for all the Sections. For the Casier Sud Section haul distances were greater and averaged about 12 km. For the Kaolieo-Chinaimo Section there were borrow pits located fairly close to both ends of the section.
Construction quantities completed in the section were:
a total of 18 632 m3 of compacted dike earthworks;
thirteen structures were either rehabilitated or newly constructed. The majority being to allow drainage of storm water from small urban catchment areas through the dike to the Mekong River. Other structures included a short length of timber post and plank upper river bank protection to protect and recover dike width which had been lost through bank erosion. This was also located above an area of dumped rock bank protection and vegetation was planted which has since taken and appears to be very stabilized;
2 223m3 of dumped rock river bank toe protection was completed at six locations. Although two locations have since been taken out and improved by a new reach of Municipality bank protection works at Wat Sop, a recent inspection showed that it has performed satisfactorily over the two wet seasons since implementation.
The final cost of the works in Contract VPFP-01 was US$ 179 921.44 and Kip 166 643 194. It is difficult to give an accurate single figure cost as there was significant devaluation of the Kip against the US$ during the contract, but it was less than US$ 350 000.
Casier Sud section
The main problems with this Section were caused by river bank erosion and trying to find local borrow pits so that haul distances were reduced.
An earthworks total of 70 755 m3 of laterite was compacted for:
simple dike raising to improve the worn crest, improving the freeboard were required, particularly on the downstream half of the dike, and the improvement of access roads and ramps onto the dike at villages and for farm access;
dike realignments of 520 m at Km 13 caused by another reach of severe river bank erosion and also that necessary for the Hat Dokkeo river bank protection works;
there were two lengths of existing dike were excessive seepage was known to occur under the dike at high water levels. Therefore cut-offs were constructed by the addition of deeper and wider dike sections adjacent to them.
Again, a minimum crest width of 4.2 m was attempted but not always possible in some village areas that had grown around the dike. In other reaches the dike is much wider were it also is the access road.
Only four existing structures were rehabilitated, two cross-drains and two irrigation conveyance structures required to convey irrigation water pumped from the Mekong to paddy areas inside the dike defences. Two new structures were constructed, one to improve drainage to the Mekong River and one irrigation conveyance necessary at the km 13 dike realignment.
Thanaleng-Dong Phosey section
A total of 115 981 m3 of laterite was compacted for the two new sections on either side of the Mekong Bridge. The dike crest width was 4.2 m over most of the length and 6 m where it followed the route of the Thanaleng to That Luang road over about a 600 m length. The dike height was a maximum of 7.3 m at the Huai Kha cross drain near the Lao Star Earth Satellite Station and on average about 1.7 m.
Twenty one new structures were completed and four existing structures were rehabilitated. The majority of the structures were to allow drainage to the Mekong River and to allow irrigation conveyance through the dike. Other structures included:
a 135 m length of flood wall behind the Caltex fuel station and storage compound, agreed with the owner as there was a problem with available land for dike construction. This consisted of concrete blocks filled with reinforcement and concrete which will act as a mass concrete wall;
a 200 m length of existing irrigation canal which was improved where it passed alongside the dike in Ban Dong Phosey;
other minor structures included culverts for access road drains and gate and security fencing necessary for the various premises along the Thadeua Road.
Of note is that six of the larger cross-drainage structures had “Coplastix” flap valves installed which were supplied from Simon-Hartley of the UK. These flap valves were meant to be examples of the latest “state of the art valves”. They are light-weight and will therefore crack open at lower head differences than heavier valves which are fabricated in Vientiane workshops and which can also have poor hinge mechanisms. The doors of the UK valves are also corrosion free and will have less maintenance requirements.
River bank protection works at Ban Hat Dokkeo
Briefly, the works consisted of the following:
clearing the river bank of vegetation, rubbish and loose material, then excavating and trimming the bank to a 2:1 slope with a 4 m wide berm half way down the slope. The excavated material was used for a coffer dam for the works at the toe of the bank;
the placing a Terram geotextile filter to prevent the loss of fine material but allow ground water and draw down water to flow through it;
the placing of rock filled Reno mattresses, 6 × 2 × 0.3 m thick galvanized wire baskets;
the placing of rock for transitions at the upstream and downstream ends of the mattresses and also at the toe of the works.
Probably the main design consideration with river bank protection works is the cost as they are expensive to construct on rivers such as the Mekong. The use of rock and gabions is far cheaper than methods using concrete. However the geotextile and mattresses have to be imported and, as there are at present no rock quarries developed near Vientiane, rock must be hauled from the Van Vieng area, some 90 km from Vientiane. The cost of the works was over US$ 1 300 per m length.
Another important consideration is the planning of the works as they must be completed when the river is at its lowest levels in February, March and April.
Other project outputs
Other project outputs included on-the-job training, a study tour to Thailand, procurement of vehicles, office equipment, etc., and preparation of reports. The reports included three volumes of a Water Management Report which presented all environmental and hydrological findings along with a strategy for the operation and maintenance of the flood protection facilities. Other reports included annual work plans and routine progress reports. The study tour to Thailand was kindly arranged by the Royal Irrigation Department which was very co-operative. Thailand was chosen as Mekong flood and river bank protection facilities are far more developed than those in Lao PDR, particularly along the Nong Khai to Vientiane reach.
Design elevations and freeboard
The Project Terms of Reference were very clear in that the Urgent Phase works should rehabilitate the existing flood protection works to their original design criteria. The works were not intended to increase the level of protection which should be taken up at a later stage. Therefore the Urgent Phase works provide flood protection to a safe freeboard above the historic peak Mekong flood elevations recorded on 4 September 1966 at Vientiane and Nong Khai. These were the design elevations used by agencies for USAID for the dikes constructed in the 1960s and 1970s. The 1966 flood caused considerable damage and is estimated at 26 000 m3/s at Vientiane and to have a return period of about 1 in 100 years. However, since 1966 there has been construction of flood protection dikes on both the Lao and Thai banks between Vientiane and Nong Khai. Therefore, floods of the same discharge to that prior to 1966 are now reaching higher water level elevations as the dikes contain the flood water and prevent over bank flows. Consequently, at the Vientiane km 4 gauging station, the 1966 flood elevations are now estimated to correspond to a 1 in 6 year return period flood of discharge about 20 000 m3/s.
The freeboard allowance included that for waves generated by wind, rutting and settlement, super-elevation from curving flow and a safety factor for errors in procedure giving a total of 0.80 m. The freeboard was reduced within reaches for situations such as when the dike is inland and sheltered from villages and trees and there will be no effect from waves, and on the inside of bends and straight reaches when there is no need for an allowance for super-elevation.
Administrative and financial
Very briefly, the main administrative and financial problems included:
a shortage of finance on several occasions. This resulted in a slow start to project activities and later on, late payments to contractors which nearly led to the PMU being in breach of contract;
the initial request from the GoL for the additional finance for the extra river bank protection works was made in November 1995 and the signing of the agreement took place in August 1996;
with the grant being in Ecu, the main project bank account being in French Francs and local expenditure being in US$ and Kip, a strengthening of the US$ against the French Franc meant that local costs were increased in French Francs and Ecu terms. This occurred quite significantly in 1997, near the end of the project and was a concern at the time. However the project was completed under budget with an estimated 97% expenditure of available funds.
Technical: River Bank Erosion and Protection Works
The main technical problems encountered were those with the existing designs in the inception period and the problems with the meandering of the Mekong River. The problems with the existing designs also contributed to a slow start to the project and the requirement of the four month extension.
This workshop concerns flood management and mitigation and not river morphology. However river bank erosion should be a major concern when planning flood protection works and is worthy of further comment. This is because it is desirable to locate the dikes as near to the river bank as possible in order to maximize the areas protected. The top-soils on the river banks are generally very fertile and intensively farmed. However if the dikes are located close to an eroding river bank then they will eventually be undermined and collapse. For example it may be considered appropriate to locate a dike 50 m from a river bank. However Mekong River bank erosion can be up to 20 m a year and if this is the case, then the dike will have to be moved in less than three years. As stated previously, bank protection works are expensive and cannot be automatically considered in a flood protection project, particularly in rural areas, where in economic terms the cost can be higher than the value of the assets which are to be protected.
The lowland Vientiane Plain consists of alluvial deposits, it is characterized by nearly all flat land with slight undulations. Consequently the Mekong River's gradient is low and the river is characterized by its winding meandering channel and natural instability with alternating sequences of erosion of the river bed and banks and then sedimentation. When viewing the river it can be observed that the upper layers of the river bank consist of strong clay deposits which are erosion resistant and can stand nearly vertically. However the clay deposits often overlay weak lower stratus of alluvial gravels, sands and silts which are near the toe of the bank.
The main cause of river bank erosion is the meandering of the river. Generally, erosion takes place on the outside of bends of the river channel and sedimentation occurs on the inside. The main channel of the river (or thalweg) is often located on the outside of the bend and the velocity of flow is greater. During floods, the sediment load of the river is higher as is the flow velocity causing a continual cycle of scouring of the river bed and then the deposition of sediments. When the bed of the main channel is immediately adjacent to the river bank, the scouring of the bed causes undermining of the toe of the bank and the bank is then left unsupported and collapses.
In the project area the erosion problem was considerably increased when there is an island in the river as was the case in front of the Agricultural Research Station at Ban Hat Dokkeo and then further downstream which caused the 520 m realignment of the Casier Sud dike at km 13.5. At Hat Dokkeo the river previously took a main route on the Thai side of the island and a large meander was created. However when the meander became long, the distance between the start and finish of the meander increased to an extent that the velocity of flow within the reach reduced and increased sedimentation took place. Then the river took a sort of “loop cut” between the two points and the main discharge took the easier, shorter distance between the island and the Lao bank at Hat Dokkeo. The result was excessive erosion of the Lao bank and the island which was previously Lao is now part of Thailand.
At the km 13 realignment the problem was more simple in that the more centrally located island divided discharges and forced the channels towards both the river banks. There is significant river bank protection works on the Thai bank at this location.
There are no braided channels in the project area, where bank erosion will also be a problem. However the Mekong has features of a braided stream in that it is unusually wide for its depth, it has low sinuosity and it has channel bars or islands.
The erosion process is then increased by drainage of water through the river bank. This drainage takes place particularly during the draw-down of the river water level when there is flow from the saturated river bank back to the lower river level. This drainage is increased and erosion aggravated when there are irrigation facilities or fish ponds located on the river bank as was the case at the Hat Dokkeo Agricultural Research Station. The erosion process is then increased when the ground water flow washes out the sands, silts and gravels in the lower river bank strata. The higher strong clay river bank is again left unsupported and collapses.
Bank collapse can also take place during the first rains. This is when tension cracks are left in the top of the bank after partial undermining from the previous flood season. The rainfall runoff can enter the cracks and cause a block to collapse into the river
It can be observed from maps that where Vientiane Municipality is located, there are many bends in the Mekong River and consequently bank erosion is occurring in many locations. It has been estimated that over 30 km of Lao river bank requires to be protected within the Municipality between Vientiane and Nong Khai but mainly because of the high costs, there has only been less than 2.5 km of bank protection works completed to date.
This leads to the consideration of other points concerning the sustainability of the works.
Requirement for operation and maintenance
The flood protection works will not be sustainable if the dikes and structures are poorly operated and maintained through lack of operation and maintenance. Maintenance programmes are necessary to ensure that the flood defence facilities are fully serviceable before flood periods. Operation procedures are necessary for the closing and opening of the water control structure gates or stop logs.
In the Project's Water Management and Final Reports it was recommended that a Vientiane Plain Flood Protection Operation and Maintenance Agency (O&M Agency) be formed by the Vientiane Municipality. The O&M Agency must have finance, staff and resources to undertake many activities including the continual inspection of all the dike facilities, to carry out routine and periodic maintenance, be able to perform emergency activities if necessary during the flood season, undertake system operation, monitor rates of river bank erosion and prepare survey, design, drawing and cost estimates for special works which may require additional inputs from consultants.
It was recommended that donor assistance is required for technical and financial assistance with O&M activities. Technical assistance is necessary to assist the proposed Agency with on-the-job and formal training and the rehabilitation of a number of existing water control structures located downstream of the Urgent Phase area that are in an extremely poor condition and in an urgent need of repair. Donor finance is required to fully equip and operate the Agency for several years and for the rehabilitation of the structures.
Dikes and structures
The previous flood protection works and those undertaken by the Urgent Phase Project have had obvious environmental benefits in terms of the prevention of loss of life, damage to infrastructure, property and crops.
The very nature of the construction of flood defences will cause increased flood problems either on the opposite river bank, if there are no defences there, or downstream of the works if there are defences on both banks. The previous works will have raised typical flood water levels in the vicinity of Vientiane. This must tend to raise flood water levels in the immediate upstream reach. However there is no evidence that this has caused significant increased flooding problems in upstream areas.
The construction of dikes will lead to greater depths of flood flows and hence in theory, to higher flow velocities and an increased risk of river bed and bank erosion. However this effect is difficult, if not impossible to measure because it only occurs on the occasion of rare flood peaks which would otherwise have overtopped the banks. Nonetheless, the Mekong River is meandering and river bank erosion is a natural process.
The previous flood protection works have reduced the frequency of periodic flooding of the wetland areas to the east of Vientiane from high Mekong River flows. However, this flooding is not integral to the survival of these wetlands and so it is difficult to quantify if the direct reduction in flooding has had any adverse affects.
The gated outfall structures across tributaries and drains, which prevent flood waters from backflowing into these waterways, will have prevented any seasonal migration of fish that was occurring between the river and the waterways.
The Urgent Phase works have had few negative impacts. No significant habitats have been affected. With only minor exceptions, trees have been retained. Apart from the construction of the new Thanaleng-Dong Phosey section, which will increase downstream flood water levels, the overall impact on the Mekong River channel is minimal. A limited area of additional land, gardens and agricultural, has been taken for the dikes, for which compensation was not paid.
The rehabilitated and new dikes have generally improved access for local residents in both the urban and rural areas. The project made every effort to construct new access ramps to the dike and improve existing ramps whenever possible. There are some exceptions in the urban Kaolieo-Chinaimo section where the dike heightening and widening between closely spaced houses has made access for pedestrians on to the dike more difficult. There are the associated traffic hazards from the dike being used as a road, particularly where the dike is located close to the river bank, houses, schools and amenity buildings.
There were some obvious temporary adverse affects during construction principally from dust.
River bank protection works
There is a general concern that river bank protection works in one reach may cause increased erosion at another unprotected reach, and even on the opposite river bank, by changing river flow patterns. However it is considered that such effects are extremely localized and, because of the large width of the Mekong River, it is certainly very unlikely that they would lead to increased erosion on the opposite bank. However, as the river is naturally meandering and bank erosion occurs naturally, it is difficult, if not impossible, to measure the effects of bank protection works on other reaches.
River bank protection works will affect fish habitats at their immediate location. They have the positive impact of saving valuable land which would of been otherwise lost to the river and it should be noted that this is an important consideration for the GoL.
Monitoring and evaluation
Monitoring and evaluation of the works completed are important for long term management of the flood defence system, for setting priorities for new works and adjusting O&M requirements.
The monitoring of important physical processes affecting the flood protection facilities must include:
rates of river bank erosion along the dike and in particular immediately upstream and downstream of river bank protection works:
dike crest levels and width;
As stated several times, river bank erosion is the process that most threatens the flood protection facilities. Without such erosion the dikes would only require routine maintenance, except when overtopped by rare flood events. If river bank erosion undercuts the dike during high river levels then flooding may occur through breaches in the protection. Erosion rates must be measured at locations all along the dikes so that the examination of the records will indicate priority areas which require action soonest and those which appear stable or at least require no action for several years.
Beyond the simple use of the rate of erosion, it was recommended that specialists be periodically requested/hired for an interpretation and evaluation of the records. A specialist may perhaps separate out various influences such as unusual bank materials, perhaps changes to the natural river regime, for example those caused by local mining of sand and gravels from the river, unusually high drainage discharges through the river bank caused by irrigation facilities and most importantly, the large, powerful effects of river meanders.
Inspection and monitoring of areas immediately upstream and downstream of river bank protection works are important as often the length of works is affected by available finance and it may be that erosion is continuing upstream and downstream and that the works may become isolated and fail from the ends.
Dike crest levels should be surveyed at least every two years or more frequently in areas subjected to excessive vehicle use, for example when mined sands and gravels are transported. The crest levels should not be allowed to fall more than 0.2 m below design levels as this is the amount of freeboard allowed for dike wear and tear. Excessive wear and tear in particular locations should be investigated. In sheltered reaches, the freeboard allowance for waves was reduced and if circumstances change, such as buildings demolished or trees cleared, and such areas become exposed to waves, then the dike should be raised accordingly.
Recommendations for further work
It is again emphasized that the Urgent Phase works have improved the flood protection facilities protecting Vientiane and have reduced the risk of flooding but not removed it. Therefore in the Project's Final Report recommendations for further work included:
investigations for increasing the level of flood protection to Vientiane to those compatible to other capital cities, at least for a 1 in 40 or 50 year flood and perhaps to a 1 in 100 year flood. The final decision as to what level is chosen will require a great deal of consideration by all concerned as it is estimated that protection for a 1 in 100 year flood will require the dikes to be risen by over 2 m. This would change the face of Vientiane considerably. Therefore, so that it is more accurately known what design elevations are required along the Vientiane river front, it was recommended that additional river level gauging stations should be installed as soon as possible;
the construction of further lengths of river bank protection works in critical areas to secure the flood protection facilities and prevent the loss of urban and agricultural land;
assistance with O&M of the dikes, structures and bank protection works to maintain the level of protection.
Investigations for the river bank protection works should include the possible development of a rock quarry nearer to Vientiane as the haul distances from the existing sources are long and add considerably to costs.
It is concluded that the Urgent Phase works have reduced the risk of Vientiane being flooded by the Mekong River but not removed it. Considerable further inputs are required to maintain the present level of flood protection, increase the level of protection to that required by a capital city and also to secure the protection by the construction of further lengths of river bank protection works.
The Mekong River is actively meandering and the main technical problem during the Project was from river bank erosion undermining the flood protection dikes. Therefore estimates of past and future erosion rates should be an important part of the investigations for all flood protection projects on the Mekong River.