Practical action balapur dyke construction


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Practical action balapur dyke construction

  1. 1. Back Ache and Bamboo: An alternative approach to dyke construction in flood prone communities in Nepal Background Throughout Nepal the theory and practice of gabion box construction to protect river banks and hill slopes is well known. In areas where rocks are abundant their construction represents a cost effective and locally replicable way for communities to utilize relatively abundant natural resources for local risk reduction and protection purposes1. Practical Action has used this technique in previous DRR2 projects, but during 2007/8, in its DIPECHO3 supported Bardia and Banke programme, it was faced with a number of constraints. In Bardia in particular vulnerable riverside communities are some distance from the hills meaning that rivers do not have sufficient power to carry large rocks to their locations, instead depositing them in the foot hills and first few kilometers of the plains. As such rocks are either unavailable locally or only at a prohibitive cost. This makes gabion spur and dyke construction expensive and ultimately unsustainable, as beyond the resources of communities to replicate independently. Additionally, due to the profusion of vertical river banks in the area, the construction of traditional gabion spurs and dykes is simply likely to protect one area at the expense of another. The stone structures simply deflecting the destructive power of the river either down-stream or to the opposing bank (in Practical Actions previous DDR programme in Chitwan and Nawalaparasi this was not a major consideration, as the far bank consisted entirely of national park, where Cleaning bamboo before use. A practice which the thick natural vegetation was largely impervious to community subsequently found unnecessary. periodic inundation). Under this scenario there has been a need for an alternative strategy if the project is to stop erosion and provide a technology and approach communities can replicate long term. The theory Regular river bank erosion and collapse take place in the Terai4 primarily due to two factors; 1. The under cutting of vertical banks by flood waters (the water undermining the high bank, leading to subsidence and collapse) 2. The creation of a rotational slippage plane, due to river bank soils becoming saturated by rain fall and surface run-off, leading to sections of bank losing their integrity and shearing off.
  2. 2. To stop this occurring both factors had to be addressed within the project. As such an approach incorporating a number of interlinked components was proposed. An example of how typical river bank failure occurs, due to; 1. Under cutting of a vertical river bank by flood waters 2. The creation of a slippage plane, caused by inundation and soil saturation. Bamboo protection has been used elsewhere in Nepal, as has the use of sloping banks and sand bag protection. What was not clear to Practical Action was if all three had been used in combination before, and as such the idea was floated with communities as a possible ‘pilot’. Looking at Balapur, a village within Ward 6 of Gulariya, areas of the bank looked suitable to try this combination, with a number of factors influencing the decision; o Near vertical banks, of relatively weak clay/soil mix, experiencing annual under-cutting and collapse, irrespective of the severity of the monsoon period flood/high water. o The possible availability of land behind the bank for cutting back, to form a sloping bank, as not intensely cultivated at present (instead being used mainly for rough grazing and pasture) o The bank side being open to the full erosive power of the river, but not being at a structurally weak point (such as the outside of a bend, where most cutting is likely to occur) o High community mobilization and motivation to take action. o The cost of stone gabion work being beyond the means of communities to support
  3. 3. The approach Before the pilot could begin community members had to be convinced of the effectiveness of the approach, particularly as it would involve a ‘voluntary’ loss of up to 8 metres (see below) of land before the dyke could even be constructed. Given the extent of land loss experienced every year anyway, community members eventually agreed, though there was skepticism from adjacent communities and the residents of Balapur went as far as obtaining a written agreement from the landowner on whose land they were testing the idea that he would not prosecute them if the project failed and even more land was lost ! “People were not convinced. We had to make a contract with land holders mentioning they would not claim against us if their land was further lost !” - Kali Parsad Tharu (PIC President. Ward 6 Gulariya). River bank at Balapur before slope cutting by community volunteers. To produce a stable slope in the sand/clay mix soil it was calculated a minimum angle of 35 degrees (from horizontal) was required, though obviously, as a general rule, the less the degree from horizontal and the shallower the slope, the more stable it will be and the less likely to erode. “Previously people from other places said that we were mad, but now they say they appreciate what we have achieved and they also want to copy us” – Kali Prasad. President Ward 6 project implementation committee. The same piece of river bank during cutting of the sloping bank As the vertical bank was about 6 metres high (at low water) this was eventually cut back by 8 metres, giving an angle in the low 30 degrees. This was achieved by first digging a channel at 30 degrees, as a guide and marker for the rest of the bank, which the rest of the slope copied.
  4. 4. The objective was to ultimately create not a barrier, to fight erosion (as in the case of a stone gabion dyke), but a structure immune to erosion through its allowance of the water to non-destructively rise up it (and later down it) without causing any obstruction likely to attract erosion by the river. Once the slope had been created bamboo poles when fixed both below and above the water line. These were set up in several rows parallel to the river bank. Strips of bamboo were then woven between these poles to act as a barrier. Once this was completed old cement bags were filled with sand from the river which also, as a side effect, helped in part to lower the river bed in the immediate area of the dyke. These were used to ‘face’ the slope, making it even more impervious to the destructive powers of the river. To stabalise and create long term strength to the bank live bamboo and lemon grass seedlings were also planted along the bank, between the sand bags, to ultimately ‘naturalise’ the bank. Finally, to stop soil saturation and possible slippage due to water seeping in to the slope from the landward side, a lateral catchment drain was dug at the top of the bank. This was dug so that water emanating from the landward side of the slope could be drained to either end of the dyke, preventing it from percolating down and into the substance of the slope, so weakening it from within.
  5. 5. 6 1. Bank before the intervention 2. Calculation for cutting back of bank to create optimum slope 3. Positioning of bamboo poles, bamboo matting fences and sand bags
  6. 6. 4. Completion of dyke with construction of lateral drainage channel In original calculations it was estimated that Gabion work necessary to protect this 150 meter length of bank would cost 1,700,000Nrs (approx £13,600/€19,000/$27,200)5. This was beyond the budget of the project, but more importantly beyond the resources of the community to replicate themselves, un-aided, long-term. Sand bag filling and fence weaving during low water at Balapur during June 2008 In constructing the sloping protection work using local materials only 92,866 Nrs was spent in total (£750/€1050/$1500). 56,000Nrs was spent on bags, while bamboo was purchased at a subsidised price (of about 50% of market rate) from the communities own collectively owned resources at 28,185Nrs. The remainder of the cost was made up of the cost of string, to tie bags, and a small amount for transportation.
  7. 7. On top of this labour equivalent to 160 people working for 16 days (= 2,560 person days) was contributed by the community. At a low daily wage rate of 125 Nrs per day this equates to 320,000 Nrs. However, as the community members contributed this free of charge, it was not factored into the ‘costs’ by the community themselves. Who contributed ? Interestingly nearly all families contributed some labour, with families and individuals with no real likelihood of benefiting from the dyke (living far from the river bank) also taking part. The only minor incentive was 700Kgs of rice contributed by a local NGO, BASE, from supplies provided by the WFP. This equated to a one off distribution of 4.5 kg per person. At a per kg price of 35Nrs this equated to just 158Nrs per person, or a total of 24,500Nrs for all labour volunteered. Within the community they developed a programme where everyone contributed to all activities, rather than individuals just working on one aspect of the programme. As such everyone contributed some time to shelter construction, setting up early warning systems and dyke construction, so collective ownership, as well as skills development and understanding were spread evenly. The experience of the 2008 monsoon During both early and late August 2008 water levels of 4.5 metres, as measured on recently installed flood gauges were recorded. This is about 3.5 metres from the top of the bank (so 8 meters on the locally installed gauges is the critical level for local flood, as this is the point at which river water would over flow the banks). Following the floods the following issues and impacts were observed; 1. Some ripping and deterioration of sand bags was observed, but oddly this was at the higher levels of the bank, rather than the lower where the bags were almost totally submerged. After discussion it was decided this was most likely the result of the fabric of the bags breaking down under extreme sun light (whereas they were protected under water). The bags used were old cement bags, so in future Practical Action will advise on the use of new, rice-type bags, as these are thought to be of better quality and more durable. Additionally the possibility of using bags made of natural fibers may be explored. All damage to the bags caused by the floods of July/August 2008 has already been repaired by the community, at very little cost. Damaged bags on the upper level of the dyke.
  8. 8. 2. At the extreme down stream end of the dyke the lateral drain at the top was not fully completed before the on-set of the monsoon. As such water percolating through the dyke caused the leading edge of the dyke to collapse and a section of bags to slip about a meter (though they were not washed away). This has proved the value and effectiveness of the drain and as such this has now been extended to ensure protection of the whole dyke. Slippage caused by saturation of the dyke from the landward side 3. Bamboo matting woven between the posts has almost totally been washed away, as have some posts. This was foreseen and is acknowledged by the community as a component which will have to be replaced annually. However the posts have also caught a lot of floating debris which community members are convinced has added to the strength and resilience of the dyke. The posts and matting have also encouraged the deposition of sand and silt (due to flood waters being forced to slow down), which have also added strength to the dyke. “We missed the fact that we should use the bamboo immediately after cutting, as live it can create a living barrier………this dyke performed very well this year because the dyke sieves and catches mud and other materials” – Kali Prasad. President Ward 6 project implementation committee. Bamboo shoots beginning to spontaneously sprout from posts on the Balapur dyke. August 2008 5. As the communities were not totally convinced of the effectiveness of the approach stone spurs were build both upstream and down stream of the sloping bank. Interestingly the gabion
  9. 9. spur down stream in particular was badly distorted by this years flood (indicating the floods power and ferocity) and is unlikely to last many years before being completely destroyed. Beyond this the existing ‘natural’ vertical bank has again been cut back, by between 1.5 and 2 meters, with this likely to continue year on year. In reality however this level of erosion is very small, as only a small distance further down stream bank up to a depth of 20 meters has been lost, which is more ‘normal’. Badly damage stone spur immediately below the sand bagged slopping dyke. Community members are convinced the relatively small amount of land lost in the area immediately down stream of the dyke is the result of the main channel of the river being redirected across towards the far bank, where it was some years previously. While it is not possible to claim this is solely the result of the dyke, the fact that the sloping bank allows water to rise unrestricted up it, where previously the vertical bank gave only the opportunity to undermine it, does seem to have effected the rivers morphology and made it more likely to deposit materials than remove them. “We never expected the result we have achieved…. now we have seen the reality and seen the real effects, this is why this work is so wonderful and exciting for us” - Sobendra Chaudhary – PIC secretary The Future The community of Balapur are now keen to extend the sloping bank work both up and down stream of the existing pilot plot and have already approached local NGOs and the municipality for support. Similarly communities from further down stream, from Ward 10, are also interested in using the approach, while staff from the DWIDP (Department for Water induced Disaster Preparedness) among others have also visited, to assess the approach and gather information, as this is not an approach currently practiced by themselves. Practical Action itself intends to further explore technical aspects of the approach, such as alternative materials and the influence of different soil types, while it’s also committed to using the expertise which now exists within the community in Balapur to further disseminate the approach both in terms of the technology used and the approach to community mobilisation. Learning and future developments o Using this approach dykes can be added to, improved, or repaired incrementally as time and resources permit. In the case of gabion work this type of construction will always require a major financial outlay.
  10. 10. o Unless banks are cut back to a sufficiently low angle undermining may still occur. The angle is likely to be less critical in structurally less critical locations, such as the inside of bends, but far more so in highly vulnerable areas such as the outsides of bends, where maximum erosion occurs. Balapur dyke mid-monsoon. Post-monsoon the stone gabion work was badly damaged while the sloping bank remained almost totally intact. o The creation of upslope drainage seems critical to bank stability. To stint on this aspect is likely to be counter productive and could result in total slope failure. o In the case of Balapur the line of sandbags currently ends about 2 metres below the top/line of the lateral drain. Community members now realise this gives further opportunity for surface run-off and rain water ingress into the dyke and as such will soon extend this line of bags to the very top. o Making the slope of the bank inconsistent could offer the potential for flood water to exploit an area of weakness. As such the community in Balapur have decided to take greater care in maintaining a completely uniform slope angle in future dyke work. o The use of non light-degradable bags seems fairly critical. In future experimentation with other technologies might also be worth exploring, such as the covering of slopes with matting or netting and the use of natural rather than man-made fibers. o Originally it was planned that bags would only be 2/3 filled, with the unfilled section underlying the next bag above. In the event, due to bags being smaller than ideal, all bags were fully filled. While these have served satisfactorily in future the original plan is likely to be reverted to, as overlaying can only add strength. o It is too early to assess the success of planting bamboo within the fabric of the slope. Long term however it will be worth experimenting with other plant varieties as well as looking at
  11. 11. creating ‘live’ barriers by artificially encouraging barrier plants to inter-grow and combine with structural measures such as the vertical bamboo poles already used. In the construction of the bamboo components of the dyke bamboo which had been cut 15-20 days previously, and allowed to dry, was mainly used. However 6 or 7 ‘fresh’ poles were used which have subsequently sprouted and are setting down roots. In the past it was the practice to cut off all the “eyes” or “knots” on the bamboo but now the community believe they should use unprocessed, freshly cut bamboo, so it can grow. Keeping community participation records. “We will extend this work on the other bank also, by which we will save our village. To achieve this we will collect local materials such as bamboo, sticks and other raw materials” – Kali Prasad. President Ward 6 project implementation committee. “Yesterday (27th August) we visited the site along with other PIC members, villagers and people who use the place as a ferry crossing. The river level was 3.4 metres so we went to observe the level and to see the work was still intact. It was” – Sobendra Chaudhary – PIC secretary. 1 See the Practical Action “Technical Brief” on Gabion box construction in Practical Actions previous DIPECHO project in Chitwan and Nawalaparsi at 2 Disaster Risk Reduction 3 DIPECHO is the disaster preparedness component/funding line within the European Commissions broader Humanitarian Aid department (ECHO) 4 The Terai is a flat flood plain extending the entire length of Nepal, along its boarder with India. 5 All prices and exchange rates used are as of 1st September 2008.