Mitigating flood disaster through river course stabilisation
Bangladesh is a country of rivers over which three major continental rivers -- the Ganges, the Jamuna and the Meghna -- are flowing together with their numerous tributaries and distributaries before falling in sea (Bay of Bengal). Scour-deposition and sediment transport processes, channel development and its shifting and abandonment are very rapid in these floodplain rivers. Due to such drastic nature of the river systems in Bangladesh, almost every year, bank lines are destroyed at several locations along the major rivers during monsoon flood because of the failure of artificially constructed embankments and spur-dikes
Failures of such structures create severe problem to the local people and often within the entire country. Millions of people are experiencing flood disasters this year and sufferings are increasing gradually. People are expressing their views for humanitarian assistance to the flood victims through different news media. This kind of assistance would definitely reduce the sufferings, but flood would come again and again unless we can wipe out the root of the problem.
We have to keep in mind that outsiders would never be successful in solving such unique problem experienced in our country. A combined effort within the country is necessary to solve such recurrent national problem
Channel width vs. channel depth
In Bangladesh, over the years, channel width is increasing due to erosion and depth is decreasing due to siltation because of its unfavourable geographic location and discharge control by the countries in the upper reaches. It is very difficult and even impossible to maintain in-stream flow requirement in the major rivers. Therefore, rivers are loosing their navigability and water-ways are severely obstructed during the dry season.
On the other hand, conveyance capacity of rivers is reducing and is insufficient for safe and expeditious passage of flood water and sediment discharge during the monsoon. As a result, country had experienced severe flood disasters during the past such as in 1988 and 1998. However, this year the situation seems to get more severe compared with the past events.
Siltation of river beds
Earthen embankments are often used as flood control measures along the major river banks. Floodplains and river banks are developed from recent deposits consisting mostly silt and fine sand that are highly susceptible to erosion. As a result, the main channel often shifts towards the earthen embankment often resulting in its failure. Moreover, due to the construction of such embankments, siltation of river beds has accelerated. But the embankment height remains almost unchanged since its construction. Therefore, the intensity and frequency of embankment breaching and over topping by peak flow are increasing.
Impermeable groins or revetments are used as a method of river training and bank protection in order to give additional protection to earthen embankments against bank erosion. Usually, river training structures are constructed during low flow period in order to get them functional during the upcoming monsoon and onward. Changes related to local scours around structures and interaction between local scour and riverbed morphology are very rapid during the first monsoon due to river's reaction against such sudden interventions. Rives need sufficient time to adjust with the modified condition towards the formation of stable courses
However, the scale of river's responses against such artificial interventions depends on the scale of the imposed disturbances. In small rivers (in terms of channel width), the lateral length of impermeable groins would be reasonably manageable from the hydraulic viewpoints if the economic condition of the respective community can afford the required cost involved for the implementation of such projects.
However, these kinds of structures are often extrapolated towards the larger rivers on the basis of the experiences in the smaller rivers in Europe. Very often, the extrapolated structures for large rivers are too big that create huge local disturbances and can rarely guide the entire river system towards its stable form. Also, these are too expensive to implement along the entire river reaches in the developing countries like Bangladesh. Therefore, location of priority sites (most vulnerable to erosion) are usually identified and recommended to protect first.
Such kinds of intervention create local changes leading to changes in the entire river reaches and in the long run, stabilised courses can never be formed. Rather intermittent local interventions would make the problem even more complex. In addition to these factors, it is already proved in some developed countries that the above conventional methods can never provide environmentally suitable solutions even though these are proved to be effective against bank erosion and to some extent stream restoration (narrowing and deepening the base-flow-channels) in smaller rivers.
Owing to these difficulties of adopting the conventional methods for alluvial river course stabilisation and restoration, alternative low cost methods need to be developed that can be adaptive within local socio-economic condition and would be environmentally compatible solution as well. Some researchers discussed the need for the development of such approaches from their experiences at the Rhine River in the Netherlands and at three main large rivers in Bangladesh. To this extent, a research group discussed the preliminary idea on the possibility of use of bandals for the formation of stable river course
It would be a gradual method that allows the river enough time to adjust during each stages of small intervention using bandals instead of conventional intervention using groins or revetments at one time. Also, it would be a cost effective solution for stabilising macro scale sand bars along the major rivers in Bangladesh where about 600,000 people use to live.
Concept of bandals
Bandals are one of the local structures developed in the Indian sub-continent that obstruct flow near the water surface and allow it to pass near the riverbed. These are made of naturally available materials such as bamboo and wood that are regarded as inexpensive method over conventional structures and mostly applied for the improvement of navigational channels during the low flow season. Information available on bandals so far is from field experiences and features of flow and sediment transport around them are still unknown.
The sediment materials of an alluvial river are transported both as bed load and suspended load. Even in the case of suspended load, most of the sediment is transported near the bed. This feature of sediment transport is the key to use bandals. Within the lower half of the flow depth, major portion of the sediment flow is concentrated, while, the reverse is true for the water flow discharges. The essential characteristics of bandals are that they are positioned at an angle with main flow and there is an opening below it while the upper portion is blocked. As a thumb rule, the blockage of the flow section at upper part should be about 50 per cent in order to maintain the flow acceleration. The surface flow is forced to the upstream creating significant pressure difference between the upstream and downstream side of bandal The bottom flow is directed perpendicular to bandal resulting near bed sediment transport along the same direction.
Therefore, much sediment is supplied towards the one side of channel and relatively much water is transported to the other side. The reduced flow passing through the opening of bandals are not sufficient to transport all the sediment coming towards this direction, resulting sedimentation over there (bank side). On the other hand, more water flows with little sediment moves towards the main channel that develop deeper navigational channels there.
From the key results of recent experimental studies using bandal-like structures, it was found that flow diversion towards the main channel can be achieved both from the upstream and downstream side of the bandal resulting deeper main channel as compared with the conventional structures. Sediment coming from the main channel was deposited in the bandal fields.
Bandals are found to be very effective for navigational channel formation and land reclamation near the bank lines. Therefore, the bandal-like structures would be capable in forming stable river course that would ensure deep navigational channel and bank protection as well.
Less expensive solution
Bandal-like structures would be less expensive solution over conventional methods. Another important feature of these structures is that the lateral intervention can be extended gradually (only idea is developed, not tested yet) that cannot be possible using conventional structures. During each stage of small intervention enough time would be allowed so that river flow and bed topography can adjust with the changed environment.
The gradual encroachment towards the lateral direction using bandal-like structure creates fewer disturbances to the river and the river can get sufficient time for its adjustment and new main channel and bankline development through scour-deposition processes. In addition to this, the cost effective solutions for the stabilisation of macro scale sand bars where many people use to live in big alluvial rivers may be obtained through this gradual process using bandal-like structures. During the processes of river system stabilisation, lost land would be reclaimed, land elevation would be raised and river courses would be deeper gradually.M. M. Rahman (29 July, 2004)
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