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PROTECTION METHODS AGAINST RIVER BANK EROSION
(With particular reference to the Amu River)
Prof. Dr. Jahangir Bakhteri F.ASCE, CMSEI, P.E. M’sia, MIE & CEng, MISET
Erosion and sedimentation are natural phenomenon and processes, but often are in conflict with our use of the shorelines and river banks. The most noticeable problem created by erosion is the loss of waterfront i.e. river bank’s property. This presentation provides basic information for assessing erosion problems and selecting appropriate solution for protection and rehabilitation with particular reference to the Amu River.
Amu River which stats from Zarkul in north-east of Afghanistan and it constitute 1200 km international boundary of Afghanistan with Tajikistan, Uzbekistan, and Turkmenistan up to Khamaab and then flow toward north-west and pour into the Aral lake. Its total length is 2500 km out of which 1300 km is inside Turkmenistan. Its average discharge at Karki is about 2050 m3/sec. The depth and width of Amu River are variable. It has more than 12.0m depth of water in some parts and its width at Tashguzar, Afghanistan is about 5000m where its depth is between 1.5 to 2.0m. Amu River erodes a large amount of soil on its lift bank (Afghanistan side) annually which causes the destruction of tremendous properties and land on the Afghan side. Therefore, the improvement and rehabilitation of its bank is very much necessary.
Amu River banks’ erosion is caused primarily by high flow velocity, wind driven waves and to a minor extent by wakes from operating boats in some parts. Wind velocity, duration, and the expanse of open water (fetch) the wind blows over are the predominant factors generating waves that attack and erode the river banks. The basic progression of erosion resulting from wave action includes:
(a) Attack by waves. (ii) Erosion of a bank and beach causing undercutting. (iii) Slumping of the bank.
(iv) Removal, transportation, and deposition of the bank sediments along the shoreline.
Depending on the size and condition of each eroded part of the Amu River bank and the water flow / discharge and its velocity in the river, a particular protection method has to be employed.
Different methods of rehabilitation are used for the protection of river banks against erosion and some of the efficient methods have been presented in the following parts of this paper.
Important factors for designing and selecting the suitable protection types are:
1 – The eroded bank length.
2 – Proper alignment (to avoid flow disturbance).
3 – Proper land use (especially upland the protected area).
4 – The design should be friendly with the environment.
5 – Availability of the suitable construction materials such as stone size, quality and thickness.
6 – Suitable filter layer/geotextile separator to prevent migration of base materials through revetment.
7 – Back fill (fine sand, silt and clay with proper compaction).
8 – Water elevation (the protection should be at least one meter above maximum water level).
9 – Slope should not be steeper than 3 horizontal and 2 vertical (340) for better stability purposes.
TYPES OF STRUCTURES USED IN THE RIVER BANK IMPROVEMENT
Few basic types of structures are designed to stabilize a bank such as different filter structures and wall structures. Filter structures reduce the level of the wave's strength while keeping soil from passing through to the water. Wall structures are impervious vertical walls that separate the natural shoreline from water and wave action. The success of each type depends upon adequate design and construction.
Filter type structures are designed to reduce the energy of the incoming waves and water flow as they strike the river banks, while at the same time, hold the soil beneath it in place. Filtering qualities result from the use of layers of varying sized stone and other materials. In construction, the bank is first graded to achieve the shape required for the structure being installed. A filter cloth / geotextile separator is placed on and attached to the graded bank. This cloth is similar in weave and texture to tightly woven burlap but is made of a non-deteriorating plastic. On top of the layer of filter cloth a six to eight inch layer of stone is placed. This layer of stone holds the filter cloth in place and becomes the bottom layer of the actual structure. A variety of outer layers are then placed on top of the stone. This type of structure is preferred to bulkheads where groundwater is part of the erosion process.
A stone revetment shown in the figure below is constructed by placing progressively larger blocks or pieces of stone on filter cloth or fine gravel. The armor layer must be stable against movement.
Gabions are rectangular wire baskets filled with stone. Gabions are very versatile and they may be used as revetments, groins and offshore breakwaters. Figure below shows gabions being utilized as a wall type erosion control structure.
There are different types of gabions such as mattresses and upper level baskets. Mattresses are baskets which are usually 9 to 12 inches thick and provide a foundation for the upper level baskets. Upper level baskets should be about 6, 9, and 12 feet in lengths and 1, 1.5, and 3 feet in heights.
At the construction site, gabion baskets are unfolded and assembled by lacing the basket edges together with wire. Individual baskets are then laced together, stretched, and filled with stone. The lids are closed and then wired to other baskets. The result is a large heavy mass that is not as easily moved by waves or current as single stones might be. Generally, gabion walls are suitable on sites where bulkheads or revetments are acceptable.
The baskets are made of galvanized and polyvinyl chloride (PVC) coated steel wire in a hexagonal mesh. The stones used to fill the baskets are usually in the range of 4 to 8 inches.
Gabions are suggested for use in brackish and freshwater environments, where corrosion of the wire will be minimal. The baskets should be staggered and joined, much like the courses of a brick wall, in order to form a stronger structure. It is also recommended that the end of the mattresses be anchored with large stones or anchor screws. Damage to the baskets should be repaired immediately. Missing stones should also be replaced from time to time to maintain a tightly packed basket. This will minimize stone movement which can cause abrasion damage to the basket wires. The main advantage is that the construction of gabions may be accomplished without heavy equipment. The structure is flexible and continues to function properly even if the foundation settles. Adding stones to the baskets is an easy maintenance procedure. The cost of using gabions may be low compared to other protection methods depending on the distance of the stone from the job site.
This type of protection is composite of small blocks of concrete which are placed on the banks slide over a filter / geotextile separator as shown in the figure below. It is usually used when the stone are not available or expensive. The blocks founded on dumped stones may be prone to foundation failure.
(d) STONE GABION MATTRESS
Mattresses made of heavy galvanized (or polyethylene) meshes; typically one meter wide with a thickness of 20cm to 25 cm is laid on top of a stone toe as shown in the figure below. The backfilled slope should be first covered with a filter layer/geotextile separator. The mattress is filled with relatively small size stones covered with mesh and wired together. However, this method of protection will suffer from scouring of damped stones toe at high flow velocities which leads to foundation failure.
(e) SACK REVETMENT
Jute sacks filled with sand and / or soil have been widely used for emergency flood protection as shown in the figure below. Sacks from plastic and other materials are also used recently. Sometimes soil-cement or sand-cement mixture is used to fill the sacks. This type of protection although is inexpensive, but it is less durable and prone to foundation failure by scouring.
(f) CONCRETE COUNTERFORT WALL SYSTEMS
These types of reinforced concrete retaining wall are well suited for retaining materials having height more than 7.0m. As shown in the figure below, both the base slab and face of wall span horizontally between the counterforts and the upright and heel slabs do not act as the cantilever but act as continuous slabs supported by the counterforts. The counterforts are spaced at intervals and act as tension members to support the stem. A counterfort retaining wall is very similar to a cantilever wall, except that this type of retaining wall has a triangular shaped wall that connects the top of the wall to the back of the footer. This is necessary added support. The wall is hidden within the earthen or gravel backfill of the wall. The footer, retaining wall and support wall must be tied together with reinforcing steel. The support walls add a great deal of strength to the retaining wall and make it virtually impossible for the wall to become detached from the footer. Usually, the aspect ratio of height to span (H / L) is about 1.0 and considering expansion / contraction problems, the length of the walls should be between 25m to 30m. Counterfort walls can be used effectively for river banks protection against erosion where the river is very deep. For longer length of river bank, the counterfort walls may be connected together by anti-rust steel hooks.
CHOICE OF METHODS
Depending on the slope, types of soil of the river bank, amount of discharge, and velocity of the flow, appropriate method of improvement has to be employed. In cases of river banks with mild slope and shallow depth of water in the river, methods such as concrete blocks revetment, and stone gabion mattress, are recommended. Sack revetments are widely used for emergency flood protection. In rivers with bulkheads where groundwater is also part of the erosion stone revetment will be appropriate. In cases where the depth of water is more with bulkheads gabion wall structures is a preferred solution. When the depth of water in the river is more than 7.0m reinforced concrete counterfort wall will a more appropriate solution to prevent bank erosion.
Note: The details of the design for different size and types of the reinforced concrete counterfort walls well be presented in separate paper / presentation.
1 –For Amu River banks improvement & rehabilitation, first and far most a feasibility study is
needed which has to be carried out.
2 - In order to carry out different rational river bank improvement designs for Amu River, a team of
expert / designers are needed to be assigned / selected. The team should include geotechnical and
slope stability experts, geologist, structural engineer, hydrologist and river engineering experts,
environment impact assessment specialist, and agricultural specialist.
3 - Based on conditions of flow, nature of land, profile of the river, types of soil, and availability of
construction materials, individual and relevant bank improvement types have to be chosen, designed,
4 - After the construction and completion of the river bank improvement at different parts of the river
regular maintenances have to be carried out.