The New Nation - Internet Edition

Moazzem Hossain Khan

Bangladesh is located in the deltaic zone of the three major rivers like the Bramaputra/ Jamuna, the Ganges and the Meghna with innumerable tributaries and distributaries. This river network system is greatly contributing to the economy of the country in various ways like navigation facility, fisheries development, irrigation, Hydro power generation, operation of thermal power plants, industries, supply of potable and other types of water requirements

2.Most part of the country has been formed with silt carried by the major rivers during flood. Because of high discharges of the major rivers their width are also considerably large. Although velocities of these rivers are not very high but because of alluvial soil banks erode easily.As per hydrological report maximum velocity is observed at Mawa where no bank erosion is observed because the bank soil is sticky clay.Similarly 20miles long canal between Fatepur and Haridaspur of greater Faridpur District, dug out during old British days, shows no erosion in the bank because of sticky peat soil. This is evident that erosion could be stopped if we could protect the bank soil. The erosion problem is so severe that this is considered as the worst catastrophes of the country. As per Bangladesh water Development Board source about half of the 64 district headquarters, one sixth of 460 upazila headquarters and about 100 business centers have been subjected to serious erosion .

The reasons of erosions are (a) Velocity,(b)wave (c)Swirl and (d) soil saturated with rain water help in falling of bank during receding and rising water level.

3.On close observation it will reveal that revetment works under BWDB are normally carried out with C.C. blocks, concrete slabs and sand filled geo bags which are placed over the slope of embankment starting from the top and up to the lowest water level. Because of the visibility problem under water portion of the bank and toe cannot be covered satisfactorily in this method.Normally the method can protect the bank from current and wave wash during higher water level. However the revetment , Groyne/spur works remain vulnerable to erosion due to swirl/scour in the under water portion of the river bank, toe and bed . Annual loss of land due to bank erosion is about 10,000 hectars.Most of the displaced persons losing land and home take shelters in the slum areas of cities to lead hard life in one hand.This also cause problem in the city planning on the other.

It may be mentioned here that no substantial protection work has yet been attempted along main river banks and some major township of special nature.In this regard mention may be made of Chand pur town where no attempt in the past either through construction of spur/groyes with loose boulder and C.C. blocks, revetment and dumping of sand filled geo bags against the steep bank slope and deep scour at toe (upto around 150') could stand the fury of the combined flow of the Brahmaputra,the Padma and the Meghna during the high monsoon.It is considered wise that before attempting to further protection work of this town ship of great commercial importance flow should be diverted through Gorai river, Arial Khan river-Modhumati Beel route (20miles long canal between Fatepur and Haridaspur in greater Faridpur District dug out during old British days which because of peat soil shows no erosion in the bank ) and old Brahmaputra river.These routes should be maintained properly.In our country spurs and groyens are constructed to divert flow of the river away from the bank to avoid erosion.Normally such diverted flow hits other places of the bank requiring further construction of structure in new locations In addition deep scour mostly uproot such structure resulting in its meaningless investment. However if it seems successful for some time this pose problem for good navigation facility with the bank .

4.Our rivers mostly follow sign curve in which erosions appear in the periphery of the outer curves. So possible erodable bank area can mostly be identified during low water stage when proper protective work are possible. Instead due to various reasons our normal practice is fire fighting type i.e. protection works are mostly attempted during monsoon when erosion starts. Obviously this can not ensure satisfactory work and cost also cannot be controlled accurately .In this method erosion of underwater portion of the bank cannot be prevented satisfactorily without which all expenditure either on construction of flood control embankment or revetment work prove of little value.

It is understood that the Mississippi river of USA has similar alluvial soil as that of our river banks. The US army core of Engineers has carried out many years research and developed articulated concrete slab mats of 3" thick. They cast the mats of sizes 4'x25' (as per their convenient standard with 16 number slabs connected with 3/16" diameter steel wires) on plain land, cure for 28 days to attain strength of 4000 P.S.I and keep the mats in stack. During low water level the mats are carried to vulnerable eroding area where banks are properly sloped and compacted.The mats are connected over sloping deck barge both in length and width. Top ends of the of the assembled mats are anchored with the bank just over lowest water line with the help of wires and anchor plates. Placing of the mat are done with the help of rollers and slopping deck of the barge to cover up to deepest scouring part of the under water bank and toe. After placing first piece of mat subsequent sections of mats are placed overlapping the first section .The process is continued until the entire vulnerable portion of the bank is covered. Since vulnerable part of the bank and toe is covered with concrete slab mats further erosion is stopped.

The bare portion of the bank above lowest water level is however covered either with riprap or as now practiced in our country.

The US army core of Engineers found the method most effective and economical (cost per square feet of mat is about the same as that of a good carpet).They are practicing the method since 1928 and already covered over 2700 km of both eroding banks of the Mississippi river. Besides More than 600km of meandering bank have also been shortened and protected.

Mississippi River bank protection works have following advantages;

a) It is economical (cost per sft is about that of the cost of floor carpet) and effective.

b) Its maintenance cost is very low. About 2% per anum

c) Since the articulated concrete slab mat can stop erosion of under water portion of bank and toe deep water channel can be brought close to the bank for better navigation.

d) With Mississippi river method spur can be constructed on the opposite side of shallow channel to increase velocity of the flow to help navigation in one hand and also to scour away any deposited silt from the top of the mat on the other.

e) Since concrete slab mats are flexible it can follow and cover any contour of under water bank and toe.(This can reduce present imaginable dumping of bags and materialas in the under water portion of river bank)

f) Since the casting of the mats is done in advance in plain land it is measurable and its quality can be controlled.

Here are some suggestion:

a) In our country comprehensive river bank protection work should be considered priority. Protection measure should include protection of the under water portion of embankment without which only protection of above low water portion mostly prove futile and in such situation construction of Flood protection embankment without proper protective measure may also prove meaningless.

(b) Bangladesh Water Development Board should follow the River bank under water protection measures as practiced in Mississippi River of U.S.A. to stop major bank erosion problem and help in the stabilization of bank. It is hoped that the method if adopted will solve most of our present problem. In case of rivers with high waves the suggested concrete slab mats may be further loaded with heavy concrete blocks to help keep the mat in position. Wires may be inserted in these blocks during casting .These blocks with wires could be tied with mats at the time of placing from the top of the barges.

(c) The suggested diversion of concentrated flow at Chandpur may help in the protection work of the town which has very commercial importance. Besides helping in the protection work the areas through which these rivers will flow will be benefited economically.

(d) The practice of dumping geo bag/jute bag/polythine bags, filled with sand may however continue as temporary and emergency measure like fire fighting but this should not be considered as permanent solution to our problem.

e) It is considered that the Mississippi river bank protection works if adopted in our country bank erosion for most of our small rivers could be stopped.In case of large rivers with high waves the articulated mats could be super loaded with heavy concrete blocks with wires which could be tied with mats at the time placing from top of the barges.

f)It is understood that for Mississippi River bank protection work only one mat laying barge of 200 feet length and one 75 ton capacity crane for handling materials are used. In our case more than one unit of smaller size mat laying barges and smaller capacity cranes could be used.In this case available barges in the country could be made suitable for the purpose and available cranes could also be used to reduce new investment cost.