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USE OF BANDALLING AS A RIVER BANK EROSION STRUCTURE

Md. Lutfor Rahman1, Dr. B.C.Basak

2, Dr. Md.Showkat Osman

3 and Md. Altaf Hossain

4

1Ph.D.Student, Department of Civil Engineering, DUET, Gazipur, Bangladesh

2,3 Professor, Department of Civil Engineering, DUET, Gazipur, Bangladesh.

4 Md.Altaf Hossain, Director General, River Research Institute, Faridpur, Bangladesh

ABSTRACTBandalling is a locally bamboo made structure used for the river bank erosion protection. Due to locally

available low cost materials, the construction cost of the Bandalling is also low. These structures are placed at theerosion prone bank of the river Jamuna near the Bangabandhu Bridge East Guide Bund, Bhuapur, Tangail, Bangladesh.The bandalling structures are placed with an angle of the 45 degrees in the direction of water flow with the spacing attwo times of the Bandalling length. It was observed that water flow is diverted towards the main channel and low flow

velocity near the bank. Due to low flow velocity, sedimentation occurred near the river bank between the bandalsindicates that bandalling can be used successfully as river bank erosion protection structures as like the conventiongroin/spur like structures.

INTRODUCTION

River bank erosion is one of the disaster in Bangladesh. The global climate is changing which has an effect inBangladesh for the river bank erosion. So it is needed to protect river bank erosion. It is mention here that every yearthe river bank is being eroded & the intensity of erosion is increasing day by day. A pilot test basis project was takenwith the application of the low cost bandalling structures in the Jamuna river at the upstream East Guide Bund of theBanghabandhu Bridge near the Shaheed Salahuddin Cantonment area to protect the river bank erosion protection. The

low cost Bandalling structures are placed at the river bank at an angles with water flow direction for the spacing 2times the Bandals length. It was observed that water flow diverted towards the main river due to Bandals resultingmaximum velocity accumulated towards the main river whereas comparatively less velocity appeared near then river bank where Bandals were placed resulting sediment deposition. The sedimentation was resulted near the river bank thatgave an indication for the river bank erosion protection.

Bank erosion and channel shifting of the untrained alluvial rivers of Bangladesh are big problems to the socio-

economic and environmental sector of the country. During 1960’s, a number of earthen embankments were constructed

along the major rivers for the protection of rural people and agricultural lands from flooding. Since then theembankments were retired several times due to river bank erosion and bank protection are often required during themonsoon and post-monsoon season. Conventionally, groynes and revetments are applied as a method of bank protection. Very recently the concept of hard points (strong revetment type structure) at the most vulnerable locationsalong the Jamuna river are considered, while in between hard points spurs or permeable groynes are recommended

(Klaassen, 2002). In some reaches, channel widths along the major rivers are 2-3 times wider than the so called regimewidth while the water depth is around 1/3

rd of the regime depth which indicating the degree of instability of rivers inBangladesh. If the bank protection structures such as groynes, revetments or spurs are applied in such rivers the utmost

success may be achieved protecting river bank locally. But these structures will create problem somewhere elseresulting far away bank erosion and additional instability to the sand bars (CHARS in local name) where a number ofrural people used to live in. Therefore, applying these conventional methods of countermeasure, the river bank erosionat the short term basis can be obtained, whereas, the long term stable channel or regime channel can never bedeveloped. Alternative solutions that can be locally adaptive and friendly to environment need to be developed for the

long-term stabilization of river channels. The possibility of using bandals for long-term channel stabilization isexamined using field data and laboratory investigation (Rahman et al., 2003).The responses of large scale alluvial rivers

against sudden changes created by conventional structures are not suitable for the overall stabilization of river courses.Therefore, it is important to have alternative long-term solution for river stabilization that will create minimumdisturbance to river courses.

WORKING PRINCIPLE OF BANDALSThe working principles of bandals for the control of water and sediment flow are shown schematically in

Figure-1, where sediments are transported as bed load and suspended load. Within the lower half of the flow depth,

major portion of the sediment flow is concentrated, whereas, within the upper half water discharges are more. Bandalsare commonly applied to improve or maintain the flow depths for navigation during low water periods in alluvial riversof Indian sub-continent. The essential characteristics of bandals are that they are positioned at an angle with maincurrent and there is an opening below it while the upper portion is blocked. As an empirical rule the blockage of the

flow section should be about 50% in order to maintain the flow acceleration. The surface current is being forced to theupstream face creating significant pressure difference between the upstream and downstream side of bandal. The flow

near the bed is directed perpendicular to the bandal resulting near bed sediment transport along the same direction.

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Therefore, much sediment is supplied to the one side of channel and relatively much water is transported to the otherside. The reduced flow passing through the opening of bandals is not sufficient to transport all the sediment comingtowards this direction, resulting sedimentation over there. On the other side, more water flows with little sediment,resulting bed erosion of the channel on that side.

Velocity ProfileSuspended sediment

sediment flow away from bandal

sediment flow under bandal

water flow away from bandalwater flow under bandal

The quantity of water and sediment flow is expressed by arrow size.

Figure 1. Working principles of bandals.

BANDALS CONSTRUCTION & ITS PERFORMANCEThe initial river bank conditions were seen as in figure 2 in below. The condition of the river bank before

taking the erosion protection was very steep slope. The channel near the river bank was very deep. In this situation bamboo bandalling was constructed.

.

Figure-2: River bank without bamboo bandalling during the pre-monsoon condition

The bamboo bandals were constructed from the river bank of the Jamuna branch channel and protrudedtowards the main river channel near the Shaheed Salahuddin Cantonment, Bhuapur, Tangail. The low cost Bandalling

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structures were placed on the left side in the Jamuna river branch channel from up stream to downstream at 45 degreeangles with the water flow direction i.e. the bank line is shown in figure 3.

Figure-3: The bandalling placed at 45 degree angle with water flow in which bamboos are driving into the river bed

Figure-4: Director General of River Research Institute visited the construction site.

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Figure-5: The constructed vertical bamboo are shown in the above

Figure 6: Officers visiting the functions of Bamboo bandallings during food in the Jamuna river

The officers of the Bangladesh Bridge Authority (BBA), River Research Institute, Faridpur as well as the BangabandhuBridge Special Orginization (BBSO) were visiting the bandalling construction site as in figure-6 in above during thefood.

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Figure-7: River Bank Erosion Protection by sedimentation during flood

The erosion-siltation due to the effect of the bandals are also shown in the figure in below:

Comparison of Bed Level for Cross-Section78

0.00

2.00

4.00

6.00

8.00

10.0012.00

14.00

0.00 25.00 50.00 75.00 100.00 125.00 150.00 175.00

Distance in meter along the River Cross-Section

E l e v a t i o n i n m P W D

Pre-monsoon 2007

Post-monsoon 2007Post-monsoon 2008

Figure 8. shows the erosion & siltation after flood 2007

RESULT & DISCUSSIONSIt is seen from the above figure-2 that there is non-cohesive soil with steep slope in the river bank. It is also

seen from the figure-3, 4 & 5 that the construction of bandalling is going on. It is seen in figure-6 &7 that the top of the bamboo bandalling is blocked by the bamboo fencing for which the velocity near the river bank is low than that of lessvelocity away from the river channel. It is concluded from the figure-7 & 8 that there is a siltation near the river bank

where as there is deep pool away from the river bank. So it can be concluded that the bandals are working as a river bank erosion protection structures.

CONCLUSIONBandals are capable for protecting river banks by flow diversion towards the main channel leading to deep

navigational channel formation in the main river. On the other hand, flow velocities are higher at the main channel

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increased the depth of the navigational channel that ensure the navigational channel development. If the bandalstructure functions optimistically, the river can get sufficient time for its adjustment and new main channel and bankline development.

ACKNOWLEDGEMENTThe authors are grateful to the River Research Institute, Faridpur, and Government of Bangladesh for the

financial assistance to conduct study for such type of research.

REFERENCES

Klaassen, G.J., Douben, K., van der Waal, M.: Novel approaches in river engineering, River Flow-2002, pp. 27-43,

2002.

Rahman, M.M., Nakagawa, H., Ishigaki, T. and Khaleduzzaman, ATM.: Channel stabilization using bandalling,

Annuals of Disaster Prevention Research Institute, Kyoto University, No. 46 B, pp. 613-618, 2003.

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