River bank erosion and bank protection review

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ROYAL NETHERLANDS EMBASSY - DHAKA

BANGLADESH WATER DEVELOPMENT BOARD

Technical Mission

CHANDPUR TOWN - LOWER MEGHNA

Project funded by the Royal Netherlands Government

APRIL 2002 ■■■ -!...!....!----■■■ ROYAL HASKONING

MISSION REPORT

Barbarossastraat 35 P.O. Box 151 6500 AD Nijmegen The Netherlands Telephone +31 24 3284284 Telefax +31 24 3239346

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HASKONING NEDERLAND BV COASTAL & RIVERS

Barbarossastraat 35 P.O. Box 151 Nijmegen 6500 AD The Netherlands +31 (0)24 328 42 84 Telephone + 31 (0) 24 360 96 34 Fax info@nijmegen.royalhaskoning.com E-mail

Document title Mission Report

Chandpur Town - Lower Meghna

Status Final Report

Date 16 April 2002

Project name River Bank Erosion and Bank Protection

Project number 9M0835.21

Author(s) FCM

Client Royal Netherlands Embassy Dhaka

Reference 9M0835.21/R001/FCM/Nijm

Drafted by F. Carvajal Monar Checked by Date/initials check Approved by Date/initials approval F. Carvajal Monar

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J.H. Laboyrie

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www.royalhaskoning.com Internet Arnhem 09122561 Coe

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FOREWORD AND ACKNOWLEDGEMENTS EXECUTIVE SUMMARY

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BACKGROUND AND APPROACH SETTING OF THE MISSION 2.1

2.2

General Objectives

LOWER MEGHNA RIVER AND EROSION AT CHANDPUR TOWN

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3 3 5

3.1 Attack on Chandpur: past, present and future 5

3.2 Bank erosion and planform changes along the Lower Meghna

River 12

4 CHANDPUR TOWN PROTECTION: WORKS CARRIED OUT AND

5 6 EXPERIENCES 17 4.1 4.2 4.3 4.4 4.5 General

Nutan Bazar protection Puran Bazar protection

Evaluation of the observed damages Evaluation of the executed surveys PRIORITY MEASURES

5.1 Criteria for urgent repair works

5.2 Priority measures in the Dakatia River

5.3 Priority measures along the Meghna Bank of Nutan Bazar

5.4 Monitoring activities

ASSESSMENT OF RISKS AT CHANDPUR TOWN

6.1 General

6.2 6.3 6.4 6.5

Origin of hazards and risks

Risk identified due to natural conditions Risks associated to design

Risks associated to administrative procedures and delays

17 17 19 19 23 25 25 26 28 29 32 32 32 33 34 35

7 PRELIMINARY UPDATE OF ECONOMIC EVALUATION OF THE CHANDPUR

TOWN PROTECTION PROJECT (PURAN AND NUTAN BAZAR) 36

7.1 General 36

7.2 Update of the analysis carried out in FAP-9B 36

7 .3 Benefits of the project 37

7.4 Investment costs 39

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7.5

Cashflow

PROPOSED SHORT-TERM AND LONG TERM APPROACH

8.1 General

8.2 8.3

Short-term measures Long-term strategy

CONCLUSIONS AND RECOMMENDATIONS

APPENDICES

Appendix - 1 Terms of Reference

Appendix - 2 Composition of the team and activities carried out

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Appendix - 3 Short-term predictions for the morphological change around the confluence of the Padma and Upper Meghna Rivers

Appendix - 4 Assessment of future planform changes and bank erosion Appendix - 5 Monitoring in relation to maintenance

Appendix - 6 Preliminary update of economic analysis

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FOREWORD AND ACKNOWLEDGEMENTS

On the 19th of September a sudden bank slide on the left bank of the Dakatia River destroyed the BIWTA terminal, the fish market and reduced the distance between the Lower Meghna and the Dakatia to less than 45m. It was considered urgent to review the past and present trend of planform changes in the Padma-Lower Meghna system and to make an expert evaluation of present bank stability of existing bank protection on the Lower Meghna at Chandpur Town.

Subsequently a request was made by BWDB to the Royal Netherlands Embassy (RNE) on 6 December 2001 requesting the input of Netherlands technical expertise to review the long-term and river bank erosion trends in and around Chandpur town, the RNE confirmed the financing for fielding of a technical assistance mission as from March 2002.

A technical mission for the Chandpur technical mission, on river bank erosion and river bank protection review visited Bangladesh from March 27-April 10, 2002. Members of the mission were: Messrs. Gerrit J Klaassen (River morphologist/Delft Hydraulics, Marteen van de Wal (river training works/Rijswaterstaat), Erik van Meerendonk (river training works), Fortunato Carvajal (river engineer) and Mr S R Khan (river engineer) from the BETS Ltd. local Consultants in Dhaka. Mr Bert te Slaa joined the mission in the Netherlands from 11 to 15 April. The mission representative of BWDB, Mr Aminul Hoque (Superintending Engineer-Design Circle VI) and representatives of BWDB-Chandpur also was part of the Mission. Together they visited the project site between March 30-31, visiting the Chandpur area in the vicinity of Nutan and Puran Bazar, the Dakatia River, the BIWTA terminal, the Chandpur Irrigation Project (CIP) and the Meghna Donagoda Irrigation Project (MDIP).

During the visit to Bangladesh, the mission held meetings with the Secretary MOWR, Royal Netherlands Embassy, Director General, Addl. Director General Planning, Chief Planning of Bangladesh Water Development Board (BWDB), Chief Engineer North Eastern Zone, BWDB-Comilla, Design Circle-BWDB, the Command Area Development Project-PMU, and the Center for Environment and Geographic Information Systems (CEGIS).

The mission thanks all the agencies and offices which were contacted during the visit for their close cooperation and hospitality. Discussions and deliberations were informative and productive. Appendix-1 presents the composition of the mission and the details on activities carried out and organizations and persons contacted during the period of the mission. An expanded version of the findings and recommendations of the mission is presented below under separate headings.

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EXECUTIVE SUMMARY General □□□ -□•□- □□□ ROYAL HASKONING

Chandpur Town (Puran and Nutan Bazar) is located along the left bank of the Lower Meghna River, one of the largest rivers in the world, which conveys the combined discharge and sediment load of the three major rivers of Bangladesh (Brahmaputra, Ganges and Meghna) into the Bay of Bengal.

On the 19th of September 2001 a sudden bank slide on the right bank of the Dakatia River destroyed the Bangladesh Inland Water Transport Authority (BIWTA) terminal, the fish market, and reduced the distance between the Lower Meghna and the Dakatia to less than 45m which is considered to be critical for the stability of the Town. This narrow distance between the left bank of the Lower Meghna and the Dakatia River and the existing instabilities of the riverbank of both rivers, due to scour processes, poses a great risk to Chandpur Town. Without permanent river training works the Lower Meghna could outflank the mole head at Nutan Bazar at any time during the next monsoon periods.

The combination of planform changes of the Padma/Lower Meghna Rivers, the recent increase in scour depth in front of Nutan Bazar and the bank slide of the right bank of the Dakatia River prompted the need to carry out a technical evaluation of the present situation on the Dakatia river and the Lower Meghna.

Bank erosion along the right bank of the Lower Meghna has a significant

socio-economic impact. The loss of land, infrastructure, crops and property has led to landless and impoverishment for thousands of households. If Chandpur is not permanently protected, the economic value of the losses to the regional and national economy from bank erosion along the right bank of the Lower Meghna is considerable. Losses consist not only of eroding urban land at Chandpur Town (Nutan & Pu ran Bazar) with severe damage to property (houses, shops, public and private buildings) and infrastructure (roads and bridges, power and telephone lines, and railway embankments). Chandpur is an important administrative and trading canter for the region with population above 100,000 (census 1991 ). It has agro-industries, the most important river fish market in the country, a railway station and a BIWTA terminal. An important part of the losses would be the loss of large extensions of fertile land developed within the GIP and MDI projects, only protected now because Chandpur Town protection has not yet failed. The Draft Final National Water Management Plan, Volume 2 (article 3.7.2) Main Report, Halcrow et. al., October 2001; states that "River bank erosion causes immense

hardship to those affected and the Government is committed to mitigate this problem", and this is certainly the case for the poor people living in constant risk along the left bank of the Lower Meghna.

In view of the above a technical mission, funded by the Netherlands Government, was organized to evaluate the present situation at Chandpur Town along the Lower Meghna River. The mission report has been prepared as per objectives described in Terms of Reference and provides the Royal Netherlands Embassy and the BWDB with all the data and a thorough background of the findings of the mission:

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1 to enable them to present a justifiable case to the International Financial Institutions for a permanent protection of Chandpur Town and

2 to recommendations for the interim period to improve the present practice for executing urgent maintenance of existing temporary protections including making timely a sufficient budget available .

River Morphology and Bank Erosion

The river reach upstream of Chandpur, presently the confluence of the Padma and the Upper Meghna Rivers, has undergone substantial changes over the last centuries. As a consequence the direction of the main flow in the Padma River makes a substantial angle with the course of the Lower Meghna near Chandpur as shown in Figure 1. This almost right angle, possibly in combination with nee-tectonic activity, induces a gradual shifting of the Lower Meghna River into eastern direction. This eastward motion is well documented and extensively described in Annex C of the FAP 9B report (Haskoning et al, 1992). As a consequence of this movement bank erosion is taking place over a substantial length of the Lower Meghna. Chandpur acts as a control point and at the same time as a major protrusion into the river. The flow of the Lower Meghna is forced to flow along the western side of Chandpur.

Over the years however the erosion situation has become increasingly serious for two reasons. On the one hand the upstream reaches continue to move eastward, whereby the protrusion is becoming more pronounced. The other reason is that over the years the maintenance of Chandpur Town Protection is not sufficient, amongst others due to late implementation and too low maintenance budgets in view of what would be needed.

The overall effect of the limited maintenance is probably that over the years:

I. the slope of the bank protection works has became steeper,

II. the underwater longitudinal profiles of the bank protection works have become more irregular.

This in turn causes that the scour holes deepen, in which way the risk of major slides increases continuously.

In the FAP9B (1992) report an extensive analysis was carried out of the cause of the scour holes near Chandpur and an explanation was given for the changes over the years. These appear to be linked to the planform development of the Padma and Lower Meghna River upstream of Chandpur. In that analysis the impact of the irregularity of the temporary bank protection works was not explicitly taken into account.

In addition, Nutan Bazar is not only attacked on the Meghna side, but (as is

demonstrated by the occurrence of the slide on the 19th of September 2001) also from the backside, i.e. by the conditions in the Dakatia River.

The findings of a short CEGIS study (change elsewhere also in CGES), commissioned by the Mission and of the Mission itself after analysis of trends for the coming monsoon period(s) imply that at least for the next monsoon season the main attack on Chandpur will be the outflanking channel along the left bank, and consequently the scour depth will not reduce significantly.

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Figure 1. Lower Meghna/Padma Rivers (satellite image January 2002)

For the longer run, and assuming that the eastward movement of the Lower Meghna is not stopped, the outflanking of the left channel would continue though on an intermittent basis. It may therefore be expected that after say one decade the conditions at

Chandpur may take an even more unfavourable turn.

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At present as in the past substantial bank erosion has occurred along the Lower Meghna River. Details on bank erosion locations and rates in the period until 1990 are provided in the FAP9B report (Haskoning et al, 1992). Bank erosion locations and rates are closely linked to the river's planform (the overall shape of the river and the pattern of islands and channels). Changes in planform of the confluence of Padma and Upper Meghna and of the Lower Meghna can be observed from satellite images covering the period 1973-2002.

Temporary river works at Chandpur Town

The mission observed that temporary revetments near Chandpur have been damaged by slides causing cavings, flow attack, wave attack, and deeper scour. The most dangerous of these phenomena are slides, because they develop rapidly, and they erode large areas, whereas the moment and the frequency of occurrence are difficult to predict.

The mission found that different types of temporary repair measures have been executed often on an emergency basis during the last decade. However, due to financial and time constraints these measures improved mainly the upper part of the protection layer near the water surface and not the critical deeper parts. The limited volumes of dumped materials (CC blocks) do not result in the required reduction of the slope steepness of the revetment to the designed slope of 1 V:2H.

Timely planning of activities of repair measures is very important, because after a late start of the construction the total construction period is too short to complete the works before the start of the monsoon. During the monsoon it is not possible to execute repair works. The planning of pre-work activities should be done in the months October and November, followed by the design and construction activities. Construction work should start before the first week of February. The Mission considers this date as a very important deadline to ensure that the works are implemented in time.

Risks

Basically there are six areas of from which hazards, and therefore risks, may originate:

1. no implementation of maintenance works in the Dakatia River and in Nutan Bazar (mole-head) this year,

2. present condition (close to limit state) of the temporary protection.

3. natural conditions: rainfall, flood, planform developments, ground conditions, earthquakes,

4. administrative or legal procedures, restrictions in the amount of maintenance work and delays,

5. contractor's hazards like poor performance, breakdown of equipment, delays in getting materials during construction of the maintenance works or emergency works,

6. hazards related to design or non-completion of the works,

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The risks within the context of the temporary protection can be listed as follow: Destruction of the mole head (by several mechanisms) followed by aggravated attack on Puran Bazar, causing extra erosion there and increased erosion of the left bank of the Lower Meghna downstream of Chandpur.

Shifting and deepening of the scour hole in front of Chandpur. Jt could shift further downstream or in the upstream direction. Most likely in the downstream direction because the river plan form changes so that total discharge of the Lower Meghna may become more concentrated next to Chandpur. This is important to come to an efficient design of the recurrent temporary maintenance works of both Nutan and Puran Bazar before the monsoon 2002.

The depth of the scour hole and its location is very important for the assessment of the possibility of new slides at the Dakatia River even after the current slide along the BIWTA terminal is repaired.

If the slide on the Dakatia is not repaired before the monsoon 2002, the risk of further slides is increased and could also be the trigger for eroding, from the back side, of the mole head.

During next monsoon, the temporary protection works of the mole head should be closely monitored. The risk for collapse partial or total is always there without permanent works. Recording of settlements of the protection would be meaningful. A contingency plan should be in place and plans for the period immediately after the a possible collapse of the mole head.

If there is a collapse of the mole head at Nutan Bazar, the area of Pu ran Bazar will be strongly attacked by the flow of the Lower Meghna. At present the mole head gives a temporary shelter to Puran Bazar.

At present the conditions at Nutan Bazar are extremely dangerous (temporary works, steep slopes, high flow velocities, deep scour hole and limited repair works} and create a high risk of failure of the mole-head. Immediate mitigative actions should be taken including an emergency plan which based on the hazards and mechanisms of failure outlines what to do. This emergency plan needs to be prepared as an early warning plan. Both technical matters (materials and equipment which should be available) and even evacuation plans should be specified in the emergency plan.

Priority measures in the Dakatia River

Upon the occurrence of the slide of the left bank portion of the Dakatia of the 19th of September 2001, the Chandpur O&M Division of the BWDB has correctly decided to take up the restoration of the bank and its protection with highest priority.

The current cavity endangers the stability of the Mole head in several distinctive ways. Firstly, it reduces in itself the stability of the bank protection at the Dakatia side of the Mole head due to the reduced structural support at the down stream corner of the cavity. Secondly, at the same point, attack by the river flow endangers the protective structure, since the point forms part of the outer band of the Dakatia river. Finally there is the possibility of a breakthrough towards or from the side of the Meghna bank, leaving the Mole head an isolated and extremely vulnerable island.

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Priority measures along the Meghna Bank of Nutan Bazar

Considering the observations made in the field and the data from the cross-sections profiles received from the Chandpur O&M Division of the BWDB, the Mission's advice is that the existing cavities in the Meghna Bank should be filled at the earliest possible occasion. However, since the latest soundings date back from the early stages of the previous monsoon, it is strongly recommended that an accurate inventory of the current state of the embankments be made before the start of the repair works. This course of activities will greatly enhance the efficiency of those works.

Preliminary update of economic evaluation

For the support of the project the mission decided to carry out a quick update of the economic analysis done in the FAP-9B (1992). The preliminary update of the economic analysis of the Lower Meghna protection works compares the cost and benefits of protecting Chandpur town, the Chandpur Irrigation Project and the Meghna Donagoda Irrigation Project.

Similarly as Sirajganj Town protection project under the River Bank Protection Project (IDA), appraised in 1995, Chandpur Town is a vital point to stop the shifting at large of the Lower Meghna and its importance must be highlighted within the next phase of any river bank protection project in Bangladesh. Past efforts to stabilize the Lower Meghna at Chandpur have resulted in creating a hardpoint. The permanent preservation of this hardpoint is considered essential if bank erosion of the Lower Meghna is to be checked.

For the predicted planform development period 2002 to 2032 (Section 3.2), the area along the left bank including the whole of Chandpur town that would be eroded, without protecting Chandpur by the year 2032, is estimated to be around 23,600 ha of which some 20,900 ha would be eroded at the CIP and MDIP. The total area which would be eroded is equivalent to 44% of the total productive area of the CIP. Therefore in a country like Bangladesh where there is a scarcity of land and increasing population density, the loss of Chandpur and its negative impact on the CIP and MDIP will have huge economic and social costs for the country.

Only considering the benefits related to land prices, loss of crops, loss of infrastructure and properties, it is highly justified to initiate a short-term project for the construction of permanent protection at Chandpur, as it has been done already to protect Sirajganj Town.._ and to develop a long-term strategic plan for controlling the whole left bank of the Lower Meghna, from Mawa to Eklashpur/Mohampur and Haimchar.

One critical note should be placed with this update of the economic analysis, as it is very likely that the value of benefits is somewhat underestimated. The valuation is based on a survey of damage during the period 1988 to 1991 , and although an inflation correction was applied, it is more than likely that during the period 1991 to 2002

damages exceeded the levels recorded.

The results of the update show that the Economic Internal rate of Return of the project is 32%, the Net Present Value (at a 12% discount rate) arrives at Taka 12.8 billion and the Benefit-Cost Ratio using a 12% discount rate is 4.07. This means that there are strong reasons to justify the project as viable and worthwhile executing.

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The overall conclusion of the basis of preliminary update of the analysis is that the project is viable. The fact that the damage to the infrastructure of Chandpur town was probably underestimated means that the viability is likely to exceed the 32% EIRR estimated. The EIRR found for the permanent bank protection of Chandpur Town is only slightly lower than the BRE (1995) for Sirajganj Town protection1 that was estimated to be 45%.

The sensitivity analysis shows that cost increases of 30% and benefit reduction of also 30% still result in a viable project. Also, it is shown that with a 60% reduction of the area likely to be eroded if no action is taken the project is viable, still having a EIRR of 19%.

Permanent protection works at Chandpur Town (short-term)

Considering the importance of Chandpur town protection for the town itself and the region and the economic viability of the permanent works, the view of the mission is that permanent bank protection works should be prepared as soon as possible for Chandpur Town.

By designing and constructing permanent works with under water slope 1: 3.5, the effect is that the scour hole will reduce its depth and the risk of eroding Chandpur Town and the banks of the Dakatia and Lower Meghna rivers will be substantially reduced. The permanent works will ensure the sustainability of development of the region and the CIP and MDIP. The practical view and recommendation of the mission towards the short-term permanent solution is:

I. The GOB should re-formulate its present RBPP-11 (IDA) including Chandpur as a priority location along the Lower Meghna or, as an alternative, to formulate a new RBBP for the Lower Meghna including MDIP, CIP and Chandpur Town, This project could be discussed with other donors/co-financiers (i.e, ADB in combination with bilateral donors),

II. Implementation of the priority permanent works under RBBP-11 or on the basis of a new RBBP-11 would take at least 4 to 5 years from now. Therefore in the interim period, it is necessary to improve present methods used for recurrent maintenance works. Preferable the interim maintenance works mentioned should be part of the permanent works.

Ill. The design of the permanent works for Chandpur Town can also be carried out taking into account phased implementation of the project. The first phase to be the permanent works at Nutan Bazar and the second at Puran Bazar.

IV. To start as soon as possible the update of the feasibility and detail design of the permanent works proposed in FAP-9B with the target to implement the works in the next 4 to 5 years.

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Staff Appraisal Report, Bangladesh River Bank Protection Project. World Bank, November 10, 1995

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Conclusions and Recommendations

The Mission's conclusions from their review of the recent Chandpur Town erosion, the present state of the Chandpur Town Protection works and the past recurrent

maintenance efforts are the following:

On the present conditions and required urgent measures:

• The recent slide in the Dakatia River underlines the serious dangers threatening Chandpur in this period.

• Unless major action is taken at short notice there is a large risk that other parts of Chandpur Town Protection will fail as well with dire consequences for both Nutan and Puran Bazar, and in due time also for increasing areas under command of

MDIP and CIP.

• The remaining time before the monsoon starts should be used to a maximum to carry out maintenance works

• A continuation of the temporary recurrent measures with limited budget and often too late will not serve to protect Chandpur on the longer run,

On short-term measures

• The possibility of a continued recurrent maintenance option should be excluded on grounds of technical impracticability and extreme cost while it does not ensure the sustainability of the regional development,

• Properly designed and constructed permanent protection works should be implemented as soon as possible; in practice this will take at least 4 to 5 years. • A feasibility and detailed design project for these permanent protection works

should in an early stage look at other measures to alleviate the attack on Chandpur Town.

• The choice of permanent protection works at Chandpur Town is limited to two sites, Nutan and Puran Bazar. Nutan Bazar (mole-head) is indisputably the priority location for any works. Pu ran Bazar will be secured from higher risk of erosion when Nutan Bazar is permanently protected,

• The engineering conditions for the implementation of permanent works are similar to those which have proved soluble by the Jamuna river training works design and construction, and hence there are possibilities for competitive prices and minimum annual maintenance costs in future.

• Socio-economic and environmental damage in such a highly populated and cultivated area will be huge and probably irreversible unless sufficient measures are taken to protect Chandpur Town; to maintain Chandpur is vital for the two irrigation projects (MDIP and CIP) as well,

• Displacement of people on a significant scale will be the result of erosion of the Town and parts of the CIP and MDIP,

• Permanent protection works at Chandpur can act as part of the control of the Lower Meghna, will have technical advantage in the longer term,

• the risk and damage scenario for the protection of Chandpur Town is substantial, and it is likely that a full economic evaluation will find the project viable,

• the project for permanent protection works is required in a period when development of the Northeast and Southeast Regions is encouraged by the expansion of infrastructure works,

• Although outside their scope of works, the Mission has done a quick update of economic analysis as a simple extension of the FAP-9B produced an

approximation of the protection returns on the likely investment {Appendix-6),

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suggesting an NPV of US$ 218.8m at 12 percent discount rate, and an EIRR of 32 percent, over a 30 year evaluation period,

• If this conclusion is supported by the proposed Feasibility Study (preferably to be carried out under a revised RBBP-11), the project could form the basis for the attraction of donors and bilateral participation.

On long-term measures:

• A long-term strategy is required for the protection of the left bank of the Lower Meghna River, in which required defence works are specified and a

implementation schedule is developed; Chandpur Town Protection and probably also works at Eklashpur and Haimchar should be part of the long-term strategy • Such a long-term strategy should be flexible to allow for developments either in

river behaviour or in socio-economy, which might necessitate to an update of the strategy

• Part of a long-term strategy should be an appropriate monitoring strategy, regular reporting and if needed maintenance works

The consequent recommendations are the following:

• Early action should be taken to define, approve budget and implement the interim urgent recurrent measures to protect Chandpur Town (mole-head) and Dakatia River this year and until the permanent works are complete.

• The conditions at Nutan Bazar are extremely dangerous (temporary works, steep slopes, high flow velocities, deep scour hole and limited repair works) and create a high risk of failure of the mole-head. Immediate mitigative actions should be taken including an emergency plan which based on a number of possible failure mechanisms of the protection works outlines what to do.

• Post implementation surveys and reviews, during the period of urgent recurrent maintenance works should be undertaken of under water slopes and critical areas. This will be a matter of enhancement of the processes at Chandpur and for ensuring that the risk of collapse is controlled as well.

• Emergency plans should be developed for the case that a major failure occurs during the next monsoon(s) and the necessary measures should be taken in advance to limit the subsequent damages. Both technical matters (materials and equipment which should be available) and even evacuation plans should be specified in the emergency plan.

• Bearing in mind the present situation and risks during the monsoon 2002 it is also recommended to have a Provisional Sum in the budget for this year in order to have emergency works carried out if the need arises at the downstream end of the temporary protection works of Nutan Bazar or elsewhere at the hardpoint. • The permanent protection works at Chandpur Town should be actively

progressed, with Nutan Bazar as the site to start the works.

• A full Feasibility Study and long-term strategic planning should be undertaken within the next phase of RBPP as soon as is practical in order to define the technical details and demonstrate its economic justification.

• The base case for detailed study of permanent protection works should be avoidance of huge negative economic and social impacts for the region and the country,

• Part of the proposed Feasibility should be the development of a long-term strategy to protect the left bank of the Lower Meghna River from river erosion, incorporating also defence works at Eklashpur, Haimchar and possibly elsewhere.

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BACKGROUND AND APPROACH

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Chandpur Town is a very important regional center for the South East Region and plays a key role in the economic development of the region and the country. Furthermore it also has a very important historical background being the main river port of the country and especially for the coastal area. The location of Chandpur Town and its protection works over the last decades has played a crucial role in avoiding that the Lower Meghna River shifted at large in eastward direction. For more than 20 years the Railway

Department and BWDB have made great efforts to keep this control point of the mighty Lower Meghna. The investment in keeping the control point to protect Chandpur and large extensions of land within the CIP has been very high already. However the security against erosion of the temporary works is under a permanent threat from the continuing increase in scour depths near the works, and the limited maintenance and repair works done on annual basis (limited budget) and the unfavorable planform development of the Padma-Upper Meghna confluence(s).

At present the permanent protection of Chandpur is a national issue. Without permanent protection there is a high risk that Chandpur will be washed away. The adverse social and economic impacts if nothing is done to avoid this risk will be high, because not only the town will disappear but also a major part of two flood control and irrigation projects (Chandpur Irrigation Project and Meghna Donagoda Irrigation Project). These depend on the existence of Chandpur as a man-made hardpoint to control the eastward shifting of the Lower Meghna. This undoubtedly will lead to the displacement of the affected people, enhance poverty and will reduce the economic development of a very large part of the country. Therefore BWDB has expressed the need find solutions for a permanent protection at Chandpur. In the past a similar approach has been followed to protect Sirajganj along the right bank of the Jamuna River under IDA funding.

The recent increase in scour depth in front of Nutan Bazar and bank slide of the right bank of the Dakatia River has prompted the need to carry out an evaluation of the present situation on the Dakatia river and the Lower Meghna. On the 19th of September 2001 a sudden bank slide on the right bank of the Dakatia River destroyed the BIWTA terminal, the fish market, and reduced the distance between the Lower Meghna and the Dakatia to less than 45m (Figure 1.1 ). The present narrow distance between the left bank of the Lower Meghna and the Dakatia River and the existing instabilities of the riverbank of both rivers, due to scour processes, poses a great threat to a large area of Chandpur Town. At present there is a high risk that without permanent river training works the Lower Meghna will outflank the mole head at Nutan Bazar at any time during the next monsoon period.

Therefore it was considered urgent to review the past and present trend of planform changes in the Padma-Lower Meghna system and to make a quick estimate of present bank stability of existing bank protection on the Lower Meghna at Nutan Bazar.

This technical examination of the present situation and evaluation of the future planform developments is needed in order to advise on measures to be taken in order to avoid an economic and social disaster in the commercial and urban area of Chandpur town to prevent irreversible and major losses of agricultural land in the Chandpur Irrigation Project (CIP).

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DOD

~

DOD

ROYAL HAIKONING

2 SETTING OF THE MISSION

2.1 General

The execution of the technical mission was organized to meet the objectives described in section 2.2 and to ensure that the Royal Netherlands Embassy and the BWDB are provided with all the data and a thorough background of the findings to enable them to present a justifiable case to the International Financial Institutions for a permanent protection of Chandpur Town and in the interim period to improve the present practice for maintenance of the existing temporary protection.

The organization of the mission study was presented in the Terms of Reference original proposal and slightly updated during the mission period in order to accommodate the proposed planning of future actions and urgent measures prior to the monsoon 2002, in particular:

• The analysis of past and present river planform of the Padma-Lower Meghna Rivers.

• The analysis of surveys and repair works done previously by the BWDB. • The proposed urgent measures for repairing the slide on the right bank of the

Dakatia River and the temporary maintenance around the mole head at Nutan Bazar.

The work of the mission commenced with a broad review and analysis of all data available with different organizations. Simultaneously, and as an integral part thereof, the preparation of morphological predictions on river planform developments, was commissioned to CEGIS. During the subsequent activities of the Mission

recommendations on urgent measures to reinforce temporary river training works and combination of options for implementation of a sustainable short-term and long-term programme to stabilize the right bank of the Lower Meghna were drafted and discussed with Director General BWDB. The report of the Mission was finalized in The

Netherlands.

2.2 Objectives

The specific objectives of the mission are as follows:

i. Analysis of critical trends in the erosion pattern around Chandpur identified by an update on recent changes in the shape (planform) of the Lower Meghna River,

ii. Assessment of the stability of the river bank along the Padma-Lower Meghna River system,

iii. Prediction of planform changes and bank erosion along the left bank of the Lower Meghna system,

iv. Identification of critical areas,

v. Assess the stability of the left bank along the Dakatia River.

Final Report . 3.

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□□□

-□•□

--□□□

ROYAL HASKONING

The short-term and long-term and more general objective of the mission is the definition and implementation of an integrated approach to control river bank erosion in the Lower Meghna with the purpose to formulate an optimal plan for international donors and co-financiers (in particular with ADB) and the Netherlands in view of their expertise in river engineering.

The specific outputs of the mission can be summarized as follow:

i. Proposal for urgent measures to reduce or stop the risk of failure or outflanking of the temporary Chandpur Town protection that is under permanent attack of the Lower Meghna on Puran and Nutan Bazar and could generate extensive loss of land and damages to the Meghna Donagoda and Chandpur irrigation projects.

ii. Recommendations for the approach to the reduction of bank erosion and improvement of the safety of the temporary bank protection works in the short-term taking into account the above mentioned outputs, experiences in the recent past (including the Command Area Development (CAD) and Meghna Estuary Study-II (MES-II) projects and the outcome of the Project Preparation Technical Assistance (PPTA) exercise of the ADB financed Meghna-Donagoda project.

Final Report . 4.

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□□□

---□•□--□□□ ROYAL HASKONING

3 LOWER MEGHNA RIVER AND EROSION AT CHANDPUR TOWN 3.1 Attack on Chandpur: past, present and future

Chandpur is located along the left bank of the Lower Meghna River, one of the largest rivers in the world, which conveys the combined discharge and sediment load of the three major rivers of Bangladesh (Brahmaputra, Ganges and Meghna) into the Bay of Bengal.

The river reach upstream of Chandpur, presently the confluence of the Padma and the Upper Meghna Rivers, has undergone substantial changes over the last centuries:

• until the end of the 18th century the flow of the Brahmaputra entered the reach via

the present Upper Meghna course,

• in less than one century the Brahmaputra has shifted to the present Jamuna course,

joined the Ganges at Aricha to form the Padma and created a new connection with the Meghna River downstream of Mawa.

As a consequence the direction of the main flow in the Padma River makes a substantial angle with the course of the Lower Meghna (see Figure 3.1.1 (a), which shows the reach near Chandpur in the year 2002). This almost right angle, possibly in combination with nee-tectonic activity, induces a gradual shifting of the Lower Meghna River into eastern direction.

This eastward motion is well documented and extensively described in e.g. Annex C of the FAP 9B report (Haskoning et al, 1992). As a consequence of this movement bank erosion is taking place over a substantial length of the Lower Meghna. The celerity of this bank erosion varies in time and space, and - as demonstrated convincingly in the FAP 9B report - is linked to the pattern of channels and islands and their movement (hereafter referred to as planform and planform changes). Figure 3.1.1 (b) presents the results of a comparison of the planform of the river reach near Chandpur for two years almost three decades apart. Substantial changes can be noticed along the left bank of the Lower Meghna River (on the right side of the image) both upstream and

downstream of Chandpur. Upstream of Chandpur the bank erosion has varied between about 2.5 km in total (corresponding to 100 m/year) near Eklashpur to about 0.5 km in total (about 20 m/year) near Chandpur.

Downstream of Chandpur the total bank erosion in about 30 years has varied between zero near Chandpur to about 3.5 km in total near Haimchar.

As can be observed in Figure 3.1.1 (a) Chandpur acts as a control point and at the same time as a major protrusion into the river. The flow of the Lower Meghna is forced to flow

along the western side of Chandpur. Old maps (Rennell's map of the late 18th century

and later maps like the 1860 map, see FAP 9B) indicate that Chandpur has always been located along the river bank, and already for a long time Chandpur is threatened by river erosion.

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(a) River planform near Chandpur beginning 2002

Legend

- 2002 Channel

ijf/j 1971 and 2002 Channel

1973 Channel 2002 BankUne - 1973 Banklina Scale = Km a • □□□ □ • □

---□ ---□ o ROYAL HAIKONING

{b) Planform changes of the river reach near Chandpur in period 1973-2002

Figure 3.1.1 River planform and planform changes near Chandpur

Over the years however the situation has become increasingly serious for two reasons. On the one hand the upstream reaches continue to move eastward, whereby the protrusion is becoming more pronounced. The other reason is that over the years the maintenance is not sufficient, amongst others due to too low maintenance budgets in view of what would be needed. This is elaborated in more detail in the next Section. The overall effect of the limited maintenance is probably that over the years:

• the slope of the bank protection works has became steeper,

• the underwater longitudinal profiles of the bank protection works have become more irregular.

This in turn causes that the scour holes deepen, in which way the risk of major slides increases continuously. This is illustrated in BWDB Morphology information presented in Figure 3.1.2, providing (a) the depth of the scour hole along Nutan Bazar, and (b) the location of the deepest scour holes in front of the bank protection at Nutan Bazar in two

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□□□

-□•□-□□□ ROYAL HASKONING

years, notably 1996 and 2001. The chainage is from West to East. As can be observed the scour depth has increased substantially, especially at the tip of the protection. The location of the scour holes has not changed to much, indicating much steeper slopes. See also Section 2.3 for more information.

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(b) Comparison location of deepest scour holes upstream of Nutan Bazar in February 1996 and in June 2001

Figure 3.1.2 Scour upstream of Nutan Bazar in 1996 and 2001 (data from BWDB Morphology)

It is relevant to note here that in the late 80-ies of the last century the bank protection of Chandpur was improved substantially (see Halcrow et al, 1990). Since then the bank protection surface must have become much more irregular. Hence a tentative

conclusion might be that the present problems at Chandpur are more serious than would be the case when the maintenance could be carried out more systematically and in a less haphazard way as happens since the early 90-ies.

In the FAP9B study an analysis was carried out of the scour depth near Chandpur and its development over time, and the cause of the deep scour holes observed. Figure 3.1.3 provides an updated version of one of the Figures in the FAP9B report, presenting the scour depth near Chandpur over the years. Here the scour depth is measured

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□□□

-□•□-_□□□

ROYAL HASKONING

relative to the average flood level which is about 5 M + PWD at Chandpur. The data are derived from BIWTA sounding maps (a check carried out showed that the deepest points correspond reasonably well to the BWDB Morphology soundings). A difference is made between the scour depth near Nutan Bazar and near Puran Bazar. As can be observed the scour depths near Nutan Bazar are at present higher than they ever were and appear to increase over the years. The maximum scour depth near Puran Bazar is fairly constant. 80 ... 70 E ♦ ~ 60

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Figure 3.1.3 Change of maximum scour depth near Nutan and Pu ran Bazar (based on BIWTA sounding maps)

In the FAP9B report an extensive analysis was carried out of the cause of the scour holes near Chandpur and an explanation was given for the changes over the years. These appear to be linked to the planform development of the Padma and Lower Meghna River upstream of Chandpur. In that analysis the impact of the irregularity of the bank protection works was not explicitly taken into account.

Nutan Bazar is not only attacked on the Meghna side, but (as is demonstrated by the occurrence of the slide on the 19th of September 2001) also from the backside, i.e. by the conditions in the Dakatia River. Only limited data are available on the Dakatia River. According to some spokesmen, the char in front of the BIWTA has grown over the years. The Mission has analyzed some BWDB-cross-sections (from 1996 and 2001) and data gathered by the Anwesha during two surveys. There are some doubts as to the chainage of the cross-sections, but the data presented in Figure 3.1.4 at least give some indications. According to Figure 3.1.4{a) the location of the thalweg has not moved towards the right bank, which seems to suggest that the char has not grown. This is also not to be expected as this char is a point bar, its dimensions being governed by river morphological processes within the Dakatia River. Hence the change in char dimensions might not be the cause of the recent Dakatia slide.

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(a) Change in thalweg position Dakatia River

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Figure 3.1.4 Changes in bed topography in Dakatia River in period 1996-2001

Figure 3.1.4(b) presents data on the thalweg depth in 1996 and in three periods in 2001, collected by BWDB Morphology and by a MES (Meghna Estuary Study) measuring campaign. Although there is some doubt on the chainages used, it can be seen that in 2001 the Dakatia River was much deeper than in 1996. This could have been the cause of the slide of the 19th of September. The larger depth in the Dakatia River is probably caused by the deep scour hole occurring in the Meghna River, and hence the slide of the 19th is indirectly linked to the very unfavorable conditions in the Meghna River. A highly relevant question to answer here is whether the attack on Chandpur and more particularly on Nutan Bazar will remain to be so serious or whether any developments can be foreseen which over time will reduce the scour depths substantially. This should be answered the very short term (the next years) and for the farther future.

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□□□

-□•□-

For the very short term use an assessment was made with the assistance of CEGIS. Recently CEGIS has developed a prediction method to assess morphological changes in the Jamuna River. Because the Padma/Lower Meghna River system downstream of Mawa shows the typical pattern of a braided river (FAP 9B, see Haskoning et al, 1992), the Mission decided to request CEGIS to apply this method also to the area of interest upstream of Chandpur. CEGIS managed to do this at very short notice, though not sufficient time was available to properly calibrate the method for the Padma as well. A revised version of the CEGIS study is included in this report as Appendix-3. The main findings of the CEGIS study are (see Figure 3.1.5) are the following trends for the coming monsoon period(s):

• Channel Ch1 will decline to some extent • Bar Br3 will migrate downstream.

• Both channels Ch4 and Ch3 will survive in the next year and enlargement of Ch4 is likely.

• Both Ch5 and Ch6 will remain for a few more years and the prevailing trend of declination of Ch5 will continue in the next year.

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asa:m □□□ ---□ • □ □□□ ROYAL HASKONING

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Final Report

IRS 1 C LISS Ill Image

January 2002

Scale

Km

0 10

Figure 3.1.5 Nomenclature of channels and bars used during the Prediction of future morphological changes

11

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□□□

-□• □

--□ --□ --□ ROYAL HASKONING

This implies that at least for the next monsoon season the main attack on Chandpur will be the outflanking channel along the left bank, and consequently the scour depth will not reduce significantly. For the next years the channel upstream of Chandpur along the right bank of the Lower Meghna might develop further. Under those conditions

Chandpur would again be located at a confluence as was the case in the period around 1990. Scour depths at Nutan Bazar might reduce slightly under those conditions. For the longer run, and assuming that the eastward movement of the Lower Meghna is not stopped, the outflanking of the left channel would continue though on an intermittent basis. It may therefore be expected that after say one decade the conditions at

Chandpur may take an even more unfavorable turn. In particular when the repair works continues to be carried out in a haphazard way, creating much additional local scour due to the turbulence generated by the uneven surface of the present (repaired) bank protection works.

3.2 Bank erosion and planform changes along the Lower Meghna River

In the past substantial bank erosion has occurred along the Lower Meghna River. Details on bank erosion locations and rates in the period until 1990 are provided in the FAP9B report (Haskoning et al, 1992). Moreover predictions are made in this study. Recently this information was updated for the reach between the confluence with the Upper Meghna River and about Chandpur under the River Erosion Prevention and Morphology Study of Meghna Dhonagoda Project (DHV, 2000) and the

Jamuna-Meghna River Erosion Mitigation Project (ADB Consultants, 2001 and Halcrow, 2002). It is noteworthy to note that the predictions made under the FAP9B project (an initial slowing down of the bank erosion, followed by increased erosion in the later period until about 2004) did indeed occur, which gives confidence in the prediction methods applied by Haskoning et al {1992).

As shown in the FAP9B study bank erosion locations and rates are closely linked to planform (the overall shape of the river and the pattern of islands and channels). Changes in planform of the confluence of Padma and Upper Meghna and of the Lower Meghna can be observed from Figure 3.2.1, where four {low-water) satellite images covering the period 1973-2002 are shown. As an example for the close link between bank erosion and planform consider Eklashpur. Over the last years substantial erosion has occurred near Eklashpur, caused by a char which has gradually moved in eastern direction and has subsequently narrowed the channel in front of Eklashpur, forcing the channel to move in eastern direction as well and thus causing major bank erosion. In general the Lower Meghna upstream of Chandpur seems to behave in response to the avulsion of the Brahmaputra to its present Jamuna course. For more details on bank

erosion rates upstream and downstream of Chandpur reference is made to Section 3.1.

In Figure 3.1.1 (b) also the total bank erosion over the period 1973-2002 is shown.

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Figure 3.2.1 Changes in planform of the confluence and Lower Meghna River over period 1973-2002

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□□□

-□• □

--□ --□--□ ROYAL HASKONING

Hence different scenarios for river bank erosion can be imagined depending how successful the different river defence works (including the Chandpur Town Protection) will be. For the time being four scenarios are proposed for further study here. These scenarios are given in Table 3.2.1 and they range from "No works" to "Maximum protection", though it is remarked that maximum protection probably is only reached when some intermediate supporting "strong" points are created between Eklashpur and Chandpur and between Chandpur and Haimchar.

Scenario 1 2 3 4

Short description No works Only "Maximum" Only

Chandpur protection Eklashpur

protected

Eklashpur (Temporary) Works X X

effective

Continued bank X X

erosion

Chandpur Permanent works X X

No works X X

Haimchar Effective works X

implemented

Continued bank X X X

erosion

Table 3.2.1 Assumed scenarios during assessment of future planform changes and related loss of land

For all four scenarios assessments were made on the basis of extrapolations and engineering judgement. Extrapolations were done on the basis of the historical trend between 1973 and 2002. Engineering judgement is required for:

• the gradual erosion of Chandpur town when not further maintained as a hard point; • the shape that eroded banks develop upstream of strong points;

• the accuracy of the prediction, which is determined by the overall accuracy of the prediction method used, the occurrence of major floods in the 30 year period considered, and the possible non-uniformity of the soils which would be eroded in the considered period.

The result of the assessment is presented in Figure 3.2.2. Note that not only lines for the average bank erosion are provided but also an indication of the minimum and maximum erosion is given. Note furthermore that only lines for the year 2032 are presented. As stated earlier the bank erosion rates changes in time and space so a straightforward interpolation for intermediate years will be less accurate. For more details reference is made to Appendix -3.

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(a) Scenario 1: No works

(c) Scenario 3: Maximum protection

ODD

□•□

ODD

ROYAL HASKONHIG

{b) Scenario 2: Only Chandpur protected

( d) Scenario 4 : Only Eklashpur protected

Figure 3.2.2 Predicted planform changes in period 2002-2032

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00 □

~

00 □

ROYAL HASKONING

The upstream end of the protection has been extended a few years back with a protection layer made of boulders and gabions filled with boulders. Inhabitants of threatened houses at the edge of the revetment have strengthened the protection around their houses by a nearly vertical wall of wall of boulders. In general, it is not recommended to apply boulders on slopes 1V:2H, because they loose easily their stability on such steep slopes. In addition it is mentioned that boulders press the underlying geotextile only in a few contact points to the subsoil, while pitched cc-blocks exert an almost evenly distributed pressure on a geotextile. This means that the subsoil under a geotextile covered by boulders starts early to erode compared with a subsoil under a protection layer of a geotextile under cc-blocks.

Some maps indicate that on the mole head an old small submerged groin {2?} had been constructed before 1973. The present situation of this groin is not clear. Might be it has been damaged and eroded in due time.

The whole protection is a combination of different approaches, and the steepest slopes are 1 V:O.SH to 1 V:1 H, which is very steep and might loose its stability anytime. These steepest slopes are likely part of a caving resulted from an underwater slide. It also indicates that the underwater protection of the revetment does not function as a falling apron. Mainly because the presence of large remains of eroded houses and the foundation by gunny bags, geotextile bags. For a proper falling process mixing of the sliding blocks with the subsoil is essential. The advantage of a falling apron of cc-blocks is the even slope of the resulting protection layer of about 1V:2H. The cavings after slides gradually disappear in the falling process, as has been observed from the falling aprons in Kamarjani, along the right bank of the Jamuna River (FAP 21/22 project).

4.3 Puran Bazar protection

The main protection of Puran Bazar is a revetment with a toplayer of cc-blocks: above low water level stone pitching, below low water level dumping of blocks. It is less extensive as the protection of Nutan Bazar. A submerged groine is a main part of the protection and it is still functioning. In recent years the protection has been extended in downstream direction.

4.4 Evaluation of the observed damages

The revetments near Chandpur have been damaged by slides causing cavings, flow attack, wave attack, and deeper scour. The most dangerous of these phenomena are slides, because they develop in several hours, and they erode large areas, whereas the moment and the frequency of occurrence are difficult to predict.

Slides

In general two types of local subsoil instability processes are distinguished:

i. slide in densely packed subsoil and

ii. liquefaction in loosely packed fine sand although also mixed forms are also possible.

Some criteria have been defined for sub-soils which are sensitive for slides. In a critical situation a small event can trigger the start of it. The main characteristics of a slide are

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ODO

---□•□-_□□□ ROYAL HAlkONING

its location, the volume of eroded/displaced material and the size of the resulting caving.

It is assumed that in Chandpur several slides have occurred in the past, however, they have not been documented systematically.

The Mission received some information on the following recent slides:

• 19 September 2001 a volume of 100.000 m3 (approximately an area 100 m by 50 m and 20 m deep) at the Dakatia River near the terminal building,

• 26 February 2001 a smaller slide occurred at the tip of the mole head, • around 1999 a large slide occurred just upstream of the mole head.

Probably this small list of slides from which cavings are visible from the ground can be extended, consulting old reports and other documents.

It seems that below the water surface the revetment has been damaged repeatedly by small slides, leaving behind cavings with steep slopes over a height of a few meters.

The documentation on these damages is limited and not all was available during the period of the mission. A consequence of these damages is that the surface is rather irregular resulting in a high hydraulic roughness of that surface. This creates additional negative turbulence and slows down the positive flow velocities above the revetment. The risk on a slide is often increased by steep slopes of a revetment and rapidly increasing scour depths at the toe of a revetment. Examples of trigger phenomena are excessive rainfall, an extreme tide (drop in water level) or a ferry hitting the mooring berths with more than normal force. The tendency of an increased risk on a slide as the slopes becomes steeper is shown in figure 4.3.1. A reduction of the present steep slopes of 1 V:0.SH to 1 V:3.SH will reduce the risk on a slide already considerably. From an analysis of slides in the Jamuna River it seems that the critical rate of scour is around 0.2 to 0.4 m/day2, see figure 4.3.2. It is not possible to slow the scour velocity directly, because it depends on the hydrological processes. However, if the absolute scour depth is reduced, for example by reducing the slope of the revetment, than also the scour velocity will reduce. The relation between the slope of the revetment and the maximum local scour depth is presented in figure 4.3.3.

Local scour depth has been defined as the difference between the bed level in the approach channel and the deepest point in the scour hole. In case the steep slope of a protection 1 V:1 H is reduced to 1 V:2H the scour hole will be approximately 1 0 % less deep. A further reduction to 1 V:3.SH results in a total 25 % reduction in local scour depth.

2 _{Rhein-Ruhr Ing. Et al, Bank Protection Pilot Project FAP 21, Reports on monitoring and Adaptation,}

Kamarjani, 1995, Bahadurabad 1997.

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1 0.9 0.8 0.7 0.6 design BWD ~ 0.5 Ill ·;: 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 slope revetment (1V:xH)

Figure 4.3.1 Risk on a slide as function of the slope of a revetment

1 0.9 0.8 0.7 0.6 ~ _0.5 ·;:: 0.4 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8

rate of scour (m/day)

□□□

---□• □--_□□□

■ OYAL HAIKONING

Figure 4.3.2. Risk of a slide as function of the rate of increase of scour depth.

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1.2 .c:

...

1 Q. G) 'C

_...

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...

0 0 1 2 3 4 5 6 7 8 9 10 11 slope (1 :x)

Figure 4.3.3. Approximate relation between the maximum scour depth and the slope of the revetment

□□□

---□• □ _ODO

--ROYAL HAIKONING

The present tendency that the scour hole near Nutan Bazar deepens and extends in North-Eastern direction contributes to an increase of the risk on a slide. Erosion of the scour hole increases the slope of the revetment which can result in slides. Another effect of deepening is an increase of the size of the scour hole horizontally, for example extending into the Dakatia River, where slides had widened the mouth of the river. Over the last years the thalweg has not shifted towards the right bank near the BIWTA

terminal building. In recent years no damage had been reported by wave action, because the size of blocks is sufficient and no information on direct under water damage by flow action was available at the time of the mission.

The risks due to the present condition of the temporary works and planform development in front of Chandpur Town are:

Destruction of the mole head (by several mechanisms) followed by aggravated attack on Puran Bazar, causing extra erosion there and increased erosion of the left bank of the Lower Meghna downstream of Chandpur.

Shifting and deepening of the scour hole in front of Chandpur. It could shift further downstream or in the upstream direction. Most likely in the downstream direction because the river plan form changes so that total discharge of the Lower Meghna may become more concentrated next to Chandpur. This is important to come to an efficient design of the recurrent temporary maintenance works of both Nutan and Puran Bazar before the monsoon 2002.

The depth of the scour hole and its location is very important for the assessment of the possibility of a new slides at the Dakatia River even after the current slide along the BIWTA terminal is repaired.

If the slide on the Dakatia is not repaired before the monsoon 2002, the risk of further slides is increased and could also be the trigger for eroding, from the back side, of the mole head.

Final Report -22

-9M0835.21 /R/FCM/Nijrn 16 April 2002