Why do flood protection embankments fail? | The Daily Star
12:00 AM, September 03, 2014 / LAST MODIFIED: 01:53 AM, March 08, 2015

Why do flood protection embankments fail?

Why do flood protection embankments fail?

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IN the last week of August this year the flood protection embankment at Shariakandi of Bogra District failed.  As a result several hundred villages in Bogra and Sirajganj were inundated with loss to properties and crops. Several upazillas of Kurigram, Jamalpur, Gaibandha, Bogra, Sirajganj, Tangail, Pabna and Manikganj lie in between the flood embankments built on both banks of the Jamuna.  People living in between those embankments are used to living with floods, but they are more afraid of river erosion, which can occur even without floods.  

This year, there was no forecast for severe floods. For the last few years, the monsoon winds caused heavy rainfall in the Indus basin, so there were severe floods in Pakistan. This year there was drought, so there was forecast for floods in the Ganges and Brahmaputra basins. It occurred, but it was not severe. But still there is time. The whole of September is the month of floods. This year Nepal experienced flood, but the waters flowing over Bihar did not reach Bangladesh. Teesta basin had heavy rainfall, which caused floods in Jalpaiguri, Kuchbehar in India, and in our Lalmonirhat and Kurigram districts.  Floods occurred in Assam also at the same time, affecting Gaibandha, Jamalpur, Bogra and Sirajganj. The floodwaters could run off between the embankments of both banks of the Jamuna River, but the havoc was created due to the embankment failure at Shariakandi.      

Why do flood protection embankments fail?  One simple answer can be: they are earthen structures, so they can give in due to water pressure. But when Bangladesh Water Development Board (BWDB) puts embankments along the banks of a flooding river by acquiring public lands, it owes responsibility to the people. Because, they create two types of public life and economy: one within, and the other, outside the embankments. Those within the embankments face floods every year. They have to grow crops which can withstand floods, i.e., jute, sugar cane etc. They grow oil seeds and vegetables during winter. Those outside the embankments are supposed to be flood free, so they grow paddy and vegetables round the year. Their homesteads are built on lower grounds compared to the people in flood prone areas.  Both the types are suitable. But areas affected by erosion are subject to unforeseen calamities. River erosion not only devours pubic properties, it breaks through the embankments and creates havoc in the flood free areas.      

Most of the flood protection embankments fail due to river erosion. But if they are poorly maintained they can fail under water pressure from the river side.  However, a river will erode its banks upstream or downstream of any bank protection work. That is its nature. That why National Water Policy 1999 Clause 4.2p directs to, 'Designate flood risk zones and take appropriate measures to provide desired levels of protection for life, property, vital infrastructure, agriculture and wetlands.'  

The major reasons behind failure of bank protection structures are construction and design faults. A bank protection work has two portions, one, well placed concrete blocks sloping from above the water towards the riverbed; the other, assorted blocks of different sizes placed under the water roughly, towards the riverbed. The blocks above the water protect the bank from wave erosion. The blocks under the water slide down to form a rough slope towards the riverbed as defense shield when erosion approaches. A bank protection work fails when any of the two parts are poorly constructed. Minor defects can be addressed by taking immediate measures. Major defects cannot be addressed by emergency works.

The figure 1 shows a typical riverbank protection work design. Sufficient amount of blocks are to be placed depending on the maximum flow of the river. However to ensure it, these amount of blocks are to be placed by February. In March and April the blocks on the slopes shall have to be placed.  Major defect in the structure remains when blocks in the slope are placed first in February or March, and blocks in the bed are dumped during April and May, when water rises in the river. It gives scope for defalcation by the contractor to claim total bill, without placing required number of blocks under the water.  These types of works are certainly to fail when erosion occurs.

Use of geo-textile sheets as filter materials in the bank protection works is a major design defect.  The defect was noticed by the writer when he was Chief Engineer, Design of BWDB in 2004. Geo-textile sheets were used in the late seventies to construct the Khulna Mongla Road. These sheets were placed horizontally between layers of muddy soil for constructing the road. Thus the muddy soil did drain down but did not flow out laterally. The importers of this material at that time lured the design engineers of BWDB to use them in the embankment slopes. Beforehand, embankment slopes were filtered by sand and brick chips. In that case any repair work or replacement could be done easily.

The problem of geo-textile is, first, its pores get clogged by mud and it loses its filter quality, secondly, if erosion hits, repair work is impossible without removing the whole sheet, and thirdly, if erosion continues, it pulls down the whole work. Such scenes are rampant at many bank protection works of BWDB. The writer banned the use of geo-textile during his time in office but had to face the consequences. The use of geo-textile returned after his transfer and is being used indiscriminately all over the country.     

The corporate culture reigning over BWDB is no less responsible for failure of its bank protection works.  Before the nineteen nineties, an Executive Engineer (ex-en) of BWDB in the field was given all powers and was held responsible for all his works. Since the mid nineteen nineties, corporate culture was introduced by the WB and ADB to float tenders and appoint contractors as per their formula, and to pay as per report of their consultants. Now an ex-en of BWDB has no power but works as a postmaster. He gets his works as part of a package program, design is given in favor of the material suppliers, contractors are appointed by the political masters, bills are prepared as per direction of the consultants, and advances and payments are made by the accountants.  As a result, bills are being paid fully, though the works remain unfinished or defective at many places. In this corporate culture, if any work or embankment fails, no responsibility or accountability can be fixed on anybody.

The writer is Chairman, Institute of Water & Environment
Email: minamul@gmail.com  

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