‘Nowhere else in the world was there such a need for extensive river training’

Former vice-chancellor of Brac University Professor Ainun Nishat was a member of the expert panel on the Padma Multipurpose Bridge project. For being someone who primarily oversaw the river training work in the project, he discussed his journey of such a monumental task in an exclusive interview with The Business Post’s Mehedi Al Amin

You were a consultant for the river training work in the Padma Multipurpose Bridge project. Given that it is a river with strong currents, can you briefly tell us how it was done?

Alluvial rivers may erode the banks when the force of the river is very high. The erosion rate depends on the composition of the bank material, i.e. soil, clay or sand. Loose sand is highly erosive. The Padma Bridge spans an alluvial river with highly erosive materials.

We had analysed the movement of the river and established mathematically that at the location where the bridge is being built, the river swings over a distance of six kilometres over a period of 12 years. In other words, the water, which was flowing close to the eastern bank when bridge construction began, slowly flowed westward. In another seven to eight years’ time, it will be flowing along the western bank.

Based on the found data, we decided to make the bridge six kilometres over the river. The bridge is nine kilometres long because there are 1.5 kilometres of viaducts on each side of the approach roads, adding three more kilometres to the length of the bridge that is over the land. To ensure that the river remained within the six kilometres, we needed to build control structures on both banks. We built this for the Jamuna River. British engineers had built similar kinds of structures for the Hardinge Bridge. We call it “guide bond, “ a technology adopted on alluvial rivers. The technology was developed by British engineers working in the subcontinent.

At the river’s upstream end, in front of the guide bond, the river would add an additional length or distance. In engineering, it is referred to as “flaring”. The length of the guide bond—the protective structure—depends on the location of the approach road and the bank’s soil composition.

On the Mawa side, we have a 1.5-kilometre-long guide bond, and we have another 11.5-kilometre-long guide bond on the Madaripur side to keep the river from swaying away from this particular distance. And on Jamuna Bridge, we have a 2.5-kilometre-long guide bond. So this is the method that was adopted to ensure that the river always flows underneath the bridge. And this whole process is called river training.

What were some of the differences you noticed while working on river training for the Padma and Jamuna rivers, both of which have different characteristics?

You see, the Brahmaputra carries a flood discharge of 1.5 million cubic feet per second and may be higher, maybe 1.7 million cubic feet per second, during flood time. And there is another river called the Ganges that also carries a similar discharge. And they meet in Aricha, and the combined flow comes to Mawa.

So Mawa has a discharge which is double that of Jamuna in Sirajganj. Therefore, the force of the river is higher. The velocity is much higher. The scour depth is much higher. So we had to make provision for the high scour depth, which could be close to 70 metres. In the case of Jamuna, the designed scour depth was 30 metres.

So we had to make provision for higher depth and much higher velocity, and therefore the elements of the design and the elements of the river training were much stronger and bigger. And the design and construction of the scour depth was an engineering challenge. Nowhere else in the world could such a big channel be controlled and there was a need for such extensive river training. At the river bank, we also experienced very high velocity. So the stones that we have used each carry a one-tonne weight. For the geotextile bags, we used 800 kilograms of sand in each bag. So both the design and the construction according to the design were very challenging, but this has been accomplished successfully.

The river training work will be completed in 2023. Is there any need to continue monitoring it after its completion?

River training or anything related to rivers cannot be completed permanently. It has to be monitored regularly so that the erratic behaviour of the river is observed and any unforeseen impact on the bank can be tackled to make it stronger.

At Jamuna Bridge, it is being monitored regularly, especially in the monsoon months. Regular surveys are done in the case of Padma Bridge. We also recommend that regular surveys be conducted. And whenever some weakness is found, the plan necessary to repair it has also been completed.