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Cyclone-time silt to fill Payra channel in only two days

German marine geologist Hermann Kudrass tells FE


Syful Islam | Published: February 11, 2019 09:37:28 | Updated: February 12, 2019 21:33:38


The Payra Port Authority (PPA) last month signed a deal with Belgian firm 'Jan De Nul' for capital dredging of the port's main access channel.

The dredging of the 75-kilometre long and 100-metre wide channel will start within six months.

Officials said the dredging will be part of a trial to assess the siltation rate before deciding on whether to build a deep-sea port or a normal seaport there.

However, experts find challenges to building a port at the point selected by the government.

They think the high rate of silt deposits will make maintenance of the channel difficult and will involve huge spending every year, thus making the port financially unviable.

The FE interviewed German marine geologist Hermann Kudrass on the challenges facing the port.

He was an adviser for the Bangladesh delegation for the Law of Sea negotiations in Hamburg and was an invited speaker at the first "Blue Economy" conference in Dhaka.

The excerpts from the interview with Mr Kudrass are as follows:

Q: What challenges do you see to Payra Port construction?

A: The main challenge is the maintenance of the 75km-long access channel, which connects the planned harbour with the open sea with water depth of more than 15 metres.

This channel cuts across the main transport path of the sediment (sand, silt, clay) from the mouth of the Meghna to the west. Approximately 500-700 million tonnes of sediment are annually moved by waves and currents in a westward direction. These masses cannot cross the artificial channel, which is a perfect sediment trap and disturbs the natural dynamic equilibrium of the shallow water surface.

Cyclones are the main drivers of this silt transport and the channel will completely disappear during any cyclone-induced waves and currents. As per a Japanese modelling of the April 1991 cyclone, the waves reached a height of 4-6m in the water depth of only 5-10m with wind velocities of 50m/sec resulting in a water current of about 1m/sec. Huge amounts of sediment are moved by cyclones and a portion of that sediment is finally deposited in the canyon 'Swatch of No Ground'. Another larger portion is deposited on a depositional ramp beyond 15m, which grows seaward by 15m/year. Thus, the tiny scratch of the access channel will totally be overwhelmed by the mobilised sediment and the channel, which will be constructed by using more than 10 dredgers in an year-long operation, will be filled within two days.

In addition, tidal currents tend to restore the dynamic equilibrium of the huge subtidal flats. The sediment mobilised by tidal currents and deposited from both sides into the channel will need permanent maintenance dredging along the entire length of the channel. If a channel depth of 10m is guaranteed, a decrease in the channel depth by 1.0m or 2.0m must be avoided. The Indian shipping ministry so far issued the only official statement on the siltation problem, which is rather small for Kolkata in comparison with the Payra Port.

Q: How viable will the port be in case of high sedimentation?

A: After the passage of an intense cyclone, the port will not be accessible for a long time and ships there be trapped. The excavation of the filled channel will cost another 500 million euro and will need 11 dredgers to move into the area and operate for a year to reopen the channel. Even weak cyclones combined with tides will fill the channel during the few days of their passage.

Q: Do you think Payra can be built as a deep-sea port? Is it possible to dredge 16m depth at that point of Bay of Bengal? How costly the maintenance dredging will be? Will it be financially viable?

A: Payra Port can be built onshore, no problem with that construction. It is the access channel which is the main problem. There is no place in the world, which is more unsuitable for such a long dredged channel. The channel is in the path of the world's largest sediment transport and it will be levelled by cyclones and monsoonal floods.

The costs have already almost doubled. In November 2017, it was estimated as 550 million euro in the agreement between the Belgian government and the PPA. And in January 2019, it increased to 853 million euro for more or less the same channel. What are the reasons behind that sudden increase? In the public-private partnership, Jan de Nul holds 49 per cent and has a guarantee of the Belgian governmental assurance group for its share. That share probably corresponds to the true operational costs of dredging about 70 million cubic metres, which corresponds to a rate of 5.0 euro per cubic metre, which has to be paid in euro for dredging.

Thus, Jan de Nul has been secured of any loss, but the rest of the money has to be paid to the financing HSBC Bank and may be used for the maintenance dredging. That gives a first approximation of the annual costs which are necessary to keep the channel depth at 10m. In case of an intense cyclone, however, the entire process has to start from the beginning.

Q: Is it possible to ensure 10m draft in that place to build a normal sea port? How viable will that be considering the cost of maintenance dredging?

A: A shallower channel is not so expensive, but it has the same problem as a deep channel. A smaller channel will be even filled in a much shorter time by the sediment moved from the highly mobile seafloor sediment on both sides of the channel. Also a smaller channel is not viable at that position in the transport path. It is against nature: the mighty rivers with huge sediment load in combination with powerful cyclones cannot be mastered in shallow sea close to the Meghna's mouth.

Q: Do you want to add any other thing?

A: Unfortunately, Bangladesh has no scientific expertise in the sea. All their expertise is presently concentrated in the onshore delta. The Bangladesh Water Development Authority is confined to the coastal area. The maritime university has just started, but without a ship and a long-time experience on the changes in the offshore area, it is extremely difficult to advise the government on these issues. Consequently, the government relied on the expertise of the dredging company, which has a strong vested interest in the task and cannot be regarded as an independent unbiased partner.

Since 1993, I have been studying the offshore area of Bangladesh, observing the amount of sediment passing from the rivers into the depositional areas of the submarine canyon and the submarine delta. Since the first reliable map from 1876, almost no sediment has accumulated near the river mouth (creating new islands) and all that material has been transferred into deeper waters. This constant huge loss of sediment into the sea is a serious problem for the subsiding part of Bangladesh also with respect to the global rise in sea level. The delta can cope with that sea level rise, if more sediment is retained in the polders of the coastal area. This will be the main task for the coming generation of redesigning the dikes of the polders.

syful-islam@outlook.com

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