Why artificial oyster reefs are the answer to our coastal embankments problems

There are about 40 kilometres of coastal embankment in Kutubdia, Cox's Bazar. Bangladesh Water Development Board (BWDB) spent Tk 130 crores to rebuild around 18 kilometres of broken dams. But the newly built embankment is being washed away by the tide of the full moon. 

Locals are being badly impacted as the embankment reportedly broke in hundreds of places along about eight kilometres, due to the tide last month. When more areas of the dam are at risk of being washed away, and geobags are miserably failing at saving Laboni Beach in Cox's Bazar, one scientist has been offering Bangladesh a nature-based solution – artificial oyster reef – to protect the embankments and stave off coastal erosion. 

After years of dilly-dallying, the water resource ministry decided a few weeks ago to commission a report on the potential of upscaling Dr Shah Nawaz Chowdhury's eco-engineering research project on breakwater oyster reef as nature-based solution to prevent coastal erosion in Bangladesh. 

In an interview with The Business Standard, Dr Chowdhury (Associate Professor, Institute of Marine Sciences at the University of Chittagong) explains how his research can help the BWDB save its embankments and help Bangladesh elevate more lands. 

The Tk130 Crore BWDB dam in Kutubdia is vanishing in full moon tide. You have been researching sustainable alternatives to prevent coastal erosion. How do you evaluate BWDB's steps to prevent coastal erosion?

BWDP is the authority for building dams or embankments in Bangladesh. They have expertise in developing dams in coastal areas. 

South of Cox's Bazar where my research is based, we see four types of dams: earthen embankment, block embankment, concrete tripod (we see this at the Marine Drive) and geotube, as temporary solutions. During my research in Kutubdia, I saw the embankment, which is roughly 40 km – most of it is earthen; and its concrete part ranges only about five kilometres. 

Kutubdia is an exposed area – it has a channel on one side, and the Bay of Bengal on three other sides. As a result, it receives high wave energy, particularly in monsoon. During frequent storm surge that this area braced for in monsoon, larger waves hit the embankment. 

BWDP applies the same engineering techniques, in regards to embankment in Kutubdia – an exposed land – that it applies in other places. But such earthen embankments are less effective in such an exposed area, in comparison to concrete blocks. 

What we are trying to say is that even if you make the entire embankment concrete, the dam can fall for two reasons: rising sea level and the direct hit of the waves at the foreshore and base of the dam. If the soil at the base and front of the dam is eroded, the dam will not sustain. It will collapse. So if we cannot protect the foreshore in front of the dam, no matter how strong a dam is, it cannot survive long. 

This is where our theory comes: breaking the wave before it reaches the embankment. If we can dampen the wave before it reaches the dam, the sustainability of the dam will increase and its protection will require less maintenance. The government is spending a lot of money on embankments, yet we don't have a remedy for erosion. 

We, the Institute of Marine Sciences, have been striving for nature-based solutions using elements of nature. In this regard, we had investigated the potentials of building living dams utilising oysters. This can be a good supplement to traditional embankments. What BWDP is doing is good but something should be there in the foreshore area to subdue the wave energy. Our eco engineering techniques can play a role there. 

Your research has been featured on national and international media as a sustainable alternative. Do the authorities have any plan to use artificial oyster reefs, on a trial basis, on a larger scale?

Yes, our research has been published in renowned scientific journals and the global media has taken us very seriously. Both national and international media have covered it with enthusiasm. We have presented it in bigger seminars and symposiums in Europe and America. 

We have been expecting the government's attention since. I remember at the project stage, we invited concerned officials to share our idea. But we didn't receive any positive response for a long time. 

However, last week I got a call from the water resource ministry. The Water Resources Planning Organisation (WARPO)'s Chittagong divisional officer has contacted me. 

I have been informed that our research has been selected as an agenda. The Chittagong divisional WARPO office has been asked to submit a report on the potential of upscaling the research implementation. 

I am happy that the ministry, even though late, has finally focused on our research.  

I have told them that we are very confident about our research findings, but for your [ministry's] confidence in the research, upscale preferably 10 kilometres. If not 10, then at least one kilometre to verify its applicability [first]. And if you find our research findings sustain in the upscaled level, then go for mass scale application. 

We can do it in an island, or to the southern coast of Bangladesh, where oysters are abundant. Such conversation is in progress now. 

What roles can artificial oyster reefs play on our coast? Would we still need regular dams if oyster reefs are created?

It can play an important role in protecting our coast from erosion. When we build an embankment, we design it for protection. Traditional embankments have some negative effects on the environment. But the sort of embankment that we plan will not only protect, it will preserve nature. Our breakwater oyster reefs can accelerate the sedimentation process in intertidal zones that can help plant mangrove behind the reefs for example – which plays a massive role protecting Bangladesh from cyclones and tides. 

Also thanks to this beneficial ecological system, fisheries production also increases in the area. We have found in our research how fishermen increasingly found more fish near the dam areas because it shelters the fish by providing different ecosystem services. It enhances the productivity of nature. So, the artificial reefs provide protection, preservation and production simultaneously. This is why the American scientists call this 'living shorelines,' and the European scientists call this 'building with nature.' 

We build the embankment in the intertidal zones. So these embankments wouldn't stop the flooding, but it would stabilise the soil to protect the embankments and reduce the cost incurred by erosion. 

The BWDB dams are important primary protection dike. But every year they cost a fortune in maintenance. As per our findings, oyster reefs will reduce the expenses and make the primary dike (the dams) sustainable by taking secondary assistance from oyster reefs. It will significantly reduce the maintenance costs. 

Are oyster reefs cost-effective or expensive in comparison to regular dams? How much time does it take to make a coast resilient to erosion with oyster reefs?

In my opinion, the oyster reefs won't take even one tenth of the expenses that traditional embankments require. It is very cost effective. However, to find the exact figure in terms of cost-effectiveness estimation, we need a pilot project. When we researched, many people volunteered with their services. But in a pilot phase of a commercial project, you will have to pay them. But considering everything, the costs will be less than one-tenth still. 

After years of research, we have found that in the southeast coast of Bangladesh, where salinity of the water is more than 10 PPT  [average ocean salinity is 30-35 PPT, but it comes down when river water mixes with it], we can develop healthy oyster reefs. 

So the pace at which an oyster reef grows depends on the salinity. If salinity is 5 PPT, it grows a little slowly, if 10 PPT, fast, but if salinity is around 30-35 PPT, it will be very fast. In such areas, it shouldn't take more than 2-3 years to build oyster reefs. 

However, in areas like Kutubdia, it can take up to three years. But from our experiences in Kutubdia, the work of the reefs – oyster settling – actually began a day after we put the reef structures such as concrete ring in water for oyster settlement. Our rings were entirely covered with oysters in two years. So we can say it takes around two years. 

But the speciality of such an embankment is that as time goes on it will grow stronger, unlike the conventional embankments that grow weaker with time. Moreover, it can outpace the sea level rise as the reef grow with rising sea level, which never can be achieved from conventional engineering approaches.

In a previous interview, you said your research found that 29 centimetres of land annually can be elevated through the sedimentation process by installing oyster reefs at intertidal zones. Can you elaborate on that?

This is the most interesting question. You know Bangladesh is a delta. A big part of our land was created through sedimentation by Ganges-Brahmaputra-Meghna River system. You will be surprised to know that billions of tons of sediments are transported to Bangladesh's coast by these rivers each fall. These sediments are flowing into the ocean through our rivers in front of our eyes. This sediment is a huge resource. Hypothetically speaking – if a country like the Netherlands had such a resource for example, I think they would double the size of their country by 50 years.

In winter when the sea is calm, the sediments are deposited at the coast. But from March to August when the sea is turbulent, the sediments are washed away due to the waves. If we could cash in on the blessings that nature has endowed us with, we could increase the size of our country by the end of a year.

You should know that in the Noakhali region, some natural islands are growing. The water development board is planting some vegetation there to accelerate the process. But I think using technologies like oyster reefs, the process can be sped up. 

If we could accelerate the sedimentation process using eco-engineering techniques to trap the sediments, we could create a new country, I would say. During our research we were able to accrete the land by 29 centimetres every year by using our technology. If a project can be stretched for a few kilometres, its impact will be higher. 

Not only the oysters, there are some other valuable ecosystems (such as saltmarsh, mangroves etc.) which are known as eco-system engineers that can modify eco-morphology of their surrounding areas. These natural strengths can be integrated with conventional engineering approach for preventing coastal erosion problem in Bangladesh. We are ready to help the water engineers from Bangladesh Water Development Board with our knowledge to develop a sustainable nature based solution to protect our coastlines from erosion in changing climate.