OUTLINE 1. Introduction 1.1 Eco‐engineering: from concepts to applications 1.2 The general objectives 2. Trends in tropical cyclones/depressions in Bangladesh 3. Erosion in the Kutubdia Island 4. The pilot project 4.1 Oyster reef design 4.2 Spat (young oysters) settlement and growth 4.3 Shoreline accretion 4.4 Food and income 4.5 Biodiversity and habitat 5. The outcomes and impact 6. Future directions
1
1. Introduction The low‐lying, densely populated coastal areas of Bangladesh are under great threat due to increasing risk of storm‐surge flood‐ ing and future sea‐level rise. Every year, about 30‐70% of the country is normally flooded, and also land losses due to erosion is a chronic problem in many coastal areas and offshore islands, such as the Kutubdia, Sandwip, etc. Traditional engineering ap‐ proach, for example, to achieve long‐term protection and/or curb erosion with earthen dikes is neither resilient nor sustainable, and often suboptimal with respect to other func‐ tions and even resulted in negative or un‐ foreseen impacts on local ecology. Instead, a combination of existing methods and strategies are gaining importance, especially in developing countries, to promote more resilient solutions that are robust, sustain‐
able, adaptable, multifunctional and yet eco‐ nomically feasible, as illustrated in figure 1. 1.1 Eco‐engineering: from concepts to applica‐ tions Eco‐engineering or the so‐called Building with Nature, also often referred to Living Shoreline, is a novel soft‐engineering ap‐ proach that combines with existing coastal defence structures for protecting coastal areas and communities from erosion, and enhancing the natural defense of the coast using living organisms, at the same time pro‐ viding various ecosystem services and prod‐ ucts to people. One method of eco‐ engineering is known for decades, i.e. the green belt or shields of mangroves, how‐ ever, newer methods are emerging, for ex‐ ample, protecting the coast and enhancing shoreline accretion by building oyster reef (Figure 2). An oyster reef protects shorelines quite in a different way than the mangrove does. It helps dampen the wave and current energy, which are the most prominent causes of erosion in coastal areas, and trap sediment. Living oyster reefs grow with time and self‐repair any damage, and therefore, they require almost no maintenance. Fur‐ thermore, oyster reefs provide shelter for many marine organisms like a coral reef does, delivering similar biodiversity and pro‐ tection benefits, in addition to providing food (e.g. crab, shrimp, fish, oyster and mus‐ sels) to local communities. The possibilities of eco‐engineering approach are not just limited to mangroves or oyster reefs, other viable living organisms (such as marsh grasses, mussels, etc.) and a variety of their combinations can be used to protect coasts, enhance accretion, conserve biodiversity and provide livelihood for communities.
Figure 1. The interactions of society, ecosystems and engineering in response to natural disasters and climate change, and the overlapping areas illustrate opportunities for combined adaptation and resilience strategies for coastal areas (source: Cheong et al. 2013)
2
Figure 2. An illustrated diagram of oyster reefs on a mudflat. The self‐renewing oyster reefs can contribute to the development of salt marshes and mangroves, and sustainable coastal defence with increased accretion and stability of intertidal flats (acknowledgement, Md Sakibul Islam)
Incidentally, for the first time, the con‐ cept of eco‐engineering has been tested on a pilot scale in the erosion prone offshore island of Kutubdia, Bangladesh under the project ECOBAS (Eco‐engineered coastal defence integrated with sustainable aquatic food production in Bangladesh). This pilot project was/is implemented by the Institute of Marine Sciences and Fisheries (University of Chittagong, Bangladesh), together with scientists from IMARES (Institute for Marine Resources & Ecosystem Studies) and LEI (Agricultural Economics Research Institute) of Wageningen University and engineering firm Royal HaskoningDHV, the Netherlands. The ECOBAS project is financed by the Dutch Partners for Water Programme and the
Royal Netherlands Embassy in Bangladesh. Figure 3, shows a photograph of the mem‐ bers of ECOBAS team.
Figure 3. The team members of ECOBAS at the ex‐ periment site, the Kutubdia Island of Bangladesh
3
1.2 The general objectives To provide coastal managers of Bangla‐ desh with an alternative solution of coastal protection for extreme events and climate change adaptation, by using the natural re‐ sistance of oysters against hydrodynamic forces in order to protect the coast from erosion and flooding, and at the same time deliver a source of aquatic food that could be used by local communities for food and livelihood. 2. Trends in tropical cyclones/depressions in Bangladesh Heat condition of the ocean in the form of sea surface temperature (SST) is one of the most important variables used in climate change monitoring programs and is often related to other variables such as sea level change and hurricane intensity (Vinogradova 2009). Incidentally, Bay of Bengal is a poten‐ tially energetic region for the development of cyclonic storms accounting for about 7% of the global annual tropical cyclones with two cyclone seasons in a year (Yesubabu et al. 2014). Chowdhury et al. (2012) reported that night SST has been increased by 0.30‐ 0.48°C over 25 years, from 1985‐2009 at a rate between 0.0126°C and 0.0203°C per year. It is revealed that early summer tem‐ perature is dropping at low and mid‐latitude zones, while the late summer temperature is rising quickly. Conversely, in other months and at other latitude zones, SST is consis‐ tently rising at a rate of about 0.02°C per year. The cyclone seasons in the Bay of Ben‐ gal are likely to be prolonged as the cooler months continued to be much warmer than average. Moreover, as cooler high latitude zones get warmer, cyclones will get larger replenishment area for gaining heat energy, thus increasing the risk of cyclones along the coast of Bangladesh. The frequency of tropi‐ cal cyclones and probable linkage of in‐ creased SST is shown in figure 4.
Figure 4. Long‐term trends of tropical cyclones in the Bay of Bengal (Chowdhury et al. 2012)
3. Erosion in the Kutubdia Island A geospatial assessment of the island's geomorphological changes (Chowdhury, unpublished data) reveal that the island has shrunk from its 79 km2 in 1950 to 68.5 km2 in
Figure 5. Map showing erosion prone areas at the Kutubdia Island of Bangladesh (Courtesy: Chowd‐ hury, unpublished)
4
2009, the fastest rate of erosion being felt in the 1990s. The severe cyclone and accompa‐ nying storm surge of 1991 which topped the island completely and literally washed away everything on its path, is primarily responsi‐ ble for the rapid erosion and disappearance of almost the entire southern tip of the is‐ land during this decade (Figure 5). Despite localized accretions taking place in few ar‐ eas the island remains extremely vulnerable to further erosion facing sea‐level rise and enhanced storm activity in the Bay of Ben‐ gal. 4. The pilot project The first phase of ECOBAS project, from April‐November 2012, primarily relating to settlement of spats (young oysters), and the ability of oyster reefs to grow at the Kutub‐ dia and Moheshkhali islands, has been posi‐ tive, i.e. the areas are suitable for natural spatfall, and support favourable environ‐ mental conditions for oyster growth and survival. However, this study was carried out on a very small area using reefs made of bamboo mattresses containing four differ‐
ent substrates (such as dead oyster shells, living oysters, window pane shells and stones) that were vulnerable to destruction by hydrodynamic forces, in particular in the monsoon season, and smothering of fine sediments and silt on the substrates, suffo‐ cating the oysters. Subsequently, in the sec‐ ond phase of the project (April 2013‐ November 2014), an experimental oyster reef (45 m long) made of concrete at the Kutubdia Island was put on the shore. 4.1 Oyster reef design For this, concrete ring structures were constructed and installed along the coasts of Kutubdia Island. The concrete reef was elevated above the sediment level that can serve as suitable substrate for oyster spat settlement, and placed on the mudflat, in front of an earthen embankment (Figure 6a,b). The technical design of the reef struc‐ tures and their positioning on the mudflat were worked out by the scientists and engi‐ neers of the partnering institutions involved in the project.
Figure 6a. The design of reef structures
5
Figure 6b. Manufacturing and installation of reef
4.2 Spat (young oysters) settlement and growth Spat settlement on the concrete rings (Figure 7), both on the inner as well as outer side of the ring, was counted (spat/m2) and expressed in total spat on all rings. The data
show that there is recruitment all year round as every month the numbers of spats are increasing (Figure 8), while the peak recruit‐ ment is noted in April/May and October/ November. For measuring growth of spats, 25 rings
6
Figure 7. Algal growth (top‐left), settled oyster (top‐right), including fouling barnacles (bottom‐left) on the concrete rings, and showing a confined area (red marked, bottom‐right) that sampled on regular intervals to measure growth and survivability of spats
Figure 8. Oyster spat densities (count/m2) on the con‐ crete ring
Figure 9. The growth of oyster spats
7
out of 69 rings were selected. Spats were randomly selected from specified zones (Figure 7, right) on the inner and outer rings, and the shell length and width were meas‐ ured using a digital caliper. The selection of spats was random, therefore, it was not pos‐ sible to monitor individual growth, but actu‐ ally measured mean spat size (Figure 9). On the inner side of the rings more oys‐ ters were present than on the outer side. Besides oysters, other species were also present on the surface of the reef, i.e. non‐ target species (Figure 7). Barnacles were abundantly present on the inner as well as the outer side of the concrete rings. Also, sea anemones and zones with green algae were seen. Where algal cover is present, there are no spats and barnacles observed, although algae were not present on all rings. However, their abundant occurrence, espe‐ cially barnacles might limit available space for settlement of oyster larvae. 4.3 Shoreline accretion For measuring the profile of mudflat (or, muddy shore), a simple equipment was de‐
veloped by the research team using cost effective locally available materials (Chowdhury et al. 2014). The equipment is a type of flexible U‐tube manometer that uses liquid columns in vertical tubes to measure differences in elevation, and the supporting frame is constructed from wooden poles with base disks, which hold measuring scales and a PVC tube (Figure 10). This beach pro‐ filer was proven to be less time consuming and easy to use in the field, operated either by 2 persons or by a single person. The preliminary results confirmed an enhanced sedimentation on the mudflat, suggesting that oyster reefs have the poten‐ tial to offer an excellent form of coastal pro‐ tection as sediment forms naturally behind oyster reefs. This particular area is clearly elevated compared to the area in the fore‐ ground and background of the photograph, and according to statistical analysis (Figure 11). The main factor causing this less dy‐ namic area behind the reef is most likely the wave dampening effect of the reef. Accumu‐
Figure 10. An illustration of locally made beach/shore profiling equipment (left), and field measurements are carrying out by the equipment (right), source Chowdhury et al. 2014
8
lation is seasonal as mud tends to erode pri‐ marily in the monsoon season. In July, there is very limited fine sediment deposit. On the other hand, more deposition of fine sedi‐ ment is observed in October . However, over time, oysters will build three dimensional reef structures (i.e. oysters will grow and attach themselves to one another, eventu‐
ally forming a hard reef structure) that will even more effective in dissipating wave en‐ ergy and protecting the underlying sediment from erosion. The successful outcome of eco‐engineering concept, in the long run, will help to build a resilient coastal zone in Bangladesh.
Figure 11. The visible elevated area behind the reef during flood (top), and lifting of shore elevation in the first year of the experiment, 2013‐2014 (bottom)
9
4.4 Food and income There are also other benefits of oyster reefs, such as provision of aquatic food and livelihoods for coastal residents. The reef seems to give additional benefits as crabs get trapped in the rings during low tide and are collected (Figure 12). Crabs are valuable trades on the international market. While oysters have had a reputation as a delicacy for many centuries, only tribal community living in coastal areas consume oysters from wild catches in Bangladesh. The meat of oys‐ ters is an excellent source of vitamins and minerals, in particular, rich in zinc which is known to promote brain development of children. So, oyster reefs can deliver a
source of aquatic food (oysters) that at the same time can improve the nutrition, health and well being of humans. In addition, it sup‐ ports productions of economically impor‐ tant fisheries that can contribute to the live‐ lihoods of coastal community. 4.5 Biodiversity and habitat Between the mangrove saplings zone and the concrete reef, there is a under de‐ veloped zone of salt marsh vegetation. By an increased accretion and the stabilization of tidal flats, as noted in this study, oyster reefs could support the growth of essential coastal, or near shore, vegetation like salt marsh bed and create accreted zone for mangroves. Altogether, they could form a cascading protection zone (oyster reefs – salt marshes – mangroves) to minimize ero‐ sion of the earthen embankment. The other ecological roles of oyster reefs include im‐ provement of water quality (i.e. remove ni‐ trogen from the water column, filter out suspended solids and lowers turbidity), pro‐ viding habitat for numerous aquatic species, enhancement of biodiversity, and contribu‐ tion to a healthier ecosystem with multiple benefits and functions (Figure 13a,b). It is, therefore, very much important to consider the oyster reef in this broader context.
Figure 12. Crabs get stuck in the ring (top), and growing oysters (also anemones) on reef structure (bottom)
Figure 13a. A young mangrove sapling behind the reef
10
Figure 13b. Gastropod and bivalve molluscs biodiversity
5. The outcomes and impact The preliminary results show that oyster reefs are causing local sedimen‐ tation as predicted that can mitigate shoreline erosion, while enhancing fish‐ eries productivity and biodiversity by creating new habitats. The visible conse‐ quences will be the resiliency of coastal community against erosion and flood‐ ing, improved food and nutrition secu‐ rity, diversified income generating op‐ tions and strengthened livelihoods. An example of economic evaluation of an oyster reef restoration project is shown in figure 14. Since the climatic conditions in Bang‐ ladesh are more extreme compared to other parts of the world, it is expected that, the ECOBAS project will generate valuable information on the feasibility of this new technique to other Delta’s in the developing world, such as in Viet‐ nam and Mozambique, as well as to a further application in Bangladesh.
Figure 14. Economic valuation of an oyster reef restoration project implemented in the northern Gulf of Mexico (source: Cheong et al. 2013) \
11
6. Future directions Pilot scale ECOBAS project, which was started in April 2013, will be financed by the donors until November 2014, but to realize the full impact of artificial oyster reefs as eco‐engineering tool in coastal protection as well as valuation of ecosystem services pro‐ vided, it is required to maintain the reefs for an extended duration. Generally, adult oysters can grow to colonize a reef in 2‐3 years, and the reefs become living and self‐sustainable struc‐ tures (i.e. support all life stages: settlement, growth, and survival) and stabilize tidal flats
in about 8‐10 years time. In a recent study, scientists observed that oyster reefs can grow fast enough than the most extreme predictions of sea‐level rise (Rodriguez et al. 2014). Similarly, results derived from both DEB (dynamic energy budget) and DORG (dynamic oyster reef growth) models showed a promising growth of oyster reefs at Kutubdia Island (the ECOBAS project loca‐ tion) that is even faster than sea‐level rise (Figure 15). This result is extremely encour‐ aging, therefore, a shoreline with protected oyster reefs hold great promise in Bangla‐ desh.
Figure 15. The predictions of oyster growth (length, cm) with the DEB model (left), and oyster reef growth rate (height, cmyr‐1) with the DORG model (right) at ECOBAS site, the Kutubdia Island
References Cheong S‐M, Silliman B, Wong PP, van Wesen‐ beeck B, Kim C‐K, Guannel G (2013) Coastal adaptation with ecological engineering. Na‐ ture Climate Change 3: 787‐791. Chowdhury SR, Hossain MS, Shamsuddoha M and Khan MMH (2012). Coastal Fishers' Livelihood in Peril: Sea Surface Temperature and Tropical Cyclones in Bangladesh. CPRD, Dhaka, Bangla‐ desh. 54pp. Chowdhury SR, Hossain MS, Sharifuzzaman SM (2014) A simple and inexpensive method for muddy shore profiling. Chinese Journal of Oceanology and Limnology 32(6): 1383‐1391. Rodriguez AB, Fodrie FJ, Ridge JT, Lindquist NL, Theuerkauf EJ, Coleman SE, Grabowski JH, Brodeur MC, Gittman RK, Keller DA, Kenwor‐
thy MD (2014) Oyster reefs can outpace sea‐ level rise. Nature Climate Change 4: 493‐497. Vinogradova NT (2009). Integrated sea surface temperature products within a coastal ocean observing system. Yesubabu V, Srinivas CV, Prasad KBRRH, Rama‐ krishna SSVS (2014) Impact of variational data assimilation for simulating tropical cy‐ clones over Bay of Bengal using WRF‐ARW. In: Mohanty et al. (eds), Monitoring and Pre‐ diction of Tropical Cyclones in the Indian Ocean and Climate Change, Springer Nether‐ lands, pp.236‐245.
12