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Using Melaleuca fences as soft coastal engineering for mangrove restoration in Kien Giang, Vietnam
Ecological Engineering 81 (2015) 256–265
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Using Melaleuca fences as soft coastal engineering for mangrove restoration in Kien Giang, Vietnam Chu Van Cuong a, * , Sharon Brown b , Huynh Huu To b , Marc Hockings a a b
School of Geography, Planning and Environmental Management – University of Queensland, St Lucia QLD 4072, Australia GIZ Kien Giang Biosphere Reserve Project, 320 Ngo Quyen, Rach Gia City, Kien Giang Province, Viet Nam
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 September 2014 Received in revised form 28 January 2015 Accepted 5 April 2015 Available online xxx
Planting mangrove is an important work to respond to mangrove loss and erosion, a serious issue and exacerbated from climatic changes in coastal area of the Mekong Delta. However, the previous efforts have repeatedly been unsuccessful because planted seedlings have suffered complete loss due to the lack of protection from wave action and seasonal sediment movement. Two Melaleuca (Melaleuca cajuputi) fences (wave barrier and silt trap fences) were designed and set up for testing their effectiveness in assisting mangrove restoration in the erosion-prone area of the Kien Giang Province. The wave height of the shoreline behind the Melaleuca fence can reduced by 63% compared to the open coast. After 3 years, they retained from 45 to 47 cm of mud inside the fence line. The survival rate of Avicennia alba (62% and 44%) was much higher than that of Rhizophora apiculata (35% and 14%) in both two fenced treatments. Growth rate of Avicennia seedlings was also much higher than Rhizophora in both treatments. There was a significant statistical difference between live seedlings and growth of Rhizophora and Avicennia in the two fenced treatments. Wild seedlings of Avicennia started colonizing the fenced area after 1.5 years of fence construction and regenerated seedling density varied from 2300 seedlings/ha in to 7100 seedlings/ha after 3 years. Species richness of benthos inside 2 fenced areas approached that in the natural forest area 1.5 years after fence construction. The study demonstrates the high potential of using Melaleuca fences to facilitate regeneration of mangrove and improves coastal protection in Kien Giang Province. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Melaleuca fences Mangrove restoration Erosion Kien Giang
1. Introduction The protective functions of mangrove ecosystem in natural disaster mitigation particularly in relation to tsunamis have been widely recognized (Alongi, 2008; Danielsen et al., 2005; Kathiresan and Rajendran, 2005; Othman, 1994; Sanford, 2009; Tanaka, 2009). Effective management and restoration of mangrove forest to maintain ecosystem services are also widely advocated (Alongi, 2002; Bosire et al., 2008; Duke et al., 2007). Maintaining strong and healthy mangrove ecosystems provides adaptive capacity to protect coastal areas against erosion and flooding due to sea level rise, storm surge or wave action and enhance climate change mitigation through acting as a carbon sink (Duarte et al., 2013; Ellison, 1999; Raghavendra, 2009; Sarwar and Khan, 2007). Vietnam is one of the most vulnerable country to climate change impacts and the Mekong Delta, the rice bowl of Vietnam
* Corresponding author at: School of Geography, Planning and Environmental Management, The University of Queensland, Room 301, Building 3, Brisbane 4072, Australia. Tel.: +61 73346 7784. E-mail address: [email protected] (C. Van Cuong). http://dx.doi.org/10.1016/j.ecoleng.2015.04.031 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.
and one of the largest rice export regions in the world, is already subject to severe inundation by sea water (IPCC, 2007). The Ministry of Natural Resource and Environment (MONRE, 2012) predicts that 39% of the delta will be inundated if the sea level rises 100 cm by 2100. In the Mekong Delta, mangrove forests along the coast play an important role as buffer strips to protect sea dykes, productive agriculture land and local community land, but this coastal defense is fragmented and degraded as a result of defoliants used between 1961 and 1971 (Hong, 2001; Hong and San, 1993), land use conversion, illegal logging (Hong and San, 1993; Sam et al., 2005) and particularly, expanding shrimp ponds and other aquaculture projects (EJF, 2003; Hong and San, 1993; Joffre and Schmitt, 2010). As a result, over half of original mangrove forest area in the delta has been lost; particularly, in the period 1989–2006 (Hoa et al., 2013). More severely, 40% of the coastline (310.6 out of 768 km) in Mekong Delta is eroding, and coastal retreat is up to 40 m year1 in some areas (VN FOREST, 2012). In response to the loss of coastal areas, the need to restore mangroves has been recognized (Alongi, 2002; Field, 1999; Hoang Tri et al., 1998; Kaly and Jones, 1998). Active planting has been the most common method employed, but this approach has repeatedly
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been unsuccessful at a large scale in many countries (Lewis, 2000, 2005). For example, the survival rate of plantation in West Bengal, India was 1.52% (Sanyal, 1998); 10–20% in Philippines (Primavera and Esteban, 2008; Samson and Rollon, 2008); approximately, 50% in Bangladesh (Saenger and Siddiqi, 1993). In Vietnam, there has been some attempts to restore mangroves in erosion-prone area, but the success rate of mangrove planting in depositional areas was less than 50% (Duke et al., 2010; Hong, 1994; Que et al., 2012). There are a great number of factors contributing to the large scale failure of mangrove plantation. Lewis (2005) and Samson and Rollon (2008) found that limited hydrological knowledge was the most common factor leading to catastrophic loss of planted seedlings. Additionally, young planted seedlings were impacted by lack of protection from strong wind and wave action (Kamali and Hashim, 2011), burial of seedlings by deposited sediment (Erftemeijer and Lewis, 1999; Tamin et al., 2011) and human disturbance from activities such as fishing (Biswas et al., 2009). All these impacts on planting success have been observed in the Mekong Delta (Cuong and Brown, 2012; Que et al., 2012). 1.1. Wave barriers – artificial solutions for mangrove restoration in highly eroded areas Restoration of mangrove in actively eroding coastlines is very difficult (GIZ Kien Giang Project, 2012; Naohiro et al., 2012). The problem is most evident in exposed, severe erosion area where new seedlings have to contend with strong wave action, wind and unfavorable hydraulic conditions (Chan et al., 1998; Kamali and Hashim, 2011; Tamin, 2005). Highly eroded areas have been affected by issues such as, complete loss of all seedlings in relatively short time-frames (Cuong and Brown, 2012; Erftemeijer and Lewis, 1999). Thus, potentially expensive solutions to reduce the force of the waves by damping the wave height are needed if restoration work is to succeed (Tamin, 2005; Tamin et al., 2011). Wave barriers to control coastal erosion and enhance sediment accretion have become the dominant process in the restoration site (Tamin et al., 2011). Working similarly to a living mangrove fringe, these wave barriers are considered as the infrastructural solutions in the coastal areas. They absorb and dissipate wave height and wave energy (Kamali and Hashim, 2011; Tamin, 2005), and then create a calm and stable substrate for mangrove establishment (Albers and Lieberman, 2011; Hashim et al., 2010; Stewart and Fairfull, 2008). Hard infrastructure solution to stop erosion from the wave has been most commonly applied for mangrove restoration in high erosion sites. These infrastructures have been applied in several sites in Vietnam (Ca Mau DARD, 2012), Malaysia (Hashim et al., 2010; Tamin, 2005) and Australia (Stewart and Fairfull, 2008). Although, hard wave barriers are needed when critical, high value land and property are threatened, the low survival rate of seedlings (Kamali and Hashim, 2011) coupled with the high cost of this method (Naohiro et al., 2012) hinders the use of this approach for scaling-up coastal protection and mangrove restoration. This paper presents the results of a cost-effective and practical methodology of fence construction using Melaleuca poles (Melaleuca cajuputi) to provide a soft barrier for coastal protection and mangrove restoration in Kien Giang Province, Mekong Delta, Vietnam.
2013). The tidal range along the previously stable coastline is 0.5–0.8 m (Hong and San, 1993). However, erosion has increased as a result of canal development for flood drainage to the west and expansion of aquaculture in early 1990s that resulted in clearing of the coastal mangrove fringe (Cuong et al., 2010; Hoa et al., 2013). It is estimated that over half, the coastline is subject to erosion or already seriously eroded (Duke et al., 2010; Russell et al., 2013). Flat low land along with the extensive loss of mangroves in recent years, and predicted extreme impacts of sea level rise make Kien Giang one of the two most vulnerable provinces to climate change in the Mekong Delta (MRC, 2010; ADB, 2011; MONRE, 2012). A sea dyke system was built along the coast behind the mangrove belt to protect highly productive agricultural land, property and local communities from salt water intrusion, storm surge and strong sea currents (VN FOREST, 2012). The existing dyke system in the province consists of 2.5–3 m high and 2–3 m wide dykes that are also used as transportation road for local people. However, the using of non-homogeneous materials such as, soil and mud (mixed of soft mud and organic) and inappropriate construction method (particularly, extracting materials from the seaward side of the dyke) have led to the weakening of the dyke and its resistance and stability under wave action (Heiland and Schüttrumpf, 2009). The average temperature in Kien Giang of over 27 C and high humidity makes the region favorable for the development and growth of mangroves. Annual rainfall is around 2000 mm with most falling in the wet season from April to November when south west (SW) winds blowing from the sea push waves into the coastal dykes causing the general erosion and breaching of the dykes. During the wet season, silt and sand are deposited along the foreshore. In the dry season, the north east (NE) winds blowing from the land move sediments that were deposited from the previous wet season out to sea. The restoration study was conducted in Vam Ray Village, Binh Son commune, Hon Dat district, 40 km north of Rach Gia city, Kien Giang Province. This area has a low topography with site elevation from 0.48 m (at the dyke’s foot) to 0.705 m (at seaward end transect) relative to mean sea level while the nearby mangrove area is consistently approximately 0.45 m higher from the coastline out to 45 m from the shore(Fig. 1). The study area is characterized by semi diurnally irregular regimes and coastal shorelines are mainly influenced by SW wind-induced waves. The lowest tide is in NE monsoon winds (0.2 m) and the highest is 1.5 m SW monsoon. Although, storms and typhoon are rare, strong SW winds in the wet season can create strong waves of 2–2.5 m offshore. Average wind speed in NE season is 2 m/s and SW is 5 m/s (Russell et al., 2013).
2. Material and methods 2.1. Research location and history of restoration attempts Located in the western part of the Mekong Delta within the Gulf of Thailand, Kien Giang Province has 206 km of coastline with mangrove forest forming a thin green line of salt-tolerant vegetation (Duke et al., 2010; Hong and San, 1993; Russell et al.,
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Fig. 1. Topography of restoration site.
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The study site has been strongly impacted by coastal erosion and mangrove loss in recent years. Hydrological modeling undertaken by ADB (2011) indicates medium erosion along the coast in this area in both NE and SW monsoons. Hoa et al. (2013) have recently estimated the loss rate of coastlines recently is up to 2.7% year1 or 24 m year1 which has been exposing earth sea dykes to strong sea wave and wind action. This directly threatens high productivity agricultural land behind the dykes. Kien Giang Department of Agriculture and Rural Development (Kien Giang DARD) had previously undertaken planting attempts at this site to protect the sea dyke. The first planting in 2004 of normal small seedlings that have often been used for planting in depositional areas failed after several months because of deep tidal inundation and strong wave action. In 2005, a second attempt using saplings that had been raised in a nursery and then protected from the wave action by anchoring with 3–4 small sticks surrounding, the trees also failed because the strings attaching the seedling to the supporting sticks ring-barked the seedlings as a result of wind and wave action. These failures led to the dyke being broken in 2006 and 2007 (Fig. 2) causing significant damage to the local properties, crops, fruit trees and fresh water fish ponds (GIZ Kien Giang Project, 2012). Kien Giang DARD rebuilt the dyke late 2008 to prevent further sea water incursion. Transect surveys to identify plant species and seed sources in the remained mangrove patches nearby the restoration site found 16 species of 11 families distributed in the areas. Nine of the plants species identified in the survey are true mangroves, dominated by Avicennia alba. This area had previously been at a higher elevation where survey data suggests that the mangrove forests were in a late successional stage (Hong and San, 1993).
(Table 1). Melaleuca is an inexpensive material that is readily available and commonly used in civil construction and house building in the Mekong Delta. This construction material was also selected because of the wide ranging environmental services (e.g., prevention of acidification of topsoil and surface water, storage and renovation of fresh water and flood and erosion mitigation), it is provided during the production, as well as for the long resistance of the timber in the inundated and saline conditions of mangrove habitats (To et al., 2011).
2.2. Research design
2.2.1. Treatment 1: wave barrier fence A wave barrier fence (Fig. 4a) was set up 60 m offshore from the dyke. It consisted of two parallel rows of Melaleuca poles with 0.5–1 m gap between them. Large poles (>5.5 cm in diameter; Table 1) were used to construct the landward fence. A post driver was used to push the poles 3.0–3.5 m into the mud with 1.5–2.0 m remaining above the substrate to establish a fence of poles for wave and wind resistance. Gap between adjacent poles was approximately 10 cm meaning that six thousand large poles were needed to construct 1 km of fence. The seaward fence was constructed with smaller poles (pole 4 or 5; Table 1) sunk 2.0–2.5 m into the mud. A larger pole positioned in every 1.5 m was included in the seaward fence to help resist wave action. One kilometer of sea ward fence, required approximately 13,000 pole 4 and 340 large poles. Bamboo matting (1.0–1.2 m high, 1.5 m long) and fine fishing net (1 m high, 3–5 m long) were attached to the inside of the seaward fence. After attaching the bamboo mat and fishing net, tree branches and small diameter poles (0.5 m3) were added into the gap between the rows. The bamboo mat and fishnet needed to be supplemented annually for up to 3 years to maintain a functional fence. The construction cost of the wave barrier fence is US$21,000/km with annual maintenance costs of US$4200/km.
An experimental site was set up in 2009 encompassing an area of approximately 2.2 ha (360 m long and 60 m wide). Three treatments including two designs of Melaleuca fencing in association with planting mangroves and a control site were established (Fig. 3). The design was innovatively developed based on the local work and experience of using simple timber fences to prevent erosion along river/canal banks in both fresh and sea water situations. Constructed wave barrier aimed to reduce wave height and partly trap sediment in the erosion area while silt trap fence was designed to trap the mud behind the wave barrier or in the depositional areas. We constructed the fences using poles with different diameters and lengths of Melaleuca (M. cajuputi) – a dominant native tree on seasonal inundated acid sulphate soils in the Mekong Delta
2.2.2. Treatment 2: wave barrier and silt trap fence A 1.5–2.0 m height silt trap fence (Fig. 4b) was set up inside the wave barrier fence (use constructed in treatment 1), approximately 20–25 m offshore from the dyke. The silt trap fence had one row of Melaleuca (pole 4; Table 1) sunk 2.0–2.5 m into the mud and placed next to each other (13,000 poles/km long) and created a small gap from 2–5 cm between adjacent poles. A layer of bamboo matting (1.0–1.2 m high, 1.5 m long) and fine fishing net was placed in front of the pole row. A frame (3 m long and 1.5 m high) made from Melaleuca (pole 3) was attached to the front and back of the row of poles. This kept the bamboo mat and fishing net in place despite wave action. Building cost of the silt trap fence is US$10,000/km, plus US$2000/km for damaged repairing and maintenance after 2 years.
Fig. 2. Dyke breached after planting attempts failed in 2006 and 2007.
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Fig. 3. Treatment design.
Table 1 Classification of Melaleuca poles using for fence construction. No
Product type
Length (m)
Top diameter (cm)
1 2 3 4
Large pole Pole 5 Pole 4 Pole 3
5.0 4.8 3.8 2.7
>5.5 4.5–5.4 4.5–5.4 2.5–5.4
2.2.3. Control site: the area with a single fence previously built by Kien Giang DARD in 2006 2.2.3.1. Tree planting. In treatments 1 and 2: after fence construction, 7–8 month old, 30–40 cm high seedlings of 2 species (A. alba and Rhizophora apiculata) were planted between the fences and the shoreline. Seedlings were prepared in plastic bags in the nursery before planting. Planted density was 25,000
Fig. 4. Melaleuca fences (a) wave barrier (b) silt trap using inside the wave barrier.
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seedlings/ha (0.5 m 0.8 m spacing) with the initially mixed proportion of 4 Rhizophora seedings with 1 Avicennia. In the control site: no planting was undertaken due to the past failures of planting without protecting fences. However, mud accumulation and natural regeneration were monitored and measured in this site. 2.3. Data collection and analysis Wave attenuation effectiveness of the fences and mangrove belt in this area was measured by setting up a transect line with observation points that were placed in front and behind fringe mangroves and fences at 15 m intervals. At each observation point, a 5 m wooden stake with calibration at accuracy of 1 cm was used to measure wave height by observer at each point. Wave height was measured at each point with five readings taken within 5 min, and the highest reading recorded. The measurement was conducted in the normal SW monsoon condition with the average wave height was 0.64 m. Wave reduction was calculated based on Mazda et al. (1997): r¼
Hs HL Hs
where r is wave attenuation between two measurement points of the transect; Hs is the wave height at the sea edge of the first point (outside mangrove/fences); HL is wave height at the second point. A system of 5 concrete meter-marked poles was set up across the demonstration site (two for each fenced treatment and one for control site) to monitor mud accumulation. These poles were staked in the ground by banger to ensure that they do not sink into the mud for the whole period of measurement. Sediment accumulation was monitored and recorded weekly from photographs and direct measurement. Survival rate, growth of planted seedlings and natural recruitment of seedlings were measured in four plots of 25 m2 (5 5 m) in each of two treatments (4 plots for each fenced treatment). Seedlings were measured in every 6 months over a period of 3 years. One plot was set up in the control site to monitor natural regeneration. Seedlings growth and regeneration were also monitored from a series of weekly photographs. In April and May 2011, a benthic diversity survey was conducted by Dragon Institute – Can Tho University and GIZ Kien Giang Project to evaluate the impact of the fences on zoo benthos biodiversity. An Ekman–Birge grab with surface area of 0.02 m2 was used to collect zoo benthos. Forty sample plots were set up in 2 fenced treatments, control site and nearby natural forest. Based on the map and prior field visit, 10 plots were randomly selected across each site. At each plot, 5 samples of benthic animals were collected at 2 h intervals during high tide. Benthos samples were screened with a 0.5 mm sieve and the collected specimens were fixed in a 10% formaldehyde solution for laboratory analysis. The survival rate and growth of planted Rhizophora and Avicennia seedlings in the two fence designs were statistically analyzed in SPSS using an independent sample T test. 3. Results 3.1. Wave attenuation There was an increase in wave attenuation cross shore distance in mangrove forests, wave barriers and open area (control site). Wave attenuation rates increased to 67% in mangrove fringe, 63% behind the wave barrier and 38% in open area within 60 m cross shore distance (Fig. 5). Significant differences were found in wave attenuation between mangrove and wave barrier recordings and
Fig. 5. Wave attenuation across shore distance in mangrove, Melaleuca barrier and open area.
the attenuation across the open mudflat (t = 9.23 and 18.12; P = 0.003 < 0.05), but no significant difference existed between mangrove fringe and wave barrier (t = 1.7; P = 0.22 > 0.05) areas. 3.2. Mud accumulation Constructed fences significantly enhanced deposition of sediment in the study area. Mud accumulation was significant different between fence treatments and control (t = 3.81; P = 0.002 < 0.05). Three years after construction, 44 cm of mud had been deposited in treatment 1 (wave barrier fence) and 42 cm in treatment 2 (silt trap fence). In contrast, only 0.3 cm of mud was accumulated in the control area, and only within 5 m of the shore. In both fence treatments, mud accumulated mostly in the first year (29 cm in treatment 1 and 33 cm in treatment 2) and slowed down in the second and third year (Fig. 6). Fig. 7 shows that there was a strong dynamic of the sediment accretion and erosion processes in the study site. In the fence areas, mud mainly deposited in the wet season and continued accumulating in the dry season. In contrast, a depth layer (17 cm) of sediment in the open (control) area that was accelerated at the earlier months of the wet season but it was washed away within one to two months at the end of this season. 3.3. Survival rate and growth of planted seedlings Survival rate of 2 planted species differed significantly in both fenced treatment sites (Fig. 8). After 3 years, survival rate of Avicennia seedlings was 44% in treatment 1 and 62% in the treatment 2. The figure was much lower for Rhizophora (14% and 35%, respectively). Fig. 6 shows that the highest mortality occurred in the first year for both species, particularly for Rhizophora in both 2 fence treatments (81% and 55%, respectively compared to 37% and 54% of Avicennia seedlings). The survival rate of seedlings was stable after that except for Rhizophora in treatment 1. For individual planted species of Rhizophora and Avicennia, there was a significant difference of trees alive in two treatments. Results of Independent – sample T test show that there is a significant difference in survival rate of R. apiculata and A. alba in the wave break fence and silt trap fence (t = 4.079 for Rhizophora and 5.257 for Avicennia; P < 0.001 for both species). Planted Avicennia seedlings also grew faster than Rhizophora in the erosion site (Fig. 9). Three years after planting Avicennia seedlings averaged 394 cm high in treatment 1 and 560 cm in treatment 2, whist Rhizophora averaged only136 cm and 226 cm, respectively. Independent – sample T test indicates the significant effect of designed fences in the growth of both planted seedlings
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Fig. 6. Mud accumulation over 3 years: fenced site (left) and control area (right).
2012 (Fig. 11). In contrast, there was no seedling recruitment in the control area. At the beginning of natural colonization process, number of regenerated Avicennia seedlings was much higher in the silt trap fence or treatment 2 (6000 trees/ha) compared to the wave breaking fence or treatment 1 (200 seedlings/ha). New seedlings were recruiting in the small dense patches nearby the remaining mother trees (Fig. 12). While, a great proportion of new recruited Avicennia (63%) in the treatment 2 survived and grew well, all seedlings in the treatment 1 died. New natural regeneration of Avicennia, however, continued occurring in treatment 1 with the density of 1300 seedlings/ha in November 2011 and increased to 2400 seedlings/ha in November 2012. Average height of regenerated Avicennia seedlings after 12 months was 195 cm in treatment 1 and 209 cm in treatment 2 but there were no statistical differences between the two treatments (t = 0.73; P = 0.942 > 0.05). Fig. 7. Sediment accumulation in the study site over a period of 12 months.
3.5. Zoobenthos biodiversity (t = 6.131 and P < 0.001 for Rhizophora and t = 2.361; P = 0.019 < 0.05 for Avicennia) (Fig. 10). 3.4. Natural recruitment Propagules of Avicennia were found in the restoration site at the initial stage of fence construction in October 2009, but no new seedling could survive or establish in the restoration site in the first one and half year after fence construction. Recruited seedlings were found in both fenced treatments after 18 months of fence building. Regenerated Avicennia seedlings reached to 2400/ha in treatment 1 and 7100 seedlings/ha in treatment 2 by November
Zoobenthos was generally low in the study area, even in the natural mangrove forest (Sam et al., 2005). Heavy disturbance of the study site by illegal logging, fishing and erosion might be a major factor leading to the depauperate benthic fauna. 10 benthos species, belonging to 5 classes (Bivalvia, Crustacea, Polychaeta, Gastropoda and Insecta) were found in 40 surveyed points (0.2 m2) of 2 treatments, control site and the nearby natural forest area. Of these, polychaete (Namalycastis longicirris) was the most common species across the study site. The lowest species richness was found in the wave break fence (treatment 1) followed by the control area. In the double fence site (treatment 2), species
Fig. 8. Survival rate of planted seedlings in 2 fence treatments (a) Rhizophora apiculata and (b) Avicennia alba. Error bars denote standard deviation.
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Fig. 9. Growth of the planted seedlings in the treatments: Rhizophora appiculata (a) and Avicennia alba (b). Error bars denote standard deviation.
Fig. 10. Time series of the restoration site (changes of fenced area) in November 2010 (a), November 2011-(b) and November 2012 (c); control in November 2010 (d), 2011 (e) and 2012 (f).
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Fig. 11. Recruited Avicennia seedlings in the fenced areas (a) treatment 1 and (b) treatment 2. Error bars denote standard deviation.
Fig. 12. Naturally recruited Avicennia seedlings in the treatment 2 in May 2011 and May 2012.
Fig. 13. Benthic diversity in the different treatments and nearby natural forest. Error bars denote standard deviation.
richness nearly approached that of the nearby natural mangrove patches (Fig. 13). 4. Discussion Mangrove restoration in high erosion areas is greatly facilitated by the use of protective Melaleuca fences. Constructed fences significantly contribute to reduce wave energy, trap and fix unstable and weak mud in the restoration area. As hydrological conditions improve and silt becomes stable, planted seedlings and propagules are able to survive and grow. The biodiversity of plants and benthic animals in the area has increased significantly.
Melaleuca fences produced similar rate of wave attenuation to remnant mangrove patches at the study site. Working as a “soft” barrier, Melaleuca fences absorb and reduce wave height when the waves cross this constructed structure. Wave height immediately reduced by 56% when the wave crossed through the fence. In contrast, this rate was much lower (30%) to the mangrove forest. Wave attenuation capacity depends on the mangrove characteristics such as tree density, root density, canopy diameter and basal area and tide condition (Bao, 2011; Mazdz et al., 1997). The thin and fragmented mangrove belt (50–60 m width) in the study site as a result of severe erosion and heavy disturbance might have reduced its wave damping capacity. Measurements were only conducted under the normal SW monsoon conditions. Different monsoon and climate conditions may affect to the results and conclusions of the wave attenuation effectiveness of Melaleuca fences and mangrove fringes. Generally, Melaleuca fences play a vital role in trapping and accumulating sediment. However, high level of mud accumulation and active movement also has led to the high mortality rate of seedlings (Affandi et al., 2010; Ellison, 1999; Hashim et al., 2010; Naohiro et al., 2012; Tamin et al., 2011). At the study site, most planted seedlings died in the first year after planting because small seedlings were uprooted from the erosion of surface sediment in the dry, NE wind season and then were buried by deep and active movement of sediment. In addition, small planted seedlings were often deeply inundated in the first year after planting which may have contributed to the high mortality, especially of Rhizophora seedlings in treatment 1. When stress factors (wave action, sediment movement and accumulation and inundation) were removed and substrate was stable, seedlings started establishing well. In the late 2011 and 2012, less sediment accretion and stabilization of the deposited mud in the fenced area reduced the
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mortality of the remaining seedlings and created suitable growing conditions. The study demonstrated that constructed Melaleuca fences serve to protect newly established seedlings in the restoration sites. Rubbish and near shore fishing are the most significant driving factors of high mangrove seedling mortality in Vietnam (Que et al., 2012). Newly small seedlings could die if they are buried and smothered by rubbish or damaged by fishing boats. Hence, fences can help to prevent both drifting rubbish from the sea (Cuong and Brown, 2012) and human access to the regeneration areas (Biswas et al., 2009). Field observation found that stems of Avicennia are supple and bend in the waves. When seedlings are pushed over, they can grow upwards. The high survival rate demonstrated in our study and growth of Avicennia seedlings indicate that this species is best able to withstand high energy waves and should be preferred as the pioneer restoration species in tidal areas. Balke et al. (2013) also report success in using Avicennia in restoration. Only Avicennia seedlings were successfully recolonized in the fenced treatments. The absence of regenerated seedlings of later successional species such as Rhizophora,Brugeria and Ceriops in the site is likely to be because of the short time since fence construction. Other studies have found that even when a seed source is available, natural regeneration only occurred when hydrodynamic forces and sediment movement were reduced below threshold levels of sedimentation and water depth (Balke et al., 2013; Tamin, 2005). Melaleuca fences provided temporal protection solutions for progagules from physical disturbance. Inundation, hydrology and sediment dynamic are critical factors contributing to the high mortality of propagule seedlings in the restoration site. Balke et al. (2011) found that strong waves and sediment movement (accretion or erosion) lead to toppling and dislodgement and can bury new A. alba propagules. Thus, constructed fences can absorb and dissipate wave energy, reduce mud movement and create a stable period that allows new established seedlings or propagules to develop and resist to the stressed physical conditions. 5. Implications Fence construction techniques now need to be tested under different conditions (erosion, accretion, sea currents, tidal ranges and water depth) that exist in other coastal areas to determine how the design principles can be adaptively applied elsewhere. The results of this study site provide a new option in shoreline erosion control and mangrove restoration. The methods developed here can assist people in coastal regions to mitigate the impacts of climate change by addressing the effects of increasing coastal erosion, and inundation resulting from sea level rise and extreme climatic events. Our method proved effective in promoting natural recruitment in restoration areas through fence building may be a better choice than the common practice of planting trees. Successful mangrove restoration might not need artificial planting (Kamali and Hashim, 2011). It significantly reduces cost of both nursery and planting activities while improving quality of the regenerated forest because wild seedlings often grow better than direct planted ones (Field, 1998). This study has shown that Melaleuca fences can assist natural regeneration in both erosion (by using a structure of 2 fences – wave breaking fence and silt trap fence) and accretion sites (silt trap fence). Planting should be the final activity in mangrove restoration program (Lewis and Marshall, 1997). If planting must be undertaken, it should be done carefully with right site matching and species selection (Field, 1998). Pioneer species such as Avicennia spp. should be selected for initial planting because this
species performs better in the exposed conditions of the restoration sites. The study shows that to ensure the high survival of seedlings, planting should be conducted when the substrate of the restoration site has stabilized after fence construction. Practical methods for fence establishment have been developed and tested in over 3 years in restoration site of Kien Giang Province. The design is being applied by IUCN in collaboration with GIZ, Department of Natural Resources and Environment, Forest Protection Management Board and local communities under the project “Building Resilience to Climate Change Impacts in Coastal Southeast Asia” that constructed 900 m Melaleuca fences near the study site to protect mangrove belt from erosion and support for mud accumulation and recruitment (Hung and To, 2014). Construction of fences for mangrove restoration is also planned in Kien Giang, (Kien Giang DARD, 2010) funded by ODA loan projects of German Bank (KFW) and Asian Development Bank (ADB) for the period 2011–2020. These projects will provide a test of how these techniques can be scaled up for mangrove forest restoration, coastal rehabilitation and shoreline erosion control at the scale needed to effect improvements in coastal management in the tropics. Mangrove restoration is expensive and is only cost effective if it follows good planning design (Lewis, 2005). Construction costs often vary with the materials and construction methods used. Melaleuca fences are much cheaper than other wave reduction barriers (see e.g., Stewart and Fairfull, 2008; Hashim et al., 2010; Albers and Lieberman, 2011; Ca Mau DARD, 2012). In Kien Giang, the proposed planting cost of traditional planting method in depositional site is 2450 USD/ha (Kien Giang DARD, 2010) compared to 1000–1600 USD/ha for fence construction (silt trap fence). In eroding sites, planting advanced saplings costs 7400 USD/ha compared to 6000–6500 USD/ha for fencing (wave barrier and silt trap fences), including damaged repairing and maintenance cost. Melaleuca is a native in the Mekong Delta and the most appropriate plant species that can tolerate the extreme acid sulphate soil conditions of the delta. Melaleuca also plays a vital role in prevention to the formation process of acid sulphate soil (Cuong and Dart, 2011; To et al., 2011). Using local materials like Melaleuca poles for fence construction will increase the monetary value of this species which has significantly declined in the market recently (To et al., 2011; Trung, 2008). Perhaps, it might encourage farmers to maintain Melaleuca plantations for both income and environmental benefits. Furthermore, all branches of Melaleuca are used in building wave break fences. This does not only bring additional income for farmers but also reduces gas emissions from the common practice of burning residuals after harvesting. Acknowledgement Mangrove restoration site was undertaken from October 2009 as a part of the mangrove management and restoration program by GIZ – Conservation and Development of the Kien Giang Biosphere Reserve Project, funded by AusAID. We would like to thank the project for support for this study. References ADB, 2011. Climate Change Impact and Adaptation Study in The Mekong Delta – Part A Final Report: Climate Change Vulnerability and Risk Assessment Study for Ca Mau and Kien Giang Provinces, Vietnam. Affandi, N.A.M., Kamali, B., Rozainah, M.Z., Tamin, N.M., Hashim, R., 2010. Early growth and survival of Avicennia alba seedlings under excessive sedimentation. Sci. Res. Essays 5, 2801–2805. Albers, T., Lieberman, N.V., 2011. Current and Erosion Modelling Survey. Deutsche Gesellschaft für Internationale Zusammenarbeit (GiZ) GmbH Management of Natural Resources in the Coastal Zone of Soc Trang Province.
C. Van Cuong et al. / Ecological Engineering 81 (2015) 256–265 Alongi, D.M., 2002. Present state and future of the world’s mangrove forests. Environ. Conserv. 29, 331–349. Alongi, D.M., 2008. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuarine Coastal Shelf Sci. 76, 1–13. Balke, T., Bouma, T.J., Horstman, E.M., Webb, E.L., Erftemeijer, P.L.A., Herman, P.M.J., 2011. Windows of opportunity: thresholds to mangrove seedling establishment on tidal flats. Mar. Ecol. Progress Ser. 440, 1–9. Balke, T., Webb, E.L., Denelzen, E., Galli, D., Herman, P.M.J., Bouma, T.J., Frid, C., 2013. Seedling establishment in a dynamic sedimentary environment: a conceptual framework using mangroves. J. Appl. Ecol. 50, 740–747. Bao, T.Q., 2011. Effect of mangrove forest structures on wave attenuation in coastal Vietnam. Oceanologia 53, 807–818. Biswas, S.R., Mallik, A.U., Choudhury, J.K., Nishat, A., 2009. A unified framework for the restoration of Southeast Asian mangroves – bridging ecology, society and economics. Wetlands Ecol. Manage. 17, 365–383. Bosire, J.O., Dahdouh-Guebas, F., Walton, M., Crona, B.I., Lewis, R.R., Field, C., Kairo, J. G., Koedam, N., 2008. Functionality of restored mangroves: a review. Aquat. Bot. 89, 251–259. Ca Mau Department of Agriculture and Rural Development (Ca Mau DARD), 2012. Planting trees ...in the sea [Trông rùng trên . . . biên]. Lao Dong Newspapers, 18 April 2012.
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Hung, P.V., To, H.H., 2014. Coatal protection using Melaleuca fences for erosion control and mangrove development. Presentation at the Third Annual Coastal Forum: Building Resilience to Climate Change Impacts in Coastal Southeast Asia, Preah Sihanouk Province, Cambodia, 23–25 October, 2014.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Using Melaleuca fences as soft coastal engineering for mangrove restoration in Kien Giang, Vietnam Chu Van Cuong a, * , Sharon Brown b , Huynh Huu To b , Marc Hockings a a b
School of Geography, Planning and Environmental Management – University of Queensland, St Lucia QLD 4072, Australia GIZ Kien Giang Biosphere Reserve Project, 320 Ngo Quyen, Rach Gia City, Kien Giang Province, Viet Nam
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 September 2014 Received in revised form 28 January 2015 Accepted 5 April 2015 Available online xxx
Planting mangrove is an important work to respond to mangrove loss and erosion, a serious issue and exacerbated from climatic changes in coastal area of the Mekong Delta. However, the previous efforts have repeatedly been unsuccessful because planted seedlings have suffered complete loss due to the lack of protection from wave action and seasonal sediment movement. Two Melaleuca (Melaleuca cajuputi) fences (wave barrier and silt trap fences) were designed and set up for testing their effectiveness in assisting mangrove restoration in the erosion-prone area of the Kien Giang Province. The wave height of the shoreline behind the Melaleuca fence can reduced by 63% compared to the open coast. After 3 years, they retained from 45 to 47 cm of mud inside the fence line. The survival rate of Avicennia alba (62% and 44%) was much higher than that of Rhizophora apiculata (35% and 14%) in both two fenced treatments. Growth rate of Avicennia seedlings was also much higher than Rhizophora in both treatments. There was a significant statistical difference between live seedlings and growth of Rhizophora and Avicennia in the two fenced treatments. Wild seedlings of Avicennia started colonizing the fenced area after 1.5 years of fence construction and regenerated seedling density varied from 2300 seedlings/ha in to 7100 seedlings/ha after 3 years. Species richness of benthos inside 2 fenced areas approached that in the natural forest area 1.5 years after fence construction. The study demonstrates the high potential of using Melaleuca fences to facilitate regeneration of mangrove and improves coastal protection in Kien Giang Province. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Melaleuca fences Mangrove restoration Erosion Kien Giang
1. Introduction The protective functions of mangrove ecosystem in natural disaster mitigation particularly in relation to tsunamis have been widely recognized (Alongi, 2008; Danielsen et al., 2005; Kathiresan and Rajendran, 2005; Othman, 1994; Sanford, 2009; Tanaka, 2009). Effective management and restoration of mangrove forest to maintain ecosystem services are also widely advocated (Alongi, 2002; Bosire et al., 2008; Duke et al., 2007). Maintaining strong and healthy mangrove ecosystems provides adaptive capacity to protect coastal areas against erosion and flooding due to sea level rise, storm surge or wave action and enhance climate change mitigation through acting as a carbon sink (Duarte et al., 2013; Ellison, 1999; Raghavendra, 2009; Sarwar and Khan, 2007). Vietnam is one of the most vulnerable country to climate change impacts and the Mekong Delta, the rice bowl of Vietnam
* Corresponding author at: School of Geography, Planning and Environmental Management, The University of Queensland, Room 301, Building 3, Brisbane 4072, Australia. Tel.: +61 73346 7784. E-mail address: [email protected] (C. Van Cuong). http://dx.doi.org/10.1016/j.ecoleng.2015.04.031 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.
and one of the largest rice export regions in the world, is already subject to severe inundation by sea water (IPCC, 2007). The Ministry of Natural Resource and Environment (MONRE, 2012) predicts that 39% of the delta will be inundated if the sea level rises 100 cm by 2100. In the Mekong Delta, mangrove forests along the coast play an important role as buffer strips to protect sea dykes, productive agriculture land and local community land, but this coastal defense is fragmented and degraded as a result of defoliants used between 1961 and 1971 (Hong, 2001; Hong and San, 1993), land use conversion, illegal logging (Hong and San, 1993; Sam et al., 2005) and particularly, expanding shrimp ponds and other aquaculture projects (EJF, 2003; Hong and San, 1993; Joffre and Schmitt, 2010). As a result, over half of original mangrove forest area in the delta has been lost; particularly, in the period 1989–2006 (Hoa et al., 2013). More severely, 40% of the coastline (310.6 out of 768 km) in Mekong Delta is eroding, and coastal retreat is up to 40 m year1 in some areas (VN FOREST, 2012). In response to the loss of coastal areas, the need to restore mangroves has been recognized (Alongi, 2002; Field, 1999; Hoang Tri et al., 1998; Kaly and Jones, 1998). Active planting has been the most common method employed, but this approach has repeatedly
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been unsuccessful at a large scale in many countries (Lewis, 2000, 2005). For example, the survival rate of plantation in West Bengal, India was 1.52% (Sanyal, 1998); 10–20% in Philippines (Primavera and Esteban, 2008; Samson and Rollon, 2008); approximately, 50% in Bangladesh (Saenger and Siddiqi, 1993). In Vietnam, there has been some attempts to restore mangroves in erosion-prone area, but the success rate of mangrove planting in depositional areas was less than 50% (Duke et al., 2010; Hong, 1994; Que et al., 2012). There are a great number of factors contributing to the large scale failure of mangrove plantation. Lewis (2005) and Samson and Rollon (2008) found that limited hydrological knowledge was the most common factor leading to catastrophic loss of planted seedlings. Additionally, young planted seedlings were impacted by lack of protection from strong wind and wave action (Kamali and Hashim, 2011), burial of seedlings by deposited sediment (Erftemeijer and Lewis, 1999; Tamin et al., 2011) and human disturbance from activities such as fishing (Biswas et al., 2009). All these impacts on planting success have been observed in the Mekong Delta (Cuong and Brown, 2012; Que et al., 2012). 1.1. Wave barriers – artificial solutions for mangrove restoration in highly eroded areas Restoration of mangrove in actively eroding coastlines is very difficult (GIZ Kien Giang Project, 2012; Naohiro et al., 2012). The problem is most evident in exposed, severe erosion area where new seedlings have to contend with strong wave action, wind and unfavorable hydraulic conditions (Chan et al., 1998; Kamali and Hashim, 2011; Tamin, 2005). Highly eroded areas have been affected by issues such as, complete loss of all seedlings in relatively short time-frames (Cuong and Brown, 2012; Erftemeijer and Lewis, 1999). Thus, potentially expensive solutions to reduce the force of the waves by damping the wave height are needed if restoration work is to succeed (Tamin, 2005; Tamin et al., 2011). Wave barriers to control coastal erosion and enhance sediment accretion have become the dominant process in the restoration site (Tamin et al., 2011). Working similarly to a living mangrove fringe, these wave barriers are considered as the infrastructural solutions in the coastal areas. They absorb and dissipate wave height and wave energy (Kamali and Hashim, 2011; Tamin, 2005), and then create a calm and stable substrate for mangrove establishment (Albers and Lieberman, 2011; Hashim et al., 2010; Stewart and Fairfull, 2008). Hard infrastructure solution to stop erosion from the wave has been most commonly applied for mangrove restoration in high erosion sites. These infrastructures have been applied in several sites in Vietnam (Ca Mau DARD, 2012), Malaysia (Hashim et al., 2010; Tamin, 2005) and Australia (Stewart and Fairfull, 2008). Although, hard wave barriers are needed when critical, high value land and property are threatened, the low survival rate of seedlings (Kamali and Hashim, 2011) coupled with the high cost of this method (Naohiro et al., 2012) hinders the use of this approach for scaling-up coastal protection and mangrove restoration. This paper presents the results of a cost-effective and practical methodology of fence construction using Melaleuca poles (Melaleuca cajuputi) to provide a soft barrier for coastal protection and mangrove restoration in Kien Giang Province, Mekong Delta, Vietnam.
2013). The tidal range along the previously stable coastline is 0.5–0.8 m (Hong and San, 1993). However, erosion has increased as a result of canal development for flood drainage to the west and expansion of aquaculture in early 1990s that resulted in clearing of the coastal mangrove fringe (Cuong et al., 2010; Hoa et al., 2013). It is estimated that over half, the coastline is subject to erosion or already seriously eroded (Duke et al., 2010; Russell et al., 2013). Flat low land along with the extensive loss of mangroves in recent years, and predicted extreme impacts of sea level rise make Kien Giang one of the two most vulnerable provinces to climate change in the Mekong Delta (MRC, 2010; ADB, 2011; MONRE, 2012). A sea dyke system was built along the coast behind the mangrove belt to protect highly productive agricultural land, property and local communities from salt water intrusion, storm surge and strong sea currents (VN FOREST, 2012). The existing dyke system in the province consists of 2.5–3 m high and 2–3 m wide dykes that are also used as transportation road for local people. However, the using of non-homogeneous materials such as, soil and mud (mixed of soft mud and organic) and inappropriate construction method (particularly, extracting materials from the seaward side of the dyke) have led to the weakening of the dyke and its resistance and stability under wave action (Heiland and Schüttrumpf, 2009). The average temperature in Kien Giang of over 27 C and high humidity makes the region favorable for the development and growth of mangroves. Annual rainfall is around 2000 mm with most falling in the wet season from April to November when south west (SW) winds blowing from the sea push waves into the coastal dykes causing the general erosion and breaching of the dykes. During the wet season, silt and sand are deposited along the foreshore. In the dry season, the north east (NE) winds blowing from the land move sediments that were deposited from the previous wet season out to sea. The restoration study was conducted in Vam Ray Village, Binh Son commune, Hon Dat district, 40 km north of Rach Gia city, Kien Giang Province. This area has a low topography with site elevation from 0.48 m (at the dyke’s foot) to 0.705 m (at seaward end transect) relative to mean sea level while the nearby mangrove area is consistently approximately 0.45 m higher from the coastline out to 45 m from the shore(Fig. 1). The study area is characterized by semi diurnally irregular regimes and coastal shorelines are mainly influenced by SW wind-induced waves. The lowest tide is in NE monsoon winds (0.2 m) and the highest is 1.5 m SW monsoon. Although, storms and typhoon are rare, strong SW winds in the wet season can create strong waves of 2–2.5 m offshore. Average wind speed in NE season is 2 m/s and SW is 5 m/s (Russell et al., 2013).
2. Material and methods 2.1. Research location and history of restoration attempts Located in the western part of the Mekong Delta within the Gulf of Thailand, Kien Giang Province has 206 km of coastline with mangrove forest forming a thin green line of salt-tolerant vegetation (Duke et al., 2010; Hong and San, 1993; Russell et al.,
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Fig. 1. Topography of restoration site.
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The study site has been strongly impacted by coastal erosion and mangrove loss in recent years. Hydrological modeling undertaken by ADB (2011) indicates medium erosion along the coast in this area in both NE and SW monsoons. Hoa et al. (2013) have recently estimated the loss rate of coastlines recently is up to 2.7% year1 or 24 m year1 which has been exposing earth sea dykes to strong sea wave and wind action. This directly threatens high productivity agricultural land behind the dykes. Kien Giang Department of Agriculture and Rural Development (Kien Giang DARD) had previously undertaken planting attempts at this site to protect the sea dyke. The first planting in 2004 of normal small seedlings that have often been used for planting in depositional areas failed after several months because of deep tidal inundation and strong wave action. In 2005, a second attempt using saplings that had been raised in a nursery and then protected from the wave action by anchoring with 3–4 small sticks surrounding, the trees also failed because the strings attaching the seedling to the supporting sticks ring-barked the seedlings as a result of wind and wave action. These failures led to the dyke being broken in 2006 and 2007 (Fig. 2) causing significant damage to the local properties, crops, fruit trees and fresh water fish ponds (GIZ Kien Giang Project, 2012). Kien Giang DARD rebuilt the dyke late 2008 to prevent further sea water incursion. Transect surveys to identify plant species and seed sources in the remained mangrove patches nearby the restoration site found 16 species of 11 families distributed in the areas. Nine of the plants species identified in the survey are true mangroves, dominated by Avicennia alba. This area had previously been at a higher elevation where survey data suggests that the mangrove forests were in a late successional stage (Hong and San, 1993).
(Table 1). Melaleuca is an inexpensive material that is readily available and commonly used in civil construction and house building in the Mekong Delta. This construction material was also selected because of the wide ranging environmental services (e.g., prevention of acidification of topsoil and surface water, storage and renovation of fresh water and flood and erosion mitigation), it is provided during the production, as well as for the long resistance of the timber in the inundated and saline conditions of mangrove habitats (To et al., 2011).
2.2. Research design
2.2.1. Treatment 1: wave barrier fence A wave barrier fence (Fig. 4a) was set up 60 m offshore from the dyke. It consisted of two parallel rows of Melaleuca poles with 0.5–1 m gap between them. Large poles (>5.5 cm in diameter; Table 1) were used to construct the landward fence. A post driver was used to push the poles 3.0–3.5 m into the mud with 1.5–2.0 m remaining above the substrate to establish a fence of poles for wave and wind resistance. Gap between adjacent poles was approximately 10 cm meaning that six thousand large poles were needed to construct 1 km of fence. The seaward fence was constructed with smaller poles (pole 4 or 5; Table 1) sunk 2.0–2.5 m into the mud. A larger pole positioned in every 1.5 m was included in the seaward fence to help resist wave action. One kilometer of sea ward fence, required approximately 13,000 pole 4 and 340 large poles. Bamboo matting (1.0–1.2 m high, 1.5 m long) and fine fishing net (1 m high, 3–5 m long) were attached to the inside of the seaward fence. After attaching the bamboo mat and fishing net, tree branches and small diameter poles (0.5 m3) were added into the gap between the rows. The bamboo mat and fishnet needed to be supplemented annually for up to 3 years to maintain a functional fence. The construction cost of the wave barrier fence is US$21,000/km with annual maintenance costs of US$4200/km.
An experimental site was set up in 2009 encompassing an area of approximately 2.2 ha (360 m long and 60 m wide). Three treatments including two designs of Melaleuca fencing in association with planting mangroves and a control site were established (Fig. 3). The design was innovatively developed based on the local work and experience of using simple timber fences to prevent erosion along river/canal banks in both fresh and sea water situations. Constructed wave barrier aimed to reduce wave height and partly trap sediment in the erosion area while silt trap fence was designed to trap the mud behind the wave barrier or in the depositional areas. We constructed the fences using poles with different diameters and lengths of Melaleuca (M. cajuputi) – a dominant native tree on seasonal inundated acid sulphate soils in the Mekong Delta
2.2.2. Treatment 2: wave barrier and silt trap fence A 1.5–2.0 m height silt trap fence (Fig. 4b) was set up inside the wave barrier fence (use constructed in treatment 1), approximately 20–25 m offshore from the dyke. The silt trap fence had one row of Melaleuca (pole 4; Table 1) sunk 2.0–2.5 m into the mud and placed next to each other (13,000 poles/km long) and created a small gap from 2–5 cm between adjacent poles. A layer of bamboo matting (1.0–1.2 m high, 1.5 m long) and fine fishing net was placed in front of the pole row. A frame (3 m long and 1.5 m high) made from Melaleuca (pole 3) was attached to the front and back of the row of poles. This kept the bamboo mat and fishing net in place despite wave action. Building cost of the silt trap fence is US$10,000/km, plus US$2000/km for damaged repairing and maintenance after 2 years.
Fig. 2. Dyke breached after planting attempts failed in 2006 and 2007.
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Fig. 3. Treatment design.
Table 1 Classification of Melaleuca poles using for fence construction. No
Product type
Length (m)
Top diameter (cm)
1 2 3 4
Large pole Pole 5 Pole 4 Pole 3
5.0 4.8 3.8 2.7
>5.5 4.5–5.4 4.5–5.4 2.5–5.4
2.2.3. Control site: the area with a single fence previously built by Kien Giang DARD in 2006 2.2.3.1. Tree planting. In treatments 1 and 2: after fence construction, 7–8 month old, 30–40 cm high seedlings of 2 species (A. alba and Rhizophora apiculata) were planted between the fences and the shoreline. Seedlings were prepared in plastic bags in the nursery before planting. Planted density was 25,000
Fig. 4. Melaleuca fences (a) wave barrier (b) silt trap using inside the wave barrier.
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seedlings/ha (0.5 m 0.8 m spacing) with the initially mixed proportion of 4 Rhizophora seedings with 1 Avicennia. In the control site: no planting was undertaken due to the past failures of planting without protecting fences. However, mud accumulation and natural regeneration were monitored and measured in this site. 2.3. Data collection and analysis Wave attenuation effectiveness of the fences and mangrove belt in this area was measured by setting up a transect line with observation points that were placed in front and behind fringe mangroves and fences at 15 m intervals. At each observation point, a 5 m wooden stake with calibration at accuracy of 1 cm was used to measure wave height by observer at each point. Wave height was measured at each point with five readings taken within 5 min, and the highest reading recorded. The measurement was conducted in the normal SW monsoon condition with the average wave height was 0.64 m. Wave reduction was calculated based on Mazda et al. (1997): r¼
Hs HL Hs
where r is wave attenuation between two measurement points of the transect; Hs is the wave height at the sea edge of the first point (outside mangrove/fences); HL is wave height at the second point. A system of 5 concrete meter-marked poles was set up across the demonstration site (two for each fenced treatment and one for control site) to monitor mud accumulation. These poles were staked in the ground by banger to ensure that they do not sink into the mud for the whole period of measurement. Sediment accumulation was monitored and recorded weekly from photographs and direct measurement. Survival rate, growth of planted seedlings and natural recruitment of seedlings were measured in four plots of 25 m2 (5 5 m) in each of two treatments (4 plots for each fenced treatment). Seedlings were measured in every 6 months over a period of 3 years. One plot was set up in the control site to monitor natural regeneration. Seedlings growth and regeneration were also monitored from a series of weekly photographs. In April and May 2011, a benthic diversity survey was conducted by Dragon Institute – Can Tho University and GIZ Kien Giang Project to evaluate the impact of the fences on zoo benthos biodiversity. An Ekman–Birge grab with surface area of 0.02 m2 was used to collect zoo benthos. Forty sample plots were set up in 2 fenced treatments, control site and nearby natural forest. Based on the map and prior field visit, 10 plots were randomly selected across each site. At each plot, 5 samples of benthic animals were collected at 2 h intervals during high tide. Benthos samples were screened with a 0.5 mm sieve and the collected specimens were fixed in a 10% formaldehyde solution for laboratory analysis. The survival rate and growth of planted Rhizophora and Avicennia seedlings in the two fence designs were statistically analyzed in SPSS using an independent sample T test. 3. Results 3.1. Wave attenuation There was an increase in wave attenuation cross shore distance in mangrove forests, wave barriers and open area (control site). Wave attenuation rates increased to 67% in mangrove fringe, 63% behind the wave barrier and 38% in open area within 60 m cross shore distance (Fig. 5). Significant differences were found in wave attenuation between mangrove and wave barrier recordings and
Fig. 5. Wave attenuation across shore distance in mangrove, Melaleuca barrier and open area.
the attenuation across the open mudflat (t = 9.23 and 18.12; P = 0.003 < 0.05), but no significant difference existed between mangrove fringe and wave barrier (t = 1.7; P = 0.22 > 0.05) areas. 3.2. Mud accumulation Constructed fences significantly enhanced deposition of sediment in the study area. Mud accumulation was significant different between fence treatments and control (t = 3.81; P = 0.002 < 0.05). Three years after construction, 44 cm of mud had been deposited in treatment 1 (wave barrier fence) and 42 cm in treatment 2 (silt trap fence). In contrast, only 0.3 cm of mud was accumulated in the control area, and only within 5 m of the shore. In both fence treatments, mud accumulated mostly in the first year (29 cm in treatment 1 and 33 cm in treatment 2) and slowed down in the second and third year (Fig. 6). Fig. 7 shows that there was a strong dynamic of the sediment accretion and erosion processes in the study site. In the fence areas, mud mainly deposited in the wet season and continued accumulating in the dry season. In contrast, a depth layer (17 cm) of sediment in the open (control) area that was accelerated at the earlier months of the wet season but it was washed away within one to two months at the end of this season. 3.3. Survival rate and growth of planted seedlings Survival rate of 2 planted species differed significantly in both fenced treatment sites (Fig. 8). After 3 years, survival rate of Avicennia seedlings was 44% in treatment 1 and 62% in the treatment 2. The figure was much lower for Rhizophora (14% and 35%, respectively). Fig. 6 shows that the highest mortality occurred in the first year for both species, particularly for Rhizophora in both 2 fence treatments (81% and 55%, respectively compared to 37% and 54% of Avicennia seedlings). The survival rate of seedlings was stable after that except for Rhizophora in treatment 1. For individual planted species of Rhizophora and Avicennia, there was a significant difference of trees alive in two treatments. Results of Independent – sample T test show that there is a significant difference in survival rate of R. apiculata and A. alba in the wave break fence and silt trap fence (t = 4.079 for Rhizophora and 5.257 for Avicennia; P < 0.001 for both species). Planted Avicennia seedlings also grew faster than Rhizophora in the erosion site (Fig. 9). Three years after planting Avicennia seedlings averaged 394 cm high in treatment 1 and 560 cm in treatment 2, whist Rhizophora averaged only136 cm and 226 cm, respectively. Independent – sample T test indicates the significant effect of designed fences in the growth of both planted seedlings
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Fig. 6. Mud accumulation over 3 years: fenced site (left) and control area (right).
2012 (Fig. 11). In contrast, there was no seedling recruitment in the control area. At the beginning of natural colonization process, number of regenerated Avicennia seedlings was much higher in the silt trap fence or treatment 2 (6000 trees/ha) compared to the wave breaking fence or treatment 1 (200 seedlings/ha). New seedlings were recruiting in the small dense patches nearby the remaining mother trees (Fig. 12). While, a great proportion of new recruited Avicennia (63%) in the treatment 2 survived and grew well, all seedlings in the treatment 1 died. New natural regeneration of Avicennia, however, continued occurring in treatment 1 with the density of 1300 seedlings/ha in November 2011 and increased to 2400 seedlings/ha in November 2012. Average height of regenerated Avicennia seedlings after 12 months was 195 cm in treatment 1 and 209 cm in treatment 2 but there were no statistical differences between the two treatments (t = 0.73; P = 0.942 > 0.05). Fig. 7. Sediment accumulation in the study site over a period of 12 months.
3.5. Zoobenthos biodiversity (t = 6.131 and P < 0.001 for Rhizophora and t = 2.361; P = 0.019 < 0.05 for Avicennia) (Fig. 10). 3.4. Natural recruitment Propagules of Avicennia were found in the restoration site at the initial stage of fence construction in October 2009, but no new seedling could survive or establish in the restoration site in the first one and half year after fence construction. Recruited seedlings were found in both fenced treatments after 18 months of fence building. Regenerated Avicennia seedlings reached to 2400/ha in treatment 1 and 7100 seedlings/ha in treatment 2 by November
Zoobenthos was generally low in the study area, even in the natural mangrove forest (Sam et al., 2005). Heavy disturbance of the study site by illegal logging, fishing and erosion might be a major factor leading to the depauperate benthic fauna. 10 benthos species, belonging to 5 classes (Bivalvia, Crustacea, Polychaeta, Gastropoda and Insecta) were found in 40 surveyed points (0.2 m2) of 2 treatments, control site and the nearby natural forest area. Of these, polychaete (Namalycastis longicirris) was the most common species across the study site. The lowest species richness was found in the wave break fence (treatment 1) followed by the control area. In the double fence site (treatment 2), species
Fig. 8. Survival rate of planted seedlings in 2 fence treatments (a) Rhizophora apiculata and (b) Avicennia alba. Error bars denote standard deviation.
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Fig. 9. Growth of the planted seedlings in the treatments: Rhizophora appiculata (a) and Avicennia alba (b). Error bars denote standard deviation.
Fig. 10. Time series of the restoration site (changes of fenced area) in November 2010 (a), November 2011-(b) and November 2012 (c); control in November 2010 (d), 2011 (e) and 2012 (f).
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Fig. 11. Recruited Avicennia seedlings in the fenced areas (a) treatment 1 and (b) treatment 2. Error bars denote standard deviation.
Fig. 12. Naturally recruited Avicennia seedlings in the treatment 2 in May 2011 and May 2012.
Fig. 13. Benthic diversity in the different treatments and nearby natural forest. Error bars denote standard deviation.
richness nearly approached that of the nearby natural mangrove patches (Fig. 13). 4. Discussion Mangrove restoration in high erosion areas is greatly facilitated by the use of protective Melaleuca fences. Constructed fences significantly contribute to reduce wave energy, trap and fix unstable and weak mud in the restoration area. As hydrological conditions improve and silt becomes stable, planted seedlings and propagules are able to survive and grow. The biodiversity of plants and benthic animals in the area has increased significantly.
Melaleuca fences produced similar rate of wave attenuation to remnant mangrove patches at the study site. Working as a “soft” barrier, Melaleuca fences absorb and reduce wave height when the waves cross this constructed structure. Wave height immediately reduced by 56% when the wave crossed through the fence. In contrast, this rate was much lower (30%) to the mangrove forest. Wave attenuation capacity depends on the mangrove characteristics such as tree density, root density, canopy diameter and basal area and tide condition (Bao, 2011; Mazdz et al., 1997). The thin and fragmented mangrove belt (50–60 m width) in the study site as a result of severe erosion and heavy disturbance might have reduced its wave damping capacity. Measurements were only conducted under the normal SW monsoon conditions. Different monsoon and climate conditions may affect to the results and conclusions of the wave attenuation effectiveness of Melaleuca fences and mangrove fringes. Generally, Melaleuca fences play a vital role in trapping and accumulating sediment. However, high level of mud accumulation and active movement also has led to the high mortality rate of seedlings (Affandi et al., 2010; Ellison, 1999; Hashim et al., 2010; Naohiro et al., 2012; Tamin et al., 2011). At the study site, most planted seedlings died in the first year after planting because small seedlings were uprooted from the erosion of surface sediment in the dry, NE wind season and then were buried by deep and active movement of sediment. In addition, small planted seedlings were often deeply inundated in the first year after planting which may have contributed to the high mortality, especially of Rhizophora seedlings in treatment 1. When stress factors (wave action, sediment movement and accumulation and inundation) were removed and substrate was stable, seedlings started establishing well. In the late 2011 and 2012, less sediment accretion and stabilization of the deposited mud in the fenced area reduced the
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mortality of the remaining seedlings and created suitable growing conditions. The study demonstrated that constructed Melaleuca fences serve to protect newly established seedlings in the restoration sites. Rubbish and near shore fishing are the most significant driving factors of high mangrove seedling mortality in Vietnam (Que et al., 2012). Newly small seedlings could die if they are buried and smothered by rubbish or damaged by fishing boats. Hence, fences can help to prevent both drifting rubbish from the sea (Cuong and Brown, 2012) and human access to the regeneration areas (Biswas et al., 2009). Field observation found that stems of Avicennia are supple and bend in the waves. When seedlings are pushed over, they can grow upwards. The high survival rate demonstrated in our study and growth of Avicennia seedlings indicate that this species is best able to withstand high energy waves and should be preferred as the pioneer restoration species in tidal areas. Balke et al. (2013) also report success in using Avicennia in restoration. Only Avicennia seedlings were successfully recolonized in the fenced treatments. The absence of regenerated seedlings of later successional species such as Rhizophora,Brugeria and Ceriops in the site is likely to be because of the short time since fence construction. Other studies have found that even when a seed source is available, natural regeneration only occurred when hydrodynamic forces and sediment movement were reduced below threshold levels of sedimentation and water depth (Balke et al., 2013; Tamin, 2005). Melaleuca fences provided temporal protection solutions for progagules from physical disturbance. Inundation, hydrology and sediment dynamic are critical factors contributing to the high mortality of propagule seedlings in the restoration site. Balke et al. (2011) found that strong waves and sediment movement (accretion or erosion) lead to toppling and dislodgement and can bury new A. alba propagules. Thus, constructed fences can absorb and dissipate wave energy, reduce mud movement and create a stable period that allows new established seedlings or propagules to develop and resist to the stressed physical conditions. 5. Implications Fence construction techniques now need to be tested under different conditions (erosion, accretion, sea currents, tidal ranges and water depth) that exist in other coastal areas to determine how the design principles can be adaptively applied elsewhere. The results of this study site provide a new option in shoreline erosion control and mangrove restoration. The methods developed here can assist people in coastal regions to mitigate the impacts of climate change by addressing the effects of increasing coastal erosion, and inundation resulting from sea level rise and extreme climatic events. Our method proved effective in promoting natural recruitment in restoration areas through fence building may be a better choice than the common practice of planting trees. Successful mangrove restoration might not need artificial planting (Kamali and Hashim, 2011). It significantly reduces cost of both nursery and planting activities while improving quality of the regenerated forest because wild seedlings often grow better than direct planted ones (Field, 1998). This study has shown that Melaleuca fences can assist natural regeneration in both erosion (by using a structure of 2 fences – wave breaking fence and silt trap fence) and accretion sites (silt trap fence). Planting should be the final activity in mangrove restoration program (Lewis and Marshall, 1997). If planting must be undertaken, it should be done carefully with right site matching and species selection (Field, 1998). Pioneer species such as Avicennia spp. should be selected for initial planting because this
species performs better in the exposed conditions of the restoration sites. The study shows that to ensure the high survival of seedlings, planting should be conducted when the substrate of the restoration site has stabilized after fence construction. Practical methods for fence establishment have been developed and tested in over 3 years in restoration site of Kien Giang Province. The design is being applied by IUCN in collaboration with GIZ, Department of Natural Resources and Environment, Forest Protection Management Board and local communities under the project “Building Resilience to Climate Change Impacts in Coastal Southeast Asia” that constructed 900 m Melaleuca fences near the study site to protect mangrove belt from erosion and support for mud accumulation and recruitment (Hung and To, 2014). Construction of fences for mangrove restoration is also planned in Kien Giang, (Kien Giang DARD, 2010) funded by ODA loan projects of German Bank (KFW) and Asian Development Bank (ADB) for the period 2011–2020. These projects will provide a test of how these techniques can be scaled up for mangrove forest restoration, coastal rehabilitation and shoreline erosion control at the scale needed to effect improvements in coastal management in the tropics. Mangrove restoration is expensive and is only cost effective if it follows good planning design (Lewis, 2005). Construction costs often vary with the materials and construction methods used. Melaleuca fences are much cheaper than other wave reduction barriers (see e.g., Stewart and Fairfull, 2008; Hashim et al., 2010; Albers and Lieberman, 2011; Ca Mau DARD, 2012). In Kien Giang, the proposed planting cost of traditional planting method in depositional site is 2450 USD/ha (Kien Giang DARD, 2010) compared to 1000–1600 USD/ha for fence construction (silt trap fence). In eroding sites, planting advanced saplings costs 7400 USD/ha compared to 6000–6500 USD/ha for fencing (wave barrier and silt trap fences), including damaged repairing and maintenance cost. Melaleuca is a native in the Mekong Delta and the most appropriate plant species that can tolerate the extreme acid sulphate soil conditions of the delta. Melaleuca also plays a vital role in prevention to the formation process of acid sulphate soil (Cuong and Dart, 2011; To et al., 2011). Using local materials like Melaleuca poles for fence construction will increase the monetary value of this species which has significantly declined in the market recently (To et al., 2011; Trung, 2008). Perhaps, it might encourage farmers to maintain Melaleuca plantations for both income and environmental benefits. Furthermore, all branches of Melaleuca are used in building wave break fences. This does not only bring additional income for farmers but also reduces gas emissions from the common practice of burning residuals after harvesting. Acknowledgement Mangrove restoration site was undertaken from October 2009 as a part of the mangrove management and restoration program by GIZ – Conservation and Development of the Kien Giang Biosphere Reserve Project, funded by AusAID. We would like to thank the project for support for this study. References ADB, 2011. Climate Change Impact and Adaptation Study in The Mekong Delta – Part A Final Report: Climate Change Vulnerability and Risk Assessment Study for Ca Mau and Kien Giang Provinces, Vietnam. Affandi, N.A.M., Kamali, B., Rozainah, M.Z., Tamin, N.M., Hashim, R., 2010. Early growth and survival of Avicennia alba seedlings under excessive sedimentation. Sci. Res. Essays 5, 2801–2805. Albers, T., Lieberman, N.V., 2011. Current and Erosion Modelling Survey. Deutsche Gesellschaft für Internationale Zusammenarbeit (GiZ) GmbH Management of Natural Resources in the Coastal Zone of Soc Trang Province.
C. Van Cuong et al. / Ecological Engineering 81 (2015) 256–265 Alongi, D.M., 2002. Present state and future of the world’s mangrove forests. Environ. Conserv. 29, 331–349. Alongi, D.M., 2008. Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuarine Coastal Shelf Sci. 76, 1–13. Balke, T., Bouma, T.J., Horstman, E.M., Webb, E.L., Erftemeijer, P.L.A., Herman, P.M.J., 2011. Windows of opportunity: thresholds to mangrove seedling establishment on tidal flats. Mar. Ecol. Progress Ser. 440, 1–9. Balke, T., Webb, E.L., Denelzen, E., Galli, D., Herman, P.M.J., Bouma, T.J., Frid, C., 2013. Seedling establishment in a dynamic sedimentary environment: a conceptual framework using mangroves. J. Appl. Ecol. 50, 740–747. Bao, T.Q., 2011. Effect of mangrove forest structures on wave attenuation in coastal Vietnam. Oceanologia 53, 807–818. Biswas, S.R., Mallik, A.U., Choudhury, J.K., Nishat, A., 2009. A unified framework for the restoration of Southeast Asian mangroves – bridging ecology, society and economics. Wetlands Ecol. Manage. 17, 365–383. Bosire, J.O., Dahdouh-Guebas, F., Walton, M., Crona, B.I., Lewis, R.R., Field, C., Kairo, J. G., Koedam, N., 2008. Functionality of restored mangroves: a review. Aquat. Bot. 89, 251–259. Ca Mau Department of Agriculture and Rural Development (Ca Mau DARD), 2012. Planting trees ...in the sea [Trông rùng trên . . . biên]. Lao Dong Newspapers, 18 April 2012.
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