More than 60% of the world’s population live in coastal areas. Here, mangroves are critical for preventing coastal erosion, attenuating wind-induced waves and protecting adjacent land-use from coastal disturbances. However, mangroves are being destroyed and degraded by urban settlements, aquaculture and agriculture activities. The loss of mangrove habitats increases the threat to human settlements and livelihoods from coastal hazards. Mangrove destruction also depletes resources, such as fish stocks, which coastal communities are heavily dependent on. For sustainable shoreline management, therefore, it is critical to understand the functional role of coastal mangroves in mitigating shoreline erosion and wave actions. This information is important for sustainable coastal management and policy.
The aim of this thesis was to advance the understanding of the drivers of change in coastal mangroves and their protective functions in attenuating tidal wind-induced waves and shoreline erosion scale. I addressed this aim using Kien Giang coast, Mekong Delta River, Vietnam, as a case study. The study coast is low-lying with an altitude ranging from 0.3 - 0.6 m above sea level and susceptible to salinity intrusion in the dry season and flooding in the rainy season. Mangroves in the region are being threatened by coastal retreat and land-use change. As coastal mangroves are destroyed and degraded, earthen dykes are being heavily eroded and will be further damaged unless appropriate mitigation actions are taken.
First, I reviewed the relationship between mangrove dynamics and anthropogenic and natural-induced factors in Southeast Asia, the driving factors of coastal shoreline change, and protective functions of coastal mangroves worldwide. The review revealed that current knowledge is limited by lack of research examining the driving factors of coastal shoreline change and by a lack of sound scientific basis for determining of how large an area of mangroves is needed to buffer waves effectively.
Second, the relationship between spatial-temporal change in mangrove extent and adjacent shoreline land-use in Kien Giang, Vietnam was examined by using four Landsat TM images (1989, 1992, 2003 and 2006) and three SPOT images (1995, 2003 and 2009). I found that mangrove losses were significantly correlated with adjacent land-use change. Further investigation also found that mangrove destruction resulted from high economic returns in other coastal land-uses and ineffective land-use policies in the past. My research suggests that the implementation of payments for mangrove ecosystem services and stronger regulations on mangrove management and land-use change could significantly enhance the effectiveness of mangrove conservation.
Third, the factors driving coastal shoreline change were examined using generalised linear mixed effects modelling. I found that mangrove cutting, dominant mangrove genus, adjacent land-use change and change in the width of mangrove belts significantly influenced the rates of shoreline change. My research suggests that the implementation of the stronger regulation on mangrove cutting and unsustainable land-use change could strengthen the effectiveness of mangrove conservation and coastal adjacent land-use management. This result also suggests that direct mangrove planting with presence of breakwater structures could promote mangrove density and help retain coastal sediments, thereby helping to stabilise eroding fringe shorelines.
Finally, I quantified the relationship of mangrove structures and tidal wind-induced wave and wind velocity behaviours. I used generalised linear mixed-effects models to quantify the influence of mangrove structures on tidal wave and wind velocity attenuation. I found that stem density, canopy diameter, root density and basal area were the most important factors affecting wave attenuation, while an interaction of canopy diameter and canopy closure, species composition, diameter of trunk at 1.3 m and stem height were the most influential factors on wind velocity reduction. The relationship between the width of mangrove belts and tidal waves was further investigated using an exponential regression integrated model to predict how large an area of mangroves was required for effective wave buffering. The results showed that a belt of coastal mangroves with a minimum width of 176 m along the Hon Dat coast was most effective in attenuating tidal wave height from 300 cm to 25 cm. For other coastal districts of Kien Giang, protective strips of coastal mangroves ranged from 80 - 177 m wide.
This thesis makes an important contribution to research and mangrove management, both internationally and in Vietnam. From an international perspective, this study highlights the importance of coastal mangroves in shoreline stabilisation and tidal wind-driven wave attenuation and defining the critical width of mangrove belts for buffering waves. From the Vietnam and particularly the Kien Giang perspective, this study provides further understanding of human and natural impacts on mangroves, in particular the role of coastal land-use planning policies, to better understand the driving factors of coastal shoreline change. It directly informs coastal decisions and policies attempting to reduce potential risks of coastal disturbance to the high-density of human settlements in the region.