University of Twente Student Theses


Quantifying physical stressors controlling mangrove seedling dynamics : a combined observational and numerical analysis

Gelderland, M. (2020) Quantifying physical stressors controlling mangrove seedling dynamics : a combined observational and numerical analysis.

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Abstract:Mangrove forests are exposed to a wide range of physical conditions and forces, such as waves, currents, sediment supply, bed level changes, etc. These mangroves are an important ecological habitat, provide food and wood, sequester carbon and attenuate waves and surges. Vegetation in mangrove forests has an important role in attenuating hydrodynamic forces and contributes to the reduction of coastal erosion. Yet, a mechanistic understanding of feedbacks between hydro- and morphodynamical stresses and mangrove seedling dynamics is lacking. By combining field work and modelling work these processes can be analysed to understand how these processes affect the long-term development and resilience of mangrove forests and the stability of mangroves. Firstly, the driving factors of bed level changes are determined. Waves, tides, flow velocities, seedlings and bed level changes itself have been measured in the Sungei Buloh Wetland Reserve mangroves in Singapore. Field observations show that the water depth combined with waves are the main driving factors of the bed level changes. During low waters, waves are the main cause of bed level changes. Additionally, the measured number of seedlings decrease during the whole fieldwork period. At the inland location 10 to 20 less seedlings per plot were found. However, the inland plot shows higher seedlings diameters and heights. Via a multiple linear regression (MLR), the main cause of the decrease in the number of seedlings, are bed level changes. Based on the MLR, an empirical growth formula is composed in order to simulate the growth of the number of seedlings as a function of the bed level changes. A Delft-FM model is set-up, with a new made vegetation growth module. The model is calibrated based on the observed dynamics. Via the vegetation module, the number of seedlings is modelled for every week, using the found growth formula. Additionally, wave scenarios are set-up to analyse the effect of the increase of wave heights on bed level changes and seedling establishment. The model does represent the observed seedling dynamics, but does not show any differences as a consequences of increased wave heights. The increased waves causes erosion to happen, which is not occurring with a constant wave height of 0.02 m. This Thesis combines fieldwork and numerical modelling to get a better understanding of seedling establishment and the effect of increased wave heights on seedling establishment. Furthermore, the new model is one of the first of its types for mangrove systems. The model, and field observations, shows that waves cause the most effects on bed level changes, which have the most effect on seedling establishment. Changes in waves cause erosion, but do not affect seedling establishment. This research contributes to modelling of mangrove seedlings using parameterizations based on observed physical processes.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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