University of Twente Student Theses


Cliff erosion of salt marshes : experimental evaluation of the effect of vegetation characteristics and sediment properties of erodibility

Rahman, A. (2015) Cliff erosion of salt marshes : experimental evaluation of the effect of vegetation characteristics and sediment properties of erodibility.

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Abstract:Salt marshes are the typical types of coastal wetlands found in the high latitude areas (i.e. the temperate climatic region). These wetlands are subjected to hydrodynamic forces and inundation by saline water during flood tides. Hence, salt marsh ecosystems are home to distinctive plant species that are resistant to these conditions. These salt marsh wetlands are of engineering significance acting as natural defence against storm surges and waves. In addition, they have great ecological value providing food and sheltered nesting places for birds and animals. Hydrodynamics and sediment dynamics are important to the development of these salt marshes. The salt marsh plants increase the drag forces on tidal currents and wave actions. This way, salt marsh vegetation directly impacts the hydrodynamics and subsequently the sediment dynamics within the salt marsh wetlands. The sediment dynamics in the salt marshes help its lateral and vertical extension. Due to the continuous sediment deposition within the salt marsh vegetation, elevation gradients can be developed between the mudflat area at the seaward side and the elevated salt marshes at the landward side. This increasing gradient positively affects the growth of salt marshes as salt stresses and tidal currents are reduced in the higher elevated parts of the marsh. With the continuous accumulation of sediments in this elevated area, the slope at the edge of the salt marsh vegetation becomes increasingly steep, prone to the wave action. High energy waves, created by storms, can induce (substantial) erosion at the salt marsh edge, resulting in the formation of a cliff. The erosive processes within the salt marsh environment are important to understand, as they determine whether a salt marsh will develop or decline. Two types of erosion processes can be distinguished: top soil erosion and cliff erosion. Topsoil erosion occurs all over the marsh surface and this process is determined by the bed shear stresses. The cliff erosion refers to the lateral erosion of the salt marsh cliff and is rather important in the gradual loss of the salt marshes. Knowledge about the cliff erosion process and the relevant parameters is still limited. Therefore, this study is dedicated to the quantification of cliff erosion rates, focusing on the impacts of sediment properties and vegetation characteristics on these rates while considering a wide range of field conditions. With this research, knowledge will be obtained about how the cliff erosion varies depending on the salt marsh vegetation species, their density and the amount of aboveground and belowground biomass. Additionally, this research will also consider the impact of the sediment grain sizes and organic carbon content on the cliff erosion. Laboratory experiments were carried out to quantify the cliff erosion of salt marsh substrates. Samples were collected from five field sites covering three vegetation species: Spartina anglica, Scirpus maritimus and Phragmites australis. Moreover, within each site, samples were collected from three density zones: the densely vegetated zone, the sparsely vegetated zone and the mudflat (un-vegetated) zone. For all study sites, sediment sizes and organic carbon content were analysed and vegetation properties were quantified. Cliff like marsh edges were reconstructed in the experimental wave tanks. The generated wave conditions in the wave tank were representative for potential field conditions. The collected samples were placed in the wave tanks and at intervals pictures were taken of the eroded sediment samples to record the erosion of the sediments from time to time due to the wave action. Obtained pictures were processed with a 3D image analysis method using Visual SFM and Meshlab. This procedure resolved the sediment volumes that were eroded from the samples during every time interval. The cliff erosion in this study was quantified using two characteristic coefficients: ‘erosion maximum’ and ‘erosion rate’. The erosion maximum resembles the maximum amount of sediments that would be eroded from a sample if it would be exposed to the simulated wave actions for an infinite period of time. Erosion rate is a rate coefficient and measures the speed by which it approaches to erosion maximum. The results of our research show clear differences in the maximum amount of sediment loss depending upon the salt marsh plant species, their density and the amount of aboveground and belowground biomass. Among the three species studied, the smaller erosion maxima were obtained for Spartina anglica with a 0.01-0.02 iv volume fraction of the sample. The erosion maxima for the other two species, Scirpus maritimus and Phragmites australis, were around in same order of magnitude, 0.02-0.11 volume fraction. The presence of vegetation clearly caused the erosion maximum to reduce, with obtained erosion maxima for the densely vegetated zones in the range of 0.01-0.11, whereas the erosion maxima of the samples for the un-vegetated mudflat zone were 0.55-1.0 of the total sample volume. The erosion maxima of the salt marsh cliffs showed a clear relation with the vegetation species, the vegetation density, and the amount of dry belowground biomass present in the salt marsh substrates. The best relation was found for the amount of dry belowground biomass and the erosion maxima showing an exponential decrease with an increase of the amount of dry belowground biomass. Besides, the vegetation density also showed this type of exponential relation with erosion maximum. The grain sizes and the organic carbon content of the substrate showed linear relation with the erosion maxima only for the mudflat zones. No significant relation was found between the erosion rates and different vegetation characteristics (i.e. vegetation species, vegetation density and the amount of aboveground and belowground biomass). Additional, no relation could be found between the erosion rates and median grain size or the organic carbon content of the salt marsh substrates. However, we found that the median grain size and organic carbon content are the best predictors of erosion rates for the mudflat sediments. Overall, the erosion maxima of the salt marsh substrates are found to be significantly reduced by the presence of plants and their characteristics. Both erosion maxima and erosion rates, are influenced by median grain size and organic carbon content for mudflat locations, but similar relations could not be found for the vegetated areas because the effect of plants was dominated in these areas. Cliff erosion of salt marshes is an intrinsic natural phenomenon in coastal wetlands. Cliff erosion is important to determine whether a salt marsh will extent laterally or retreat. The results of this study show how the cliff erosion relates to the presence of vegetation and its characteristics. Among the three species used for this research, Spartina anglica induced the slowest and least severe cliff erosion, meaning a more active contribution to coastal protection. Our results show that sediment properties such as grain size and organic carbon content do not affect cliff erosion from salt marsh substrates. Nevertheless, these properties are found to be important factors for the cliff erosion in mudflat areas. Un-vegetated sediments from the mudflat with smaller median grain size and larger amounts of organic carbon content are less prone to cliff erosion. The results were compared to the available results of Feagin et al., (2009). Feagin et al., (2009) found that the presence of salt marsh plants does not significantly reduce the cliff erosion, instead it improves the soil properties that reduces the cliff erosion. Our results hardly support the findings of Feagin et al., (2009). However, the output of this research still supports the concept that the presence of salt marsh plants help to reduce erosion by binding the soil and impart in the coastal management issues.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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