University of Twente Student Theses


Modelling the influence of sand-mud interaction and waves on salt marsh development

Vreeken, Wouter (2015) Modelling the influence of sand-mud interaction and waves on salt marsh development.

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Abstract:Sea level rise requires innovative solutions in flood protection. Salt marshes could be such a solution because of their adaptive ecological characteristics. Salt marshes are tidally influenced ecosystems between land and sea. They attenuate waves and decrease fetch length. In addition, salt marshes are able to capture sediment and rise in elevation, making it an effective flood protection. The recent interest in salt marshes leads to questions and this thesis therefore analyses the significance of different processes in salt marsh development. This could eventually help in maintenance of current marshes but also in the construction of artificial marshes. FINEL2D is used for analysing long-term development of salt marshes. FINEL2D is a depth-average numerical model developed by Svašek Hydraulics. With the use of FINEL2D, 100 years of salt marsh development is simulated. The dimensions of the test case are based on Paulinaschor, a relative small salt marsh located in the Western Scheldt, Netherlands. First a simulation is done with sand as only sediment fraction. Then the interaction between sand and mud was implemented in the model. Sand and mud show cohesive behaviour when a certain percentage of mud is present in the soil. This cohesive behaviour leads to higher critical shear stresses. Finally plants were added. Vegetation influences flow velocities as higher plant densities cause more friction. Also growth and decay of vegetation is present in FINEL2D. The simulation with vegetation in combination with sand-mud interaction functioned as reference simulation to compare with adapting processes. Results show that the implementation of mud is essential for a salt marsh environment to arise. Mud leads to significant higher bed levels due to settling lag. Plants establish on locations where no creeks are present. Vegetation determines the creek pattern as plants obstruct flow leading to more creeks which are needed to discharge all water after high tide. Validation of results showed similarities with Paulinaschor. The height of the marsh platform and the location of the marsh edge correspond well with Paulinaschor but there are also some differences. The simulations showed a marsh edge of about 4 meters high as there is no mudflat. Paulinaschor has an edge of 2 meters high. This difference could be caused by only taking two sediment fractions in consideration. Therefore is recommended to further research the effect of implementing multiple sediment fractions. The following processes are evaluated: tidal amplitude, mud availability, critical deposition shear stress (continuous deposition), critical mud content (determines when bed is cohesive), settling velocity and maximum bottom slope. The availability of mud determines lateral expansion of the marsh, while tidal amplitudes influence the expansion of the marsh but also the height. Because of high critical shear stresses sediment is able to constantly deposit and this results in an extended marsh towards the channel. The effect of a lower critical mud content was minimal, however, also this simulation showed a slight expansion compared to the reference simulation. When a low settling velocity for mud was implemented, mud was not able to settle and a small salt marsh arose. Nevertheless, the simulation with low settling velocities was the only simulation which shows higher mud percentages with distance from the channel. With a maximum bottom slope is meant that when a certain slope is reached, erosion occurs according to avalanche formulations. In this simulation a maximum bottom slope of 0.5 is applied. This results in erosion of the marsh edge. All simulation show establishment of plants over the entire marsh platform except for locations where creeks are present. Subsequently one-day storm events are modelled on the bed level, mud content and plant density obtained from the reference simulation. These storm events caused erosion of the marsh edge. Plants were not able to survive the storm. The effect of waves on plant establishment and decay is not extensively studied yet, and therefore this result cannot be validated.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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