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Accelerated morphological modelling : a schematized case study into the medium- and long-term morphological acceleration techniques Morfac and Mormerge

Wilmink, R.J.A. (2015) Accelerated morphological modelling : a schematized case study into the medium- and long-term morphological acceleration techniques Morfac and Mormerge.

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Abstract:In simulating the long-term morphology of a natural coastal and offshore system, the most restrictive element is the available computational capacity. Computations for long-term simulations of the morphology take a long time and a lot of computational power. To overcome this crucial disadvantage for long-term morphological simulations, input reduction for real-time measurement signals and an acceleration of morphological changes can be applied. This thesis attempts to test both approaches. Based on progressive insight in morphodynamic modelling, two techniques for accelerating the morphological changes are commonly used nowadays, i.e. Morfac and Mormerge. The performance of both methods and their possibilities and limitations for a uniform sandy coast, including a navigation channel and a harbour entrance is the subject of this study. This thesis first provides answers to introductory questions as which input conditions should be combined into ‘input scenarios’ to run morphological simulations that represent the reality and how do Morfac and Mormerge simulate the medium- and long-term morphology. Then, using a schematized Delft3D model in 2DH mode for the coastal and offshore area of IJmuiden, there has been investigated in what way a tidal signal can be reduced, how many wave classes should be included to obtain accurate results, which acceleration factor is still acceptable to achieve accurate results and what are the distinguishing elements that decide which method is most appropriate for a certain simulation. All simulations will produce a ten yearly morphodynamic development of the study area by tidal forcing (and if included, waves as well). The simulations are compared to a reference simulation. To quantify the accuracy, four performance indicators will be calculated. These performance indicators are: Nash-Sutcliffe coefficient (NS), Root Mean Square error (RMS), linear correlation coefficient (R) and the slope of the linear regression line (B). An acceptable accurate result is obtained when the NS is above 0.95, the RMS lower than 0.5, R over 0.99 and B lower than 1.05. By means of a Multi-Criteria-Analysis (MCA) a comparison will be performed between Morfac and Mormerge. The assessment criteria of the MCA are: accuracy, ease of use, applicability (physical justification of what is happening) and simulation time. The reference simulation chosen in this study is named the brute-force simulation. This brute-force simulation uses the reduced tidal input condition, a measured wave signal and an acceleration factor of ten to simulate the morphological development for ten years. Incorporating these elements for the brute-force simulation is the most time efficient and closest to reality as possible for the time limit of this study. Morfac and Mormerge both accelerate the sediment transport rate by a factor. This approach is considered legitimate because of the differences in timescales of the hydro- and morphodynamics. The difference between the techniques is in the computation of particular conditions (for example varying wave heights or directions). Morfac calculates the conditions one after another using the bathymetry of the previous condition at the start of the new condition. The acceleration factor applied is for every condition multiplied by a weight (percentage of occurrence of that particular condition). The sequence of conditions is determined randomly. Mormerge calculates these conditions parallel by weighting the bottom change of all conditions every flow time step by its percentage of occurrence of that condition. A fixed acceleration factor is applied in Mormerge. For the input reduction, a (double consecutive) morphological tide has been derived which matches the morphological development of the study area for approximately one spring-neap tidal cycle simulation as close as possible in terms of the linear correlation coefficient. This morphological tide has been made A schematized case study into the medium- and long-term morphological acceleration techniques Morfac and Mormerge harmonic to avoid shocks of the model from one tide to another and it consists of the M2, M4, M6 and M8 tidal constituents. For the long-term simulations, the harmonic morphological tide is easy to implement. The validation of the harmonic morphological tide, when focusing on water levels and flow velocities, matches the double tide of the spring-neap tidal cycle very well. The Nash-Sutcliffe coefficients averaged over 41 temporal observation points in the domain for the water levels and flow velocities are 0.98 and 0.95 respectively. The Brier Skill Score of the morphological development by the harmonic morphological tide compared to the morphological development of the simulated spring-neap tidal cycle is 0.96 (were a value of 1 is a perfect prediction with respect to the brute-force simulation). In addition, a real-time wave signal has been converted into directional and magnitudinal bins. The results of short simulations of all combinations of directional and magnitudinal bins (wave conditions) have been used to determine the sequence of importance for the morphological development of the study area by different wave conditions. This sequence is determined by the OPTI-routine which resulted in four wave scenarios (including one, two, six or ten wave classes). The long-term simulations performed showed the formation of nearshore banks, migration of the nautical channel, sedimentation around the harbour moles and a deep scour hole directly in front of the harbour moles. This study showed that for water depths of approximately > 6 m, the acceleration factor and acceleration method are not decisive for the accuracy of the results with respect to the brute-force simulation (in water depths over 6 m, the sediment transport is more tide driven). The model is accelerated to its maximum factor possible using the input reduction for the tide; at least one harmonic morphological tide has to be simulated for the result to be valid. No deviating results were obtained. The inclusion of additional wave classes (determined by the OPTI-routine) resulted in an increased model performance. At least six wave classes had to be included to obtain results within the acceptable range of accuracy. A Multi-Criteria Analysis has been performed to determine which method is most appropriate to use and what are the distinguishing elements that are decisive in choosing a particular method. Each performance criterion (accuracy, ease of use, applicability and simulation time) has a specific weight in the MCA and are ranked from + + to - -. The outcomes of the MCA are therefore a weighted average of the rankings. Accuracy is important for the choice of a particular method. However, because the small difference in accuracy when including six or ten wave classes, this performance criteria was not decisive. More decisive in choosing a particular acceleration method are the run times. For Mormerge, especially when including additional wave classes, the run times are much shorter (when enough computational capacity is available). Ease of use and applicability for a particular situation contribute only to a minor extent to the overall score, both in favour of Mormerge. Mormerge is thus more beneficial than morfac, especially when including additional wave classes (which in turn leads to a more accurate result). For the nearshore zone (water depth approximately < 6 m) no clear results in terms of accuracy of the acceleration method and the amount of wave classes to be included were obtained. For this zone additional research is recommended.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/67609
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