بررسي انتقال رسوب و تغييرات ريخت‌شناسي در ناحيه نزديك ساحل

Investigation of Sediment Transport an‎d Morphological Changes in the Coastal Nearshore Zone

مهندسي عمران

Sediment transport an‎d seabed morphology evolution in the nearshore zone are among the most significant topics in coastal engineering. To improve the accuracy of predicting sediment transport rates an‎d, consequently, shoreline morphological evolution, it is essential to identify an‎d quantify the detailed characteristics of wave motion. In process-based models used for estimating sediment transport, which utilize wave orbital velocity parameters, the accurate estimation of the magnitude, duration, an‎d direction of shoreward (uprush) an‎d seaward (backwash) velocities is crucial for determining sediment transport an‎d morphological changes in the nearshore zone. In this thesis, the characteristics of wave orbital velocities in the nearshore zone are investigated by analyzing measured data from a large-scale flume an‎d through numerical modeling using OpenFOAM software. The orbital velocity features of both regular an‎d irregular wave trains—including maximum velocity values an‎d the duration of shoreward an‎d seaward phases, which significantly influence sediment displacement—have been examined. Analysis of the experimental data indicates that the ratio of maximum shoreward velocity to maximum seaward velocity (Uc/Ut) generally decreases as the waves approach the shoreline. This ratio is greater than one prior to wave breaking an‎d becomes less than one after breaking. The duration of the shoreward velocity phase (Tc) remains relatively constant across various measurement locations in the nearshore zone. The duration of the seaward velocity phase (Tt) is also nearly constant until the breaking zone, where it increases. In the numerical simulations of wave propagation in the nearshore zone, both the RANS an‎d LES turbulence models demonstrated acceptable accuracy in estimating the water surface elevation. However, the LES model provided superior accuracy in predicting the mean, minimum, an‎d maximum velocities, as well as the vertical velocity distribution. The confidence level of the LES numerical model was 95% for regular waves an‎d 78% for irregular waves. Based on the analysis of experimental data an‎d numerical simulations, a new empirical relationship for calculating wave orbital velocities in the nearshore zone was derived. This relationship provides a more accurate estimation of velocity characteristics compared to previous models. Subsequently, established an‎d commonly used models for estimating sediment transport potential in the nearshore zone were compared. The accuracy of sediment transport rate estimation was improved by modifying the value of the critical Shields parameter during the seaward half-cycle. This improvement is likely influenced by the enhanced effect of pore water pressure on suspending sediment grains during the wave backwash. Applying a correction factor of 0.17 to the critical Shields parameter in the backwash phase reduced the error in measured sediment volume by up to 70% across different sediment transport calculation methods. Using this correction, the Bagnold model provided a better estimation of sediment transport than more complex models. This comparison clearly highlights the importance of accurately estimating velocity details an‎d the varying sensitivity of different sediment transport formulae to the precision of wave orbital velocity input. Finally, by discretizing the beach profile, solving the continuity equation, an‎d utilizing the modified sediment transport formulae an‎d velocity data, temporal changes in the beach profile were computed. The Van der A (with the lowest error of 0.76) an‎d Bagnold (error of 0.81) rela beach profiles. Furthermore, the temporal sequence

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