Coastal permeable groins have been used to protect beaches from erosion for centuries. However, the hydraulic functioning of permeable groins has not been fully understood and their design heavily depends on engineering experiences. In this study, numerical experiments were executed to investigate the effects of layout configurations of a permeable groin system on longshore currents. The non-hydrostatic SWASH (Simulating WAve till SHore) model was employed to carry out the numerical simulations. Two data sets obtained from physical laboratory experiments with different permeable groin layouts on different slopes are used to validate the accuracy of the model. Then, the longshore current reduction by the permeable groin system with varying configuration parameters (e.g., groin spacing, groin length) was numerically investigated under different environmental conditions (e.g., a slight or a moderate wave climate). From the calculation results of numerical experiments, it is indicated that permeable groins function efficiently to reduce the maximal longshore current velocity under the condition that the groin length ranges from 84% and 109% of the wave breaker zone width. The longshore current reduction rate monotonously decreases with the increase in groin spacing; permeable pile groin functions best to reduce longshore current with the minimal groin spacing-groin length ratio 1:1 among the range between 1:1 and 2:1. When the groin spacing–groin length ratios are 1:1 and 1.5:1, the longshore current reduction is not sensitive to the investigated wave conditions in this study. When the spatial ratio is 2:1, the permeable pile groin system functions worse under a moderate wave climate than under a slight wave climate, from the view of longshore current reduction.@en

Mangroves can function as a ‘bio-shield’ to protect coastal communities from harsh environments because of their strong ability to attenuate wave energy. However, as mangroves are usually oversimplified as rigid cylinders in antecedent studies, the effects of complex mangrove morphology on wave attenuation have not been well researched. Although increasing attention has been paid to the wave dissipation induced by varying mangrove morphologies, most of them focus on the bottom trunk and root components of mature mangrove trees. There are few investigations about the contributions of the canopies of young saplings and/or short species to wave attenuation. To bridge this knowledge gap, a series of laboratory experiments under regular waves were conducted to examine the hydrodynamic variations affected by varying mangrove morphology configurations. Three water depths were considered to explore the influences of the vertical-varying submerged volume of mangroves when the artificial mangrove models are submerged, nearly emergent, and fully emergent. The mangrove forest model is 2 m long at a 1:10 scale. Three mangrove configurations, i.e. with no canopy, sparse canopy, and dense canopy were applied and compared to isolate the wave attenuation contributed by mangrove canopies. The results highlight the wave energy attenuation attributed to the canopy density. A linear correlation is found between the wave damping factor and a new variable named hydraulic submerged volume index (HSVI). The bulk drag coefficient, including canopy effects, was calculated to characterize mangrove-induced wave attenuation when the mangrove canopy is submerged. The relationships between the bulk drag coefficient C_D and the characteristic hydraulic numbers (i.e., Reynolds number, Keulegan–Carpenter number, Ursell number) are discussed in detail. Consequently, new generic formulas of C_D were deduced considering the effects of the submerged canopy. The employment of new C_D formulas improves the reliability of the prediction of the wave attenuation ability by mangroves since the canopy effects are incorporated.

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In the present study a new multi-fractional, depth-averaged sediment transport module was developed and embedded into a morphodynamic model for a sand-silt mixed shallow water environment. Subsequently, the model was applied to the case of the Jiangsu coast, which features a silt enriched sedimentary environment bordered by two large-scale geomorphological units: the Old Yellow River Delta (OYRD) in the north and the Radial Sand Ridge Field (RSRF) in the south. Based on this case, the predictive abilities of the present model are
assessed on both the short-term and the long-term. Comparisons with measurements over two successive tidal cycles indicate that the present model produces very good results on short-time scales. The model performance is
extended and further validated by comparing the overall annual Suspended Sediment Concentration (SSC) pattern, the annual morphological changes, the annual sediment budget and the evolution trend of the bed composition. Also, these long-term results agree well with existing observations over the past several decades. Hence, an essential feature of the present modelling approach is the ability to simulate sediment transport and morphological changes over a relatively long time span (i.e., time scale of years) in a sand-silt mixed sedimentary environment, based on its validated short-term performance.@en

The Abandoned Yellow River Delta (AYD), which formed when the Yellow River flowed into the Southern Yellow Sea between 1128 and 1855 AD, is a representative example of the sensitivity of deltas to modifications in their environments. In this study, we established a process-based morphodynamic model to explore the morphological evolution of one such largescale fine-grained delta (the AYD before 1855). The uncertainties in the model settings, which are inevitable when historical data are insufficient, were assessed together with the corresponding influences on the evolution of the deltaic system by considering a series of scenarios. The results indicate that the strength of local tidal forcing is the key factor that determines the shape and evolutionary trend of the delta. Sediment input discharge and the slope of the initial coastal profile have a considerable effect on the overall size of the delta and the relative ratio between subaerial and subaqueous parts of the delta, respectively. Based on the evaluation of the uncertainties and a comparison with historical maps, the simulated AYD was evaluated to be reliable. Through an analysis of the temporal delta evolution and residual sediment transport, the morphological evolution of the AYD before 1855 AD was investigated. The southern delta grew as the shoals merged with the mainland, which is in agreement with an existing hypothesis (Zhang, 1984), whereas the accretion of the northern delta was independent from the shoals in the northern part. Additionally, suggestions are made regarding the distribution of the AYD at the end of its progradation stage, which provides fundamental information for analyzing subsequent erosion processes since 1855 AD. This study differs from existing studies on the AYD, which are all based on geological approaches. It provides insight into the evolution of the AYD through an alternative means, viz. a process-based morphodynamic-modeling approach. @en

The effective roughness height is an important parameter in coastal sediment transport models. It has been extensively investigated in the past but few research results are related to the high-concentrated flows which often occur in a silty coast. A series of experiments has been carried out in a wave-current flume with silt-sized sediment bed. The mean velocity profiles were measured under different combined wave-current conditions. The effective roughness heights were calculated based on the curve fitting of measured velocity profiles by following the velocity profile model of You (1994). The accuracy of three empirical models, namely, Grant and Madsen (1982), Li and Amos (1998) and You (1996) was examined with the 'measured' effective roughness heights. The results show that all the models are not much accurate for the high-concentrated flows, particularly in the case with a relatively small sediment size. Therefore, cautions should be taken when applying those models in the silty coast, particularly during the extreme events.@en

In this study, we aim to investigate the overall morphological evolution of the Jiangsu coast after 1855, when the Yellow River shifted northward. We focus on fine sediment transport between two large-scale geomorphological units, i.e., the Abandoned Yellow River Delta (AYD) and the Radial Sand Ridges (RSRs). An existing morphodynamic model, which was established for reproducing the development of the AYD before 1855, is modified and extended. In addition to the tidal forcing, waves and human interventions (i.e., revetments) are considered in the model. The model results are compared with the existing data. Both the evolution trend of the Jiangsu coast and the spatial distribution of the offshore shoals show good agreement. The simulated fine sediment depositions in different periods are consistent with the geological measurements. The results reveal that the old Yellow River-derived sediment not only contributes to the sedimentation in the RSRs but can also be transported to the adjacent zones, especially farther south/southeast. Moreover, the spatial distribution of fine sediment deposits varies in the RSRs. The different sedimentary environments in the Dongsha and Tiaozini ridges result in significant grain size differences in these two neighboring ridges. A sensitivity analysis indicates that tides play a key role in dominating the long-term morphological evolution of the Jiangsu coast and the total erosion from the AYD. On smaller scales, the effect of revetments (built since the 1930s) on the evolution of the nearshore zone and the effect of wind waves on the erosion of offshore shoals are relatively important. The effect of a gradual coarsening process of bottom sediment along the Jiangsu coast, which may be due to continuous fine sediment removal, is identified. Fine sediment depositions in the Tiaozini ridge and in the northern offshore zone of the RSRs are relatively more sensitive to the coarsening trend of bottom sediment than other areas. @en

The Optical Backscatter Sensor (OBS) has been widely used to measure suspended sediment concentration in both field and laboratory conditions, even though it is very sensitive to many factors. The most significant factor suggested is the grain size. In order to enhance the quality of OBS data, an improved approach is proposed, based on the "mixture of linear component response" method (Green and Boon III, 1993) to account for the effect of grain size. In addition to an original sediment sample, which commonly serves as a single calibration material, an accompanying sediment sample is necessary to calibrate OBS sensors. A multi-fraction sediment model is used to predict the grain size distribution in suspension. Compared with existing methods, the improved approach does not require a sieving procedure nor the assumption that the sediment fractions of the calibrated sediment sample exhibit the same sensitivities as those of the suspended sediments. The applicability of our method has been verified by a series of laboratory experiments over silt-sand mixtures. The results show that this method successfully yields continuous concentration profiles, which agree well with the measurements using a suction method. The converted concentrations of the time-averaged OBS measurements by traditional method and by the improved method, respectively, are compared with the suction measurements. The results of linear regression analyses show that the coefficient of determination increases (e.g. from 0.55 to 0.92 for wave-current conditions) and the Root Mean Square Error decreases (e.g. from 0.97 to 0.39 for wave-current conditions). It demonstrates that improved method enhances the quality of OBS conversion. Furthermore, suggestions on selecting the accompanying sediment sample (i.e. on grain size and composition) are given.@en