Tidal bores are a natural phenomenon with high flow velocity and destructive potential. A spur dike is a widely used river regulation and embankment protection structure that minimizes erosion, promotes deposition, and protects riverbanks. However, the scouring mechanism around a spur dike under the action of a tidal bore is not fully understood, and accurately estimating the maximum scour depth has always been a challenge, limiting the construction and maintenance of spur dikes in estuarine and marine environments. This study analyzed the scouring characteristics around a spur dike induced by tidal bores via field observation and model experiments. The results show that the scours around a spur dike can be divided into dike head scour and upstream side scour. The scour depth is related to the geometric shape of the spur dike and its surrounding riverbed, hydrodynamic forces, and sediment characteristics. The least squares method obtained the fitting formulas for the depth of the scour at the dike head and upstream side. Overall, the present study indicates an agreement of the scouring characteristics around a spur dike between the field observations and laboratory experiment, and the fitting formulas can be effectively applied to engineering practices of a macro-tidal estuary.

Subterranean termite activity can increase the hydraulic conductivity and water infiltration of filling soil, and therefore affects the stability of an earth embankment and subsequent safety. As a physical barrier for sustainable termite management, NaCl-laden soil barrier (NLSB) is a promising alternative for subterranean termite control in earth embankments. This novel technology can prevent tunneling and penetration of subterranean termites into the interior of an embankment and has been widely employed for more than 20 years in Zhejiang Province, China. The efficacy and longevity of NLSB depend on the long-term presence of NaCl concentration in soil barriers. The aim of this study is to develop an understanding of water flow and salt transport in NLSB based on the two-dimensional Richards’ equation and convection dispersion equation using the HYDRUS software package. Conceptual and numerical models of NLSB are modeled using scenario analysis according to water level fluctuations, saturated hydraulic conductivity, and rainfall infiltration conditions. Furthermore, the center and spread variance of a solute mass over a 100-year period are quantified using moment analysis. As flood frequency, saturated hydraulic conductivity, and rainfall infiltration flux increase, salt desalination in NLSB significantly increases. When the rainfall infiltration flux is 1% of the annual average rainfall, the total amount of salt transport and leaching can increase by 55%. Moreover, these results facilitate better long-term sustainable management of existing sites and optimal design of future NLSBs.

Tidal flats are dynamic coastal ecosystems continually reshaped by natural processes and human activities. This study investigates the application of Empirical Orthogonal Function (EOF) analysis to the long-term profile evolution of tidal flats along the Jiansan Bend of the Qiantang River Estuary, China. By applying EOF analysis to profiles observed from 1984 to 2023, this study identifies dominant modes of variability and their spatial and temporal characteristics, offering insights into the complex sediment transport and morphological evolution processes. EOF analysis helps unravel the complex interactions between natural and anthropogenic factors shaping tidal flats, with the first three eigenfunctions accounting for over 90% of the observed variance. The first spatial eigenfunction captures the primary trend, while the subsequent two eigenfunctions reveal secondary and tertiary modes of variability. A conceptual model developed in this study elucidates the interplay between hydrodynamic forces and morphological changes, highlighting the rotation and oscillation of tidal flat profiles in response to seasonal variations in hydrological conditions. The findings emphasize the effectiveness of EOF analysis in capturing significant geomorphological processes and underscore its potential in enhancing the understanding of tidal flat dynamics, thereby informing more effective management and conservation strategies for these critical coastal environments.

Tidal gates cause adverse effects on the ecological environment through the downstream acceleration of sediment deposition. This deposition process and its quantitative analysis have become a hot topic in marine science. A large-scale physical model experiment of morphological responses to the construction of a tidal gate in an estuarine tributary was performed in this study. The sediment distribution characteristics, morphologic patterns, and further analysis of the adjacent intertidal flats were investigated, triggered by the construction of tidal gates at the mouth of the Nanxi River in Zhejiang, China. The results indicated that the flood and ebb tide velocities change dramatically after the construction of the tributary tidal gates. Large amounts of sediment from the mouth of the estuary could not be entirely carried away from the flood phase to the ebb phase, resulting in downstream deposition in the vicinity of the gates. The ebb tide caused erosion of the tidal flats’ surface in the silted intertidal zone that gradually developed into a tidal channel network. The tidal channel bifurcated in the middle–upper part of the intertidal flats through continuous headward erosion, thus promoting the landward expansions of channels. Applying a set of logistic growth models to the mean sinuosity, Hausdorff dimension, and junction number data indicated that the tidal channel development progressively tended toward a state of morphodynamic equilibrium.

Tidal bores, defined by sudden upstream surges of tidal water in estuaries, exert significant hydrodynamic forces on riverbeds, leading to complex sedimentary responses. This study examines the dynamic response and liquefaction potential of riverbeds subjected to tidal bores in macro-tidal estuaries. An analytical model, developed using the generalized Biot theory and integral transform methods, evaluates the dynamic behavior of riverbed sediments. Key factors such as permeability, saturation, and sediment properties are analyzed for their influence on momentary liquefaction. The results indicate that fine sand reduces liquefaction risk by facilitating pore water discharge, while silt soil increases sediment instability. Additionally, the study reveals that pressure gradients induced by tidal bores can trigger momentary liquefaction, with the maximum liquefaction depth predicted based on horizontal pressure gradients being five times that predicted based on vertical pressure gradients. This research highlights the critical role of sediment characteristics in riverbed stability, providing a comprehensive understanding of the interactions between tidal bores and riverbed dynamics. The findings contribute to the development of predictive models and guidelines for managing the risks of tidal bore-induced liquefaction in coastal and estuarine environments.

Using Landsat 8 OLI remote sensing images in November 2016, October 2017 and December 2017, relevant quantitative remote sensing statistical models were adopted under the ENVI software platform to invert and evaluate the chlorophyll-a (Chl-a) concentration in East Dongting Lake, China. The results show that from 2016 to 2017, the maximum concentration of Chl-a was 28 mg/m³, with an average of 16 mg/m³. The water temperature is relatively high in summer, and the photosynthesis of algae is enhanced, which is conducive to the growth and reproduction of algae. The concentration is relatively high in flood season, and the non-flood season is relatively low. In the northwest of East Dongting Lake, there is no free exchange of water with the shore, and the water in the lake has a relatively low purification capacity for pollutants; Chl-a concentration in this region is higher than that in other regions, while the water in the channel connecting South Dongting Lake has strong fluidity with low concentration. In the flood season of July 2017, the trophic state index (TSI) value of the west inshore side of the lake can reach 60-70, and there is a problem of water bloom. The TSI of non-flood season is 40-50. The degree of eutrophication has decreased recently, which should be related to the overall management of water environment.