Tidal flats are accumulations of fine-grained sediment formed under the action of tides and play a very important role in coastal protection. The northern part of Jiangsu coast, as a typical example of muddy coasts found all over the world, has experienced serious erosion since the Yellow River shifted northward, and the range of erosion has been gradually extending southward, now reaching the south of the Sheyang River estuary (SYRE). In order to address coastal erosion near the SYRE through protective measures, there is an urgent need for research on the spatial and temporal variation of coastal erosion processes and their control mechanisms in the SYRE and adjacent coastal areas. For this study, the tidal flats on the south side of the SYRE were selected as the study area, and the sediment dynamics in the upper and lower intertidal flat were observed in different seasons to investigate the erosion processes and their dynamic mechanisms. The results show that the tidal current and wave action in the observed intertidal flats are stronger in winter than in summer, and these intertidal flats erode under the combined action of waves and currents. During winter, the net transport of the near-bottom suspended sediment and bedload is primarily towards the southeast, while in summer, the direction tends toward the north and northeast. The net transport fluxes are larger in the lower part of the intertidal flat than in the upper part in summer and also larger in winter than in summer within the lower intertidal flat. Furthermore, the tidal flat erosion in the study area manifests as shoreline retreat and flat surface erosion. The average shoreline retreat rate increased from 23.3 m/a during 2014–2019 to 43.5 m/a during 2019–2021, and the average erosion depth of the lower and upper parts of the intertidal flat over a tidal cycle is, respectively, 1.98 cm and 0.24 cm in winter and 1.65 cm and 0.26 cm in summer. The ratio of the wave-induced bottom shear stress to the tidal current-induced bottom shear stress is 0.40~0.46 in the lower intertidal flat and increases to 0.66~0.67 in the upper intertidal flat, indicating that the intertidal flat erosion in the study area is primarily driven by tidal currents, with significant contributions from wave action, especially in the upper intertidal flat.

Under the combined pressures of natural variability and human activities, the area of tidal flats has been gradually decreasing, with most muddy coasts experiencing varying degrees of erosion. The central coast of Jiangsu Province, a world-renowned region for extensive tidal flats, has witnessed intensifying erosion of its muddy coasts in recent years. To mitigate further coastal erosion, an ecological submerged breakwater (ESB) was constructed in the intertidal zone north of the Sheyang River estuary to reduce wave impact on the shoreline. This study evaluates the wave attenuation performance of the ESB based on wave observations conducted at stations deployed on the seaward and landward sides of the structure in May 2025. Results indicate that the breakwater effectively reduces wave height, but its performance exhibits significant dynamic characteristics. During the observation period, the maximum attenuation rate for significant wave height (H₁/₃) reached 76.3%, with an average rate of 33.8%. Wave dissipation efficiency was closely related to sea state: under calm conditions (H₁/₃ < 0.4 m), the average attenuation rate was only 18.4%, whereas under severe sea states (H₁/₃ ≥ 0.4 m), it increased markedly to 57.6%. The wave transmission coefficients (K_t) span a wide range from 0.20 to 0.99, indicating a significant dynamic variability in the wave attenuation performance of the ESB. The performance of the ESB was primarily controlled by two key factors: incident wave height and submergence depth of the structure. Compared to “zonated” natural ecosystems such as oyster reefs, coral reefs, salt marshes, and mangroves, the ESB, as a “linear” engineered structure, achieves comparable wave attenuation within a limited spatial footprint. A promising future strategy involves using the ESB as a frontline defense, integrated with landward ecological restoration measures like salt marsh rehabilitation, to establish a hybrid “grey-green” coastal protection system that synergistically enhances both coastal resilience and ecological function. This study provides a scientific basis for the design and performance evaluation of ecological engineering solutions for protecting eroding muddy coasts.

Tidal flats are shrinking and eroding due to sea-level rise and human activities. Ecological submerged breakwaters (ESBs) offer a novel solution combining coastal protection and ecological restoration, but their effects on sediment dynamics lack field evidence. This study presents synchronous in situ measurements from an inner tidal flat (WN01) and an outer shallow area (WN02) of a newly built riprap slope-type ESB on the northern coast of the Sheyang River Estuary, Jiangsu, China. Using Acoustic Doppler Velocimeters (ADVs) and wave-tide gauges, we examined hydrodynamics, suspended sediment concentration (SSC), bed shear stress, erosion–accretion, and sediment transport under normal-weather and strong wave events. Within the constraints of a 14-day observation at two stations, our results indicate that: (1) The ESB reduced wave height and weakened currents, shifting dominant bed shear stress from wave-dominated outside to tide-dominated inside. Under normal weather, both sides were accretive. (2) Strong wave events caused sharp increases in bed shear stress, net erosion on both sides, and a 2–3-fold SSC rise, breaking the normal balance. (3) Suspended sediment transport direction remained northwest inside during strong wave events but shifted to northeast/southeast outside, demonstrating effective isolation of wave-driven anomalies. Bedload was trapped inside, resulting in no net sediment loss, in contrast to the unprotected southern tidal flat. (4) We recommend moderately lowering the ESB crest elevation to prevent excessive accretion and implementing “grey-green” restoration (salt marshes or oyster reefs) to enhance coastal resilience against sea-level rise.