Downdrift shoreline recession associated with the construction of a shore-crossing hard structure represents one of coastal erosional hotspots that must be addressed for an integrated, sustainable coastal zone management. To prevent siltation within the navigation channel, two rubber-mounted jetties were installed at the Sheyang River mouth on the open mesotidal muddy coast in Jiangsu province, China, in October 2013. The north jetty is 7.9 km long, while the south jetty is 7.8 km long. The net longshore sediment transport is from the north to the south due to flood-tide dominance. As disclosed by high-resolution satellite images, a 36-km-long downdrift shoreline stretch had experienced remarkable retreats at alongshore varying rates by March 2019. The eroding shoreline planform does not resemble a classic “S” shape, a crescentic shape, or a parabolic shape but an irregularly indented curved shape. Transect topographic survey also reveals an almost immediate response of the downdrift coast from the original accretionary scenery to an erosional regime, with the erosion front translocating downcoast at a much faster speed than a normal speed of 1–1.5 km/yr. Using FVCOM and SWAN, 2DH process-based numerical simulations are performed to simulate the flow, the sediment transport, and the yearly-magnitude accretion/erosion distribution in the jetty-affected area by a representative tidal force and an annual-magnitude wave force. The results demonstrate that the reciprocal tidal flow is predominantly responsible for the muddy sediment accretions at downdrift intertidal and surf zones shallower than a 4.0-m isobath, whereas big wind waves play a decisive role in triggering and developing the downdrift erosional process. The predicted spatial extent of the downdrift erosional segment matches closely the actual eroding front. The loss of the net annual longshore sediment transport volume, i.e., 3.08 million m³ due to the blockage by the twin jetties is recovered from a much larger spatial extent than the 36-km-long retreating shoreline stretch. With regard to the Bruun model, the one-line model, the headland-bay model, and the 2DH numerical model, the potential maximum recession length and the planform shape of the downdrift erosional shoreline arc are further elaborated to gain new insights into the spatial and temporal impact of a hard structure on the adjacent shoreline and flat (beach).

The processes of sediment particle movement were studied through numerical simulation using a coupled method with focus on discussing the characteristics of sliding or rolling and saltation sediment particles, respectively. Turbulent flow was simulated using large eddy simulation (LES). The sediment particle was simulated using the combined finite-discrete element method (FDEM). The interaction forces of turbulent flow and sediment particle were calculated using the immersed boundary method (IBM). It indicated that the collisions of saltating particle with low concentration increase the saltation length and flight time. In response, sediment particle velocity also increases. The particle angular velocity is largest at the takeoff moment, and decreases gradually in the saltation progress. The drag and lift forces near the bed are large, and away from the bed decrease and trend to be a stable value, gradually. From the relative magnitudes of the drag and lift forces, the lift force plays a more important role than the drag force in the sediment saltation. The relative magnitudes of drag and lift forces influence the incident and takeoff angles. The sediment transport rate calculated based on the characteristics of saltation sediment particles is overestimated, ignoring the effect of sliding or rolling sediment particles and inter-particle collisions.

Sediment deposition in the reservoir of run-of-the-river power station is severe, in this paper we take upper Marsyangdi reservoir as an example to analyze sedimentation and desilting process according field data measured from September 2016 when the reservoir had just been impounded in order to find strategy for managing reservoir sedimentation. The ratio of Upper Marsyangdi reservoir capacity and volume of sediment into the reservoir is about 0.2. The reservoir arrived silt-stable in a year after impoundment with a depth of 12m deposition at the dam site. Most of sediment deposit in the periods that at the initial and the end of flood reason and it is found the flow rate is the key factor influencing trap efficiency because that due to damming velocity of medium flow decreased significantly compared to natural condition which caused numerous deposition. Based on result of analysis of deposition the desilting condition is determined. Empty flushing is proposed to release deposition after flood season when flow rate is greater than 100m³/s and the new capacity will last to next flood season. In order to reduce sediment concentration into diversion channel a desilting should be done in flood season when flow rate is larger than 200m³/s and flow rate for impound should not be more than 1/10 of that into reservoir which can avoid deposition during impoundment near dam site.

The bank-full discharge along the Dongbatou-Aishan river reach of the Lower Yellow River is significantly lower than that of the upper reach and is the flow rate constraint reach that dictates the flood safety of the Lower Yellow River. The water and sediment matching relationship is the important factor that affects scour and siltation along this reach. It is necessary to control the relationship between water and sediment by retaining a portion of sediments in the upper reach in order to increase the overflow capacity in this reach. We first analyse the relationship between the water and sediment processes of the Lower Yellow River and the scouring and siltation of the constraint reach and verify the water and sediment conditions that require sediment retention in the upper reach. By analysing the characteristics of the river, we determined the area of sediment retention to be the reach that stretches from Xiaolangdi to Huayuankou. The relationship between the incoming sediment coefficient at Huayuankou Station and the percentage of sediment discharged and deposited in the lower reach is analysed. The results showed that when the incoming sediment coefficient is more than 0.04 kg.s/m⁶, sediment should be retain in the Xiaolangdi-Huayuankou river reach to maintain non-siltation in the constraint reach. Using bank-full discharge in the river reach from Xiaolangdi to Huayuankou and the channel stability as constraints we propose that the sediment retention in this river reach should be less than 0.144 billion m³.

Tidal-flat changes during typhoon events are controlled by compound interactions among waves, tides, runoff, sediment transport, and vegetation resistance. However, rapid prediction remains challenging because high-resolution process-based morphodynamic models are computationally expensive. This study developed an observation-supported coastal morphodynamic emulator for rapid prediction of typhoon-induced tidal-flat erosion and deposition in the Jiuduansha Wetland, Yangtze Estuary. Multi-source field observations collected during Typhoons Bebinca and Pulasan in September 2024 were first used to validate a coupled MIKE21 FM model. The validated model was then applied to generate hydrodynamic and morphodynamic samples for emulator training and testing. Generalized Lagrangian mean velocity and bottom shear stress were selected as physically meaningful inputs. Current-timestep bed-level change was predicted using a UNet model enhanced with the Convolutional Block Attention Module (CBAM), hereafter referred to as CBAM-UNet. The numerical model reproduced the observed processes with acceptable accuracy, with Skill values of 0.98–0.99 for water level, 0.83–0.84 for wave height, and 0.82 for suspended sediment concentration. Compared with the conventional UNet, CBAM-UNet reduced the final cumulative RMSE from approximately 17.2 mm to 8.8 mm, corresponding to an error reduction of about 49%. Under prescribed wave, runoff, and tidal perturbations, the emulator reproduced the main erosion–deposition patterns, with final cumulative RMSE values of approximately 6.67 mm, 5.43 mm, and 10.68 mm, respectively. Across validation cases, replacing the morphodynamic module with the emulator reduced the average runtime by 42.50%. These results indicate that observation-supported morphodynamic emulation can support rapid tidal-flat assessment under compound typhoon forcing.