River longitudinal profile, a key morphological characteristic of the river channel, is subject to river mouth progradation. Given the increasing influence of human activities and climate change on this critical downstream control, understanding its effects on the evolution of the longitudinal profile is imperative. A general theoretical framework is proposed to quantify the relevant effects, which is tested by numerical experiment and compared with field, numerical and laboratory data from the literature. The results suggest the existence of a critical ratio of accommodation space to sediment supply of approximately 0.5, above which the typical concave upward profile tends to form. Further analyses show that sea level rise tends to increase the concavity of the longitudinal profile of a river with a relatively low equilibrium bed slope and progradation rate.@en

It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion
timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for
engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.@en

Rivers, regardless of their scales and geographic locations, are characterized with natural and human-induced variability in their discharges. While previous studies have established the effects of both interannual and intra-annual variabilities of unsteady river discharge on delta morphological evolution, the long-term cumulative effects of intra-annual unsteadiness on the progradation of delta lobes has remained hitherto elusive. To address this issue, numerical experiments using simplified unsteady discharges were performed in Delft3D and compared with those assuming constant bank-full discharges. A modified box model was further used to explore the effects of varying intra-annual unsteadiness on the progradation of delta lobes at reduced computational cost. While the overall trends of the progradation and the ultimate area created were found to be similar between the unsteady discharge scenarios and their corresponding constant bank-full discharge scenarios, the nuances of intermittent zig-zag variation in natural delta lobe area were well reproduced by model simulations assuming unsteady river discharges. In addition, long-term predictions suggested the potential existence of a tipping point in the area growth trajectory beyond which the delta lobe area declines during periods of low discharge. When confounding factors such as waves and variable sediment capture ratio were further taken into consideration, simulation results for unsteady river discharge scenarios exhibit significant deviations from constant bank-full discharge scenarios. The implications of the modeling results for delta protection and restoration measures, such as the water-sediment regulation scheme in the Yellow River and artificial channel diversions in the Mississippi River Delta, are also discussed.@en

• Background and Aims Ecosystem-based flood defence including salt-marsh as a key component is increasinglyapplied worldwide due to its multifunctionality and cost-effectiveness. While numerous experiments haveexplored the wave-attenuation effects of salt-marsh plants critical to flood protection, little is known about thephysiological and biochemical responses of these species to continuous wave exposure.
• Methods To address this knowledge gap, we developed a shallow-water wave simulator to expose individualSpartina alterniflora plants to waves in a greenhouse for 8 weeks. S. alterniflora individuals were partially submergedand experienced horizontal sinusoidal motion to mimic plant exposure to shallow water waves. A factorialexperiment was used to test the effects of three wave heights (4.1 cm, 5.5 cm and a no-wave control) and two waveperiods (2 s and 3 s) on the following key physiological and biochemical plant parameters: plant growth, antioxidantdefence and photosynthetic capacity.
• Key Results Comparison of wave treatment and control groups supported our hypotheses that wave exposureleads to oxidative stress in plants and suppresses plant photosynthetic capacity and thereby growth. In response,the wave-exposed plants exhibited activated antioxidant enzymes. Comparison between the different wave treatmentgroups suggested the wave effects to be generally correlated positively with wave height and negativelywith wave period, i.e. waves with greater height and frequency imposed more stress on plants. In addition, waveexposedplants tended to allocate more biomass to their roots. Such allocation is favourable because it enhancesroot anchorage against the wave impact.
• Conclusions Simulated wave exposure systems such as the one used here are an effective tool for studying theresponse of salt-marsh plants to long-term wave exposure, and so help inform ecosystem-based flood defence projectsin terms of plant selection, suitable transplantation locations and timing, etc. Given the projected variabilityof the global wave environment due to climate change, understanding plant response to long-term wave exposurehas important implications for salt-marsh conservation and its central role in natural flood defence.@en

River mouth bar formation, a key process in fluvial-deltaic morphodynamics, is subject to both river discharge and waves. Given the increasing variability of both forcings under continuous climate change and human interventions, assessing their combined effects on mouth bar formatio n is an imperative issue. In this study, an extensive set of combined high and low river flows coupled with varying wave conditions and sediment grain sizes was assumed for numerical experiments conducted in Delft3D-SWAN. The results suggested that three regimes existed for mouth bar formation, namely, stable, ephemeral, and absent. These regimes corresponded to consistently weak, initially-weak-then-strong, and initially strong relative wave strengths, respectively, during the onset and reworking stages. Suppression of mouth bar formation further led to the inhibition of deltaic distributary networks. These findings have important implications for water and sediment management strategies, such as water diversion and dam regulation, in estuaries and deltas to prevent coastal erosion.@en