Salt marsh wetlands can serve as an efficient buffer against storm waves. However, accurate quantification of the wave attenuation process is challenging. Previous modeling approaches generally ignored the posture of flexible vegetation and the impact of wave-induced vertical velocity, which may lead to underestimated wave attenuation. To address this issue, we developed a new hybrid model framework for wave attenuation by flexible vegetation. This framework incorporates a machine learning predictor for vegetation posture via a bending coefficient and a modified semi-analytical model that accounts for wave attenuation by bent rigid vegetation. This model framework is compared with our own experimental data and additional data from literature. The model shows that flexible vegetation can induce up to 21 % greater wave attenuation than rigid vegetation in near-emergent testing conditions due to the contribution of wave-induced vertical velocity. Our model framework can accurately reproduce such a contribution (up to 60 % of the total wave energy), and avoid underestimation of attenuation (R2 = 0.96). This study develops a novel tool integrating mechanical analysis and data-driven approaches to quantify wave attenuation by flexible vegetation, enabling the assessment of coastal protection by salt marshes and seagrass wetlands.

Recent remote sensing analysis has revealed extensive loss of tidal flats, yet the mechanisms driving these large-scale changes remain unclear. Here we show the spatiotemporal variations of 2,538 tidal flat transects across China to elucidate how their morphological features vary with external factors, including suspended sediment concentration (SSC), tidal range, and wave height. We observe a correlation between flat width and SSC distribution, and between flat slope and tidal range. A nation-wide decline in flat width is observed together with SSC reduction between 2002 and 2016. Intriguingly, sediment-rich flats exhibit more rapid width losses if SSC reduces, but slower width gain if SSC increase compared to sediment-starved flats. These dynamics resemble stretched (sediment-rich) or compressed (sediment-starved) springs that tend to return to equilibrium, which can be explained by synthetic morphodynamic modeling. Similar patterns can be observed from Indonesia, the United States, and Australia, implying that the impact of sediment supply change is wide-spread and large-scale sediment allocation plan based on equilibrium concept can help preserving intertidal ecosystems.

Laboratory experiments of wave propagation over rigid and flexible vegetation fields, with the same configurations, were conducted to understand the effect of vegetation flexibility on the drag coefficient (C_D). The direct method and the least squares method (LSM), based on force and flow measurements, are applied to calculate the C_D in the experimental conditions. The formulations of both methods are extended to estimate the C_D for flexible vegetation cases. A video analysis was performed to account for the swaying motion. Typically, wave dissipation is lower for flexible than for rigid vegetation of the same configuration, under the same flow condition. Therefore, a proportional effect in the corresponding C_D results, obtained from common C_D calibration to wave dissipation without considering vegetation motion, is usually observed. However, the present results show that although the wave dissipation was 34% lower for flexible relative to rigid vegetation, the respective C_D values were close. C_D estimations considering vegetation motion and inertia suggest that C_D of flexible vegetation was up to 13% higher relative to rigid vegetation. Accounting for inertia reduced the C_D for rigid vegetation up to 7%, while raised the C_D for flexible vegetation up to 13%.

Tidal flats are shrinking in extent globally. The dynamics of the response of estuarine tidal flats to global environmental changes remain unclear. Tidal-flat morphology is shaped by the interplay among wave and tidal forces, river discharge and sediment supply, and preservation of tidal flats requires a balance between erosional and depositional processes be maintained. Here we assess tidal-flat morphodynamic changes of 40 globally distributed estuaries with contrasting tidal amplitudes between 1986 and 2011 from analyses of 4,939 satellite images. We consider both vegetated and unvegetated intertidal areas. From comparisons with remote-sensing-derived turbidity estimates, we identify a critical turbidity threshold indicative of a minimum sediment supply along with the hydrodynamic forces, which is necessary to maintain tidal flats. Tidal flats in intertidal areas in estuaries with low turbidity face retreat, with the critical turbidity threshold increasing with increasing tidal amplitudes. By contrast, estuaries with high turbidity tend to exhibit laterally or vertically expanding tidal flats. However, despite estuaries with limited tidal ranges having relatively low turbidity thresholds, environmental or anthropogenic alterations can still adversely affect the morphology of tidal flats. Our findings demonstrate the need to consider sediment supply in integrated estuarine management strategies to maintain the ecological integrity and flood defence function of tidal flats.

The authors regret that there was a typo in Equation (14b).