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Estuarine salt intrusion greatly threatens freshwater supply in surrounding lands. Physical barriers, which reduce the estuary length (L), are widely constructed to control salt intrusion. Yet, the role of L in salt intrusion remains unknown. Using a process‐based, idealized, semi‐analytical three‐dimensional model, we systematically investigate for the first time this unknown for tide‐dominated, periodically weakly stratified estuaries. Results show decreasing L significantly reduces salinities for short estuaries (L < Lw/4, with Lw the dominant tidal wavelength), but not for long estuaries. Tidal pumping remains a key salt importer in most estuaries, regardless of L. However, substantial decreases in L relative to Lw/4 can change the dominant landward salt importer from tidal pumping to horizontal diffusion. The latter, together with gravitational circulation, weakens responses of salt intrusion to changes in tidal and river forcing in short estuaries. This study highlights the importance of considering L to understanding and mitigating salt intrusion. Plain Language Summary: Salty seawater can enter rivers through estuaries, where rivers meet the sea, thus affecting the surrounding freshwater supply and the healthy functioning of estuarine ecosystems. To mitigate the adverse impact of salt intrusion under aggravating climate change and human activities, expensive salinity barriers are commonly built around the world. These barriers result in an abrupt change in the estuary length and can substantially affect estuarine salt intrusion. This study uses a simplified computer model to understand the dependency of salt intrusion on estuary length in estuaries with small top‐to‐bottom salinity differences. Our results show that estuary length substantially influences the salinity distribution and estuaries of different lengths can respond to changes in tides and river flow contrastingly. Our study highlights the importance of considering the impact of estuary length to effectively mitigate estuarine salt intrusion. Key Points: Estuary length strongly affects tidal propagation, residual circulation, and salt intrusion in periodically weakly stratified estuariesThe dominant salt balance may change when substantially reducing estuary lengthSteady lateral shear dispersion can reduce responses of salt intrusion to forcing changes in short estuaries with weak stratification [ABSTRACT FROM AUTHOR]

Asymmetric tidal turbulence (ATT) strongly influences estuarine health and functioning. However, its impact on the three-dimensional estuarine dynamics and the feedback of water motion and salinity distribution on ATT remain poorly understood, especially for short estuaries (estuarine length « tidal wavelength). This study systematically investigates the abovementioned interactions in a short estuary for the first time, considering periodically weakly stratified conditions. This is done by developing a three-dimensional semi-analytical model (combining perturbation method with finite element method) that allows a dissection of the contributions of different processes to ATT, estuarine circulation, and salt transport. The generation of ATT is dominated by (i) strain-induced periodic stratification and (ii) asymmetric bottom-shear-generated turbulence, and their contributions to ATT are different both in amplitude and phase. The magnitude of the residual circulation related to ATT and the eddy viscosity--shear covariance (ESCO) is about half of that of the gravitational circulation (GC) and shows a ''reversed'' pattern as compared to GC. ATT generated by strain-induced periodic stratification contributes to an ESCO circulation with a spatial structure similar to GC. This circulation reduces the longitudinal salinity gradients and thus weakens GC. Contrastingly, the ESCO circulation due to asymmetric bottom-shear-generated turbulence shows patterns opposite to GC and acts to enhance GC. Concerning the salinity dynamics at steady state, GC and tidal pumping are equally important to salt import, whereas ESCO circulation yields a significant seaward salt transport. These findings highlight the importance of identifying the sources of ATT to understand its impact on estuarine circulation and salt distribution. [ABSTRACT FROM AUTHOR]

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments. [ABSTRACT FROM AUTHOR]