Abstract. Hypertidal estuaries are very dynamic environments characterised by high tidal ranges (>6 m) that can experience rapid rates of bank retreat. Whilst a large body of work on the processes, rates, patterns and factors driving bank erosion has been undertaken in fluvial environments, the process mechanics affecting the stability of the banks with respect to mass failure in hypertidal settings are not well documented. In this study, the processes and trends leading to bank failure and consequent retreat in hypertidal estuaries are treated within the context of the Severn Estuary (UK) by employing a combination of numerical models and field-based observations. Our results highlight that the periodic fluctuations in water level associated with the hypertidal environment drive regular fluctuations in the hydrostatic pressure exerted on the incipient failure surfaces that range from a confinement pressure of 0 kPa (at low tide) to ~100 kPa (at high tide). However, the relatively low transmissivity of the fine-grained banks (that are typical of estuarine environments) results in low seepage inflow/outflow velocities (~3x10-10 m s-1), such that variations in positive pore water pressures within the saturated bank are smaller, ranging between about 10 kPa (at low tide) to ~43 kPa (at high tides). This imbalance in the resisting (hydrostatic confinement) versus driving (positive pore water pressures) forces thereby drives a frequent oscillation of bank stability between stable (at high tide) and unstable states (at low tide). This transition between stability and instability is found not only on a semidiurnal basis, but also on a longer timeframe. In the spring to neaps transitional period, banks experience the coexistence of high degrees of saturation due to the high spring tides and decreasing confinement pressures favoured by the still moderately high channel water levels. This transitional period creates conditions when failures are more likely to occur.
Abstract Many large estuaries are threatened by intensifying hypoxia. However, due to the limited duration of available observations, uncertainties persist regarding the level of contemporary hypoxia intensity in a longer-term context and the relative contributions of climate versus human factors. Here we present sediment records for the hypoxia intensity and associated environmental parameters in the Yangtze Estuary over the past three centuries. The results show that the hypoxia intensity has been increasing during the last half century due to anthropogenic eutrophication, but the current hypoxia condition is not as severe as some preindustrial periods due to weaker stratification in the water column. Our findings suggest that if anthropogenic and climatic forcing coincide in the foreseeable future, the hypoxia intensity of the Yangtze Estuary may reach unprecedented levels.
Meandering rivers are complex systems that support high rates of biodiversity and the livelihoods of millions of inhabitants through their ecological services. Meandering rivers are often located in remote locations and cover long distances. As a result, observational satellites are crucial for investigating and monitoring meandering river dynamics. Satellite remote sensing technology is responsible for many advances in our knowledge about the variables that affect these rivers and their interaction with their surrounding floodplains. Furthermore, new sensors and the advent of cloud computing are allowing researchers to revisit theories that have hitherto lacked observational evidence to support them. In this paper, we review articles that have applied remote sensing techniques to analyse river meander migration processes. Our findings show that the majority of articles analysed the meandering rivers of the Ganges/Brahmaputra (29.0% of all articles) and the Amazon Basin (26.1%). We propose that these two locations are popular for different reasons: to improve management in highly populated floodplains of Ganges/Brahmaputra, and to investigate the meandering mechanisms without major anthropogenic interference in the Amazon Basin. Furthermore, most of the articles used Landsat for river monitoring (80.7%) and tracked the river changes throughout time using satellite time series (82.0%). However, the incorporation of Synthetic Aperture Radar satellites in papers was minimal, and only a small fraction (13%) of studies utilized cloud computing platforms for processing satellite images. Finally, we discuss new possibilities in terms of sensors and processing that might in the future advance our knowledge of river geomorphology.
Abstract Submarine channels are key features for the transport of flow and nutrients into deep water. Previous studies of their morphology and channel evolution have treated these systems as abiotic, and therefore assume that physical processes are solely responsible for morphological development. Here, a unique dataset is utilised that includes spatial measurements around a channel bend that hosts active sediment gravity flows. The data include flow velocity and density, alongside bed grain size and channel‐floor benthic macrofauna. Analysis of these parameters demonstrate that while physical processes control the broadest scale variations in sedimentation around and across the channel, benthic biology plays a critical role in stabilising sediment and trapping fines. This leads to much broader mixed grain sizes than would be expected from purely abiotic sedimentation, and the maintenance of sediment beds in positions where all the sediment should be actively migrating. Given that previous work has also shown that submarine channels can be biological hotspots, then the present study suggests that benthic biology probably plays a key role in channel morphology and evolution, and that these need to be considered both in the modern and when considering examples preserved in the rock record.
Abstract. In coastal regions, floods can arise through a combination of multiple drivers, including direct surface run-off, river discharge, storm surge, and waves. In this study, we analyse compound flood potential in Europe and environs caused by these four main flooding sources using state-of-the-art databases with coherent forcing (i.e. ERA5). First, we analyse the sensitivity of the compound flooding potential to several factors: (1) sampling method, (2) time window to select the concurrent event of the conditioned driver, (3) dependence metrics, and (4) wave-driven sea level definition. We observe higher correlation coefficients using annual maxima than peaks over threshold. Regarding the other factors, our results show similar spatial distributions of the compound flooding potential. Second, the dependence between the pairs of drivers using the Kendall rank correlation coefficient and the joint occurrence are synthesized for coherent patterns of compound flooding potential using a clustering technique. This quantitative multi-driver assessment not only distinguishes where overall compound flooding potential is the highest, but also discriminates which driver combinations are more likely to contribute to compound flooding. We identify that hotspots of compound flooding potential are located along the southern coast of the North Atlantic Ocean and the northern coast of the Mediterranean Sea.
The time-averaged and instantaneous flow velocity structures of flood waters are not well understood for irregular surfaces such as are created by the presence of roots and fallen branches on forested floodplains. Natural flow structures commonly depart systematically from those described for idealised roughness elements, and an important knowledge gap exists surrounding the effects of natural flow structures on vertical exchanges of fluid and momentum. An improved understanding of the flow structure is required to model flows over forested floodplains more accurately, and to distinguish their dynamics from non-vegetated floodplains or indeed floodplains with other vegetation types, such as reed or grass. Here we present a quantification of the three-dimensional structure of mean flow velocity and turbulence as measured under controlled conditions in an experimental flume using a physical reproduction of a patch of forested floodplain. The results conform in part to existing models of local flow structure over simple roughness elements in aspects such as flow separation downstream of protruding roots, flow reattachment, and the lowering of the velocity maximum further downstream. However, the irregular shape of the surface of the floodplain with exposed roots causes the three-dimensional flow structure to deviate from that anticipated based on previous studies of flows over idealised two-dimensional roughness elements. The results emphasise varied effects of inheritance of flow structures that are generated upstream-the local response of the flow to similar obstacles depends on their spatial organisation and larger-scale context. Key differences from idealised models include the absence of a fully-developed flow at any location in the test section, and various interactions of flow structures such as a reduction of flow separation due to cross-stream circulation and the diversion of the flow over and around the irregular shapes of the roots.
We present an integrated analysis of bank erosion in a high‐curvature bend of the gravel bed Cecina River (central Italy). Our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes, the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations. The results identify two mechanisms that explain how most bank retreat usually occurs after, rather than during, flood peaks. First, in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases, reducing sidewall boundary shear stress and fluvial erosion at peak flow stages. Second, bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events.
The authors analysis is an interesting and timely attempt to account for the effects of bank stability on the geometry of stable gravel rivers, as the neglect of the width dimension is a major limitation of channel morphology models. In the development of their model the authors encounter the problem that there are apparently more dependent variables than equations available for solution. To make the solution statistically determinate, they recognize that an additional equation is required and, therefore, use an extremal hypothesis. The authors acknowledge that the use of extremal hypotheses has been widely criticized on the grounds that the method lacks a physical basis. The main cause for concern is that the method only provides a method to calculate the channel width, although it does not suggest a mechanism by which width adjustment to a stable value is achieved (Bettess et al. 1988). In spite of this criticism, the authors argue that the use of an extremal hypothesis is justified on two grounds. First, they suggest that the method has enjoyed predictive success. Second, they argue that the inclusion of additional relations describing boundary shear stress and bank stability are insufficient to close the problem and that one extra relationship, in this case an extremal hypothesis, is still required. We would first like to discuss the apparent empirical success of the various extremal hypothesis approaches. There are a large number of extremal hypotheses to choose from, including the maximization of sediment transport rate, minimization of energy, and maximization of friction factor. Most studies have attempted to validate their results using comparisons between predicted and observed stable channel geometries, usually with a reasonable degree of success. However, no concerted effort was made until recently to directly verify the validity of various extremal hypotheses, by observing trends in the relevant parameters in unstable channels as they evolve toward equilibrium. Direct observations of the morphology and flow discharges of evolving channels were obtained in two diverse disturbed fluvial environments: the steep, high-energy coarse-grained Toutle River system in Washington, disturbed by the 1980 eruption of Mount Saint Helens; and the low-gradient, low-energy, fine-grained Obion-Forked Deer catchments in West Tennessee (Simon 1992), disturbed by channelization in the 1960s and 1970s. These data can be used in conjunction with step-backwater models to estimate temporal trends of flow energy and roughness variables in evolving channels. Simon (1992) showed that in both environments the stream power, total mechanical energy, and energy dissipation rate decreased with time toward minimum values, providing strong direct evidence in favor of the minimization of energy hypotheses. To illustrate this, Fig. 7 shows examples of temporal trends of the energy dissipation rate (energy slope) from the Toutle River system, in both aggrading and degrading reaches. As the authors recognized, the hypotheses of minimization of energy and maximization of sediment transport rate have been shown to be equivalent (Davies and Sutherland 1983); thus, this result also supports the maximization of sediment transport rate hypothesis, which was the one used by the authors in their analysis. However, support for other hypotheses is less clear. In particular, the estimated temporal trends of the Darcy-Weisbach friction factor in the Toutle River system following the 1980 eruption of Mount Saint Helens show a tendency to decrease or remain constant with time (Fig. 7), in contradiction to the maximization of friction factor hypothesis (Simon and Thorne, in press), which, according to Davies and Sutherland (1983), is also equivalent to the minimization of energy hypotheses. Shiqiang et al. (1986) conducted a comparison of the predictive abilities of the various extremal hypothesis methods. Of the tested hypotheses, the principles of minimum stream power, or maximum sediment concentration, gave the best agreement with the field data, which is consistent with the hypotheses. Although these results support the authors' choice of the maximization of sediment transport rate hypothesis, it appears that the basis of some of the extremal hypotheses may be open to question. Further, extremal hypotheses may give rise to very broad maximums or minimums, so that predicted channel equilibriums can exist over a large range, as is also suggested by the data shown in Fig. 7. This may make it difficult to obtain precise predictions of channel morphology when using the various extremal hypothesis approaches in practice. Independent of the validity and predictive power of the various extremal hypotheses, we
Abstract Bank retreat plays a fundamental role in fluvial and estuarine dynamics. It affects the cross‐sectional evolution of channels, provides a source of sediment, and modulates the diversity of habitats. Understanding and predicting the geomorphological response of fluvial/tidal channels to external driving forces underpins the robust management of water courses and the protection of wetlands. Here, we review bank retreat with respect to mechanisms, observations, and modeling, covering both rivers and (previously neglected) tidal channels. Our review encompasses both experimental and in situ observations of failure mechanisms and bank retreat rates, modeling approaches and numerical methods to simulate bank erosion. We identify that external forces, despite their distinct characteristics, may have similar effects on bank stability in both river and tidal channels, leading to the same failure mode. We review existing data and empirical functions for bank retreat rate across a range of spatial and temporal scales, and highlight the necessity to account for both hydraulic and geotechnical controls. Based on time scale considerations, we propose a new hierarchy of modeling styles that accounts for bank retreat, leading to clear recommendations for enhancing existing modeling approaches. Finally, we discuss systematically the feedbacks between bank retreat and morphodynamics, and suggest that to move this agenda forward will require a better understanding of multifactor‐driven bank retreat across a range of temporal scales, with particular attention to the differences (and similarities) between riverine and estuarine environments, and the role of feedbacks exerted by the collapsed bank soil.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)