1,721,096 research outputs found
Reappraising the geomorphology-ecology link
No full textThis thematic Virtual Special Issue highlights a personal selection of 18 recent (2007-2009) contributions to Earth Surface Processes and Landforms. These papers provide a flavour of recent research that is concerned with furthering our understanding of the many ways in which the biosphere interacts with the physical and chemical processes of sediment transfer/transformation. Much of this research has focused on understanding the mechanics by which the biota can modulate sediment transport and the strength of earth surface materials, often with the aim of applying that knowledge to enhance bioremediation methods of erosion control. This work continues to be fundamentally important in enhancing our understanding of earth surface processes, but often treats the biosphere and physical world as uncoupled entities. This selection therefore also provides samples of work that point to an ongoing but significant disciplinary reappraisal in which it is the interactions between ecological and geomorphological realms that are of primary interest
The pace of human-induced change in large rivers: stresses, resilience and vulnerability to extreme events
No full textThe world’s great rivers are threatened by a range of anthropogenic stresses - climate change being just one - that are driving a major sustainability crisis. As rivers react to these stressors, declining resilience makes them more vulnerable to extreme events, amplifying their effects and driving system change from which there may be no recovery. Future governance must recognise both the rate of change associated with singular and compound anthropogenic stressors, and the potential for extreme events to transgress sustainability thresholds. <br/
Computational Fluid Dynamics (CFD) modelling to estimate fluvial bank erosion—a case study
No full textRiver bank erosion models are an important prerequisite for understanding the development of river meanders and for estimating likely land-loss and potential danger to floodplain infrastructure. Although bank erosion models have been developed that consider large-scale mass failure, the contribution of fluvial erosion (the process of particle-by-particle erosion due to the shearing action of the river flow) to bank retreat has not received as much consideration. In principle, such fluvial bank erosion rates can be quantified using excess shear stress formulations, but in practice, it has proven difficult to estimate the parameters involved. In this study, a series of three-dimensional Computational Fluid Dynamics (CFD) simulations for a meander loop on the River Asker (200 m long) at Bridport in southern England were undertaken to elucidate the overall flow structures and in particular to provide estimates of the applied fluid shear stress exerted on the riverbanks. The CFD models, which simulated relatively low and relatively high flow conditions, were established using Fluent 6.2 software. The modelling outcomes show that the key qualitative features of the flow endure even as flow discharge varies. At bank full, the degrees of velocity and simulated shear stresses within the inner bank separation zones are shown to be higher than those observed under low flow conditions, and that these elevated shear stresses may be sufficient to result in the removal of accumulated sediments into the main downstream flow
Effectiveness of grade-control structures in reducing erosion along incised river channels: the case of Hotophia Creek, Mississippi
No full textHerein, we undertake a geomorphological analysis in which spatial and temporal trends of bed and bank erosion along an 18-km length of Hotophia Creek, Mississippi, are estimated for the period between 1961 and 2050. The evaluation was undertaken for two scenarios of channel response to channelization during 1961-1963. One scenario represents the 'actual' response of the channel and includes the effects of installing a series of grade-control structures (GCS) between 1980 and 1996, while the other represents a hypothetical scenario in which the channel is left to adjust naturally. This allows the effectiveness of GCS in reducing in-channel erosion to be assessed. The analysis relies on the availability of channel survey data to develop empirical bed and bank response models for each adjustment scenario, supplemented by bank stability modelling to predict future rates of bank erosion. Results indicate that channel erosion rates decline nonlinearly with respect to time since 1961, for both adjustment scenarios. However, by the year 2050, the ''with'' GCS adjustment scenario results in the cumulative removal of some 663,000 (9%) extra tonnes of sediment relative to the ''without'' GCS scenario. Most (63%) of this excess is derived from enhanced bed erosion during 1976-1985 and 1985-1992, with the remainder derived from increased bank erosion during 1985-1992. Detailed analysis of the patterns of erosion and deposition, and their association with the GCS, provides evidence to support the view that GCS installed along Hotophia Creek have, for the most part, been ineffective in reducing channel erosion rates. This is because the GCS were installed too late to prevent bed degradation, caused by the 1961-1963 channelization, migrating upstream. In addition, some structures have disrupted the downstream transmission of bed material from eroded reaches upstream, exacerbating bed degradation and bank erosion in incised reaches downstream
Effects of Holocene climate and sea-level changes on coastal gully evolution: insights from numerical modelling
No full textGullies are known to be sensitive to a wide range of environmental disturbances so their geomorphology can provide insight into the environmental history of the surrounding landscape. Coastal gullies are of particular interest in that they are strongly influenced by both terrestrial and marine processes. For example, the coastal gullies found on the Isle of Wight, UK, known locally as ‘Chines’, are highly dynamic, with episodes of sea cliff erosion frequently causing the rejuvenation of the channel network. Consequently a key factor in the long-term evolution of the Chines is the relative balance between rates of cliff retreat (driven primarily by Holocene sea-level rise) and headwards incision caused by knickpoint migration (driven primarily by Holocene climate via its impact on runoff). In this paper we explore the Holocene erosional history of the Chines using a numerical landscape evolution model that has been modified to include a cliff recession function. Knickpoint recession rates are simulated using a detachment-limited channel erosion law wherein erosion rate is a power function of drainage area and stream gradient with model parameters defined using empirically-derived data. Hindcast simulations, from 12000 cal. years BP to present, are undertaken for a range of scenarios of Holocene climate change and sea-level rise. Plausible erosional histories are extracted from scenarios in which simulated and observed Chine and landscape forms match. The results suggest that the rate of sea-level rise is the key control on Chine formation and that it is only in this late Holocene period, and specifically in the last 2000 years, that sea-level rise has slowed sufficiently for knickpoint recession rates to exceed cliff recession rates and create sustainable gully networks. Our interpretation that the Isle of Wight gullies are of relatively modern is in agreement with previous studies. Finally, the simulations also indicate that contemporary Chine gully systems are close to a critical threshold, suggesting that future gully evolution is likely to be sensitive to small changes in future rates of effective precipitation and/or sea-level rise
An empirical-conceptual gully evolution model for channelled sea cliffs
No full textIncised coastal channels are a specific form of incised channel that are found in locations where stream channels flowing to cliffed coasts have the excess energy required to cut down through the cliff to reach the outlet water body. The southern coast of the Isle of Wight, southern England, comprises soft cliffs that vary in height between 15 and 100 m and which are retreating at rates ? 1.5 m a? 1, due to a combination of wave erosion and landslides. In several locations, river channels have cut through the cliffs to create deeply (? 45 m) incised gullies, known locally as ‘Chines’. The Chines are unusual in that their formation is associated with dynamic shoreline encroachment during a period of rising sea-level, whereas existing models of incised channel evolution emphasise the significance of base level lowering. This paper develops a conceptual model of Chine evolution by applying space for time substitution methods using empirical data gathered from Chine channel surveys and remotely sensed data. The model identifies a sequence of evolutionary stages, which are classified based on a suite of morphometric indices and associated processes. The extent to which individual Chines are in a state of growth or decay is estimated by determining the relative rates of shoreline retreat and knickpoint recession, the former via analysis of historical aerial images and the latter through the use of a stream power erosion model.<br/
Evaluating sustainable adaptation strategies for vulnerable mega-deltas using system dynamics modelling: rice agriculture in the Mekong Delta’s An Giang Province, Vietnam
No full textAbstractChallenging dynamics are unfolding in social-ecological systems around the globe as society attempts to mitigate and adapt to climate change while sustaining rapid local development. The IPCC's 5th assessment suggests these changing systems are susceptible to unforeseen and dangerous ‘emergent risks’. An archetypal example is the Vietnamese Mekong Delta (VMD) where the river dyke network has been heightened and extended over the last decade with the dual objectives of (1) adapting the delta's 18 million inhabitants and their livelihoods to increasingly intense river-flooding, and (2) developing rice production through a shift from double to triple-cropping. Negative impacts have been associated with this shift, particularly in relation to its exclusion of fluvial sediment deposition from the floodplain. A deficit in our understanding of the dynamics of the rice-sediment system, which involve unintuitive delays, feedbacks, and tipping points, is addressed here, using a system dynamics (SD) approach to inform sustainable adaptation strategies. Specifically, we develop and test a new SD model which simulates the dynamics between the farmers' economic system and their rice agriculture operations, and uniquely, integrates the role of fluvial sediment deposition within their dyke compartment.We use the model to explore a range of alternative rice cultivation strategies. Our results suggest that the current dominant strategy (triple-cropping) is only optimal for wealthier groups within society and over the short-term (ca. 10years post-implementation). The model suggests that the policy of opening sluice gates and leaving paddies fallow during high-flood years, in order to encourage natural sediment deposition and the nutrient replenishment it supplies, is both a more equitable and a more sustainable policy. But, even with this approach, diminished supplies of sediment-bound nutrients and the consequent need to compensate with artificial fertilisers will mean that smaller-scale farmers in the VMD are more vulnerable to accruing debt
Beyond just floodwater
No full textFlooding, already the largest hazard facing humankind, is becoming more frequent and affecting more people. Adapting to flooding must consider more than just water to encapsulate the effects of sediment movement, re-imagine flooding through a sociogeomorphic lens and expand approaches to knowing about floods
Surface water transitions 1984–2022: a global dataset at annual resolution
No full textRecent advances in satellite technology and cloud computing have enabled global-scale monitoring oflong-term surface water changes. The dynamic nature of surface water, driven by seasonal fluctuationsand climatic events, presents challenges for accurately interpreting these dynamics. Here, we introducethe first global dataset that identifies the timing, at annual resolution, of surface water advance or recession from 1984 to 2022. Our approach focuses on identifying persistent changes in surface water features by filtering out seasonal or shorter-term fluctuations. Using a novel algorithm, we mapped the timing of surface water transitions globally, including rivers, lakes, reservoirs, flooded agriculture, and coastal regions. In the dataset each 30 m × 30 m pixel records whether water advance or recession occurred and specifies the year of transition. This dataset enables users to visualize the location, type, and magnitude of changes, while its focus on timing provides new insights into the drivers of water dynamics. Designed for accessibility, the dataset supports scientific research as well as NGOs, policymakers, and water managers in addressing surface water-related challenges
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