1,721,000 research outputs found
Modelling the effects of climate change and sea level rise on the evolution of incised coastal gullies
No full textUnder projections of future (next ~100 years) anthropogenic climate change, it is predicted that marked changes in environmental driving conditions, with relation to baseline (1961 - 1990) climates, will be experienced. Such changes have the potential to induce substantial geomorphological and ecological change. Numerical models of landscape evolution provide powerful tools to assess the impacts that environmental changes may have on landscape morphology. Accordingly, this research seeks to utilise landscape evolution models (LEMs) to understand how projected changes in climate will affect the geomorphic response of a series of incised coastal gullies found on the Isle of Wight, UK. Incised coastal gullies are known to be dynamic and sensitive landscape features which intersect the terrestrial - marine boundary; as such their evolution is influenced by changes in both terrestrial (i.e. precipitation) and maritime (i.e. sea level and wave height) climates. In order to ensure the processes driving incised coastal gully evolution are represented within the LEM, an existing LEM was modified to include processes of soft cliff erosion. This represents the first such inclusion of coastal processes within a LEM framework. The modified LEM was forced with ensemble projections of precipitation, sea level and wave height downscaled from HadCM3 and CGCM2 Global Climate Model (GCM) outputs for two emissions scenarios (A2 and B2). Comparison against a baseline scenario based on the 1961-1990 climatology allows for climate induced changes in system response to be quantified. To constrain the uncertainties associated with the application of landscape models and downscaled GCM data, a Monte Carlo analysis framework is employed, resulting in ~22000 model runs. This method also permits the development of probabilistic results describing geomorphological change in gully systems. Results suggest that the likelihood of extreme loss in gully extent will increase by up to 61%. Furthermore, it is projected that extreme rates of coastal erosion will increase by 22% by 2100 (under HadCM3 runs forced with the A2 emissions scenario). However, under certain scenarios the possibility of extension of the gully systems exists, with likely (>66% probability) increases in gully length of 13.7 m projected under CGCM2 runs forced with the A2 emissions scenario. The novel application of a Monte Carlo methodology with a LEM framework permits the identification of key climatic parameters responsible for causing extreme changes within these gully systems, allowing the relative importance of each climate parameter in driving incised coastal gully evolution to be assessed. Furthermore, the successful application of this technique suggests it may be applicableto other studies applying LEMs to scenarios of future climate change
The occurrence of obtuse junction angles and changes in channel width below tributaries along the Mekong River, South-East Asia
No full textClassic descriptions of drainage patterns suggest that confluence angle is determined by the shape of the drainage basin unless constraining factors, such as the geological structure, affect stream flow. Downstream changes in channel width below tributary junctions have long been associated with tributary inputs of flow and sediment. Analysis of tributary junction geometry and channel width changes in large rivers and over large reaches is sparse. The Lower Mekong Basin exhibits a generally dendritic drainage network despite flowing through a diverse array of geological settings. Publicly available SPOT?5 imagery from Google Earth was used to identify and catalogue junction geometries and downstream changes in channel width below tributary junctions along a ~2200 km reach of the Mekong River. Of the 284 junctions identified, the majority (66.2%) were acute. However 12 (4.8%) were found to be normal (90°) and 75 (30%) were found to be obtuse. This latter number is in contrast to previous studies over similar spatial scales which found little evidence of obtuse junctions. Meander extension of the incoming tributary and deflection of the tributary across bedrock shoulders were found to be the dominant geomorphological causes of obtuse tributary junctions. The relationship between the width of the tributary channels and the width of the mainstem upstream and downstream of the confluences was analysed. It was observed that, over the whole reach, a slight narrowing occurred immediately below tributary junctions. Although the changes themselves were small, the slight net narrowing is shown to be statistically significant. The observed relationship is shown to vary considerably with geology. The geological control suggests that complex factors play important roles in determining changes to channel width across large systems and that simple cause–effect relationships do not hold in such complicated geological settings
Modelling the response of soft cliffs to climate change: a statistical, process-response model using accumulated excess energy
No full textWater and suspended sediment discharges for the Mekong Delta, Vietnam (2005-2015)
No full textThis dataset describes hourly time series of discharge and suspended sediment flux at four sites in the Vietnamese Mekong Delta (Chau Doc, Tan Chau, Can Tho and My Thaun) for the period 2005 – 2015. This data was calculated from historic Acoustic Doppler Current Profiler (aDcp)data obtained as part of routine flood monitoring conducted by the Vietnamese Hydrological Agency. The data were collated by the authors.
The data were processed to back out sediment fluxes through the delta through calibration of the acoustic backscatter signal to suspended sediment concentrations collected in Chau Doc (May 2017) and Can Tho (September 2017). For each aDcp instrument acoustic backscatter signal was calibrated to observed suspended sediment concentrations (SSCs). These concentrations values were then matched to measured acoustic backscatter values (dB) from the depth at which each sample was taken to generate power law calibration curves. To generate daily fluxes, the point specific ADCP fluxes were used to generate sediment ratings curves between sediment flux (kg/s) and discharge (m3/s). These ratings curves were then propagated over recorded daily discharge values measured by the Vietnamese hydrological agency to provide daily fluxes over the period of record.
The work was funded through NERC grant reference NE/P008100/1 - Deciphering the dominant drivers of contemporary relative sea-level change: Analysing sediment deposition and subsidence in a vulnerable mega-delta</span
Landscapes on the edge: examining the role of climatic interactions in shaping coastal watersheds using a coastal-terrestrial landscape evolution model
No full textIncised coastal gullies (ICGs) are dynamic features found at the terrestrial-coastal interface. Their geomorphic evolution is driven by the interactions between processes of fluvial knickpoint migration and coastal cliff erosion. Under scenarios of future climate change the frequency and magnitude of the climatological drivers of both terrestrial (fluvial and hillslope) and coastal (cliff erosion) processes are likely to change, with an adjunct impact on these types of coastal features. Here we explore the response of an incised coastal gully to changes in both terrestrial and coastal climate in order to elucidate the key process interactions which drive ICG evolution.We modify an extant landscape evolution model, CHILD, to incorporate processes of soft-cliff erosion. This modified version, termed the Coastal-Terrestrial-CHILD (CT-CHILD) model, is then employed to explore the interactions between changing terrestrial and coastal driving forces on the future evolution of an ICG found on the south west Isle of Wight, UK. It was found that the magnitude and frequency of storm events will play a key role in determining the future trajectory of ICGs, highlighting a need to understand the role of event sequencing in future projections of landscape evolution. Furthermore, synergistic (positive) and antagonistic (negative) interactions were identified between coastal and terrestrial parameters, such as wave height intensity and precipitation duration, which act to modulate the impact of changes in any one parameter. Of note was the role played by wave height intensity in driving coastal erosion, which was found to play a more important role than sea-level rise in determining rates of coastal erosion. This highlights the need for a greater focus on wave height in studies of soft-cliff erosion
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
River bank instability from unsustainable sand mining in the lower Mekong River
Full text linkRecent growth of the construction industry has fuelled the demand for sand, with considerable volumes being extracted from the world’s large rivers. Sediment transport from upstream naturally replenishes sediment stored in river beds, but the absence of sand flux data from large rivers inhibits assessment of the sustainability of ongoing sand mining. Here, we demonstrate that bedload (0.18 ± 0.07 Mt yr
−1) is a small (1%) fraction of the total annual sediment load of the lower Mekong River. Even when considering suspended sand (6 ± 2 Mt yr
−1), the total sand flux entering the Mekong delta (6.18 ± 2.01 Mt yr
−1) is far less than current sand extraction rates (50 Mt yr
−1). We show that at these current rates, river bed levels can be lowered sufficiently to induce river bank instability, potentially damaging housing and infrastructure and threatening lives. Our research suggests that on the Mekong and other large rivers subject to excessive sand mining, it is imperative to establish regulatory frameworks that limit extraction rates to levels that permit the establishment of a sustainable balance between the natural supply/storage of sand and the rate at which sand is removed.
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The influence of flow discharge variations on the morphodynamics of a diffluence-confluence unit on a large river
Full text linkThis is the author accepted manuscript. The final version is freely available from Wiley via the DOI in this record.Bifurcations are key geomorphological nodes in anabranching and braided fluvial channels, controlling local bed morphology, the routing of sediment and water, and ultimately defining the stability of their associated diffluence–confluence unit. Recently, numerical modelling of bifurcations has focused on the relationship between flow conditions and the partitioning of sediment between the bifurcate channels. Herein, we report on field observations spanning September 2013 to July 2014 of the three-dimensional flow structure, bed morphological change and partitioning of both flow discharge and suspended sediment through a large diffluence–confluence unit on the Mekong River, Cambodia, across a range of flow stages (from 13 500 to 27 000 m3 s−1).
Analysis of discharge and sediment load throughout the diffluence–confluence unit reveals that during the highest flows (Q = 27 000 m3 s−1), the downstream island complex is a net sink of sediment (losing 2600 ± 2000 kg s−1 between the diffluence and confluence), whereas during the rising limb (Q = 19 500 m3 s−1) and falling limb flows (Q = 13 500 m3 s−1) the sediment balance is in quasi-equilibrium. We show that the discharge asymmetry of the bifurcation varies with discharge and highlight that the influence of upstream curvature-induced water surface slope and bed morphological change may be first-order controls on bifurcation configuration. Comparison of our field data to existing bifurcation stability diagrams reveals that during lower (rising and falling limb) flow the bifurcation may be classified as unstable, yet transitions to a stable condition at high flows. However, over the long term (1959–2013) aerial imagery reveals the diffluence–confluence unit to be fairly stable. We propose, therefore, that the long-term stability of the bifurcation, as well as the larger channel planform and morphology of the diffluence–confluence unit, may be controlled by the dominant sediment transport regime of the system. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.Natural Environment Research Council. Grant Numbers: NE/JO21571/1, NE/JO21881/1, NE/JO21970/
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