Modelling long term basin scale sediment connectivity, driven by spatial land use changes

Changes in land use can affect local geomorphology and sediment dynamics. However, these impacts could conceivably lead to changes in geomorphological processes beyond the area of land use change, thereby evidencing a geomorphic connectivity in the landscape. We conduct a numerical modelling experiment, using the CAESAR landscape evolution model, to investigate the extent and nature of such connectivity in the River Swale basin. Six simulations are run and analysed. Two of these are reference simulations, where the basin has a hypothetical total grassland cover or total forest cover. In the other four simulations, half of the basin is subjected to either deforestation or reforestation during the simulation. Simulations are analysed for temporal trends in sediment yield and for spatial trends in erosion and deposition across the basin. Results show that deforestation or reforestation in one half of the basin can indeed affect the geomorphology of the other half, thus implying a geomorphological connectivity across the basin. This connectivity is locally very high, with significant morphological impacts close to where de- or re-forestation occurs. Changes are observed both downstream and upstream of the areas where the land use changes occurred. The impacts are more pronounced in the downstream direction and are still apparent in the basin scale sediment yields, as deforestation of half the basin can increase decadal sediment yields by over 100%, whilst reforestation of half the basin can lead to 40% decreases. However, our results also indicate a reverse connectivity whereby erosion and deposition in upstream headwaters and tributaries can, for the first time, be conclusively attributed to land use changes several kilometres downstream, due to alterations in the valley floor base level resulting from incision and alluviation

Implementation of geotechnical and vegetation modules in TELEMAC to simulate the dynamics of vegetated alluvial floodplains

Water Qualit

Morphological and morphometrical differentiation of processes on crater walls in Eastern Utopia Planitia, Mars

This study identifies a variety of processes associated with erosional and depositional structures within impact craters in eastern Utopia Planitia, Mars. Differentiation of the morphological characteristics of erosional and depositional structures within five structures suggests that four types of landforms develop on craters walls: debris flows, linear or dendritic channels resembling gullies, head-cut channels, and dry flows. Previous studies have mostly focused on the orientation characteristics of gully-type landforms and the environmental conditions that contributed to their formation. Most of these studies favored the term gully for all “wet” processes affecting crater walls, although debris flows have also recently been described. The full development of these structures shows that the wet-member structures (e.g., temporary channels resembling gullies) and mixed types (e.g., debris flows) evolved under different environmental conditions than that of present-day Mars. Dry flows can form in the current environmental conditions, but their presence near to the wet-member forms and the structural relationships among these wet and dry forms suggest that they formed within the same periods during fluctuations in atmospheric conditions. The morphometrical characteristics of flows on craters walls show that there is a relationship between the accumulation area and slope of processes, which indicate a morphometric threshold between the wet and dry types of erosion; with gully channels developing on low angle colluvial slopes while the debris flows are forming on more abrupt slopes. It is suggested that the most important controlling factors for flow initiation and development on the crater walls are first related to the morphometry of craters walls, and then to water availability and exposure of bedrock within crater walls

Global Sensitivity Analysis of Parameter Uncertainty in Landscape Evolution Models

The evaluation and verification of landscape evolution models (LEMs) has long been limited by a lack of suitable observational data and statistical measures which can fully capture the complexity of landscape changes. This lack of data limits the use of objective function based evaluation prolific in other modelling fields, and restricts the application of sensitivity analyses in the models and the consequent assessment of model uncertainties. To overcome this deficiency, a novel model function approach has been developed, with each model function representing an aspect of model behaviour, which allows for the application of sensitivity analyses. The model function approach is used to assess the relative sensitivity of the CAESAR-Lisflood LEM to a set of model parameters by applying the Morris method sensitivity analysis for two contrasting catchments. The test revealed that the model was most sensitive to the choice of the sediment transport formula for both catchments, and that each parameter influenced model behaviours differently, with model functions relating to internal geomorphic changes responding in a different way to those relating to the sediment yields from the catchment outlet. The model functions proved useful for providing a way of evaluating the sensitivity of LEMs in the absence of data and methods for an objective function approach.</p

Calculating flux to predict future cave radon concentrations

Cave radon concentration measurements reflect the outcome of a perpetual competition which pitches flux against ventilation and radioactive decay. The mass balance equations used to model changes in radon concentration through time routinely treat flux as a constant. This mathematical simplification is acceptable as a first order approximation despite the fact that it sidesteps an intrinsic geological problem: the majority of radon entering a cavity is exhaled as a result of advection along crustal discontinuities whose motions are inhomogeneous in both time and space. In this paper the dynamic nature of flux is investigated and the results are used to predict cave radon concentration for successive iterations. The first part of our numerical modelling procedure focuses on calculating cave air flow velocity while the second part isolates flux in a mass balance equation to simulate real time dependence among the variables. It is then possible to use this information to deliver an expression for computing cave radon concentration for successive iterations. The dynamic variables in the numerical model are represented by the outer temperature, the inner temperature, and the radon concentration while the static variables are represented by the radioactive decay constant and a range of parameters related to geometry of the cavity. Input data were recorded at Driny Cave in the Little Carpathians Mountains of western Slovakia. Here the cave passages have developed along splays of the NE–SW striking Smolenice Fault and a series of transverse faults striking NW–SE. Independent experimental observations of fault slip are provided by three permanently installed mechanical extensometers. Our numerical modelling has revealed four important flux anomalies between January 2010 and August 2011. Each of these flux anomalies was preceded by conspicuous fault slip anomalies. The mathematical procedure outlined in this paper will help to improve our understanding of radon migration along crustal discontinuities and its subsequent exhalation into the atmosphere. Furthermore, as it is possible to supply the model with continuous data, future research will focus on establishing a series of underground monitoring sites with the aim of generating the first real time global radon flux maps.The authors would like to thank Peter Zvonár, Sara Argerich-Bergada, Amanda Keen-Zebert, Lenka Thinová, and Petr Otáhal as well as the reviewers whose constructive comments have helped to improve the clarity of the manuscript. This study was conducted with support from the long term conceptual development research organisation RVO: 67985891. It is published in the framework of CzechGeo-EPOS “Distributed system of permanent observatory measurements and temporary monitoring of geophysical fields in the Czech Republic” (MŠMT Project: LM2010008).Peer reviewe

Two decades of numerical modelling to understand long term fluvial archives: Advances and future perspectives

The development and application of numerical models to investigate fluvial sedimentary archives has increased during the last decades resulting in a sustained growth in the number of scientific publications with keywords, 'fluvial models', 'fluvial process models' and 'fluvial numerical models'. In this context we compile and review the current contributions of numerical modelling to the understanding of fluvial archives. In particular, recent advances, current limitations, previous unexpected results and future perspectives are all discussed. Numerical modelling efforts have demonstrated that fluvial systems can display non-linear behaviour with often unexpected dynamics causing significant delay, amplification, attenuation or blurring of externally controlled signals in their simulated record. Numerical simulations have also demonstrated that fluvial records can be generated by intrinsic dynamics without any change in external controls. Many other model applications demonstrate that fluvial archives, specifically of large fluvial systems, can be convincingly simulated as a function of the interplay of (palaeo) landscape properties and extrinsic climate, base level and crustal controls. All discussed models can, after some calibration, produce believable matches with real world systems suggesting that equifinality - where a given end state can be reached through many different pathways starting from different initial conditions and physical assumptions - plays an important role in fluvial records and their modelling. The overall future challenge lies in the development of new methodologies for a more independent validation of system dynamics and research strategies that allow the separation of intrinsic and extrinsic record signals using combined fieldwork and modelling

Defining recovery potential in river restoration: a biological data-driven approach

Scientists and practitioners working on river restoration have made progress on understanding the recovery potential of rivers from geomorphological and engineering perspectives. We now need to build on this work to gain a better understanding of the biological processes involved in river restoration. Environmental policy agendas are focusing on nature recovery, reigniting debates about the use of “natural” reference conditions as benchmarks for ecosystem restoration. We argue that the search for natural or semi-natural analogues to guide restoration planning is inappropriate due to the absence of contemporary reference conditions. With a catchment-scale case study on the invertebrate communities of the Warwickshire Avon, a fifth-order river system in England, we demonstrate an alternative to the reference condition approach. Under our model, recovery potential is quantified based on the gap between observed biodiversity at a site and the biodiversity predicted to occur in that location under alternative management scenarios. We predict that commonly applied restoration measures such as reduced nutrient inputs and the removal of channel resectioning could be detrimental to invertebrate diversity, if applied indiscriminately and without other complementary measures. Instead, our results suggest considerable potential for increases in biodiversity when restoration measures are combined in a way that maximises biodiversity within each water bod

Simulating bank erosion over an extended natural sinuous river reach using a universal slope stability algorithm coupled with a morphodynamic model

Meandering river channels are often associated with cohesive banks. Yet only a few river modelling packages include geotechnical and plant effects. Existing packages are solely compatible with single-threaded channels, require a specific mesh structure, derive lateral migration rates from hydraulic properties, determine stability based on friction angle, rely on nonphysical assumptions to describe cutoffs, or exclude floodplain processes and vegetation. In this paper, we evaluate the accuracy of a new geotechnical module that was developed and coupled with Telemac-Mascaret to address these limitations. Innovatively, the newly developed module relies on a fully configurable, universal genetic algorithm with tournament selection that permits it (1) to assess geotechnical stability along potentially unstable slope profiles intersecting liquid-solid boundaries, and (2) to predict the shape and extent of slump blocks while considering mechanical plant effects, bank hydrology, and the hydrostatic pressure caused by flow. The profiles of unstable banks are altered while ensuring mass conservation. Importantly, the new stability module is independent of mesh structure and can operate efficiently along multithreaded channels, cutoffs, and islands. Data collected along a 1.5-km-long reach of the semialluvial Medway Creek, Canada, over a period of 3.5 years are used to evaluate the capacity of the coupled model to accurately predict bank retreat in meandering river channels and to evaluate the extent to which the new model can be applied to a natural river reach located in a complex environment. Our results indicate that key geotechnical parameters can indeed be adjusted to fit observations, even with a minimal calibration effort, and that the model correctly identifies the location of the most severely eroded bank regions. The combined use of genetic and spatial analysis algorithms, in particular for the evaluation of geotechnical stability independently of the hydrodynamic mesh, permits the consideration of biophysical conditions for an extended river reach with complex bank geometries, with only a minor increase in run time. Further improvements with respect to plant representation could assist scientists in better understanding channel-floodplain interactions and in evaluating channel designs in river management projects