GM2.10

The ephemeral streams, which drain steep and metamorphic catchments, experience rapid and torrential runoff with high sediment loads. These processes cause important morphological changes in the channels. This work proposes a methodological approach to verify the change patterns in the magnitude and frequency of the hydrological events that geomorphologically model this type of channels. A gravel-bed ephemeral stream, the Rambla de la Azohía, located in the coastal area of the Betic Mountains (southeastern Spain), has been chosen as a study case for the method validation. This approach focuses first on relationships between peak discharges and sediment budgets measured at checkpoints for specific events from 2018 to 2020 and then runoff data and sediment yields obtained using the GeoWEPP model for the same cases after calibration/validation. Water depths and concentrations of suspended sediment recorded during the events of 2018 and 2019 were used for model calibration and validation, respectively. For the calibration stage, a sensitivity analysis was carried out in order to detect the parameters that most influence the model output and are, therefore, suitable for calibration. Finally, the results obtained in the calibration and validation periods were evaluated using the Nash-Sutcliffe efficiency (NS) and percent bias (PBIAS). Values of NS and PBIAS equal to 0.86 and  7.81%, respectively, were found in the calibration period, while these indices were  0.81 and  -4.1% in the validation period. All these values confirm the model’s capacity to simulate peak flow and erosion in the experimental conditions. Topographical variations and sediment budgets, verified combining high-resolution digital terrain models (HRDTMs) with ortophotographs and point clouds dated in 2018, 2019 and 2020, and ground-based surveys, were analyzed in relation to changes in discharge in order to determine geomorphic flow thresholds. According to these thresholds, three classes of morphological adjustments were defined: 1. global changes caused by  discharges over the bankfull depth; 2. large alterations at the bankfull stage driven by a noticeable vertical bed accretion and lateral erosion; 3. moderate adjustments during sub-bankfull flows that are able to modify alluvial bars; and 4. minor events, in which the accretion of these bars ceases and shallow scouring and washing actions prevail. These geomorphic thresholds were then applied to the complete series of discharges simulated using GeoWEPP at the event scale during the period 1997-2019. The results revealed a significant increase in the number of events that are capable to produce bed aggradation and bank erosion. This research was funded by FEDER / Spanish Ministry of Science, Innovation and Universities - State Research Agency (AEI) / Projects CGL2017-84625- C2-1-R and CGL2017-84625-C2-2-R; State Program for Research, Development and Innovation Focused on the Challenges of Society.

How to cite: Conesa-García, C., Martínez-Salvador, A., Martínez-Capel, F., Puig-Mengual, C., Pérez-Cutillas, P., Zema, D. A., and Bombino, G.: Using GeoWEPP model, high-resolution 3D models and ground-based survey to detect sedimentation changes and morphological adjustments in an ephemeral stream, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2843, https://doi.org/10.5194/egusphere-egu21-2843, 2021.

This work extends the established geochronology of the Mammoth Cave region, Kentucky, USA, spatially and temporally, to infer evolution of the karst landscape and to consider the shifting drainage basins of the Barren River and the Green River in relation to regional drainage rearrangements. Previous studies have focused on the Mammoth Cave System and used cosmogenic radionuclide dating to link the incision history of the Green River and the Cave as far back as 3.25 Ma. We posit that prior to the wide-spread karstification that produced Mammoth Cave, drainage consisted of a purely fluvial stream network flow on the youngest clastic rocks. When this caprock was breached, carbonate dissolution ensued and the system transitioned to fluviokarst. Relict large trunk passages that originated at that time can be found in features such as Prewitts Knob, Bald Knob, and Huckleberry Knob. We intend to use sediments and speleothems collected from Crystal Onyx Cave in Prewitts Knob to constrain the age of this stage of karst development and to provide an estimate of the long-term erosion rate of the Sinkhole Plain surrounding the knob. These relict trunks were also used for cave stream profile reconstruction in combination with the east-west trending uvalas and sets of steep, deep sinkholes. We interpret that paleodrainage as having been west-flowing into the Barren River which then served as regional base level. Thus, we infer that as the rivers incised, this drainage was pirated to the north and began flowing to the Green River. The system then evolved into a more mature karst, large conduits near the surface collapsed, and dissected the landscape into isolated depressions. The collapsed limestone formed red soil and the sandstone produced angular clasts scattered throughout that soil. The retreating Chester Cuesta, marking the boundary between the Sinkhole Plain and the sandstone-capped Chester Upland, eroded most rapidly where limestone was exposed to the surface and more slowly where it was sandstone-capped leaving abandoned isolated cave trunk passage segments in remnant knobs. The results of this work have implications for understanding timescales of the evolution of karst systems in unconfined carbonate sequences as well as the interaction of karst areas with the transience in drainage networks.

How to cite: Bosch, R. and Ward, D.: Chasing sand: evolution of the surface and subsurface drainage of the Sinkhole Plain, Central Kentucky Karst, USA, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-957, https://doi.org/10.5194/egusphere-egu21-957, 2021.

Resolving Earth’s surface at the meter scale is essential for an improved understanding of topographic signatures generated by debris-flow activity in high-relief mountainous terrains. Here, we explore the applicability and potential of digital elevation models (DEMs) derived from stereo-photogrammetry for debris-flow detection in the southern Central Andes of NW Argentina. Our analysis relies on a high-resolution (3 m) DEM created from SPOT-7 tri-stereo satellite data. We carefully validated DEM quality with ~5000 differential GPS points for an area of 245 km² in the Quebrada del Toro basin within the Eastern Cordillera. 

We build upon previous work that suggests that debris flows have a distinct signature in the drainage area and slope framework: debris-flow channels exhibit a nearly constant slope (no channel curvature), while channels dominated by fluvial transport processes show a negative power-law behavior in log-log space. Drainage-area approaches in geomorphic analysis are fast and efficient tools to distinguish signatures of debris-flow and fluvial transport processes, yet they might introduce an averaging bias because upstream areas are analyzed jointly. 

For a more precise localization and assessment of debris-flow activity, we evaluate topographic signatures of individual channels. We present a new approach that relies on connected components of similar slope that can be attributed to different transport regimes. Debris-flow activity reflects particularly steep segments of medium connected-component lengths in small drainage areas. The spatial occurrence and lengths of these segments are controlled by geologic and lithologic boundary conditions and we find that the highest debris-flow activity corresponds with steep slopes in areas documented Quaternary tectonic activity and the exposure of pervasively fractured bedrock. Comparing our results to topographic signatures of the corresponding catchments in log-log space, we show that individual channel approaches allow to better detect intra-catchment variability. These are imperative for understanding erosion and sediment-transport processes in the river channel. Since high-resolution data are needed to reliably resolve debris-flow channels, our meter-scale DEMs greatly improve the localization and prediction of debris-flow activity. Thus, for evaluations of recurring hazardous debris-flow activity in extensive, remote, and sparsely vegetated mountainous landscapes, stereo-photogrammetry presents a very suitable and cost-efficient alternative to airborne lidar data. 

How to cite: Mueting, A., Bookhagen, B., and Strecker, M. R.: Mapping debris-flow channels in the southern Central Andes using high-resolution topographic data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7520, https://doi.org/10.5194/egusphere-egu21-7520, 2021.

Evaluating the future stability and land denudation rates of natural or anthropogenic landforms is paramount for sustainable land use practices. Landform evolution models can be powerful tools in this endeavour.  In this study we used the well-established landform evolution model SIBERIA and the newly developed coupled soilscape-landform evolution model SSSPAM to simulate the evolution of a proposed post mining landform. SIBERIA uses a cellular digital elevation model to simulate annual average fluvial and diffusive erosion on landforms using annual average precipitation. However it does not simulate the soil profile evolution on the evolving landform. The new SSSPAM coupled soilscape-landform evolution model has the ability to assess the overall erosion rates of catchment scale landforms either using short term precipitation events, variable precipitation or time averaged precipitation rates. In addition, SSSPAM is able to simulate the evolution of the soil profile of the evolving landform using pedogenetic processes such as physical weathering and armouring.

            To assess the reliability of SSSPAM, model predictions at 100 and 10000 years were compared with SIBERIA predictions at the same times. During the long term (10000yr) simulation the effect of armouring and weathering on the landform evolution was also assessed. The results obtained from these different simulations were compared and contrasted. Comparison of the short term simulations revealed that SSSPAM results compare well with the simulation results of the more established SIBERIA model. Long term simulation showed that SSSPAM simulation results also compares well with SIBERIA simulations while the erosion rates predicted by both models are close to the land denudation rates measured in the field. The soil profile characteristics and channel forms simulated by SSSPAM long term simulations were examined using several landform cross-sections. This analyses revealed that SSSPAM produces deep incised channels with very low soil thickness in upper reaches of the catchment and shallow channels with relatively thick soil layers in the lower reaches of the catchment. These SSSPAM simulated channels match well with the channel forms and distribution of bedrock channels and alluvial channels observed in the field. The analysis of the catchment cross-sections also showed that SSSPAM is capable of reproducing complex subsurface soil evolution and stratification and spatial variability of soil profile characteristics typically observed in the field.

How to cite: Welivitiya, W. D. D. P., Willgoose, G., and Hancock, G.: Evaluating the stability and evolution of a proposed post mining landform., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8569, https://doi.org/10.5194/egusphere-egu21-8569, 2021.

Tailings are a by product of mining and the processing of minerals. Tailings are generally highly erodible as they have a fine particle size. They can also contain elevated concentrations of unwanted minerals and process chemicals. Consequently, if released to the environment they can be a significant environmental problem. There have been several high profile cases which have highlighted the human and environmental risk of tailings. A common way to manage tailings is to store them in ‘tailings dams’ where they will remain in perpetuity. There has been little investigation of the long-term erosional behaviour of a tailings dams. Computer based Landscape Evolution Models (LEMs) can provide insight into these new geomorphological entities. LEMS provide information on erosion rates, type of erosion and where erosion is likely to occur and can provide guidance on long-term behaviour. Here a LEM is used to assess tailings dam designs using a range of different surface covers and climates. The modelling and methods here provide a template for tailings dam assessment at other sites globally. The methods here will improve tailings dam design and reduce environmental risk.

How to cite: Hancock, G.: Using Landscape Evolution Models to assess the long-term erosional stability of tailings dams, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8558, https://doi.org/10.5194/egusphere-egu21-8558, 2021.

Sequences of coral reef terraces result from the interplay between biogenic and clastic sedimentary production, relative sea level (RSL) variations, wave erosion and tectonic forcing. Reefal sequences are gold standard proxies for paleo-sea level and tectonic reconstructions, but their contribution is usually restricted to a bijective approach, correlating the single elevation and age of their inner edge to single sea level stands or coseismic offsets, and reciprocally. The increase of available data, such as coral datings and high resolution topography revealed major deviations from this bijective approach (corals from a single MIS on several terraces, and conversely, or MIS highstands not represented in a sequence).

The Cape Laundi sequence, Sumba island, Indonesia, demonstrates such deviations, with outcrops of corals from MIS 5e on as many as three terraces instead of a single terrace as commonly expected. A preliminar numerical model of coral reef terrace profile has been developed, integrating reef growth, wave erosion, RSL variations and tectonic deformation. The interplay between reef growth rate, tectonic displacements and RSL variations provides a plausible explanation for these numerous occurrences. The low growth rate of this reef appears to prevent coral from  saturating the accommodation space generated during sea level transgression, leading to the preservation of drowned platforms and reefal construction of similar age during regressions.

Preliminary results from numerical modeling reveal complex feedbacks between the processes shaping these morphologies. Tectonic deformation has a major influence on reef development, by favoring reef preservation at high uplift rates and controlling the available accommodation space for reef growth.. By taking into account the numerous feedbacks controlling reef morphology, we can investigate the significance of RSL variations, continuous and punctual rock uplift, biogenic activity, and clastic inputs on coral terrace morphology and chronostratigraphy. Our approach can bring crucial constraints to the rates and frequency of RSL variations. To do so, we further develop our numerical model in order to provide more robust insights on the controls of reefal sequences morphologies. 

How to cite: Pastier, A.-M., Malatesta, L., Huppert, K., and Chauveau, D.: Towards a dynamic approach of sequences of coral reef terraces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15976, https://doi.org/10.5194/egusphere-egu21-15976, 2021.

Mid-latitude mountain ranges such as the Eastern Alps are characterized by a strong topographic imprint of Pleistocene glaciations. The characteristic geometry of glacial landforms has been quantified in various ways, but studies about the evolution of glacial landscape metrics are lacking. However, such information is needed to interpret the evolutionary state of glacial topography.

By employing a landscape evolution model for cold climate processes, we trace the fluvial-to-glacial transformation of a synthetic landscape. Our simulations inspired by alpine glaciations of mid-latitude mountain ranges with peaks and ridges towering above the glacier network lead to a general increase in relief. This is expressed as the formation of overdeepened valleys with steepened flanks. Overdeepening starts at the glacier front and progressively extends upstream with ongoing glacial erosion.

The topographic signature of the progressively transforming landscape is characterized by an increase of mean channel slopes and its variance. However, above the steep flanks, the initial fluvial topography is persisting. Whereas the initial fluvial mountain range is characterized by a monotonic increase of channel slope with elevation, a transition from increasing to decreasing channel slope with elevation emerges above the equilibrium line altitude where (tributary-)headwalls transition to ridges and summits. This turning point and a high slope variance becomes progressively distinctive with ongoing glacial occupation.

By comparing landscape metrics derived from model time series with those of the Eastern Alps, we found that the temporal transition observed in our numerical experiments occur as spatial transition from the fully glaciated western to a minorly glaciated eastern part of the Alps. Thus, slope-elevation plots serve as a diagnostic tool for interpreting the glacial - fluvial influence in mountain landscapes. However, catchments of the unglaciated part of the Eastern Alps show also turning points in their slope-elevation distributions, but the variance of slope is significantly smaller at all elevation levels, when compared to the glaciated part.

How to cite: Liebl, M., Robl, J., Egholm, D., Prasicek, G., Stüwe, K., Gradwohl, G., and Hergarten, S.: Progressive landscape transformation from a fluvial to a glacial topography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8271, https://doi.org/10.5194/egusphere-egu21-8271, 2021.

Many laws have been developed to describe the different aspects of landscape evolution at large spatial and temporal scales. Natural landscapes have heterogeneous properties (lithologies, climates, tectonics, etc.) that are associated with multiple coexisting processes. In turn, this can demand different mathematical expressions to model landscape evolution as a function of time and or space. Landscape Evolution Models are mostly designed to facilitate the combination of different landscape-wide laws in a plug-and-play way and many frameworks are being developed in this aim. However, most current frameworks cannot capture important landscape processes such as lake dynamics and full sediment tracing because they are optimized for speed and handle fluxes separately. Several processes require information from more than the immediate neighboring cells within a time step and demand an integrated knowledge from the entire upstream trajectory. Lakes for example require knowledge of all upstream water and sediment fluxes to be filled. These can only be known if all the laws controlling those have been processed. Tackling these situation with a grid logic requires substantial amount of numerical refactoring from existing models.

We present an alternative method to tackle landscape evolution modelling in heterogeneous landscapes with a framework inspired from Lagrangian and cellular automaton methods. Our framework only relies on the assumption that upstream nodes needs to be processed before the downstream ones, including lakes with outlets, in order to process all selected governing equations on a pixel-to-pixel basis. This way, we ensure that the true content of sediment and water fluxes can be known and tracked at any points. We first utilise graph theory to (i) find the most comprehensive path to reroute water through depressions and (ii) determine a generic multiple flow topological order (any node is processed after all potential upstream ones). Particles that register and track all fluxes simultaneously can then "roll" on the landscape and merge between each other while interacting with the grid.

This formulation makes possible a number of generic features. (i) The laws can be dynamically adapted to the environment (e.g. switching from single to multiple flow function of water content, adapting erodibility function of the sediment composition and quantity), (ii) Depressions can be explicitly managed, filled (or not) and separated from the rest of the landscape (e.g. sedimentation or evaporation in lakes) as a function function of inputted fluxes and parameters, (iii) full provenance, transport time, and deposition tracking as the particle can always keep in memory where the fluxes are from and in what proportions. In this contribution, we demonstrate the impact the importance of considering these additional elements in landscape evolution. In particular, lake dynamic can significantly impact the long-term signal propagation from source to sink.

How to cite: Gailleton, B., Malatesta, L., Braun, J., and Cordonnier, G.: Dynamic modelling framework to track sediment provenance and solve lakes in long-term landscape evolution models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9392, https://doi.org/10.5194/egusphere-egu21-9392, 2021.

Predicting future relief is a longstanding challenge in the field of geomorphology. Past denudation and incision rates can be reconstructed and modelled from field data such as thermochronometers, cosmogenic nuclides or optically stimulated luminescence, whereas future rates are then, by definition, fully unknown. Predicting future landscape evolution is further complicated by the dynamic nature of drainage networks, as well as the necessity of constraining properties such as erodibility in order to make sensible predictions. One of the few constraints available for future landscape properties is the underground stratigraphy imaged by wells or geophysical methods. The 3D rock structure will eventually be exhumed and can be utilised to constrain the future states of model simulations.

In this contribution, we present a landscape evolution model capable of ingesting 3D lithologic information and adapting to alternative channel networks, and demonstrate it using a study area in the Swiss Jura Mountains. The model calculates local relief using steady state solutions of the stream power incision model, and also quantifies hillslope relief using a very simple critical slope gradient where hillslope angles are set to a critical value on pixels that have a small drainage area. Further, drainage divides are allowed to migrate to minimize sharp breaks in relief across drainage divides.

We calibrate the values of erodibility, K, for each lithological unit by extracting ranges of apparent K value from the present-day landscape based on drainage area and gradient along the drainage network. This is then further refined by i) using a Monte Carlo approach to create combinations of K based on these ranges, and ii) comparing the real and model landscape for each combination with the aim to minimise the difference between the two. We then run selected model simulations of future base level fall and potential drainage reorganisation events, highlighting the effects of i.) spatially variable erodibility and ii.) lateral changes of the main channel axis on divide migration.

How to cite: Graf, E., Mudd, S., Kober, F., Landgraf, A., and Ludwig, A.: 3D lithological structure in a steady state model drives divide migration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8875, https://doi.org/10.5194/egusphere-egu21-8875, 2021.

Insolation differences play a primary role in controlling microclimate and vegetation cover, which together influence the development of topography. Topographic asymmetry (TA), or slope differences between terrain aspects, has been well documented in small-scale, field-based, and modeling studies. Here we combine a suite of environmental (e.g., vegetation, temperature, solar insolation) and topographic (e.g., elevation, drainage network) data to explore the driving mechanisms and markers of TA on a global scale. Using a novel empirical TA analysis method, we find that (1) steeper terrain has higher TA magnitudes, (2) globally, pole-facing terrain is on average steeper than equator-facing terrain, especially in mid-latitude, tectonically quiescent, and vegetated landscapes, and (3) high-elevation and low-temperature regions tend to have terrain steepened towards the equator. We further show that there are distinct differences in climate and vegetation cover across terrain aspects, and that TA is reflected in the size and form of fluvial drainage networks. Our work supports the argument that insolation asymmetries engender differences in local microclimates and vegetation on opposing terrain aspects, which broadly encourage the development of asymmetric topography across a range of lithologic, tectonic, geomorphic, and climatic settings.

How to cite: Smith, T. and Bookhagen, B.: Climatic and Biotic Controls on Topographic Asymmetry at the Global Scale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9758, https://doi.org/10.5194/egusphere-egu21-9758, 2021.