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GM4.4

A key goal within geomorphic research is understanding the processes linking topographic form, erosion rates, and sediment production, transport and deposition. Numerical modelling, by allowing the creation of controlled analogues of natural systems, provides exciting opportunities to explore landscape evolution and generate testable predictions.

In this session, we invite contributions that use numerical modelling to investigate landscape evolution in a broad sense, and over a range of spatial and temporal scales. We welcome studies using models to constrain one or more of: erosion rates and processes, sediment production, transport and deposition, and sediment residence times. We also particularly wish to highlight studies that combine numerical modelling with direct Earth surface process monitoring techniques, such as topographic, field, stratigraphic, or geochronological data. Contributions using numerical models to unravel the interaction between environmental variables such as precipitation and lithology are further encouraged. There is no geographical restriction: studies may be focused on mountain environments or sedimentary basins, or they may establish links between the two. Studies beyond planet Earth are welcome too.

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Co-organized by SSS10
Convener: Fiona ClubbECSECS | Co-conveners: Benjamin CampfortsECSECS, Boris GailletonECSECS, Kimberly HuppertECSECS, Jörg Robl
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| Attendance Tue, 05 May, 08:30–10:15 (CEST)

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Chat time: Tuesday, 5 May 2020, 08:30–10:15

D1104 |
EGU2020-701
Yuval Shmilovitz, Efrat Morin, Yair Rinat, Itai Haviv, Genadi Carmi, Amit Mushkin, and Yehouda Enzel

Talus-pediment slopes are a common morphologic feature in arid areas and constitute a prominent runoff and sediment source at the watershed and channel scales. The evolution of talus-pediment sequences (talus flatirons) was often linked to climatic cycles, although the physical processes that may account for such a link remained obscure. Our approach is to integrate field measurements, high-resolution radar rainfall data and numerical modeling to link the frequency of storms and the resulted hillslope runoff and sediment transport. We present a quantitative hydrometeorological analysis of rainstorms and their geomorphic impact, potentially involved in the evolution of arid talus-pediment slopes in the Negev desert (Israel). Artificial rainstorms were designed based on intensity-duration-frequency curves and simulated in the field using a rainfall simulator. Then, the obtained experimental results were up-scaled to the entire slope length using a fully distributed hydrological model. In addition, natural storms and their hydro-geomorphic impacts were monitored using X-band radar and time-lapse cameras.

These integrated analyses constrain the rainfall threshold for local runoff generation at rain intensity of 14-22 mm h-1 for a duration of 5 min for the study area conditions. We characterized small-scale runoff-generating convective rain cells using an X-band radar and found that small convective cells (~30 km2), having extremely high internal spatial gradients in rainfall intensity and low velocity (<10 m s-1), have the potential to generate local hillslope runoff. The frequency of local runoff-producing rainstorms is ~1-3 per year, but most of these storms activate only small parts of the hillslope. Modeling results indicate that a full extent hillslope runoff occurs under much rarer rainstorms of at least 100-years return interval (1% or less). During such rainstorms, the shear stress produced by the runoff flow (sheetwash) is capable of transporting surface clasts at a distance of ~80 m downslope. However, transport of coarse clasts in the upper parts of the slopes is most probably gravitationally controlled. The erosion efficiency of discrete rare events (1% or less) on the lower part of the slopes highlights their potential to trigger incision and lead to cliff dissection. This study results support the hypothesis that a climatic shift in terms of the properties and frequency of extreme rainstorms, rather than the common views of it as changing precipitation means, can play an important role in shaping and in transforming landscapes in such arid setting.

How to cite: Shmilovitz, Y., Morin, E., Rinat, Y., Haviv, I., Carmi, G., Mushkin, A., and Enzel, Y.: Linking frequency of rainstorms, runoff generation and sediment transport across hyperarid talus-pediment slopes , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-701, https://doi.org/10.5194/egusphere-egu2020-701, 2019

D1105 |
EGU2020-987
Lena Katharina Öttl, Peter Fiener, Florian Wilken, and Michael Sommer

Hummocky landscapes under intensive arable use are substantially affected by erosion processes. Data from the Quillow catchment (size: 196 km2; mean annual precipitation: 500 mm) in North-East Germany are used to estimate landscape-scale water and tillage erosion with the model SPEROS-C. Recent results show that tillage erosion causes substantial soil redistribution that can distinctively exceed water erosion. In consequence, truncated soil profiles can be found on hilltops and steep slopes, whereas colluvial material is accumulated in depressions and along downslope field boarders. The resulting spatial variability of soil types with different properties and conditions is known to influence crop growth and leads to a highly variable biomass pattern in hummocky landscapes under highly mechanised arable cultivation.

The main goal of our study is to link tillage-induced erosion rates to landscape development at centennial time scales. By modelling the development of the hummocky moraine landscape of North-Eastern Germany, we explain the spatial distribution of the current soil erosion state. Furthermore, the soil erosion induced impact on crop biomass patterns and the redistribution of soil organic carbon since the beginning of human land use in this area is assessed. To address this goal, a new model component is implemented into SPEROS-C that iteratively rejuvenates topography backwards in time considering modelled erosion and deposition rates. Afterwards, modelling forward in time allows estimating carbon fluxes due to soil redistribution. Furthermore, the extent and location of truncated soils will be validated with historic aerial photographs at different time steps.

The benefits of implementing landscape development into SPEROS-C are that (i) an annual update of topography generates a more realistic soil erosion pattern, (ii) the current crop biomass pattern may be explained by erosion history, and (iii) estimates about the future development of crop yield patterns considering ongoing tillage practices can be drawn from a validated soil erosion and landscape development model.

How to cite: Öttl, L. K., Fiener, P., Wilken, F., and Sommer, M.: What role does tillage erosion play regarding landscape evolution of an intensively used hummocky landscape?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-987, https://doi.org/10.5194/egusphere-egu2020-987, 2019

D1106 |
EGU2020-17653
Thomas Brunner, Anna Zeiser, Andreas Klik, and Peter Strauss

On agricultural fields, management (especially tillage) operations with a distinct orientation often lead to a corresponding preferred orientation of surface runoff and associated sediment transport. When deriving surface properties like flow directions and slope for runoff modelling from digital elevation model (DEM) data with grid sizes larger than 1m, these features of the surface will usually remain undetected and by default predict runoff and sediment transport patterns based on the topographic slope and flow directions alone.

A methodology proposed by (Takken et al., 2001) involves calculating 1) topographic slope and flow directions and 2) slope and flow directions assuming that surface runoff takes place exclusively along the tillage orientation. A decision algorithm then decides for each grid cell, whether 1) or 2) is to be used, based on cell slope, oriented roughness and the angle between topographic and tillage-controlled flow directions. An exception is made for distinct thalweg situations, where 1) is always used.

For larger areas, where the manual assignment of the management direction of individual fields (e.g. based on orthophotos) is not feasible, automatic estimation of a field’s tillage orientation is done using field geometry parameters and assuming tillage taking place in the direction of the longest field extent.

The output of the methodology is to be used subsequently in grid-based soil erosion modelling and is expected to provide more realistic results of surface runoff and soil loss patterns. Initial tests using the output flow directions and slope of the method as input for an MMF (Morgan-Morgan-Finney) based soil erosion model in a small experimental catchment (0.7 km²) show surface runoff and soil loss concentrating on the field borders (headlands) for some fields, potentially leading to a shift of priority for protection of either whole individual fields or particularly affected portions of fields.

The improved modelling results can in some situations be significant for decisions on the placement of best management practices (BMP) that intend to limit either soil loss from the field or sediment input into adjacent surface water bodies (e.g. vegetated filter strips, grassed waterways or the feasibility of contouring), since these measures might be rendered useless, when their placement is based on topographic flow directions alone, as is the default practice.

How to cite: Brunner, T., Zeiser, A., Klik, A., and Strauss, P.: Deriving tillage-controlled runoff patterns for agricultural fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17653, https://doi.org/10.5194/egusphere-egu2020-17653, 2020

D1107 |
EGU2020-12033
Tingray Liu and Chunhung Wu

This research focuses on the long-term geomorphologic change in the upstream of the silt dams in the Lan-daw rivers watershed in central Taiwan, adopts the long-term rainfall records in the Lan-daw rivers watershed to calculate the 1-day, 2-days, 3-days accumulated rainfall with different return period, and analyzes the relationship between the geomorphologic change and the accumulated rainfall. This research builds the Digital Surface Models based on the photos shot by UAV at 9 different times. The river in upstream of the Lan-daw rivers watershed is sinuous. The research classifies 3 time periods from 2010 to August, 2019, including the first time period from 2010 to June, 2017, the second time period from June, 2017 to Nov. 2018, and the third time period from Nov. 2018 to Aug. 2019. The target in the first time period is to observe the geomorphologic change after the first dam removal, that in the second time period to observe the geomorphologic change in the 2 years after dam removal, and that in the third time period to observe the geomorphologic change after the second dam removal.

The longitudinal slopes in the first, second, and third time periods are -30.3%, 14.8%, and 5.98%, and the knickpoint in the longitudinal profile in the first and second time periods occur in the upstream 20 m of the silt dam and that in the third time periods occurs in the upstream 45 m of the silt dam. The research classifies the cross-sections profiles into 3 groups, including the first group from C1 to C7 cross sections, the second group from C8 to C14 cross sections, and the third group from C15 to C22 cross sections. The geomorphologic change in the first group near the silt dam is the most obvious in the three groups. The geomorphologic change in the three groups in the first time period are -6.43 m to -8.13 m (scouring), those in the second time period are 0.23 m to 0.34 m (deposition), and those in the third time period are 0.46 m (deposition) to -1.78 m (scouring). Based on the analysis of the long-term rainfall record in the Lan-daw river watershed, the return period of the heaviest rainfall from 2015 to Aug. 2019 is less than 20-year return period. This means that the geomorphologic change in upstream of silt dam in the Lan-Daw river watershed is easy induced in the short time after dam removal.

How to cite: Liu, T. and Wu, C.: Long term change observation of river terrain due to dam removal in Central Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12033, https://doi.org/10.5194/egusphere-egu2020-12033, 2020

D1108 |
EGU2020-7671
Giulia Battista, Peter Molnar, Fritz Schlunegger, and Paolo Burlando

The identification of preferential sediment production areas within a river basin is essential to improve predictions of sediment load and its sources, and to identify sources of potential water pollution. The role of these localized sediment sources is especially relevant in the sediment budget of alpine basins, where erosion in highly non-uniform and mass movements play a major role in the mobilization of sediments. While sediment tracers are useful to assess the origin of river-borne sediments, currently very few spatially distributed sediment transport models include the sediment production from a variety of sources and track sediment from source to outlet.

In this work, we present a new approach to include the production of sediment from localized sources, in addition to diffusive overland flow erosion, in a spatially distributed sediment production and transport model. This extension of the hydrological model Topkapi-ETH simulates the mobilization of sediments by (i) overland flow erosion, (ii) sediment pickup from landsliding areas by overland flow and (iii) river discharge, and (iv) sediment pickup from deeply incised valleys by channel flow. Landslides and incised valleys were identified from geological/geomorphological maps and a high resolution DEM of the study basin. To model the contribution of landslides, we introduce a parameter λ for gully competence, which describes the effectiveness of overland flow in mobilizing the sediments. Overall, λ affects the contributions of the different sediment production processes to the modelled sediment load at the basin outlet. To estimate a value of λ for the case study, we propose the local surface roughness to quantify the gully development onto the landslide surfaces. Additionally, we use available 10Be measurements across the basin to assign a concentration to each sediment production process and select the end member value of λ that best reproduces the observed 10Be concentrations at the outlet.

Our simulations indicate that including the production of sediments from localized sources with processes (ii) to (iv) is essential to capture the highest observed concentrations with the model. Moreover, the same observed suspended sediment concentrations at the outlet may be obtained with different combinations of sediment production processes in function of the gully competence. Finally, the local surface roughness analysis and the use of 10Be concentration as a sediment tracer suggest that channel processes are dominant over hillslope sediment production in the study basin.

In conclusion, our work shows that combinations of physically-based sediment transport modelling with geomorphological mapping of localized sediment sources, high-resolution topographic information and point measurements of cosmogenic radionuclide concentrations allow to infer the dominant sediment production processes in river basins.

How to cite: Battista, G., Molnar, P., Schlunegger, F., and Burlando, P.: Diffused and localized sediment production processes in a distributed transport model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7671, https://doi.org/10.5194/egusphere-egu2020-7671, 2020

D1109 |
EGU2020-4821
Roger Clavera-Gispert and Aaron Micallef

Groundwater has been implicated as an important geomorphic agent in landscape evolution. The link between groundwater seepage and landscape evolution remains controversial and poorly quantified, however. Groundwater weathering and erosion processes have not been quantified in terms of mechanisms, rates or resulting morphologies. Experimental and numerical analyses of these processes have been based on simplistic assumptions about flow processes and hydraulic characteristics. There is also a paucity of process-based observations and detailed instrumental studies of seepage erosion and weathering due to the long timescales involved and the complexity of the process. Numerical modelling, in particular Landscape Evolution Modelling (LEM), is a valuable tool that can allow us to better understand the spatial and temporal evolution of landscapes by groundwater seepage, particularly when integrated with field data.

 

Here we report preliminary results from a study focusing on the Canterbury coast of the South Island, New Zealand. The study area, located between the Ashburton and Rakaia Rivers, comprises a 20 m high coastal cliff of sandy gravels with isolated sand bodies that features a series of box canyons. Field visits carried out in 2017 and 2019 allowed us to characterise the geological framework of the area and monitor the formation and evolution of box canyons by groundwater seepage. We used Landlab, an open source framework written in python, to build a LEM for the study area.  The code includes a simplified groundwater model using the Dupuit approximation, the calculation of the drainage area, as well as erosion processes using diffusion and a power law functions.

 

The model computes the evolution of the coastal landscape during 1 year. The initial topography is obtained from a 1x1m DEM and the initial conditions are derived from the fieldwork. Several examples have been run using different aquifer recharge rates and hydraulic conductivity. The results suggest that the factor that controls the inception erosion is the spatial variability in permeability and initial topography, whereas the evolution of the canyon is controlled by the seepage flow, which depends on the hydraulic conductivity and the erosivity of the sediments.

How to cite: Clavera-Gispert, R. and Micallef, A.: Numerical modelling of groundwater seepage and landscape evolution along the Canterbury coast, South Island, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4821, https://doi.org/10.5194/egusphere-egu2020-4821, 2020

D1110 |
EGU2020-13732
Elco Luijendijk

The role of groundwater flow in determining overland flow, drainage density and landscape evolution has long been debated. Landscape models often only address groundwater as a simplified storage term and do not explicitly include lateral groundwater flow, although recently some model codes have started to include lateral flow. However, the role of groundwater flow on landscape evolution has not been explored systematically to my knowledge. Here I present a new numerical and analytical model that combines groundwater flow, saturation overland flow, hillslope diffusion and stream erosion. A number of model experiments were run with different values of transmissivity and groundwater recharge. The model results demonstrate that transmissivity, groundwater flow and the depth of the watertable strongly govern overland flow, the incision of stream channels and erosion rates. The results imply that the permeability and transmissivity of the subsurface are important parameters for explaining and modelling landscape evolution.  

How to cite: Luijendijk, E.: Modelling the effects of permeability, groundwater flow and water table depth on landscape evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13732, https://doi.org/10.5194/egusphere-egu2020-13732, 2020

D1111 |
EGU2020-4236
Ariane Mueting, Bodo Bookhagen, and Manfred R. Strecker

Mountainous high-relief terrains in climatically sensitive regions are often subjected to natural extreme events such as debris flows and landsliding. With people and infrastructure at risk, it is important to identify, measure, and comprehend the driving forces and mechanisms of slope movements in these environments at regional scale. Geomorphologic analyses and hazard assessments in these regions are, however, often limited by the availability of good-quality high-resolution digital elevation models (DEMs). Publically available data often have lower spatial resolution and are distorted in high-relief areas. In contrast, airplane-based lidar (light detection and ranging) data provide highly accurate information on 3D structure, yet, acquisition is costly and limits the size of the respective study area. Finding adequate, economical alternatives for creating high-resolution DEMs is therefore essential to study Earth-surface processes at regional scale, which may enable the detection of spatial variations, clusters and trends.

In areas with sparse vegetation, stereogrammetry has proven to be a viable tool for creating high-resolution DEMs. Here, we use SPOT-7 tri-stereo satellite imagery to create DEMs at 3 m spatial resolution for the Quebrada del Toro (QdT) in the Eastern Cordillera of NW Argentine Andes, an area with extreme gradients in topography, rainfall and erosion. Over 5000 GPS points collected during fieldwork ensure the spatial coherence of our DEMs.

Field observations in this high-elevation area show that the hillslopes of the deeply incised QdT gorge are characterized by debris flow deposits of various extent. Debris flows have a specific slope-drainage area relationship that curves in log-log space. Using high-resolution topographic data, we are able to provide further evidence for this phenomenon and characterize the distinct topographic signature of debris flows. We specifically focus on the transition zone between debris-flow and fluvial processes, which is variable in the different catchments. The transition is characterized by a pronounced kink revealed in slope-drainage plots, as well as an increase of slope scatter in the drainage area logbins. We propose that the presence and location of this kink reflects the nature of the dominating transport processes in the corresponding catchments. In light of these observations we discriminate between debris-flow and fluvially dominated catchments in the QdT and identify regions that primarily exhibit slope movement. Our new results reveal a cluster of fluvial catchments to the SE of our study area – an area that receives significantly more moisture than upstream regions. In contrast, debris flows are prominent in areas of sparse vegetation, where occasional extreme rainfall events are efficient in transporting large amounts of talus downhill. These observations are key to a better understanding of the relationships between the impact of extreme rainfalls at high elevation and the formation of large volumes of sediment in the arid highlands of the Andes.

How to cite: Mueting, A., Bookhagen, B., and Strecker, M. R.: Using high-resolution DEMs for debris flow detection based on topographic signatures: A case study in the Quebrada del Toro, NW Argentina, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4236, https://doi.org/10.5194/egusphere-egu2020-4236, 2020

D1112 |
EGU2020-9913
Odin Marc, Hussain Alqattan, and Sean Willett

 Many long-term landscape evolution models are currently combining equations describing the evolution of the surface under fluvial incision (using the so-called stream-power incision model) and hillslope transport (often modeled as linear diffusion). Some models combine these two terms (e.g., Fastscape) and implicitly contain a transition from hillslope to fluvial processes dependent on the ratio of the diffusive and fluvial erosional parameters, D and K respectively (Perron et al., 2009). Other models require as input a hillslope-fluvial transition length (e.g., DAC) and apply hillslope erosion from the ridge-top to this lengthscale and fluvial incision only downstream of it. Still, in both cases the influence of non-linear processes such as landslide and debris-flow on this transition are not accounted.

We have analyzed the scaling between slope gradient and drainage areas in LIDAR-derived high-resolution DEM for >30 catchments, with apparent steady-state morphology, and where long-term denudation estimates, E, were estimated from cosmogenic nuclides . The catchments span different lithology, climate and denudation rates from ~0.05 to ~3 mm/yr but show a consistent pattern where substantial portion of upstream channels exhibit slope gradient roughly constant with drainage area, and transition towards a negative scaling between slope and area (characteristic of fluvial processes) after a critical drainage area, Ac. Previous work (Stock and Dietrich, 2003) suggested the portion with constant slope may be dominated by erosion due to debris-flow processes, maintaining the channel at a critical slope, Sdf.

Here we show that both Sdf, and Ac, are strongly correlated to the long-term denudation, E. Further, we find that Sdf seems to saturate at a critical slope angle, Sc , near 40° when denudation rates reach about 1mm/yr consistent with predictions for the slope of a non-linear diffusive hillsllopes (Roering et al., 2007). Combining this expression with the empirical model for the steady-state slope of Stock and Dietrich, 2003, and enforcing the consistency with a stream-power-law downstream we find that the steady state values for Sdf and Ac can be fully expressed as analytical functions of E, K, D and Sc. We assess the validity of these expressions with independent estimate of K and D extracted from local channel steepness and hilltop curvature. 

As the impact of debris flow on landscape morphology seems ubiquitous on landscape with more than 0.1 mm/yr of erosion, the classical landscape evolution formulation may need to be upgraded to correctly represent steady-state morphology of the upstream part of catchment (i.e., <1km2). Even if it still lack physical basis, we propose a formulation that adequately represent the steady state morphology from ridge to large drainage area. We show that it yield a new definition of Chi that may be better match the morphology of channel approaching ridges and we also discuss how to implement this new-steady state formulation in landscape evolution models.

How to cite: Marc, O., Alqattan, H., and Willett, S.: A morphologically-consistent expression for the transition from stream-power-law regime to a debris-flow regime, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9913, https://doi.org/10.5194/egusphere-egu2020-9913, 2020

D1113 |
EGU2020-9027
Patricia Saco, Juan Quijano, Mariano Moreno-de las Heras, Garry Willgoose, and Jose Rodriguez

Vegetation not only controls but is also controlled by erosion processes. This tight feedback effect leads to the coevolution of vegetation and erosion patterns that modulate landform shape, and regulate many other landscape processes. These tight interactions are particularly important in semiarid landscapes. We have studied these interactions using a landform evolution model that accounts for the effect (and feedbacks) of spatially and temporally varying hydrologic and vegetation patterns.

We apply the modelling framework to improve our understanding of the coevolution of landforms and vegetation patterns in different semiarid landscapes in Australia. The vegetation of the selected sites is Acacia Aneura (Mulga) which covers vast areas of Australia.  These sites display a sparse vegetation cover and strong patterns of water redistribution, with sources located in the bare areas and sinks in the vegetation patches which characterize the observed hydrologic connectivity. This effect triggers high spatial variability of erosion/deposition rates that affects the evolving topography and induces feedbacks to the dynamic vegetation patterns. We run simulations for 1000 years using local rainfall and erosion and vegetation parameters previously calibrated for similar sites in the Northern territory. Our numerical modelling results are validated by comparing simulated and observed patterns of vegetation and landforms obtained from satellite, airborne remote sensing and field data. We further investigate the effect of alterations in hydrologic connectivity induced by climate change and/or anthropogenic activities, which affect water and sediment redistribution and can be linked to loss of resources leading to degradation.

Our simulations are able to reproduce observed banded vegetation and landform patterns for the Northern territory in Australia. We show that an increase in hydrologic connectivity can trigger changes in vegetation patterns inducing feedbacks with landforms leading to degraded states. These transitions display non-linear behaviour and in some cases can lead to thresholds with an abrupt reduction in productivity. Critical implications for effective long-term restoration efforts are discussed.

How to cite: Saco, P., Quijano, J., Moreno-de las Heras, M., Willgoose, G., and Rodriguez, J.: The effect of vegetation dynamics on erosion processes, sediment dynamics, and landscape evolution in semiarid areas with sparse plant cover, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9027, https://doi.org/10.5194/egusphere-egu2020-9027, 2020

D1114 |
EGU2020-5900
Andreas Ludwig, Wolfgang Schwanghart, Florian Kober, and Angela Landgraf

The topographic evolution of landscapes strongly depends on the resistance of bedrock to erosion. Detachment-limited fluvial landscapes are commonly analyzed and modelled with the stream power incision model (SPIM) which parametrizes erosional efficiency by the bulk parameter K whose value is largely determined by bedrock erodibility. Inversion of the SPIM using longitudinal river profiles enables resolving values of K if histories of rock-uplift or base level change are known. Here, we present an approach to estimate K-values for the Wutach catchment, southern Germany. The catchment is a prominent example of river piracy that occurred ~18 ka ago as response to headward erosion of a tributary to the Rhine. Base level fall of up to 170 m triggered a wave of upstream migrating knickpoints that represent markers for the transient response of the landscape. Knickpoint migration along the main trunk stream and its tributaries passed different lithological settings, which allows us to estimate K for crystalline and sedimentary bedrock units of variable erodibility.

How to cite: Ludwig, A., Schwanghart, W., Kober, F., and Landgraf, A.: Evaluating the effect of variable lithologies on rates of knickpoint migration in the Wutach catchment, southern Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5900, https://doi.org/10.5194/egusphere-egu2020-5900, 2020

D1115 |
EGU2020-10307
Nikos Theodoratos and James W. Kirchner

Theoretical analysis of the governing equations of numerical models can reveal relationships between topographic properties, such as drainage area, slope, and curvature, in simulated landscapes. These relationships are testable predictions; they can diagnose whether real-world landscapes could potentially arise from similar mechanisms. For example, the stream-power incision model is consistent with drainage area and slope data that plot as straight lines on logarithmic axes.

Here we graph theoretical relationships between topographic curvature and the steepness index, which depends on drainage area and slope. These relationships plot as straight lines for steady-state landscapes that have evolved according to a model that combines stream-power incision, linear diffusion, and uplift. Further, they link topography (drainage area, slope, and curvature) to characteristic length scales of the landscape, which depend on the competition between the processes of incision, diffusion, and uplift.

Adding an incision threshold to the model changes the relationship between the steepness index and topographic curvature. We examine these changes graphically and we show that they shed light on how incision thresholds influence topographic and scaling properties of landscapes. Specifically, we present a graphical method that consists of plotting steepness index–curvature lines and of tracing their intersections with each other and with the coordinate axes. This simple method reveals both how topography and process competition are influenced by the incision threshold, and how these influences vary within a given landscape and across different landscapes.

How to cite: Theodoratos, N. and Kirchner, J. W.: A graphical method to interpret how incision thresholds influence topographic and scaling properties of landscapes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10307, https://doi.org/10.5194/egusphere-egu2020-10307, 2020

D1116 |
EGU2020-9474
Benoit Bovy, Jean Braun, Guillaume Cordonnier, Raphael Lange, and Xiaoping Yuan

The name “FastScape” has been used to describe a landscape evolution model as well as a set of efficient algorithms to simulate various processes of erosion, transport and deposition (e.g., fluvial, hillslope and marine). We also use this name for a set of software components (https://github.com/fastscape-lem) aimed at making those models and algorithms readily accessible to a wide range of users, from experts in landscape evolution modelling to scientists, researchers and teachers in the broader Earth science community. Those software components are organised as a stack where each level has a distinct scope. At the bottom of this stack, “fastscapelib-fortran” is the original, full-featured implementation of the FastScape model, which provides a Fortran API as well as Python bindings. Its successor “fastscapelib” is a library written in modem C++ that directly exposes the FastScape algorithms (e.g., flow-routing, depression-resolving, channel erosion, hillslope diffusion) through basic APIs in C++, Python and potentially other languages such as R or Julia in the future. Built on top of those core libraries, “fastscape” is a high-level yet flexible tool that helps anyone who wants to quickly build, extend or simply run FastScape model variants in a user-friendly, interactive environment. Through its xarray-centric interface, it is deeply integrated with the rest of the Python scientific ecosystem, therefore offering great capabilities at user’s fingertips for pre/post-processing, visualisation and simulation management. One of our primary concern is following good practices (API design, testing, documentation, distribution...) while developing each of these tools. We show through a gallery of examples how the FastScape software stack has been used in research and outreach projects. We plan to provide better integration with other tools for topographic analysis/modelling (e.g., Landlab, LSDTopotools) in the future and we also greatly encourage contributions from the broader community.

How to cite: Bovy, B., Braun, J., Cordonnier, G., Lange, R., and Yuan, X.: The FastScape software stack: reusable tools for landscape evolution modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9474, https://doi.org/10.5194/egusphere-egu2020-9474, 2020

D1117 |
EGU2020-7201
Jörg Robl and Stefan Hergarten

The majority of the highest mountain peaks on Earth is located at the dissected rim of large orogenic plateaus such as the Tibetan Plateau or the Altiplano. The striking spatial coexistence of deep, incised valleys and extraordinary high peaks located at the interfluves led to the idea of a common formation even a hundred years ago: focused erosion in valleys triggers the rise of mountain peaks due to erosional unloading and isostatically driven uplift. Ridgelines rise at the interfluves parallel to major rivers, but an additional ridgeline forms perpendicular to the principal flow direction separating the dissected rim from the undissected center of the plateau. As major rivers originate within the plateau and bypass the highest peaks, the latter rigdeline does not form a principal drainage divide. However, it forms a strong orographic barrier with wet conditions at the windward and dry conditions towards the plateau center at the leeward side. The height of the ridgeline is controlled by valley incision via erosional unloading and isostatic uplift.  If the precipitation pattern responsible for localized valley incision is controlled by the geometry of orographic barriers, a series of complex feedbacks between precipitation, erosion and ridgeline uplift (including the evolution of the highest peaks) occurs.

In this study, we present first results of a novel numerical model, which couples (a) fluvial erosion based on the stream power law, (b) flexural isostasy including viscous relaxation and (c) orographic precipitation based on the advection and diffusion of moisture and its reaction on topographic barriers. Originating from a simple model setup with a plateau in the center of the model domain and moisture transported along a predominant wind direction, we explore the co-formation of valleys and the rise of ridgelines including the growth of extraordinary high peaks. As the evolving topography controls the precipitation pattern, erosion rates are high at the wet windward side of the ridgeline, which parallels the plateau rim, while the leeward side towards the plateau center is characterized by low precipitation and very low erosion rates. As it prevents elevated low-relief areas from dissection, we suggest that this mechanism is a principal cause for the longevity of orogenic plateaus.

How to cite: Robl, J. and Hergarten, S.: The rise of high mountain peaks: Feedbacks between orographic precipitation, fluvial erosion and flexural isostasy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7201, https://doi.org/10.5194/egusphere-egu2020-7201, 2020