Flood magnitude-frequency and lithologic control on bedrock river incision in post-orogenic terrain

Mixed bedrock-alluvial rivers - bedrock channels lined with a discontinuous alluvial cover - are key agents in the shaping of mountain belt topography by bedrock fluvial incision. Whereas much research focuses upon the erosional dynamics of such rivers in the context of rapidly uplifting orogenic landscapes, the present study investigates river incision processes in a post-orogenic (cratonic) landscape undergoing extremely low rates of incision (> 5 m/Ma). River incision processes are examined as a function of substrate lithology and the magnitude and frequency of formative flows along Sandy Creek gorge, a mixed bedrock-alluvial stream in arid SE-central Australia. Incision is focused along a bedrock channel with a partial alluvial cover arranged into riffle-pool macrobedforms that reflect interactions between rock structure and large-flood hydraulics. Variations in channel width and gradient determine longitudinal trends in mean shear stress (τb) and therefore also patterns of sediment transport and deposition. A steep and narrow, non-propagating knickzone (with 5% alluvial cover) coincides with a resistant quartzite unit that subdivides the gorge into three reaches according to different rock erodibility and channel morphology. The three reaches also separate distinct erosional styles: bedrock plucking (i.e. detachment-limited erosion) prevails along the knickzone, whereas along the upper and lower gorge rock incision is dependent upon large formative floods exceeding critical erosion thresholds (τc) for coarse boulder deposits that line 70% of the channel thalweg (i.e. transport-limited erosion). The mobility of coarse bed materials (up to 2 m diameter) during late Holocene palaeofloods of known magnitude and age is evaluated using step-backwater flow modelling in conjunction with two selective entrainment equations. A new approach for quantifying the formative flood magnitude in mixed bedrock-alluvial rivers is described here based on the mobility of a key coarse fraction of the bed materials; in this case the d84 size fraction. A 350 m3/s formative flood fully mobilises the coarse alluvial cover with τb200-300 N/m2 across the upper and lower gorge riffles, peaking over 500 N/m2 in the knickzone. Such floods have an annual exceedance probability much less than 10- 2 and possibly as low as 10- 3. The role of coarse alluvial cover in the gorge is discussed at two scales: (1) modulation of bedrock exposure at the reach-scale, coupled with adjustment to channel width and gradient, accommodates uniform incision across rocks of different erodibility in steady-state fashion; and (2) at the sub-reach scale where coarse boulder deposits (corresponding to <i>τ</i><sub>b</sub> minima) cap topographic convexities in the rock floor, thereby restricting bedrock incision to rare large floods. While recent studies postulate that decreasing uplift rates during post-orogenic topographic decay might drive a shift to transport-limited conditions in river networks, observations here and elsewhere in post-orogenic settings suggest, to the contrary, that extremely low erosion rates are maintained with substantial bedrock channel exposure. Although bed material mobility is known to be rate-limiting for bedrock river incision under low sediment flux conditions, exactly how a partial alluvial cover might be spatially distributed to either optimise or impede the rate of bedrock incision is open to speculation. Observations here suggest that the small volume of very stable bed materials lining Sandy Creek gorge is distributed so as to minimise the rate of bedrock fluvial incision over time

Deep learning methods applied to digital elevation models: state of the art

Deep Learning (DL) has a wide variety of applications in various thematic domains, including spatial information. Although with limitations, it is also starting to be considered in operations related to Digital Elevation Models (DEMs). This study aims to review the methods of DL applied in the field of altimetric spatial information in general, and DEMs in particular. Void Filling (VF), Super-Resolution (SR), landform classification and hydrography extraction are just some of the operations where traditional methods are being replaced by DL methods. Our review concludes that although these methods have great potential, there are aspects that need to be improved. More appropriate terrain information or algorithm parameterisation are some of the challenges that this methodology still needs to face.Functional Quality of Digital Elevation Models in Engineering’ of the State Agency Research of SpainPID2019-106195RB- I00/AEI/10.13039/50110001103

Subglacial floods beneath ice sheets.

Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future

Glacial Geomorphology of southern Alberta, Canada

During deglaciation from the Last Glacial Maximum three terrestrial ice streams within the south western sector of the Laurentide Ice Sheet competed and coalesced in southern Alberta; the High Plains Ice Stream (HPIS), Central Alberta Ice Stream (CAIS) and the east lobe. The ice streams are characterised by smoothed corridors along which lie lineations that identify multiple flow events, transverse ridges of thrust and push origin, esker networks and large sequences of parallel and transverse meltwater channels. The CAIS and HPIS were dynamic and transitory in nature creating a ‘time-transgressive’ imprint. The CAIS terminated within southern Alberta creating a wealth of landforms composed of controlled, hummocky, push and thrust block moraines, along with doughnut hummocks, ice walled lake plains, recessional meltwater, tunnel and large spillway channels. The CAIS margin is interpreted to have been polythermal in nature, creating a continuum of landforms that is dominated by active marginal recession. The methodologies used were placed within an overarching ‘scale approach’, whereby the research initially focused on a small, regional scale and gradually moved to large scale, local investigations. Firstly, Shuttle Radar Topography Mission (SRTM) and Landsat 7 Enhanced Thematic Mapper (Landsat ETM+) data sets were used to map the regional picture. Then, Aerial Photo Investigation (API), ground truthing and sedimentary analyses were employed to provide a detailed, localised focus into the landform sediment assemblages in southern Alberta

The identification of former terrestrial ice stream dynamics from geomorphic evidence and till architecture: A case study of southwestern Saskatchewan

A multidimensional study, utilising geomorphological and sedimentological techniques, is conducted to investigate the former dynamics and regional till architecture of terrestrial ice streams during the last (Late Wisconsinan) deglaciation of the Laurentide Ice Sheet. Detailed mapping over a 57,400 km2 area of southwestern Saskatchewan reaffirmed previous proposals of a southwest trending ice stream demarcated by a corridor of megaflutes and mega-scale glacial lineations (Ice Stream 1). Extending from the Canadian shield to southwestern Saskatchewan this ice stream is cross cut by three (one previously unrecognised) southeast trending ice streams (Ice Streams 2A, B and C). Analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals a superimposed till and associated deposits comprising 17 stratigraphic units. A 3D stratigraphic model of the 57,400 km2 swath was constructed, by extrapolating data away from boreholes using a nearest-neighbour approach. Using this model the thickness, extent and distribution of these stratigraphic units was delineated allowing the depositional history of the region to be reconstructed and thus the extent of till emplaced during ice stream operation through time and space to be inferred. Reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction recorded by the cross cutting corridors of Ice Streams 1, 2A, B and C. Thin tills at the centre of the trunk zone of Ice Stream 1 in many places lie unconformably over stratified sediments. This suggests widespread basal sliding may have been subordinate to subglacial sediment deformation but the general thickening of tills towards the lobate terminal margins is consistent with subglacial deformation theory. In addition, variations in till thickness are also recognised on a more localised scale. These variations are attributed to three processes; 1. down-ice thickening associated with buried valley margins; 2. upland thinning; and 3. thickening as a result of overridden glacial-marginal landforms. The significance of newly interpreted patterns of till deposition resulting from ice streaming are then considered and a model of ice stream till deposition is presented. This model provides a generalised view of the pattern of deposition resulting from fast flow over a unlithified sediment bed which may be used to infer the dynamic behaviour of other former ice sheets from their sediment imprint

Large Scale Terrain Generation from Tectonic Uplift and Fluvial Erosion

International audienceAt large scale, landscapes result from the combination of two major processes: tectonics which generate the main relief through crust uplift, and weather which accounts for erosion. This paper presents the first method in computer graphics that combines uplift and hydraulic erosion to generate visually plausible terrains. Given a user-painted uplift map, we generate a stream graph over the entire domain embedding elevation information and stream flow. Our approach relies on the stream power equation introduced in geology for hydraulic erosion. By combining crust uplift and stream power erosion we generate large realistic terrains at a low computational cost. Finally, we convert this graph into a digital elevation model by blending landform feature kernels whose parameters are derived from the information in the graph. Our method gives high-level control over the large scale dendritic structures of the resulting river networks, watersheds, and mountains ridges

Reconstructing subglacial meltwater dynamics from the spatial and temporal variation in the form and pattern of eskers

Meltwater drainage beneath glaciers and ice sheets is intimately linked to their dynamics. Meltwater may increase ice velocity if it acts to lubricate the bed; conversely, an efficient subglacial meltwater drainage system may preclude meltwater induced ice acceleration by limiting the amount of water available to facilitate sliding. Thus, understanding the nature of meltwater flow beneath ice masses is crucial for predicting how ice sheets and glaciers will react to increased meltwater input. However, direct observation of subglacial meltwater drainage systems is extremely difficult, meaning that indirect methods such as remote sensing, numerical modelling, dye tracing and geophysical survey are the only way to observe this environment. These methods often suffer from excessive uncertainty and poor spatial and, particularly, temporal resolution. This thesis presents the results of an alternative approach, using the geomorphological record of eskers to understand the former behaviour of meltwater beneath the Laurentide Ice Sheet (LIS) in Canada, and at Breiðamerkurjӧkull in Iceland. Eskers are elongate, sinuous ridges of glaciofluvial sand and gravel deposited in glacial drainage channels. Despite a large body of research on eskers, no systematic analysis of the large-scale properties of eskers, or the implications this may have for understanding subglacial meltwater, has yet been undertaken. Eskers are mapped at the ice sheet (continental) scale in Canada from 678 Landsat ETM+ images and at high resolution (~30 cm) from 407 aerial photographs of the Breiðamerkurjӧkull foreland, in order to address three outstanding questions: (i) What controls the pattern and morphology of eskers? (ii) How did subglacial drainage systems evolve during ice sheet deglaciation? (iii) How can eskers be used to further our understanding of subglacial hydrology? Over 20,000 eskers are mapped in Canada, revealing that esker systems are up to 760 km long, and are surprisingly straight. The spacing between eskers is relatively uniform and they exhibit little change in elevation from one end to another. As the LIS deglaciated between 13 cal ka and 7 cal ka, eskers increased in frequency, which is interpreted to represent an increase in meltwater drainage in channelized, rather than distributed, systems. Eskers are abundant over the resistant rocks of the Canadian Shield and also show a strong preference for formation in areas covered with till. Esker length, sinuosity and spacing appear to be unrelated to the underlying geology. Finally, two types of complex esker systems are proposed: esker fan complexes and topographically constrained esker complexes. The formation of esker complexes is dependent on sediment and meltwater supply and the pre-existing topography controls the overall shape of the esker systems

Constraint energies for the adaptation of 2d river borderlines to airborne laserscanning data using snakes

The German Authoritative Topographic Cartographic Information System (ATKIS) stores the height and the 2D position of the objects in a dual system. The digital terrain model (DTM), often acquired by airborne laser scanning (ALS), supplies the height information in a regular grid, whereas 2D vector data are provided in the digital landscape model (DLM). However, an increasing number of applications, such as flood risk modelling, require the combined processing and visualization of these two data sets. Due to different kinds of acquisition, processing, and modelling discrepancies exist between the DTM and DLM and thus a simple integration may lead to semantically incorrect 3D objects. For example, rivers may flow uphill. In this paper we propose an algorithm for the adaptation of 2D river borderlines to ALS data by means of snakes. Besides the two basic energy terms of the snake, the internal and image energy, 3D object knowledge is introduced in the constraint energy in order to guarantee the semantic correctness of the rivers in a combined data set. The image energy is based on ALS intensity and height information and derived products. Additionally, features of rivers in the DTM, such as the flow direction or the river profile, are formulated as constraints in order to fulfil the semantic properties of rivers and stabilize the adaptation process. Furthermore, the known concept of twin snakes exploits the width of the river and also supports the procedure. Some results are given to show the applicability of the algorithm