ALBERT

Description: Heavy rainfall is expected to intensify with increasing temperatures, which will likely affect the rainfall spatial characteristics. The spatial variability of rainfall can affect streamflow and sediment transport volumes and peaks. Yet, the effect of climate change on the small-scale spatial structure of heavy rainfall and how those impacts hydrology and geomorphology remains largely unexplored. In this study, the sensitivity of the hydro-morphological response to heavy rainfall at the small-scale of minutes and hundreds of meters was investigated. A numerical experiment was conducted, in which synthetic rainfall fields representing heavy rainfall events of two types, stratiform and convective, were simulated using a space-time rainfall generator model. The rainfall fields were modified to follow different spatial rainfall scenarios, associated with increasing temperatures, and used as inputs into a landscape evolution model. The experiment was conducted over a complex topography medium-size (477 km2) Alpine catchment in central Switzerland. The results highlight that the response of the streamflow and sediment yields are highly sensitive to changes in the rainfall structure at the small-scale, in particular to changes in the areal rainfall intensity and in the area of heavy rainfall, which alters the total rainfall volume, and to a lesser extent to changes in the peak rainfall intensity. The hydro-morphological response is enhanced (reduced) when the local peak rainfall intensified and the area of heavy rainfall increased (decreased). The hydro-morphological response was found to be more sensitive to convective rainfall than stratiform rainfall because of localized runoff and erosion production. It is further shown that assuming heavy rainfall to intensify with increasing temperatures without introducing changes in the rainfall spatial structure might lead to over-estimation of future climate impacts on basin hydro-morphology.

Description: Quantifying bedload transport is paramount to the effective management of rivers with sand or gravel-dominated bed material. However, a practical and scalable field methodology for reliably estimating bedload remains elusive. A popular approach involves calculating transport from the geometry and celerity of migrating bedforms, extracted from time-series of bed elevation profiles acquired using echosounders. Various echosounder sampling methodologies of how to extract bed elevations profiles exist. Using two sets of repeat multibeam sonar surveys with large spatio-temporal resolution and coverage, we compute bedload using three field techniques (one actual and two simulated) for acquiring bed elevation profiles: repeat multi-, single-, and multiple single-beam sonar. Significant differences in flux arise between repeat multibeam and single beam sonar. Mulitbeam and multiple single beam sonar systems can potentially yield comparable results, but the latter relies on knowledge of bedform geometries and flow that collectively inform optimal beam spacing and sampling rate. These results serve to guide design of optimal sampling, and for comparing transport estimates from different sonar configurations.

Description: Submarine Groundwater Discharge (SGD) influences ocean chemistry, circulation, spreading of nutrients and pollutants, and shapes seafloor morphology. In the Baltic Sea, SGD was linked to the development of terraces and semi-circular depressions mapped in an area of the southern Stockholm Archipelago, Sweden, in the 1990s. We mapped additional parts of the Stockholm Archipelago, areas in Blekinge, southern Sweden, and southern Finland using high-resolution multibeam sonars and sub-bottom profilers to investigate if the seafloor morphological features discovered in the 1990s are widespread and to further address the hypothesis linking SGD to their formation. Sediment coring and seafloor photography conducted with a Remote Operated Vehicle (ROV) and divers add additional information to the geophysical mapping results. We find that terraces, with general bathymetric expressions of about 1 m and lateral extents of sometimes 〉 100 m, are widespread in the surveyed areas of the Baltic Sea and are consistently formed in glacial clay. Semi-circular depressions, however, are only found in a limited part of a surveyed area east of the island Askö, southern Stockholm Archipelago. Our study supports the basic hypothesis of terrace formation initially proposed in the 1990s, i.e. groundwater flows through siltier permeable layers in glacial clay to discharge at the seafloor, leading to the formation of a sharp terrace when the clay layers above seepage zones are undermined enough to collapse. By linking the terraces to a specific geologic setting, our study further refines the formation hypothesis and forms the foundation for a future assessment of SGD in the Baltic Sea that may use marine geological mapping as a starting point. We propose that SGD through the sub-marine seafloor terraces is most likely intermittent and linked to periods of higher groundwater levels, implying that to quantify the contribution of freshwater to the Baltic Sea through this mechanism, more complex hydrogeological studies are required.

Description: The evolution of the drainage system in the Eastern Alps is inherently linked to different tectonic stages of the alpine orogeny. Crustal scale faults imposed east-directed orogen parallel flow on major rivers, whereas late orogenic surface uplift increased topographic gradients between foreland and range and hence the vulnerability of such rivers to be captured. This leads to a situation where major orogen-parallel alpine rivers such as the Salzach River or the Enns River are characterized by elongated east-west oriented catchments south of the proposed capture points, whereby almost the entire drainage area is located west of the capture point. To determine the current stability of drainage divides and to predict the potential direction of divide migration, we analyzed their geometry at catchment, headwater and hillslope scale. Therefore, we employ chi mapping for different base levels, generalized swath profiles along drainage divides and Gilbert metrics. Our results show that almost all drainage divides are asymmetric with steeper channels west and flatter channels east of a common drainage divide. Interpreting these results, we propose that drainage divides migrate from west towards east, so that the Inn catchment grows on expense of the Salzach catchment and the Salzach catchment consumes the westernmost tributaries of the Mur and Enns catchments. While Gilbert metrics show the same trend at hillslope scale at the Salzach–Enns and Salzach–Mur drainage divide, they show no significant asymmetry at the Inn–Salzach drainage divide. As topography at the latter divide is dominated by glacial landforms such as cirques and U-shaped valleys, we interpret the missing hillslope scale asymmetry of this divide as a result of Pleistocene climate modulations, which locally obscured the large-scale signal of drainage network reorganization. We suggest that the east-directed divide migration progressively leads to symmetric catchment geometries, where eventually tributaries west and east of the capture point contribute equally to the drainage area. To test this assumption we have reconstructed the proposed drainage network for different time slices. Chi mapping of these reconstructed drainage networks indicates a progressive stability of the network topology in the Eastern Alps towards the present-day situation.

Description: Here we examine the landscape of New Zealand's Marlborough Fault System, where the Australian and Pacific plates obliquely collide, in order to consider landscape evolution and the controls on fluvial patterns at complicated plate tectonic boundaries. Based on topographic patterns, we divide the study area into two geomorphic domains, the Kaikōura and Inland Marlborough regions. We present maps of drainage anomalies and channel steepness, as well as an analysis of the plan view orientations of rivers and faults, and find abundant evidence of structurally-controlled drainage and a history of capture and rearrangement. Channel steepness is highest in a zone centered on the Kaikōura domain, including within the low-elevation valleys of main stem rivers and at tributaries near the coast. This pattern is consistent with an increase in rock uplift rate toward a subduction front that is locked on its southern end. Based on these results and a wealth of previous geologic studies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the Kaikōura domain, Miocene folding above blind thrust or reverse faults generated prominent mountain peaks and formed major transverse rivers early in the plate collision history. A transition to Pliocene dextral strike-slip faulting and widespread uplift led to cycles of river channel offset, deflection and capture of tributaries draining across active faults, and headward erosion and captures by major transverse rivers within the Inland Marlborough domain. Despite clear evidence of recent rearrangement of the Inland Marlborough drainage network, rivers in this domain still flow parallel to the older faults, rather than along orthogonal traces of younger, active faults. Continued flow in the established drainage pattern may indicate that younger faults are not yet mature enough to generate the damage and weakening needed to reorient rivers. We conclude that faulting, uplift, river capture and drainage network entrenchment all dictate drainage patterns and that each factor should be considered when assessing tectonic strain from landscapes, particularly at long-lived and complex tectonic boundaries.

Description: Marine sedimentary archives are well dated and often span several glacial cycles; cosmogenic 10Be concentrations in their detrital quartz grains could thus offer the opportunity to reconstruct a wealth of past denudation rates. However, these archives often comprise sediments much finer (

Description: Allometry refers to a physical principle in which geometric (and/or metabolic) characteristics of an object or organism are correlated to its size. Allometric scaling relationships typically manifest as power laws. In geomorphic contexts, scaling relationships are a quantitative signature of organisation, structure, or regularity in a landscape, even if the mechanistic processes responsible for creating such a pattern are unclear. Despite the ubiquity and variety of scaling relationships in physical landscapes, the emergence and development of these relationships tend to be difficult to observe – either because the spatial and/or temporal scales over which they evolve are so great, or because the conditions that drive them are so dangerous (e.g., an extreme hazard event). Here, we use a physical experiment to examine dynamic allometry in overwash morphology along a model coastal barrier. We document the emergence of a canonical scaling law for deposit (washover) length versus area. Comparing the experimental features, formed during a single forcing event, to four decades of change in real washover morphology from the Ria Formosa barrier system, in southern Portugal, we show that features forming at the event scale might exhibit a different pattern of change over longer time scales. This work reinforces the potential importance of initial conditions in landscape evolution, such that a landscape may reflect characteristics associated with an equilibrium or steady-state condition even when features within that landscape do not.

Description: The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment redistribution over a 1-km2 area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out 3 joint gravity-photogrammetry surveys, once a year in 2015, 2016 and 2017 over a 1-km2 area in southern Taiwan featuring both a wide meander of the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface displacements and air pressure variations. Then, we inverted the density of the sediment, with an attempt to distinguish the density of the landslide from the density of the river sediments. We eventually estimate an average loss of 4.7 ± 0.4 × 109 kg of sediment from 2015 to 2017, mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long surveys, new instrumentation progresses shall enable dense and continuous measurements at lower cost, making this method relevant to improve the estimation of erosion, sediment transfer and deposition in landscapes.

Description: Species distribution and richness ultimately result from complex interactions between biological, physical and environmental factors. It has been recently shown for a static natural landscape that the elevational connectivity, which measures the proximity of a site to others with similar habitats, is a key physical driver of local species richness. Here we examine changes in elevational connectivity during mountain building using a landscape evolution model. We find that under uniform tectonic and variable climatic forcing, connectivity peaks at mid-elevations when the landscape reaches its geomorphic steady-state and that orographic effect on geomorphic evolution tends to favour low connectivity on leeward facing catchments. Statistical comparisons between connectivity distribution and results from a metacommunity model confirm that landscape elevation connectivity explains to the first order species richness in simulated mountainous regions. Our results also predict that low connectivity areas which favour isolation, a driver for in-situ speciation, are distributed across the entire elevational range for simulated orogenic cycles. Rapid adjustments of catchment morphology after cessation of tectonic activity should reduce speciation by decreasing the number of isolated regions.