ALBERT

Description: Nature Geoscience 9, 42 (2016). doi:10.1038/ngeo2600 Authors: Robert Emberson, Niels Hovius, Albert Galy & Odin Marc A link between chemical weathering and physical erosion exists at the catchment scale over a wide range of erosion rates. However, in mountain environments, where erosion rates are highest, weathering may be kinetically limited and therefore decoupled from erosion. In active mountain belts, erosion is driven by bedrock landsliding at rates that depend strongly on the occurrence of extreme rainfall or seismicity. Although landslides affect only a small proportion of the landscape, bedrock landsliding can promote the collection and slow percolation of surface runoff in highly fragmented rock debris and create favourable conditions for weathering. Here we show from analysis of surface water chemistry in the Southern Alps of New Zealand that weathering in bedrock landslides controls the variability in solute load of these mountain rivers. We find that systematic patterns in surface water chemistry are strongly associated with landslide occurrence at scales from a single hillslope to an entire mountain belt, and that landslides boost weathering rates and river solute loads over decades. We conclude that landslides couple erosion and weathering in fast-eroding uplands and, thus, mountain weathering is a stochastic process that is sensitive to climatic and tectonic controls on mass wasting processes.

Description: We present a new, seismologically consistent expression for the total area and volume of populations of earthquake-triggered landslides. This model builds on a set of scaling relationships between key parameters, such as landslide spatial density, seismic ground acceleration, fault length, earthquake source depth and seismic moment. To assess the model we have assembled and normalized a catalogue of landslide inventories for 40 shallow, continental, earthquakes. Low landscape steepness causes systematic over-prediction of the total area and volume of landslides. When this effect is accounted for, the model predicts the total landslide volume of 63% of 40 cases to within a factor 2 of the volume estimated from observations ( R 2 =0.76). The prediction of total landslide area is also sensitive to the landscape steepness, but less so than the total volume, and it appears to be sensitive to controls on the landslide size-frequency distribution, and possibly the shaking duration. Some outliers are likely associated with exceptionally strong rock mass in the epicentral area, while others may be related to seismic source complexities ignored by the model. However, the close match between prediction and estimate for about two-thirds of cases in our database suggests that rock mass strength is similar in many cases and that our simple seismic model is often adequate, despite the variety of lithologies and tectonic settings covered. This makes our expression suitable for integration into landscape evolution models, and application to the anticipation or rapid assessment of secondary hazards associated with earthquakes.

Description: In humid, forested mountain belts, bedrock landslides can harvest organic carbon from above ground biomass and soil (OC modern ) while acting to refresh the landscape surface and turnover forest ecosystems. Here we assess the impact of landslides on organic carbon cycling in 13 river catchments spanning the length of the western Southern Alps, New Zealand, over four decades. We combine spatial and temporal landslide maps with the observed distribution and measured variability of hillslope OC modern stocks. On average, we estimate that landslides mobilised 7.6 ± 2.9 tC km -2  yr -1 of OC modern , ~30% of which was delivered to river channels. Comparison with published estimates of OC modern export in river suspended load suggests additional erosion of OC modern by small, shallow landslides or overland flow in catchments. The exported OC modern may contribute to geological carbon sequestration if buried in sedimentary deposits. Landslides may have also contributed to carbon sequestration over shorter timescales (〈100 years). 5.4 ± 3.0 tC km -2  yr -1 of the eroded OC modern was retained on hillslopes, representing a net-carbon sink following re-vegetation of scar surfaces. In addition, we find that landslides caused rapid turnover of the landscape, with rates of 0.3 % of the surface area per decade. We measured high rates of net ecosystem productivity in this forest of 94 ± 11 tC km -2  yr -1 , which is consistent with rapid landscape turnover suppressing ecosystem retrogression. Landslide-OC modern yields and rates of turnover vary between river catchments and appear to be controlled by gradients in climate (precipitation) and geomorphology (rock exhumation rate, topographic slope). Copyright © 2011 John Wiley & Sons, Ltd.

Description: Abstract Large earthquakes initiate chains of surface processes that last much longer than the brief moments of strong shaking. Most moderate‐ and large‐magnitude earthquakes trigger landslides, ranging from small failures in the soil cover to massive, devastating rock avalanches. Some landslides dam rivers and impound lakes, which can collapse days to centuries later, and flood mountain valleys for hundreds of kilometers downstream. Landslide deposits on slopes can remobilize during heavy rainfall and evolve into debris flows. Cracks and fractures can form and widen on mountain crests and flanks, promoting increased frequency of landslides that lasts for decades. More gradual impacts involve the flushing of excess debris downstream by rivers, which can generate bank erosion and floodplain accretion as well as channel avulsions that affect flooding frequency, settlements, ecosystems, and infrastructure. Ultimately, earthquake sequences and their geomorphic consequences alter mountain landscapes over both human and geologic time scales. Two recent events have attracted intense research into earthquake‐induced landslides and their consequences: the magnitude M 7.6 Chi‐Chi, Taiwan earthquake of 1999, and the M 7.9 Wenchuan, China earthquake of 2008. Using data and insights from these and several other earthquakes, we analyze how such events initiate processes that change mountain landscapes, highlight research gaps, and suggest pathways toward a more complete understanding of the seismic effects on the Earth's surface.

Description: Sediments in rivers record the dynamics of erosion processes. While bulk sediment fluxes are easily and routinely obtained, sediment caliber remains underexplored when inferring erosion mechanisms. Yet, sediment grain-size distributions may be the key to discriminating their origin. We have studied grain-size-specific suspended sediment fluxes in the Kali Gandaki, a major trans-Himalayan river. Two strategically located gauging stations enable tracing of sediment caliber on either side of the Himalayan orographic barrier. The data show that fine sediment input into the northern headwaters is persistent, while coarse sediment comes from the High Himalayas during the summer monsoon. A temporally matching landslide inventory similarly indicates the prominence of monsoon-driven hillslope mass wasting. Thus, mechanisms of sediment supply can leave strong traces in the fluvial caliber, which could project well beyond the mountain front and add to the variability of the sedimentary record of orogen erosion.

Description: Nature Geoscience 7, 682 (2014). doi:10.1038/ngeo2224 Authors: Kristen L. Cook, Jens M. Turowski & Niels Hovius

Description: Step‐pool systems are ubiquitous in mountain regions, but their formation is debated. A field test of competing step formation models reveals that steps form by jamming in narrow channel reaches and around keystones in wide channel reaches. Abstract Steep streams often feature a step‐pool morphology where the steps determine channel stability and dissipate the stream's energy, and thus are important for local flow hydraulics and bedload transport. Steps also play a key‐role for the coupling of channels and adjacent hillslopes by controlling hillslope stability. Although step‐pool systems have been investigated in various modelling and experimental efforts, the processes of step formation and destruction remain under debate. Theories of step formation consider a wide range of dominant drivers and fall into three groups favouring hydraulic controls (HC), granular interactions during flow (GI) or random drivers (RD) as relevant factors for step location. A direct evaluation of these models with field observations is challenging, as step formation cannot be directly observed. Based on the physical mechanisms of the various formation models we derive diagnostic morphometric parameters and test them with a field data set from a steep stream in Switzerland. Our results suggest that one class of alluvial steps form due to jamming in narrow and narrowing sections of the channel, while steps in wide and widening sections form around rarely mobile keystones. These two models of step formation apply in our study reach at the same time in different locations of the channel. A third class of steps is forced by logs. Such steps are typically located close to the original growth position of the tree and therefore reflect strong channel‐hillslope coupling. Wood‐forced steps make up a minor fraction of the step population, but contribute significantly to the cumulative step height and are therefore relevant to reach‐scale flow resistance of the channel. © 2019 John Wiley & Sons, Ltd.

Description: ABSTRACT We provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation. Using air photos, field surveys, and hydrological data, we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by Da'an River in Western Taiwan. This reach of the river experienced 10 m of uplift during the 1999 Chi-Chi earthquake. For five years following the earthquake, bedload was prevented from entering the uplift zone, the knickpoint was static and little incision took place. Bedload transport across the uplift zone resumed in 2004, initiating extremely rapid incision, with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008. This change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation. Once bedload tools became available, knickpoint propagation was influenced by geological structure, lithology, and drainage organization. In particular, a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Earthquake-induced landslide (EQIL) inventories are essential tools to extend our knowledge of the relationship between earthquakes and the landslides they can trigger. Regrettably, such inventories are difficult to generate and therefore scarce, and the available ones differ in terms of their quality and level of completeness. Moreover, access to existing EQIL inventories is currently difficult because there is no centralized database. To address these issues, we compiled EQIL inventories from around the globe based on an extensive literature study. The database contains information on 363 landslide-triggering earthquakes and includes 66 digital landslide inventories. To make these data openly available, we created a repository to host the digital inventories that we have permission to redistribute through the U.S. Geological Survey ScienceBase platform. It can grow over time as more authors contribute their inventories. We analyze the distribution of EQIL events by time period and location, more specifically breaking down the distribution by continent, country and mountain region. Additionally, we analyze frequency distributions of EQIL characteristics, such as the approximate area affected by landslides, total number of landslides, maximum distance from fault rupture zone, and distance from epicenter when the fault plane location is unknown. For the available digital EQIL inventories, we examine the underlying characteristics of landslide size, topographic slope, roughness, local relief, distance to streams, peak ground acceleration, peak ground velocity, and Modified Mercalli Intensity. Also, we present an evaluation system to help users assess the suitability of the available inventories for different types of EQIL studies and model development.