Description: Gullies have been a common phenomenon in semi-arid northern Ethiopia for the last centuries. On the other hand, soil and water conservation (SWC) structures have been implemented since a long time to curb soil erosion. However, like most of the affected areas worldwide, density and distribution of gullies and SWC structures, their causes and interrelations are poorly understood. The aims of this study were to develop a technique for mapping these densities of gullies and SWC structures, to explain their spatial distribution and to analyze changes over the period 1935 – 2014. Aerial photographs from 1935-36 and Google Earth images from 2014 of the 5142 km 2 Geba catchment were used. Transect lines were established to count gullies and SWC structures in order to calculate densities. On average, a gully density of 1.14 km km -2 was measured in 1935-36 of which the larger portion (75%) were vegetated, indicating they were not very active. Over 80 years, gully density has significantly increased to 1.59 km km -2 with less vegetation growing in their channel, but 66% of these gullies were treated with check dams. There were ca. 3 km km -2 of indigenous SWC structures ( daget or lynchets) in 1935-36 whereas a high density (20 km km -2 ) of introduced SWC structures (mainly stone bunds and terraces) were observed in 2014. The density of gullies is positively correlated with slope gradient and shrubland cover and negatively with cropland cover, whereas the density of SWC structures significantly increased with increasing cropland cover. Density maps of gullies and SWC structures indicate sensitive areas to gully formation and priority areas for the implementation of soil and water conservation structures in Geba catchment. The obtained results illustrate the feasibility of the methods applied to map the density of gullies and SWC structures in mountainous areas.

Description: Cuesta escarpment retreat is a principal mode of exhumation in regions of layered sedimentary rock. On the Colorado Plateau, this process acts as a mechanism for maintaining high-relief topography and facilitating drainage divide migration. Quantitative estimates of cuesta evolution are difficult to evaluate over glacial-interglacial timescales, and thus rates of geomorphic change along individual escarpments have mostly been constrained over millions of years. Several studies have addressed this problem by dating colluvium-mantled talus flatirons. However, this technique has not been applied systematically on the Colorado Plateau. This study quantifies geomorphic change along a single Colorado Plateau cuesta using 36 Cl surface exposure dating. We present 33 ages from a single generation of talus flatirons below the Coal Cliffs of central Utah. Landscape evolution is further constrained using 14 ages from in-situ bedrock, 3 ages from boulders on gully interfluves, and two ages from terrace alluvium. Results suggest a colluvial apron was deposited below the cuesta beginning as early as Marine Isotope Stage 3, and the latest depositional phase occurred near the Last Glacial Maximum. A switch from apron deposition to incision initiated flatiron formation sometime between 19.7 ± 2.5 and 11.8 ± 1.6 ka, broadly coincident with the transition from glacial to interglacial conditions. Our results have several important implications. Climatic changes during the end of the last glacial period appear to have shifted the balance between deposition and erosion below the Coal Cliffs, emptying the sediment reservoir at their base and increasing their height via bedrock incision. The climatic forcing could be imparted by several mechanisms, including local controls on debris generation / mobilization and base level changes exerted by transverse streams. Similar processes may have occurred during previous glacial-interglacial transitions, implying that the escarpment retreat processes may be partially modulated by orbitally-controlled variations in Earth's climate over larger timescales.

Description: When studying the evolution of landscape, it is difficult to discriminate the influence of anthropogenic from natural causes, or recognise changes caused by different sources of human action. This is especially challenging when the influence of certain sources is overprinted. For instance, although dam closure is the most common method of altering river courses, dam construction is often preceded by hydro technical works such as channel straightening, embankment construction or sediment mining. Both dam construction and the hydro technical works that precede dam closure can result in changes in the balance between sediment supply and transport capacity, and often, changes in river planform (Lane, 1955). The main objective of this study was to verify whether the works preceding dam closure are an important driver of river planform changes on the lower Drava River (Hungary). The case study is based on geological and geophysical surveys, as well as the analysis of historical maps covering an anabranching, 23 km long valley section. We show that channel straightening conducted prior to dam closure resulted in a transition from a meandering to sinuous planform with channel bars. Dam construction itself then caused enhanced incision, exposure of bar surfaces, vegetation encroachment and the formation of an anabranching planform. Based on this study, we developed models of alluvial island and channel planform evolution downstream of dams. Dam construction enhances channel incision, narrowing, and the reduction of flow caused by earlier hydro technical works. Many rivers downstream of dams experience episodes of anabranching or wandering, with a multi-thread pattern replacing sinuous, braided and meandering courses. When incision continues, river patterns evolve from anabranching to sinuous via the attachment of alluvial islands to floodplains. However, the timing and sequence of these changes depend on hydrological and sediment supply regimes, geomorphic settings and anthropogenic actions accompanying dam construction.

Description: The effect of a step change in macro-roughness on the saltation process under sediment supply limited conditions was examined in the atmospheric boundary layer. For an array of roughness elements of roughness density λ=0.045 (λ=total element frontal area/total surface area of the array) the horizontal saltation flux was reduced by 90% (±7%) at a distance of ≈150 roughness element heights into the array. This matches the value predicted using an empirical design model and provides confidence that it can be effectively used to engineer roughness arrays to meet sand flux reduction targets. Measurements of the saltation flux characteristics in the vertical dimension, including: saltation layer decay (e-folding) height and particle size, revealed that with increasing distance into the array, the rate of mass flux change with increasing height decreased notably, and (geometric) mean particle diameter decreased. The distribution of the saltation mass flux in the vertical remains exponential in form with increasing distance into the roughness array, and the e-folding height increases as well increasing at a greater rate as particle diameter diminishes. The increase in e-folding height suggests the height of saltating particles is increasing along with their mean speed. This apparent increase in mean speed is likely due to the preferential removal, or sequestration, of the slower moving particles, across the size spectrum, as they travel through the roughness array. This article is protected by copyright. All rights reserved.

Description: Landscape adjustment to tectonic, lithologic and climatic forcing leads to drainage reorganization and migration of divides. The respective contribution of these forcings, especially on carbonate landscapes is not well defined. Here, we have addressed this issue by combining field observations, satellite image interpretation and DEM quantitative analysis to assess drainage response to spatially heterogeneous rainfall, asymmetric uplift, and normal faulting on an emerging carbonated platform (Sumba Island, Indonesia). We map geomorphic markers of fluvial dynamics and drainage rearrangement and compute a χ parameter that incorporates the contributions of unevenly distributed precipitation and asymmetric uplift to estimate erosional disequilibrium across drainage divides. We find that asymmetric emergence of Sumba Island created an initial parallel drainage, asymmetric across a divide that propagates landwards. Soon after establishing itself on the emerging slopes this drainage was disturbed by normal faulting, which has become the main force driving drainage rearrangement. Vertical offsets across normal fault scarps first triggered aggradation within valleys over the hanging walls, and then disconnected upstream reaches from downstream reaches, leading to the formation of wind gaps atop the fault scarps and upstream perched sedimentary basins. The defeat of rivers by growing fault scarps was catalysed by the possibility for surface water to be rerouted near the fault scarps into underground water networks inside the underlying carbonates. At the end of the process, the opposite drainage across the main water divide captured the struggling drainage. Capture mechanisms include initial groundwater capture of the perched alluvial aquifers, followed by ground sapping at the head of the opposite drainage and surface stream diversion by avulsion. Finally, normal faulting is the main driving force of drainage rearrangement allowing avulsion and karstic rerouting whereas asymmetric uplift and climate forcings have shown a low efficiency. The role of karstification is more ambiguous, catalyzing or inhibiting drainage rearrangement.

Description: To maintain a reasonable sediment regulation system in the middle reaches of the Yellow River, it is critical to determine the variation in sediment deposition behind check–dams for different soil erosion conditions. Sediment samples were collected by using a drilling machine in the Fangta watershed of the loess hilly–gully region and the Manhonggou watershed of the weathered sandstone hilly–gully ( pisha ) region. On the basis of the check–dam capacity curves, the soil bulk densities and the couplet thickness in these two small watersheds, the sediment yields were deduced at the watershed scale. The annual average sediment deposition rate in the Manhonggou watershed (702.0 mm/(km 2 ·a)) from 1976 to 2009 was much higher than that in the Fangta watershed (171.6 mm/(km 2 ·a)) from 1975 to 2013. The soil particle size distributions in these two small watersheds were generally centred on the silt and sand fractions, which were 42.4% and 50.7% in the Fangta watershed and 60.6% and 32.9% in the Manhonggou watershed, respectively. The annual sediment deposition yield exhibited a decreasing trend; the transition years were 1991 in the Fangta watershed and 1996 in the Manhonggou watershed (P 〈 0.05). In contrast, the annual average sediment deposition yield was much higher in the Manhonggou watershed (14011.1 t/(km 2 ·a)) than in the Fangta watershed (3149.6 t/(km 2 ·a)). In addition, the rainfalls that induced sediment deposition at the check–dams were greater than 30 mm in the Fangta watershed and 20 mm in the Manhonggou watershed. The rainfall was not the main reason for the difference in the sediment yield between the two small watersheds. The conversion of farmland to forestland or grassland was the main reason for the decrease in the soil erosion in the Fangta watershed, while the weathered sandstone and bare land were the main factors driving the high sediment yield in the Manhonggou watershed. Knowledge of the sediment deposition process of check–dams and the variation in the catchment sediment yield under different soil erosion conditions can serve as a basis for the implementation of improved soil erosion and sediment control strategies, particularly in semi-arid hilly–gully regions.

Description: Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes — events occurring once per year or less frequently with discharges larger than a few hundred of cubic metres per second — are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a 1-D numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed- and suspended-load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D  = 256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges.

Description: This paper demonstrates that the Belgian Continental Shelf and coastal plain occupy a key position between the depositional North Sea Basin and the erosional area of the Dover Strait as it is an area where erosional landforms and fragmented sedimentary sequences provide new evidence on northwest European landscape evolution. The study area hosts 20-30 m thick penultimate to last glacial sand-dominated sequences that are preserved within the buried palaeo-Scheldt Valley. Here, we build on the results of previous seismo- and lithostratigraphical studies, and present new evidence from biostratigraphical analysis, OSL dating and depth-converted structure maps, together revealing a complex history of deposition and landscape evolution controlled by climate change, sea-level fluctuations and glacio-isostasy. This study presents strong new supportive evidence on the development of the incised palaeo-Scheldt Valley landform that became established towards the end of the penultimate glacial period (MIS 6; Saalian) as a result of glacio-isostatic forebulge updoming, proglacial lake drainage and subsequent collapse of a forebulge between East Anglia and Belgium following ice-sheet growth, disintegration and retreat in areas to the north. The majority of the incised-valley fill is of estuarine to shallow marine depositional context deposited during the transgression and high-stand of the last interglacial (MIS 5e: Eemian). A thin upper part of the valley fill consists of last glacial (MIS 5d-2: Weichselian) fluvial sediments that show a gradual decrease and retreat of fluvial activity to inland, upstream reaches of the valley system until finally the valley ceases to exist as the combined result of climate-driven aeolian activity and possibly also glacio-isostatic adjustment. Thus, strong contrasts exists between the palaeo-Scheldt Valley and estuary systems of the penultimate glacial maximum to Last Interglacial (Saalian, Eemian), the begin of the Last Glacial (Weichselian Early Glacial and Early-Middle Pleniglacial), and the Last Glacial Maximum to Holocene.

Description: Soil erosion hinders the recovery and development of ecosystems in semiarid regions. Rainstorms, coupled with the absence of vegetation and improper land management, are important causes of soil erosion in such areas. Greater effort should be made to quantify the initial erosion processes and try to find better solutions for soil and water conservation. In this research, 54 rainfall simulations were performed to assess the impacts of vegetation patterns on soil erosion in a semiarid area of the Loess Plateau, China. Three rainfall intensities (15 mm/h, 30 mm/h and 60 mm/h) and six vegetation patterns (arbors-shrubs-grass -A-S-G-, arbors-grass-shrubs -A-G-S-, shrubs-arbors-grass -S-A-G-, shrubs-grass-arbors -S-G-A-, grass-shrubs-arbors -G-S-A- and grass-arbors-shrubs -G-A-S-) were examined at different slope positions (summits, backslopes and footslopes) in the plots (33.3%- 33.3%- 33.3%), respectively. Results showed that the response of soil erosion to rainfall intensity differed under different vegetation patterns. On average, increasing rainfall intensity by 2 to 4 times induced increases of 3.1 to 12.5 times in total runoff and 6.9 to 46.4 times in total sediment yield, respectively. Moreover, if total biomass was held constant across the slope, the patterns of A-G-S and A-S-G (planting arbor at the summit position) had the highest runoff (18.34 l·m -2 ·h -1 ) and soil losses (197.98 g·m -2 ·h -1 ), while S-A-G had the lowest runoff (5.51 l·m -2 ·h -1 ) and soil loss (21.77 g·m -2 ·h -1 ). As indicated by redundancy analysis (RDA) and Pearson correlation results, a greater volume of vegetation located on the back- and footslopes acted as effective buffers to prevent runoff generation and sediment yield. Our findings indicated that adjusting vegetation position along slopes can be a crucial tool to control water erosion and benefit ecosystem restoration on the Loess Plateau and other similar regions of the world.

Description: Summary statistics derived from the frequency-area distribution (FAD) of inventories of triggered landslides allows for direct comparison of landslides triggered by one event (e.g., earthquake, rainstorm) to another. Such comparisons are vital to understanding links between the landslide-event and the environmental characteristics of the area affected. This could lead to methods for rapid estimation of landslide-event magnitude, which in turn could lead to estimates of the total triggered landslide area. Previous studies proposed that the FAD of landslides follows an inverse power-law, which provides the basis to model the size distribution of landslides and to estimate landslide-event magnitude ( mLS ), which quantifies the severity of the event. In this study, we use a much larger collection of earthquake-induced landslide (EQIL) inventories (n=45) than previous studies to show that size distributions are much more variable than previously assumed. We present an updated model and propose a method for estimating mLS and its uncertainty that better fits the observations and is more reproducible, robust, and consistent than existing methods. We validate our model by computing mLS for all of the inventories in our dataset and comparing that to the total landslide areas of the inventories. We show that our method is able to estimate the total landslide area of the events in this larger inventory dataset more successfully than the existing methods.