Bio‐geomorphic effects on tidal channel evolution: impact of vegetation establishment and tidal prism change

The long‐term (10–100 years) evolution of tidal channels is generally considered to interact with the bio‐geomorphic evolution of the surrounding intertidal platform. Here we studied how the geometric properties of tidal channels (channel drainage density and channel width) change as (1) vegetation establishes on an initially bare intertidal platform and (2) sediment accretion on the intertidal platform leads to a reduction in the tidal prism (i.e. water volume that during a tidal cycle floods to and drains back from the intertidal platform). Based on a time series of aerial photographs and digital elevation models, we derived the channel geometric properties at different time steps during the evolution from an initially low‐elevated bare tidal flat towards a high‐elevated vegetated marsh. We found that vegetation establishment causes a marked increase in channel drainage density. This is explained as the friction exerted by patches of pioneer vegetation concentrates the flow in between the vegetation patches and promotes there the erosion of channels. Once vegetation has established, continued sediment accretion and tidal prism reduction do not result in significant further changes in channel drainage density and in channel widths. We hypothesize that this is explained by a partitioning of the tidal flow between concentrated channel flow, as long as the vegetation is not submerged, and more homogeneous sheet flow as the vegetation is deeply submerged. Hence, a reduction of the tidal prism due to sediment accretion on the intertidal platform, reduces especially the volume of sheet flow (which does not affect channel geometry), while the concentrated channel flow (i.e. the landscape forming volume of water) is not much affected by the tidal prism reduction. Copyright © 2012 John Wiley & Sons, Ltd.     

Control of sediment dynamics by vegetation as a key function driving biogeomorphic succession within fluvial corridors

Riparian vegetation responds to hydrogeomorphic disturbances and environmental changes and also controls these changes. Here, we propose that the control of sediment erosion and deposition by riparian vegetation is a key geomorphological and ecological (i.e. biogeomorphic) function within fluvial corridors. In a 3 year study, we investigated the correlations between riparian vegetation and hydrogeomorphic dynamics along a transverse gradient from the main channel to the floodplain of the River Tech, France. Sediment erosion and deposition rates varied significantly along the transverse gradient as a function of the vegetation biovolume intercepting water flow. These effects, combined with the extremely strong mechanical resistance of pioneer woody structures and strong resilience of pioneer labile herbaceous communities, Populus nigra and Salix spp., explain the propensity of biogeomorphic succession (i.e. the synergy between vegetation succession and landform construction) to progress between destructive floods. This geomorphological function newly identified as an ‘ecosystem function’ per se encompasses the coupling of habitat and landform creation, maintenance and change with fundamental ecosystem structural changes in space and in time. Three different biogeomorphic functions, all related to the concept of ecosystem engineering, were identified: (i) the function of pioneer herbaceous communities to retain fine sediment and diaspores in the exposed zones of the active tract near the water resource, facilitating recruitment of further herbaceous and Salicacea species; (ii) the function of woody vegetation to drive the construction of forested islands and floodplains; and (iii) the function of stabilised riparian forests to act as ‘diversity reservoirs’ which can support regeneration after destructive floods. Overall, this study based on empirical data points to the fundamental importance of sediment flow control by pioneer riparian vegetation in defining fluvial ecosystem and landform organisation in time and in space. Copyright © 2009 John Wiley & Sons, Ltd.     

Fluvial sediment supply and pioneer woody seedlings as a control on bar‐surface topography

Plants influence river channel topography, but our understanding of the interaction among plants, flow, and sediment is limited, especially when sediment supply is variable. Using laboratory experiments in a recirculating flume with live seedlings in a mobile sand bed, we demonstrate how varying the balance between sediment supply and transport capacity shifts the relationship between plants and bar‐surface topography. Each experimental trial contrasted two sediment conditions, in which initially supply was maintained in equilibrium with transport via sediment recirculation, followed by sediment deficit, in which transport capacity exceeded supply, which was set to zero. For both sediment balances, the topographic response was sensitive to plant size, with larger plants inducing greater aggradation relative to a baseline condition. During sediment equilibrium, the positive relationship between plant size and topographic change also depended on species morphology (multi‐stemmed shrubs versus single‐stemmed plants). Plant morphology effects disappeared when the sediment balance shifted to a deficit, but the presence of plants had a greater impact on the magnitude of change compared to the topographic response under sediment equilibrium. Our results suggest that the interactions among sediment supply, plants, and topography may be strongest on rivers with a balance in sediment supply and transport capacity. Because of the large variability in fluvial sediment supply resulting from natural and anthropogenic influences, these interactions will differ spatially (e.g. longitudinally through a watershed) and at different temporal scales, from single flood events to longer time periods. Copyright © 2016 John Wiley & Sons, Ltd.     

Implications of long-term changes in valley geomorphology on the behavior of small-volume pyroclastic flows

Stratigraphic mapping in the lower 3km of the Vazcún Valley on the NE flank of Volcán Tungurahua (Ecuador) provides insight into the effects of long-term geomorphic changes on pyroclastic flow behavior. Exposures of deposits in the Vazcún Valley record activity over the last 2000years, during which time significant changes in the geomorphology of the valley have occurred. Two sets of terraces are present in the lower 2–3km of the valley, the older of which grades into a small debris fan at the mouth of the valley. Each terrace formed during a period of frequent activity that was separated by a long period of quiescence during which the Río Vazcún eroded a channel as deep as 40m reaching the previous base level. The pyroclastic flows from historical eruptions appear to have been largely contained within the channel that is cut through the higher terraces and debris fan. Their surface forms the lower terrace located upstream from the head of the debris fan. Thin pyroclastic deposits exposed within the city of Baños are mostly related to ash cloud surges that detached from the main flows as they slowed down within the channel. The lower reach of the present channel of the Rio Vazcún is very sinuous and deeply incised into the two sets of terraces. The winding channel would severely impede the mobility of future pyroclastic flows resulting in the deposition of thick deposits. Detachment of an overriding ash cloud surge could also occur in this region. Such a surge could be more likely to surmount the channel banks and travel over the surface of the terraces and debris for up to several kilometers from the channel.     

The effect of landform variation on vegetation patterning and related sediment dynamics

Semi‐arid ecosystems are often spatially self‐organized in typical patterns of vegetation bands with high plant cover interspersed with bare soil areas, also known as ‘tiger bush’. In modelling studies, most often, straight planar slopes were used to analyse vegetation patterning. The effect of slope steepness has been investigated widely, and some studies investigated the effects of microtopography and hillslope orientation. However, at the larger catchment scale, the overall form of the landscape may affect vegetation patterning and these more complex landscapes are much more prevalent than straight slopes. Hence, our objective was to determine the effect of landform variation on vegetation patterning and sediment dynamics. We linked two well‐established models that simulate (a) plant growth, death and dispersal of vegetation, and (b) erosion and sedimentation dynamics. The model was tested on a straight planar hillslope and then applied to (i) a set of simple synthetic topographies with varying curvature and (ii) three more complex, real‐world landscapes of distinct morphology. Results show banded vegetation patterning on all synthetic topographies, always perpendicular to the slope gradient. Interestingly, we also found that movement of bands – a debated phenomenon – seems to be dependent on curvature. Vegetation banding was simulated on the slopes of the alluvial fan and along the valley slopes of the dissected and rolling landscapes. In all landscapes, local valleys developed a full vegetation cover induced by water concentration, which is consistent with observations worldwide. Finally, banded vegetation patterns were found to reduce erosion significantly as compared to other vegetation configurations. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Reworking of basin fill deposits along a tributary of the upper Yellow River: Implications for changes to landscape connectivity

Recent emphasis on sediment connectivity in the literature highlights the need for quantitative baseline studies on the patterns and distribution of sediment stores to facilitate understanding of how sediment moves through the landscape at various temporal and spatial scales. This study evaluates the distribution and make‐up of sediment stores within the dramatically incised landscapes of the upper Yellow River, where basin fill deposits up to 1200 m in depth have been extensively reworked following incision by the Yellow River. Field and GIS analyses highlight the discontinuous distribution of sediment stores in Garang catchment, a 236 km² tributary of the upper Yellow River. Volumetric estimates of sediment storage were obtained through a combination of field mapping, GPR transects, and GIS analyses. Sediment stores cover 20% of the Garang catchment, with an estimated volume of 474.0 × 10⁶ m³, and inferred residence times from OSL and ¹⁴C dating of 10³–10⁴ years. Fans and terraces reworked from basin fill deposits, and associated cut and fill terrace features, are the dominant forms of sediment storage (~90% of total). A space‐for‐time argument is used to assess stages of basin infilling and subsequent landscape responses to incision, outlining a dramatic example of changes to sediment dynamics and connectivity relationships within the upper Yellow River. Sediments within the upper catchment lie above the regional basin fill level, offering a glimpse of pre‐incisional conditions. This contrasts markedly with the enduring influence of basin incisional history seen within the middle catchment, and the contemporary landscapes of the lower catchment where nearly all available sediment has been excavated from the basin and the landscape effectively operates under post‐incisional conditions. The need to contextualise catchment‐scale studies in terms of landscape history is emphasised. Copyright © 2017 John Wiley & Sons, Ltd.     

A twenty‐one‐year record of surface change on a Namib linear dune

Repeated surveying of two sites on a Namib linear dune between 1980 and 2001 provides a 21‐year record of dune surface change. The surveys con?rm the view that the dunes are not inactive relics but are responding to the present‐day wind regime. They also provide no evidence that the dunes are migrating laterally. Examination of wind data for the survey period provides some evidence that the form of the crest of the dunes is actively responding to the natural year‐by‐year climate variability, such that an increase in the frequency of easterly winds leads to the development of a double‐crested form while fewer easterly winds lead to a single‐crested form. Copyright © 2003 John Wiley & Sons, Ltd.     

Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude

This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley & Sons, Ltd.     

Variations in the response of the dune coast of northern France to major storms as a function of available beach sediment volume

A series of airborne topographic LiDAR data were obtained from May 2008 to January 2014 over two coastal sites of northern France (Bay of Wissant and east of Dunkirk). These data were used with wind and tide gauge measurements to assess the impacts of storms on beaches and coastal dunes, and particularly of the series of major storms that hit western Europe during the fall and early winter of 2013. Our results show a high variability in shoreline response from one site to the other, but also within each coastal site. Coastal dune erosion and shoreline retreat occurred at both sites, particularly on the coast of the Bay of Wissant where shoreline retreat up to about 40 m was measured. However, stability or even shoreline advance were also observed despite the occurrence of an extreme water level with a return period >100 years during the storm Xaver in early December 2013. Comparison of shoreline change with variations of coastal dune and upper beach volumes revealed only weak relationships. Our results nevertheless showed that shoreline behavior seems to strongly depend on the initial sediment volume on the upper beach before the occurrence of the storms. According to our measurements, an upper beach volume of about 30 m³ m^?1 between the dune toe and the mean high water level is sufficient at these sites to protect the coastal dunes from storm waves associated with high water levels with return periods >10 years. The identification of such thresholds in terms of upper beach width or sediment volume may represent valuable information for improving the management of shoreline change by providing an estimate of the minimum quantity of sand on the upper beach necessary to ensure shoreline stability in this region. Copyright © 2016 John Wiley & Sons, Ltd.     

Testing geomorphic signal of active normal faulting: The case of the Cittanova Fault (Calabria,southern Italy)

Extracting tectonic signals from the landscape is an important challenge for constraining the style and rate of deformation associated with active faults, especially where their displacement history cannot be independently determined. Based on previous paleoseismological data coupled with new geomorphological field work and ¹⁴C dating of geomorphic markers, we analysed the geomorphic signal of the along‐strike differential throw of the Cittanova Fault in southern Calabria (Italy), the recent activity of which is already well documented and constrained. Through DTM‐derived stream power law parameters (SL and χ), we provide evidence of drainage network disequilibrium and reorganization in response to fault growth and deformation style. Furthermore, a methodological test of the reliability of the χ metric as a proxy for the differential throw along the strike of active normal faults provided good preliminary results, consistent with a strong inverse linear correlation with fault throw. Copyright © 2018 John Wiley & Sons, Ltd.     

Vegetation and hydrogeomorphic features of a large lowland river: NDVI patterns summarizing fluvial dynamics and supporting interpretations of ecological patterns

The interactions among hydrogeomorphic features of large lowland rivers regulate the spatio-temporal fluvial dynamics and influence the occurrence and understanding of the ecological patterns in these systems. We studied the hydrogeomorphic features that explain the fluvial dynamics of the Paraná River floodplain and investigated the usefulness of NDVI patterns in summarizing these dynamics. Information layers related to elevation, drainage network, geomorphic units, runoff dynamics, and NDVI patterns of the study area were generated from multi-source remote sensing data and fieldwork measurements. All these layers were integrated and analysed in a GIS environment, and the statistical association among them was corroborated. In our study area, the interaction among hydrogeomorphic features determined a centrifugal flooding pattern through which inundation occurs when water from remote courses, fluvial lakes, and secondary tributaries reaches the main channel. The areas closest to the main channel are flooded only when it overflows. Five NDVI patterns statistically different in terms of their means and standard deviations were identified and characterized. These NDVI patterns were significantly associated with elevation, geomorphic units, and runoff dynamics, highlighting their usefulness to characterize the fluvial dynamics and support understanding of the ecological patterns. © 2020 John Wiley & Sons, Ltd.     

Dynamics of grains falling on a sloping granular medium: application to the evolution of a talus

Numerous field surveys have provided quantitative information on the characteristics of talus deposits. Much less has been done to quantify the basic dynamics processes of blocks involved in talus evolution. In this work, we perform a set of experiments at the reduced scale of some metres using an inclined board covered with a loose granular medium. The complexity of the processes forming a talus is simplified by studying the interaction of only two kinds of grain sizes at a time. Grains of one class size are dropped from a fixed height onto the board covered by a layer of grains of a different class size, and their final distribution is recorded. We find that when small grains fall on large grains, the granular abundance decreases rapidly as a function of the distance from the fall point, which is explained by the effect of multiple bouncing on the irregular surface. In contrast, large grains falling on a bed of smaller grains lose much more energy at impact. They may stop at once, or roll down slope, often reaching the whole board length; as a consequence, their abundance peaks in the fall zone and at the change of slope. Experiments also show that grains travel longer with increasing fall height and sloping angle. The results clarify in a physical manner one mechanism that might explain why large blocks are typically found in the distal part of a talus slope, while smaller blocks remain near the fall zone. Based on these and previous experiments, a schematic view of talus evolution is discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Remotely sensed rivers in the Anthropocene: state of the art and prospects

The rivers of the world are undergoing accelerated change in the Anthropocene, and need to be managed at much broader spatial and temporal scales than before. Fluvial remote sensing now offers a technical and methodological framework that can be deployed to monitor the processes at work and to assess the trajectories of rivers in the Anthropocene. In this paper, we review research investigating past, present and future fluvial corridor conditions and processes using remote sensing and we consider emerging challenges facing fluvial and riparian research. We introduce a suite of remote sensing methods designed to diagnose river changes at reach to regional scales. We then focus on identification of channel patterns and acting processes from satellite, airborne or ground acquisitions. These techniques range from grain scales to landform scales, and from real time scales to inter-annual scales. We discuss how remote sensing data can now be coupled to catchment scale models that simulate sediment transfer within connected river networks. We also consider future opportunities in terms of datasets and other resources which are likely to impact river management and monitoring at the global scale. We conclude with a summary of challenges and prospects for remotely sensed rivers in the Anthropocene. © 2019 John Wiley & Sons, Ltd.     

The influence of ground ice distribution on geomorphic dynamics since the Little Ice Age in proglacial areas of two cirque glacier systems

Holocene glaciers have contributed to an abundance of unstable sediments in mountainous environments. In permafrost environments, these sediments can contain ground ice and are subject to rapid geomorphic activity and evolution under condition of a warming climate. To understand the influence of ground ice distribution on this activity since the Little Ice Age (LIA), we have investigated the Pierre Ronde and Rognes proglacial areas, two cirque glacier systems located in the periglacial belt of the Mont Blanc massif. For the first time, electrical resistivity tomography, temperature data loggers and differential global positioning systems (dGPS) are combined with historical documents and glaciological data analysis to produce a complete study of evolution in time and space of these small landsystems since the LIA. This approach allows to explain spatial heterogeneity of current internal structure and dynamics. The studied sites are a complex assemblage of debris‐covered glacier, ice‐rich frozen debris and unfrozen debris. Ground ice distribution is related to former glacier thermal regime, isolating effect of debris cover, water supply to specific zones, and topography. In relation with this internal structure, present dynamics are dominated by rapid ice melt in the debris‐covered upper slopes, slow creep processes in marginal glacigenic rock glaciers, and weak, superficial reworking in deglaciated moraines. Since the LIA, geomorphic activity is mainly spatially restricted within the proglacial areas. Sediment exportation has occurred in a limited part of the former Rognes Glacier and through water pocket outburst flood and debris flows in Pierre Ronde. Both sites contributed little sediment supply to the downslope geomorphic system, rather by episodic events than by constant supply. In that way, during Holocene and even in a paraglacial context as the recent deglaciation, proglacial areas of cirque glaciers act mostly as sediment sinks, when active geomorphic processes are unable to evacuate sediment downslope, especially because of the slope angle weakness. Copyright © 2014 John Wiley & Sons, Ltd.     

Restoring earth surface processes through landform design. A 13‐year monitoring of a geomorphic reclamation model for quarries on slopes

The application of geomorphic principles to land reclamation after surface mining has been reported in the literature since the mid‐1990s, mostly from Australia, Canada and the USA. This paper discusses the reclamation problems of contour mining and quarries on slopes, where steep gradients are prone to both mass movement and water erosion. To address these problems simultaneously, a geomorphic model for reclaiming surface mined slopes is described. Called the ‘highwall–trench–concave slope’ model, it was first applied in the 1995 reclamation of a quarry on a slope (La Revilla) in Central Spain. The geomorphic model does not reproduce the original topography, but has two very different sectors and objectives: (i) the highwall–trench sector allows the former quarry face to evolve naturally by erosion, accommodating fallen debris by means of a trench constructed at the toe of the highwall; (ii) the concave‐slope base sector, mimicking the landforms of the surrounding undisturbed landscape, promotes soil formation and the establishment of self‐sustaining, functional ecosystems in the area protected from sedimentation by the trench. The model improves upon simple topographic reconstruction, because it rebuilds the surficial geology architecture and facilitates re‐establishment of equilibrium slopes through the management and control of geomorphic processes. Thirteen years of monitoring of the geomorphic and edaphic evolution of La Revilla reclaimed quarry confirms that the area is functioning as intended: the highwall is backwasting and material is accumulating at the trench, permitting the recovery of soils and vegetation on the concave slope. However, the trench is filling faster than planned, which may lead to run‐off and sedimentation on the concave slope once the trench is full. The lesson learned for other scenarios is that the model works well in a two‐dimensional scheme, but requires a three‐dimensional drainage management, breaking the reclaimed area into several watersheds with stream channels. Copyright © 2010 John Wiley & Sons, Ltd.     

Impact of vegetation on erosion: Insights from the calibration and test of a landscape evolution model in alpine badland catchments

While it is well recognized that vegetation can affect erosion, sediment yield and, over longer timescales, landform evolution, the nature of this interaction and how it should be modeled is not obvious and may depend on the study site. In order to develop quantitative insight into the magnitude and nature of the influence of vegetation on catchment erosion, we build a landscape evolution model to simulate erosion in badlands, then calibrate and evaluate it against sediment yield data for two catchments with contrasting vegetation cover. The model couples hillslope gravitational transport and stream alluvium transport. Results indicate that hillslope transport processes depend strongly on the vegetation cover, whereas stream transport processes do not seem to be affected by the presence of vegetation. The model performance in prediction is found to be higher for the denuded catchment than for the reforested one. Moreover, we find that vegetation acts on erosion mostly by reducing soil erodibility rather than by reducing surface runoff. Finally, the methodology we propose can be a useful tool to evaluate the efficiency of previous revegetation operations and to provide guidance for future restoration work. © 2019 John Wiley & Sons, Ltd.     

Sediment yield of the lower Tana River,Kenya, is insensitive to dam construction: sediment mobilization processes in a semi‐arid tropical river system

Dam construction in the 1960s to 1980s significantly modified sediment supply from the Kenyan uplands to the lower Tana River. To assess the effect on suspended sediment fluxes of the Tana River, we monitored the sediment load at high temporal resolution for 1 year and complemented our data with historical information. The relationship between sediment concentration and water discharge was complex: at the onset of the wet season, discharge peaks resulted in high sediment concentrations and counterclockwise hysteresis, while towards the end of the wet season, a sediment exhaustion effect led to low concentrations despite the high discharge. The total sediment flux at Garissa (c. 250 km downstream of the lowermost dam) between June 2012 and June 2013 was 8.8 Mt yr^‐1. Comparison of current with historical fluxes indicated that dam construction had not greatly affected the annual sediment flux. We suggest that autogenic processes, namely river bed dynamics and bank erosion, mobilized large quantities of sediments stored in the alluvial plain downstream of the dams. Observations supporting the importance of autogenic processes included the absence of measurable activities of the fall‐out radionuclides ⁷Be and ¹³⁷Cs in the suspended sediment, the rapid lateral migration of the river course, and the seasonal changes in river cross‐section. Given the large stock of sediment in the alluvial valley of the Tana River, it may take centuries before the effect of damming shows up as a quantitative reduction in the sediment flux at Garissa. Many models relate the sediment load of rivers to catchment characteristics, thereby implicitly assuming that alterations in the catchment induce changes in the sediment load. Our research confirms that the response of an alluvial river to external disturbances such as land use or climate change is often indirect or non‐existent as autogenic processes overwhelm the changes in the input signal. Copyright © 2015 John Wiley & Sons, Ltd.