Human impact on fluvial systems in Europe with special regard to today’s river restorations

Climate, geology, geomorphology, soil, vegetation, geomorphology, hydrology, and human impact affect river–floodplain systems, especially their sediment load and channel morphology. Since the beginning of the Holocene, human activity is present at different scales from the catchment to the channel and has had an increasing influence on fluvial systems. Today, many river–floodplain systems are transformed in course of river restorations to “natural” hydrodynamic and morphodynamic conditions without human impacts. Information is missing for the historical or rather “natural” as well as for the present-day situation. Changes of the “natural” sediment fluxes in the last centuries result in changes of the fluvial morphology. The success of river restorations depends on substantial knowledge about historical as well as present-day fluvial morphodynamics. Therefore, it is necessary to analyze the consequences of historical impacts on fluvial morphodynamics and additionally the future implications of present-day human impacts in course of river restorations. The objective of this review is to summarize catchment impacts and river channel impacts since the beginning of the Holocene in Europe on the fluvial morphodynamics, to critically investigate their consequences on the environment, and to evaluate the possibility to return to a “natural” morphological river state.

Using morphometric and geomorphic indices to assess Western São Francisco Craton neotectonic traces

Recently, geomorphometric properties of river networks and catchments have been described and applied as an efficient tool in the investigation of the landforms' response to neotectonics. Geometric parameters of the Cotovelo River catchment extracted from an Alos-Palsar digital elevation model were used to compute morphometric and geomorphic indices to investigate whether the bedrock structure and recent active tectonics influence the local drainage network. The Cotovelo catchment is situated in the Middle to Upper Proterozoic western foreland basin of the São Francisco craton, in northwestern Minas Gerais, Southeastern Brazil; it is presumed to be a stable piece of earth’s crust. The automatically generated streams were processed at the sub-catchment scale to calculate the hypsometric integral, relief ratio, stream frequency, and drainage density morphometric indices as well as supported a geomorphic study based on the basin shape, asymmetry factor, valley floor width-to-height ratio, mountain front sinuosity, transverse topographic symmetry factor, and stream-length gradient index. Achieved results revealed recent and low-rate tectonic activity and structural control on the fluvial morphology. Prominent knickpoints, aligned with mapped fault scarps, disclose straight erosive fronts away from stratigraphic borders, indicating these features are unrelated to lithological changes. Despite the catchment location, the area exhibits impressive fluvial anomalies, and dissection occurs preferentially along ancient faults and fractures densely occurring in the rocky strata. Channel parallelism in context of medium to high relief and steep slopes, remarkably structurally drive fluvial dissection, asymmetric and elongated drainage catchments, and aligned landforms suggest neotectonic influence on the drainage network.

Diachronism as process-inherent part of Pleistocene river terrace formation: First results based on luminescence dating for testing a well-established theoretical concept and possible implications for practical field work

<p>There are two main statements of a long-accepted paradigm of fluvial morphodynamics formalized inter alia by S.A. Schumm: (i) changes in fluvial systems strongly depend on exceeding external and / or internal thresholds ​​and (ii) they are always characterized by a nonlinear and asynchronous character. While the first aspect of this paradigm is part of numerous studies on fluvial morphology and river dynamics, the second aspect has so far tended to be sidelined in practical geomorphology.</p><p>With particular respect to the field of paleo-environmental research, this is evident from studies that aim at determining ages of Pleistocene river terraces in order to provide a time frame for the reconstruction of paleo-environmental conditions. Typically, numerical dating approaches are only applied to a single location that is supposed to be exemplary for the respective terrace level. Numerical ages determined for this specific location are then extrapolated and interpreted in a generalizing way to derive "THE age" of the river terrace as a whole.</p><p>With respect to the concept of asynchronism of fluvial reactions to environmental changes, such an approach seems problematic. In fact, asynchronism implies different sections of a river showing different and specific reactions to environmental changes at a given point in time. For fluvial terraces, this means that the processes controlling their formation may already have started in some sections of a valley, while in other sections they do not yet have any impact on landscape evolution.</p><p>In this contribution, we present luminescence ages of fluvial deposits originating from an Upper Pleistocene river terrace in a small valley located in the headwater of the Main River, Germany. Here, several samples from various locations throughout the river longitudinal course have been analysed. The luminescence ages determined for the lowermost part of the valley are significantly older than those from the middle section, which in turn are older than those from the valley’s upper reaches.</p><p>Our results suggest a diachronic alignment of sedimentation ages for fluvial deposits, starting with old ages close the mouth of a river and getting progressively younger for locations approaching the upper reaches. If these findings are confirmed in other fluvial systems and are not only the result of specific local conditions, they will be of great relevance for geomorphological research in fluvial landscapes. As a result, the widespread approach of deriving age estimates for fluvial terraces from numerical results merely determined for a single location appears to be inadequate and should be subjected to a critical review.</p>

Meso- to macro-scale landscape modelling with SfM-MVS photogrammetry: a case study from the Urft Lake water reservoir (Eifel Mountains, western Germany)

<p>3D landscape reconstruction derived from imagery acquired by unmanned aerial systems (UAS) is an increasingly applied method within the field of geosciences. Low-cost UAS and subsequent Structure from Motion (SfM) and multi-view stereo (MVS) processing provides the opportunity to study landforms and processes in high detail; for instance mapping of river terraces (Li et al. 2019) or landslide monitoring (Devoto et al. 2020).</p><p>Due to an almost complete drainage of the Urft Lake reservoir in the northern Eifel mountains (W-Germany) in the autumn of 2020, the lake’s entire ground could be surveyed using a low-cost UAS.</p><p>The lake stretches for 12 km and has a maximum impoundment volume of approximately 45 million m³. Its shape is characterized by multiple fluvial bends and steep slopes, which required an elaborated flight layout. A DJI Phantom 4 Pro V2.0 was used. Each flight was carried out in two parallel heights (90 and 120 m), 80° camera inclination, and in double-grid pattern. Five full days of surveying yielded over 6,000 aerial images. Despite the difficulty to access the drained reservoir, 154 evenly distributed ground control points were taken using a Leica RTK dGPS instrument (accuracy <5 cm). SfM-MVS photogrammetric processing was conducted with Agisoft Metashape Professional 1.6, using an optimized workflow based on USGS (2017) and James et al. (2020).</p><p>The resulting 3D model features high accuracy and precision making it suitable for further detailed stationary as well as multi-temporal geomorphologic analyses. The derived DEM features a spatial resolution of <6 cm and will be used to calculate geometric changes of the reservoir body since its construction in 1905; in particular, due to sedimentation and mass movements along the hillslopes. Moreover, the products can be used to study the anthropogenic influences of the water reservoir on the fluvial morphology of the Urft.</p><p> </p><p>References:</p><p>Devoto, S., Macovaz, V., Mantovani, M., Soldati, S., Furlani, S., 2020. Advantages of Using UAV Digital Photogrammetry in the Study of Slow-Moving Coastal Landslides.  Remote Sensing 2020, 12, 3566. https://doi.org/10.3390/rs12213566  </p><p>James, M.R., Antoniazza, G., Robson, S., Lane, S.N., 2020. Mitigating systematic error in topographic models for geomorphic change detection: accuracy, precision and considerations beyond off-nadir imagery. Earth Surface Processes and Landforms 45, 2251–2271. https://doi.org/10.1002/esp.4878</p><p>Li, H., Lin, C., Wang, Z., Yu, Z., 2019. Mapping of River Terraces with Low-Cost UAS Based Structure-from-Motion Photogrammetry in a Complex Terrain Setting. Remote Sensing 2019, 11, 464. https://doi.org/10.3390/rs11040464</p><p>United States Geological Survey (USGS), 2017. Unmanned Aircraft Systems Data Post Processing: Structure-from-Motion Photogrammtery. Section 2 – MicaSense 5-band MultiSpectral Imagery. USGS National UAS Project Office. https://uas.usgs.gov/nupo/pdf/PhotoScanProcessingMicaSenseMar2017.pdf (Retrieved: 24 July 2020).</p>

River Styles and Stream Power Analysis Reveal the Diversity of Fluvial Morphology in a Philippine Tropical Catchment

Characterisation of hydromorphological attributes is crucial for effective river management. In the Philippines, such applications are usually solely based on water quantity and quality. This paper uses the River Styles Framework as an alternative template for identifying the diversity of river morphodynamics as a valuable input to river management. Eight distinct River Styles (river types) were identified in the Bislak catchment (586 km2), north-west Luzon, indicating considerable geomorphic diversity within a relatively small catchment area. Three River Styles in Confined valley settings occupy 57% of the catchment area, another three in Partly-confined valley settings occupy 37%, and two in the remaining 6% are found in Laterally-unconfined valley settings. Five characteristic downstream patterns of River Styles were identified within the tributaries. We find that variation in channel slope for a given catchment area (i.e. total stream power) is insufficient to differentiate the type of river in a given reach. Hence, topographic analyses should be complemented with broader-framed, catchment-specific approaches to predict river character and behaviour. Geomorphologically-informed analyses can support management applications in the Philippines, explicitly incorporating understanding of river diversity and dynamics.

Data-fusion of satellite and ground sensors for river hydro-morphodynamics monitoring

<p>Changes in fluvial morphology, such as the migration of channels and sandbars, are driven by many factors e.g. water, woody debris and sediment discharges, vegetation and management practice. Nowadays, increased anthropic pressure and climate change are accelerating the natural morphologic dynamics. Therefore, the monitoring of river changes and the assessment of future trends are necessary for the identification of the optimal management practices, aiming at the improvement of river ecological status and the mitigation of hydraulic risk. Satellite data can provide an effective and cost-effective tool for the monitoring of river morphology and its temporal evolution.</p><p>The main idea of this work is to understand which remote sensed data, and particularly which space and time resolutions, are more adapt for the observation of sandbars evolution in relatively large rivers. To this purpose, multispectral and Synthetic Aperture Radar (SAR) archive data, with different spatial resolution, were used. Preference was given to satellite data freely available. Moreover, the observations extracted by the satellite data were compared with ground data recorded by a fixed camera.</p><p>The study case is a sandy bar (area about 0.4 km<sup>2 </sup>and maximum width about 350 m) in a lowland reach of the Po River (Italy), characterized by frequent and relevant morphological changes. The bar shoreline changes were captured by a fixed video camera, installed on a bridge and operating for almost two years (July 2017 - November 2018). To this purpose, we used: Sentinel-2 multispectral images with a spatial resolution of 10 m, Sentinel-1 SAR images with a resolution of 5 x 20 m and CosmoSkyMed SAR images with a resolution of 5 m. It is worth noting that the Sentinel data of the Copernicus Programme are freely available while the CosmoSkyMed data of the Italian Space Agency (ASI) are freely distributed for scientific purpose after the successful participation to an open call. In order to validate the results provided by Sentinel and CosmoSkyMed data, we used very high resolution multispectral images (about 50 cm).</p><p>Multispectral images are easily interpreted, but are affected by the presence of cloud cover. For instance, in this analysis, the expendable multispectral images were equal to about 50% of the total archive. On the other hand, the SAR images provide information also in the presence of clouds and at night-time, but they have the drawback of more complex processing and interpretation. The shorelines extracted from the satellite images were compared with those extracted from photographic images, taken on the same day of the satellite acquisition. Other comparisons were made between different satellite images acquired with a temporal mismatch of maximum two days.</p><p>The results of the comparisons showed that the Sentinel-1 and Sentinel-2 data were both adequate for the shoreline changes observation. Due to the higher resolution, the CosmoSkyMed data provided better results. SAR data and multispectral data allowed for automatic extraction of the bar shoreline, with different degree of processing burden. The fusion of data from different satellites gave the opportunity of highly increase the sampling rate.</p>

Changes in braided river morphology driven by flood sequencing

<p>Sequential observations of channel adjustments in relation to short-term flow variability are required to evaluate the effects of temporal ordering of hydrologic events on channel form. With the increasing hydroclimate variability due to global climate change, fluvial morphology might also exhibit adjustments toward changing equilibria. By combining flume and numerical modelling we examine the mechanism of bed morphology changes of braided rivers to a sequence of low to moderate magnitude flood events. Over 60 runs were performed in a mobile bed flume (10 m x 2.5 m), with constant longitudinal slope (0.015) and mean grain size (0.45 mm) in the Total Environment Simulator at the University of Hull, UK. The outcomes of each run were characterized by a detailed digital elevation model, digital imagery and continuous monitoring of the sediment transported through the flume outlet. Sediment conditions included floods with equilibrium and deficit loads. Rivers were allowed to evolve from an initially flat-bed to a self-organized, steady state. The rate of change and rate of bed load movement against time were indicative of the gradual approach to equilibrium. The Delft3D code in depth-averaged (2-D) mode was used to reproduce different aspects of the braiding process over an up-scaling of the laboratory river. Data analysis allowed us to assess the effect of discharge variation on the braiding dynamics and on the width-to-depth ratio of channels, which although variable in time, fluctuated among defined values. Once in equilibrium, net changes in reach-averaged width and depth values were relatively minor. The adjustment of the river morphology through time was well fitted by an exponential decay expression, and we tested diffusive relationships held within our braided river system for both constant and varying discharge conditions. In long term process-response systems, climatic changes introduce sequences of disruption of equilibria such as those analysed in this study. The results might provide then a useful basis for analysing the similar but more complex long-term dynamics found in natural rivers.</p><p> </p>

Reach scale analysis of riparian vegetation interactions with fluvial morphology using UAV based laser scanning and multispectral imaging

<p>River corridors are greatly influenced by vegetation, whether it be through direct interactions with flow, influencing the stability of banks, or contributing to floodplain roughness. With vegetation present across many of the world’s river corridors in one form or another, it is a vital component of the active river corridor that receives relatively less attention than the flow and morphological components. This is partly because the routine monitoring of the very complex and temporally dynamic structure of vegetation is challenging.  Terrestrial Laser Scanning (TLS) and Airborne Laser Scanning (ALS) have been used to monitor fluvial vegetation across scales. However, whilst UAVs and Structure from Motion (SfM) techniques have recently bridged the gap between fine scale local surveys and coarse larger surveys for fluvial morphology, they are not well suited to characterising complex vegetation.</p><p>A UAV based laser scanning and imagery system has been developed which enables the collection of high resolution (> 300 points m2) point cloud data (first and last return) to analyse vegetation structure alongside simultaneous multispectral imagery data, including the red edge band. Such data can be collected on scales from metres to kilometres depending on the needs of the user, and is capable of picking out vegetation structure using metrics such as stand height, vertical distribution, canopy health, plant density etc. Moreover, the collection of this data through time will allow the evaluation of how these factors change across seasons, subsequently filling a void in data collection between spatially limited TLS and temporally limited ALS. Here we show some examples of how the data can be used to establish interactions between vegetation, flow and fluvial morphology from a series of flights over a 1 km reach of the River Teme, UK. These examples highlight how the data enables us to begin to establish a more detailed conceptual understanding of temporally evolving fluvial-vegetation interactions along river corridors.</p>

Concept and Performance Evaluation of a Novel UAV-Borne Topo-Bathymetric LiDAR Sensor

We present the sensor concept and first performance and accuracy assessment results of a novel lightweight topo-bathymetric laser scanner designed for integration on Unmanned Aerial Vehicles (UAVs), light aircraft, and helicopters. The instrument is particularly well suited for capturing river bathymetry in high spatial resolution as a consequence of (i) the low nominal flying altitude of 50–150 m above ground level resulting in a laser footprint diameter on the ground of typically 10–30 cm and (ii) the high pulse repetition rate of up to 200 kHz yielding a point density on the ground of approximately 20–50 points/m2. The instrument features online waveform processing and additionally stores the full waveform within the entire range gate for waveform analysis in post-processing. The sensor was tested in a real-world environment by acquiring data from two freshwater ponds and a 500 m section of the pre-Alpine Pielach River (Lower Austria). The captured underwater points featured a maximum penetration of two times the Secchi depth. On dry land, the 3D point clouds exhibited (i) a measurement noise in the range of 1–3 mm; (ii) a fitting precision of redundantly captured flight strips of 1 cm; and (iii) an absolute accuracy of 2–3 cm compared to terrestrially surveyed checkerboard targets. A comparison of the refraction corrected LiDAR point cloud with independent underwater checkpoints exhibited a maximum deviation of 7.8 cm and revealed a systematic depth-dependent error when using a refraction coefficient of n = 1.36 for time-of-flight correction. The bias is attributed to multi-path effects in the turbid water column (Secchi depth: 1.1 m) caused by forward scattering of the laser signal at suspended particles. Due to the high spatial resolution, good depth performance, and accuracy, the sensor shows a high potential for applications in hydrology, fluvial morphology, and hydraulic engineering, including flood simulation, sediment transport modeling, and habitat mapping.