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1.
Human activities influence watershed sediment dynamics in profound ways, often resulting in excessive loading of suspended sediment to rivers. One of the primary factors limiting our ability to effectively manage sediment at the watershed scale has been our inability to adequately measure relatively small erosion rates (on the order of millimeters to centimeters per year) over annual and sub‐annual time scales on spatially‐extensive landforms, such as river banks and bluffs. Terrestrial laser scanning (TLS) can be employed to address this need. TLS collects high‐resolution data allowing for more accurate monitoring of erosion rates and processes, and provides a new opportunity to make precise measurements of geomorphic change on vertical landforms like banks and bluffs, but challenges remain. This research highlights challenges and limitations of using TLS for change detection on river banks and bluffs including the presence of vegetation, natural surface crenulations, and difficulties with creating benchmarks, and provides solutions developed to overcome these limitations. Results indicate that data processing algorithms for change detection can have a significant impact on the calculated erosion rates, with different methods producing results that can vary by over 100%. The most accurate change detection technique compares a point cloud to a triangulated irregular network (TIN) along a set of vectors that accommodate bluff curvature. This paper outlines a variety of methods used to measure bluff change via TLS and explains the accompanying error analysis that supports these methods. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
Geomorphic river design strives for natural resilience by encouraging geomorphic form complexity and morphological processes linked to greater habitat diversity. Increasing availability of high-resolution topographic data and spatial feature mapping methods provide advantages for morphological analysis and river restoration planning. We propose and evaluate an approach to quantifying topographic variability of geomorphic form and pixel-level surface roughness resulting from channel planform geometry differences using spatially continuous variety computation applied to component metrics including flow direction, aspect and planform curvature. We define this as the geomorphic form variation (GFV) approach and found it scalable, repeatable and a multi-stage analytical metric for quantifying physical aspects of river-bed topographic variability. GFV may complement process-based morphological feature mapping applications, hydraulic assessment indices and spatial habitat heterogeneity metrics commonly used for ecological quality evaluation and river restoration. The GFV was tested on controlled synthetic channels derived from River Builder software and quasi-controlled sinuous planform flume experiment channels. Component variety metrics respond independently to specific geometric surface changes and are sensitive to multi-scaled morphology change, including coarser-grained sediment distributions of pixel-level surface roughness. GFV showed systematic patterns of change related to the effects of channel geometry, vertical bed feature (pool-bar) frequency and amplitude, and bar size, shape and orientation. Hotspot analysis found that bar margins were major components of topographic complexity, whereas grain-scale variety class maps further supported the multi-stage analytical capability and scalability of the GFV approach. The GFV can provide an overall variety value that may support river restoration decision-making and planning, particularly when geomorphic complexity enhancement is a design objective. Analysing metric variety values with statistically significant hotspot cluster maps and complementary process-based software and mapping applications allows variety correspondence to systematic feature changes to be assessed, providing an analytical approach for river morphology change comparison, channel design and geomorphic process restoration.  相似文献   

3.
In the twentieth century Polish Carpathian rivers were considerably modified by channelization and gravel mining, with significant detrimental effects to their ecological integrity, vertical stability of the streambeds and flood hazard to downstream river reaches. Restoration of the rivers is thus necessary to improve their ecological status and re‐establish geomorphic dynamic equilibrium conditions. Various approaches to defining hydromorphological reference conditions, proposed to date in river restoration literature, have serious deficiencies. In particular, environmental changes that took place in the catchments of Carpathian rivers during the twentieth century invalidate the historical state of the rivers as reference for their restoration. This is illustrated by a change from bar‐braided to island‐braided channel pattern that occurred in the past century in unmanaged sections of the Czarny Dunajec in response to a reduction in flow and sediment dynamics of the river. We indicate that reference conditions should be defined as those which exist or would exist under present environmental conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river which is to be restored. This assumption was tested through the evaluation of hydromorphological river quality of the Czarny Dunajec according to the European Standard EN‐14614. The evaluation confirmed a high‐status hydromorphological quality in an unmanaged channel section, which can thus be used as a reference for restoration of impacted river sections. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

4.
In recent years airborne laser scanning (ALS) evolved into a state‐of‐the‐art technology for topographic data acquisition. We present a novel, automatic method for water surface classification and delineation by combining the geometrical and signal intensity information provided by ALS. The reflection characteristics of water surfaces in the near‐infrared wavelength (1064 nm) of the ALS system along with the surface roughness information provide the basis for the differentiation between water and land areas. Water areas are characterized by a high number of laser shot dropouts and predominant low backscatter energy. In a preprocessing step, the recorded intensities are corrected for spherical loss and atmospheric attenuation, and the locations of laser shot dropouts are modeled. A seeded region growing segmentation, applied to the point cloud and the modeled dropouts, is used to detect potential water regions. Object‐based classification of the resulting segments determines the final separation of water and non‐water points. The water‐land‐boundary is defined by the central contour line of the transition zone between water and land points. We demonstrate that the proposed workflow succeeds for a regulated river (Inn, Austria) with smooth water surface as well as for a pro‐glacial braided river (Hintereisfernerbach, Austria). A multi‐temporal analysis over five years of the pro‐glacial river channel emphasizes the applicability of the developed method for different ALS systems and acquisition settings (e.g. point density). The validation, based on real time kinematic (RTK) global positioning system (GPS) field survey and a terrestrial orthophoto, indicate point cloud classification accuracy above 97% with 0·45 m planimetric accuracy (root mean square error) of the water–land boundary. This article shows the capability of ALS data for water surface mapping with a high degree of automation and accuracy. This provides valuable datasets for a number of applications in geomorphology, hydrology and hydraulics, such as monitoring of braided rivers, flood modeling and mapping. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

5.
The worldwide availability of digital elevation models (DEMs) has enabled rapid (semi-)automated mapping of earth surface landforms. In this paper, we first present an approach for delineating valley bottom extent across a large catchment using only publicly available, coarse-resolution DEM input. We assess the sensitivity of our results to variable DEM resolution and find that coarse-resolution datasets (90 m resolution) provide superior results. We also find that LiDAR-derived DEMs produce more realistic results than satellite-derived DEMs across the full range of topographic settings tested. Satellite-derived DEMs perform more effectively in moderate topographic settings, but fail to capture the subtleties of valley bottom extent in mild gradient, low-lying topography and in narrow headwater reaches. Second, we present a semi-automated technique within ArcGIS for delineating valley bottom segments using DEM-derived network scale metrics of valley bottom width and slope. We use an unsupervised machine-learning technique based on the k-means clustering algorithm to solve a conundrum in GIS-based geomorphic analysis of rivers: the delineation of valley bottom segments of variable length. The delineation of valley bottom segments provides a coarse-scale entry point into automated geomorphic analysis and characterization of river systems. © 2020 John Wiley & Sons, Ltd.  相似文献   

6.
The surface of the earth is being transformed by a new force in the form of technological systems and processes that move significant quantities of mass large distances. Because movement of mass is perhaps the most basic geomorphic process, and because the continuing rise of technology appears to characterize a new epoch in earth evolution (the Anthropocene), it is of interest to compare technological and natural mass transport mechanisms. A purely dynamical ‘mass‐action’ metric, representing the product of mass displaced, distance moved, and mean speed of displacement, is used to compare the transport effectiveness of selected systems. Systems with large mass‐action tend to be advective, and systems with small mass‐action diffusive. Local environments are conditioned by mass‐action through the introduction of transport corridors, such as roads and rivers, which put constraints on mass transport by embedded diffusive systems. Advection also subjects local environments to externally determined time scales, such as the times for delivery of unit mass of water or sediment to a river mouth, and supports the emergence of associated dynamical processes there, for example those of human activity or delta construction, that are too rapid to be sustained by diffusion. Most of the world's mass‐action is generated by the motion of fluids of global or continental extent, as in atmospheric circulation or river flow. Technological mass‐action exceeds that of all land‐based geomorphic systems except rivers. Technological systems with large mass‐action tend to be comprised of discrete, self‐powered units (e.g. trucks). Discretization of transported mass reflects the different locomotion strategy required for transport of solids on land, compared with the transport requirements of spatially extensive fluids in nature. The principle of maximum entropy production may provide a framework for understanding the emergence of advective, technological mass‐transport systems. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

7.
Historical range of variability (HRV) describes the range of temporal and spatial variations in river variables such as flow regime or channel planform prior to intensive human alteration of the ecosystem. In mountainous river networks, HRV is most usefully applied to spatially differentiated geomorphic process domains with distinctive form and process. Using the Colorado Front Range as an example, three examples of how knowledge of HRV can assist river management and restoration are discussed. The examples involve instream wood load and channel morphology, beaver colonies and valley‐bottom form and process, and flow thresholds in regulated rivers. The question of what a river should look like – that is, what range of process and form the river included prior to intensive human alteration – can be addressed by (i) placing the river within a process domain, (ii) establishing correlations between form parameters that can be remotely sensed and reach‐scale process and form, so that the spatial extent, connectivity, and rarity of process domains within a river network or a region can be quickly assessed, (iii) inferring characteristics of the river prior to intensive alteration by documenting characteristics of the least altered reference rivers and by using proxy indicators of pre‐alteration conditions, and (iv) establishing process thresholds that must be exceeded to maintain form (e.g. flow thresholds to mobilize bed sediment). Once this context has been established, resource managers can better evaluate the options for restoring altered riverine form and function. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
In the last few centuries humans have modified rivers, and rivers have responded with noticeable changes in sedimentary dynamics. The objective of this study is to assess these responses of the sedimentary dynamics. Therefore, we calculated a sediment budget for eroded and deposited sediment volumes in a ~12‐km long floodplain section of the largest semi‐natural embanked but still dynamic lower Rhine distributary, for ~50‐years time slices between ad 1631 and present. This is the period during which embanked floodplains were formed by downstream migration of meander bends between confining dykes. Our sediment budget involves a detailed reconstruction of vertical and lateral accretion rates and erosion rates of floodplain sediment. To do so, we developed a series of historical geomorphological maps, and lithogenetic cross‐sections. Based on the maps and cross‐sections, we divided the floodplain into building blocks representing channel bed and overbank sediment bodies. Chronostratigraphy within the blocks was estimated by interpretation of heavy metal profiles and from optically stimulated luminescence (OSL) dating results. Sediment budgets were hence calculated as a change of volume of each building block between time steps. The amount of lateral accretion initially increased, as a result of island and sand bar formation following embankment. From the eighteenth century onwards, there was a decrease of lateral processes in time, which is a result of straightening of the river by human activities, and a reduction of water and sediment supply due to the construction of a new upstream bifurcation. With straightening of the river, the floodplain area grew. Artificial fixation of the channel banks after ad 1872 prevented lateral activity. From then on, overbank deposition became the main process, leading to a continuous increase of floodplain elevation, and inherent decrease of flooding frequency and sediment accumulation rate. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

9.
IMOUNTAINE~ON-MENTSANDSEDIMENTGeologistsandgeomorphologistsareabletomakeatleasttWoimportantcontributionstomitigatinghazardsassociatedwithsedimentprocessessuchasfloodsordebrisflows.First,geomorphologistscaninterpretgeologicrecordsofthehiStoryofseddrientprocesses.Theserecordsmayprovideinsightintothemagnitude,frequency,andlocationofsedimentsourcesandtransport,aswellashillslopeandchannelresponsestosedimentprocesses.Informationonpastsedimentproductionandmovement,andchannelresponse,mayb'…  相似文献   

10.
Earth scientists have traditionally conceptualized rivers and streams as geomorphic machines, whose role is to transfer sediment and to sculpt the landscape. Steady‐state relationships between sediment supply and transport capacity have traditionally been considered normative in fluvial systems. Rivers are hydrological entities, however, whose function is to redistribute excess moisture on land. The geomorphic work of the river – erosion, transport, deposition, etc. – is a byproduct of the hydrological job of the river. There is therefore no reason to expect any particular relationship between sediment supply and transport capacity to develop as a normative condition in fluvial systems. The apparent steady‐state equilibrium slope adjustments of rivers are a byproduct of four basic phenomena: (1) hydraulic selection, which favors channels and branching networks over other flux patterns; (2) water flows along the available path of least resistance; (3) energy dissipation; and (4) finite relaxation times. Recognizing converging trends of stream power or slope and sediment supply as common (but far from inevitable) side effects rather than self‐regulation has important implications for interpreting and predicting fluvial systems, and for river management and restoration. Such trends are variable, transient, contingent, and far from universal. Where they occur, they are an emergent byproduct of fundamental physical mechanisms, not a goal function or attractor state. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

11.
Anthropogenic climate change is expected to change the discharge and sediment transport regime of river systems. Because rivers adjust their channels to accommodate their typical inputs of water and sediment, changes in these variables can potentially alter river morphology. In this study, a hierarchical modeling approach was developed and applied to examine potential changes in reach‐averaged bedload transport and spatial patterns of erosion and deposition for three snowmelt‐dominated gravel‐bed rivers in the interior Pacific Northwest. The modeling hierarchy was based on discharge and suspended‐sediment load from a basin‐scale hydrologic model driven by a range of downscaled climate‐change scenarios. In the field, channel morphology and sediment grain‐size data for all three rivers were collected. Changes in reach‐averaged bedload transport were estimated using the Bedload Assessment of Gravel‐bedded Streams (BAGS) software, and the Cellular Automaton Evolutionary Slope and River (CAESAR) model was used to simulate the spatial pattern of erosion and deposition within each reach to infer potential changes in channel geometry and planform. The duration of critical discharge was found to control bedload transport. Changes in channel geometry were simulated for the two higher‐energy river reaches, but no significant morphological changes were found for a lower‐energy reach with steep, cohesive banks. Changes in sediment transport and river morphology resulting from climate change could affect the management of river systems for human and ecological uses. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

12.
Structure‐from‐Motion (SfM) photogrammetry is now used widely to study a range of earth surface processes and landforms, and is fast becoming a core tool in fluvial geomorphology. SfM photogrammetry allows extraction of topographic information and orthophotos from aerial imagery. However, one field where it is not yet widely used is that of river restoration. The characterisation of physical habitat conditions pre‐ and post‐restoration is critical for assessing project success, and SfM can be used easily and effectively for this purpose. In this paper we outline a workflow model for the application of SfM photogrammetry to collect topographic data, develop surface models and assess geomorphic change resulting from river restoration actions. We illustrate the application of the model to a river restoration project in the NW of England, to show how SfM techniques have been used to assess whether the project is achieving its geomorphic objectives. We outline the details of each stage of the workflow, which extend from preliminary decision‐making related to the establishment of a ground control network, through fish‐eye lens camera testing and calibration, to final image analysis for the creation of facies maps, the extraction of point clouds, and the development of digital elevation models (DEMs) and channel roughness maps. The workflow enabled us to confidently identify geomorphic changes occurring in the river channel over time, as well as assess spatial variation in erosion and aggradation. Critical to the assessment of change was the high number of ground control points and the application of a minimum level of detection threshold used to assess uncertainties in the topographic models. We suggest that these two things are especially important for river restoration applications. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

13.
The alluvial cover in channels with non-alluvial beds is a major morphologic feature in these rivers and has important geomorphic and ecologic functions. Although controls on the extent of the alluvial cover have been previously researched, little is known about the role of channel meanders in shaping the three-dimensional morphology and bedload transport rates in these rivers. Flume experiments were conducted in a fixed-bed sinuous channel scaled from an engineered urban river. A fully graded sediment supply mixture was fed into the bare channel at rates ranging between 0.3 and 1.2 times the estimated channel capacity under constant discharge. The three-dimensional morphology and surface texture of the alluvial cover were captured using photogrammetry, and the sediment output was periodically measured and sieved. A stable alluvial cover was achieved under all sediment supply conditions that coincided with a sediment transport equilibrium. The sediment supply rate controlled the final areal extent, mass and volume of the alluvial cover, while cover developed as a periodic series of stable bars ‘fixed’ by the channel planform. The alluvial cover development followed consistent trajectories relative to angular position around bends but developed to a greater degree and higher elevation with increasing sediment supply. The stable cover extent had a logarithmic relationship with the relative sediment supply, while the final mass, volume and bar height had linear relationships. The final channel morphology was characterized by fine-textured point bars with flat tops and steep margins connected by coarse riffle features. The outside of banks between bend apexes remained bare, even at sediment supply conditions exceeding the channel capacity. The length of the exposed outer banks followed predictable linear relationships with the total cover extent. Insights from this study can provide guidance for the management of channels with non-alluvial boundaries and provide validation for models of sinuous bedrock channel abrasion. © 2020 John Wiley & Sons, Ltd.  相似文献   

14.
The sediment flux from a catchment is driven by tectonics and climate but is moderated by the geomorphic response of the landscape system to changes in these two boundary conditions. Consequently, catchment response time and the non‐linear behavior of landscapes in response to boundary condition change control the downstream propagation of climatic or tectonic perturbations from catchments to neighboring basins. In order to investigate the impact of catchment response time on sediment flux, we integrated a spatially‐lumped numerical model PaCMod, with new routines simulating the evolution of landscape morphology and erosion rates under tectonic and climatic forcing. We subsequently applied the model to reconstruct the sediment flux from a tectonically perturbed catchment in central Italy. Finally, we coupled our model to DeltaSim, a process‐response model simulating fluvio‐deltaic stratigraphy, and investigated the impact of catchment response time on stratigraphy, using both synthetic scenarios and a real world system (Fucino Basin, central Italy). Our results demonstrate that the differential response of geomorphic elements to tectonic and climatic changes induces a complex sediment flux signal, and produces characteristic stratigraphic architectures and shoreline trajectories. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

15.
Large wood tends to be deposited in specific geomorphic units within rivers. Nevertheless, predicting the spatial distribution of wood deposits once wood enters a river is still difficult because of the inherent complexity of its dynamics. In addition, the lack of long‐term observations or monitored sites has usually resulted in a rather incomplete understanding of the main factors controlling wood deposition under natural conditions. In this study, the deposition of large wood was investigated in the Czarny Dunajec River, Polish Carpathians, by linking numerical modelling and field observations so as to identify the main factors influencing wood retention in rivers. Results show that wood retention capacity is higher in unmanaged multi‐thread channels than in channelized, single‐thread reaches. We also identify preferential sites for wood deposition based on the probability of deposition under different flood scenarios, and observe different deposition patterns depending on the geomorphic configuration of the study reach. In addition, results indicate that wood is not always deposited in the geomorphic units with the highest roughness, except for low‐magnitude floods. We conclude that wood deposition is controlled by flood magnitude and the elevation of flooded surfaces in relation to the low‐flow water surface. In that sense, the elevation at which wood is deposited in rivers will differ between floods of different magnitude. Therefore, together with the morphology, flood magnitude represents the most significant control on wood deposition in mountain rivers wider than the height of riparian trees. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

16.
Historical archives of grey‐scale river channel imagery are extensive. Here, we present and test a methodology to extract detailed quantitative topographic data from such imagery of sand‐bed rivers. Extracting elevation information from rivers is difficult as they are characterized by a low relative relief (<4 m); the area of interest may be spatially extensive (e.g. active channel widths >500 m in large braided rivers); the rate of change of surface elevation is generally low except in the vicinity of individual channel banks where the rate of change is very high; there is the complication that comes from inundation; and there may be an added complication caused by blockage of the field of view by vegetation. Here, we couple archival photogrammetric techniques with image processing methods and test these for quantification of sand‐bed braided river dynamics, illustrated for a 500 m wide, 3 km long reach of the South Saskatchewan River, Canada. Digital photogrammetry was used to quantify dry areas and water edge elevations. A methodology was then used to calibrate the spectral signature of inundated areas by combining established two media digital photogrammetric methods and image matching. This allowed determination of detailed depth maps for inundated areas and, when combined with dry area data, creation of complete digital elevation models. Error propagation methods were used to determine the erosion and deposition depths detectable from sequential digital elevation models. The result was a series of elevation models that demonstrate the potential for acquiring detailed and precise elevation data from any historical aerial imagery of rivers without needing associated calibration data, provided that imagery is of the necessary scale to capture the features of interest. We use these data to highlight several aspects of channel change on the South Saskatchewan River, including bar movement, bank erosion and channel infilling. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

17.
Riverine sediments have played an important role in the morphological evolution of river channels and river deltas. However, the sediment regime in the many world's rivers has been altered in the context of global changes. In this study, temporal changes in the sediment regime of the Pearl River were examined at different time scales, that is, annual, seasonal, and monthly time scales, using the Mann–Kendall test. The results revealed that precipitation variability was responsible for monthly and seasonal distribution patterns of the sediment regime and the long‐term changes in the water discharge; however, dam operation has smoothed the seasonal distribution of water discharge and resulted in decreasing trends in the annual, wet‐season, and dry‐season sediment load series since the 1950s. Due to the different regulation magnitudes of dam operation, differences were observed in sediment regime changes among the three tributaries. In addition, human activities have altered the hysteresis of seasonal rating curves and affected hysteresis differences between increasing and decreasing water discharge stages. Sediment supply is an important factor controlling river channel dynamics, affecting channel morphology. From the 1950s to the 1980s, siltation was dominant in river channels across the West River and North River deltas in response to the sediment increases; however, scouring occurred in the East River deltas due to sediment reduction. Significant erosion occurred in river channels in the 1990s, which was mostly due to downcutting of the river bed caused by sand excavations and partly because of the reduced sediment load from upstream. Although sand excavations have been banned and controlled by authority agencies since 2000, the erosion of cross sections was still observed in the 2000s because of reduced sediment caused by dam construction. Our study examines the different effects of human activities on the sediment regime and downstream channel morphology, which is of substantial scientific importance for river management.  相似文献   

18.
Knowledge on spatio-temporal variations in planform, hydraulic geometry, and bed-level variations of alluvial streams is required for planning and development of hydraulic structures and bank protection works. In the current study, a Geographic Information System (GIS) has been used to analyze topographical maps, multi-temporal remotely sensed imagery, and hydrologic and hydraulic data to extract the morphological parameters of the Upper Tapi River, India. The river has been found to have consistent migration towards the northern direction, with erosion/deposition on right/left banks. The river has not experienced any major meander except in the lower reaches of the Upper Tapi Gorge and minor braiding conditions at the location where the river emerges from mountainous topography to the plain region. The analyzed river cross sections were found to be depth dominated, and contain large flows within the channel banks. The cross-sections exhibited moderate channel bed adjustments in 1994, 2006, and 2007 wherein excessive sediment flux and stream power were capable of causing morphological changes in the river. High intensity rainfall in the subcatchment resulted in high sediment flux into the river during 1994, which was reported to cause significant aggradation at the downgauging station. The analysis of sediment flux into the river in conjunction with decadal land use land cover, revealed that sediment yield from the catchment was reduced during 2000–2010 due to an increase in water bodies in the form of minor hydraulic structures. The entry of comparatively less sediment laden water into the river, resulted in moderate bed degradation especially in 2006 and 2007 as observed at the downstream station. The methodology applied in the current study is generic in nature and can be applied to other rivers to identify their morphological issues.  相似文献   

19.
The effects of large floods on river morphology are variable and poorly understood. In this study, we apply multi‐temporal datasets collected with small unmanned aircraft systems (UASs) to analyze three‐dimensional morphodynamic changes associated with an extreme flood event that occurred from 19 to 23 June 2013 on the Elbow River, Alberta. We documented reach‐scale spatial patterns of erosion and deposition using high‐resolution (4–5 cm/pixel) orthoimagery and digital elevation models (DEMs) produced from photogrammetry. Significant bank erosion and channel widening occurred, with an average elevation change of ?0.24 m. The channel pattern was reorganized and overall elevation variation increased as the channel adjusted to full mobilization of most of the bed surface sediments. To test the extent to which geomorphic changes can be predicted from initial conditions, we compared shear stresses from a two‐dimensional hydrodynamic model of peak discharge to critical shear stresses for bed surface sediment sizes. We found no relation between modeled normalized shear stresses and patterns of scour and fill, confirming the complex nature of sediment mobilization and flux in high‐magnitude events. However, comparing modeled peak flows through the pre‐ and post‐flood topography showed that the flood resulted in an adjustment that contributes to overall stability, with lower percentages of bed area below thresholds for full mobility in the post‐flood geomorphic configuration. Overall, this work highlights the potential of UAS‐based remote sensing for measuring three‐dimensional changes in fluvial settings and provides a detailed analysis of potential relationships between flood forces and geomorphic change. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

20.
Many urban rivers receive significant inputs of metal‐contaminated sediments from their catchments. Restoration of urban rivers often creates increased slack water areas and in‐channel vegetation growth where these metal‐contaminated sediments may accumulate. Quantifying the accumulation and retention of these sediments by in‐channel vegetation in urban rivers is of importance in terms of the planning and management of urban river restoration schemes and compliance with the Water Framework Directive. This paper investigates sediment properties at four sites across three rivers within Greater London to assess the degree to which contaminated sediments are being retained. Within paired restored and unrestored reaches at each site, four different bed sediment patch types (exposed unvegetated gravel, sand, and silt/clay (termed ‘fine’) sediments, and in‐channel vegetated sediments) were sampled and analysed for a range of metals and sediment characteristics. Many samples were found to exceed Environment Agency guidelines for copper (Cu), lead (Pb) and zinc (Zn) and Dutch Intervention Values for Cu and Zn. At all sites, sediments accumulating around in‐channel vegetation were similar in calibre and composition to exposed unvegetated fine sediments. Both bed sediment types contained high concentrations of pseudo‐total and acetic acid extractable metal concentrations, potentially due to elevated organic matter and silt/clay content, as these are important sorbtion phases for metals. This implies that the changed sediment supply and hydraulic conditions associated with river restoration may lead to enhanced retention of contaminated fine sediments, particularly around emergent plants, frequently leading to the development of submerged and emergent landforms and potential river channel adjustments. High pseudo‐total metal concentrations were also found in gravel bed sediments, probably associated with iron (Fe) and manganese (Mn) oxyhydroxides and discrete anthropogenic metal‐rich particles. These results highlight the importance of understanding the potential effects of urban river restoration upon sediment availability and channel hydraulics and consequent impacts upon sediment contaminant dynamics and storage. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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