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Quantification of channel bed morphology in gravel-bed rivers using airborne multispectral imagery and aerial photography
Abstract
The potential for mapping in-channel morphology within shallow gravel-bed rivers using airborne multispectral imagery and aerial photography is illustrated using a case study from the River Tummel, Scotland. The technique described relies on a good correlation between observed light reflectance levels from a water body and water depth. Measured water depths are regressed against reflectance levels derived from airborne multispectral imagery and black-and-white aerial photographs, to obtain equations that can be used for mapping channel bathymetry. The technique has a great deal of potential for wide-ranging applications, including detailed morphological surveys, assessing in-channel changes and mapping riverine habitats.© 1997 John Wiley & Sons, Ltd.
... Traditional methods of collecting these data involve making measurements along lateral cross-sections of the river. Downstream extrapolations are then made between these cross-sections to estimate the depth, slope, velocity, and width values between them (Winterbottom and Gilvear 1997). These measurements are both time consuming and expensive. ...
... These measurements are both time consuming and expensive. Also, obtaining a time series of data for change analysis is difficult (Marcus 2002, Winterbottom andGilvear 1997). ...
... Researchers (e.g. Lyzenga 1981, Winterbottom and Gilvear 1997, Holden and LeDrew 2000 have demonstrated that depth measurements can be made with aerial photography. Also, many GIS tools are available for hydrologic modeling and representation. ...
Effective management of river environments requires efficient means of gathering data on the rivers, including stream power and other hydraulic attributes. Traditional methods of data collection are spatially limited and can be restrictively expensive. This study utilizes hydraulically‐calibrated aerial photography and GIS to calculate cross‐sectional and mean stream power on a stretch of the Brazos River in central Texas. Field measured water depths are regressed against image brightness values to establish a highly accurate depth to reflectance curve. GIS‐derived water surface slopes are then combined with estimated water depths to map fully two‐dimensional hydraulic processes. This type of image‐based river monitoring provides both an advance in measurement accuracy and in temporal monitoring.
... Common techniques to better visualise naturally transparent flow include adding coloured dye (Winterbottom and Gilvear, 1997;Gran and Paola, 2001;Tal and Paola, 2007) and/or pigments in powder form such as titanium dioxide (TiO 2 ) to make it opaque (Martin et al., 2009). Adding colour/opacity to the flow makes it much easier to identify both qualitatively and quantitatively from images based on the colour value of each pixel (RGB and HSV; colour value can be read and manipulated using image processing software such as Photoshop and ImageJ or using a program that treats images as matrices such as Matlab). ...
... A constant dye concentration can be used to estimate the flow depth based on colour intensity (Winterbottom and Gilvear, 1997;Gran and Paola, 2001;Tal and Paola, 2007). This technique requires uniform light-coloured sediment; the optimal dye concentration and the variation of colour intensity with flow depth need to be calibrated for every experimental setup. ...
Introduction Bedload transport Channel morphology and flow dynamics Bed topography and flow depth Conclusions Acknowledgements References
... The measure of the flow depth using color-intensity variations of the dyed water (Winterbottom & Gilvear, 1997;Tal et al., 2012). The water-surface elevation can then be measured for the whole duration of the flume experiment based on overhead pictures. ...
Abandoned channels coarse-grained fill deposits are studied using flume experiments and field surveys. A detailed model of the channel plug architecture is proposed, and controls on its formation identified from flume experiments. Hydraulic effects, controlled by the asymmetry and diversion angle of the bifurcation, determine the sedimentation pattern and extent of the bedload deposits in abandoned channels. These relationships are quantified for the first time. Influence of base level, abandoned channel length or slope is discussed. Field surveys of abandoned channels demonstrate that the bedload channel fill architecture can be preserved in time and that the experimental relationship is reliable to predict the channel plug coarse deposits minimal extent.
... L'objectif de cette partie est de tester l'intérêt des outils géomatiques pour détecter et caractériser les méso-habitats aquatiques à large échelle. Plusieurs travaux (Brasington et al., 2003 ;Bryant et Gilvear, 1999 ;Carbonneau et al., 2006 ;Chaponniere, 2004 ;Lejot et al., 2007 ;Lorang et al., 2005 ;Marcus et al., 2003 ;Whited et al., 2002 ;Winterbottom et Gilvear, 1997) ont clairement montré l'intérêt des techniques de télédétection pour identifier les méso-habitats et les caractériser à l'échelle du tronçon en termes de taille, géométrie plane ou même bathymétrique. ...
According to the Water Framework Directive (WFD), the good ecological status of all aquatic environments should be achieved by 2015. This status is evaluated largely using biological parameters, which are controlled by the physico-chemical and hydromorphological conditions of water bodies. To achieve the objectives of the WFD, assessing the physical condition of the hydrographic network appears to be a prerequisite, yet this issue still requires research of a fundamental nature. This stage in the basin of the Rhône raises a number of scientific issues which it is necessary to answer before considering practical application. In fact, knowledge of geomorphology and hydrology is now sufficient at a local scale and we can envisage applying this information at a regional scale. However, the change of scale results in important methodological constraints linked in particular to the great heterogeneity of environments.This thesis has two objectives. The first one is to implement geomatic tools to obtain an overall and homogeneous characterisation of the physical network. We propose to develop a methodology to extract information. Second, these tools and methodology will be tested and applied to the Rhône River system to evaluate their effectiveness and to provide reliable evidence for the characterization of large-scale physical conditions.
... They are based on passive sensors (aerial photos) and the technique of ortho-restitution (Rinner, 1969; Fryer, 1983), or on the calibration of a depth– spectral-variation relationship of images defined according to the Beer-Lambert law (e.g. Winterbottom & Gilvear, 1997; Carbonneau et al., 2003; Marcus et al., 2003; Legleiter, 2011). ...
Estimating underwater features of channel bed surfaces without the use of bathymetric sensors results in very high levels of uncertainty. A novel approach to create more accurate and detailed Digital Terrain Models (DTMs) integrates LiDAR-derived elevations of dry surfaces, water depth of wetted areas derived from aerial photos and a predictive depth–colour relationship. This method was applied in three different sub-reaches of a northeastern Italian gravel-bed river (Brenta) before and after flood events occurred in November and December 2010 (recurrence interval: 8 and 10 years). From the data collected through channel field survey, a regression model which calculates channel depths using the correct intensity of three colour bands was implemented. LiDAR and depth points were merged and interpolated into a DTM which features an average error of ±18 cm. The morphological evolution and the sediment volume change calculated through a difference of DTMs shows deposition and erosion areas, indicating a deficit which reduces as it goes downstream.
... The recent development of bathymetric LiDAR technologies has enabled reliable modeling of river bed surfaces, nevertheless, these types of sensors come at an extremely high cost, produce scarce data quality and relatively rough DA VID PUBLISHING D resolutions [22]. To avoid these problems, the topography of wetted areas can be measured by calibrating depth-reflectance relationships using grey-scale [23], colour-scale [14] [15] and multispectral imagery [24] [25]. Depth-reflectance relationships need to be further calibrated using control points derived from field survey contemporary to image acquisition. ...
The magnitude of river morphological changes are better analyzed through the use of quantitative approaches, wherein resolution accuracy and uncertainty assessment are treated as crucial key-factors. In this sense, the creation of precise DEMs (Digital Elevation Models) of rivers represents an affordable tool to analyze geomorphic variations and budgets, except for wetted areas, where reliable channel digitalization can normally be obtained only using expensive bathymetric surveys. The proposed work aims at improving channel surface models without having available bathymetric sensors, by deriving dry areas elevations from LiDAR data and water depth of wetted areas from aerial photos through a predictive depth-colour relationship. The methodology was applied to two different sub-reaches of the Piave River, a gravel-bed river which suffered severe flood events in 2010. Erosion and deposition patterns were identified through DEM differencing, showing a predominance of scour processes which can lead to channel instability situations. The bathymetric output was compared to other previously-derived models confirming the accuracy of the in-channel elevation estimates. Finally, a discussion on the role played by longitudinal protections during the studied flood events is proposed, focusing the attention on the incidence of two major bank erosions that removed significant volumes of stable areas.
... The survey of wetted areas can be thus approached using techniques based on the calibration of a depth-reflectance relationship of images, which can be in grey-scale (e.g. Winterbottom and Gilvear, 1997), coloured (Carbonneau et al., 2006 and Moretto et al., 2012) or multispectral (Legleiter et al., 2011). All the solutions need a field survey, contemporary to the flight, to allow the availability of calibration depth points. ...
Risk management and flood protection are frequently assessed through geo-morphometric evaluations resulting by floods events. If we aim at elevation models with high resolutions and covering large areas, airborne LiDAR surveys can represent a good compromise among costs, time and uncertainty. The major limitation of the nonbathymetric LiDAR surveys consists in the detection of wet areas. Indeed, accounting for more than 20 cm of water depth, LiDAR signal increases exponentially its error. In this paper we present a comparison of the results concerning the application of a colour bathymetry methodology for the production of hybrid DTMs (HDTM). These elevation models were derived by merging LiDAR data for the dry areas and colour bathymetry for the wet areas. The methodological approach consists in a statistical regression between water depth and RGB band intensity values from contemporary aerial images. This methodology includes the use of filters in order to reduce possible errors due to the application of the model, to estimate precise “in-channel” points. The study areas are three different human impacted gravel-bed rivers of the North-East of Italy. This methodology has been applied in three sub-reaches of Brenta River, two of Piave River and two of Tagliamento River before and after relevant flood events with recurrence interval ≥ 10 years. Potentials and limitations of the applied bathymetric method, the comparison of its use in different fluvial contexts and its possibility of employment for geo-morphometric evaluations, were then tested. DGPS control points (1841, 2638, 10473 respectively for Brenta, Piave and Tagliamento River) were finally used to evaluate the accuracy of wet areas. Results showed that, in each model, wet areas vertical errors were comparable to those featured by LiDAR data for the dry areas.
... restricted by logistical issues such as legal access, flow conditions of depth, velocity and water temperature, GPS signal quality, and the time required to make each measurement. Optical airborne remote sensing surveys have recently been used with some success to map bathymetry in local reaches of channels [e.g., Lyon et al., 1992; Gilvear et al., 1995; Winterbottom and Gilvear, 1997; Lane, 2000; Marcus et al., 2003; Legleiter et al., 2004; Marcus and Fonstad, 2008; Feurer et al., 2008; Legleiter et al., 2009]. These passive sensors must accommodate spatial and temporal variability in water column optical properties (e.g., from turbidity), bottom reflectivity, sun glint off the water surface, shadowing by overhanging stream bank vegetation, and mixed subaerial-subaqueous pixels on the margins of flowing water and generally require some local calibration [Feurer et al., 2008; Gao, 2009; Legleiter et al., 2009]. ...
New remote sensing technologies and improved computer performance now allow numerical flow modeling over large stream domains. However, there has been limited testing of whether channel topography can be remotely mapped with accuracy necessary for such modeling. We assessed the ability of the Experimental Advanced Airborne Research Lidar, to support a multi-dimensional fluid dynamics model of a small mountain stream. Random point elevation errors were introduced into the lidar point cloud, and predictions of water surface elevation, velocity, bed shear stress, and bed mobility were compared to those made without the point errors. We also compared flow model predictions using the lidar bathymetry with those made using a total station channel field survey. Lidar errors caused < 1 cm changes in the modeled water surface elevations. Effects of the point errors on other flow characteristics varied with both the magnitude of error and the local spatial density of lidar data. Shear stress errors were greatest where flow was naturally shallow and fast, and lidar errors caused the greatest changes in flow cross-sectional area. The majority of the stress errors were less than ± 5 Pa. At near bankfull flow, the predicted mobility state of the median grain size changed over ≤ 1.3% of the model domain as a result of lidar elevation errors and ≤ 3% changed mobility in the comparison of lidar and ground-surveyed topography. In this riverscape, results suggest that an airborne bathymetric lidar can map channel topography with sufficient accuracy to support a numerical flow model
... Thus, optical bathymetric mapping is a practical method to model the river bed elevation at submeter resolution (Marcus and Fonstad, 2008; Williams et al., 2011). Optical bathymetric mapping requires a correlation between the water's depth and the water's color (Winterbottom and Gilvear, 1997) and works well for shallow water depth and minimal turbidity. To develop a river bed map of the inundated areas, three general steps are taken. ...
... [4] The availability of data sets that capture changes in the topography of rivers over time has allowed the application of the morphological method [Ashmore and Church, 1998] as an alternative to directly measuring sediment transport. Several techniques have been previously used to collect these repeating data sets, including hand and total station surveys of cross sections [Milne and Sear, 1997], real-time kinetic global positioning system (RTK-GPS) [Brasington et al., 2000], aerial photography [Winterbottom and Gilvear, 1997;Lane, 2003;Westaway et al., 2003], ground-based lidar [Wheaton et al., 2010], and combinations of several methods ]. ...
[1] The spatial distribution of riparian vegetation can strongly influence the geomorphic evolution of dryland rivers during large floods. We present the results of an airborne lidar differencing study that quantifies the topographic change that occurred along a 12 km reach of the Lower Rio Puerco, New Mexico, during an extreme event in 2006. Extensive erosion of the channel banks took place immediately upstream of the study area, where tamarisk and sandbar willow had been removed. Within the densely vegetated study reach, we measure a net volumetric change of 578,050 ± ∼ 490,000 m3, with 88.3% of the total aggradation occurring along the floodplain and channel and 76.7% of the erosion focusing on the vertical valley walls. The sediment derived from the devegetated reach deposited within the first 3.6 km of the study area, with depth decaying exponentially with distance downstream. Elsewhere, floodplain sediments were primarily sourced from the erosion of valley walls. Superimposed on this pattern are the effects of vegetation and valley morphology on sediment transport. Sediment thickness is seen to be uniform among sandbar willows and highly variable within tamarisk groves. These reach-scale patterns of sedimentation observed in the lidar differencing likely reflect complex interactions of vegetation, flow, and sediment at the scale of patches to individual plants.
... Digital photogrammetry (Lane, 2000; Westaway et al., 2000) and airborne light detection and ranging (LiDAR; Charlton et al., 2003; Kinzel et al., 2007) can be used to remotely acquire reach-scale digital elevation models (DEMs) but each of these is subject to large uncertainties when surveying submerged topography (Brasington et al., 2003; Lane et al., 2003). Optical reflectance depth monitoring (Winterbottom and Gilvear, 1997; Fonstad and Marcus, 2005) spectral remote sensing (Legleiter et al., 2009) and bathymetric green LiDAR (Guenther et al., 2000; Bailly et al., 2010 ) present alternative surveying strategies for the submerged zone but large uncertainties remain. More accurate reach-scale survey methods employ total stations (Milne and Sear, 1997) and real time kinematic differential global positioning systems (RTK-DGPS; Brasington et al., 2000) which can be time consuming, and terrestrial laser scanners (TLS) which Milan et al. (2007) showed are subject to large vertical errors when surveying submerged topography at the reach-scale. ...
Natural gravel surfaces are spatially variable. Measurement of their detailed structure is essential for understanding the interaction of roughness with near-bed flows and the sediment entrainment process. However, the acquisition of high resolution topographic data of a river bed is technically demanding where the bed is not regularly exposed by fluctuating water levels. Often the most geomorphologically active portion of a gravel bed river remains submerged even at low stages. Optical reflectance depth monitoring and through-water photogrammetry have been employed to map bed topography over relatively shallow submerged zones. This study presents laboratory and field experiments to demonstrate that through-water terrestrial laser scanning can also be used to provide high resolution DTMs of submerged gravel beds. The resulting point cloud data must be corrected for refraction before the registration process takes place. Additional errors arise from the internal architecture of the scanner as the offset between the arbitrary origin and the point from which the laser emanates must be calculated before refraction correction. These DTMs can be seamlessly embedded within larger sub aerial reach-scale surveys and can be acquired alongside flow measurements to examine the effects of three-dimensional surface geometry on turbulent flow fields.
... Often-cited advantages of this approach include improved efficiency relative to traditional fieldbased surveys, and the ability to obtain quantitative, spatially distributed measurements of key channel attributes at high resolution over larger, watershed extents [Marcus and Fonstad, 2008]. Although a number of case studies have demonstrated the potential to characterize river bathymetry via remote sensing [e.g., Winterbottom and Gilvear, 1997; Marcus et al., 2003; Carbonneau et al., 2004; Lejot et al., 2007], most previous research has been empirical and sitespecific , lacking generality. More recent work has attempted to overcome this obstacle by seeking to understand the manner in which solar energy interacts with the atmosphere, air-water interface, water column, and substrate, and is ultimately measured by a remote detector. ...
Remote sensing offers an efficient means of mapping bathymetry in river systems, but this approach has been applied primarily to clear-flowing, gravel bed streams. This study used field spectroscopy and radiative transfer modeling to assess the feasibility of spectrally based depth retrieval in a sand-bed river with a higher suspended sediment concentration (SSC) and greater water turbidity. Attenuation of light within the water column was characterized by measuring the amount of downwelling radiant energy at different depths and calculating a diffuse attenuation coefficient, K d. Attenuation was strongest in blue and near-infrared bands due to scattering by suspended sediment and absorption by water, respectively. Even for red wavelengths with the lowest values of K d, only a small fraction of the incident light propagated to the bed, restricting the range of depths amenable to remote sensing. Spectra recorded above the water surface were used to establish a strong, linear relationship (R 2 = 0.949) between flow depth and a simple band ratio; even under moderately turbid conditions, depth remained the primary control on reflectance. Constraints on depth retrieval were examined via numerical modeling of radiative transfer within the atmosphere and water column. SSC and sensor radiometric resolution limited both the maximum detectable depth and the precision of image-derived depth estimates. Thus, although field spectra indicated that the bathymetry of turbid channels could be remotely mapped, model results implied that depth retrieval in sediment-laden rivers would be limited to shallow depths (on the order of 0.5 m) and subject to a significant degree of uncertainty.
... The other category, in which the illumination is provided by the sun, includes passive methods such as photogrammetric and spectral methods. In photogrammetric methods the image geometry is used to extract information (Westaway et al., 2001(Westaway et al., , 2003Feurer et al., 2007), whereas in spectral methods, water depth is related to image reflectance to derive bathymetry (e.g., Winterbotton and Gilvear, 1997;Westaway et al., 2003;Carbonneau et al., 2003;Lyzenga et al., 2006). Lane (2000Lane ( , 2001 provides a history and review of photogrammetry in river applications. ...
Recent advances in river instrumentation, including acoustic and remote sensing methods for measuring river hydrodynamics and morphology, are revolutionizing our capabilities to understand, describe and model river systems. The instrument development outpaces their thorough evaluation and assimilation such that a rigorous review is a daunting task. While not exhaustive, the present paper focuses on the new generation of acoustic and remote sensing methods and their synergy in providing new insights into the river hydrodynamic and morphologic macro-characteristics. These instruments can quickly and efficiently provide detailed multi-dimensional measurements that uniquely enable investigation of complex river processes that are related to critical issues such as river hydrodynamics, sediment transport, habitat availability and stream ecology. The paper reviews modern instruments including Acoustic Doppler Velocimeter (ADV), Acoustic Doppler Current Profiler (ADCP), Large Scale Particle Image Velocimetry (LSPIV), Multibeam Echosounder (MBES) and Airborne Bathymetry LiDAR (ABL). The use of two acoustic methods, namely ADCP and MBES, is then presented for describing multi-dimensional river hydrodynamics and morphology by using data from three river reaches (Brazos River in Texas, Kissimmee River in Florida and Mississippi River in Iowa) in the United States. ADCP and MBES provide two and three dimensional datasets that have opened the door to many new research opportunities in the past decade. A brief description of these new opportunities along with the challenges that these new datasets bring is also discussed.
Remote sensing enables us to measure fluvial systems without disrupting their dynamics. Small-scale physical models of rivers allow us to observe their geomorphic evolution, but we need remote sensing methods to monitor these laboratory landscapes without altering their flow or topography, just as with field-scale rivers. In this paper, we review how experimental geomorphologists have adapted remote sensing for the laboratory. We consider how remote methods to monitor model topography, flow depth, velocity and planform have been employed, enabling uninterrupted experimental evolution. We also explore the transfer of techniques between field-scale and experimental remote sensing; the controlled conditions in the lab aided the development of some methods, while others benefited from airborne deployment. We consider recent developments offered by laboratory remote sensing, including through-water laser scanning and adaptations of structure-from-motion photogrammetry; we also consider new challenges associated with these developments, such as computational power. Finally, we discuss new research problems that laboratory remote sensing is opening up to geomorphology. We hope this review will be useful for experimentalists seeking to collect data remotely, continuously and/or cost-effectively. © 2023 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Balkan rivers are described as hotspots of biodiversity due to the limited glaciation to a few high-altitude cirques during the last ice age. Although many studies have examined local-scale highlights in biota and biodiversity, an overall hydro-morphological characterization and development assessment of these rivers during the Holocene are missing. Aim of the presented study is to put together the present and recent river dynamics and Quaternary landscape development for a comprehensive understanding of geomorphic processes and the current river morphological characteristics of the Vjosa river. This was done by linking satellite imagery and hydro-dynamic modelling to read the landscape, i.e., to determine the development of landscape units, and (i) the active channel, (ii) the active floodplain and (iii) the morphological floodplain. The digital terrain of the Vjosa River catchment, as an example of the Balkan rivers, was based on a 25 m × 25 m digital elevation model, and hydrodynamic numerical analysis was conducted with a one-dimensional step-backwater model. The application of this assessment approach enabled a clear differentiation of four different sections along the longitudinal profile of the Vjosa—mainly differentiated by the slope and partially corresponding with the confluence of tributaries. We also found that periods of glaciation imposed a strong impact on the hydro-morphological characteristics of the Vjosa River. In particular, the delineation of the active channel and the active floodplain exhibited two different morphological reaches. The first reach exhibits a clear incision of the river into fluvial deposits when the historical sediment supply was high, and the second reach occurs downstream where the current sediment supply is equal or higher than that during the Pleistocene or earlier periods. These differences in hydro-morphological development exert a strong impact on the floodplain development and the human use of these reaches. Thus, despite the given uncertainties due to the lack of bathymetric accuracy, the hybrid assessment approach is useful for the hydro-morphological characterization of the Vjosa river and the identification of landscape forming processes on the catchment scale.
This paper addresses the underlying bottleneck of unknown materials and material characteristics for assessing the life cycle of an existing structure or considering interventions. This done by classifying and characterizing common building materials with two readily accessible, remote sensing technologies: multispectral imaging and Light Detection and Ranging (LiDAR). A total of 142 samples, including concrete of 3 different water/cement ratios, 2 mortar types, and 2 brick types (each type fired at 3 different temperatures) were scanned using a 5-band multispectral camera in the visible, RedEdge, and Near Infrared range and 2 laser scanners with different wavelengths. A Partial Least Squares Discriminant Analysis model was developed to classify the main materials and then the subcategories within each material type. With multispectral data, an 82.75% average correct classification rate was achieved (improving to 83.07% when combined with LiDAR intensity data), but the effect was not uniformly positive. This paper demonstrates the potential to identify building materials in a non-destructive, non-contact manner to aid in in-situ facade material labelling.
Most Alpine rivers have undergone strong alteration of flow and sediment regimes. These alterations have notable effects on river morphology and ecology. One option to mitigate such effects is the flow regime management, specifically by the re-introduction of channel-forming discharges. The aim of this work is to assess the morphological changes induced in the Piave River (Italy) due to two different controlled flood strategies, the first characterized by a single artificial flood per year and the second by higher magnitude, but less frequent, floods. The work was carried out applying a 2D reduced-complexity morphodynamic model (CAESAR-LISFLOOD) to a 7 km-long reach, characterized by a braided pattern and highly regulated discharges. The numerical modelling allowed the assessment of morphological changes for four long-term scenarios (2009–2034). The scenarios were defined taking into account the current flow regime and the natural regime, which was estimated by a stochastic physically-based hydrologic model. Changes in channel morphology were assessed by measuring active channel width and braiding intensity. Comparing controlled flood scenarios to a baseline scenario (i.e., no controlled floods) it turned out that artificial floods had small effects on channel morphology. The highest channel widening (13.5 %) was produced by the release strategy with higher magnitude floods, while the other strategies produced lower widening (8.6 %). Negligible change was observed in terms of braiding intensity. Results pointed out that controlled floods may not represent an effective solution for morphological recovery in braided rivers strongly impacted in their flow and sediment regimes.
Any decision-making and planning in coastal areas and ports require a series of basic information in which bathymetry is an important part of it. The concern of most scholars who deal with or research in this area is to access accurate, up-to-date, and cost-effective information. One of the methods to achieve such information is to use remote sensing technologies. The purpose of this study is to use Landsat 8 Operational Land Imager and Random Forest algorithm by using regression and classification methods for depth prediction in a part of Persian Gulf region (Bushehr port, Kharg Island, and its surroundings). For verification of depth prediction in these two methods, two indices of root mean square error and mean absolute error in the regression method and the Kappa index (KAPPA) for classification method were used. The results of these two methods show that in both regression and classification methods, the best combination of bands for depth prediction was the band combination (1–2–3–4) and the Landsat 8 satellite image has the ability to obtain depth with a fairly acceptable accuracy to depth of around 10 m. From the depths of 10 m onward, the measurement error will increase relative to the depth.
A reliable representation of river terrains is essential to river research. Field surveys of river channel geometry are time‐consuming, costly, and logistically constrained and thus would encounter difficulties to achieve sufficient spatial coverage, resolution, and frequency of resurvey. This paper aims to demonstrate an efficient approach to building a river terrain model (RTM), the emphasis being on how to combine bathymetry and topography derived from satellite images captured at different flow stages. A method for calculating the difference between high and low stages (DHLS) based on the uniform‐flow theory is proposed. Calculations are carried out for a 13‐km long meandering section of the gravel‐bed Goulais River in Canada, which features pools and riffles, alternating point bars, and midchannel bars. A RTM for this complex section has been successfully produced. It consists of three data components: bathymetry at low stage, topography at high stage, and DHLS. The results capture realistic characteristics, including thalweg shift, steep outer banks, and gradual inner banks. They also show realistic longitudinal and lateral locations of pools and riffles. To illustrate potential applications of RTM, this paper has computed extreme bed shear stresses at bankfull discharge through hydrodynamics simulations of depth‐averaged flow in the river section and further estimated bed‐sediment grain‐size distributions. The estimates compare well with field measurements. The DHLS can vary significantly along a river channel. The proposed method for determining it is not site‐specific, and hence applicable to other rivers. The novelty of the methodology discussed lies in combining remote sensing techniques with physical flow laws.
It is not new to recognise that data from remote sensing platforms is transforming the way we characterise and analyse our environment. The ability to collect continuous data spanning spatial scales now allows geomorphological research in a data rich environment and this special issue (coming just 7 years after the 2010 special issue of ESPL associated with the remote sensing of rivers) highlights the considerable research effort being made to exploit this information, into new understanding of geomorphic form and process. The 2010 special issue on the remote sensing of rivers noted that fluvial remote sensing papers made up some 14% of the total river related papers in ESPL. A similar review of the papers up to 2017 reveals that this figure has increased to around 25% with a recent proliferation of articles utilising satellite based data and structure from motion derived data. It is interesting to note, however that many studies published to date are proof of concept, concentrating on confirming the accuracy of the remotely sensed data at the expense of generating new insights and ideas on fluvial form and function. Data is becoming ever more accurate and researchers should now be concentrating on analysing these early data sets to develop increased geomorphic insight challenging paradigms and moving the science forward. The prospect of this occurring is increased by the fact that many of the new remote sensed platforms allow accurate spatial data to be collected cheaply and efficiently. This is providing the individual researcher or small research grouping with tremendous opportunity to move the science of fluvial geomorphology forward unconstrained to a large degree of the need to secure substantial research funding. Fluvial geomorphologists have never before been in such a liberated position! As techniques and analytical skills continue to improve it is inevitable that Marcus and Fondstad's (2010) prediction that remotely sensed data will revolutionising our understanding of geomorphological form and process will prove true, altering our ideas on the very nature of system functioning in the process.
IntroductionThe Context: Environmental Changes Affecting Valley Floors and Floodplains in the Last 1000 YearsValley-floor and Floodplain ResponseRecent DevelopmentsConclusions
References
This chapter reviews various types of spatial data used for geomorphological mapping, with reference to their basic characteristics, historical background and some examples of mapping using the data. Data types include text descriptions; hand-drawn illustrations; data from ground surveying using triangulation (plane table, level, global navigation satellite systems, total station, laser range finder and terrestrial laser scanner); existing topographic maps; ground and aerial photographs and videos for visual interpretation; satellite and aerial imagery for visual interpretation; height data from analogue, analytical and digital photogrammetry; height data from airborne/satellite light detection and ranging and interferometric synthetic aperture radar; and compiled digital elevation models. This chapter also discusses recent trends, problems and future perspectives concerning the use of data for geomorphological mapping, with reference to various aspects of data including format (analogue versus digital), spatial and temporal resolution, error and availability. Technical and strategic issues related to data selection and conversion, combined use of various data, application of manual and automated methods for mapping and labour/cost of work are also discussed.
Introduction: The Hyperspatial Perspective Hyperspatial image acquisition Issues, potential problems and plausible solutions From data acquisition to fluvial form and process understanding Conclusion Acknowledgements References
Multispectral satellite remote sensing can predict shallow-water depth distribution inexpensively and exhaustively, but it requires many in situ measurements for calibration. To extend its feasibility, we improved a recently developed technique, for the first time, to obtain a generalized predictor of depth. We used six WorldView-2 images and obtained a predictor that yielded a 0.648 m root-mean-square error against a dataset with a 5.544 m standard deviation of depth. The predictor can be used with as few as two pixels with known depth per image, or with no depth data, if only relative depth is needed.
Work Package 2 of REFORM focuses on hydromorphological and ecological processes and interactions within river systems with a particular emphasis on naturally functioning systems. It provides a context for research on the impacts of hydromorphological changes in Work Package 3 and for assessments of the effects of river restoration in Work Package 4.
Deliverable 2.1 of Work Package 2 proposes a hierarchical framework to support river managers in exploring the causes of hydromorphological management problems and devising sustainable solutions. The deliverable has four parts. Part 1 (this volume) provides a full description of the hierarchical framework and describes ways in which each element of it can be applied to European rivers and their catchments. Part 2 includes thematic annexes which provide more detailed information on some specific aspects of the framework described in Part 1. Part 3 includes catchment case studies which present the application of the entire framework described in Part 1 to a set of European catchments located in different biogeographical zones. Part 4 includes catchment case studies which present a partial application of the framework described in Part 1 to a further set of European catchments.
The rapid proliferation of remote sensing and geographic information systems (GIS) into geomorphologic mapping has increased the objectivity and efficiency of landform segmentation, measurement, and classification. The near ubiquitous presence of Earth-observing satellites provides an array of perspectives to visualize the biophysical characteristics of landscapes, access inhospitable terrain on a predictable schedule, and study landscape processes when conditions are hazardous. GIS technology has altered the analysis, visualization, and dissemination of landform data due to the shared theoretical concepts that are fundamental to geomorphology and GIScience. The authors review geospatial technology applications in landform mapping (including emerging issues) within glacial, volcanic, landslide, and fluvial research.
Objective
Estimating river's underwater bed elevations is a necessary but challenging task. The objective of this study is to develop a revised approach to generate accurate and detailed Digital Terrain Models (DTMs) of a river reach by merging LiDAR data for the dry area, with water depth indirectly derived from aerial imagery for wet areas.
Methods
This approach was applied along three sub-reaches of the Brenta River (Italy) before and after two major flood events. A regression model relating water depth and intensity of the three colour bands derived from aerial photos, was implemented. More than 2400 in-channel depth calibration points were taken using a differential Global Positioning System (dGPS) along a wide range of underwater bed forms.
Results
The resulting DTMs closely matched the field-surveyed bed surface, and allowed to assess that a 10-year recurrence interval flood generated a predominance of erosion processes. Erosion dominated in the upper part of the study segment (− 104,082 m3), whereas a near-equilibrium is featured on the lower reach (− 45,232 m3). The DTMs allowed the detection of processes such as riffle–pool downstream migration, and the progressive scour of a pool located near a rip-rap.
Conclusion
The presented approach provides an adequate topographical description of the river bed to explore channel adjustments due to flood events.
Practice
Combining colour bathymetry and dGPS surveys proved to represent a useful tool for many fluvial engineering, ecology, and management purposes.
Implications
The proposed approach represents a valuable tool for river topography description, river management, ecology and restoration purposes, when bathymetric data are not available.
Although visible bands of high-resolution multispectral imagery are used for bathymetry, the relative utility of different bands is poorly understood. Therefore, we evaluated the relative utility of the six visible bands of WorldView-2. We statistically selected the visible bands that gave the best accuracy under different situations, tallying how often each band was included in the best combination. The average frequency was greater than 50% for every band and differed between bands by only 17%. We conclude that all visible bands are useful for remote sensing of water depth, although the utility depends on the image and number of training pixels available.
A high-resolution or complete bathymetric map of shallow water based on sparse point measurements (depth soundings) is often needed. One possible approach to such maps is passive remote sensing of water depth by using multispectral imagery in the popular method proposed by Lyzenga et al. [2006]; however, its application has been limited due to insufficient accuracy. To improve accuracy, we have developed 3 extensions of Lyzenga's method by addressing unrealistic optical and statistical assumptions in the method. The purpose of this paper is to compare the accuracy of Lyzenga's method, the 3 extensions, and the combination of the 3 extensions. The accuracy comparison test was performed for 2 coral reef sites by using cross validation. The results indicated that for both sites, the extended methods were more accurate than Lyzenga's method when sufficient training data were available. The most accurate extension was the one derived by modeling the spatial autocorrelation in the error term of the regression model used in Lyzenga's method. The combination of the 3 extensions was even more accurate than the extensions. The implementations of the extended methods are not difficult in terms of software availability and computational cost.
Fine-scale (submeter) resolution digital elevation models (DEMs) created from high precision (subcentimeter) instruments (e.g., total station, rtkGPS, and laser scanning) have become ubiquitous in the field of fluvial geomorphology. They permit a diverse range of spatially explicit analyses including hydraulic modeling, habitat modeling, and geomorphic change detection. While previous studies have assessed the quality of specific topographic survey methods at individual sites or across a limited number of sites, an intercomparison of survey technologies across a diverse range of wadeable streams could help clarify which techniques are feasible, as well as which work best under what circumstances and for what purposes. Although a wealth of existing studies and protocols explain how to undertake each individual technique, in this study we seek to provide guidance on what techniques to use in which circumstances. We quantified the relative quality and the amount of effort spent collecting data to derive bare earth topography from an array of ground-based and airborne survey techniques. We used topographic survey data collected over the summer of 2010 from six sample reaches of varying complexity in the Lemhi River basin, Idaho, USA. We attempted to conduct complete, replicate surveys at each site using total station (TS), real-time kinematic (rtk) GPS, discrete return terrestrial laser scanner (TLS), and airborne LiDaR surveys (ALS). We evaluated the precision and accuracy of derived bare earth DEMs relative to the higher precision total station point data. Discrepancies between pairwise techniques were calculated using propagated DEM errors thresholded at a 95% confidence interval. Mean discrepancies between total station and rtkGPS DEMs were relatively low (≤ 0.05 m), yet TS data collection time was up to 2.4 times longer than rtkGPS. The ALS DEMs had lower accuracy than TS or rtkGPS DEMs, but the aerial coverage and floodplain context of the ALS data set was superior to all other techniques. The TLS bare earth DEM accuracy and precision were lower than any other technique because of vegetation returns misinterpreted as ground returns. Our results are helpful for understanding the strengths and weaknesses of different approaches.
Accurate terrain models are a crucial component of studies of river channel evolution. In this paper we describe a new methodology for creating high-resolution seamless digital terrain models (DTM) of river channels and their floodplains. We combine mobile laser scanning and low-altitude unmanned aerial vehicle (UAV) photography-based methods for creating both a digital bathymetric model of the inundated river channel and a DTM of a point bar of a meandering sub-arctic river. We evaluate mobile laser scanning and UAV-based photogrammetry point clouds against terrestrial laser scanning and combine these data with an optical bathymetric model to create a seamless DTM of two different measurement periods. Using this multi-temporal seamless data, we calculate a DTM of difference that allows a change detection of the meander bend over a one-year period.
Understanding river form and behavior requires an efficient means of
measuring channel morphology. This study evaluated the potential to map
the bathymetry of two clear-flowing, shallow (<3 m deep) gravel bed
rivers <60 m wide from 2 m-pixel WorldView2 satellite images. Direct
measurements of water column optical properties were used to quantify
constraints on depth retrieval. The smallest detectable change in depth
was 0.01-0.04 m and the maximum detectable depth was 5 m in green bands
but <2 m in the near-infrared; lower sensor radiometric resolution
yields less precise estimates over a smaller range. An algorithm for
calibrating a band ratioX to field measurements of depth d proved
effective when applied to spectra extracted from images (R2 =
0.822 and 0.594 for the larger and smaller stream, respectively) or
measured in the field (R2 = 0.769 and 0.452). This procedure
also identified optimal wavelength combinations, but different bands
were selected for each site. Accuracy assessment of bathymetric maps
produced using various calibration approaches and image types indicated
that: 1) a linear d vs. Xrelation provided depth estimates nearly as
accurate as a quadratic formulation; 2) panchromatic and pan-sharpened
multispectral images with smaller 0.5 m pixels did not yield more
reliable depth estimates than the original images; and 3) depth
retrieval was less reliable in pools due to saturation of the radiance
signal. This investigation thus demonstrated the feasibility, as well as
the limitations, of measuring the bathymetry of clear, shallow gravel
bed rivers from space.
The multispectral method for the remote sensing of water depth proposed by Lyzenga has been widely applied to shallow-water bathymetry by researchers. The predictor of water depth used in this method is a linear function of image-derived variables for each visible band. The coefficients of the predictor are estimated by using a number of pixels with known depth as training data; this depth information is usually obtained by performing in situ depth measurements. Theoretically, if an appropriate set of coefficients is chosen, the predictor can be insensitive to some variations in the optical properties of the bottom material and water. However, it is sensitive to variations in atmospheric and water surface transmittance and sun and satellite elevations. Consequently, a single set of coefficients cannot always be applied to multiple images. In this letter, we propose a simple method to estimate a general set of coefficients for Lyzenga's predictor that is relatively less affected by the aforementioned factors. We derive and utilize the theoretical fact that these factors affect only the intercept (constant term) of the predictor function. We demonstrate the effectiveness of the proposed method using WorldView-2 images of coral reefs. The proposed method will enable the application of a single set of coefficients (except for the intercept) to a broad range of images. This will significantly reduce the number of pixels with known depth required for the prediction of an image and thereby improve the feasibility of remote sensing of water depth.
Acquiring high resolution topographic data of natural gravel surfaces is technically demanding in locations where the bed is not exposed at low water stages. Often the most geomorphologically active surfaces are permanently submerged. Gravel beds are spatially variable and measurement of their detailed structure and particle sizes is essential for understanding the interaction of bed roughness with near-bed flow hydraulics, sediment entrainment, transport and deposition processes, as well as providing insights into the ecological responses to these processes. This paper presents patch-scale laboratory and field experiments to demonstrate that through-water terrestrial laser scanning (TLS) has the potential to provide high resolution digital elevation models of submerged gravel beds with enough detail to depict individual grains and small-scale forms. The resulting point cloud data requires correction for refraction before registration. Preliminary validation shows that patch-scale TLS through 200 mm of water introduces a mean error of less than 5 mm under ideal conditions. Point precision is not adversely affected by the water column. The resulting DEMs can be embedded seamlessly within larger sub-aerial reach-scale surveys and can be acquired alongside flow measurements to examine the effects of three-dimensional surface geometry on turbulent flow fields and their interaction with instream ecology dynamics. Copyright © 2011 John Wiley & Sons, Ltd.
A severe decline in Columbia River salmonid populations and subsequent
Federal listing of subpopulations has mandated both the monitoring of
populations and evaluation of the status of available habitat. Numerous
field and analytical methods exist to assist in the quantification of
the abundance and quality of in-stream habitat for salmonids. These
methods range from field 'stick and tape' surveys to spatially explicit
topographic and aerial photographic surveys from a mix of ground-based
and remotely sensed airborne platforms. Although several previous
studies have assessed the quality of specific individual survey methods,
the intercomparison of competing techniques across a diverse range of
habitat conditions (wadeable headwater channels to non-wadeable mainstem
channels) has not yet been elucidated. In this study, we seek to
enumerate relative quality (i.e. accuracy, precision, extent) of habitat
metrics and inventories derived from an array of ground-based and
remotely sensed surveys of varying degrees of sophistication, as well as
quantify the effort and cost in conducting the surveys. Over the summer
of 2010, seven sample reaches of varying habitat complexity were
surveyed in the Lemhi River Basin, Idaho, USA. Complete topographic
surveys were attempted at each site using rtkGPS, total station,
ground-based LiDaR and traditional airborne LiDaR. Separate high spatial
resolution aerial imagery surveys were acquired using a tethered blimp,
a drone UAV, and a traditional fixed-wing aircraft. Here we also
developed a relatively simplistic methodology for deriving bathymetry
from aerial imagery that could be readily employed by instream habitat
monitoring programs. The quality of bathymetric maps derived from aerial
imagery was compared with rtkGPS topographic data. The results are
helpful for understanding the strengths and weaknesses of different
approaches in specific conditions, and how a hybrid of data acquisition
methods can be used to build a more complete quantification of salmonid
habitat conditions in streams.
This paper combines archived remotely sensed data (airborne lidar and
digital color air photographs) with non-synchronous ground observations
(including observations of topographic form and vegetation cover and
growth) to test the hypothesis that colonization of exposed river
sediments by riparian trees has an impact on channel form and to
quantify any impact that is identified. This is achieved along a 21 km
reach of the braided, gravel-bed Tagliamento River, northeast Italy,
where the width of the braided corridor typically exceeds 800 m. Lidar
data are analyzed to extract a 2 m resolution DEM and determine riparian
vegetation extent, height and structure within the active corridor.
Aerial photographs are used to map the topography of the submerged parts
of the corridor. These data are divided into 1 km length sub-reaches,
which possess strong contrasts in vegetation height and extent. Joint
analysis of vegetation and morphological properties of these sub-reaches
reveals significant associations between vegetation properties and reach
morphology. Residuals from a gamma function fitted to the topographic
data for each sub-reach show a good fit with poorly vegetated reaches,
but a weakening fit with increasing vegetation cover, largely as a
result of the appearance of secondary peaks in the elevation frequency
distribution associated with the heavily vegetated areas. Furthermore,
the overall skewness and kurtosis of the elevation frequency
distribution within each of the sub-reaches are both significantly
correlated with vegetation extent, height, median elevation and growth
rate, indicating a clear topographic signature of vegetation development
along this braided river that reflects sediment accumulation within and
around the vegetated patches.
A fundamental challenge in river analysis and modeling is the lack of readily available and reliable information on river bank geometry. Traditional survey methods are expensive and time consuming and often difficult to execute in many river systems due to hazardous terrain or lack of access. However as high quality aerial and satellite imagery becomes available for more of the globe, it is increasingly possible to extract these bank locations directly from imagery. The most direct method of doing this involves manually designating edges based on visual criterion. This however is often prohibitively time consuming and labor intensive and the quality is dependent on the individual doing the task. This paper describes a quick and fully automated method for locating water surface and river banks in high resolution aerial imagery without recourse to any multispectral information, by segmenting based on the local entropy of the image. This method is demonstrated on imagery of several rivers and its advantages and limitations are discussed.
This study represent an assessment of the feasibility of using optical and near infra-red wavelengths to map water depth and substrate type which are two of the primary components of river physical habitat. The objective was met by measuring reflectance using field spectroscopy of exposed and then progressively submerged (0-1 m) artificial substrates during rising tides on a tidal reach of the River Forth; this approach provided absolute control for substrate type. The field data were used to simulate multi-spectral response as measured using Airborne Thematic Mapper (bands 1 to 7; 420nm to 900nm) since airborne remote sensing provides the way forward for synoptic mapping of small streams (,20 m), such as those typically found in Scotland. In the presence of sunlit conditions significant differences in reflectance over a broad range of wavelengths was evident with strong correlations with water depth and substrate. Differences in reflectance were most marked under shallow conditions (,0.75 m) and then progressively diminished. Application of simple linear correlation, band ratios, the Lyzenga algorithm and multiple discriminant analysis all suggested the possibility of mapping shallow river channel water depths. The results also showed that substrate type and the analysis technique selected also affected the optimum spectral wavelength for detection of channel substrate type and water depth. Overall the work suggests mapping channel depth and substrate type is possible using remotely sensed data in optical and near infra-red wavelengths, using high spatial and spectral resolution multi-spectral imagery. However it may be restricted to depths of less than 1 m where there is high organic colour and/or a significant periphyton cover.
This paper reviews various types of remote sensing that can be applied to mapping and monitoring riverine and coastal geohazards. It focuses on flooding, ground instability, erosion and sedimentation, mainly drawing on examples from the UK. Both airborne and space-borne systems are examined, with assessments of the merits, limitations and relative costs of each system. Remote sensing offers a wide range of useful techniques for mapping coastal and riverine features, particularly where there are access problems associated with sites prone to flooding or slope instability.
Riparian vegetation can significantly influence the morphology of a river, affecting channel geometry and flow dynamics. To examine the effects of riparian vegetation on gravel bed braided streams, we conducted a series of physical experiments at the St. Anthony Falls Laboratory with varying densities of bar and bank vegetation. Water discharge, sediment discharge, and grain size were held constant between runs. For each run, we allowed a braided system to develop, then seeded the flume with alfalfa (Medicago sativa), allowed the seeds to grow, and then continued the run. We collected data on water depth, surface velocity, and bed elevation throughout each run using image-based techniques designed to collect data over a large spatial area with minimal disturbance to the flow. Our results show that the influence of vegetation on overall river patterns varied systematically with the spatial density of plant stems. Vegetation reduced the number of active channels and increased bank stability, leading to lower lateral migration rates, narrower and deeper channels, and increased channel relief. These effects increased with vegetation density. Vegetation influenced flow dynamics, increasing the variance of flow direction in vegetated runs and increasing scour depths through strong downwelling where the flow collided with relatively resistant banks. This oblique bank collision also provides a new mechanism for producing secondary flows. We found it to be more important than the classical curvature-driven mechanism in vegetated runs.
The availability of new geomatics technologies and methods has transformed the investigation of sediment transport rates using the morphological approach. Terrestrial laser scanning (TLS), in particular, has transformative potential for mapping river change in braided rivers since much of the bed is sub-aerially exposed at low flows. TLS offers the opportunity to collect high-precision and high-accuracy data over large spatial extents, at temporal frequencies commensurate with individual flood events. When coupled with a suitable bathymetric mapping technique, high-resolution digital elevation models (DEMs) can be developed for both dry and wet areas of the braidplain. This chapter presents results from the ReesScan Project, where a methodology has been developed to apply TLS and empirical–optical bathymetric mapping to monitor the evolution of the braided Rees River, New Zealand, through a sequence of competent flood events. A reach-scale sediment budget is calculated and subjected to a statistical method to estimate a threshold level of detection to distinguish spurious changes in the DEM of difference. Tests at all the confidence levels considered (84%–99%) show that the reach is aggrading, with deposition volumes exceeding erosion volumes by 9% for the 84% confidence interval. This equated to a mean aggradation rate of 11mm. The DEM of difference is rich in detail and, even when subjected to stringent statistical testing, subtle geomorphological changes are evident. The results show that the coupling of TLS with empirical–optical bathymetric mapping is a compelling approach for quantifying topographic change in unprecedented detail.
The rapid proliferation of remote sensing and geographic information systems (GIS) into geomorphologic mapping has increased the objectivity and efficiency of landform segmentation, measurement, and classification. The near ubiquitous presence of Earth-observing satellites provides an array of perspectives to visualize the biophysical characteristics of landscapes, access inhospitable terrain on a predictable schedule, and study landscape processes when conditions are hazardous. GIS technology has altered the analysis, visualization, and dissemination of landform data due to the shared theoretical concepts that are fundamental to geomorphology and GIScience. The authors review geospatial technology applications in landform mapping (including emerging issues) within glacial, volcanic, landslide, and fluvial research.
This phase of the study investigated the relation of average and peak flows in the Rio Grande to changes in channel, sand bar, and island widths as measured on historical aerial photographs taken from 1972 to 2006. The study reach lies between Bernalillo Bridge and the Isleta Diversion Dam, with a focus on the river between the North and South Albuquerque Metropolitan Arroyo Flood Control Authority (AMAFCA) Diversion Channels. Digitized and georeferenced photographs were analyzed using a geographic information system (GIS), with particular attention paid to quantifying potential measurement error and its propagation through estimates of channel areas and bank erosion rates. Average active channel widths decreased from 169 +/- 5 m in 1972 to 130 +/- 5 m in 2006. Narrowing concentrated in the upper study reach and in areas where the 1972 channel was relatively wide. Variability in channel width also decreased over the study period. Decreases in channel width and area coincide with periods of low flows, although the area changes are associated with large errors. Island areas have increased since 1972, although islands per se were also lost during the later study period by bank attachment. Bank erosion estimates have large associated errors. Erosion rates appear to be generally decreasing over time, but accelerated during the 2005 high flows.
Contemporary (event to decadal-scale) morphological changes in two large braided rivers in Canterbury, New Zealand, are described, along with laboratory studies that support the field observations. In the process, some new developments in field and laboratory methods for investigating morphological change in braided rivers are presented, and Paola's (2001) hypothesis that braiding tendency should be influenced by a river's ability to turn over its bed within the characteristic time for riparian vegetation to establish and grow to a mature, scour-resistant state is examined. The lower Waitaki River has been regulated for hydropower since 1935, and since then vegetation has encroached over the riverbed and braiding intensity for a given discharge has reduced. Measurements of vegetation removal by floods indicate that floods are not able to turn over the bed fast enough to contain vegetation encroachment, and the present braiding state is held by virtue of a regular spraying programme. In contrast, on the unregulated and sparsely vegetated Waimakariri River, remotely sensed high-resolution topographic surveys using LiDAR and digital photogrammetry have shown that even sub-annual floods turn over large proportions of the braidplain. The laboratory studies show that a braided river will evolve into a single-thread channel when its bed is invaded by vegetation and floods are too infrequent to contain the vegetation growth. Collectively, the field and laboratory evidence confirms that Paola's (2001) dimensionless time-scale parameter is a reasonable first-order predictor of whether floods or vegetation will achieve ascendancy, driving a river towards either braided or single-thread end-points, respectively.
Reaches extending for tens or hundreds of channel widths along a river are normally seen as stable and homogeneous from a regime perspective, but other approaches emphasise channel change and within-reach spatial variability in flow and physical habitat. Tensions between different perspectives are discussed, with particular emphasis on limitations of traditional regime approaches. Cross-cutting issues include reconciling different scales of self-organisation in gravel-bed rivers, the need to treat bed characteristics as a degree of freedom, and within-reach spatial variability and temporal fluctuation. New ways to tackle these and other issues have been enabled by ever-increasing computing power. Non-uniform and/or unsteady fluvial processes can be modelled numerically, and remote-sensing methods have been developed to acquire dense spatially distributed measurements. But neither models nor observations are infallible, and models of different complexity need to be compared and assessed carefully.
In the first decades of the 20th century, the Ebro River was the Iberian channel with the most active fluvial dynamics and the most remarkable spatial-temporal evolution. Its meandering typology, the dimensions of its floodplain, and the singularities of its flow regime produced an especially interesting set of river functions.
The largest dynamics of the Ebro River are concentrated along the meandering profile of the central sector. During the 20th century, this sector experienced a large alteration of its geomorphological structure. We present here an analysis of this evolution through the cartographic study of a long segment of the river (~250 km) in 1927, 1956 and 2003. The results show a large reduction in bank sinuosity, a progressive loss of fluvial territory, and a large decrease in channel width. These changes are especially clear in the areas previously most ecologically connected with the active channel. The fluvial territory of the river in 2003 was approximately half that found during the first decades of the 20th century. Forest plantations, which were non-existent in 1927, occupied more than 1500 ha of the study area in the last decade.
This intense geomorphological transformation becomes ecologically visible in (i) a 35% reduction of the area occupied by riparian vegetation; (ii) a loss of the heterogeneity of riparian forest spots, which were formerly structured in an irregular mosaic far from the river thalweg; and (iii) a modification of the riparian forest structure, which is currently linear, uniform, thin and very close to the river axis. The ecomorphological alteration was intensified by the remarkable reduction in bank length (13%) and the reduced dynamism of the present river system, indicated by an increase in the percentage of fluvial territory occupied by riparian forests and a reduction in the area occupied by the active channel. Copyright
IntroductionNumerical Models of Braided Rivers at the System-ScaleThe Scaling Characteristics of Braided River SystemsBraided River Morphology and Morphological ChangeConclusions: Perspectives for the Next 10 YearsAcknowledgementsAppendixReferences
The structure and function of rivers have long been characterized either by: (1) qualitative models such as the River Continuum Concept or Serial Discontinuity Concept which paint broad descriptive portraits of how river habitats and communities vary, or (2) quantitative models, such as downstream hydraulic geometry, which rely on a limited number of measurements spread widely throughout a river basin. In contrast, authors such as Fausch et al. (2002) and Wiens (2002) proposed applying existing quantitative, spatially comprehensive ecology and landscape ecology methods to rivers. This new framework for river sciences which preserves variability and spatial relationships is called a riverine landscape or a ‘riverscape’. Application of this riverscape concept requires information on the spatial distribution of organism-scale habitats throughout entire river systems.
Built in 1930 on the Matawin River (5770 km²) to supply hydroelectric plants located downstream on the Saint-Maurice River, the Taureau Reservoir caused an inversion of the natural hydrological cycle: maximum flows occur in winter and minimum flows occur in spring during snowmelt. This inversion induces a very rapid rise of the reservoir level (base level for the channel upstream from the reservoir) in April–May during the filling-up process. The goal of the study is to analyze the effect of the rapid rise of the base level on the morphological evolution of the 7-km-long sandy section of the Matawin River located immediately upstream from the reservoir and to compare it with Jiongxin's general model. Comparison of aerial photos taken before (1928) and after (1937, 1950, 1965, 1975, 1983, and 1995) reservoir construction reveals no significant change in the bankfull mean width, sinuosity, and bankfull depth of the studied section. It follows that the morphological evolution of the Matawin River channel upstream from the dam is not consistent with the Jiongxin model. The effect of the inverted hydrological regime in fact produces high channel stability. This stability will probably contribute to a slow but progressive reduction of the channel width and an increase of the sinuosity. This morphological evolution seems typical of the semialluvial streams found upstream from the inverted-type reservoir of the Canadian Shield.Research highlights► Hydrologic regime inversion. ► Substantial rise in base level in spring upstream from reservoir. ► This rise does not change the channel morphology (width, depth and sinuosity).
The reflectance of shallow water areas to solar illumination is a function of the water depth, the water optical properties and the bottom reflectance. Assuming the water optical properties to be uniform over a given scene area, the signals recorded by a multispectral scanner system may be combined to obtain information on the water attenuation and bottom reflectance parameters without knowledge of the water depth. These techniques are described and evaluated for a test site near North Cat Cay in the Bahamas.
The hydraulics of hydraulic geometry has been sadly neglected by most geomorphologists. The links are explored first for a station variation and then for downstream hydraulic geometry. There are comments on maximum efficiency channels and bank stability in mobile bed channels. Downstream hydrology is empirically more regular than at a point variation, though harder to explain rationally. - K.Clayton
The study demonstrates successful remote determination of shallow water depth by measuring wave refraction changes and using the Fourier transform plane for wavelength measurements with data obtained at a Lake Michigan test site. The study shows that the technique is suitable for use from spacecraft altitudes, provided that water waves of suitable length occur in the region of interest.
Most river cross-sections display some degree of asymmetry for which there is no adequate quantitative definition. Indices of asymmetry are derived using two principles based on areal differences and maximum depth displacement relative to the channel centreline. The indices are assessed firstly by comparing values for a set of constructed channel shapes which are ranked in order of increasing visual asymmetry, and secondly by analysing a series of 50 cross-sections from a small mountain stream. Two indices, A* and A2, are probably the most valuable, and could be used to test and generate hypotheses regarding the related changes to cross-sectional and planimetric form which occur during the transition from a straight to a meandering channel, particularly if parallel measures of flow asymmetry are defined.
Part II of this paper, concerned with ‘Mode of Development and Local Variation’ will be published in a subsequent issue of this journal.
An analysis of old maps and documentary sources reveals that major changes in river channel planform have occurred over the last 200 years on the River Tay system, Scotland, UK. Reaches showing natural river channel planform change, however, are relatively small and a stable planform is characteristic of many sections of the river. River planform instability appears to be controlled by channel bed slope, sediment load and the enhanced vulnerability of former river channel courses to erosion. Flood protection embankments built in the 19th and 20th centuries modified unstable multichannel wandering gravel bed river sections to narrower single-channel reaches, with limited lateral migration. On the River Tummel, 20th century impoundment has caused further geomorphological change in response to clearwater erosion close to the dam and aggradation processes within the regulated river downstream, but isolation of the effects of impoundment from those of channelization are problematic. An examination of the geomorphic effects of a high magnitude flood event in 1990 and historical accounts of earlier large floods reveal that the 1990 flood was the third largest since 1800 in the study area. Despite river regulation and bank protection the zones naturally characterized by instability are still susceptible to planform changes causing flood embankments to be breached, channel shifts and development of gravel bars.
Alternative indices of cross-section asymmetry are compared. Observed values of two preferred indices for 567 natural sections across eleven confined upland channels are predictably low along such low sinuosity reaches. Variation in asymmetry of sections located around confined, unconfined but stable, and freely-migrating bends is evaluated. Consideration of the influence of bed topography emphasizes local plan curvature of bed forms as an important control on degree of asymmetry. Variation in indices for bed forms in similar planform locations indicates the levels of asymmetry at different stages of the transformation of channel geometry over time.
This thesis examines historical river channel change on a 12km study reach of the Rivers Tay and Tummel Scotland via the development of GIS and remote sensing techniques. Firstly, historical maps were combined using GIS rectification techniques in order to examine channel changes over the period 1755 to 1975. Secondly, also using GIS methodology, channel planforms as depicted in a series of aerial photographs were overlain to study recent channel change (1971 to 1994) including that caused by two major flood events. The study formed part of wider investigations into the hydrology and geomorphology of the River Tay, following the 1990 and 1993 flood events commissioned by organisations involved with management of the river. The study reach in 1863 and 1899 was shown to have alternating, highly divided sections with multiple mid-channel islands, and stable single-thread sections although, overall, the channel was less braided than depicted on 18th century maps. By 1975, the multi-channel sections had changed to a predominantly single-thread character and it is proposed that this had occurred in response to flood embankment construction and bank protection leading to channel narrowing and incision. This has wider implications for the management of the River Tay as channel instability supports diverse natural habitats with high conservation value. Once recent river planform changes on the study reach had been identified, stable and unstable reaches were defined allowing the determination of the degree and nature of instability using GIS methodology which included quantification of active channel widths and gravel area, braiding indices, sinuosity and channel occupancy indices. A number of unstable reaches were also studied in the field to examine the processes responsible for river bank erosion. In addition, the effect of in-channel morphology on river planform changes was examined by applying image analysis to bands 3, 5, 6 and 8 of airborne multi-spectral imagery (Daedalus ATM) to map channel bathymetry. The results showed that changes in channel planform and position occurred almost entirely in response to extreme flood events and that areas of greatest channel change were in zones of historical instability resulting from the presence of less cohesive sediments along the courses of former river channels. A meander-like alternation of pool-riffle sequences controlled the local distribution of bank erosion along most of these reaches by deflecting thalwegs against outer banks. The information derived from the study was used to construct an erosion hazard map. Using raster-based GIS techniques, these data were combined with measurements of distance from river channel and flood return periods, to create a model which enabled spatial mapping of river bank erosion probabilities. These probabilities were then mapped for hypothetical floods of 5, 10 and 25 year recurrence interval.