Meander Bend Research Articles

Simple Terrestrial Laser Scanner for Measuring Streambank Retreat

AbstractHigh-resolution measurements of streambank retreat (SBR) rates are important for engineering applications, such as infrastructure planning and stream restoration. Conventional methods of measuring SBR, such as erosion pins or total stations, lack sufficient point density to accurately capture the spatial variability of bank retreat over an entire streambank surface. This paper presents a newly developed terrestrial laser scanner (TLS) that is affordable, robust, and simple. The TLS was employed to profile a ∼9-m wide portion of a meander bend on Clear Creek in Coralville, Iowa. Bank profiles were acquired in July 2012 and July 2013; the resulting subtraction yielded a mean SBR rate of 2.13 m/year, with a maximum of 3.7 m at the upper portion of the bank.

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

AbstractCompound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three‐dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one‐third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations. Copyright © 2016 John Wiley & Sons, Ltd.

The influence of meander bend evolution on the formation of multiple cutoffs: findings inferred from floodplain architecture and bend geometry

AbstractThe evolution of meander bends and formation of cutoffs, including a series of cutoffs developed simultaneously in a number of bends, have been investigated by many researchers. However, relatively little is known about factors that lead to the development of multiple cutoffs that are formed subsequently at one location. The present study aims to determine the influence of meander bend development on multiple chute cutoff formation in a single bend. The research is based on the sedimentary record of meander migration and cutoffs preserved in a lowland river floodplain (the lower Obra River, Poland). Analysis of changes in meander geometry was conducted to describe the influence of their migration on cutoff formation and in other rivers where multiple cutoffs occurred. The results showed that multiple cutoffs in the lower Obra River have occurred during the last 3000 years, owing to the interaction of upstream and downstream controls: migration of meander bends in opposing directions accompanied by an increase of flood frequency and sediment supply. The flow and sediment supply has been further altered since the nineteenth century due to anthropogenic impacts: an artificial cutoff of the downstream bend and elevation of channel levées. Similar mechanisms driving the formation of multiple cutoff have been found in other river courses, despite significantly higher energy of the compared rivers. Moreover, development of a confined‐shape bend (caused by artificial barrier or autogenic bend behaviour) may also favour the formation of multiple cutoffs. However, counter migration of meanders enhanced by increased flood frequency and sediment supply are primary triggers for such events. Copyright © 2015 John Wiley & Sons, Ltd.

Streambed and water profile response to in‐channel restoration structures in a laboratory meandering stream

AbstractIn‐channel structures are often installed in alluvial rivers during restoration to steer currents, but they also modify the streambed morphology and water surface profile, and alter hydraulic gradients driving ecologically important hyporheic exchange. Although river features before and after restoration need to be compared, few studies have collected detailed observations to facilitate this comparison. We created a laboratory mobile‐bed alluvial meandering river and collected detailed measurements in the highly sinuous meander before and after installation of in‐channel structures, which included one cross vane and six J‐hooks situated along 1 bar unit. Measurements of streambed and water surface elevation with submillimeter vertical accuracy and horizontal resolution were obtained using close‐range photogrammetry. Compared to the smooth gradually varied water surface profile for control runs without structures, the structures created rapidly varied flow with subcritical to supercritical flow transitions, as well as backwater and forced‐morphology pools, which increased volumetric storage by 74% in the entire stream reach. The J‐hooks, located along the outer bank of the meander bend and downstream of the cross vane, created stepwise patterns in the streambed and water surface longitudinal profiles. The pooling of water behind the cross vane increased the hydraulic gradient across the meander neck by 1% and increased local groundwater gradients by 4%, with smaller increases across other transects through the intrameander zone. Scour pools developed downstream of the cross vane and around the J‐hooks situated near the meander apex. In‐channel structures significantly changed meander bend hydraulic gradients, and the detailed streambed and water surface 3‐D maps provide valuable data for computational modeling of changes to hyporheic exchange.

Stratigraphic Expression of Intra-Point-Bar Erosion and Rotation

Abstract Meander belts are characterized by a complex amalgam of point bars and associated depositional elements, such as oxbow-lake fills. Point-bar deposits are composite elements often characterized by complex scroll-bar patterns formed in response to processes of expansion, rotation, and translation of a meander bend. Intra-point-bar erosion is commonly observed in modern river systems, yet its product is rarely described from the ancient record. Late Cretaceous meander-belt deposits, consisting of a transition from a point-bar to counter-point-bar deposit, as well as an associated abandoned channel fill, crop out in south central Alberta, Canada. The strata are characterized by widespread evidence for intra-point bar deposit erosion and punctuated rotation. In the stratigraphic record, intra-point bar deposit erosion and punctuated rotation is expressed by steeply dipping discordant surfaces (up to 20°), which truncate previously deposited lateral-accretion packages. Across these erosion surfaces the direction of bar migration often rotates significantly, up to 50°. Subsequent accretion surfaces onlap intra-point-bar-deposit erosion surfaces. The analysis reveals that point-bar surfaces are formed through a complex interplay of erosional and depositional processes along their length, resulting in stratigraphic surfaces that may be highly composite. This study departs from more simplistic process models of point bars that consider deposits to be the product of continuous lateral accretion, and as such has important implications for paleoenvironmental reconstructions and subsurface reservoir characterization.

Video observations of bed form morphodynamics in a meander bend

A new optical remote sensing technique for estimating water depth from an oblique camera view is described. The water surface and the bed were imaged simultaneously to create time-dependent maps of the water surface velocities and the bed elevations that can be used to validate numerical models at high spatial and temporal resolution. The technique was applied in a sandy meander bend at the University of Minnesota Saint Anthony Falls Laboratory Outdoor StreamLab. The root mean square differences between optical estimates of the bed and in situ observations ranged between 0.01 and 0.03 m. Mean bed form wavelength was 0.73 m and mean crest height was 0.07 m, but both varied with distance around the meander bend. Bed form classification varied with distance downstream, and sinuosity of bed forms varied with local radius of curvature. Bed form roughness scaled similarly to other natural riverine environments although wavelength and height magnitude and variability were larger than predicted by empirical formulations for straight reaches. Bed form translation rate varied between 1 and 5 mm s−1. Estimates of velocity from particle image velocimetry (PIV) on the water surface were ∼10% higher than in situ observations collected ∼0.05 m below the water surface. Using the PIV observations to drive simple equations for bed load sediment flux, we explained up to 72% of the observed variance in downstream sediment flux. The new methodology described here provides nonintrusive, high spatial and temporal resolution measurements of both the bed and the flow.

Spatial variability of streambed vertical hydraulic conductivity and its relation to distinctive stream morphologies in the Beiluo River, Shaanxi Province, China

Streambed vertical hydraulic conductivity (K v) is a key parameter in the analysis of interactions between streams and aquifers, and of stream ecosystems. However, knowledge of the streambed hydraulic conductivity associated with different stream morphologies is relatively scarce. An in-situ standpipe permeameter test method was used to determine the spatial variability in streambed K v measured along 18.5 km of stream reach in the Beiluo River, Shaanxi Province, China. The 59 total measurements were conducted at four test sites in three different stream morphologies: straight channel, anabranching channels and a nearby meander bend. There was great spatial variability in K v among the four test sites and three K v distribution patterns can be determined: (1) higher K v values appear on the erosional bank in contrast to lower K v values on the depositional bank at the two sites near meander bends; (2) the K v distribution in straight channels shows that the higher K v values occur in the center of the channel; (3) the K v values are generally highest on the branch with more alluvial forms in the anabranching channels. Moreover, grain-size analysis results illustrate that the average grain-size distributions of streambed sediments are significantly different on each side of the channel. The erosional and depositional forms are one of the driving dynamics for the distribution of streambed grain size that trigger the differences in the distribution of the K v.

A study of flow dynamics and implications for benthic fauna in a meander bend of a lowland river

ABSTRACTChannel curvature and riffle-pool bathymetry in meandering streams control complex hydrodynamic and morphodynamic processes. This study investigates how spatial and temporal heterogeneities in flow hydraulics influence benthic fauna in a meander bend of a lowland sand-bed river. Spatial heterogeneity of riverbed morphology and secondary flow, induced by channel curvature, make pools hydraulically more diverse compared with riffles. Numerical simulations demonstrate that velocity reversal between riffles and pools in this meander bend produces spatially variable flow with complex temporal variations. Patterns of macro-invertebrates indicate an increase in population density from riffle to pool, reflecting an increase in diversity of abiotic factors. For most invertebrate species the observed patterns persisted during temporal variations of the flow. Considerable changes were observed only in some groups with specific preferences.

Three Dimensional Numerical Simulation of Vortex Structures in Barak River

Natural channel have complex three dimensional flow structures particularly at the outer bank cell due to the combined effects of secondary currents and higher velocity profiles. In this paper Computational fluid dynamics is used to study the meandering bend of the Barak River. Numerical modeling is done using Reynolds averaged continuity and Navier Stokes equation. These equations are solved by finite volume method. Appropriate representation of counter-rotating secondary flow in the channel bend requires both the suitable treatment of the free water surface and a turbulence model that can resolve the anisotropy of turbulence. Hence the volume of fluid method (VOF) was used to model the free surface and reynolds stress turbulence model (RSM) has been used to close the RANS equations. Higher velocity profiles were prominent at the outer bank. Skew induced stream wise vorticity was observed close to the outer bank which confirms the existence of corner induced secondary current. The vortices formed were found to be of Prandtl’s first kind.

A lithologic control on active meandering in bedrock channels

Topographic evidence requires that some rivers actively meander in bedrock, yet the way in which rivers can erode laterally and meander within bedrock banks is not well understood. Lateral channel migration, and especially lateral channel migration via active bedrock meandering, is commonly responsible for the preservation of unpaired strath terraces. A process-level understanding of lateral channel migration and active meandering in bedrock rivers is key to interpreting the climatic and tectonic significance of unpaired strath terraces and the planform shape of bedrock rivers. In this study, we compare erosional processes in two adjacent bedrock channels in the Santa Cruz Mountains, California. The main differences between these channels are that Pescadero Creek actively meanders within mudstone, while Butano Creek is straight and incises sandstone. Laboratory rock strength and slake durability tests show that while the two lithologies have similar tensile strengths before drying, the meander-supporting mudstone loses strength dramatically when dried and rewet (slakes), while the sandstone does not. The slaked mudstone bank rock was easily detached without the need for bed-load tools during in situ erosion tests, while mudstone that had not dried and sandstone were not detachable. The depth of bank rock detached solely from rewetting of previously dried mudstone ranges between 1 and 8 mm, which is well in excess of annual background erosion in the Santa Cruz Mountains. In addition, hand samples of the mudstone rapidly disintegrated upon wetting and drying in the laboratory, whereas sandstone hand samples remained intact. In the meandering stream, there is a consistent pattern of scoured bedrock (exposed to drying and slaking) along the outside “cutbank” of meander bends and forced bars that grade into soil and vegetation, which protect the bedrock from slaking along the inside of bends. Additionally, in the meandering stream, subaerially exposed mudstone clasts are often found to be disintegrating on the surface of bars. Taken together, these observations suggest that slaking allows for bedrock meandering in two fundamental ways. First, by rapidly disintegrating coarse hillslope-derived sediment that is deposited in the channel, slaking suppresses the negative feedback on lateral channel migration that would otherwise result from the buildup of talus along a retreating bedrock valley wall on the outside of a meander bend. Second, at cutbanks where scour exposes bare bedrock to drying, slaking weakens a layer of bank rock to the point where it can be eroded by clear-water flows. In these ways, slaking enables erosion into bedrock banks in response to curvature-driven fluid shear stress perturbations, as in alluvial rivers.

Hydraulic and geomorphic processes in an overbank flood along a meandering, gravel‐bed river: implications for chute formation

AbstractHydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.

The life of a meander bend: Connecting shape and dynamics via analysis of a numerical model

Analysis of bend-scale meandering river dynamics is a problem of theoretical and practical interest. This work introduces a method for extracting and analyzing the history of individual meander bends from inception until cutoff (called “atoms”) by tracking backward through time the set of two cutoff nodes in numerical meander migration models. Application of this method to a simplified yet physically based model provides access to previously unavailable bend-scale meander dynamics over long times and at high temporal resolutions. We find that before cutoffs, the intrinsic model dynamics invariably simulate a prototypical cutoff atom shape we dub simple. Once perturbations from cutoffs occur, two other archetypal cutoff planform shapes emerge called long and round that are distinguished by a stretching along their long and perpendicular axes, respectively. Three measures of meander migration—growth rate, average migration rate, and centroid migration rate—are introduced to capture the dynamic lives of individual bends and reveal that similar cutoff atom geometries share similar dynamic histories. Specifically, through the lens of the three shape types, simples are seen to have the highest growth and average migration rates, followed by rounds, and finally longs. Using the maximum average migration rate as a metric describing an atom's dynamic past, we show a strong connection between it and two metrics of cutoff geometry. This result suggests both that early formative dynamics may be inferred from static cutoff planforms and that there exists a critical period early in a meander bend's life when its dynamic trajectory is most sensitive to cutoff perturbations. An example of how these results could be applied to Mississippi River oxbow lakes with unknown historic dynamics is shown. The results characterize the underlying model and provide a framework for comparisons against more complex models and observed dynamics.

Momentum transport and bed shear stress distribution in a meandering bend: Experimental analysis in a laboratory flume

The paper concerns the mechanisms underlying the distribution of the bed shear stress in meandering bends. Literature indicates that cross-stream circulation strongly affects the redistribution of the downstream velocity, but the feedback between them is still poorly understood. The aim of this paper is to gain some insight into how the momentum transport by cross-stream circulation contributes to the bed shear stress redistribution. Experimental analysis, based on a detailed dataset collected in a large-amplitude meandering laboratory flume, is presented. From these data an evaluation is made of the terms in the depth-averaged momentum equations and the analysis is especially devoted to terms including the momentum transport by cross-sectional motion. Results confirm that these terms exert an important role in bed shear stress estimation and, thus, they have to be adequately included in the depth-averaged models. Based on measured data, an equation expressing the interrelation between the cross-sectional momentum and the downstream velocity is also introduced.

Turbulence anisotropy in a compound meandering channel with different submergence conditions

The understanding of the physical processes related to flows on compound meandering channels is a challenge given their highly 3D and complex characteristics. Three-dimensional ADV measurements were made in three cross-sections on a 1:20 physical Froude model of a real reach in River Mero (A Coruña) for bankfull flow and flood conditions. General characteristics and processes within the flow are herein described and characterized, such as momentum and mass exchange between the main channel and the floodplains. Time-averaged velocities and Reynolds stresses are presented and discussed. The spatial distribution of turbulence in several positions along a meander bend is analyzed in this paper. The characterization of the turbulent field in these 3D complex flows highly depends on the used reference system, and the intense local variation of turbulence makes a global and fixed coordinate system of petty use. An independent technique, regardless the coordinate system of the measurements, is thus the best way to analyze these flows. The anisotropy invariants technique was used to analyze the evolution of the magnitude and nature of anisotropy along the meander. The degree and nature of anisotropy was identified, and their relation to flow structures, such as vortices in the contact between the main channel and the floodplains, was analyzed using the quadrant analysis technique.

Two‐dimensional and three‐dimensional computational models in hydrodynamic and morphodynamic reconstructions of a river bend: sensitivity and functionality

AbstractThis study assesses hydrodynamic and morphodynamic model sensitivity and functionality in a curved channel. The sensitivity of a depth‐averaged model to user‐defined parameters (grain size, roughness, transverse bed slope effect, transport relations and secondary flow) is tested. According to the sensitivity analysis, grain size, transverse bed slope effect and sediment transport relations are critical to simulated meander bend morphodynamics. The parametrization of grain size has the most remarkable effect: field‐based grain size parametrization is necessary in a successful morphodynamic reconstruction of a meander bend. The roughness parametrization method affects the distribution of flow velocities and therefore also morphodynamics. The combined effect of various parameters needs further research. Two‐dimensional (2D) and three‐dimensional (3D) reconstructions of a natural meander bend during a flood event are assessed against field measurements of acoustic Doppler current profiler and multi‐temporal mobile laser scanning data. The depth‐averaged velocities are simulated satisfactorily (differences from acoustic Doppler current profiler velocities 5–14%) in both 2D and 3D simulations, but the advantage of the 3D hydrodynamic model is unquestionable because of its ability to model vertical and near‐bed flows. The measured and modelled near‐bed flow, however, differed notably from each other's, the reason of which was left open for future research. It was challenging to model flow direction beyond the apex. The 3D flow features, which also affected the distribution of the bed shear stress, seem not to have much effect on the predicted morphodynamics: the 2D and 3D morphodynamic reconstructions over the point bar resembled each other closely. Although common features between the modelled and measured morphological changes were also found, some specific changes that occurred were not evident in the simulation results. Our results show that short‐term, sub‐bend scale morphodynamic processes of a natural meander bend are challenging to model, which implies that they are affected by factors that have been neglected in the simulations. The modelling of short‐term morphodynamics in natural curved channel is a challenge that requires further study. Copyright © 2014 John Wiley & Sons, Ltd.

Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions

AbstractRecent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd.

Annual bank and point bar morphodynamics of a meandering river determined by high‐accuracy multitemporal laser scanning and flow data

AbstractThe knowledge has been insufficient concerning the effects of peak flows, and local bend and flow characteristics on annual morphodynamics of consecutive bends in meandering rivers. Therefore, it was determined how flow peak magnitude and duration affect morphodynamics, how the short‐term spatial evolution of a given meander bend associates with the neighboring bends, and how local bend and flow characteristics affect morphodynamics. The annual bank and point bar morphodynamics of eight consecutive bends of a subarctic meandering river were analyzed between 2009 and 2012 on the basis of high‐accuracy multitemporal data, measured by mobile and terrestrial laser scanning and an Acoustic Doppler Current Profiler. According to the results, multiple years of highly accurate data are crucial for a broader picture of meandering channel evolution. The results showed for the first time in detail that none of the years were similar in terms of point bar and bank morphodynamics. The duration of point bar submergence and maximum water stage was more important for evolution of the meandering channel than the local effects of each bend. The detailed topographical data of the present study confirmed that the higher the flow and water stage peak the more deposition occurred on point bars. More importantly, the independence of the short‐term spatial evolution of meander bends from the association with neighboring bends was confirmed. Erosion patterns did not relate particularly to the sinuosity or radius of curvature. A clear relation between velocity and bend curvature, on which some meander migration models rely, was not found.

Estimation of floodplain aboveground biomass using multispectral remote sensing and nonparametric modeling

Floodplain forests serve a critical function in the global carbon cycle because floodplains constitute an important carbon sink compared with other terrestrial ecosystems. Forests on dynamic floodplain landscapes, such as those created by river meandering processes, are characterized by uneven-aged trees and exhibit high spatial variability, reflecting the influence of interacting fluvial, hydrological, and ecological processes. Detailed and accurate mapping of aboveground biomass (AGB) on floodplain landscapes characterized by uneven-aged forests is critical for improving estimates of floodplain-forest carbon pools, which is useful for greenhouse gas (GHG) life cycle assessment. It would also help improve our process understanding of biomorphodynamics of river-floodplain systems, as well as planning and monitoring of conservation, restoration, and management of riverine ecosystems. Using stochastic gradient boosting (SGB), multivariate adaptive regression splines (MARS), and Cubist, we remotely estimate AGB of a bottomland hardwood forest on a meander bend of a dynamic lowland river. As predictors, we use 30-m and 10-m multispectral image bands (Landsat 7 ETM+ and SPOT 5, respectively) and ancillary data. Our findings show that SGB and MARS significantly outperform Cubist, which is used for U.S. national-scale forest biomass mapping. Across all data-experiments and algorithms, at 10-m spatial resolution, SGB yields the best estimates (RMSE=22.49tonnes/ha; coefficient of determination (R2)=0.96) when geomorphometric data are also included. On the other hand, at 30-m spatial resolution, MARS yields the best estimates (RMSE=29.2tonnes/ha; R2=0.94) when image-derived data are also included. By enabling more accurate AGB mapping of floodplains characterized by uneven-aged forests, SGB and MARS provide an avenue for improving operational estimates of AGB and carbon at local, regional/continental, and global scales.

Hyperspectral remote sensing of aboveground biomass on a river meander bend using multivariate adaptive regression splines and stochastic gradient boosting

Research on aboveground biomass (AGB) retrieval via remote sensing in floodplain forests, in particular, is urgently needed for improved understanding of carbon cycling in such areas. AGB estimation is particularly challenging in floodplain forests, which are characterized by high spatial variability in AGB resulting from biogeomorphodynamic processes. In this study, we perform remote AGB retrieval for a deciduous riparian forest on a river meander bend based on hyperspectral/high-dimensional Hyperion bands and other input variables. We compare multivariate adaptive regression splines (MARS)-, stochastic gradient boosting (SGB)- and Cubist-based AGB estimates. Results show that MARS- and SGB-derived estimates are significantly more accurate than Cubist-based AGB. The most accurate MARS and SGB estimates have a coefficient of determination, R2, of 0.97 and 0.95, respectively, whereas the Cubist estimate with the lowest error has an R2 of 0.85. MARS and SGB AGB are not significantly different, however. These modelling approaches are applicable across scales and environments.

Control of wave climate and meander dynamics on spit breaching and inlet migration

Chaumillon E., Ozenne F., Bertin X., Long N., Ganthy F., 2014. Wave climateand inlet channel meander bend control spit breaching and migration of a new inlet: La Coubre Sandspit, France. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 109–114, ISSN 0749-0208.This study focuses on the mid-term (infra annual and pluri annual; over the period 1999–2013) morphological evolutions of a sandspit (La Coubre Sandspit, West coast of France), with a particular emphasis on spit breaching, new inlet opening and migration. Morphological evolutions are observed from a large number of aerial photographs and satellite images and are compared with wave parameters and extreme events (periods where significant wave height exceeds the 1% largest waves with water level exceeding the 90% highest water level) obtained from a high resolution hindcast wave modelling. It appears that extreme events are likely to be responsible for spit breaching and new inlet opening. Once opened, the new inlet migrates downdrift, but its migration rate is not correlated with the wave climate variations. Detailed geomorphological observations suggest that the main control on downdrift inlet migration is related to orientation of the meander bend of the tidal inlet main channel. When the meander bend is convex in an updrift direction, it counteracts the littoral drift and slows the inlet migration. Oppositely, when the meander bend is convex in a downdrift direction, the meander-induced transport is in the same direction as wave-induced longshore transport, which allows the inlet to migrate much faster.