Description: Arctic–alpine blockfields in the northern Swedish Scandes: late Quaternary – not Neogene Earth Surface Dynamics, 2, 383-401, 2014 Author(s): B. W. Goodfellow, A. P. Stroeven, D. Fabel, O. Fredin, M.-H. Derron, R. Bintanja, and M. W. Caffee Autochthonous blockfield mantles may indicate alpine surfaces that have not been glacially eroded. These surfaces may therefore serve as markers against which to determine Quaternary erosion volumes in adjacent glacially eroded sectors. To explore these potential utilities, chemical weathering features, erosion rates, and regolith residence durations of mountain blockfields are investigated in the northern Swedish Scandes. This is done, firstly, by assessing the intensity of regolith chemical weathering along altitudinal transects descending from three blockfield-mantled summits. Clay / silt ratios, secondary mineral assemblages, and imaging of chemical etching of primary mineral grains in fine matrix are each used for this purpose. Secondly, erosion rates and regolith residence durations of two of the summits are inferred from concentrations of in situ-produced cosmogenic 10 Be and 26 Al in quartz at the blockfield surfaces. An interpretative model is adopted that includes temporal variations in nuclide production rates through surface burial by glacial ice and glacial isostasy-induced elevation changes of the blockfield surfaces. Together, our data indicate that these blockfields are not derived from remnants of intensely weathered Neogene weathering profiles, as is commonly considered. Evidence for this interpretation includes minor chemical weathering in each of the three examined blockfields, despite consistent variability according to slope position. In addition, average erosion rates of ~16.2 and ~6.7 mm ka −1 , calculated for the two blockfield-mantled summits, are low but of sufficient magnitude to remove present blockfield mantles, of up to a few metres in thickness, within a late Quaternary time frame. Hence, blockfield mantles appear to be replenished by regolith formation through, primarily physical, weathering processes that have operated during the Quaternary. The persistence of autochthonous blockfields over multiple glacial–interglacial cycles confirms their importance as key markers of surfaces that were not glacially eroded through, at least, the late Quaternary. However, presently blockfield-mantled surfaces may potentially be subjected to large spatial variations in erosion rates, and their Neogene regolith mantles may have been comprehensively eroded during the late Pliocene and early Pleistocene. Their role as markers by which to estimate glacial erosion volumes in surrounding landscape elements therefore remains uncertain.

Description: Morphological and sedimentological response of a mixed-energy barrier island tidal inlet to storm and fair-weather conditions Earth Surface Dynamics, 2, 363-382, 2014 Author(s): G. Herrling and C. Winter The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 μm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.

Description: Intertidal finger bars at El Puntal, Bay of Santander, Spain: observation and forcing analysis Earth Surface Dynamics, 2, 349-361, 2014 Author(s): E. Pellón, R. Garnier, and R. Medina A system of 15 small-scale finger bars has been observed, by using video imagery, between 23 June 2008 and 2 June 2010. The bar system is located in the intertidal zone of the swell-protected beaches of El Puntal Spit, in the Bay of Santander (northern coast of Spain). The bars appear on a planar beach (slope = 0.015) with fine, uniform sand ( D 50 = 0.27 mm) and extend 600 m alongshore. The cross-shore span of the bars is determined by the tidal horizontal excursion (between 70 and 130 m). They have an oblique orientation with respect to the low-tide shoreline; specifically, they are down-current-oriented with respect to the dominant sand transport computed (mean angle of 26° from the shore normal). Their mean wavelength is 26 m and their amplitude varies between 10 and 20 cm. The full system slowly migrates to the east (sand transport direction) with a mean speed of 0.06 m day -1 , a maximum speed in winter (up to 0.15 m day -1 ) and a minimum speed in summer. An episode of merging has been identified as bars with larger wavelength seem to migrate more slowly than shorter bars. The wind blows predominantly from the west, generating waves that transport sediment across the bars during high-tide periods. This is the main candidate to explain the eastward migration of the system. In particular, the wind can generate waves of up to 20 cm (root-mean-squared wave height) over a fetch that can reach 4.5 km at high tide. The astronomical tide seems to be important in the bar dynamics, as the tidal level changes the fetch and also determines the time of exposure of the bars to the surf-zone waves and currents. Furthermore, the river discharge could act as input of suspended sediment in the bar system and play a role in the bar dynamics.

Description: Automated landform classification in a rockfall-prone area, Gunung Kelir, Java Earth Surface Dynamics, 2, 339-348, 2014 Author(s): G. Samodra, G. Chen, J. Sartohadi, D. S. Hadmoko, and K. Kasama This paper presents an automated landform classification in a rockfall-prone area. Digital terrain models (DTMs) and a geomorphological inventory of rockfall deposits were the basis of landform classification analysis. Several data layers produced solely from DTMs were slope, plan curvature, stream power index, and shape complexity index; whereas layers produced from DTMs and rockfall modeling were velocity and energy. Unsupervised fuzzy k means was applied to classify the generic landforms into seven classes: interfluve, convex creep slope, fall face, transportational middle slope, colluvial foot slope, lower slope and channel bed. We draped the generic landforms over DTMs and derived a power-law statistical relationship between the volume of the rockfall deposits and number of events associated with different landforms. Cumulative probability density was adopted to estimate the probability density of rockfall volume in four generic landforms, i.e., fall face, transportational middle slope, colluvial foot slope and lower slope. It shows negative power laws with exponents 0.58, 0.73, 0.68, and 0.64 for fall face, transportational middle slope, colluvial foot slope and lower slope, respectively. Different values of the scaling exponents in each landform reflect that geomorphometry influences the volume statistics of rockfall. The methodology introduced in this paper has possibility to be used for preliminary rockfall risk analyses; it reveals that the potential high risk is located in the transportational middle slope and colluvial foot slope.

Description: Morphological coupling in multiple sandbar systems – a review Earth Surface Dynamics, 2, 309-321, 2014 Author(s): T. D. Price, B. G. Ruessink, and B. Castelle Subtidal sandbars often exhibit alongshore variable patterns, such as crescentic plan shapes and rip channels. While the initial formation of these patterns is reasonably well understood, the morphodynamic mechanisms underlying their subsequent finite-amplitude behaviour have been examined far less extensively. This behaviour concerns, among other aspects, the coupling of alongshore variable patterns in an inner bar to similar patterns in a more seaward bar, and the destruction of crescentic patterns. This review aims to present the current state of knowledge on the finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar systems. In this context we include results from our recent study, based on a combination of remote-sensing observations, numerical modelling and data–model integration. Morphological coupling is an inherent property of double sandbar systems, where the inner bar may attain a type of morphology not found in single bar systems. Coupling is governed by water depth variability along the outer-bar crest and by various wave characteristics, including the offshore wave height and angle of incidence. In recent research, the role of the angle of wave incidence for sandbar morphodynamics has received more attention. Numerical modelling results have demonstrated that the angle of wave incidence is crucial to the flow pattern, sediment transport, and thus the emerging morphology of the coupled inner bar. Moreover, crescentic patterns predominantly vanish under high-angle wave conditions, highlighting the role of alongshore currents in straightening sandbars and challenging the traditional conception that crescentic patterns vanish under high-energy, erosive wave conditions only.

Description: Comparison between experimental and numerical stratigraphy emplaced by a prograding delta Earth Surface Dynamics, 2, 323-338, 2014 Author(s): E. Viparelli, A. Blom, C. Ferrer-Boix, and R. Kuprenas A one-dimensional model that is able to store the stratigraphy emplaced by a prograding delta is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e., a condition in which the stratigraphy emplaced by the delta front is entirely stored in the deposit. The migration of the delta front and the deposition on the delta top are modeled with total and grain-size-based mass conservation models. The vertical sorting on the delta front is modeled with a lee-face-sorting model as a function of the grain size distribution of the sediment deposited at the brinkpoint, i.e., at the downstream end of the delta top. Notwithstanding the errors associated with the grain-size-specific bedload transport formulation, the comparison between numerical and experimental results shows that the model is able to reasonably describe the progradation of the delta front, the frictional resistances on the delta top, and the overall grain size distribution of the delta top and delta front deposits. Further validation of the model in the case of variable base level is currently in progress to allow for future studies, at field and laboratory scale, on how the delta stratigraphy is affected by different changes of relative base level.

Description: Coastal vulnerability of a pinned, soft-cliff coastline – Part I: Assessing the natural sensitivity to wave climate Earth Surface Dynamics, 2, 295-308, 2014 Author(s): A. Barkwith, C. W. Thomas, P. W. Limber, M. A. Ellis, and A. B. Murray The impact of future sea-level rise on coastal erosion as a result of a changing climate has been studied in detail over the past decade. The potential impact of a changing wave climate on erosion rates, however, is not typically considered. We explore the effect of changing wave climates on a pinned, soft-cliff, sandy coastline, using as an example the Holderness coast of East Yorkshire, UK. The initial phase of the study concentrates on calibrating a numerical model to recently measured erosion rates for the Holderness coast using an ensemble of geomorphological and shoreface parameters under an observed offshore wave climate. In the main phase of the study, wave climate data are perturbed gradually to assess their impact on coastal morphology. Forward-modelled simulations constrain the nature of the morphological response of the coast to changes in wave climate over the next century. Results indicate that changes to erosion rates over the next century will be spatially and temporally heterogeneous, with a variability of up to ±25% in the erosion rate relative to projections under constant wave climate. The heterogeneity results from the current coastal morphology and the sediment transport dynamics consequent on differing wave climate regimes.

Description: Morphodynamic regime change in a reconstructed lowland stream Earth Surface Dynamics, 2, 279-293, 2014 Author(s): J. P. C. Eekhout, R. G. A. Fraaije, and A. J. F. Hoitink With the aim to establish and understand morphological changes in response to channel reconstruction, a detailed monitoring plan was implemented in a lowland stream called Lunterse Beek, located in the Netherlands. Over a period of almost 2 years, the monitoring programme included serial morphological surveys, continuous discharge and water level measurements, and riparian vegetation mapping, from photographs and field surveys. Morphological processes occurred mainly in the initial period, before riparian vegetation developed. The initial period was largely dominated by upstream sediment supply, which was associated with channel incision upstream from the study area. Herbaceous vegetation started to develop approximately 7 months after channel reconstruction. The monitoring period included two growing seasons. A clear increase of riparian vegetation cover from first to the second year was observed. Detailed morphological and hydrological data show a marked difference in morphological behaviour between the pre-vegetation and post-vegetation stage. A linear regression procedure was applied to relate morphological activity to time-averaged Shields stress. In the initial stage after channel reconstruction, with negligible riparian vegetation, channel morphology adjusted, showing only a weak response to the discharge hydrograph. In the subsequent period, morphological activity in the channel showed a clear relation with discharge variation. The two stages of morphological response to the restoration measures may be largely associated with the upstream sediment supply in the initial period. Riparian vegetation may have played a substantial role in stabilizing the channel banks and floodplain area, gradually restricting the morphological adjustments to the channel bed.

Description: Multiple knickpoints in an alluvial river generated by a single instantaneous drop in base level: experimental investigation Earth Surface Dynamics, 2, 271-278, 2014 Author(s): A. Cantelli and T. Muto Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.

Description: Development of a meandering channel caused by the planform shape of the river bank Earth Surface Dynamics, 2, 255-270, 2014 Author(s): T. Nagata, Y. Watanabe, H. Yasuda, and A. Ito Due to a typhoon and a stationary rain front, record amounts of rain fell in September 2011, and the largest class of discharge in recorded history was observed in the Otofuke River of eastern Hokkaido in Japan, and extensive bank erosion occurred in various parts of the river channel. Damages were especially serious in the middle reaches, where part of a dike was washed out. The results of a post-flood survey suggested that the direct cause of the dike breach was lateral advance of the bank erosion associated with the development of meandering channels. As the related development mechanism and predominant factors have not yet been clarified, this remains a priority from the viewpoint of disaster prevention. A past study on the development of meandering channels was reported by Shimizu et al. (1996). In this study, the meandering channel development process was reproduced using a slope failure model that linked bank erosion with bed changes. The study attempted to clarify the meandering development mechanism in the disaster and its predominant factors by using this model. The analysis properly reproduced the characteristics of the post-flood meandering waveforms. Therefore, it is suggested that the development of meandering during the flood attributed to the propagation of meandering downstream, which is triggered by the meandering flow from the meandering channel in the upstream, which also suggests that this propagated meandering then caused a gradual increase of meandering amplitude accompanied by bank erosion in the recession period of the flood.