Bedform morphology of salmon spawning areas in a large gravel-bed river

While the importance of river channel morphology to salmon spawning habitat is increasingly recognized, quantitative measures of the relationships between channel morphology and habitat use are lacking. Such quantitative measures are necessary as management and regulatory agencies within the Pacific Northwest region of the USA, and elsewhere, seek to quantify potential spawning habitat and develop recovery goals for declining salmon populations. The objective of this study was to determine if fall Chinook salmon (Oncorhynchus tshawytscha) spawning areas in the Snake River, Idaho, USA, were correlated with specific bedform types at the pool–riffle scale. A bedform differencing technique was used to objectively quantify the longitudinal riverbed profile into four distinct pool–riffle units that were independent of discharge. The vertical location of thalweg points within these units was quantified with a riffle proximity index. Chinook salmon spawning areas were mapped and correlated with the pool–riffle units through the use of cross-tabulation tables. The results indicate that 84% of fall Chinook salmon spawning areas were correlated with riffles (χ² = 57.5, df = 3, p < 0.001), with 53% of those areas located on the upstream side of riffle crests. The majority of Snake River fall Chinook salmon spawning occurred at elevations greater than 80% of the difference in elevation between the nearest riffle crest and pool bottom. The analyses of bedform morphology will assist regional fish managers in quantifying existing and potential fall Chinook salmon spawning habitat, and will provide a quantitative framework for evaluating general ecological implications of channel morphology in large gravel-bed rivers.     

Acoustic Doppler current profiler surveys along the Yangtze River

An acoustic Doppler current profiler is used to characterize the river velocity against the morphology of the Yangtze River from Chonqing to the sea. High flow velocities occur in the Three Gorges section and lower velocities in the middle and lower reaches of the river. This is largely due to the change in river pattern from a high gradient deeply-cut valley to a flat fluvial plain. Flow velocities fluctuate in the middle Yangtze due to the presence of meander bends of different length. There are numerous smaller velocity fluctuations in the lower Yangtze channel that reflect multichannel pattern with numerous sand bars and a river morphology affected by bedrock outcrops. Water depths of 40–100 m occur in the Three Gorges valley but decrease to 15–40 m in the middle and lower Yangtze. At the Gezhou Reservoir, 30 km downstream of the Three Gorges damsite velocity drops to low (< 1.0 m s^− 1) 20 km reach. A second low velocity (< 0.5 m s^− 1) zone, about 20 km in length, is located in the lower Yangtze near the coast probably due to the tidal influence. The results from this research will serve as a datum for evaluating changes to the river once the Three Gorges dam is completed in 2009.     

Geomorphology of Lake Lisan terraces along the eastern coast of the Dead Sea, Jordan

Lake Lisan, the lake that filled the Jordan graben during the Last Glacial, left behind a well developed sequence of erosional and depositional shore terraces in the south east of the current Dead Sea. These terraces record a series of stillstands that were caused by small transgressions within an overall trend of falling lake levels. The terraces were observed in places where they had not been identified previously. The morphology of the terraces was investigated in six cross-sections using differential GPS altimetry. The levels of the terraces range between − 370 and − 148 m a.s.l. The high stand of Lake Lisan at − 148 m correlates well with the high level of − 150 m reported by Bowman and Gross [Bowman, D., Gross, T., 1992. The highest stand of Lake Lisan: ~ 150 meters below MSL. Israel Journal of Earth-Science 41, 233–237.] along the western coast of Lake Lisan. The lake terraces are horizontal, elongated and tectonically undisturbed, and have a sub-horizontal foreshore (tread) with an average slope of 8.2° and steep backshore cliff (riser) with an average slope of 17.7°. The six cross-sections show a good altitudinal correlation between their terraces. Moreover, the terraces appear in undisturbed continuity on the aerial photos. These morphological characteristics demonstrate that the retreat of the lake was a result of substantial climatic changes, not of tectonic subsidence.In-situ stromatolites were found on most of the terraces, reflecting a shallow water environment and emphasizing that these terraces are recessional. Well-developed desert varnish and Tafoni observed on blocks sitting on the terrace surfaces imply a long period of exposure and a low rate of post lacustrine erosion. The formation of Lisan terraces is constrained mainly by coastal slope, water depth and underlying lithology. The morphological analysis of these terraces allows identification of two kinds of pseudo-terraces, which were formed as a result of tread or riser destruction.U/Th and OSL dating allowed the dating of three events within the lake level curve more precisely. The high level of − 148 m occurred at 30.5 ± 0.22 ka BP, consistent with the Heinrich Event 3 and Dansgaard–Oeschger stadial 5, the coldest period in the NGRIP Greenland Ice Core record. The next lower terrace at − 154 m was formed at 22.9 ka BP ± 0.29 and corresponds to the stadial 2C, the final phase of the Last High Glacial. The correlation between the Lisan high stands and climatic stadials suggests that Northern-Hemispheric cold periods led to periods with a more positive water balance in the Near East. At ~ 10 ± 0.8 ka BP Lake Lisan experienced a sharp drop to − 200 m followed by a transgression between 9.5 to 7 ka BP.     

Dust emission by off-road driving: Experiments on 17 arid soil types, Nevada, USA

Field experiments were conducted in Nellis Dunes Recreational Area (Clark County, Nevada, USA) to investigate emission of dust produced by off-road driving. Experiments were carried out with three types of vehicles: 4-wheelers (quads), dirt bikes (motorcycles) and dune buggies, on 17 soil types characteristic for a desert environment. Tests were done at various driving speeds, and emissions were measured for a large number of grain size fractions. This paper reports the results for two size fractions of emissions: PM10 (particles < 10 μm) and PM60 (particles < 60 μm). The latter was considered in this study to be sufficiently representative of the total suspendable fraction (TSP). Off-road driving was found to be a significant source of dust. However, the amounts varied greatly with the type of soil and the characteristics of the top layer. Models predicting emission of dust by off-road driving should thus consider a number of soil parameters and not just one key parameter. Vehicle type and driving speed are additional parameters that affect emission. In general, 4-wheelers produce more dust than dune buggies, and dune buggies, more than dirt bikes. Higher speeds also result in higher emissions. Dust emitted by off-road driving is less coarse than the parent sediment on the road surface. Off-road driving thus results in a progressive coarsening of the top layer. Exceptions to this are silty surfaces with no, or almost no, vegetation. For such surfaces no substantial differences were observed between the grain size distribution of road dust and emitted dust. Typical emission values for off-road driving on dry desert soils are: for sandy areas, 30–40 g km^− 1 (PM10) and 150–250 g km^− 1 (TSP); for silty areas, 100–200 g km^− 1 (PM10) and 600–2000 g km^− 1 (TSP); for drainages, 30–40 g km^− 1 (PM10) and 100–400 g km^− 1 (TSP); and for mixed terrain, 60–100 g km^− 1 (PM10) and 300–800 g km^− 1 (TSP). These values are for the types of vehicles tested in this study and do not refer to cars or trucks, which produce significantly more dust.     

Geomorphic effectiveness of extraordinary floods on three large rivers of the Indian Peninsula

The efficacy of extreme events is directly linked to the flood power and the total energy expended. The geomorphic effectiveness of floods is evaluated in terms of the distribution of stream power per unit boundary area (ω) over time, for three very large floods of the 20th Century in the Indian Peninsula. These floods stand out as outliers when compared with the peak floods per unit drainage area recorded elsewhere in the world. We used flood hydrographs and at-a-station hydraulic geometry equations, computed for the same gauging site or a nearby site, to construct approximately stream-power curves and to estimate the total energy expended by each flood. Critical unit stream power values necessary to entrain cobbles and boulders were estimated on the basis of empirical relationships for coarse sediment transport developed by Williams [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] in order to determine the geomorphological effectiveness of the floods. The estimates indicate that the minimum power per unit area values for all three floods were sufficiently high, and stream energy was above the threshold of boulder movement (90 W m^− 2) for several tens of hours. The peak unit stream power values and the total energy expended during each flood were in the range of 290–325 W m^− 2 and 65–160 × 10⁶ J respectively. The average and peak flood powers were found to be higher or comparable to those estimated for extreme palaeo or modern floods on low-gradient, alluvial rivers.     

Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer

Saltation is a major mechanism for the transport of soil particles. In the present study, we carried out wind tunnel tests to examine the saltating trajectories of two types of natural sand collected from a beach (diameter, d = 300–500 μm and 200–300 μm respectively) as well as sand from the Taklimakan desert (d = 100–125 μm) in an atmospheric boundary layer. Consecutive images of saltating particles were recorded using a high-speed digital camera at a rate of 2000 fps with a spatial resolution of 1024 × 1024 pixels. The high temporal resolution of the acquired images enabled us to study the particle motion very close to the surface. The saltating particle trajectories were reconstructed from consecutive images, and the physical quantities characterizing the initial and final stages of the particle flight in the windward direction at friction velocities of about 10%–25% above the threshold friction velocity (u / u_t = 1.11–1.26) were analyzed statistically. In addition, the transverse deviation of the saltating particles from the main streamwise direction was evaluated. The results shed new light on the complicated motions involved in sand saltation and should prove useful in the evaluation and formulation of theoretical models.     

Geomorphic evidence of major sea-level fluctuations during marine isotope substage-5e, Cape Cuvier, Western Australia

A detailed geomorphologic and morphostratigraphic investigation of raised marine terraces at Cape Cuvier, Western Australia, reveals two morphologically distinct units. A lower, well-developed accretional reef terrace between 3 and 5.5 m above MLWS (mean low-water springs; hereafter denoted as “+”) represents an extended interval of stable sea level. An upper erosional terrace and incipient coralgal rim between + 8.5 to 10.5 m represents a brief sea-level stillstand at this higher elevation. These features suggest the lower and upper terraces developed during discrete sea-level events. In an attempt to better define the timing of emplacement of each marine unit, 20 coral samples collected along vertical and lateral reef growth axis from both terraces were analysed with U-series dating. Unfortunately, all coral samples exhibited elevated δ²³⁴U_initial values, suggesting that pervasive uptake of ²³⁴U-enriched uranium and ²³⁰Th thorium had occurred. Despite the shortcomings of absolute dating, a succession of events can be resolved though morphostratigraphic relationships. Comparison of the facies relationships, coral growth, and morphostratigraphic features between the lower and upper terraces indicates that an early to mid MIS 5e stillstand at + 3 to 5 m was followed by a late rise to + 8.5 to 10.5 m. This agrees with an emerging global view of MIS 5e sea-level history derived from stable carbonate platforms, rejecting the hypothesis that these higher sea-level benchmarks are an artefact of localized tectonic processes.     

Small valley glaciers and the effectiveness of the glacial buzzsaw in the northern Basin and Range, USA

The glacial buzzsaw hypothesis suggests that efficient erosion limits topographic elevations in extensively glaciated orogens. Studies to date have largely focussed on regions where large glaciers (tens of kilometres long) have been active. In light of recent studies emphasising the importance of lateral glacial erosion in lowering peaks and ridgelines, we examine the effectiveness of small glaciers in limiting topography under both relatively slow and rapid rock uplift conditions. Four ranges in the northern Basin and Range, Idaho, Montana, and Wyoming, USA, were chosen for this analysis. Estimates of maximum Pleistocene slip rates along normal faults bounding the Beaverhead–Bitterroot Mountains (~ 0.14 mm y^− 1), Lemhi Range (~ 0.3 mm y^− 1) and Lost River Range (~ 0.3 mm y^− 1) are an order of magnitude lower than those on the Teton Fault (~ 2 mm y^− 1). We compare the distribution of glacial erosion (estimated from cirque floor elevations and last glacial maximum (LGM) equilibrium line altitude (ELA) reconstructions) and fault slip rate with three metrics of topography in each range: the along-strike maximum elevation swath profile, hypsometry, and slope-elevation profiles. In the slowly uplifting Beaverhead–Bitterroot Mountains, and Lemhi and Lost River Ranges, trends in maximum elevation parallel ELAs, independent of variations in fault slip rate. Maximum elevations are offset ~ 500 m from LGM ELAs in the Lost River Range, Lemhi Range, and northern Beaverhead–Bitterroot Mountains, and by ~ 350 m in the southern Beaverhead–Bitterroot Mountains, where glacial extents were less. The offset between maximum topography and mean Quaternary ELAs, inferred from cirque floor elevations, is ~ 350 m in the Lost River and Lemhi Ranges, and 200–250 m in the Beaverhead–Bitterroot Mountains. Additionally, slope-elevation profiles are flattened and hypsometry profiles show a peak in surface areas close to the ELA in the Lemhi Range and Beaverhead–Bitterroot Mountains, suggesting that small glaciers efficiently limit topography. The situation in the Lost River Range is less clear as a glacial signature is not apparent in either slope-elevation profiles or the hypsometry. In the rapidly uplifting Teton Range, the distribution of ELAs appears superficially to correspond to maximum topography, hypsometry, and slope-elevations profiles, with regression lines on maximum elevations offset by ~ 700 and ~ 350 m from the LGM and mean Quaternary ELA respectively. However, Grand Teton and Mt. Moran represent high-elevation “Teflon Peaks” that appear impervious to glacial erosion, formed in the hard crystalline bedrock at the core of the range. Glacier size and drainage density, rock uplift rate, and bedrock lithology are all important considerations when assessing the ability of glaciers to limit mountain range topography. In the northern Basin and Range, it is only under exceptional circumstances in the Teton Range that small glaciers appear to be incapable of imposing a fully efficient glacial buzzsaw, emphasising that high peaks represent an important caveat to the glacial buzzsaw hypothesis.     

Estuarine shore platforms in Whanganui Inlet, South Island, New Zealand

Whanganui Inlet is a low mesotidal environment where wave energy at the shoreline is limited due to a small fetch, a narrow entrance and tidal flat accretion to intertidal elevations. Wave energy is therefore only an erosive force at high tide and under storm conditions. Despite this low-energy environment extensive shore platforms occur within the inlet. They are sub-horizontal and range in width from 4.1 to 185.2 m with an average of 44.9 m. All the platforms are formed in sandstone of low resistance (mean N-type Schmidt Hammer rebound value of 17 ± 8) and have their seaward edges buried by intertidal sediment flats. The majority of platforms occur at around MHWN level, corresponding to the elevation of those flats. Where wave energy is highest, opposite the inlet's entrance and at those sites with the largest fetch, platforms develop to 0.5–1.0 m below MSL. A higher platform level is also found at MHWS elevations, however it appears to be relict with active erosion of its seaward edge occurring and therefore is most likely related to a higher mid-Holocene sea level. Apart from the location of the lowest platforms little correspondence is found between platform morphology and wave energy. Platform evolution appears to be intrinsically linked to the intertidal sediment flats which determine the degree of surface saturation of the bedrock and, hence, the number of wetting and drying cycles the platforms may undergo. As the seaward edge is buried platform development is primarily through retreat of the landward cliff. This process can, however, be complicated by the migration of intertidal water channels onto the seaward edge of the platforms or relative sea level fall which may rejuvenate landward retreat of the low-tide cliff.     

Processes and rates of rock fragment displacement on cliffs and scree slopes in an amba landscape, Ethiopia

Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y^− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m^− 1 y^− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m^− 1 y^− 1 on rangeland but only 3.9 kg m^− 1 y^− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.     

Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel

Short-term changes in Eastern Mediterranean precipitation affecting flow regime were documented in Nahal Oren, a 35 km² ephemeral stream in Mt. Carmel, a 500 m high mountain ridge located at the NW coast of Israel. The rainy winter of the Mediterranean type climate (Csa) in Mt. Carmel is characterized by average annual rainfall of 550 mm at the coastal plain to 750 mm at the highest elevation while the summer is hot and dry. Stream flow generates after accumulated rainfall of 120–150 mm while “large floods”, defined as flows with peak discharge of > 5 m³ s^− 1 and/or > 150,000 m³ in volume, are generated in response to rainfall of over 100 mm. Hence, large floods in Nahal Oren stream occur not earlier than December. Precipitation and flow data were divided into two sub-periods: 1957–1969 and 1991–2003 and compared to each other. The results indicate a clear increase in the frequency of large floods, their magnitudes and volumes during the second period with no parallel change in the annual precipitation. Similarly, an increase in storm rainfall–runoff ratio from < 5% to > 15% and a decrease in the threshold rainfall for channel flow by 16–25% were documented. These short-term variations in flooding behavior are explained by the clear decrease in the length of the rainy season and by the resulting significant shortening in the duration of the dry-spells. The increase in the number of large rainfall events and the large floods in each hydrological year together with the increasing number of years with no floods indicate strengthening of their uncertainty of behavior.     

Morphodynamic river processes and techniques for assessment of channel evolution in Alpine gravel bed rivers

Over the past 10 years many restoration projects have been undertaken in Austria, and river engineering measures such as spur dykes and longitudinal bank protection, which imposed fixed lateral boundaries on rivers, have been removed. The EU-Life Project “Auenverbund Obere Drau” has resulted in extensive restoration on the River Drau, aimed to improve the ecological integrity of the river ecosystem, to arrest riverbed degradation, and to ensure flood protection. An essential part of the restoration design involved the consideration of self-forming river processes, which led to new demands being imposed on river management.This paper illustrates how model complexity is adapted to the solution and evaluation of different aspects of river restoration problems in a specific case. Point-scale monitoring data were up-scaled to the whole investigation area by means of digital elevation models, and a scaling approach to the choice of model complexity was applied. Simple regime analysis methods and 1-D models are applicable to the evaluation of long-term and reach-scale restoration aims, and to the prediction of kilometre-scale processes (e.g. mean river bed aggradation or degradation, flood protection). 2-D models gave good results for the evaluation of hydraulic changes (e.g. transverse flow velocities, shear stresses, discharges at diffluences) for different morphological units at the local scale (100 m–10 m), and imposed an intermediate demand on calibration data and topographic survey. The study shows that complex 3-D numerical models combined with high resolution digital elevation models are necessary for detailed analysis of processes (1 m–0.01 m), but not for the evaluation of the restoration aims on the River Drau. In conclusion, model choice (complexity) will depend on both lower limits (determined by the complexity of processes to be analysed) and upper limits (field data quality and process understanding for numerical models).     

Beach morphology and change along the mixed grain-size delta of the dammed Elwha River, Washington

Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~ 0.6 m/yr, which is equivalent to ~ 24,000 m³/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~ 0.8 m/yr during 1939–1990 to ~ 1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~ 100 mm) than the foreshore (mean grain size ~ 30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast.     

Channel morphology and its impact on flood passage, the Tianjiazhen reach of the middle Yangtze River

The Tianjiazhen reach of the middle Yangtze is about 8 km long, and characterized by a narrow river width of 650 m and local water depth of > 90 m in deep inner troughs, of which about 60 m is below the mean sea level. The troughs in the channel of such a large river are associated with regional tectonics and local lithology. The channel configuration plays a critical role in modifying the height and duration of river floods and erosion of the riverbed. The formation of the troughs in the bed of the Yangtze is considered to be controlled by sets of NW–SE-oriented neotectonic fault zones, in which some segments consist of highly folded thick Triassic limestone crossed by the Yangtze River. Several limestone hills, currently located next to the river channel, serve as nodes that create large vortices in the river, thereby accelerating downcutting on the riverbed composed of limestone highly susceptible to physical corrosion and chemical dissolution. Hydrological records indicate that the nodal hills and channel configuration at Tianjiazhen do not impact on normal flow discharges but discharges > 50,000 m³s^− 1 are slowed down for 2–3 days. Catastrophic floods are held up for even longer periods. These inevitably result in elevated flood stages upstream of prolonged duration, affecting large cities such as Wuhan and a very large number of people.     

Temporal variations in the specific stream power and total energy expenditure of a monsoonal river: The Tapi River, India

In this study, an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the Tapi River, in central India. Continuous daily discharge data (1978–1990), hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power (ω) for the Savkheda gauging site in the lower Tapi Basin. The total amount of energy generated by all the monsoon flows was estimated by integrating the area under the ω-graph derived for the monsoon season.The analyses of the 13-year daily discharge data reveal that the average and maximum ω values range from 4–20 W m^{− 2}, and 22–964 W m^{− 2} respectively. Specific stream power duration curve derived for the site shows that for 25% of the time the power per unit area is > 10 W m^{− 2}. Furthermore, unit stream power was found to be above the Williams' [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] threshold of pebble-movement (1.5 W m^{− 2}), cobble-movement (16 W m^{− 2}) and boulder-movement (90 W m^{− 2}) for 71%, 15% and 2% of the time, respectively. Computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 MJ (deficit monsoon years) to 256 MJ (excess monsoon and/or flood years). Large floods have one-third share in the total monsoon energy expenditure. In the absence of appropriate data on the yearwise geomorphic effects, the geomorphic work was evaluated in terms of the total suspended sediment load transported. The total monsoon sediment load is strongly related to the total monsoon energy. The results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons.The present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season, except during the deficient monsoon years, and there is little doubt that large-magnitude floods are effective agents of geomorphic change in monsoonal rivers.     

Weathering characteristics of arctic islands in northern Norway

The arctic islands of the Lofoten-Vesterålen archipelago in northern Norway have a wide distribution of weathered land surfaces commonly located above 250 m with several apparent similarities. In order to investigate the characteristics of (deep) weathering in this region, northern Langøya and Hadseløya were chosen for in-depth analyses. Eight weathering profiles were excavated from various surfaces, and the stratigraphies were logged in detail. Material was collected throughout the weathering horizons, and all samples were subsequently analysed for clay mineralogy (< 63 μm fraction) and grain size distribution. The sampling strategy was complemented by samples from additional saprolites and other landforms such as moraines and rock glaciers. The XRD results indicate that the presence of secondary minerals, such as gibbsite (Al(OH)₃) and kaolinite (Al₂Si₂O₅(OH)₄), are very common throughout the profiles. Gibbsite is an extreme end product of silicate weathering and usually associated with a warmer and more humid climate, as found in Scandinavia during the Tertiary. The grain size analyses (< 63 μm) show that the finer silt fractions (< 8 μm) tend to be high in the profiles (20–40%), with significant amounts of clay (5–15%) demonstrating that the regolith itself is susceptible to frost sorting mechanisms.¹⁰Be exposure dates from in situ quartz knobs on tors and boulders of local origin suggest > 40,000 years of subaerial conditions. Considering the steady surface erosion, this figure should be viewed as an absolute minimum age estimate. Mapping of the superficial sediments and geomorphological features of the study areas has revealed several common morphological features, which indicate dominance of glacial and periglacial processes in the areas lying below the lower boundary of blockfields (c. 250 m). The weathering mantles are not a periglacial end product, but rather a relict tertiary landform that were modulated by permafrost processes as well as biological processes at later stages. The regolith cover constrain the vertical extension of warm-based Quaternary ice sheets challenging the notion of a parabolic ice mass consuming every mountain top of Lofoten and Vesterålen.     

Modeling forced pool–riffle hydraulics in a boulder-bed stream, southern California

The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force.     

Beach–dune interactions on the dry–temperate Danube delta coast

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m³ m^− 1 y^− 1 to 5.1 m³ m^− 1 y^− 1) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s^− 1) and erodes during high winds (> 12 m s^− 1).     

Wind tunnel experimental investigation of sand velocity in aeolian sand transport

Sand velocity in aeolian sand transport was measured using the laser Doppler technique of PDPA (Phase Doppler Particle Analyzer) in a wind tunnel. The sand velocity profile, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence were analyzed in detail. The experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0.01 m, while a deviation occurs below 0.01 m. The mean vertical velocity of grains generally ranges from − 0.2 m/s to 0.2 m/s, and is downward at the lower height, upward at the higher height. The probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right-skewed at a height of 4 mm in the lower part of saltation layer. The vertical profile of the horizontal RMS velocity fluctuation of particles shows a single peak. The horizontal RMS velocity fluctuation of sand particles is generally larger than the vertical RMS velocity fluctuation. The RMS velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity. The particle turbulence intensity decreases with height. The present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity.     

Nature rehabilitation by floodplain excavation: The hydraulic effect of 16 years of sedimentation and vegetation succession along the Waal River, NL

The “Ewijkse Plaat” is a floodplain along the Waal River, NL. In 1988, the floodplain was excavated as part of a program for enlargement of the discharge capacity and was assigned as a nature rehabilitation area. This paper describes the combined geomorphological and vegetation evolution of the floodplain until 16 years after the initial excavation using elevation data and data on vegetation structure derived from detailed aerial stereographic imagery. The impact of these processes on flow velocity and water surface elevation was evaluated by using a hydraulic model. Within 16 years, the excavated amount of sediment was redeposited in the area. The dominant geomorphological process after excavation was vertical accretion of the floodplain which resulted in the formation of natural levees. The amount of sedimentation was correlated to the across-floodplain flow (R² = 0.89). In the research period, 41% of the sedimentation took place during two single major flood events. The creation of pioneer stages by excavation promoted softwood forest establishment, which influenced the sedimentation pattern significantly. The landscape evolved toward structure-rich vegetation. Nine years after excavation the initial hydraulic gain was lost by the combined effect of sedimentation and vegetation succession. Implications for river and nature management are discussed.