Leeside Slope Research Articles

Spatial distribution of dust deposition within a small drainage basin: Implications for loess deposits in the Negev Desert

AbstractLoessial colluvial sediments and aeolian aprons are common deposits in the Negev Desert Highlands. In an attempt to monitor the amounts and distributional pattern of loess, monthly dust measurements were carried out during 2004 to 2006 in 10 cm diameter traps located at 18 stations along four slopes, north‐facing, south‐facing, east‐facing and west‐facing in a second‐order drainage basin near Sede Boqer, Negev Desert Highlands, Israel. Annual total dust depositions ranged between 110 g and 178 g m−2 with an average of 151·1 g m−2. The average annual dust deposition in the catchment was 23·5% higher than the average amount recorded at the hilltops (122·4 g m−2) and may be a consequence of sheltering opportunities in the hilly topography. When analysed according to season and aspect, significantly higher monthly amounts were received during the wet rainy season of December to March (17·0 g m−2), in comparison with the rest of the year (8·1 g m−2). As for the aspect, while no significant differences characterized north‐facing and south‐facing slopes, east‐facing slopes received significantly higher amounts (by 43·3%) than west‐facing slopes, pointing to preferential dust deposition at the leeward slope. Concurring with the classical model that anticipates higher dust deposition at the leeside slope, but in disagreement with some reports published in the literature, the findings of this study were also supported by a field survey that showed preferential loess accumulation at the eastern and north‐eastern aspects. These findings may shed light on distributional patterns of colluvial sediments and aeolian aprons in the Negev, on soil‐forming processes and on past cycles of dust deposition.

Understanding river dune splitting through flume experiments and analysis of a dune evolution model

ABSTRACTForecasts of water level during river floods require accurate predictions of the evolution of river dune dimensions, because the hydraulic roughness of the main channel is largely determined by the bed morphology. River dune dimensions are controlled by processes like merging and splitting of dunes. Particularly the process of dune splitting is still poorly understood and – as a result – not yet included in operational dune evolution models. In the current paper, the process of dune splitting is investigated by carrying out laboratory experiments and by means of a sensitivity analysis using a numerical dune evolution model. In the numerical model, we introduced superimposed TRIAS ripples (i.e. triangular asymmetric stoss side‐ripples) on the stoss sides of underlying dunes as soon as these stoss sides exceed a certain critical length. Simulations with the model including dune splitting showed that predictions of equilibrium dune characteristics were significantly improved compared to the model without dune splitting. As dune splitting is implemented in a parameterized way, the computational cost remains low which means that dune evolution can be calculated on the timescale of a flood wave. Subsequently, we used this model to study the mechanism of dune splitting.Literature showed that the initiation of a strong flow separation zone behind a superimposed bedform is one of the main mechanisms behind dune splitting. The flume experiments indicated that besides its height also the lee side slope of the superimposed bedform is an important factor to determine the strength of the flow separation zone and therefore is an important aspect in dune splitting. The sensitivity analysis of the dune evolution model showed that a minimum stoss side length was required to develop a strong flow separation zone. Copyright © 2014 John Wiley & Sons, Ltd.

Turbulent flow characteristics and drag over 2-D forward-facing dune shaped structures with two different stoss-side slopes

The present paper explores the characteristics of turbulent flow and drag over two artificial 2-D forward-facing waveform structures with two different stoss side slopes of $$50^{\circ }$$ and $$90^{\circ },$$ respectively. Both structures possessed a common slanted lee side slope of $$6^{\circ }.$$ Flume experiments were conducted at the Fluvial Mechanics Laboratory of Indian Statistical Institute, Kolkata. The velocity data were analyzed to identify the spatial changes in turbulent flow addressing the flow separation region with recirculating eddy, the Reynolds stresses, the turbulent events associated with burst-sweep cycles and the drag over two upstream-facing bedforms for Reynolds number $$Re_h=1.44\times 10^5.$$ The divergence at the stoss side slope between the two structures revealed significant changes in the mean flow and turbulence. Comparison showed that during the flood-tide condition there was no flow separation region on the gentle lee side of the structure with smaller slope at the stoss side, while for the other structure with vertical stoss side slope a thick flow separation region with recirculating eddy was observed at the gentle lee side just downstream of the crest. The recirculating eddy induced on the lee-side had a strong influence on the resistance that the structure exerts to the flow due to loss of energy through turbulence. In contrast, a great amount of reduction in drag was observed in the case of smaller stoss side sloped structure as there was no flow separation. The quadrant analysis was also used to highlight the turbulent event evolution along the bed form structures under flood-tide conditions.

Numerical Simulations of Effects of Soil Moisture and Modification by Mountains over New Zealand in Summer

Abstract Historically most soil moisture–land surface impact studies have focused on continents because of the important forecasting and climate implications involved. For a relatively small isolated mountainous landmass in the ocean such as New Zealand, these impacts have received less attention. This paper addresses some of these issues for New Zealand through numerical experiments with a regional configuration of the Met Office Unified Model atmospheric model. Two pairs of idealized simulations with only contrasting dry or wet initial soil moisture over a 6-day period in January 2004 were conducted, with one pair using realistic terrain and the other pair flat terrain. For the mean of the 6 days, the differences in the simulated surface air temperature between the dry and moist cases were 3–5 K on the leeside slopes and 1–2 K on the windward slopes and the central leeside coastal region of the South Island in the afternoon. This quite nonuniform response in surface air temperature to a uniformly distributed soil moisture content and soil type is mainly attributed to modification of the effects of soil moisture by mountains through two different processes: 1) spatial variation in cloud coverage across the mountains ranges leading to more shortwave radiation at ground surface on the leeside slope than the windward slope, and 2) the presence of a dynamically and thermally induced onshore flow on the leeside coast bringing in air with a lower sensitivity to soil moisture. The response of local winds to soil moisture content is through direct or indirect effects. The direct effect is due to the thermal contrast between land and sea/land shown for the leeside solenoidal circulations, and the indirect effect is through the weakening of the upstream blocking of the South Island for dryer soils shown by the weakening and onshore shift of the upstream deceleration and forced ascent of incoming airflow.

Numerical Simulations of Island-Scale Airflow over Maui and the Maui Vortex under Summer Trade Wind Conditions

Abstract The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) coupled with the Noah land surface model (LSM) is employed to simulate island-scale airflow and circulations over Maui County, Hawaii, under summer trade wind conditions, during July–August 2005. The model forecasts are validated by surface observations with good agreement. In this study, it is shown that a previously known closed circulation over the Central Valley of Maui, or the Maui vortex, represents the northern cyclonic vortex of the dual-counter-rotating vortices in the lee of Haleakala, which extend up to the base of the trade wind inversion with a westerly reversed flow (>2 m s−1). At low levels, the northern cyclonic vortex is more pronounced than the southern anticyclonic vortex. The asymmetric structure of the dual vortices is related to the shape of Haleakala and the flow deflection by the West Maui Mountains. The Maui vortex has a relatively narrow east–west extent in the lowest levels, especially at night, due to the deflected strong northerly/northeasterly winds from the windward foothills of the West Maui Mountains. Unlike the lee vortices off the leeside coast of the island of Hawaii, the Maui vortex and the westerly return flow in low levels are mainly over land and are strongly modulated by the diurnal heating cycle. In addition, the location and horizontal and vertical extent are affected by the trade wind speed and latent heat release. Over the West Maui Mountains, with their height below the trade wind inversion, dual-counter-rotating vortices are present below the 1-km level in the wake, with strong downslope flow on the leeside slopes followed by a hydraulic jump. In the afternoon, downslope winds are weak, with combined westerly return/sea-breeze flow along the leeside coast. Orographic blocking is also evident over eastern Molokai with strong downslope winds, especially at night.

Numerical Simulations of Island Effects on Airflow and Weather during the Summer over the Island of Oahu

Abstract The high-resolution (1.5 km) nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and an advanced land surface model (LSM) are used to study the island-induced airflow and weather for the island of Oahu, Hawaii, under summer trade wind conditions. Despite Oahu’s relatively small area (1536 km2), there are considerable spatial variations in horizontal distribution of thermodynamic fields related to terrain, airflow, rain, cloud, and ground cover. The largest diurnal variations in temperature and moisture occur in the lee sides of mountains, especially along the western leeside coast. The island-scale surface airflow is also significantly affected by terrain and land surface forcing. The downslope winds above the leeside slopes of both the Ko’olau and Waianae Mountains are simulated with significant diurnal variations with the strongest downslope winds just before sunrise. The timing of diurnal rainfall maxima over the Ko’olau Mountains is closely related to vertical motions. The early morning rainfall maximum on the windward side is caused by anomalous rising motion due to significant flow deceleration when the land surface is the coolest. The evening rainfall maximum after sunset is related to anomalous orographic lifting due to stronger winds aloft. In the early afternoon, winds aloft are relatively weak with a relatively high level of free convection (LFC) because of vertical mixing. As a result, the rainfall over the Ko’olau Mountains exhibits an afternoon minimum. The westerly reversed flow off the western leeside coast in the afternoon is mainly thermally driven and related to land surface heating superimposed by latent heat release of persistent orographic precipitation over the Ko’olau Mountains. Rainfall along the western leeside slopes has a late afternoon maximum due to the development of the onshore/upslope flow.

Weather regimes and orographic circulation around New Caledonia

The local climate and island-scale circulation around New Caledonia is investigated using a 4-km resolution mesoscale atmospheric model in concert with QuikSCAT scatterometer winds at 12.5-km resolution. The mesoscale atmospheric weather regimes are first examined through an objective classification applied to the remote sensed winds for nine warm seasons from 1999 to 2008. Four main weather types are identified. Their corresponding synoptic-scale circulation reveals that they are strongly discernable through the position and intensity of the South Pacific Convergence zone (SPCZ), the mid-latitude systems, and the subtropical jet stream. The link between the mesoscale weather types and the two dominant large-scale modes of variability, namely the Madden–Julian Oscillation (MJO) and the El Niño-Southern Oscillation (ENSO), is also described in terms of their influence on the occurrence of each weather type. It shows that their occurrence is significantly controlled by both MJO and ENSO, through modulation of the SPCZ. The large-scale modes of variability are scaled down to island-scale circulation through synoptic and mesoscale regimes, and are eventually modulated by orographic and thermal control. The island-scale circulation is inferred in this study by applying the compositing method to both observed and simulated winds. Their comparison clearly shows the ability of the mesoscale model to capture the local circulation and its spatial and temporal variability. A scaling analysis conducted from the simulated atmospheric parameters shows that the mountain range of New Caledonia is hydrodynamically steep. As a result of trade-wind obstruction by the mountainous island, the flow is shaped by coastally trapped mesoscale responses, i.e., blocking, flow splitting and corner winds, with a spatial scale of about 150km. Two main obstacles, Mont Panié and Mont Humboldt play a significant role on the dynamical behavior of the low-level flow, while the diurnal heating cycle in the vicinity of the Mainland strongly modulates the local circulation. Moreover, nocturnal drainage flow of cold air occurs on the leeside slope of Mont Humboldt and inhibits vertical mixing over the ocean, which results in a deceleration of surface winds.

Seepage Effects on Dune Dimensions

This note presents an experimental study of seepage effects on the dimensions of a dune: its height, length and lee-side slope. The critical slope of a cohesionless material was found to be related to the ratio of the hydraulic gradient of seepage to its critical value under the quick condition. The theoretical relationship is successfully used to predict seepage effects on the lee-side slope of dunes. Measured laboratory data support the assumption that the effect of seepage on the lee-side slope of dunes is the same as that on the critical slope of the sediment. The data reveal that the slope is reduced and increased with injection and suction, respectively. Additionally, the study explores seepage effects on the development of dune height and length. Experimental results show that suction and injection increase and decrease the equilibrium height of dunes by 25 and 40%, respectively. On the other hand, the dune length is found to increase by approximately 30% and decrease by 20% under the influence of suction and injection, respectively.

Predicting Rock Mass Decay in Engineering Lifetimes: The Influence of Slope Aspect and Climate

Following the initial stress release after excavation of a road cut, weathering and erosion processes will start acting on the newly exposed slope material. As field observations show, the resulting degradation may have significant effects well before the envisaged engineering lifetime of the slope ends. The data set obtained by 12 years of fieldwork by the International Institute for Geo-Information Science and Earth Observation and Delft University of Technology was used to assess and quantify the time-related degradation of rock masses by weathering. The results of a bootstrap analysis of the data show a close relationship between the weathering rates, the slope aspect, and the prevailing wind directions during rainfall events. For the study area around Falset it can be concluded that in shales, marls, and similar materials containing clays (especially swelling clays), weathering rates are highest in slopes facing the prevailing winds during rainfall because of more frequent and intense cyclic wetting and drying. The same holds true for more resistant materials such as limestones, which are interbedded with weak materials such as shales and marls. In soluble materials such as the gypsum unit investigated here, water retention in slopes sheltered from the winds facilitates chemical weathering, and it is in these lee-side slopes that the highest weathering rates are found. These relations can be used to predict weathering rates and, therefore, also weathering degree as a function of time. With the use of a probabilistic slope-stability assessment method, the time-related slope stability can also be predicted.

Response of sand dunes to variations in tidal flow: Fraser Estuary, Canada

The morphology of dunes in rivers and estuaries often lags behind changes in river discharge and neap‐spring tides. This study extends previous research by examining the response of large subtidal dunes in the Fraser Estuary, Canada, to changing flow conditions over a semidiurnal tidal cycle. An acoustic Doppler current profiler (ADCP) is used to measure three‐dimensional velocity profiles and estimate sediment transport and a digital echosounder to measure bed profiles and dune characteristics. Mean flow velocity accelerates from 0.60 to 1.99 m/s and mean flow depth decreases from 15.1 to 12.2 m on the falling tide, followed by a decrease in mean velocity to 0.25 m/s and an increase in depth to 15.25 m on the rising tide. Estimates based on ADCP bottom‐tracking and backscatter, and the sediment transport model of van Rijn (1984a, 1984b, 1984c), indicate that bed‐material sediment transport generally follows the pattern of mean velocity over the tidal cycle. Dune length does not change significantly over the survey period. Changes in dune height, steepness and leeside slope angle precede changes in flow velocity, increasing early in the tidal fall because of scour in dune troughs that is likely caused by increased turbulence resulting from development and expansion of the flow separation/deceleration zone. Erosion of dune crests increases as peak velocity is approached near low tide, resulting in a decrease in dune height. Concentrations of sand in suspension also increase with mean velocity, leading to deposition in troughs and a further reduction in dune height and leeside slope angle. Sand falls out of suspension and drapes the dunes as high tide approaches. Dune length is not in equilibrium with the flow whereas dune height is in equilibrium with the strongest flows that occur around low tide. A comparison of bed‐material transport based on dune migration and the model of van Rijn (1984a, 1984b, 1984c) suggests that the model provides reasonable estimates of bed load in the Estuary, although the model must be used with caution because deposition from suspension also contributes to dune migration.

Contrasting models of temperate carbonate sedimentation in a small Mediterranean embayment: the Pliocene Carboneras Basin, SE Spain

Temperate carbonates consisting of bioclastic (bryozoan–bivalve–coralline algal) packstones to rudstones formed in the Carboneras Basin, a small embayment of the Mediterranean Sea in SE Spain, during the early Pliocene. Transgressive systems tract carbonate deposits exhibit three distinct sedimentary styles, with contrasting lithofacies patterns and stratal-geometry arrangements. Palaeotopography and local hydrodynamic conditions were the major factors controlling sedimentation. On the steep northern margin, affected by southeasterly wind-driven storms, a prograding platform (distally steepened ramp) with well-marked, platform-slope clinoforms developed. Bivalves extensively colonized the outer platform, and rhodolith pavements covered its edge. Coarse-grained, bioclastic sediments were frequently removed from the platform by storms and redeposited on the platform slope. On the southern margin of the basin, longshore currents driven by southeasterly winds hydrodynamically accumulated carbonate particles to form a spit-platform, on top of which some shoals developed. These shoals were dismantled from time to time by the northern storms, and the sediment was redeposited on the lee-side slope of the spit-platform. On the gentle and protected western margin, oyster banks and rhodolith pavements colonized the inner-ramp environment whereas coralline algal–bryozoan–bivalve bioconstructions formed on the outer ramp. This example shows that several sedimentary styles of temperate carbonate deposition can coexist in a single basin.

Habitat Connectivity in Reef Fish Communities and Marine Reserve Design in Old Providence-Santa Catalina, Colombia

On the insular platform of Old Providence/Santa Catalina, Colombia, we compared nearshore lagoonal patch reefs to those on the northern bank distant from the islands to determine the importance of habitat connectivity to fish community structure. Nearshore patch reefs had greater proximity to mangrove, seagrass and rocky shore habitats, and they had significantly more individuals. Nearshore reefs also tended to have a greater total biomass, more species, a higher proportion of predators of mobile invertebrates and small fishes, and a lower proportion of herbivores. Biomass of snappers and grunts at nearshore sites was four times greater compared to bank sites, and was correlated with the amount of seagrass and sand/rubble habitat within 500 m of each patch reef. We also compared length-frequency distributions and abundances of grunts and snappers among all sites (deep and shallow forereefs, patch reefs and deep and shallow leeside slopes). The results were consistent with ontogenetic migrations from shallow sites, primarily seagrass and mangrove habitats, to deeper sites and to those further out on the bank. The evidence suggests that species differed in both distance and direction of dispersal, which may be affected by the abundance and distribution of preferred habitats. Marine reserves near the islands should target nearshore nursery areas and patch reefs harboring species of limited dispersal capability. Reserves on the northern bank would protect spawners of those species showing the greatest dispersal capability. Gulf and Caribbean Research Vol 14(2), 61–77, 2003 Manuscript received January 22, 2002; accepted September 23, 2002

An experimental study of turbulent flow over a low‐angle dune

Many large, sand bed alluvial channels are dominated by dunes that possess low‐angle lee sides, often <10°, which play a critical role in the transportation of sediment and generation of significant bed form roughness. Despite the fact that these low‐angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee. Study of flow associated with low‐angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms. Results from a detailed scale model experimental study of flow over a low‐angle dune, which is based on a prototype dune from the Fraser River, Canada, present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms. Two‐dimensional laser Doppler anemometer measurements over a low‐angle dune (maximum lower lee side slope = 14°) show that dune morphology exerts a dominant control on the turbulent flow, causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above. The scale model confirms that permanent flow separation does not occur over low‐angle dunes but, instead, is replaced by a small region (here ∼7% of the dune wavelength in length) of intermittent flow reversal, which may be present for up to 4% of the time. Shear layers generated along this small zone of decelerated and/or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes. Turbulence production associated with low‐angle dunes is dominated by eddies generated along this shear layer, which produce highly variable horizontal and vertical velocities and large Reynolds stresses in this region. These results show that macroturbulence associated with low‐angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion. Velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low‐angle dunes in natural sand bed rivers. These results highlight the need to consider the temporal evolution and intermittency of shear layer behavior, often very near the bed, when interpreting the generation of macroturbulence and dispersal of sediment associated with low‐angle dunes.

Large-scale bedforms, sediment distribution and sand mobility in the eastern North Sea off the Danish west coast

Two side-scan sonar surveys were carried out 9 months apart in a 24-km 2 off Thorsminde at the Danish west coast, were water depth is 12–18 m. Sediment samples were systematically collected in the same area using a 250-m sampling grid. The North Sea study area is exposed to strong wind-induced coastal currents and high waves, and thereby characterized by significant sediment transport. The seabed data were used to analyse the large-scale dynamical bedforms found in the area, the mobility of the bedforms, the areal variation of the seabed mean grain-size and the grain-size variation across the large bedforms. The compilation of side-scan sonar mosaics revealed very large bedforms, with a mean wavelength of approximately 500 m and heights of 1–3 m. These bedforms indicate a north-going net sediment transport, caused by the Jutland Coastal Current. Large dunes ( L=10–100 m) and smaller bedforms were also recorded, as well as large shoreface-connected ridges with a spacing of 3–4 km. The bedform configuration appears to be relatively stable, only small changes having been observed between the two surveys, even though a 100-yr cyclone and several storms occurred between the two surveys. The small changes observed, also indicate a net northerly sediment transport. The surface sediments reveal a remarkably high positive correlation between the mean grain size and the position across the very large flow-transverse bedforms, from coarse sand in the troughs to fine sand at the crest and lee-side slope of the bedforms.

Quantitative visualization of flow fields associated with alluvial sand dunes: Results from the laboratory and field using ultrasonic and acoustic doppler anemometry

This paper presents results detailing the quantitative visualization of flow fields associated with natural sand dunes, Fraser River Estuary, Canada, using the complementary approaches of laboratory modelling and field instrumentation. Ultrasonic Doppler velocity profiling is used in the laboratory to elucidate the mean flow fields of low-angle dunes (leeside slope angle ~14°) that are typical of many large natural rivers. These dunes do not possess a zone of permanent flow separation in the dune leeside and have a velocity structure that is dominated by the effects of flow acceleration and deceleration generated by topographic forcing of flow over the dune form. Turbulence associated with these dunes appears linked to both longer-period shear layer flapping and eddy generation along the shear layer. The field study uses acoustic Doppler profiling to reveal similar mean flow patterns and shows that flow is dominated by deceleration in the leeside without the presence of a region of permanent separated flow.

A field study of turbulence and sediment dynamics over subaqueous dunes with flow separation

This study examines flow, turbulence and sand suspension over large dunes in Canoe Pass, a distributary channel of the Fraser River delta, Canada. Dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30°. Velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter (OBS) probes. The general patterns of time‐averaged velocity and sand suspension are consistent with previous studies, including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed, reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near‐bed velocity and sand concentration along the stoss side of the dune. Frequency spectra of near‐bed velocity and OBS records from leeside separation zones are composed of two distinct frequencies, providing field confirmation of scale relations based on flume experiments. The low‐frequency spectral signal probably results from wake flapping and the high‐frequency signal from vortex shedding. The wake‐flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand. Predictions from a depth‐scale Strouhal Law show good agreement with measured wake‐flapping frequencies. Cross‐correlations of OBS records reveal that turbulent sand suspension structures advect downstream at 23–25° above the horizontal. These advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field.

The relation between shear velocity and suspended sediment concentration over dunes: Fraser Estuary, Canada

Two types of dunes develop in the Fraser River Estuary, Canada. Symmetric dunes have stoss and lee sides of similar length, gentle lee-side slope angles and rounded crests, while asymmetric dunes have superimposed small dunes on their stoss sides, sharp crests and steeper lee sides. Symmetric dunes appear to be near-equilibrium features at higher flows. Asymmetric dunes are transitional between symmetric dunes and the small superimposed dunes that are in equilibrium with the lower velocity and sediment transport conditions. Spatially averaged velocity profiles over dunes consist of upper and lower log-linear segments. Upper segments of profiles reflect the total stress of the flow. The lower segment on symmetric dunes reflects skin friction from sand particles, but for asymmetric dunes it is skin friction plus form roughness from the superimposed dunes. Predictions from the Rouse equation indicate that sediment suspension is controlled by total stress for symmetric dunes, whereas for asymmetric dunes sediment suspension is related to stress associated with skin friction plus form roughness. It is the equilibrium bedform — the large symmetric dunes, and the small superimposed dunes in the asymmetric case — that controls sediment suspension over dunes.

Bedrock surface roughness and the distribution of subglacially precipitated carbonate deposits: implications for formation at Glacier de Tsanfleuron, Switzerland

Two carbonate deposits are identified on the exposed bedrock surface in the forefield of Glacier de Tsanfleuron, Switzerland: macrocrystalline sparite and microcrystalline micrite. Comparison of the distributions of these forms with lee-side slope facets identified by high-pass filtering of a flow-parallel bedrock profile at a range of frequencies reveals two significant results. First, while the distribution of sparite is consistent with formation in the lee side of subglacial bedrock hummocks, that of micrite is not. This contrasts with previous investigations in which both sparite and micrite have been considered to form by mineral concentration and precipitation during the refreezing of regelation-related basal meltwaters in the lee side of bedrock hummocks. Alternative mechanisms of micrite formation involving carbonate deposition and/or precipitation within subglacial bedrock hollows are proposed. Second, the distribution of sparite is most strongly correlated with the distribution of lee-side slope facets identified by filtering at a frequency equivalent to a hummock wavelength of c. 0·1 m. This correspondence indicates empirically that pressure-related melting and refreezing (regelation) operates most effectively around bedrock hummocks that are shorter than c. 0·1 m. © 1998 John Wiley & Sons, Ltd.

Heavy Rain Events over the South-Facing Slopes of Hawaii: Attendant Conditions

Abstract Heavy rain events (>100 mm day−1) over the southeast flank of Mauna Loa volcano on the island of Hawaii are examined using surface, rawinsonde, rain gauge, and satellite data. The events occur in the presence of four types of synoptic-scale disturbances that include Kona storms, cold fronts, upper-tropospheric troughs, and tropical systems. The heaviest rainfall occurs at elevations above 0.5 km over the volcano slopes facing the prevailing low-level flow with the leeside slopes experiencing considerably smaller amounts. Rainfall duration and patterns demonstrate that the rains and flash floods are the result of more than one convective cell. Soundings prior to the heavy rain events show only modest instability, with only the K index serving as a useful predictor for heavy rain among the standard stability indices. There is an increase in both the moisture content of the midlevels (750–450 mb) and the onshore flow normal to the terrain gradient prior to the heavy rain. The lack of the trade winds...

Flow and sediment transport over large subaqueous dunes: Fraser River, Canada

ABSTRACTLarge symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near‐bed velocity and bed load transport near dune crests result in crest rounding. Long, low‐angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near‐bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume‐based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows.