Down-valley Flow Research Articles

Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2–1.2 m thickness on the Cadiz slope (1–12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.

Disruption of Drift glacier and origin of floods during the 1989–1990 eruptions of Redoubt Volcano, Alaska

Melting of snow and glacier ice during the 1989–1990 eruption of Redoubt Volcano caused winter flooding of the Drift River. Drift glacier was beheaded when 113 to 121 × 10 6 m 3 of perennial snow and ice were mechanically entrained in hot-rock avalanches and pyroclastic flows initiated by the four largest eruptions between 14 December 1989 and 14 March 1990. The disruption of Drift glacier was dominated by mechanical disaggregation and entrainment of snow and glacier ice. Hot-rock avalanches, debris flows, and pyroclastic flows incised deep canyons in the glacier ice thereby maintaining a large ice-surface area available for scour by subsequent flows. Downvalley flow rheologies were transformed by the melting of snow and ice entrained along the upper and middle reaches of the glacier and by seasonal snowpack incorporated from the surface of the lower glacier and from the river valley. The seasonal snowpack in the Drift River valley contributed to lahars and floods a cumulative volume equivalent to about 35 × 10 6 m 3 of water, which amounts to nearly 30% of the cumulative flow volume 22 km downstream from the volcano. The absence of high-water marks in depressions and of ice-collapse features in the glacier indicated that no large quantities of meltwater that could potentially generate lahars were stored on or under the glacier; the water that generated the lahars that swept Drift River valley was produced from the proximal, eruption-induced volcaniclastic flows by melting of snow and ice.

Atmospheric Dispersion and Tracer Ventilation in a Deep Mountain Valley

Abstract During September and October 1984, a major meteorological and tracer study was conducted in Colorado's Brush Creek valley. The characteristics of atmospheric dispersion during the nighttime and morning transition periods are discussed in this paper. Tracer released near the valley floor did not reach the ridgetops (escape from the valley) during the nighttime but was confined to the valley, being carried in down-valley flows. After sunrise, with the onset of convective boundary-layer growth and initiation of upslope flows, the tracer within the valley was carried into the upper elevations of the valley atmosphere and ventilated from the valley. This was confirmed by the ridgetop tracer samplers and by a tracer mass budget applied to a valley atmosphere control volume. The ventilation rate of tracer from the valley atmosphere to the above-ridgetop flows was calculated from the tracer mass budget. A dimensionless form of the ventilation rate is proposed. The Gaussian plume equation adequately repre...

The vertical structure of summertime local winds in the Wright Valley, Antarctica

Intensive observations of summertime up- and down-valley winds in a “dry” valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.

Morning Transition Tracer Experiments in a Deep Narrow Valley

Abstract Three sulfur hexafluoride atmospheric tracer experiments were conducted during the post-sunrise temperature inversion breakup period in the deep, narrow Brush Creek Valley of Colorado. Experiments were conducted under clear, undisturbed weather conditions. A continuous elevated tracer plume was produced along the axis of the valley before sunrise and the behavior of the plume during the inversion breakup period was detected down-valley from the release point using an array of radio-controlled sequential bag samplers, a vertical SF6 profiling system carried on a tethered balloon, two portable gas chromatographs operated on a sidewall of the valley, and a continuous real-time SF6 monitor operated from a research aircraft. Supporting meteorological data came primarily from tethered balloon profilers. The nocturnal elevated plume was carried and diffused in down-valley flows. After sunrise, convective boundary layers grew upward from the sunlit valley surfaces, fumigating the elevated plume onto the ...

Modeling of Mountain-Valley Wind Fields in the Southern San Joaquin Valley, California

A dry three-dimensional mesoscale model was used to study the diurnal cycle of mountain-valley winds in the southern San Joaquin Valley during a summer day. A scheme for interpolating potential temperature was developed to provide hourly temperature fields to initialize and force the dynamically predicted wind fields. A simplified modeling approach was used to produce steady state solutions that are dynamically consistent with the momentum equation and supplied temperature fields. Model performance was evaluated by comparing observed and predicted surface winds. Some features of the wind field flow aloft were qualitatively examined with regard to their importance in air quality studies. The morning drainage-upslope transition and the evening reversal of upslope flow were realistically simulated throughout most of the valley. The variation of wind speeds throughout the valley and over the course of the day were simulated with an average bias of 9% of the average wind. Wind directions were simulated with an overall average bias of 5° and midday hourly correlation coefficients of typically r = 0.8. Model performance was below average during the morning and evening transition periods, when thermal forcing is at a minimum and valley winds are light and variable. At midday, the model produces strong upward vertical motions near the ridge crests and divergence-driven subsidences at the foot of the mountains typical of observations made in mountain-valley systems. During the morning, modeled drainage flow down the mountains results in a convergence zone in the southern and narrowest part of the valley, resulting in rising motions; down-valley flow, sometimes observed in mountain-valley systems, also occurs. The model is best suited for applications in mountain-valley regions for which wind observations are sparse and do not adequately reflect thermally driven circulation.

Local winds in the upper Rhone Valley

Balloon soundings during July and August 1979, 1981 and 1982 showed the vertical structure of the flow in the upper Rhone Valley. Between the low level winds up to a height of about 2000 m a s l and the gradient winds above 3000 m a s l, in 73 % of the 107 ascents, a counterflow was detected. It appeared more often in connection with down-valley flow (89 %) than with up-valley flow (38 %) above the ground. This flow pattern was found to be almost unaffected by the upper winds.

A numerical model of the time‐dependent wintertime circulation of the New York Bight

The time‐dependent flow in the New York Bight from March 7 to April 30, 1975, is calculated from approximations to the barotropic vorticity equation subject to forcing by winds observed at the John F. Kennedy airport and by an upstream flow across the Long Island shelf. Comparisons of major axis velocities to observations throughout the bight show reasonable agreement. There is also considerable model skill in the prediction of alongshore variations in the major axis velocity with relatively low absolute errors. In general, associated with northeastward winds are northeastward shelf flows and upvalley currents in the Hudson Shelf Valley (HSV). Downvalley flows in the HSV occur when the northeastward wind abates and the flow on the shelf turns southwestward. The mechanism responsible for the behavior of the valley flow appears to be topographic steering with moderation due to bottom friction. The time rate of change of relative vorticity does not appear to have large net effect over the duration of an event Consequently, steady state models represent well the dynamics for an event‐averaged flow. Discrepancies between model results and observation suggest that there is indeed a southwestward pressure gradient force operating in the New York Bight, and an adiabatic pressure condition offshore may be an over simplification that need be removed.

An externally forced barotropic circulation model for the New York Bight

The sea level and the barotropic, frictional circulation response for the New York Bight are used to demonstrate the effects of external sea-level forcing, bathymetry, and variable friction. The governing equation is the steady, integrated vorticity equation and is computed by finite differencing over a curvilinear grid conforming to the 10- and 100-m isobaths and extending for 250 km alongshore. The boundary conditions are based on the hypothesis that the dynamics of the shelf are driven by the external sea-level gradient and the coastal no-flux condition; and consequently the conditions at the lateral boundaries are dependent thereon. Therefore, the external sea-level slope must be independently specified, and the lateral boundary conditions must be dependently generated. The diabathic component of the external sea slope forces the calm wind circulation by its effect on the transport through the upstream boundary; and the parabathic component has also an important modifying effect by forcing a shelf convergent transport. The parabathic sea slope at the coast is independent of its offshore value, being instead a direct product of the coastal boundary condition. The bottom friction is expressed as related to the sea level through a bottom length parameter and a veer angle, both of which are taken to increase shoreward. An additional bottom stress component, related to the surface stress, is determined for bottom depths less than the Ekman depth. Such bottom stress variability produces significant alterations in the nearshore flow field, over the constant bottom stress formulation, by reducing it and causing it to veer downgradient and downwind in the nearshore. The model is forced by different wind directions and the results are discussed. The circulations generally conform to the observed mean flow patterns, but with several smaller-scale features. The strong bathymetric feature of the Hudson Shelf Valley causes a polarized up- and downvalley flow for winds with an eastward or westward component, respectively. Under mean westerly winds, there is a divergence in the shelf valley flow at about the 60-m isobath. The Apex gyre existing off the western tip of Long Island becomes more extensive for winds changing from northeast to southwest. Mean flow reversals (to the northeast) occur off both Long Island and New Jersey for wind directions changing counterclockwise from northwest to southeast and from west to east, respectively. Southeastward transport over the outer New Jersey shelf tends to be enhanced by wind and external sea-level conditions; and the transport over the New Jersey midshelf, particularly in the lee of the shelf valley, tends to be weak and variable also under these mean conditions.

Characterization of the transport and dispersion of pollutants in a narrow mountain valley region by means of an atmospheric tracer

Three atmospheric tracer experiments using sulfur hexafluoride (SF6), were conducted in a narrow mountain valley from 29 January–3 February 1982. These tests were performed in order to characterize the transport and dispersion of airborne pollutants around Telluride, Colorado which lies near the head of the San Miguel Valley. Particular attention was given to the influence of flow reversals upon pollutant dispersion, for which adequate numerical simulation is not yet available. Data from surface wind stations, pilot balloons and from an acoustic sounder were used to characterize the local meteorology. In this case, an upslope flow followed by a downslope drainage is required for emissions from a site on the slope to reach the upper valley about 10 km away. On the other hand, a downvalley flow followed by an upslope flow is required for emissions from the upper valley to reach the slope site. Certain aspects of the transport and dispersion characteristics associated with the mesoscale winds in the area appear to be relatively insensitive to changes in the synoptic winds. For example, even though the synoptic wind patterns in Tests One and Two were different, the 24-h average concentrations in the upper valley were found to be about 15 and 17 (kg-mole) −1h −1, respectively at the slope site. Test Three involved releasing the tracer from Telluride. The 24-h average concentration of tracer at Telluride was used, along with an estimate of the particulate emission rate, to predict the average ambient concentration of total suspended particulates. The value predicted was 71 μgm −3, which is in excellent agreement with the measured value of 63 μgm −3. After the downvalley flow gave way to the midday upslope flow, tracer concentrations were measured along a ski lift. The tracer concentration decreased linearly with increasing distance up the slope. Estimates of concentrations based upon the Gaussian model, (the parameters of which were determined from meteorological data), were in general higher than those observed. In addition to allowing direct prediction of impacts of pollutants emitted within this region, these data can be used to test the validity of numerical models.

Alluvial hydrogeology of the lower Missouri River valley

Floodplain deposits in the lower Missouri River valley alluvium can be divided horizontally into a channel belt, meander belt and flood basin; and vertically they consist of a top-stratum and a substratum. The Missouri, like most major rivers in the midwestern U.S., is an underfit stream which does not fully penetrate its alluvium. Large amounts of groundwater are stored in these alluvial deposits, in which sediment mean grain size and hydraulic conductivity both increase exponentially with depth. Digital computer models indicate, however, that only one flow system exists in the alluvium. Digital simulations and field observations of the floodplain near Glasgow, Missouri, show four types of groundwater response to river stage changes: areas of rapidly fluctuating groundwater levels, areas of slow response, areas of predominantly down-valley flow, and areas of continuing groundwater high. The bank storage-river interaction can be mainly effluent, mainly influent, or a combination of both. The most important factor controlling the floodplain groundwater system is river stage variation. Distance from the river, the time interval since the river has fallen or risen, and the geometry of the river and valley walls all affect the groundwater system. In addition, floodplain inhomogeneities, tributary creeks, and pumping may cause perturbations in groundwater levels.

No current reversal at 10,000 B.P. in the strait of Gibraltar — A discussion

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2–1.2 m thickness on the Cadiz slope (1–12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.

The Development of a Nocturnal Warm Air Layer Within a Shallow Valley

The occurrence of a warm thermal-belt across an entire valley is shown to develop in association with the onset of cool katabatic downvalley flow and the development of a local valley inversion.