Time and space distribution of megaripples and associated bedforms, Middle Atlantic Bight, North American Atlantic Shelf

Three genetically distinct size classes of lower regime transverse bedforms have long been known from laboratory studies, and from studies of the intertidal zone; ripples, megaripples, and sand waves. These features are also present on the subtidal shelf surface of the Middle Atlantic Bight, and their distribution in time and space allows us to draw inferences concerning the time and space pattern of sediment transport. Transverse bedforms in the Middle Atlantic Bight occur in response to tidal flows at estuary and inlet mouths and on tide-dominated banks; on the shelf surface, however, they are primarily responses to wind-driven flows. Ripples are the most widespread of the three classes. They are current-formed during peak storm flows, but are probably remade as oscillatory wave ripples as the flow wanes. Megaripples are found primarily on the inner shelf, also as responses to peak storm flows. Sand waves of several metres amplitude occur on the inner shelf in the vicinity of topographic highs; low amplitude sand waves (< 2m), solitary or in trains, are widespread on the inner shelf. They survive through many seasons of storm flows. Megaripples are especially interesting as records of specific flow events. They are widespread on the inner shelf during the winter, occurring in fields up to several kilometres in diameter. On a portion of the Long Island inner shelf during December 1976, megaripple fields covered approximately 15% of the shelf surface. They tend to be erased during the succeeding summer months. Both megaripples (short-term response elements) and sand waves (long-term response elements) indicate that sand transport in the Middle Atlantic Bight is directed to the southeast, parallel with the regional trend of the isobaths.     

Estimation of contaminant transport parameters for a tropical sand in a sand tank model

This research describes the goals, design and implementation of a quasi natural gradient, laboratory scale, sand tank (aquifer) model experiment. The model was used to study the transport of an inorganic tracer (Chloride) in groundwater, within a tropical aquifer (porous medium) material. Three-dimensional sand tank (1.8 m × 0.3 m × 0.8 m) experiments were conducted to investigate contaminant transport and natural attenuation within the sand tank. In all, 360 samples were collected during 24 sampling sessions, for the three days of the tracer experiments in the Sand Tank. The Owena sand is a poorly graded sand with 88.1 % sand and 11.9 % gravel. Geotechnical properties including; coefficient of uniformity C_u = 2.53, coefficient of gradation C_z = 0.181, hydraulic conductivity K = 5.76 × 10^?4 m/s, bulk density p = 1.9 Mg/m³, effective porosity n_e = 0.215 and median grain diameter D₅₀ = 0.55 mm, were determined. Other relevant hydraulic and solute transport parameters, such as dispersion coefficients and dispersivities were also established for the tropical soil.     

Sorted bedforms off Western Long Island,New York,USA: Asymmetrical morphology and twelve‐year migration record

Sorted bedforms are widely present in sediment‐starved littoral and inner shelf settings; they are indicators for hydrodynamic conditions and a primary contributor for the subsurface structure. This study investigated the morphology and migration of sorted bedforms on the inner shelf of Long Beach Barrier Island, New York, USA , by repeat geophysical and geological surveys in 2001, 2005 and 2013 (following superstorm Sandy) involving swath bathymetry, backscatter, chirp seismic reflection data and grab sampling. Swath data revealed that the western sector, comprising the western 75% of the survey region, is dominated by NNE –SSW ‐oriented, 0·5 to 1·0 km wide sorted bedforms with highly asymmetrical cross‐sections, with steeper slopes and coarser sands on the eastern (stoss) flanks. Many secondary bedforms were also observed (north–south to north‐east/south‐west oriented lineation structures) at the western edges of coarse sand zones. The eastern sector displays an unusual sorted bedform pattern that is dominated by coarse‐grained substrate, with isolated patches of fine‐grained sands oriented north‐east/south‐west which are 0·15 to 1·0 km in length and ca 30 to 200 m in width, similar in scale and orientation to the secondary bedforms in the western sector. Comparison analysis of the swath data sets indicates that the primary transverse sorted bedform morphology within the western sector was largely stable over this time frame, although the swales were deepened following the storms. The coarse/fine sand boundaries did migrate, however, moving ca 1 to 5 m eastward between 2001 and 2005, and ca 5 to 20 m westward between 2005 and 2013; the higher migration rates (up to 2·5 m year⁻¹) in the latter time period may be attributable to large storm forcing (for example, hurricanes Irene and Sandy). Significant north‐westward migration of the secondary bedforms and coarse sand patches in the western sector, as well as fine sand patches in the eastern sector were also observed; these features are far more mobile than the primary sorted bedforms, possibly because they are fine sand drifts that do not erode into the coarse substrate. Seismic reflection data revealed a transgressive ravinement beneath sorted bedforms, merging with the sea floor at the bottom of swales. The authors hypothesize that long‐term topographic migration of transverse sorted bedforms contributes to the formation and evolution of the ravinement.     

Contourite origin for shelf and upper slope sand sheet, offshore Antarctica

A widespread (3200 km²), thick (10 cm to > 100 cm) sand deposit exists on the continental shelf and upper slope offshore of the Pennell Coast, Antarctica. The sand body occurs at water depths between 200 and 1200 m. The mean grain size of the sand varies from 3·3 phi to 1·2 phi, and the composition is dominantly volcanic. The only source for this volcanic material is Cape Adare, a volcanic headland. Sands are transported up to 70 km from Cape Adare by a westward‐flowing circumpolar boundary current that impinges on the upper slope and shelf. Radiocarbon dates from the sand and from the glacial unit beneath it indicate that the deposit formed within the past 9000 years. The occurrence of this sand sheet demonstrates the ability of contour currents to assemble extensive sand bodies on the shelf and upper slope and the relatively rapid rate at which these deposits may form.     

Velocity of ground water percolation at Indaram, Godavari sub-basin, India

We present values of velocity of ground water percolation (Vg) over large depth intervals, varying from shallow to deeper depths in Indaram area of Godavari sub-basin. The velocities have been estimated using available measured geothermal data. Sub-surface temperatures were measured in seven boreholes. Terrestrial heat flow values are calculated using temperature data and measured values of thermal conductivity of core samples. The results show that Vg is ～3.4 ×10^?7 cm /sec in the top layers (70–150 m) and decreases to ～0.04×10^?7 cm/sec in the deeper levels around 350 m depth and becomes negligibly small thereafter, thereby, indicating that the overall permeability of the sub-surface layers, due to the occurrence of successions of permeable, semi-permeable layers gets reduced to more or less zero at depths around 350 m. The value of Thermal Peclet Number, which is the ratio of the heat transfer through convection to that through conduction, naturally becomes negligible around this depth in the area. The observed consistency of the magnitude of heat flow through various deep sections is a clear indicator that water percolation is practically reduced to zero at depths around 320–400 m and that conduction is the dominant mechanism of heat transfer below the inferred depth section, while the upper layers are dominated by recharge at various depths by near surface water from streams at Indaram.     

Sedimentation in the Conway Trough, a deep near-shore marine basin at the junction of the Alpine transform and Hikurangi subduction plate boundary, New Zealand

The Conway Trough is a 40 km × 10 km, 1000 m deep, rectangular, tectonically controlled sedimentary basin situated on an active plate boundary. The basin lies at the junction of the Alpine transform and Hikurangi subduction sectors of the Indo-Australian/Pacific plate boundary. It is incised into Cainozoic sedimentary rocks and contains a thick fill of late Neogene sediment. Study of the continental shelf near the Conway Trough indicates: (1) that transport mechanisms operate in a net north-easterly direction, and (2) that the neritic sediment drift system is confined to within a few kilometres of the shoreface, with the result that (3) large areas of the shelf south of the trough comprise bare bedrock, or relict late Pleistocene sediment. Despite reaching to within 3 km of shore, the Conway Trough receives little modern bedload sediment, as indicated by a paucity of sand fill. However, the trough has intercepted some Holocene bedload, since the main channel contains redeposited graded gravel and sand with a C-14 age of 4670 yr. Satellite imagery and piston cores reveal that abundant mud is provided to the trough, much of it from river sources up to 200 km to the south. Some of this sediment is inferred to move by turbid layer transport through Conway Trough and out via Kaikoura Canyon into nearby Hikurangi Trough. Nonetheless, sedimentation rates in the Conway Trough may be as high as 1-7 m (1000 yr) ¹. The Conway Trough forms a modern example of a transform basin, varying Neogene examples of which are found widely in north-eastern South Island. Sedimentation models are constructed to compare transform and transduction basins, particularly with respect to the effects of Pleistocene sea-level changes.     

Cat Island platform, Bahamas: an incipiently drowned Holocene carbonate shelf

Analyses of high resolution, seismic reflection profiles and surface sediment samples indicate that the Cat Island shelf is presently in an incipiently drowned state. This small carbonate bank is characterized by a thin (<4 m), coarse-grained, relict sediment cover, along with limited reef development, and a relatively deep (20–30 m) margin indicating that it has been unable to ‘keep-up’ with Holocene sea-level rise. Early flooding at relatively high rates of sea-level rise (4 m kyr^-1, 5–8 × 10³ yr BP) in conjunction with small bank size and relatively low elevation, led to a reduced rate of carbonate accumulation and incipient drowning. The shelf edge currently lies beneath the zone of maximum carbonate production and exposes the interior shelf to open marine conditions which may result in permanent drowning if it is unable to ‘catch-up’ with continued sea-level rise. Sediment facies patterns are largely oriented perpendicular or oblique to the shelf edge and appear to be controlled by shelf circulation patterns focused by bank-margin reentrants. In comparison with most of the northern Bahamas, the Cat Island shelf was flooded earlier and at relatively higher rates of Holocene sea-level rise which led to selective drowning, implying that carbonate platforms need not drown synchronously over widespread areas as commonly thought. The potential rock record of this incipient drowning event would be a thin, open-marine sand sheet of highly degraded cryptocrystalline and aggregate grains associated with poorly developed reefs.     

Offshore Aggregates Resources on the Northern Continental Shelf of the East China Sea

A large‐scale survey for offshore aggregates is carried out on the northern shelf of the East China Sea. Results show that most of them are directly exposed in the Yangtze Shoal and the linear sand ridges system at water depths 25–55 m and 60–120 m, respectively. The components of these deposits belong to fine aggregates in terms of the fineness modulus. The potential resources are as high as 147.8 × 10¹² kg for the Yangtze Shoal yet only 36.68 × 10¹² kg for the sand ridges area, respectively. A preliminary feasibility analysis suggests that the Yangtze Shoal is suitable for dredging under the present conditions of technology and economy.     

Evolution of the barrier islands of southern Long Island, New York

Three lines of evidence based on data from more than 400 boreholes and vibrocores have been used to reconstruct the evolution of the barrier islands during the Holocene transgression in southern Long Island, New York: (1) the Holocene transgressive stratigraphic sequence behind the present barriers, (2) the stratigraphic patterns of the inner shelf, and (3) the morphology of the now-buried late Pleistocene coastal features. The extensive preservation of backbarrier sediments, radiocarbon dated between 7000 and 8000 yr BP, on the inner shelf of southern Long Island suggests that the barriers have not retreated by continuous shoreface erosion alone, but have also undergone discontinuous retreat by in-place ‘drowning’ of barriers and stepwise retreat of the surf zone. Such stepwise retreat of the surf zone has prevented the backbarrier sediments from being reworked. Based on the presence of submerged barrier sand bodies in seismic records, it is inferred that about 9000 years ago, when the sea stood about 24 m below the present sea level, a chain of barriers developed on the present shelf about 7 km offshore of the present barriers. With continued sea-level rise, the – 24 m barrier built upward until the sea reached about – 15 m MSL, just prior to 7000 yr BP. The barriers were then submerged by the rapidly rising sea, and the surf zone shifted rapidly landward to a position about 2 km from the present shoreline. The surf zone overstepped to the landward margin of the old lagoon, which had become fixed at the steep seaward face of mid-Wisconsinan (?) or Sangamonian coastal barriers. During the past 5000 or 6000 years, the shoreface has retreated continuously by about 2 km. Evidence from southern Long Island and elsewhere in regions of coastal submergence indicates that rapid sea-level rise and low sand supply seem to favour the stepwise retreat of barriers, whereas slow rates of submergence and a greater supply of sand generally favour continuous shoreface retreat. Stationary upbuilding, or seaward progradation of barriers may occur when supply of sand is great, and/or submergence is slowed or reversed. Morphologic highs on the pretransgression surface (such as old barrier ridges) tend to fix the migrating barrier shoreline during either continuous retreat, or stepwise retreat of barriers.     

Late Quaternary core stratigraphy of the northern New South Wales continental shelf

On the southeast Australian continental margin, mixed siliciclastic and temperate carbonate sediments are presently forming along the narrow 20–35 km‐wide northern New South Wales shelf over an area of 4960 km². Here, year‐round, highly energetic waves rework inner and mid‐shelf clastic sediments by northward longshore currents or waning storm flows. The strong East Australian Current flows south, sweeping clastic and outer shelf biogenic sands and gravels. Quaternary siliciclastic inner shelf cores consist of fine to medium, lower shoreface sand and graded storm beds of fine to coarse sand. Physically abraded, disarticulated molluscs such as Donacidae and Glycymeridae form isolated gravel lags. Highstand inner shelf clastics accumulate at 0.53 m/10³ y in less than 50 m water depth. Clastic mid‐shelf cores contain well‐sorted, winnowed, medium shoreface sands, with a fine sand component. Fine sand and mud in this area is discharged mainly from New South Wales’ largest river, the Clarence. The seaward jutting of Byron Bay results in weakened East Australia Current flows through the mid‐shelf from Ballina to Yamba allowing the fine sediments to accumulate. Quaternary carbonate outer shelf cores have uniform and graded beds forming from the East Australian Current and are also influenced by less frequent storm energy. Modern clastic‐starved outer shelf hardgrounds are cemented by coralline algae and encrusting bryozoans. Clay‐sized particles are dominantly high‐Mg calcite with minor aragonite and smectite/kaolinite. Carbonate sands are rich in bryozoan fragments and sponge spicules. Distinctive (gravel‐sized) molluscs form isolated shells or shell lag deposits comprising Limopsidae and Pectinidae. The upper slope sediments are the only significant accumulation of surficial mud on the margin (18–36 wt%), filling the interstices of poorly sorted, biogenic gravels. Pectinid molluscs form a basal gravel lag. During highstand the outer shelf accumulates sediment at 0.40 m/10³ y, with the upper slope accumulating a lower 0.23 m/10³ y since transgression. Transgression produced a diachronous (14–10 ka) wave‐ravinement surface in all cores. Relict marine hardgrounds overlie the wave‐ravinement surface and are cemented by inorganic calcite from the shallow and warm East Australian Current. Transgressive estuarine deposits, oxygen isotope Stage 3–5 barriers or shallow bedrock underlie the wave‐ravinement surface on the inner and mid shelf. Northern New South Wales is an example of a low accommodation, wave‐ and oceanic current‐dominated margin that has produced mixed siliciclastic‐carbonate facies. Shelf ridge features that characterise many storm‐dominated margins are absent.     

Lithostratigraphy and depositional environment of the Permian Nowra Sandstone in the southwestern Sydney Basin,Australia

Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becomes progressively younger eastward. The sand may have been supplied by rivers along the western coast but the major source was south of the study area. The lower Nowra Sandstone is separated from the upper part of the formation by an extensive ravinement surface overlain by the Purnoo Conglomerate Member. In contrast to the lower unit, the upper Nowra Sandstone forms a westward thickening wedge that represents a backstepping nearshore sand facies that accumulated during a transgression. The upper Nowra Sandstone passes vertically and laterally eastward into the Berry Siltstone. Thus both boundaries of the Nowra Sandstone are diachronous, first younging eastward and then westward as a response to a regressive‐transgressive episode.     

Onshore record of Hudson River drainage to the continental shelf from the late Miocene through the late Wisconsinan deglaciation, USA: synthesis and revision

Stanford, S. D. 2009: Onshore record of Hudson River drainage to the continental shelf from the late Miocene through the late Wisconsinan deglaciation, USA: synthesis and revision. Boreas, 10.1111/j.1502‐3885.2009.00106.x. ISSN 0300‐9483. Fluvial and glacial deposits in New Jersey, Long Island, and the Hudson valley provide a record of Hudson River drainage since the late Miocene. Late Miocene fluvial deposits record southerly flow across the emerged inner New Jersey shelf. In the late Miocene–early Pliocene this drainage incised, shifted southwesterly, and discharged to the shelf south of New Jersey. During late Pliocene or Early Pleistocene glaciation, discharge to the shelf in the New York City area was established. This drainage incised and stabilized in the Early and Middle Pleistocene and remained open during pre‐Wisconsinan (Oxygen Isotope Stage 6? (OIS‐6?)) and late Wisconsinan (OIS‐2) glacial advances. During late Wisconsinan retreat, moraine deposits dammed the valley at the Narrows to form Lake Albany. From 19 to 15.5 kyr BP (all dates in ¹⁴C yr), Hudson drainage was directed eastward into the Long Island Sound lowland. Drainage of Lake Wallkill into Lake Albany at 15.5 kyr BP breached the Narrows dam and initiated the unstable phase of Lake Albany, which was controlled by eroding spillways, first on the moraine dam, then on emerged lake‐bottom in the mid‐Hudson valley. Marine incursion between 12 and 11 kyr BP limited fluvial incision of the lake bottom, stabilizing the Quaker Springs, Coveville, and upper Fort Ann spillways. Lowering sea level between 11 and 10 kyr BP allowed incision from the upper to lower Fort Ann threshold. Sediment eroded by lake outflows between 15 and 10.5 kyr BP was trapped in the glacially deepened lower valley. Little inland sediment reached the shelf after 20 kyr BP.     

Evaluation of the bearing capacity of near-surface soils using integrated methods: A case study of Otukpa,Ogbadibo Lga,Benue state,North-Central Nigeria

The load bearing capacity of soils in Otukpa area of Benue state, Nigeria was evaluated by characterizing the geotechnical, geophysical and structural properties of the soil. The evaluation was necessary because aside the area holding good prospect for future increase in human population, sparsely distributed low-weight buildings in the area have cracks and other signs of failure. Remote sensing/GIS, geophysical and geotechnical approaches were integrated to achieve the objective. 2-D tomography and processed remotely sensed data confirmed the presence of areas of strength especially to the north. Stratigraphy of the northern portion consist of four layers; loose top soil (0-0.26m), compact clayey sand (0.26m-3.33m), dry clayey sand (3.33m-74.21m) and saturated clayey sands (74.21m-201.58m) While the stratigraphy in the south is slightly different; compact topsoil (0-1.2m), loose sand (1.2m-10.81m), compact sand (10.82m-19.71m) and compact clay sand (19.71m-56.03m). The plastic limit range from 11-17% with average specific gravity of 2.64g/cm³. Permeability ranges from 7.8×10^-6m/sec to 3.36×10^-4m/sec while volume compressibility under 250kN ranges from 7.27×10^-5m²/kN to 1.3×10^-4m²/kN and the rate of consolidation under 250kN is within 12.43 m²/year to 1.33m²/year. MDD peaks at 1.85g/cm³ at an OMC of 12.11% and is lowest at 1.67g/cm³ at an OMC of 11.99%. UCS ranges from 31-45kN/m² while shear strength is between 258.81kN/m² to 338 kN/m². Due to the variability of engineering properties of soils, the southern part of the area is preferred for locating the buildings. Generally, careful foundation design in the area is necessary.     

Depositional controls on Palaeoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA

Shallowing‐upward, decametre‐scale, Palaeoproterozoic iron formation cycles in northern Wisconsin record the combined effects of tectonism and changing oceanographic conditions on a storm‐dominated shelf. Cycles consist of a lower unit of laminated, Fe‐ and Si‐rich chemical mudstone that is transitional into an upper unit dominated by trough cross‐stratified chert grainstone. Grainstone lenses become progressively thicker upwards in cycles with the largest at cycle tops, where they are sharply overlain by a unit of slumped chemical mudstone. The cycles developed through progradation when offshore‐directed storm currents transported chert sand intraclasts that were formed in nearshore settings into middle and distal shelf environments. Abrupt subsidence events, probably resulting from normal faulting associated with extensional tectonism, repeatedly terminated chert grainstone accumulation and may also have generated the slumped units at cycle boundaries. The episodic storm currents are also interpreted to have transported biologically oxygenated waters from the shallow‐water, inner shelf into otherwise anoxic bottom waters of the strongly stratified distal shelf. The consequence of such transport and mixing was rapid deposition of chemical mud, mainly as precipitated Fe‐oxide. In many cases, the resultant decrease in Fe²⁺ in the water column, together with pelagic inorganic precipitation of SiO₂ and rainout of terrigenous clays, resulted in submillimetre‐ to millimetre‐thick, chemically graded laminae. The concomitant decreasing Fe²⁺/Mn²⁺ ratio also led to increasing Mn‐compound precipitation and enrichment in the upper portions of some chemically graded layers.     

Bed thickness characteristics of inner-shelf storm deposits associated with a transgressive to regressive Holocene wave-dominated shelf, Sendai coastal plain, Japan

Holocene inner-shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north-eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave-dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) ¹⁴C ages and the origin of bed thickness variations in the inner-shelf storm deposits is explored. The Holocene succession is <30 m thick and overlies latest Pleistocene to early Holocene non-marine deposits above a transgressive ravinement surface. It comprises transgressive ravinement and inner-shelf deposits, and regressive inner shelf, shoreface, and coastal plain deposits. The inner-shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds. The average preservation interval of a single storm bed is shortest during the transgression (5·7–20·6 years), and then increases to a maximum during the early regression (83·3–250·0 years), decreasing to 7·7–31·3 years with shoreline progradation. Average accumulation rates decreased during the transgression and then increased during the regression, but the sand/mud ratio varies little, reflecting inefficient sediment segregation downdip on the inner shelf. The vertical pattern of sand-layer thicknesses also shows no relationship to position within the cycle, although small-scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and/or random storm processes. The average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low-magnitude storms during this period of low accumulation rates and extensive reworking. This preservation bias and the nature of the Sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits.     

Copper in surface waters of the Bering Sea

Copper concentrations have been measured in surface <75 m waters of the central Bering Sea. Concentrations of 2–4 nmole kg^?1 were measured in the Zhemchung Canyon region where water depths are greater than 1000 m. Concentrations are higher 2–25 nmole kg^?1 on the shallow <100 m continental shelf, inshore of a hydrographic front at the 100 m isobath. Copper-depth profiles on the continental shelf water mass are dominated by Cu concentrations increasing toward the sediments. These trends may be maintained by a flux of Cu from surficial sediments. A frontal system over the 100 m isobath acts to control the flux of Cu, a significant part of which apparently emanates from the sediments, between the continental shelf and the central Bering Sea basin. The benthic shelf Cu flux was constrained to be less than 3 nmole cm^?2 yr^?1.     

Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia

Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m³ s^?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m³ s^?1). About 3·7 × 10⁶ tonnes of suspended sediment and 3 × 10⁵ tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.     

Hydrogeology of Al-Lith Sabkha,Saudi Arabia

In Saudi Arabia, coastal sabkhas cover extensive areas along the coasts of the Red Sea and Arabian Gulf in addition to the continental sabkhas scattered in many places inland. Al-Lith sabkha is one of the typical coastal sabkhas located along the Red Sea coast. Sabkhas, in general, pose a number of geotechnical problems and need to be carefully investigated before being urbanized.A generalized geologic section in Al-Lith sabkha indicates a salty crust at the surface followed by yellowish brown silt and silty sand, olive gray silt and sandy silt and bottomed by coralline reefal limestone. Within this succession, there are several isolated lenticular bodies of sandy silt, silty sand and shelly silty sand. The clay minerals constituting the fine-grained portion of the soil are, in decreasing order, kaolinite, illite and montmorillonite in addition to minor chlorite.The depth to groundwater in 17 observation wells ranged from 0.18 to 1.81 m with a maximum fluctuation of 0.60 m between summer and winter. The permeability of the top silt layer was found to be very low with an average of 5.4×10⁻⁴ m/day. A pumping test was performed in a deep well penetrating the coralline limestone. The measured permeability is 1.1×10² m/day and the estimated storage coefficient is 4×10⁻⁵.Soil water evaporation was measured using a lysimeter constructed with undisturbed soil samples having different depths to the water level. The rate of evaporation ranges from 2.8 to 27.8 ml/day decreasing with an increase in depth to the water level.Groundwater samples were analyzed for their major anions and cations. Salt concentrations show a general increase toward the sea except for the calcium and carbonates that show a landward increase. The groundwater could be classified as a Cl+SO₄ brine. The salinity of the groundwater was determined at different depths in the pumping well and was found to be low in the top 4 m. It sharply increases until it reaches a value approximately 10 times the salinity of the top layer indicating groundwater intermixing with freshwater and salt-water intrusion. The change in the salinity during pumping was erratic but within a range of 2%.     

Characterization of the shallow groundwater system in an alpine watershed: Handcart Gulch,Colorado, USA

Water-table elevation measurements and aquifer parameter estimates are rare in alpine settings because few wells exist in these environments. Alpine groundwater systems may be a primary source of recharge to regional groundwater flow systems. Handcart Gulch is an alpine watershed in Colorado, USA comprised of highly fractured Proterozoic metamorphic and igneous rocks with wells completed to various depths. Primary study objectives include determining hydrologic properties of shallow bedrock and surficial materials, developing a watershed water budget, and testing the consistency of measured hydrologic properties and water budget by constructing a simple model incorporating groundwater and surface water for water year 2005. Water enters the study area as precipitation and exits as discharge in the trunk stream or potential recharge for the deeper aquifer. Surficial infiltration rates ranged from 0.1–6.2×10^?5 m/s. Discharge was estimated at 1.28×10^?3 km³. Numerical modeling analysis of single-well aquifer tests predicted lower specific storage in crystalline bedrock than in ferricrete and colluvial material (6.7×10^?5–2.0×10^?3 l/m). Hydraulic conductivity in crystalline bedrock was significantly lower than in colluvial and alluvial material (4.3×10^?9–2.0×10^?4 m/s). Water budget results suggest that during normal precipitation and temperatures water is available to recharge the deeper groundwater flow system.     

Seepage loss from a canal in Sri Lanka measured using tritium tracer

Using single borehole dilution technique, and tritiated water as tracer, lateral flow rate of water near the phreatic line has been measured at 24 points along the banks of an unlined canal. Permeability values range from 0.5×10^?6 m sec^?1 to 34×10^?6 m sec^?1; the median value being 5×10^?6m sec^?1. The seepage loss is estimated to be 5×10^?5 m³ sec^?1 m^?1. For the entire canal the value is 0.74 m³ sec^?1 which is 12% of the total flow. Limitations and merits of the technique are discussed.