Bed-load Systems Research Articles

Strengths and weaknesses of acoustic pipe microphone systems in ephemeral, sandy, gravel‐bed rivers

AbstractWe calibrated an acoustic pipe microphone system to monitor bedload flux in a sandy, gravel‐bed ephemeral channel. Ours is a first attempt to test the limit of an acoustic surrogate bedload system in a channel with a high content of sand. Calibrations varied in quality; significant data subsetting was required to achieve R2 values >0.75. Several data quality issues had to be addressed: (1) apparent pulses, which occur when a sensor records an impulse from sediment impacting the surrounding substrate rather than directly impacting the sensor, were frequent, especially at higher signal amplifications. (2) The impact sensors were frequently covered by gravel sheets. This prompted the development of a cover detection protocol that rejected part of the impact sensor record when at least one sensor was partially or fully covered. (3) Because of the lack of sensor sensitivity to impacts of sand‐sized particles, which was anticipated, and the considerable sand component of bedload in this channel, a grain size‐limited bedload flux was estimated. This was accomplished by sampling the bedload captured by slot samplers and evaluating the variation of grain size with increasing flow strength. This considerably improved the results when compared to attempts at estimating the flux of the entire distribution of grain sizes. This calibration is a successful first attempt, though the impact sensors required several site‐specific calibration steps. A universal set of equations using impact sensors to estimate bedload transport of fine‐gravel with a large content of sand remains elusive. Notwithstanding, our study demonstrates the utility of impact sensor data, producing relatively low root mean square errors that are independent of measurements of flow strength (i.e. discharge). These tools will be particularly useful in settings that would benefit from new methodologies for estimating bedload transport in sand‐rich gravel‐bed rivers, such as the American desert Southwest.

The late Quaternary evolution of the Negro River, Amazon, Brazil: Implications for island and floodplain formation in large anabranching tropical systems

The Rio Negro has responded significantly in the Late Pleistocene and Holocene to lagged environmental changes largely associated with activity during the last glacial in the Amazon basin. On the basis of geological structure, the Rio Negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles. No deposits of the Upper Pleniglacial were recognized in the field. The oldest recognizable Late Pleistocene alluvial unit is the Upper Terrace of Middle Pleniglacial age (ca. 65–25 ka) (reach I), tentatively correlated with the oldest terrace identified on the left bank of reach III. At that time, the river was mainly an aggradational bed load system carrying abundant quartz sand, a product of more seasonal conditions in the upper catchment. The late glacial (14–10 ka) is represented by a lower finer-grained terrace along the upper basin (reach I), which was recognized in the Tiquié, Curicuriarí, and Vaupes rivers. At that time, the river carried abundant suspended load as a response to climatic changes associated with deglaciation. Since about 14 ka, the river has behaved as a progradational system, infilling in downstream series a sequence of structurally controlled sedimentary basins or ‘compartments,’ creating alluvial floodplains and associated anabranching channel systems. Reach II was the first to be filled, then reach III, both accumulating mainly sand. Fine deposits increase downstream in reach III and become predominant in some anabranch islands of the distal reach. The lowermost reaches of the Negro (V and VI) have been greatly affected by a rising base level and associated backwater effect from aggradation of the Amazon during late glacial and recent times. Reach V has acted almost entirely as a fine sediment trap. The remarkable Anavilhanas archipelago is the product of Holocene deposition in the upper part of this sedimentary basin; however, suspended sediment load declined about 1.5 ka, prior to the lower part of this basin becoming infilled. The progradational behavior of the Rio Negro, filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones, illustrates how a large river system responses to profound changes in Late Quaternary base level and sediment supply. The most stable equilibrium conditions have been achieved in the Holocene in reaches IIb and IIIa, where an anabranching channel and erosional–relictual island system relatively efficiently convey water and sediment downstream. Reaches IIIb and V never achieved equilibrium conditions during the Holocene, characterised as they are today with incomplete floodplains and open water.

Architecture of channel bodies and paleovalley fills in high-frequency Carboniferous sequences, Sydney Basin, Atlantic Canada

ABSTRACT Fluvial channel bodies in the Pennsylvanian Sydney Mines Formation are contained within high-frequency sequences that represent relative sea-level fluctuations, linked to climate change, on a gently subsiding craton. Channel-body style within coastal-plain deposits was controlled by sea-level transit cycles, modulated by climate and substrate resistance. Coarse bed-load systems are deeply incised, multi-storey bodies up to 30 m thick. Some represent valleys backfilled during late lowstand and transgression, probably under a humid climate. Sinuous channel systems display well developed lateral accretion surfaces and ridge-and-swale topography. Distributary systems are narrow channel bodies that cut bayfill deposits. These two groups lie within highstand deposits, and represent the readvance of coastal rivers following maximum transgression. Sand-flat systems consist of plane-bedded sand sheets and downstream-accreting macroforms, laid down under ephemeral flow conditions, adjacent to deep sinuous channels. They are interpreted as channel and valley fills generated during late highstand, falling stage, and lowstand under a strongly seasonal climate. Calcrete caps on some sandflat bodies suggest that they filled prior to the main lowstand phase of paleosol development. A complex relationship is apparent between channel or valley fills and mature paleosols that bound sequences. Resistant substrates (coals, indurated limestones) influenced channel-body geometry by promoting lateral expansion and localising incision. 1 Present Address: Nova Scotia Community College, Institute of Technology Campus, P.O. Box 2210, Halifax, N S, Canade B3J 3C4; E-mail: batsonpA@it.nscc.ns.ca End_Page 138------------------------

Multiparticle simulation methods applied to the micromechanics of bed load transport

We solve the Newtonian equations of motion to follow the trajectories of each of a large number of two‐dimensional circular bed load particles as they move in response to stresses exerted by an overlying fluid. The fluid is modeled as a moving layer or “slab” which exerts a velocity‐dependent drag force on embedded particles and satisfies its own momentum balance equation. Bed load mass flux and hop length and hop height statistics of simulated beds resemble those observed in the laboratory. By marking particles originally residing on the surface, vertical mixing of sediment is investigated. An injection of fine particles into a bed load system of coarse particles under traction illustrates the early stages of the evolution of sedimentary stratigraphy. Friction angles and their distribution are investigated as a function of bed transport history and particle size. The main thrust of the paper, however, is to describe and test a microscopic bed load model which engineers, geologists, and biologists may find useful for applications where particle arrangement and relative particle motion are important.

A Predictive Model of Reservoir Continuity in Fluvial Sand Bodies of a Lacustrine Deltaic System, Colton Formation, Utah

Abstract The Paleocene/Eocene Colton Formation of east-central Utah is interpreted to be a fluvial-dominated lacustrine deltaic sequence that prograded from the southeast into ancient Lake Uinta. Predictability of reservoir continuity is based largely on understanding the nature of impermeable mudstone drapes within multilateral and multistory sandstone bodies. Fluvial sandstones are categorized into three systems according to associated facies, channel morphologies, and percent sandstone found within the section. Suspended-load systems dominate the western distal end of Colton exposures and are characterized as follows: meandering to sinusoidal fluvial systems, abundant multilateral accretion sets, and low sandstone percent (< 30%). Reservoir characteristics include poor porosity (< 9%) and permeability (< 0.6 md), poor reservoir continuity, small reservoir size, and good potential for development of numerous hydrocarbon traps. Mixed-load systems are typified by sinusoidal fluvial channels, large multilateral accretion sets, and intermediate sandstone percent (25-45%). Reservoirs display good porosity (approx. 18.5%) and permeability (< 50 md), intermediate reservoir continuity, intermediate to large reservoir size, and good potential to develop numerous hydrocarbon traps. Bed-load systems are characterized by relatively straight channels, very few point bar sequences, and high sandstone percent (< 45%). Reservoir characteristics display very good porosity (19.6-22.6%) and permeability (0.133-2.657 darcies), good reservoir continuity, large reservoir size, and less potential for hydrocarbon trap development. Distributary mouth bar sandstones display excellent reservoir characteristics including good porosity (20%) and the most homogeneous permeability (< 200 md) of the four volumetrically important sandstone facies. Recognition criteria of the four reservoir sandstone facies include sandstone percent in the section, grain size, thickness of the sandstone body, thickness of the sandstone interval, porosity and permeability of sandstones, and the nature of mudstone drapes between multilateral and multistory sandstone bodies. Improved technology in horizontal drilling may be used to link large noncommunicating multilateral accretion sets.

Preliminary Observations of Modern Point Bar Facies, Amite River, Louisiana: ABSTRACT

ABSTRACT The modern Amite River, an incised, flashy discharge stream, represents a coarse sand and gravel bedload system typical of the Florida Parishes of southeastern Louisiana. Preliminary investigation provides general descriptions of channel morphology and sedimentology which permit the recognition of distinct point-bar facies. Bedform and stratification types are used to differentiate lower bar, upper bar, and chute-bar facies. The lower bar facies contain transverse bars, scour pits, dunes, and ripples. A vertical sequence reveals poorly defined tabular and trough cross-stratifictions and horizontal stratifications. The upper bar facies appear similar to a longitudinal bar with superimposed ripples. A vertical sequence shows small scale (less than 5 cm thick) or medium scale (5 to 15 cm thick) trough and tabular cross-stratifictions, horizontal laminations, ripple-drift cross-laminations, and clay drapes. The chute-bar facies is characterized by coalescing lobate bars with superimposed ripples. A vertical sequence displays large scale (greater than 15 cm thick) tabular cross-stratifictions, small- or medium-scale trough cross-stratifications, ripple-drift cross-laminations, and clay drapes. Evaluation of observed facies characteristics indicates that the distribution of bar facies and the development of vertical sedimentary profiles appears to be related to the degree of meander curvature. Further research is being initiated to construct a semi-quantitative geomorphic and sedimentologic facies model useful to investigators of both modern and ancient fluvial systems.

Sedimentology of Volcaniclastic Deposits from 1971-1974 Eruption Cluster of Volcano Fuego, Guatemala: ABSTRACT

Volcaniclastic sedimentation during and subsequent to the 1971-74 eruption cluster of the Volcano Fuego in Guatemala has occurred in four distinct phases which are part of a 15 to 25 year cycle of sedimentation. In phase 1, the eruption cluster generated 6 × 108 cu m of tephra, one-third in the form of glowing avalanches, the remainder as an elongate airfall ash blanket west-southwest of the cone. Glowing avalanches with a volume of 5 × 107 cu m formed two fans, each 1 to 3 m thick, east and west of the crater. Further avalanches flowed down seven narrow canyons radiating to the south of the crater forming 40-m-thick deposits totaling 1.3 × 108 cu m. During phase 2, debris flows and flash floods removed about one-third of th phase 1 canyon deposits in the first 2 years following eruption. Fan deposits remained intact. Three digitate, 1 to 2.5-m-thick, debris-flow deposits (2.2 × 107 cu m) and two 1-m-thick flood fans (1.8 × 107 cu m) formed south of the crater. In phase 3, terraced, meandering, suspended-load streams were metamorphosed to braided, aggrading, bed-load systems annually eroding 6 million tons of phase 1 and 2 debris, primarily from the canyon deposits. Transport of about two-thirds of this debris to the sea has produced rapid coastal progradation. During phase 4, 15 to 25 years of phase 1 and 2 activity will remove canyon avalanche deposits, redistributing the material in stable fans on the lower volcanic slopes. Phase 1 and 2 processes become inactive while str am incision produces discontinuous terracing. Fluvial systems return to meandering, suspended-load streams. End_of_Article - Last_Page 843------------