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The erosion process at a bridge abutment may be affected by the flow constriction when the abutment occupies a significant part of the flume width. We devised a specific experimental campaign to investigate the effect of the obstruction ratio (i.e., the ratio between the abutment length and the channel width) on the erosion depth: we performed homogeneous series of clear-water scour experiments in each of which the obstruction ratio was the only parameter that varied. The experimental results are presented in light of a dimensionless framework. It was found that the effect of the obstruction ratio on the time development of local scour depth may not be large; nevertheless, significant effect can be observed, even for relatively small values of the parameter during the earliest phases. The latter are important, for example, for the step-by-step modeling of the erosion development under unsteady flow conditions. We propose a simple equation to quantify the scour enhancement due to the flow constriction in clear-water abutment scour. DOI: 10.1061/(ASCE)0733-9429(2009)135:2(140) CE Database subject headings: Bridge abutments; Scour; Erosion; Flumes.

The continuous discharge of coalbed natural gas-produced (CBNG-produced) water within ephemeral, cohesive channels in the Powder River Basin (PRB) of Wyoming can result in significant erosion. A study was completed to investigate channel stability in an attempt to correlate cohesive soil properties to critical shear stress. An in situ jet device was used to determine critical shear stress ([t.sub.c]) and erodibility ([k.sub.d]); cohesive soil properties were determined following ASTM procedures for 25 reaches. The study sites were comprised of erodible to moderately resistant clays with [t.sub.c] ranging from 0.11 to 15.35 Pa and [k.sub.d] ranging from 0.27 to 2.38 [cm.sup.3]/N x s. A relationship between five cohesive soil characteristics and [t.sub.c] was developed and presented for use in deriving [t.sub.c] for similar sites. Allowable discharges for CBNG-produced water were also derived using [t.sub.c] and the tractive force method. An increase in the allowable discharge was found for channels in which vegetation was maintained. The information from this case study is critical to the development of a conservative methodology to establish allowable discharges while minimizing flow-induced instability. DOI: 10.1061/(ASCE)0733-9429(2008)134:12(1677) CE Database subject headings: Cohesive soils; Effective stress; Shear stress; Submerged jet; Tractive force; Wyoming; River basins.

Between 1999 and 2005, drought in the western United States led to a >44 m fall in the level of Lake Powell (Arizona-Utah), the nation's second-largest reservoir. River discharges to the reservoir were halved, yet the rivers still incised the tops of deltas left exposed along the rim of the reservoir by the lake-level fall. Erosion of the deltas enriched the rivers in sediment such that upon entering the reservoir they discharged plunging subaqueous gravity flows, one of which was imaged acoustically. Repeat bathymetric surveys of the reservoir show that the gravity flows overtopped rockfalls and formed small subaqueous fans, locally raising sediment accumulation rates 10-100-fold. The timing of deep-basin deposition differed regionally across the reservoir with respect to lake-level change. Total mass of sediment transferred from the lake perimeter to its bottom equates to ~22 yr of river input.

This paper analyzes the effect of flume test section length on sediment erodibility measurements. A modular flume was constructed and experiments were conducted with two test section lengths: 0.15 and 1.10 m. The internal height and width of the flume were 0.11 and 0.13 m, respectively. A fine (7 [micro]m) commercially available quartz sediment was used for the tests. The expectation was that the shorter flume test section would experience a significantly higher erosion rate (per unit surface area) due to its greater sensitivity to edge effects (i.e., scour) at the entrance and exit of the flume test section. However, the measured erosion rates at comparable bottom stresses were only 35% greater in the short test-section tests. These results were consistent with the lack of significant scour development at the entrance or exits of the test sections. Hence, flume test section length alone does not appear to significantly affect erodibility measurements provided edge effects (i.e., scour) are minor. CE Database subject headings: Sediment transport; Cohesive sediment; Flumes; Erosion; Pollution.

The paper reports on an experimental investigation concerning two important issues: (1) local scour and (2) riprap stability at a 45[degrees] wing-wall abutment in a degrading river bed of noncohesive sediment. The abutment considered was short (that is abutment length/flow depth CE Database subject headings: Abutments; Degradation; Erosion; Scour; Riprap; Streambeds.

The Brahmaputra River slices an exceptionally deep canyon through the eastern Himalaya. Fission-track and laser-ablation U-Pb ages of detrital zircon grains from the river document very rapid erosion from this region and its impact on sediment fluxes downstream in the Brahmaputra. Downstream from the canyon, 47% of the detrital zircons in the river's modern sediment load comprise a fission-track age population averaging only 0.6 Ma. Equally young cooling ages are reported from bedrock in the canyon through the Namche Barwa-Gyala Peri massif but are absent from riverbank sands of major tributaries upstream. Simple mixing models of U-Pb ages on detrital zircons from samples taken above and below this massif independently suggest that 45% of the downstream detrital zircons are derived from the basement gneisses extensively exposed in the massif. Constraints on the extent of the source area provided by bedrock cooling ages together with sediment-flux estimates at Pasighat, India, suggest exhumation rates averaging 7-21 mm [yr.sup.-1] in an area of ~3300 [km.sup.2] centered on the massif. This rapid exhumation, which is consistent with the very young cooling ages of the detrital zircons from this area, produces so much sediment that ~50% of the vast accumulation in the Brahmaputra system at the front of the Himalaya comes from only ~2% of its drainage. This extreme localization of rapid erosion, sediment evacuation, and bedrock cooling bear on (1) common assumptions in geodynamic and geochemical studies of the Himalaya about sources of sediment, and (2) plans for hydroelectric development and flood management in southeastern Tibet and the heavily populated areas of eastern India. Keywords: Himalaya, Yarlung-Tsangpo River, Siang River, Brahmaputra River, fission-track dating, U-Pb ICP-MS dating, detrital-mineral thermochronology.

This paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence. We show that until the Neoarchean, continents were unable to sustain topography >2500 m. During the Neoarchean the continental lithosphere evolved through a rheological threshold, allowing for the development of significant topography. The consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the Earth's geochemical reservoirs. The Neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values. We propose that during the Neoarchean the exogenic envelopes, which were until then coupled to the mantle through hydrothermal processes and volcanism, also became coupled to the continental crust through reliefgenerating tectonics processes and erosion, hence changing the balance between mantle versus crustal interaction with the exogenic Earth. Keywords: Archean, tectonics, geochemistry, erosion, Earth evolution, modeling.

Streambank erosion is an important management issue, particularly for meandering rivers. Recently, bendway weirs have become popular control measures for bank erosion along small meandering streams in the agricultural Midwest. Although these structures have successfully mitigated bank erosion in some cases, there is evidence that the weirs do not always perform as anticipated. Scientific understanding of how bendway weirs influence flow dynamics, streambank erosion, and aquatic habitat is limited. Current design criteria are based primarily on expert judgment rather than a formalized technical design procedure. At field-scale studies, the present paper represents a first step toward an integrated geomorphological and engineering evaluation of the performance of bendway weirs in rivers. To accomplish this initial phase, three-dimensional (3D) velocity data were collected on Sugar Creek at Brookside Farm, Ill., and 3D numerical simulations for low-flow conditions were performed to validate the computational fluid dynamic model. Overall results show good agreement between measured and simulated data for streamwise velocities and turbulence kinetic energy. The model is less accurate at predicting the velocity and turbulence kinetic energy in the shear layer immediately downstream from the weir tips. Based on the validation for low-flow condition, 3D simulations were carried out for medium and high flows where the bendway weirs are completely submerged. These simulations indicate that 3D patterns of flow, especially flow near the outer bank, change dramatically with changes in flow stage. Flow patterns at high-flow condition indicate that bank retreat over the tops of weirs is associated with locally high-shear stresses, thus producing a 'shelf' along the base of the outer bank as observed in the field. DOh 10.1061/(ASCE)0733-9429(2008) 134:8(1052) CE Database subject headings: Bank erosion; Meandering streams; Secondary flow; Turbulent flow; Acoustic techniques; Three-dimensional flow; Hydraulic structures.

Four in situ experiments on sediment erosion characteristics were conducted at the Anacostia River that runs through Washington, D.C. Supplemental erosion rate data were also obtained by carrying out five laboratory experiments using sediment samples collected at the field. In laboratory experiments, the sediment samples were mixed with tap water and placed in the flume to form beds for finding the difference in terms of erosion characteristics caused by different sediment composition among the five samples. This approach enables the finding of erosion characteristics for the entire tidal Anacostia River with limited resources. The in situ measured critical bed-shear stresses [[tau].sub.cr] for erosion at the water-sediment interface z=0 varies from 0.03 to 0.08 Pa. Field results indicated that [[tau].sub.cr] (z) increases with the depth z and becomes more than 0.6 to 0.7 Pa with an erosion thickness of less than 1 cm. Sediment beds prepared at a laboratory appear having an upper limit on how much [[tau].sub.cr](Z) can be developed. DOI: 10.1061/(ASCE)0733-9429(2008) 134:8(1102) CE Database subject headings: Rivers; Sediment; Erosion; In situ tests; Experimentation; Washington D.C.

[1] We present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of Central Nepal: the KaliGandaki and the Arun drainage basins. Modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate. Our analysis of the stream power in the whole river network confirms the work of other authors that a 50-to 80-km-wide zone, roughly corresponding to the High Himalayan topography, is subjected to rapid rock uplift. We suggest a model where the uplift of this zone is driven by erosion and isostatic response, so that centers of maximum uplift are located within the main channels of the north-south draining rivers. We also suggest that the rate of uplift slows down with increasing distance to the main channels. Such a spatial distribution of the uplift leads ultimately to the formation of fiver anticlines as observed along all major Himalayan rivers. We propose that the formation of fiver anticlines along south draining Himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen-parallel drainages on the north side of the range. This may follow successive headward cutting into the Tibetan Plateau. The model is confirmed by differences between main channels and east-west running tributaries. Time-dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel. Citation: Robl, J., K. Stuwe, and S. Hergarten (2008), Channel profiles around Himalayan river anticlines: Constraints on their formation from digital elevation model analysis, Tectonics, 27, TC3010, doi:10.1029/2007TC002215.

An understanding of bridge scour mechanisms during floods in a fluvial river is very important for cost-effective bridge foundation design. Reliable bridge scour data for flood events are limited. In this study, field experiments were performed at the Si-Lo Bridge in the lower Cho-Shui River, the longest river in Taiwan, to collect scour-depth data using a sliding magnetic collar, a steel rod, and a numbered-brick column. By separating each scour component, a methodology for simulating the temporal variations of the total scour depth under unsteady flow conditions is proposed. The proposed total-scour model integrates three scour components, namely general scour, contraction scour, and local scour. The collected field data, comprising both general scour and total scour depths, are used to validate the applicability of the proposed model. Based on the peak flow discharges during floods, a comparison of the local scour depths calculated using several commonly used equilibrium local scour formulas indicates that most equations may overestimate the local scour depth. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(810) CE Database subject headings: Bridges, piers; Scour; Floods; Measurements; Erosion; Deposition; Unsteady flow.

A robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors is presented. The physically based model simulates the three main processes that shape incised streams: hydraulics, sediment transport, and streambed and bank adjustments. A generalized implicit Preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals. The solution method introduces several enhancements that increase its robustness, specifically to simulate flashy flows. Transport of cohesive or cohesionless graded bed material is based on a total-load concept, and suspended and bed load transport modes are accounted for through nonequilibrium effects. The model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material. The present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds, and reports simulations of published experiments on degrading and aggrading channels with graded bed material. Description and validation of the model's streambank erosion component and the application of the model to incised stream systems are presented elsewhere. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(749) CE Database subject headings: Computer models; Channel morphology; Open channel flow; Bank erosion; Sediment transport; Streams.

Comprehensive geological surveys have revealed the physiographical and sedimentological characteristics of the Kusbiro Submarine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacific. The forearc slope has a convex-upward geometry that can be divided into upper and lower parts separated by an outer-arc high (3200-3500 m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimentary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250 m of thalweg water depth) are developed on the upper slope, showing a weakly concave-upward longitudinal profile with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infill of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900 m thalweg water depth) is inactive, the lower part of the upper reaches (900-3250 m thalweg water depth) is considered to be an active conduit to the lower reaches, as determined from voluminous turbidites recovered in sediment cores (~76-yr intervals) and rockfalis observed in the canyon bottom by deep-sea camera. A number of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbidity currents, giving rise to a down-canyon increase in the frequency of flow events. The down-canyon increase in flow occurrence is related to a gradual decrease in gradient, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250-7000 m thalweg water depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon of the confluence with the Hiroo Submarine Channel, which is the largest tributary of the main canyon, indicates that the erosional force of turbidity currents decreases down-canyon. The gradient of the lower reaches largely reflects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by subduction-related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and tectonic processes along the forearc slope are important in the development of the physiography of submarine canyons upon active forearc margins. Keywords: Submarine canyon, forearc slope, turbidite, erosion, Kuril Trench, Japan.

Erodibility of a soil is defined here as the relationship between the erosion rate of a soil dz/dt and the velocity v of the water flowing over it, or the relationship between the erosion rate of a soil dz/dt and the shear stress developed by the water at the water-soil interface. This is called the erosion function. The test used to measure the erosion function of the levee soils is the erosion function apparatus test. The test consists of eroding a soil sample by pushing it out of a thin wall steel tube and recording the erosion rate for a given velocity of the water flowing over it. Several velocities are used and the erosion function is defined. A new erosion category chart is proposed to reduce the erodibility of a soil or rock to a single category number. Twenty three samples were retrieved from 11 locations at the surface of the levees around New Orleans. Thirteen were samples from Shelby tubes while ten were bag samples. The results obtained show a large variation of erosion resistance among the soils tested. Some of the levees associated with the location of the samples resisted the overtopping erosion very well; others eroded completely. On the basis of the erosion test results and of the observed behavior of the levees during the hurricane, a chart is presented which can be used to select soils for overtopping resistance. Numerical simulations were performed using the program CHEN 3D to obtain the distribution of velocity vectors in the overtopping flow and of shear stresses at the interface between the water and the levee surface. The comparison of the numerical simulation results and of the erosion function gives added credibility to the proposed levee overtopping erosion chart. CE Database subject headings: Levees; Erosion; Louisiana; Hurricanes.

The development of coral reef framework and the preservational character of both in-situ and rubble coral is strongly influenced by a range of physical, chemical and biologically-mediated taphonomic processes. These operate at, or just below, the reef framework-water interface and can be defined as having either a constructive or destructive effect upon primary reef framework (i.e., coral) constituents. Constructional activities add additional calcium carbonate to the primary framework structure via secondary framework growth and early cementation. Destructive processes, which remove or degrade primary (and secondary) framework carbonate, are associated with the effects of either physical (mainly storm) disturbance or biological erosion (termed bioerosion). Key bioeroding groups include the grazing fish and echinoid groups, as well as the activities of an array of infaunal borers. These include specific groups of sponges, bivalves and worms (termed macroborers), as well as cyanobacteria, chlorophytes, rhodophytes and fungi (termed microborers). The relative importance of each process and the rates at which they operate vary spatially across individual reef systems. In addition, many of these processes leave distinctive signatures on, or in, the coral framework. In some cases (e.g., calcareous encmsters) these are the skeletons of the organisms themselves, whilst in other cases the organism may leave behind a trace of their activity (e.g., macro- and microborers). These represent useful palaeoenvironmental tools, firstly because they often have good preservation potential and, secondly because the range and extent of many of the individual species, groups and processes involved exhibit reasonably well-constrained environment and/or depth-related distributions. As a result these taphonomically important organisms or processes can be used to delineate between reef environments in core or outcrop, and to aid the interpretation of reef depositional processes and 'events'. This review summarises current understanding regarding the distribution of these species/processes within contemporary reef settings and considers the suites of taphonomic signatures that may aid in the recognition and interpretation of depositional environments and events. Keywords: reef taphonomy; calcareous encrusters; microborers; macroborers; cements; palaeoecology; palaeoenvironments

Attempts to include river bank erosion predictors within morphological models are becoming increasingly common, but uncertainty surrounds the procedures used to couple bed deformation and bank erosion submodels in a way that maintains the mass continuity of eroded bank sediment. Herein we present a coupling procedure that comprises two discrete elements. First, immediately following bank failure, the slumped debris comes to rest at the bank toe as a planar surface inclined at an 'angle of repose.' Second, we use a fractional transport model to simulate the subsequent erosion and deposition of the failed bank material debris. The method is demonstrated with an example in which an equilibrium bed topography model is combined with a river bank erosion model to predict the morphological response of a river bend to a large flow event. CE Database subject headings: Bank erosion; Topography; Morphology; Sediment; Rivers; Deformation; Equilibrium.

[1] The southernmost tip of South America is an active continental margin where oblique convergence between plates, transcurrent motion, and tectonic rotation on land make the geodynamic setting more complex than that of the central Andes. A multichannel seismic data set has been used in conjunction with multibeam and altimetry data to clarify the regional architecture of the continental margin from 50[degrees]S to 57[degrees]S. Despite the thick sedimentary section in the trench and the slow plate convergence rate peculiar of typical accretionary margins, seismic reflection profiles image widely varying frontal wedge morphologies, different rates of accretion, a high degree of structural diversity, and different modes of continental building (offscraping, underplating, tectonic erosion). Correlation between structural parameters (depth of the decollement level, width of the wedge, accretionary rates, Moho depth) suggests large-scale structural control on margin geometry and structural diversity. The transition from tectonic accretion to erosion and general structural variations do not show any gradual trend, but rather, they occur along oblique structural trends that produce a tectonic segmentation of the margin. This is mainly related to tectonic processes on the overriding plate (block rotations along strike-slip faults), while local disturbances on the incoming plate (seamounts, fracture zones, and oceanic fabric relative orientation) add further structural complexity. Altimetry data, in conjunction with structural analysis, suggest that large-scale tectonic variations and structural development are related to trench-parallel gravity anomaly variations and, ultimately, to basal friction on the plate interface. Strong negative gravity anomalies are associated with sedimentary basins, wide wedges, and accretionary domains, while positive gravity anomalies mainly refer to transverse structural highs and narrow wedges. doi: 10.1029/2006TC001983.

HV composite polymeric insulators are being accepted increasingly for use in outdoor installations by the traditionally cautious electric power utilities worldwide. They currently represent ~60 to 70% of newly installed HV insulators in North America. The tremendous growth in the applications of non-ceramic composite insulators is due to their advantages over the traditional ceramic and glass insulators. These include light weight, higher mechanical strength to weight ratio, resistance to vandalism, better performance in the presence of heavy pollution in wet conditions, and comparable or better withstand voltage than porcelain or glass insulators. However, because polymeric insulators are relatively new, the expected lifetime and their long-term reliability are not known and therefore are of concern to users. Additionally they might suffer from erosion and tracking in the presence of severe contamination and sustained moisture. This leads to the development of dry band arcing that under certain circumstances could lead to failure of polymer insulators. In this paper a review is presented of the recent performance experience of HV composite polymeric insulators in outdoor service, testing methods, aging, the ranking of the materials, the role of fillers, the role of low molecular weight components present in the insulators, the mechanisms responsible for the loss and recovery of hydrophobicity, one of the most important properties of polymers, the mechanisms of failure, detection of faults, type and quantity of natural contaminants, effects of exposure to rain, hydrocarbons, stationary air and wind, various methods to optimize the electrical performance and a relatively new method for evaluating the performance status of polymeric insulators in the field.