Geology topics

E.E. Wohl

Publications and source records attributed to E.E. Wohl.

4 recordsLinked to original sources

USGS2013

A revised velocity-reversal and sediment-sorting model for a high-gradient, pool-riffle stream

Sediment - sorting processes related to varying channel-bed morphology were investigated from April to November 1993 along a 1-km pool - riffle and step- pool reach of North Saint Vrain Creek, a small mountain stream in the Rocky Mountains of northern Colorado. Measured cross-sectional areas of flow were used to suggest higher velocities in pools than in riffles at high flow. Three hundred and sixteen tracer particles, ranging in size from 16 mm to 256 mm, were placed in two separate pool - riffle - pool sequences and used to assess sediment - sorting patterns and sediment -transport competence variations. Tracer-particle depositional evidence indicated higher sediment -transport competence in pools than in riffles at high flow. Pool - riffle sediment sorting may be created by velocity reversals , and more localized sorting results from gravitational forces along the upstream sloping portion of the channel bed located at the downstream end of pools .

Colorado↗

USGS2007

Field-derived relationships for flow velocity and resistance in high-gradient streams

We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08-0.21) of a predominantly step-pool channel, the Rio Cordon, Italy, over a range of discharges (3-80% of the bankfull discharge). The resulting data were used to compute flow resistance. At-a-station hydraulic geometry relations indicate that in most reaches, the exponent describing the rate of velocity increases with discharge was between 0.48 and 0.6, which is within the range of published values for pool-riffle channels. The Rio Cordon data are also combined with published hydraulics data from step-pool streams to explore non-dimensional relationships between velocity and flow resistance and factors including unit discharge, channel gradient, and step geometry. Multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge (q*) is the most important independent variable overall in explaining variations in velocity and flow resistance, followed by channel slope and the ratio of step height to step length. Empirical equations are provided both for dimensionless velocity and flow resistance, but prediction of the former variable appears more reliable. ?? 2007 Elsevier B.V. All rights reserved.

Journal of Hydrology↗

USGS1999

Velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions

Keller [Keller, E.A., 1971. Areal sorting of bed-load material; the hypothesis of velocity reversal. Geological Society of America Bulletin 82, 753-756] hypothesized that at high flow, near-bed velocities in pools exceed velocities in riffles and create pool scour. Pools, however, typically have larger cross-sectional areas of flow at bankfull discharge. This condition raises an inconsistency with Keller's velocity reversal hypothesis and the one-dimensional continuity of mass equation. To address this problem, a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit-slopes of pools. According to the model, a narrow zone of high velocity occurs in the center of pools, creating scour. Along the downstream end of pools, an uphill climb of particles up the pool exit-slope promotes sediment deposition. The model is tested with field and flume measurements of velocity, water-surface elevation, and size of bed sediments in recirculating-eddy influenced pools. Local reversals of the water-surface gradient were measured in the field and a velocity reversal was created in the flume. The reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow. The distribution of bed sediments collected in the field also support the proposed model of pool maintenance.

Geomorphology↗

USGS1991

Debris flows as geomorphic agents in the Huachuca Mountains of southeastern Arizona

Numerous debris flows occurred in the Huachuca Mountains of southeastern Arizona during the summer rainy season of 1988 in areas that were burned by a forest fire earlier in the summer. Debris flows occurred following a major forest fire in 1977 as well, suggesting a causal link between fires and debris flows. Abundant evidence of older debris flows preserved along channels and in mountain front fans indicates that debris flows have occurred repeteadly during the late Quaternary in this environment. Soil development in sequences of debris-flow deposits indicates that debris flows probably recur over time intervals of several hundred to a thousand years in individual drainage basins in the study area. Surface runoff in the steep drainage basins of the Huachuca Mountains is greatly enhanced following forest fires, as the hillslopes are denuded of their vegetative cover. Water and sediment eroded from the hillslope regolith are rapidly introduced into the upper reaches of tributary channels by widespread rilling and slope wash during rainfall events. This influx of water and sediment destabilizes regolith previously accumulated in the channel, triggering debris flows that scour the channel to bedrock in the upper reaches. Following a debris flow, the scoured, trapezoidally-shaped channel gradually assumes a swale shape and the percentage of exposed bedrock declines, as material is introduced from the slopes. Debris flows do a tremendous amount of work in a very short time, however, and are the major channel-forming events. Where the tributary channels enter larger, trunk channels, the debris flows serve as the main source of very coarse sediment. The local slope and coarse particle distribution of the trunk channel depend on the competence of water flows in the channel to transport the material introduced by debris flows. Where the smaller channels drain directly to the mountain front, debris flows create extensive alluvial fans which dominate the morphology of the basin-range boundary. Time intervals between debris flows in the drainage basins of the Huachuca Mountains are probably controlled by complex interactions among climate, forest fires and slope processes. Fires destroy the protective vegetation that stabilizes the upper catchment slopes and inhibits erosion. However, not every fire that burns a catchment causes debris flows, because sufficient weathered material must accumulate in the upper channel reaches to initiate a large debris flow. If such accumulation has not occurred, the material introduced to a channel following a forest fire will move only a short distance down the channel. Thus, the episodic nature of debris flows probably depends on rates of slope weathering and erosion, which are in turn controlled by climate, both directly and through vegetation and forest fires.

Geomorphology↗