Douglas M. Thompson

Velocity characteristics along a small step–pool channel

This paper summarizes measurements of velocity along three reaches of a small mountain channel with step–pool bedforms. A one-dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph. Measurement cross-sections were located upstream from a bed-step, at the step lip, downstream from the step, and in a uniform-gradient run. Data analyses focused on characteristics of velocity profiles, and on correlations between velocity characteristics and the potential control variables bedform type, reach gradient and flow depth. To test the hypothesis that velocity characteristics are related to channel bedform types, ANOVA and ANCOVA tests were performed for the average velocity and coefficient of variation of point velocity data. Results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge. Velocity became more variable as stage increased, particularly at low-gradient reaches with less variable bed roughness. Velocity profiles suggest that locations immediately downstream from bed-steps are dominated by wake turbulence from mid-profile shear layers. Locations immediately upstream from steps, at step lips, and in runs are dominated by bed-generated turbulence. Adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation, whereas favourable pressure gradients at steps are suppressing turbulence. The bed-generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake-generated turbulence and form drag of step–pool bedforms. Copyright

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

Abstract 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 Kellers 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.

A comparison of surface sampling methods for coarse fluvial sediments

In order to characterize variability associated with sampling coarse fluvial sediment, surface grain-size distributions were characterized at eight sample sites within gravel- to boulder-bed channels. Four methods were used: (1) a random walk, (2) a sampling grid spaced at an interval equal to the intermediate diameter of the largest clast in the sampling area, (3) a sampling grid spaced at one half the interval of method 2, and (4) a randomly chosen subsection within which a patch count of most clasts at least partially exposed at the surface was conducted. At each site, six replicates of each method were performed, with each replicate consisting of 100 clasts. Sampling was performed by a single operator at six of the sites and by multiple operators at two of the same sites and at two additional sites. The research objectives were to characterize variability among replicates of a method, among methods, and among operators. Sample sites were divided into relatively well sorted (inclusive graphic standard deviation <1.4) and poorly sorted (≥1.5). The first three methods produce statistically indistinguishable values of D50 and D84 for both well-sorted and poorly sorted sites when performed by a single operator. Multiple operators may produce statistically different population measures (D50, D84, sample distributions, and variance) using any of the four methods at either type of site. Because of operator-induced variability, we caution against direct comparisons of grain-size distributions between channels or within a single channel through time if more than one operator performs the sampling.

Canyons with undulating walls

Channels incised into cohesive substrates commonly develop undulating walls characterized by regular, repetitive variations in channel width. Canyons with undulating walls are found in a variety of lithologic, climatic, and tectonic regimes, and are most commonly present downstream from a knick zone of active incision. Flume experiments and natural channel morphologic patterns lead us to infer that wall undulations are the remnants of breached, offset potholes or of sinuous longitudinal grooves formed during incision of the knick zone. Substrate characteristics seem to have little influence on these undulations. Using measurements from eight channels incised into the Navajo Sandstone of southern Utah, we found that intact rock strength, rock-mass strength, joint orientation and spacing, bedding, and porosity did not differ significantly between channel reaches with straight walls, undulating walls, and meanders. The wavelength and amplitude of wall undulations also did not correlate with any substrate characteristic. These results lead us to conclude that the primary control on formation of undulating walls is hydraulic processes. Along the study channels, deep, narrow reaches with undulating walls alternate with wider straight or meandering reaches. Using flume simulations and one- and two-dimensional hydraulic modeling, we found that the wall undulations act to reduce interreach (spatial) variability in energy expenditure, and to minimize energy expenditure within a reach. We propose that the wall undulations and flow hydraulics create a feedback whereby the wall undulations are preserved following knickpoint incision. The wall undulations then act to regulate downstream energy expenditure in a manner analogous to bedforms. This implies that generalized principles of uniform energy expenditure developed for alluvial channels may also apply at the reach scale to bedrock channels with relatively homogeneous substrates.

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. [Key words: channel morphology, fluvial geomorphology, velocity reversal...

Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels

Abstract Pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat. An understanding of this morphology is necessary for the successful design of a reconstructed or restored channel. To better understand pool geometry and sorting characteristics of New England channels, characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions. Characterizations of channel-bed sediments were also performed at 34 of these locations. Data from 120 pools were used in least-squares, multiple-regression analysis with 19 different independent variables to develop prediction equations for pool depth, length, and exit-slope gradient. These equations were then tested using the 25 remaining pools. Eighteen different geometric variables were then used to predict sediment φ 50 values in pool centers, pool exit slopes, and riffles for the subset of 34 samples. The regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit-slope width, constriction gradient, constriction width, drainage area, upstream channel width, and the exit-slope expansion ratio, with an R 2 value of 0.53. Similarly, pool length is influenced by channel gradient, location of the channel constriction, pool width, drainage area and constriction length, with an R 2 value of 0.72. The gradient of pool exit slope is influenced by drainage area, channel gradient, and constriction gradient, with an R 2 value of 0.34. However, R 2 values for exit slope increased to 0.65 when sediment d 50 data were utilized. Using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero, although moderate variations do exist. A Tukey–Kramer HSD comparison of the means for sediment φ 50 values indicates pool sediments are significantly coarser than pool exit-slope and riffle sediments for the subset of 34 pools. Regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with R 2 values that range from 0.45 to 0.62. The results suggest that pool dimensions are related to basin-wide variability and local hydraulic conditions. The findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool–riffle morphology.

The velocity-reversal hypothesis revisited

In 1971, Edward Keller proposed the velocity-reversal hypothesis to describe observations where near-bed velocities in pools increased at a faster rate than in riffles. The reversal in conditions was used to explain the maintenance of pool-riffle sequences. The hypothesis continues to draw interest, and research shows that velocity reversals occur in limited conditions, but average cross-sectional velocities in pools do not universally exceed those in riffles at near-bankfull flows. A consensus is now beginning to form that flow convergence in pools is common at high flow and helps to form an isolated region of jet flow with high velocities that maintains the characteristics of the pool-riffle sequence.

Geometric adjustment of pools to changes in slope and discharge: a flume experiment

Abstract To characterize the factors controlling pool shape, 30 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed of sand with a d50 of 0.25 mm. Pool depth and length were measured and used as dependent variables in least-squares, multiple-regression analyses. Discharge, channel-bed gradient and energy slope were the independent variables. Additional linear-regression analyses were conducted with either pool depth or length and stream power. Results indicate that both pool depth and length are primarily a function of discharge. Channel-bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth. Stream power is significantly related to both pool depth and length, but R2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power. Observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation. In contrast, pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools. The results suggest that many field studies may suffer from cross-correlation problems. In particular, channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction-influenced channels.

A Paradox of Power: Voices of Warning and Reason in the Geosciences

As a culture, we are faced with a series of increasingly complex environmental problems. As geoscientists, we are often asked to assume the role of technical experts to provide guidance or solutions to these difficult dilemmas. Unfortunately, the political, social, economic, and judicial aspects of environmental management often require geoscientists to advocate choices or provide definitive answers that are in conflict with our training as never-ending hypothesis testers and unbiased observers of nature. A Paradox of Power: Voices of Warning and Reason in the Geosciences addresses this topic by providing a collection of works that highlights various discrepancies between standard scientific procedures and the public decision-making process. The book presents a range of views and examples that provides geoscientists with a background on the issues surrounding the use of their expertise and analyses in typical law and policy decisions.