Michael Church

Paraglacial Sedimentation: A Consideration of Fluvial Processes Conditioned by Glaciation

Glaciation is schematically considered as a perturbation of “normal” fluvial conditions. Drift is unstable in a proglacial or postglacial fluvial environment, resulting in heightened sediment movement that continues as long as drift material remains easily accessible for fluvial erosion and transportation. Sediment yield bears no relation to concurrent primary production of weathered debris. Examples of such “paraglacial” denudation and sedimentation are reported from two contrasting areas. Postglacial valley alluvial deposits are widespread in central Baffin Island where rapid sedimentation continues today; estimated denudation rates are an order of magnitude higher than in comparable unperturbed areas. In south-central British Columbia, rapid sedimentation during the paraglacial period contrasts sharply with present-day conditions.

Stabilizing self-organized structures in gravel-bed stream channels: Field and experimental observations

Stable reticulate structures, which we call “stone cells,” have been observed in cobble-gravel channel beds with low bed material transport rates. Experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the Shields threshold by more than ∼2 times, so that bed material transport is low. They are constructed by particles moving from less stable positions into more stable configurations against each other. Intermediate developments include clusters and stone lines. They reduce sediment transport by orders of magnitude and are evidently a major stability-promoting mechanism in gravel channels. The timescale for their development suggests that the boundaries of many gravel-bed channels are not in equilibrium with recent competent flows but reflect the history of recent “dominant” flows.

Grain size along two gravel‐bed rivers: statistical variation, spatial pattern and sedimentary links

A new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel-bed rivers in northeastern British Columbia, Canada. A preliminary assessment of grain-size variability establishes a basis for examination of the spatial pattern of grain-size change. Standard ANOVA techniques are inappropriate because individual samples have unequal variances and are not normally distributed. Alternative tests for homoscedasticity and comparison of means are therefore utilized. Within-site, between-sample variability is not significant. The grain-size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled. In both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining. Textural variations along the two rivers studied here are complex and show negligible overall fining (in over 100 km). This is the consequence of a large number of tributary inputs and non-alluvial sediment sources which are the legacy of Late Pleistocene glaciation. The identification of lateral sources like these is fundamental for understanding textural changes within rivers. The sedimentary link (a channel reach between significant lateral sediment inputs) provides a means of isolating fluvial maturation processes (abrasion and sorting) from contingent lateral inputs. Strong fining trends are apparent in most links and classification of grain-size measurements according to their location within particular links greatly improves the statistical explanation of textural variation. Identification of sedimentary links provides a means of applying models of fluvial fining processes, so isolation of link networks will aid the development of basin-scale models of textural variation.

Channel morphology, gradient profiles and bed stresses during flood in a step–pool channel

Abstract The stability of a step–pool channel is investigated on the basis of measurements during a median annual flood. The study reaches range in gradient from 5% to 10% and the flow was 3 m3 s−1 in an 8-m wide channel. The estimated shear stress based on the channel gradient was >102 Pa, which implies that the channel ought to move half-meter class boulders. Calculations of the shear stress using flow resistance equations yield values of about 30 Pa. These figures imply that the largest boulders in the channel could be moved. In fact, the channel is stable. The effective stress remains well below the threshold to move the large, structure-forming boulders. The steps are stable structures that result from interlocking of the larger stones. They are semi-randomly located, depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks. The high variation in step–pool wavelength, step height, residual pool depth, and pool length is indicative of the random nature of step location and structure along a reach. There is no convincing evidence that special conditions govern their formation.

On the misuse of regression in earth science

Simple least-squares regression analysis is applied to almost all empirical curve-fitting problems in earth science (and related fields). Its use, however, should be restricted to predictive situations. For comparisons with theory or among fitted lines, the related technique termed functional analysisshould be employed. To apply this method, the ratio of the random components of the variances of the variables must be estimated. Principles are illustrated with examples from geomorphometry, especially the stream frequency-drainage density relation.

DISTANCE OF MOVEMENT OF COARSE PARTICLES IN GRAVEL BED STREAMS

Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein-Hubbell-Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles.

Form and stability of step‐pool channels: Research progress

[1] Research examining the hydraulics, morphology, and stability of step-pool mountain streams has blossomed in the last decade, resulting in more than a dozen dissertations. These, along with other research projects, have transformed our understanding of step-pool channels. Contributions have been made toward understanding depositional step formation and destruction, scour downstream of steps, step-pool hydraulics, and the effect of sediment transport on step stability. We propose that depositional steps exist in a jammed state whereby the boulders are structurally arranged within the channel and thereby stabilize it. Once a step has formed, a scour pool with a characteristic length and depth develops downstream, creating a zone where additional steps are unlikely to occur. Downstream of the scour hole, steps are more likely to occur as the high energy associated with the plunge pool has dissipated. Data suggest that the presence of cobbles or boulders limits pool scour as well as the degree to which well-defined, channel-spanning step-pools form. We propose a state-space for step-pools in which conditions for a step to form include (1) the ratio between width and boulder diameter (the jamming ratio), (2) the ratio between applied shear stress and the stress needed to mobilize the bed (relative Shields number), and (3) the ratio between bed material supply and discharge (bed sediment concentration). Available data suggest this model is plausible. Emerging critical research questions are discussed.

Experiments on surface structure and partial sediment transport on a gravel bed

Eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport. The experiments have two phases, a no-feed degradational phase followed by a feeding phase. A surface structure of irregular, reticulate stone nets and clusters was developed before sediment feeding commenced. Bed load transport equaled or slightly exceeded the fed supply, except at the highest feed rate. The bed structure was maintained, but bed surface texture fined with increasing sediment load. The two phenomena may coexist because the largest grains on the bed moved only very sporadically. The actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size. The difference reflects the increase in bed stability introduced by the bed structure. Between 17% and 47% of the bed shear stress is estimated to be carried by the structure, <4% being absorbed by the load, while the bed grains carried the balance of the stress. Bed material transport is exceedingly sensitive to bed surface structure and grain size, which raises concerns about the realizable precision of grain size measurements and characterization of the structure.

Bed material transport estimated from the virtual velocity of sediment

This study evaluates the possibility of determining bed material transport using the virtual rate of travel of individual particles, dimensions of the active layer of the streambed, and porosity and density of streambed material. Magnetically tagged stones and scour indicators were employed in Carnation Creek, British Columbia, to quantify transport rates. Observations cover flows up to 36 m3 s−1 (τ* = 0·081). Transport rates, ranging from 0·090 to 9·7 kg s−1 (0·12–13·2 m3 h−1), display a relatively sensitive trend with maximum stream power, as expected. Error analysis indicates that uncertainty in virtual velocity covers the majority of sample variance. An evaluation of the two measurement techniques used to delineate active layer dimensions, magnetically tagged stones and scour indicators, indicates that they yield comparable depths, widths and transport rates over the range of flows observed. Issues for further study are discussed.

Size and distance of travel of unconstrained clasts on a streambed

We have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers. Distance of movement was scaled using the mean distance of movement of the median size group of the surface material; size was scaled using the median b axis diameter of the subsurface material. The scaled data are fitted with a nonlinear function. Clasts larger than the median of surface material exhibit a steep decline of mobility, with an upper limit for mobility near Di/D50sub = 10. Smaller materials exhibit a less sensitive relation. We suppose that trapping of small clasts affects their travel distance. These results indicate that the movement of free surface stones is size preferred. A subordinate analysis of free, constrained and buried particles reveals that bed surface structure acts to reduce differential movement.