Showing papers on "Stream power published in 1992"

The Problem of Channel Erosion into Bedrock

Abstract: Summary Although river incision into the bedrock of uplifted regions creates the dissected topography of landscapes, little is known about the process of channel erosion into bedrock. Here we present a testable framework for the study of fluvial incision into bedrock that combines theory with field observation. We quantify a simple erosion law by measuring drainage areas and slopes on both principal channels and tributaries. The data suggest that both a bedrock tributary and main stem will lower at the same rate at their confluence if the ratio of main stem to tributary drainage area equals the ratio of tributary to main stem channel slope at the junction. Erosion across several tributary junctions is therefore linearly related to stream power. Tributary slopes greater than about 0.2 deviate from this linear prediction, apparentiy because debris flows scour these steep tributaries. Further field study suggests that the common elevation of tributary and main stem may result from the upslope propagation of locally steep reaches generated at tributary mouths. This propagation continues only to the point on the channel where the channel slope is too steep to preserve the oversteepened reach, or knickpoint, and debris flow scour dominates channel erosion. Our results suggest three general mechanisms by which bedrock channels erode: (1) vertical wearing of the channel bed due to stream flow, by such processes as abrasion by transported particles and dissolution; (2) scour by periodic debris flows; and (3) knickpoint propagation. Consequently, application of a single erosion law to the entire bedrock channel network may be inappropriate.

Relationship between discharge, velocity and flow area for rills eroding loose, non‐layered materials

Abstract: A relationship between discharge, flow velocity and flow area in rills is established using data from four field and laboratory studies. The proposed relationship is shown to predict successfully flow velocities measured in six other studies. Although slopes range from 0.035 to 0.45 and soil materials range from stony sands over silt loams to vertisols, mean flow velocity can be well predicted from discharge alone. Thus, there is no important influence of slope and/or soil material characteristics on flow velocities in rills. The proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments. Furthermore, it allows the calculation of unit stream power, which has been shown to be related to the transporting capacity of overland flow, in terms of slope and discharge.

Virtual rate and mean distance of travel of individual clasts in gravel-bed channels

Abstract: Travel distances in gravel-bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge. Mean distance of movement, irrespective of grain size, is weakly correlated with stream power, especially near the threshold of movement. There may be several reasons for the weak correlation, including variable effects of bed structure, varying magnitudes of sediment mobilizing events and sampling problems. Grain size itself is of marginal significance. The virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period. Better results are obtained by relating the virtual rate of travel to the first peak of the flow event only. This implies that the initial seeding of the tagged particles dominates the observations. Nonetheless, an underlying general relation is indicated by the data, which are derived from a wide range of flow regime types.

Bedrock benches and boulder bars: Floods in the Burdekin Gorge of Australia

Abstract: The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability. Slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1,200 yrs. These floods range in magnitude from 11,000 to 30,000 m 3 s -1 and are characterized by large downstream variations in hydraulics. The floods generate high values of velocity, boundary shear stress, and stream power per unit area. Downstream fluctuations in these variables help to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels. Boulder bars and flood levees form where shear stress and stream power decrease due to channel widening. Small-scale erosional features, such as potholes and troughs, are best developed at sites of channel constriction and associated increases in shear stress and stream power. The development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure, and by flow hydraulics. All of the channel features are related to the hydraulics of large floods, which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge.

Gradient irregularity in the herbert gorge of Northeastern Australia

Abstract: The seventy-kilometre-long Herbert Gorge of northeastern Australia preserves a record of past floods in slackwater deposits and palaeostage indicators. Step-backwater modelling of water-surface profiles indicates that discharges ranging from 11000 to 17000 m3s−1 have occurred six times in the gorge during the last 900 years. These flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology. In particular, the hydraulics of extreme flows can be related to boulder transport, and to the location of large boulder bars. Large boulder bars occur throughout the Herbert Gorge, being best developed at loci of stream power minima along the inside of bends, at tributary junctions, and at obstructions in the channel caused by bedrock highs. Only the flows exceeding approximately 8000 m3 s−1 are competent to transport the boulders which constitute the bars. In the straight channel reaches, the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls. The bars provide an efficient means of energy dissipation, and they are interpreted as a result of the inherent high turbulence of flow in a steep channel. The regular spacing of the bars, and their correspondence with the hydraulics of large flows, suggest that the bars and associated bedrock highs may represent a self-regulating mechanism akin to the pool-riffle sequence of alluvial channels. It may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges.

Geomorphic implications of glacier outburst flooding: Noeick River valley, British Columbia

Abstract: An outburst flood from Ape Lake in October 1984 severely affected channel and floodplain morphology of Noeick River. This event, estimated to have exceeded 103 m3∙s−1, was as much as 2.4 times greater than the probable maximum meteorological flood. Storage of water in ponds, lakes, and floodplain margins resulted in attenuation of the flood peak as it progressed downstream. As a result, flood effects were most evident in the alluvial reaches of upper Noeick River, below Ape Lake between km 10 and km 23. Here, the channel zone widened from a preflood average of 75 m to almost 200 m, the area of eroded valley bottom increased by about 2.5 times, and a cumulative length of 2.5 km of valley-side colluvial fans was eroded. Transported sediment stored in the floodplain and along valley margins resulted in aggradation of up to 0.75 m, except in restricted reaches where limited sediment supply and increased stream power caused degradation of up to 3 m. A second, and fractionally larger, outburst flood in August 1...

An Evaluation of Unit Stream Power Theory for Estimating Soil Detachment and Sediment Discharge From Tilled Soils

Abstract: Unit Stream Power (USP) and the associated Yang (1973) sediment transport equations were compared to soil detachment rates and sediment concentrations in runoff from eroding soils. Although USP was significantly correlated with rill detachment rates, potential energy dissipation rate per unit rill area was a better predictor of soil detachment rates. The Yang (1973) equations estimated sediment concentrations within a factor of 10 of the observed concentrations, if the median soil aggregate diameter was within the range of diameters used by Yang to estimate the equation parameters. When median aggregate sizes were less than 0.15 mm, the lower limit of Yang’s (1973) calibration range, the equations tended to produce unrealistically large estimates of sediment concentration.