Showing papers on "Stream power published in 1996"

Channel response to sediment wave propagation and movement, redwood creek, california, usa

Abstract: Redwood Creek, north coastal Californa , USA, has experienced dramatic changes in channel confguation since the 1950s. A series of large floods (in 1955 , 1964, 1972 and 1975) combined with the advent of widespread commercial timber harvest and road building resulted in extensive erosion in the basin and contributed high sediment loads to Redwood Creek. Since 1975, no peak flows have exceeded a 5 year recurrence interval. Twenty years of cross-sectional survey data document the downstream movement of a 'sediment wave' in the lower 26 km of this gravel-bedded river at a rate of 800 to 1600 m a I durng this period of moderately low flows. Higher transit rates are associated with reaches of higher unit stream power. The wave was initially deposited at a site with an abrupt decrease in channel gradient and increase in channel width. The amplitude of the wave has attenuated more than I m as it moved downstream, and the duration of the wave increased from eight years upstream to more than 20 years downstream. Channel aggradation and subsequent degradation have been accommodated across the entire channel bed. Channel width has not decreased significantly after initial channel widening from large ( 25 year recurrence interval) floods. Three sets of longitudinal surveys of the streambed showed the highest increase in pool depths and frequency in a degrading reach, but even the aggrading reach exhbited some pool development through time. The aggraded channel bed switched from functioning as a sediment sink to a significant sediment source as the channel adjusted to high sediment loads. From 1980 to 1990, sediment eroded from temporary channel storage represented about 25 per cent ofthe total sediment load and 95 per cent of the bedload exported from the basin.

Interactions of growing folds and coeval depositional systems

Abstract: Responses of both modern and ancient fluvial depositional systems to growing folds can be interpreted in terms of interactions among competing controlling variables which can be incorporated into simple conceptual models. The ratio of the rate of sediment accumulation to the rate of structural uplift determines whether a fold develops a topographic expression above local base level. The balance between (a) stream power and rates of upstream deposition vs. (b) bedrock resistance and rates of crestal uplift and of fold widening determines whether an antecedent stream maintains its course or is defeated by a growing structure. Modern drainage configurations in actively folding landscapes can often be interpreted in terms of these competing variables, and through analysis of digital topography, detailed topographic characteristics of these folds can be quantified. Modern examples of growing folds display both defeated and persistent antecedent rivers, deflected drainages and laterally propagating structures. The topography associated with a defeated antecedent river at Wheeler Ridge, California, is consistent with a model in which defeat results from forced aggradation in the piggyback basin, without the need to vary discharge or uplift rate. Reconstruction of the long-term interplay between a depositional system and evolving folds requires a stratigraphic perspective, such as that provided by syntectonic strata which are directly juxtaposed with ancient folds and faults. Analysis of Palaeogene growth strata bounding the Catalan Coastal Ranges of NE Spain demonstrates the synchronous growth and the kinematic history of multiple folds and faults in the proximal foreland basin. Although dominated by transverse rivers which crossed fold crests, palaeovalleys, interfan lows, structural re-entrants and saddles, and rising anticlines diverted flow and influenced local deposition. In the ancient record, drainage-network events, such as avulsion or defeat of a transverse stream, usually cannot be unambiguously attributed to a single cause. Examination of ancient syntectonic strata from a geomorphological perspective, however, permits successive reconstructions of synorogenic topography, landscapes and depositional systems.

Influence of topography on some vegetation cover properties

Abstract: We study the influence of nine local, non-local and combined quantitative topographic characteristics on vegetation cover, altitude and density. The objects under study are four areas of the Rudny Altai. Differentiation of plant properties is shown to depend on relief parameters which control migration and accumulation of water in landscape by gravity. These are landsurface curvatures, catchment area, topographic and stream power indexes. In some cases the phytocoenosis characteristics can have higher correlation with non-local and combined topographic variables than with local ones. This derives from the fact that non-local and combined relief attributes take into account a relative location of an area in a landscape, so they can better determine topographic prerequisites of substance motion. We demonstrate the advisability of the use of digital models and maps of indicated topographic characteristics in plant investigations.

Infiltration, runoff and erosion characteristics of agricultural land in extreme storm events, SE France

Abstract: Following the extreme rainfall event of 22 September 1992 in Southeastern France, an attempt was made to characterize the response of areas cultivated under vines to heavy precipitation. This response was determined in three ways by means of rainfall simulation experiments. First, the infiltration rate is calculated as the difference between rainfall and runoff rates. Secondly, the flow depths are measured to allow the flow hydraulics to be calculated. Thirdly, sediment transport, both by splash and overland flow, is sampled. These measurements were carried out as near simultaneously as possible, in order to define any possible interactions. Surface characteristics of the experimental plots were measured to define possible controls on individual responses. Buffering of response of the agricultural areas is seen both in the relatively high infiltration rates, and in the high Darcy-Weisbach friction factors observed. However, a problem is noted in the case of the latter, in that the small plot size used probably leads to the over-estimation of the friction factor. The rainfall rate and the surface slope mainly characterize the infiltration rate, whereas the friction factor is controlled by the flow Reynolds' number and the percentage of the surface covered by stones ( > 2 mm). The erosion rates measured are comparable to those found on vineyards elsewhere in the Mediterranean. Extrapolation from the measured values indicates that up to 3.4 kg m−2 of soil may have been lost in interrill flow from these areas in the event of 22 September 1992. Splash is the dominant control of sediment production in the interrill zone. The splash rates are a function of the rainfall rate, surface slope, water depth, soil shear strength, and the percentage of the surface covered either by stones or by vegetation. The sediment transport rate in overland flow can be shown to follow the characteristic equation of Julien and Simons, as a function of flow discharge and surface slope, as well particle size characteristics of the soil. Alternatively, it can be described using hydraulic parameters such as excess shear velocity, effective stream power or unit stream power, as demonstrated experimentally by Everaert.

Channel adjustment of an unstable coarse-grained stream: Opposing trends of boundary and critical shear stress, and the applicability of extremal hypotheses

Abstract: Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated by deposition of a 2.5 km3 debris avalanche and associated lahars that accompanied the catastrophic eruption of Mount St. Helens, Washington on 18 May 1980. Channel widening was the dominant process. In combination, adjustments caused average boundary shear stress to decrease non-linearly with time and critical shear stress to increase non-linearly with time. At the discharge that is equalled or exceeded 1 per cent of the time, these trends converged by 1991–1992 so that excess shear stress approached minimum values. Extremal hypotheses, such as minimization of unit stream power and minimization of the rate of energy dissipation (minimum stream power), are shown to be applicable to dynamic adjustments of the Toutle River system. Maximization of the Darcy–Weisbach friction factor did not occur, but increases in relative bed roughness, caused by the concomitant reduction in hydraulic depths and bed-material coarsening, were documented. Predictions of stable channel geometries using the minimum stream power approach were unsuccessful when compared to the 1991–1992 geometries and bed-material characteristics measured in the field. It is concluded that the predictions are not applicable because the study reaches are not truly stable and cannot become so until a new floodplain has been formed by renewed channel incision, retreat of stream-side hummocks, and establishment of riparian vegetation to limit the destabilizing effects of large floods. Further, prediction of energy slope (and consequently stream power) by the sediment transport equations is inaccurate because of the inability of the equations to account for significant contributions of finer grained (sand and gravel) bank materials (relative to the coarsened channel bed) from bank retreat and from upstream terrace erosion.

Sediment Transport in the Yellow River

Abstract: Yang's unit stream power formula is modified for the computation of total bed-material load in a sediment-laden river with a high concentration of fine suspended materials. Theoretical and laboratory studies indicate that modification can be made on the values of particle fall velocity, flow viscosity, and relative specific weight without any change of the coefficients used in Yang's original formula. Comparisons between computed results from the modified formula and field indicate that the modified formula can be used to compute total bed-material load in the Yellow River with accuracy.

Transport and deposition of plutonium-contaminated sediments by fluvial processes, Los Alamos Canyon, New Mexico

Abstract: Between 1945 and 1952 the development of nuclear weapons at Los Alamos National Laboratory, New Mexico, resulted in the disposal of plutonium into the alluvium of nearby Acid and (to a lesser degree) DP Canyons. Previous research has identified some of this material in the Rio Grande and defined its distribution in the regional river system. The purpose of this paper is to explore the connection between the disposal sites and the main river, a 20 km link formed by the fluvial system of Acid, Pueblo, DP, and Los Alamos Canyons. Empirical data from 15 yr of annual sediment sampling throughout the canyon system has produced 458 observations of plutonium concentration in fluvial sediments. These data show that, overall, mean plutonium concentrations in fluvial sediment decline from 10 000 fCi/g near the disposal area to 100 fCi/g at the confluence of the canyon system and the Rio Grande. In finer detail, the concentrations fluctuate with downstream distance depending on the trap efficiency of various reaches, as controlled by hydraulic conditions. Temporal data from sites repeatedly sampled show the passage of waves of contaminated sediment through the canyon system. Field mapping identified 108 deposits of sediment, including active bed load, flood plains, bars, channel fills, and slack-water deposits. Volumes of sediment in these deposits (calculated from field measurements of the dimensions of the features), combined with the mean concentration values, produced a first approximation of the amount of plutonium in each deposit. The geographic distribution of deposits and plutonium is clustered: of the 1000 mCi of plutonium in the canyon system, 78% is in lower Pueblo Canyon, 18% is in lower Los Alamos Canyon, and the remainder is in the upper reaches of the system. Simulations using a computer model for water, sediment, and plutonium routing in the canyon system show that discharges as large as the 25 yr event would fail to develop enough transport capacity to completely remove the contaminated sediments from Pueblo Canyon. Lesser flows would move some materials to the Rio Grande by remobilization of stored sediments. The simulations also show that the deposits and their contaminants have a predictable geography because they occur where stream power is low, hydraulic resistance is high, and the geologic and/or geomorphic conditions provide enough space for storage.

Regime theory of alluvial channels based upon the concepts of stream power and probability.

Abstract: A new rational regime theory for stable alluvial channels is developed in this paper. Based on the concepts of stream power and fluvial process probability, a basic regime equation with a modified aspect ratio relationship is analytically derived. Most existing explicit regime equations can be transformed into the format of the proposed basic regime equation. A relationship for the basic regime coefficient is derived after introducing the concept of a 'fluvial processes index'. A suitable resistance relationship and a sediment transport formula are then coupled with the basic regime equation to determine depth, width and slope of the regime channel. The new regime theory is compared with existing regime equations and is also appied to field data and laboratory data. The predicted channel dimensions are in good agreement with observed values.

Geomorphology of temperate rivers

Abstract: Small headwater tributaries usually flow within steep-sided V-shape valleys. Further down the river network the valley sides tend to be less steep and the valley bottom is often infilled with sediments. Here the river is separated from the valley sides by more or less extensive flood-plain. Sometimes bordered by terraces, remnants of former active floodplains form stair-like features up valley sides. However, this generalized change in the downstream character of rivers and river valleys masks considerable variability.