Showing papers on "Stream power published in 1988"

Bed load transport in braided gravel‐bed stream models

Abstract: Bed load transport rate was measured in ten self-formed small-scale gravel braided streams developed in a laboratory flume at several different values of steady discharge and flume gradient. The streams are approximate Froude models of typical prototype braided streams but of no particular river. Slight viscous effects may be present in the models because particle Reynolds numbers are close to 70. Total bed load discharge was measured every fifteen minutes throughout each 60 hour run. In addition, 80 channel cross-sections were measured in each run to establish the average channel geometry. Total bed load transport rate correlates well with total discharge and total stream power, although at a given stream power bed load discharge is greater when braiding is less intense and the width/depth ratio is lower. Analysis using unit stream power and cross-section average bed shear stress reveals that the laboratory data conform to existing empirical bed load transport relationships. However, comparison with field data from gravel-bed rivers shows discrepancies that may be due to differences in bed material size gradation and bed sediment structure. At constant discharge, wide fluctuations in bed load discharge occur with some regularity. Periods range from 2 to 10 hours in the models, which is equivalent to several tens of hours in a prototype. The presence of these long-period fluctuations compounds the problems of field measurement of bed load in braided streams.

Formation of inner gorges

Abstract: Under certain conditions, a distinctive type of landform, the inner gorge, evolves in drainage basins undergoing persistent base level lowering. Inner gorges are typical of basins in north coastal California, U.S.A., and one such basin was selected for detailed study. Based on analysis of along-channel distribution of rock types, streamside land-sliding, and stored sediment, as well as computation of stream power, the following set of conditions appears to accompany the formation of an inner gorge: 1. inner gorge parent materials are relatively competent, homogenous rock types; 2. in inner gorge reaches, there is an absence of any base level control that would prevent downcutting; 3. continued downcutting occurs due to continued base level fall, and this fall is most likely induced by tectonic uplift; 4. mass slope failures that form steep straight slopes (debris slides) are the main denudational hillslope process in the inner gorge; 5. sufficient stream power is available to transport landslide-derived material out of the inner gorge reach such that alluvial sediment does accumulate in the reach over time; and 6. the initial condition of the landscape is a low relief, erosionally mature land surface into which the inner gorge is cut. This initial condition implies that inner gorges develop in tectonically rejuvenated landscapes. The processes of denudation on inner gorge slopes, acting over a time scale of 105 years, result in a style of landform evolution that approximates parallel slope retreat. This type of slope retreat maintains the distinct inner gorge morphology during sustained downcutting. Though the analysis is confined to one basin in northern California, so as to provide a detailed framework to understand inner gorge landform, inner gorges appear to be commonplace in areas of moderate to high uplift where lithologic and tectonic conditions are appropriate for their formation. A global search for this landform, yet to be undertaken, is needed to confirm whether it indeed occurs where the above conditions are met.

Stream channel pattern: a threshold model

Abstract: The focus of this research is the development of a model which explains channel pattern variability in streams. Since channel pattern is commonly regarded as a qualitative phenomenon, the research employs a logistic regression model, which is advocated as an alternative to traditional graphic/discriminant analysis, since the concepts of threshold and instability have very natural expressions in the logistic regression framework. The results demonstrate that channel gradient and mean discharge can effectively explain channel pattern (i.e., whether the channel is single or multithreaded) in an environment where there is a small range of bed material size. Sediment sorting is also shown to be related to channel pattern in the study environment. Models using valley gradient rather than channel gradient are shown to be distinctly inferior, and no advantage is found in using a stream power measure as opposed to separate gradient and discharge measures. [Key Words: fluvial geomorphology, stream channels, channel...

Predicting Channel Recovery from Sand and Gravel Extraction in the Naugatuck River and Adjacent Floodplain

Abstract: Gravel extraction at five sites along the Naugatuck River in Connecticut is of sufficient magnitude that channel recovery to pre-mining morphology is expected to require up to several hundred years for instream sites and longer for riparian pits. The bases for this prediction are: a geomorphic evaluation of the channel from sequential aerial photography; hydrographers' records of gaging stations in the basin; and limited calculations of bedload transport, based upon stream power - instantaneous transport rate relations. The pre-mining response of the channel to an extreme flood in 1955 was also examined to determine how the channel might adjust to major disturbance.

Mathematical Modeling of Delta Deposits and Retrogressive Erosion in Reservoirs

Abstract: A hybrid computer model which simulates mainly delta deposition and retrogressive erosion in reservoirs is described. Basic equations that govern the flow and bed changes used in the model include the continuity equation for water, momentum equation for water, and continuity equation for sediment. Meyer-Peter bed load formula and the suspended load transport formula based on the concept of stream power per unit channel width previously developed by the first author are applied. In the numerical approach, the model combines the double sweep method of weighted implicit finite-difference scheme developed by Preismann with the backward explicit finite-difference scheme in order to account for different patterns of channel changes. Additional aspects of the subject are discussed.