Showing papers on "Stream power published in 1994"

Long river profiles, tectonism, and eustasy: A guide to interpreting fluvial terraces

Abstract: Along three rivers at the Mendocino triple junction, northern California, strath, cut, and fill terraces have formed in response to tectonic and eustatic processes. Detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level, primarily aggradation and formation of fill terraces during sea level high stands, alternating with deep incision during low stands. A eustasy-driven depositional wedge extends tens of kilometers upstream on all rivers (tapering to zero thickness). This distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere, due in part to the large size of these rivers. However, the change in gradient is nearly identical to other base level rise studies: the depositional gradient is about half that of the original channel. Middle to upper reaches of each river are dominated by the effects of long-term uplift, primarily lateral and vertical erosion and formation of steep, unpaired strath terraces exposed only upstream of the depositional wedge. Vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates. Strath terraces have steeper gradients than the modern channel bed and do not merge with marine terraces at the river mouth; consequently, they cannot be used to determine altitudes of sea level high stands. Strath formation is a continuous process of response to long-term uplift, and its occurrence varies spatially along a river depending on stream power, and hence position, upstream. Strath terraces are found only along certain parts of a coastal stream: upstream of the aggradational effects of oscillating sea level, and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load. For a given size river, the greater the uplift rate, the greater the rate of vertical incision and, consequently, the less the likelihood of strath terrace formation and preservation.

Hillslope and channel evolution in a marine terraced landscape, Santa Cruz, California

Abstract: A flight of marine terraces along the central California coastline provides a unique setting for the study of topographic evolution. Wavecut platforms mantled by 2–6 m of marine terrace cover deposits are separated by 10–50 m tall decaying sea cliffs. Paleocliff edges become more rounded with age, yet the details of the profiles and frequent bedrock exposure on the upper slopes imply weathering-limited transport. Five bedrock stream channels etched through the marine terrace sequence display one to three distinct convexities in their longitudinal profiles. Detailed hand level surveys of the hillslopes and of the stream channel longitudinal profiles constrain hillslope evolution and channel incision components of a numerical model of landscape evolution. We account for regolith production as a function of regolith depth. In accord with the field observation that hillslope processes are presently dominated by the activities of burrowing rodents, the transport process is taken to be diffusive. Stream incision is assumed to be controlled by stream power, for which we use the surrogate of local drainage area-slope product. Best fits of the numerical model to field data imply: hillslope diffusivity is 10 m2 kyr−1; regolith production rate on bare bedrock is 0.3 m kyr−1, and falls off rapidly with regolith cover, and the constant controlling the efficiency of stream incision is 5 to 7×10−7m−1 kyr1.

Longitudinal Profile Development into Bedrock: An Analysis of Hawaiian Channels

Abstract: Analysis of topographic maps of rivers incised into dated Hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power. Field observations suggest that two processes may control Hawaiian channel downcutting: (1) stream power-dependent erosion, including abrasion of the channel bed by transported particles, and (2) step-wise lowering caused by knickpoint propagation. Modeling results indicate that a simple stream power-dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of Kauai channels but is insufficient to predict two other characteristic features: the upslope propagation of knickpoints and the straight 5-8° channel slopes below the knickpoints; thus more than this single transport law is apparently required to model bedrock channel incision. Field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders. We propose that...

Controls on bedrock channel incision along nahal paran, Israel

Abstract: Nahal Paran drains 3600 km2 of Egypt's Sinai peninsula and Israel's Negev Desert. Much of the channel is alluvial, but a canyon 10·5 km long has been incised into Late Cretaceous chert and dolomite in the lower portion of the basin. Slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3 s−1 over a period of at least 350 years. Step-backwater simulations of flood-flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon, and associated variability in energy expenditure and sediment transport. These variations reflect channel cross-sectional morphology. The greatest values of stream power occur along the lower half of the study reach, in association with three pronounced knickpoints and an inner channel. The locations of these features reflect the exposure of thick, resistant chert layers along the channel. The presence of several similar, but buried and inactive, knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time, probably in response to baselevel changes associated with tectonic activity along the Dead Sea Rift. Thus, the rate and manner of channel incision along the canyon of Nahal Paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient, which in turn influence hydraulics and sediment transport.

Scale, Direction, and Pattern in Riparian Vegetation-Environment Relationships

Abstract: Because of the linear nature of riparian environments, direction and scale are closely linked. Cross-valley variation in mountain stream valleys is typically seen on a scale of meters, while variation up- and down-valley may be measured in kilometers. As a result, environmental factors at transverse scale (varying within a cross section) and at longitudinal scale (varying between cross sections) impose different patterns on vegetation. This paper compares these scale influences on the composition of woody riparian vegetation in southern California. Using cover data from 37 valley cross sections in the Transverse Ranges, and dividing these into four-meter segments, I determine segment values for the longitudinal-scale variables of elevation, years since burning, aspect, valley width and lithology, and for the transverse-scale variables of water-table depth, unit stream power (for the 20-year flood), and substrate particle size. The influence of variables at these contrasting scales on the vegetati...

Among-site variation in riparian vegetation of the southern California transverse ranges

Abstract: -Data from 37 sites in the Transverse Ranges in southern California were used to assess the influence of a variety of environmental variables on among-site variation in riparian vegetation. Ordination and regression analyses indicated that elevation, unit stream power, fire history and valley width all have significant impacts on the composition of this vegetation. TWINSPAN classification generated four species groups: a Baccharis glutinosa/Salix laevigata group that appears related to low elevations, high stream power and recent fire; a Populusfremontii/Alnus rhombifolia group associated with high stream power, but longer time since burning and higher elevations; a Salix lasiolepis/Rosa californica group typically found at high elevation sites with low stream power; and a Platanus racemosa/Adenostoma fasciculatum group that did not show a clear environmental preference.

Seepage Effects on Sand-Bed Channels

Abstract: ExperimentS were conducted to find the effects of seepage on flow over a sand bed in a straight rectangular flume under two conditions: (1) When the channel bed is plane, horizontal, and nontransporting; and (2) when the bed is transporting at a constant sediment concentration. Effects of both injection and suction, caused by seepage flow into and out of the channel bed, are studied for condition 1; and only suction effects are studied for condition 2, Three sands, sizes 0.34 mm, 0.53 mm, and 0.80 mm, are used in the study. It is found that seepage can cause an increase or decrease in the bed shear stress relative to no seepage for the two conditions. The change in bed shear stress depends on the relative magnitudes of the bed shear stress and the critical shear stress of particles under the no-seepage condition, sediment concentration, and the seepage rate. Quantitative relationships giving the ratio of bed shear stresses with and without seepage are presented for both conditions of the bed. A procedure to estimate the changes in bed shear stress, friction factor, Manning's n, and stream power due to seepage for known initial conditions of the channel and the amount of applied seepage is presented

Channel planform adjustment and stream power variations on the middle River Coe, Western Grampian Highlands, Scotland

Abstract: This paper assesses the spatial variations in types and rates of channel migration over an historical time-period on the middle River Coe, Western Grampians, Scotland. The study reach represents an exceptional example of an integrated channel system, which displays progressive downstream adjustment in both hydraulic geometry and planform characteristics from a mountain torrent environment, through a wandering gravel-bed river section to a low-angle, gravel delta within a fluvio-lacustrine environment above Loch Achtriochtan. Spatial and temporal variations in channel planform adjustment and floodplain reworking are related to variations in bankfull discharge, associated stream power and channel competence.

The influence of channel steps on coarse bed load transport in mountain torrents: case study using the radio tracer technique ‘PETSY’

Abstract: Direct measurements of the movements of bed load particles in a mountain torrent, using the radio tracer technique ‘PETSY’, show the influence of channel (bottom) steps on coarse bed load transport. This effect is demonstrated through an analysis of the step length of tagged cobbles. A significant change in the mean step length of particle is noted and the effect of marked channel slope changes on mean particle travel velocity revealed. The empirical results lead to the conclusion that particle transport is clearly controlled by local channel slope changes and that these are of fundamental importance in the transport of coarse bed load material.

Geomorphic response to channel modifications of skuna river at the state highway 9 crossing at bruce, calhoun county, mississippi

Abstract: Skuna River at State Highway 9 at Bruce, Calhoun County, Mississippi, has geomorphically responded to channel modifications by lowering of the channel bed through degradation, which heightened and steepened channel banks and induced widening. Skuna River Canal (Skuna River) has typically degraded about 16.5 feet and widened about 150 feet from 1925 (when constructed) to 1992. Old Skuna River has degraded and widened about 11 feet and 40 feet, respectively, from 1921 to 1991. Skuna River Canal tributary has degraded about 6 feet from 1921 to 1991. Most of the geomorphic response on the Old River and the tributary seems to be a consequence of modifications of the canal. The bankfull discharge of the canal has increased about 1,450 percent, and the channel slope has decreased about 34 percent from 1925 to 1989. The bankfull stream power has been decreasing since 1980. The bankfull channel width-depth ratio has been increasing since 1975, which indicates the canal has been widening more than degrading since 1975. As much as 1 foot of additional degradation and 40 feet of additional widening are projected through 2010 on Skuna River Canal in the vicinity of State Highway 9. About 70 feet of additional widening could occur before the canal reaches quasi-equilibrium, which will likely be reached after 2010. If Old Skuna River and Skuna River Canal tributary degrade as much as the canal, which is doubtful, then about 6 and 11 feet of additional degradation could occur by 2010 on the Old Skuna River and the tributary, respectively, at State Highway 9. Old Skuna River and the tributary could both widen an additional 30 feet in the next 10 to 20 years. The channel low-stage thalweg of Skuna River Canal is beginning to meander around sandbars inducing lateral erosion of the channel banks. The widening projections in this report do not directly account for lateral erosion and are considered to be a minimum for the typical channel reach. Lateral erosion will likely have a significant effect on future widening processes at this site.

Bri-stars (bridge stream tube model for alluvial river simulation)

Abstract: This NCHRP digest describes the BRIdge Stream Tube model for Alluvial River Simulation (BRI-STARS), Version 3.3, an innovative, operational computer program developed for NCHRP Project 15-11, "Computer-aided Analysis of Highway Encroachments on Mobile Boundary Streams". It is a semi-two-dimensional model capable of computing alluvial scour/deposition through subcritical, supercritical, and a combination of both flow conditions involving hydraulic jumps. This model, unlike conventional water and sediment routing computer models, is capable of simulating channel widening/narrowing phenomenon as well as local scour due to highway encroachments. It couples a fixed-width stream tube computer model, which simulates the scour/deposition process taking place in the vertical direction across the channel, with a total stream power minimization algorithm. The decision making algorithm, using rate of energy dissipation or total stream power minimization, determines whether the simulated sediment erosion satisfying the sediment continuity equation should take place in the lateral or vertical direction. It is this second component that allows the lateral changes in channel geometries. Finally, the bridge component allows computation of the hydraulic flow variables and the resulting scour due to highway encroachments. The model also contains a rule-based expert system program for classifying streams by size, bed and bank material stability, platform geometry, and other hydraulic and morphological features.

Floodflow effects on riparian vegetation in Arizona

Abstract: A relation for estimating changes in the condition of riparian vegetation as a function of stream power was developed for stream channels in central Arizona. Flood and vegetation data were collected from 13 flows at 11 sites. Stream power was computed at cross sections and plotted against the average height of vegetation for each flow. The effect of the flow - no effect, little effect, laid over, or removed - on the riparian vegetation is related to stream power and vegetation height. As vegetation height increases, the magnitude of stream power needed to affect the vegetation also increases. Stream power of about 72 Newton-meters per second per meter squared is needed to lay over 1-meter-high vegetation, and stream power of about 580 Newton-meters per second per meter squared is needed to lay over 5.5-meter-high vegetation. This relation can be used to estimate vegetation conditions at the time of peak floodflow.

Floodplain Change in the Glenmalure Valley, South-east Leinster

Abstract: The Glenmalure Valley is one of several U-shaped valleys which drain the eastern flanks of the Wicklow Mountains, 45km south west of Dublin. Five channel and floodplain reaches are recognised along a 25km stretch of the Glenmalure River on the basis of spatial changes in stream power and sediment load characteristics. The downvalley sequence of floodplain types within the river is atypical of that commonly associated with decreasing slope and sediment calibre downstream. Radiocarbon dating together with the use of anomalous concentrations of heavy metals, which are used as stratigraphic indicators of alluvial deposition post 1800, also reveal temporal changes in the rate of Holocene floodplain development within the valley. Downstream changes in floodplain processes within the valley are explained, therefore, in terms of spatial and temporal changes in the river's ability to adjust its slope, form and bed texture within the constraints of geological and Pleistocene geomorphic inheritance. The presence of a bedrock basin in the valley floor profile, infilled by Pleistocene lacustrine deposits, is believed to have caused a local change in base level leading to a significant reduction in valley slope and local stream power in the middle reaches of the valley. The subsurface bedrock form and the Pleistocene palaeolake also had a significant impact on coarse sediment transport, storage and transfer through the river system and hence on the ability of the Holocene river to regrade its slope and laterally rework Pleistocene valley fills for floodplain construction.

Sediment quality characteristics of salmonid spawning grounds

Abstract: This thesis focuses upon the sediment characteristics of salmonid spawning grounds in a number of lowland and upland catchments in England and discusses the implications of spatial and temporal variability of channel-bed sediment composition upon different stages of the salmonid life-cycle. Substrate samples were obtained by using freeze-coring techniques, thus avoiding the problems of elutriation of fine-sediments which have hindered many studies in the past. Spatial variability is assessed within-site, between-sites and between catchments. Temporal development of channel-bed sediments is assessed both between and within spawning seasons in two upland streams, one of which is regulated. Overall change in sediment quality is demonstrated by changes in armour layer particle-size and variability in the fine-grained sediment population. Vertical infiltration of fine-grained sediments was successfully quantified by analysing core-section loadings over time. Results are explained in terms of mechanisms and processes occurring at each scale. A typology of streams has been developed based upon a continuum of granulometric characteristics which ranged from coarse-grained upland sites to fine-grained (sandy) lowland sites. Sediment quality of a number of lowland streams, based upon indices derived from the literature, were classed as unsuitable by this methodology, largely because of the high loadings of fine-grained sediments (in excess of 14% sub 1.0 mm material). As these streams are known to support good salmonid populations, the usability of such indices is criticised. A model is presented to predict percent weight fine-sediment (sub 1.0 mm) from maximum unit stream power. Error margins limited this application however. The use of freeze-coring techniques for sediment quality assessment are discussed in the light of the results.

Stream Morphology and Hydraulics of the Isaac River Diversion

Abstract: A 7.8km diversion channel was constructed in 1987 to replace 9.5km of the Isaac River so that coal could be extracted from below the stream course by open cut methods. In 1988 a flood event associated with Cyclone Charlie initiated major instabilities of the diversion. This paper presents the results of an investigation to determine the cause of these instabilities and to develop strategies for future management of the diversion. In some reaches the bed of the diversion channel has widened from 40 m to 80 m, and bends have migrated laterally by up to 40 metres. Such adjustments are continuing. By comparison, the natural river upstream and downstream of the diversion is stable. Comparison of key characteristics between the diversion channel and the natural river channel forms the basis of this investigation. Deepening of the diversion channel has not occurred, so this can not explain instabilities. Bed and bank materials in the diversion are similar to those in the Isaac River so materials do not cause the instabilities. The diversion channel is lacking in fringing vegetation. Bench and bank vegetation are important stabilisers of the Isaac River and will be an important component in achieving a more stable diversion channel, but the most important factor in Isaac River instabilities is channel width. The Isaac River diversion channel was constructed to a bed width of 40 metres. This is similar to the natural channel except that the natural channel also exhibits marginal instream benches. These are low depositional features flanking the active channel and colonised by vegetation. During large flood events the vegetation is stripped and the benches are mobilised thus increasing the effective stream bed width to between 65 and 80 metres. Regime equation and stream power analyses both indicate a stable channel width of approximately 80 m.