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Mikael Attal - One of the best experts on this subject based on the ideXlab platform.
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substrate sediment and slope controls on bedrock channel geometry in postglacial streams
Journal of Geophysical Research, 2015Co-Authors: John D Jansen, K Whitbread, Paul Bishop, Mikael AttalAbstract:The geometry of channels controls the erosion rate of rivers and the evolution of topography following environmental change. We examine how sediment, slope, and substrate interact to constrain the development of channels following deglaciation and test whether theoretical relationships derived from streams reacting to tectonic uplift apply in these settings. Using an extensive data set of channel geometry measurements from postglacial streams in the Scottish Highlands, we find that a power law width-Drainage Area scaling model accounts for 81% of the spatial variation in channel width. Substrate influences channel form at the reach scale, with bedrock channels found to be narrower and deeper than alluvial channels. Bedrock channel width does not covary with slope, which may be due to downstream variations in sediment flux. Bedrock channel width-to-depth ratios increase with discharge (or Area) and sediment flux, consistent with increasing bed cover promoting lateral widening. We find steep, wide, and shallow bedrock channels immediately below lakes, which we interpret as the result of limited erosion due to a lack of sediment “tools.” Where sediment supply is sufficient to exceed transport capacity, alluvial channels develop wider, shallower geometries constrained primarily by flow hydraulics. Our results indicate that simple scaling models of channel width with Drainage Area are applicable at regional scale, but locally, channel width varies with substrate, and in the case of bedrock channels, with sediment flux.
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a statistical framework to quantify spatial variation in channel gradients using the integral method of channel profile analysis
Journal of Geophysical Research, 2014Co-Authors: Simon M Mudd, Mikael Attal, David T Milodowski, Stuart W D Grieve, Declan A ValtersAbstract:[1] We present a statistical technique for analyzing longitudinal channel profiles. Our technique is based on the integral approach to channel analysis: Drainage Area is integrated over flow distance to produce a transformed coordinate, χ, which has dimensions of length. Assuming that profile geometry is conditioned by the stream power law, defined as E = KAmSn where E is erosion rate, K is erodibility, A is Drainage Area, S is channel gradient, and m and n are constants, the slope of a transformed profile in χ-elevation space should reflect the ratio of erosion rate to channel erodibility raised to a power 1/n; this quantity is often referred to as the channel steepness and represents channel slope normalized for Drainage Area. Our technique tests all possible contiguous segments in the channel network to identify the most likely locations where channel steepness changes and also identifies the most likely m/n ratio. The technique identifies locations where either erodibility or erosion rates are most likely to be changing. Tests on a simulated landscape demonstrate that the technique can accurately retrieve both the m/n ratio and the correct number and location of segments eroding at different rates where model assumptions apply. Tests on natural landscapes illustrate how the method can distinguish between spurious channel convexities due to incorrect selection of the m/n ratio from those which are candidates for changing erodibility or erosion rates. We also show how, given erosion or uplift rate constraints, the method can be used to constrain the slope exponent, n.
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bedrock channel adjustment to tectonic forcing implications for predicting river incision rates
Geology, 2007Co-Authors: Alexander C Whittaker, Mikael Attal, Gregory E Tucker, P A Cowie, Gerald P RobertsAbstract:We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from Drainage Area upstream of the fault. Unit stream powers are ∼4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins.
J E Hurtrez - One of the best experts on this subject based on the ideXlab platform.
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effect of Drainage Area on hypsometry from an analysis of small scale Drainage basins in the siwalik hills central nepal
Earth Surface Processes and Landforms, 1999Co-Authors: J E Hurtrez, C Sol, Francis LucazeauAbstract:Hypsometry of Drainage basins (Area–elevation analysis) has generally been used to infer the stage of geomorphic development and to study the influence of varying forcing factors (i.e. tectonics, climate, lithology) on topography. However, the scale dependence of hypsometry has generally been neglected. In order to assess the scale dependence of hypsometry, this study focuses on the sensitivity of hypsometry to different Digital Elevation Model (DEM) resolutions and on the influence of Drainage Area. Hypsometry inferred from different DEMs is shown to be robust against variations of their resolution. However, hypsometry appears to be dependent on Drainage Area. We propose that this scale dependence may reflect the varying importance of river and hillslope processes with basin Area. Copyright © 1999 John Wiley & Sons, Ltd.
Richard N Weisman - One of the best experts on this subject based on the ideXlab platform.
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effects of urbanization on watershed hydrology the scaling of discharge with Drainage Area
Geology, 2006Co-Authors: Joshua C Galster, Frank J Pazzaglia, Bruce R Hargreaves, Donald P Morris, Stephen C Peters, Richard N WeismanAbstract:This study examines the effects of impervious surfaces within urbanized land on the scaling of river discharge with Drainage Area. Discharge in a river channel grows as Drainage basin Area increases following the general equation Q = kA c , where Q is river discharge, k is a measure of river base flow, A is upstream Drainage Area, and c is the scaling power dependency. Land use is a critical variable in the examination of river discharge; discharge has significant geologic and ecologic influences on fluvial systems. Discharge is assumed to scale linearly or nearly linearly with Drainage Area ( c ∼1), but in spite of its widespread application, the relationship has not been explicitly tested with respect to urbanization. Here we show that in small urban settings the scaling is nonlinear for peak flows. It is proposed that effective water loading occurs through a combination of increased runoff and an increase in the rate of transport to the rivers. These higher discharges in urban rivers have the potential to increase erosion, degrade aquatic habitats, and significantly alter channel forms.
Gregory E Tucker - One of the best experts on this subject based on the ideXlab platform.
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the importance of the catchment Area length relationship in governing non steady state hydrology optimal junction angles and Drainage network pattern
Geomorphology, 2007Co-Authors: Peter Solyom, Gregory E TuckerAbstract:Abstract Analytical and numerical models of landscape evolution in general either assume steady state hydrology or use empirically based functions in the form of a Drainage Area power law to model runoff. A method to compute non-steady state runoff on the basis of the Area–length relationship of river basins is proposed, for application in landscape evolution models. This shape-sensitive runoff function is derived analytically for environments with short storm duration (storm duration
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bedrock channel adjustment to tectonic forcing implications for predicting river incision rates
Geology, 2007Co-Authors: Alexander C Whittaker, Mikael Attal, Gregory E Tucker, P A Cowie, Gerald P RobertsAbstract:We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from Drainage Area upstream of the fault. Unit stream powers are ∼4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins.
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channel response to tectonic forcing field analysis of stream morphology and hydrology in the mendocino triple junction region northern california
Geomorphology, 2003Co-Authors: Noah P Snyder, Kelin X Whipple, Gregory E Tucker, Dorothy J MerrittsAbstract:Abstract An empirical calibration of the shear stress model for bedrock incision is presented, using field and hydrologic data from a series of small, coastal Drainage basins near the Mendocino triple junction in northern California. Previous work comparing basins from the high uplift zone (HUZ, uplift rates around 4 mm/year) to ones in the low uplift zone (LUZ, ∼0.5 mm/year) indicates that the HUZ channels are about twice as steep for a given Drainage Area. This observation suggests that incision processes are more effective in the HUZ. It motivates a detailed field study of channel morphology in the differing tectonic settings to test whether various factors that are hypothesized to influence incision rates (discharge, channel width, lithology, sediment load) change in response to uplift or otherwise differ between the HUZ and LUZ. Analysis of regional stream gaging data for mean annual discharge and individual floods yields a linear relationship between discharge and Drainage Area. Increased orographic precipitation in the HUZ accounts for about a twofold increase in discharge in this Area, corresponding to an assumed increase in the erosional efficiency of the streams. Field measurements of channel width indicate a power-law relationship between width and Drainage Area with an exponent of ∼0.4 and no significant change in width between the uplift rate zones, although interpretation is hampered by a difference in land use between the zones. The HUZ channel width dataset reveals a scaling break interpreted to be the transition between colluvial- and fluvial-dominated incision processes. Assessments of lithologic resistance using a Schmidt hammer and joint surveys show that the rocks of the study Area should be fairly similar in their susceptibility to erosion. The HUZ channels generally have more exposed bedrock than those in the LUZ, which is consistent with protection by sediment cover inhibiting incision in the LUZ. However, this difference is likely the result of a recent pulse of sediment due to land use in the LUZ. Therefore, the role of sediment flux in setting incision rates cannot be constrained with any certainty. To summarize, of the four response mechanisms analyzed, the only factor that demonstrably varies between uplift rate zones is discharge, although this change is likely insufficient to explain the relationship between channel slope and uplift rate. The calibrated model allows us to make a prediction of channel concavity that is consistent with a previous estimate from slope–Drainage Area data. We show that the inclusion of nonzero values of critical shear stress in the model has important implications for the theoretical relationship between steady-state slope and uplift rate and might provide an explanation for the observations. This analysis underscores the importance of further work to constrain quantitatively threshold shear stress for bedrock incision.
R Reid - One of the best experts on this subject based on the ideXlab platform.
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stable isotope fingerprint of open water evaporation losses and effective Drainage Area fluctuations in a subarctic shield watershed
Journal of Hydrology, 2010Co-Authors: J J Gibso, R ReidAbstract:summary Stable isotopes of water, oxygen-18 and deuterium, were measured at biweekly to monthly intervals during the open-water season in a small, headwater lake (Pocket Lake, 4.8 ha) near Yellowknife Northwest Territories, and concurrently in a nearby string-of-lakes watershed (Baker Creek, 137 km 2 ) situated in the subarctic Precambrian Shield region. As measured in water samples collected over a 12 year period (1997–2008), the levels of evaporative isotopic enrichment in both lake and watershed outflow were differentially offset, and seasonal variations were found in both to be driven by variations in open-water evaporation. Systematic differences measured in the magnitude of the offset between the lake and watershed outflow are interpreted as being caused by changes in the effective Drainage Area contributing to runoff. Based on the observed and extremely consistent relationship between isotopic compositions of lake water and watershed outflow (r 2 = 0.849, p < 0.001) we extend the analysis of open-water evapora
