Thesis (Ph.D.)--University of Washington, 1995. / Vita. Includes bibliographical references (leaves [163]-173).

Thesis (Ph. D.)--University of California, Santa Cruz, 1993. / Typescript. Includes bibliographical references (leaves 162-172).

Vegetation in rivers increases flow resistance and bank stability, reduces bed resistance and flow conveyance, improves water quality, promotes habitat diversity, and alters both mean and turbulent flow. By reducing bed resistance and altering turbulent characteristics, vegetation can change the distribution of deposition and erosion processes. To understand all above mentioned vegetation effects, more research is needed. The goal of this dissertation was to determine the impacts of vegetation on bed resistance and sediment transport and identify a best approach for quantifying vegetation induced friction resistance. To achieve this, both experimental study and numerical simulation were performed. A series of laboratory experiments were conducted in an open channel flume to investigate the impacts of vegetation density on bed resistance and bed load transport for emergent vegetation condition. The bed resistance in a mobile bed channel is equal to the summation of grain and bed form resistances. An attempt has been made to make a separation between grain and bed form resistances, which is challenging and has never been reported in literature. An alternative approach is used to calculate the grain resistance. A new iterative method was derived to calculate the bed form resistance. Empirical relations were formulated to calculate the bed form resistance and bed load transport rate using a newly defined flow parameter that incorporates the vegetation concentration. The bed elevations and bed form height were measured by the Microsoft Kinect 3D Camera. It was found that the height of bed form depends on the vegetation concentration, which determines whether ripple/dune or scour holes are dominant on the bed surface. For sparsely vegetated flows, the bed form height and resistance are decrease rapidly as the vegetation concentration was increased, and they decreased gradually when the vegetation concentration was high. To quantify the vegetation induced friction resistance, a 3D numerical simulation was conducted using the Delft3D-FLOW open source program. The study area is Davis Pond freshwater marsh area near New Orleans, Louisiana. The dominant vegetation type for the study area is Panicum hemitomon. The study area was divided into several sub-areas depending on the existence of channels, overbanks, and vegetation height. Several approaches were used to approximate the vegetation roughness; a constant Manning's n coefficient, a time-varying n or Chezy's C coefficient, and the modified momentum and k-ɛ equations for each subarea. To quantify the time varying roughness coefficients, four equations for calculating n values were incorporated in the Delft3D-FLOW program in addition to two options offered by this program to calculate C values. It is concluded that the use of the time varying roughness coefficient gives better results than other approaches. Among the selected equations to calculate the time varying vegetation roughness, the equations that account for the effect of the degree of submergence and the vegetation frontal area per unit volume, symbolized as a, gave the closet matches with the observations. The sensitivity of modeling results to the selection of vertical grid (σ–and Z-grids), a value, and grid size were analyzed. It is found that using the σ-grid yielded more accurate results with less CPU times and the best range of a value for the Panicum hemitomon vegetation type is from 8.160 to 11.220 m⁻¹. Also it was observed that the adoption of a coarse mesh gives reasonable simulation results with less CPU time compared with a fine mesh. A non-linear relation between the vegetation resistance, in terms of n value, and degree of submergence was observed.

Data compiled from standardized procedures for width measurement at established streamflow gaging stations were used to develop a power-function relation between width in feet (W(A)_), and mean discharge in acre-feet per year (Q), for high-gradient perennial streams. Highgradient channels, which generally exhibit low variability for most factors influencing the width-discharge relation, were selected to indicate a standard exponent in the power-function equation. Flume data supported extrapolation of the high-gradient relation, Q = a w(A)¹•⁹⁸, through five orders of discharge magnitude. Further support for a standard exponent of the regression equation was provided by data from Kansas streams that had very silty beds and similar gradients, climate, and vegetation. Regression analysis of data from these streams gave an exponent of 2.01. Hence, a constant exponent of 2.0 was used for the width-discharge relation of all streams. To account for the effect of sediment on channel geometry, silt-clay percentages of the bed and bank material of 98 perennial streams of the western and midwestern United States were introduced into the standard width-discharge relation. Bed and bank cohesiveness, as indicated by silt-clay content, is considered a measure of channel competence. Narrowest streams for a specific mean discharge occur where channel competence, due to fine material and other variables including channel armoring and lush riparian vegetation, is greatest. Thus, at constant discharge, stream width varies inversely with both bed and bank silt-clay content. Multiple-regression analysis yielded the equation: Q = 2.0 w(A)²•⁰ (SC(bd))⁰•²² (SC(bk))⁰•⁵⁷ where SC(bd) and SC(bk) are percent silt-clay of bed and bank material, respectively. The average standard error of estimate for the relation is 58 percent, much of which is inferred to result from excessive width caused by bank erosion of historically recent flood events. Other causes of deviation from the relation include errors associated with the collection and analysis of data, climate and riparian vegetation, discharge variability, and armoring by coarse sediment sizes. Studies showing that stream channels are widened during periods of flooding suggest that most streams subsequently narrow toward an equilibrium width at normal discharges. Assuming that about one-sixth of the data sets, those exceeding one standard deviation, indicate streams too narrow owing to unrepresentative data or recently deficient runoff, the multiple-regression equation was modified to define channel equilibrium. For known conditions of mean discharge and sediment characteristics, an equilibrium width, W(eq) , can be calculated. Comparison with the measured width, W(A), defines the instability ratio, W(A)/W(eq) , The instability ratio identifies the degree to which channel width varies from assumed equilibrium, and ranges from 1.0 to 1.5 for most perennial streams. The ratio of suspended load to bed load appears to be a principal determinant of channel morphology, whereas sediment yield affects the rapidity with which channel healing can occur after widening by flooding. Greatest channel instabilities generally occur in sandchannel streams of arid to semiarid areas. In humid areas, lush vegetation, which encourages accretion of fine sediment sizes to bank material, induces channel stability. Low discharge variability, as shown by springs and regulated streams, generally results in low values of instability. Utility of the multiple-regression equation includes estimation of discharge from ungaged basins, and prediction of short-term changes in channel morphology resulting from altered supplies of water or sediment. Isolation of the influence of sediment on the width-discharge relation also permits consideration of the effects of other variables on channel shape.

In 1983, Laursen and Flick (1) proposed an equation to predict the depth of scour at the toe of a sloping sill. The equation was developed for use with clear-water only since their experiments did not fully investigate the effect of sediment transport. Their report also left open the question of the possible ramifications of sediment transport on the depth of a scour hole. This thesis is an exploratory investigation of the effect of the sediment transport phenomenon on the depth of a scour hole. It will discuss and compare both the clear-water and sediment transport conditions and to explore if the sediment transport rates to be expected in most streams would have a substantial effect on the scour at the toe of a sloping sill.

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A study of downstream suspended sediment transport dynamics in the West River at Cape Bounty, Melville Island, Nunavut, was undertaken in 2012. The first component of the research quantified the sediment mobilized in the West River during the 2012 season. A nival bed-contact survey was undertaken to identify areas of the river in which stream flow was isolated from the bed, and was combined with a reach-based sediment budget approach to assess sediment entrainment and downstream movement. This analysis revealed the propensity of the West River to store suspended sediment through much of the season. Permafrost disturbances in 2007 inundated the West River with fine sediments, the majority of which are progressing from the headwaters as a sediment slug that is subject to substantial downstream storage. Diurnal and event hysteresis analysis from 2004-2012 demonstrate the change in sediment delivery inter-annually, transitioning from a system characterized by clockwise hysteresis prior to the 2007 disturbances, to counter-clockwise hysteresis post 2007. The latter is reflective of the important contribution of the headwater sediment slug from disturbance to downstream sediment transport and common net sediment storage in the lower reaches of the river. The second project studied the delivery of suspended sediment following late season major rainfall events (MRE) and the control antecedent catchment conditions prior to rainfall exert on the magnitude of stream runoff and suspended sediment transport. Two MREs on July 9 and July 23, totalling 35.4 and 10.6 mm, respectively, resulted in exceptionally low discharge response and sediment mobilization. Analysis of synoptic level pressure patterns and catchment soil moisture revealed low volumetric water content preceding both MREs, a result of sustained exceptional early summer warmth under stable regional high pressure. Compared to similar MREs in 2007-2009, the soil in 2012 did not become saturated, and substantial runoff did not occur. These studies contribute to an understanding of the processes of sediment transport in response to disturbances, rainfall, and antecedent catchment conditions, all of which are becoming important components of the Arctic fluvial systems but have had limited study due to the emphasis on snow melt processes and hydrological fluxes. / Thesis (Master, Geography) -- Queen's University, 2013-09-25 11:36:58.882

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