Bedload-trapping efficiencies for four types of pressure-difference bedload samplers – a standard Helley-Smith (intake-nozzle width and height of 76.2 mm x 76.2 mm), BLH-84 (76.2 mm x 76.2 mm), Elwha (203 mm x 102 mm) and Toutle River-2 (305 mm x 152 mm) a standard Helley-Smith, US BLH-84 (both with intake nozzle dimensions of 76.2 mm × 76.2 mm), Elwha (203 mm × 102 mm) and Toutle River-2 (TR-2; 305 mm × 152 mm) – were calculated from data collected during the StreamLab06 experiments in the St. Anthony Falls Laboratory Main Flume during January-March 2006. Sampler nozzle-flare ratios –the area of the nozzle's outlet divided by its inlet area – equaled 1.4 for all but the Helley-Smith sampler's nozzle-flare ratio of 3.22. A sampler's trapping coefficient quantifies its bedload-trapping efficiency. Technically supportable trapping coefficients are divided into raw trapping rates measured by the sampler to produce "true" bedload-transport rates equivalent to that which was inferred to have occurred in the absence of the sampler. Six combinations of sampler and bed types were tested; the BLH-84, Elwha, and Helley-Smith samplers were deployed atop a sand bed (D50 = 1.0 mm) during five steady flows ranging from 2.0-3.6 m3/s. The BLH-84, Elwha, and TR-2 samplers were deployed atop a gravel bed (D50 = 11.2 mm) at four steady flows ranging from 4.0-5.5 m3/s. Thirty-seven trials – repeated manual at-a-point deployments of a given bedload sampler for a given steady flow and bed type – took place. Trapping coefficients were calculated for each sampler and bed type in which it was deployed. Ergo, two of the samplers – the BLH-84 and Elwha – were each assigned two trapping efficiencies for sampling on a sand versus a gravel bed. These data were evaluated using four analytical methods: Ratio of Averages: This relatively simple and straight-forward method required calculating averages of bedload-transport rates derived for each of the 37 trials for a given bedload sampler and for up to nine combinations of weigh pans and time intervals. The computations were performed using untransformed data. Average of Ratios: This more complex method using real-space trapping data involved developing average transport rates from selected pan data for each bedload sample. Pan transport-averages were calculated for each interval equal to the duration of a single at-a-point bedload measurement, ranging from 15-180 seconds. Ratios (coefficients) were calculated by dividing each interval average into the single-sample trap rate. Those ratios were then averaged to produce a single trapping coefficient for the trial and then combined into a single average for each bedload-sampler/bed type/flow combination. Modified Thomas and Lewis Model (1993): The Thomas-Lewis Model was revised to operate using untransformed data in addition to cube-root transformed data (thus, the third and fourth analytical methods used, respectively), and to use nine pan-window combinations to calculate trapping coefficients. The original 3-step model required first regressing cube root-transformed sampler data on time-window averaged pan transport rates. The second step squared the regression residuals from the first step on the variance of the cube root of the interval-mean transport rate for the time window. The predicted values from the second-step regression were inverted and used as weights to re-estimate the first-step regression. Generalized trapping-coefficient calculations based on results from the four analytical methods for the bed-types in which the samplers were deployed follow: • BLH-84 Sampler: A 0.83 sand-bed trapping coefficient and 0.87 gravel-bed coefficient, which could be averaged to a single coefficient of 0.85. • Elwha Sampler: A 1.67 sand-bed trapping coefficient and 1.54 gravel-bed coefficient, which could be averaged to a single coefficient of 1.6 • Helley-Smith Sampler: The 3.11 sand-bed trapping coefficient could be applied as such or reasonably simplified to a value of 3.0, and • TR-2: The gravel-bed trapping coefficient equaled 1.70. An unadjusted bedload-trapping rate calculated from a sample collected by a given sampler can be divided by its trapping coefficient(s) to obtain the most reliable transport-rate value. / Ph.D.

No description available.

Bedload transport has long been known for its complexity. Despite decades of research, significant gaps of understanding exist in the ability to assess and predict bedload movement. This work introduces a comprehensive bedload database that is a compilation of field samples collected over the past 40 years; compares prediction formulae using a subset of the database; evaluates the influence of the armor layer on stream response to sediment input based on a hypothesis linked to one of the tested formulae, presents a mathematically manipulation of the empirical Pagosa Good/Fair formula for bedload transport into a format similar to the semi-empirical Parker Surface-Based 1990 formula; and addresses the complications of bedload transport by collecting bedload samples on a stream in Central Utah. A comprehensive review of available bedload data resulted in a publicly available database with more than 8,000 individual bedload samples on gravel bed streams. Each measurement included extensive and detailed information regarding channel, site, and hydraulic characteristics. A subset of this database was used to compare four calibrated (a single calibration point of a measured bedload transport rate near bankfull discharge is used to improve formula prediction accuracy) and two un-calibrated bedload prediction formulae. The four calibrated formulae include three semi-empirical (a theoretical treatment adjusted to fit bedload measurements) formulae and one empirical (solely based on regression of bedload measurements) formula; the two un-calibrated formulae are both semi-empirical. Of the formulae compared, the empirical Pagosa Good/Fair formula (a calibrated formula) provided the most accurate prediction results with an overall root mean square error of 6.4%, an improvement of several orders of magnitude over the un-calibrated formulae. The Pagosa Good/Fair formula is cast in a form similar to the Parker 1990 formula, suggesting that criticisms stating that the empirical Pagosa method lacks a theoretical basis are unfounded. The hypothesis of equal mobility that states the gradation of the average annual gravel bedload yield for a given stream matches the particle size distribution of the subsurface material is evaluated with relation to the armor layer. Equal mobility is found to correlate to armor layer such that lower armor ratios indicate a greater tendency to uphold the equal mobility hypothesis and increasing armor ratio values tending to move toward supply limited conditions. This correlation provides an upper limit for lightly armored streams. Bedload sampling efforts described in this work compare the Helley-Smith sampler with the net trap sampler and duplicate previous observations that bedload transport collected using net traps increase more rapidly with discharge than for data collected using Helley-Smith samplers. An alternative, relatively low-cost method for collecting bedload during relatively high discharges on highly urbanized streams is also proposed.

The bedload transport of mixed size sediment is an important process in river engineering. Bedload transport controls channel stability and has a significant bearing on the hydraulic roughness of the channel. The prediction of bedload transport traditionally relies upon defining some critical value of fluid force above which particles of a particular diameter are assumed to be put into transport. The suggestion here is that the transport of bed material is size dependent with large grains being more difficult to remove from the bed surface than small grains and that all grains of the same size start to move under identical conditions. While it is relatively straightforward to assess the forces required to engender transport in a bed of uniform size grains, it is not so simple where there are a number of different grain sizes present. Multitudinous experimental studies have revealed that where there are a number of grain sizes present, large grains tend to become mobilised under lower fluid forces and small grains mobilised under higher fluid forces than those required for beds of uniform material. These results led to the development of so-called hiding functions which are used to model the variation of particle mobility with its relative size within the mixture. These functions derive their name from the tendency of large grains to shelter smaller grains from the action of the flow. Determining the relative mobility of each fraction in a mixture under given hydraulic conditions is the key to predicting how the composition of the bed load will relate to that of the bed surface material. Experiments were carried out in a rectangular, glass sided channel, in a sediment recirculation mode, under varying hydraulic conditions with a set of six different sediment mixtures. Laser Doppler Anemometry (LDA) was used to attain instantaneous velocity measurements at a number of locations in the flow. A Laser Displacement Meter was used to measure the detailed topography of small sections of the bed surface. Novel analysis techniques facilitated the determination of the grain size distribution of the bed surface by a grid-by-number method. The minimum force required to entrain each grain could also be estimated by a grain pivoting analysis. This information represents the resistance of the bed grains to erosion by flowing water. With the critical conditions for the bed grains known, it is possible to estimate the proportion of each fraction entrained from the bed surface under given hydraulic conditions. To estimate the bedload composition it is first necessary to scale by the proportion each size comprises on the bed surface and then, by a function of grain diameter to account for size dependency of travel velocity. For mean hydraulic conditions the proportion of the bed mobilised can be simply determined by inspection of a cumulative distribution of critical conditions. In reality, although it may be possible to entrain some grains at the mean velocity/shear stress, the majority of transport may be anticipated to occur during high magnitude events. Turbulence may be incorporated by adopting a probabilistic approach to the prediction of grain entrainment. By considering the joint probability distribution of bed shear stress and critical shear stress, one may attain the probability of grain entrainment. Comparison of the probability of erosion of each fraction facilitates a prediction of the bedload composition. Results show that the probabilistic approach provides a significant improvement over deterministic methods for the prediction of bedload composition.

Only a small body of work currently exists regarding the geomorphology of humid tropical montane rivers. The research that does exist reports rapid geomorphic processes and high sediment loads compared to other montane rivers. This research applies traditional field survey methods combined with new applications of remote sensing techniques to examine the geomorphology and sediment dynamics of the montane portions of the Rio Pacuare in Costa Rica. A suite of geomorphic components (channel slope and width, lateral contributions and planform) are examined and a model presented that illustrates the complexity of the Rio Pacuare’s geomorphology and how the distribution of alluvial sediment varies in relation to geology (tectonics and lithology) and flow hydraulics (stream power). Next, average annual bedload sediment transport capacity is estimated using fifty-one years of daily discharge data at six different locations within the study area, including the temporal (monthly) variability of sediment flux due to dry versus wet season discharge regimes. Then, a time-step hydraulic model is created that simulates observed (modern) and potential future discharge scenarios based on regional climate change model results. The simulated discharge data for two locations within the study area is then integrated into the sediment transport model to examine how sediment flux, and thus channel geomorphology, is likely to change in response to changes in the river’s discharge regime.

The coastal wetlands at Aransas National Wildlife Refuge near Austwell, Texas, support the last migrating population of whooping cranes during the winter months (October through April). With a population currently at 216 individuals, these are the rarest cranes in the world. The wetlands in which they winter are a part of the San Antonio Bay system, a bay that receives constant fresh water flow from the Guadalupe River. Currently there is a plan for using water diverted from the Guadalupe River just before it enters San Antonio Bay as a water supply for the greater San Antonio metropolitan area located 200 km to the northwest. The Guadalupe River delivers nutrients and sediment into the estuary along with fresh water. Because of the importance of sediment within a tidal wetland ecosystem, it is imperative to understand the sediment budget and underlying forces that drive it if one is to ultimately grasp how this ecosystem functions. To document natural and anthropogenic factors exerting control over sediment movement in this system, three sites on tidal creeks near the boundary between marsh and bay were chosen. The Gulf Intracoastal Waterwayparallels the marsh edge. Over six, non-consecutive weeks water level and velocity were automatically monitored in the tidal creeks. Automated water samplers extracted water samples that were analyzed for suspended sediment. In addition, bedload traps were deployed in one creek to monitor sediment movement along the channel bottom. Inflow exceeded outflow during the study. As a result there was a net influx of suspended sediments into the marsh. Bedload material also moves with current direction, and it appears to move in response to barge induced outflow currents. Barges passing on the Gulf Intracoastal Waterway exert influence on water level, flow direction, and velocity within tidal creeks. Natural factors such as winds, tides, and freshwater input from upland runoff or river discharge also impact suspended and bedload sediments.

Conventional morphodynamic models are typically based on a coupled system of hydrodynamic equations, a bed-update equation, and a sediment-transport equation. However, the sediment-transport equation is almost invariably empirical, with numerous options available in the literature. Bed morphological evolution predicted by a conventional model can be very sensitive to the choice of sediment-transport formula. This thesis presents a physics-based model, where the shallow water-sediment-mixture flow is idealised as being divided into two layers of variable (in time and space) densities: the lower layer concerned with bedload transport, and the upper layer representing sediment in suspension. The model is referred to as a Quasi-2-Layer (Q2L) model in order to distinguish it from typical 2-Layer models representing stratified flow by two layers of different but constant and uniform densities. The present model, which does not require the selection of a particular empirical formula for sediment transport rates, is satisfactorily validated against widely used empirical expressions for bedload and total transport rates. Analytical solutions to the model are derived for steady uniform flow over an erodible bed. Case studies show that the Q2L model, in contrast to conventional morphodynamic approaches, yields more realistic results by inherently including the influence of the bed slope on the sediment transport. This conclusion is validated against experimental data from a steep sloping duct. An analytical study using the Q2L model investigates the influence of bed-slope on bedload transport; the resulting expressions are in turn used to modify empirical sediment transport formulae (derived for horizontal beds) in order to render them applicable to arbitrary stream-wise slopes. The Q2L model provides an alternative approach to studying sediment-transport phenomena, whose adequate analysis cannot be undertaken following coniv ventional approaches without further increasing their degree of empiricism. The Q2L model can also lead to the enhancement of conventional morphodynamic models. For coarse sediments and/or relatively low flow velocities, bedload transport is usually responsible for most sediment transport. Bedload transport consists of a combination of particles rolling, sliding and saltating (hopping) along the bed. Hence, saltation models provide considerable insight into near-bed sediment transport. This thesis also presents an analysis of the statistics and mechanics of a saltating particle model. For this purpose, a mathematically simple, computationally efficient, stochastic Lagrangian model has been derived. This model is validated satisfactorily against previously published experimental data on saltation. The model is then employed to derive two criteria aimed at ensuring that statistically convergent results are achieved when similar saltation models are employed. According to the first criterion, 103 hops should be simulated, whilst 104 hops ought to be considered according to the second criterion. This finding is relevant given that previous studies report results after only a few hundred, or less, particle hops have been simulated. The model also investigates sensitivity to the lift force formula, the friction coefficient, and the collision line level. A method is proposed by which to estimate the bedload sediment concentration and transport rate from particle saltation characteristics. This method yields very satisfactory results when compared against widely used empirical expressions for bedload transport, especially when contrasted against previously published saltation-based expressions.

Braiding is a mesmerizing phenomenon since flow and sediment transport interact and are able to change the morphology of a channel in a rapid and complex fashion. Conventional two-dimensional morphodynamic models estimate bedload distribution using shear stress distribution. However, it is unclear if the use of such shear stress distributions is relevant or applicable for all situations when using two-dimensional morphodynamic modelling. This thesis strives to investigate whether shear stress distributions are useful to predict bedload transport pathways. This study focuses upon prediction of bedload transport pathways using a morphodynamic model (Delft3D) of an anabranch of the Rees River (New Zealand). Observed bedload transport pathways were compared to modelled bedload transport pathways in an attempt to validate the predictive ability of the model. Results show that there is a significant correlation between predicted bedload transport pathways and the apparent bedload transport pathways derived from the field measurements. Furthermore, bedload transport predictions were in good agreement with observed data in areas where the model’s predictions of high shear stress were comparable to field observations. However, substantial bedload transport predictions in low shear stress areas were not adequately captured by the model, suggesting that the observed pathways were not due to high shear stress, but rather to other sediment supply sources.

付記する学位プログラム名: グローバル生存学大学院連携プログラム / 京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22419号 / 工博第4680号 / 新制||工||1730(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 角 哲也, 准教授 竹門 康弘, 准教授 Sameh Kantoush / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.