Fu-Chun Wu

Support vector machine approach for longitudinal dispersion coefficients in natural streams

This paper presents the support vector machine approach to predict the longitudinal dispersion coefficients in natural rivers. Collected published data from the literature for the dispersion coefficient for wide range of flow conditions are used for the development and testing of the proposed method. The proposed SVM approach produce satisfactory results with coefficient of determination=0.9025 and root mean square error=0.0078 compared to existing predictors for dispersion coefficient.

Modeling embryo survival affected by sediment deposition into salmonid spawning gravels: Application to flushing flow prescriptions

This paper presents a framework for predicting embryo survival in salmonid spawning gravels as a function of sediment deposition. This framework integrates three quantitative relationships modeling the variations of substrate permeability with sediment deposition, apparent velocity with substrate permeability, and embryo survival rate with apparent velocity. The model allows evaluation of the impacts of sediment deposition on embryo survival. The relative sensitivity of embryo survival to three selected environmental factors is investigated. The model results indicate that embryo survival is most sensitive to the composition of fine sediments (or sediment-gravel size ratio). The maximum influences of the hydraulic pressure head and the length of intragravel flow path are ;60 and 35% of the value influenced by size ratio. The proposed model is applied to determine the timing of flushing flows. The results suggest that the interval between flushing flows should be reduced when higher levels of embryo survival are prescribed or higher near-bed sediment concentrations are imposed.

Quantifying the forcing effect of channel width variations on free bars: Morphodynamic modeling based on characteristic dissipative Galerkin scheme

6 [1] The forcing effect of channel width variations on free bars is investigated in this study 7 using a two‐dimensional depth‐averaged morphodynamic model. The novel feature of 8 the model is the incorporation of a characteristic dissipative Galerkin (CDG) upwinding 9 scheme in the bed evolution module. A correction for the secondary flows induced by 10 streamline curvature is also included, allowing for simulations of bar growth and migration 11 in channels with width variations beyond the small‐amplitude regimes. The model is 12 tested against a variety of experimental data ranging from purely forced and free bars to 13 coexisting bed forms in the variable‐width channel. The CDG scheme effectively dissipates 14 local bed oscillations, thus sustains numerical stabilities. The results show that the global 15 effect of width variations on bar height is invariably suppressive. Such effect increases 16 withthedimensionless amplitudeACandwavenumberlCofwidthvariations. ForsmallAC, 17 lChas little effects on bar height; forACbeyond small amplitudes, however, the suppressing 18 effect depends on both AC and lC. The suppressing effect on bar length increases also 19 with both AC and lC, but is much weaker than that on bar height. The global effect of width 20 variations on bar celerity can be suppressive or enhancive, depending on the combination 21 ofACandlC.ForsmallerlC,theeffectonbarcelerityisenhancive;forlargerlC,barcelerity 22 tends to increase at small AC but decreases for AC beyond small amplitudes. We present 23 herein an unprecedented data set verifying the theoretical prediction on celerity enhancement. 24 Full suppression of bar growth above the theoretically predicted threshold AC was not 25 observed, regardless of the adopted amplitude of initial bed perturbation A. The global 26 effects of width variations on free bars can be quantified using a forcing factor FC that 27 integrates the effects of ACandlC. The suppressing effects on bar height and length are both 28 proportional toFC ; the global effect onbar celerity is,however, aparabolic function ofFC.

Assessment of flow regime alterations over a spectrum of temporal scales using wavelet‐based approaches

The full range of natural flow regime is essential for sustaining the riverine ecosystems and biodiversity, yet there are still limited tools available for assessment of flow regime alterations over a spectrum of temporal scales. Wavelet analysis has proven useful for detecting hydrologic alterations at multiple scales via the wavelet power spectrum (WPS) series. The existing approach based on the global WPS (GWPS) ratio tends to be dominated by the rare high-power flows so that alterations of the more frequent low-power flows are often underrepresented. We devise a new approach based on individual deviations between WPS (DWPS) that are root-mean-squared to yield the global DWPS (GDWPS). We test these two approaches on the three reaches of the Feitsui Reservoir system (Taiwan) that are subjected to different classes of anthropogenic interventions. The GDWPS reveal unique features that are not detected with the GWPS ratios. We also segregate the effects of individual subflow components on the overall flow regime alterations using the subflow GDWPS. The results show that the daily hydropeaking waves below the reservoir not only intensified the flow oscillations at daily scale but most significantly eliminated subweekly flow variability. Alterations of flow regime were most severe below the diversion weir, where the residual hydropeaking resulted in a maximum impact at daily scale while the postdiversion null flows led to large hydrologic alterations over submonthly scales. The smallest impacts below the confluence reveal that the hydrologic alterations at scales longer than 2 days were substantially mitigated with the joining of the unregulated tributary flows, whereas the daily-scale hydrologic alteration was retained because of the hydropeaking inherited from the reservoir releases. The proposed DWPS approach unravels for the first time the details of flow regime alterations at these intermediate scales that are overridden by the low-frequency high-power flows when the long-term averaged GWPS are used.

Asymmetric Effects of Subaerial and Subaqueous Basement Slopes on Self-Similar Morphology of Prograding Deltas

Deltas form over basements of various slope configurations. While the morphodynamics of prograding deltas over single-slope basements have been studied previously, our understanding of delta progradation over segmented basements is still limited. Here we use experimental and analytical approaches to investigate the deltaic morphologies developing over two-slope basements with unequal subaerial and subaqueous slopes. For each case considered, the scaled profiles of the evolving delta collapse to a single profile for constant water and sediment influxes, allowing us to use the analytical self-similar profiles to investigate the individual effects of subaerial/subaqueous slopes. Individually varying the subaerial/subaqueous slopes exerts asymmetric effects on the morphologies. Increasing the subaerial slope advances the entire delta; increasing the subaqueous slope advances the upstream boundary of the topset yet causes the downstream boundary to retreat. The delta front exhibits a first-retreat-then-advance migrating trend with increasing subaqueous slope. A decrease in subaerial topset length is always accompanied by an increase in subaqueous volume fraction, no matter which segment is steepened. Applications are presented for estimating shoreline retreat caused by steepening of basement slopes, and estimating subaqueous volume and delta front using the observed topset length. The results may have implications for real-world delta systems subjected to upstream tectonic uplift and/or downstream subsidence. Both scenarios would exhibit reduced topset lengths, which are indicative of the accompanied increases in subaqueous volume and signal tectonic uplift and/or subsidence that are at play. We highlight herein the importance of geometric controls on partitioning of sediment between subaerial and subaqueous delta components.

A heuristic probabilistic approach to estimating size-dependent mobility of nonuniform sediment

This paper presents a heuristic probabilistic approach to estimating the size-dependent mobilities of nonuniform sediment based on the pre- and post-entrainment particle size distributions (PSDs), assuming that the PSDs are lognormally distributed. The approach fits a lognormal probability density function to the pre-entrainment PSD of bed sediment and uses the threshold particle size of incipient motion and the concept of sediment mixture to estimate the PSDs of the entrained sediment and post-entrainment bed sediment. The new approach is simple in physical sense and significantly reduces the complexity and computation time and resource required by detailed sediment mobility models. It is calibrated and validated with laboratory and field data by comparing to the size-dependent mobilities predicted with the existing empirical lognormal cumulative distribution function approach. The novel features of the current approach are: (1) separating the entrained and non-entrained sediments by a threshold particle size, which is a modified critical particle size of incipient motion by accounting for the mixed-size effects, and (2) using the mixture-based pre- and post-entrainment PSDs to provide a continuous estimate of the size-dependent sediment mobility.

Anisotropy Characteristics of Exposed Gravel Beds Revealed in High-Point-Density Airborne Laser Scanning Data

The aim of this study was to examine the relationship between the anisotropy direction of exposed gravel bed and flow direction. Previous studies have shown that the anisotropy direction of a gravel bed surface can be visually determined in the elliptical contours of 2-D variogram surface (2DVS). In this letter, airborne laser scanning (ALS) point clouds were acquired at a gravel bed, and the whole data set was divided into a series of 6 m × 6 m subsets. To estimate the direction of anisotropy, we proposed an ellipse-fitting-based automatic procedure with consideration given to the grain size characteristic d50 to estimate the primary axis of anisotropy [hereafter referred to as the primary continuity direction (PCD)] in the 2DVS. The ALS-derived PCDs were compared to the flow directions (for both high and low flow) derived from hydrodynamic model simulation. Comparison of ALS-derived PCDs and simulated flow directions suggested that ALS-derived PCDs could be used to infer flow direction at different flow rates. Furthermore, we found that the ALS-derived PCDs estimated from any elliptical contour of the 2DVS exhibited a similar orientation when the contours of the 2DVS reveal the clear anisotropic structure, demonstrating the robustness of the technique.

Potential for application of an acoustic camera in particle tracking velocimetry

We explored the potential and limitations for applying an acoustic camera as the imaging instrument of particle tracking velocimetry. The strength of the acoustic camera is its usability in low-visibility environments where conventional optical cameras are ineffective, while its applicability is limited by lower temporal and spatial resolutions. We conducted a series of experiments in which acoustic and optical cameras were used to simultaneously image the rotational motion of tracer particles, allowing for a comparison of the acoustic- and optical-based velocities. The results reveal that the greater fluctuations associated with the acoustic-based velocities are primarily attributed to the lower temporal resolution. The positive and negative biases induced by the lower spatial resolution are balanced, with the positive ones greater in magnitude but the negative ones greater in quantity. These biases reduce with the increase in the mean particle velocity and approach minimum as the mean velocity exceeds the threshold value that can be sensed by the acoustic camera.