Publications

While turbulent flow characteristics in the region upstream of wall-mounted obstacles (often called the junction region) are well described for many engineering systems, uncertainty remains about flow characteristics upstream of the large, immobile boulders that are commonly found in steep gravel-bed streams. This uncertainty is largely due to the unique bed features (e.g., relatively large bed roughness, a permeable bed) and hydraulic characteristics (e.g., variable submergence) present at boulders, which may affect flow characteristics. This study reports results of laboratory volumetric particle image velocimetry (PIV) experiments performed upstream of model boulders for fully submerged (FS) and partially submerged (PS) conditions. Several atypical junction flow characteristics were documented. Mean-flow reversal and spiraling streamlines (commonly associated with horseshoe vortices) were not documented, though limitations in near-bed measurement extents contributed to this result in several cases. Nevertheless, modest increases in local rotation rate (assessed via vorticity magnitude and swirling strength) were observed near where the horseshoe vortex is typically expected. Mean-flow data also suggest that notable mass flux into the permeable bed occurred upstream of boulders. Several effects of submergence on junction flow characteristics were also identified. Compared with the FS condition, the PS condition exhibited more rapid deceleration of the mean streamwise velocity, stronger downward velocities near the bed that extended over a wider transverse area, stronger increases of the near-bed turbulent kinetic energy, and oppositely signed streamwise-vertical Reynolds stresses near the water surface. The improved understanding of flow upstream of boulders provided here will aid future river engineering and restoration efforts.

Prediction of bedload rates in gravel-bed rivers at low-to-moderate flow conditions, where bedload movement is intermittent, remains a challenging problem. While the virtual velocity concept provides a useful approach to bedload rate estimation in the intermittent movement regime, virtual velocity estimation remains hindered by a lack of tools for predicting mean sediment resting time. As a first step toward sediment resting time estimation in gravel beds, the present study develops a semi-theoretical resting time model applicable to nonuniform gravel-sized spherical particles. The model is based on the consideration that interactions of near-bed flow with bed material lead to mobilization of individual resting particles during hydrodynamic momentum transfer events (i.e., impulses). Thus, resting time is affected by impulse magnitude and timing. The primary premise underpinning model development is that an instantaneous velocity time-series generation approach based on the velocity spectrum can be used to mimic hydrodynamic impulses and simulate resting times. Based on past findings, two model parameters are considered important to advancing resting time predictions in gravel beds. First, the relative particle size allows size-fractional resting time predictions for a nonuniform sediment mixture. Second, the hindrance coefficient accounts for hiding effects and enables resting time predictions for different bed structure types. To provide calibration and verification data, laboratory experiments documenting impulse statistics and mean resting times for a range of flow and relative particle size conditions were also performed. The verified model exhibits mean resting times with similar magnitude and trends with increasing stress compared with experimental verification data.

In this work, the backward wave breaking process by the presence of flow separation vortices under a solitary wave is studied. Based on a set of non-dimensional variables defined from the Buckingham Π theorem, a set of numerical experiments are performed in order to analyze the effect of varying the submerged obstacle geometry on the surrounding flow and free surface by using a RANS-VOF model. Model simulations are tested against available experiments showing a good performance of numerical results. The structure-submergence ratio and the structure-based Reynolds number modulate the type of breaking (collapsing-plunging). The structure aspect ratio defines the location and number of backward breaking points for rectangular structures and the breaking wave direction for triangular structures (either forward or backward). Moreover, when the flow separation vortex diameter is comparable to the local water depth a backward wave breaking process is originated. The vortex behaves as a rigid body that accelerates the flow at the upper side of the vortex, accumulating mass at the downstream side where a very large slope on the free surface makes the water fall backward due to gravity.

The influence of boulders in high gradient gravel-bed rivers can be significant. Yet few studies have isolated their role in regulating bedload. It is hypothesized that boulder relative submergence and the corresponding eddies developing around boulders affect the frequency of bedload pulsations and the location of sediment deposits. Results show the occurrence of two distinct timescales in bedload exiting a boulder array, which were identified via time series analysis. These are the small-scale periodicity, P_S, and the large-scale periodicity, P_L. P_S was identified in the range of 4–6 min and is believed to result from congested bedload movement due to reduced conveyance area within the array. P_L was identified in the range of 8–46 min. It is suggested that P_L corresponds to large bedload releases around boulders, which the authors consider to be caused by feedbacks between boulder eddies and bedload deposits. It is also found that boulder submergence and Froude number, Fr, influence the location of predominant deposition. At High Relative Submergence, deposition occurs in the boulder wake regions. At Low Relative Submergence, deposition instead occurs upstream of boulders in locations that depend on Fr. For Fr < 1, material deposits in the stoss of boulders due to the necklace structure of the horseshoe vortex. However, for Fr > 1, material deposits at the flanks of boulders due to the influence of local wave crests around boulders. The trapping efficiency of boulders reduces the dimensionless mean bedload transport rate by three orders of magnitude compared to conditions without boulders.