Journal of Hydraulic Engineering

Dredging is the relocation of soil. Before the soil can be transported, it has to be loosened. This can be done hydraulically (jetting) or mechanically (cutting). Often, water jets are used to erode the soil layer. Over time, pickup functions have been derived to predict the amount of erosion corresponding to the flow conditions. However, existing pickup functions are inaccurate at high flow velocities. During the current study, erosion experiments have been done at high flow velocities (up to $4.7\mathrm{m}/\mathrm{s}$) corresponding to a bed shear stress of up to 60 Pa and a Shields parameter (θ) of up to 30. The results of these experiments were compared with a number of well-known data sets and pickup functions.

Past researchers have studied particle entrainment on a flat-bed and the flow fields in a rectangular cavity without considering particle entrainment. However, particle entrainment from a rectangular cavity, which hitherto had rarely been studied, is the focus of this study. The entrainment phenomenon is investigated by using a frequentist probabilistic approach. The spatial and temporal variations of the cavity and flat-bed flow fields are compared using measurements from the particle image velocimetry and acoustic doppler velocimetry techniques. The particle’s motion from the cavity is superimposed onto the measured flow field to examine the mechanism of entrainment. The results show that particle entrainment from a cavity subjected to the same approach flow takes place over a wide range of the Shields parameter and in highly variable time duration. Additionally, the particle’s upward trajectory reveals how the highly turbulent and unsteady flow affects the entrainment process, resulting in the observed stochastic trend.

Treatment of the free-surface pressure boundary (FSPB) remains a challenge for the three-dimensional (3D) nonhydrostatic hydrodynamic models using a vertical staggered-grid variable arrangement. Through the tests of the small-amplitude wave, it was found that the pressure of the top layer of the vertical grid is semihydrostatic for vertical staggered-grid nonhydrostatic models. Based on this finding, a modified ghost-cell FSPB treatment is proposed and a fully nonhydrostatic staggered-grid model developed. The new method is simple and easy to implement, and has the self-calibration property of its parameters. The model was further tested using a laboratory experiment of nonlinear waves propagating over a bar toward a beach. Sensitivity studies of model parameters and grid scales were performed. The vertical staggered-grid model using the new FSPB treatment is shown to require only coarse vertical grids to achieve accurate simulations of nonhydrostatic free-surface flows.

For stormwater systems, geysers in manholes are a risk to pedestrian and vehicle safety and the urban environment. Manhole covers dancing and blowing off are reported worldwide during heavy rainfall and geyser events. These manhole cover motions impact geyser characteristics but are rarely studied. This work experimentally studied geysers in covered manholes induced by air pocket release. Both fixed and mobile covers were tested, and the effects of manhole diameter and cover ventilation size were considered. The geyser regimes are classified into nongeyser, single-geyser, and multigeyser. The mobile covers can dance and blow off, resulting in pressure at the manhole headspace with characteristics of oscillation with high frequency, decline of peak magnitude, and negative pressure occurrence. The maximum positive pressure can be seven times higher for fixed covers than that for mobile covers. Increasing the manhole diameter decreases the number of geysers and the maximum positive pressure for both fixed and mobile covers. Decreasing cover ventilation reduces the number of geysers and increases the maximum positive pressure for the fixed covers, although it has limited effects for mobile covers.