Environmental Fluid Mechanics & Hydrology
School of Civil & Environmental Engineering
- Environmental fluid mechanics
- Coastal engineering
- Nearshore hydrodynamics
- Swash zone
- Boundary layers under long waves
- Direct measurement of wall shear stress
During my PhD, my research has investigated topics around the central question of how breaking waves influence the beach. We know that the swash zone – the region of the beach that becomes wet and dry with each wave – is important in the transport of sediment on timescales much longer than one wave, but how each incoming wave acts to move the sediment is a more difficult question. Most theoretical models of sediment pick-up rely on knowledge of the shear stress that the water motion exerts on the surface of the beach, so this is the quantity that we would like to know about.
Direct measurements of local bed shear stress
The nearshore region of the beach is a difficult place to make measurements. Even in the laboratory, where wave conditions can be controlled, measurement instruments face challenges in the very shallow and bubbly flow created by breaking waves. The first part of my Ph.D. work was to create a custom sensor to measure the bed shear stress field in the laboratory. We decided to take the approach of direct measurements using a shear plate sensor. A section of the flume bottom is removed and replaced with a plate that has small gap around it so that it can deflect under the stress of the water. The idea being that the stress is measured directly and the shallowness of the flow doesn’t matter.
This type of sensor typically faces difficulties if there is a pressure gradient in the flow because there are extra forces on the plate. We addressed this issue by analysing the flow around the edges of the plate when the pressure gradients are mild and found a method for first order correction for this force. The development and validation of the sensor and the analysis for the correction for extra forces can be found in the following publication: Pujara, N. & Liu, P.L.F., 2014. Direct measurements of local bed shear stress in the presence of pressure gradients. Experiments in fluids, 55(7), p.1767.
Experimental study of the swash zone
To study the swash zone at a physical scale relevant to the field is rare. We conducted experiments at the long wave flume at the Hinsdale Wave Research Laboratory at Oregon State University. The flume is over 100 m long, 3.7 m wide and 4.6 m deep and a 1:12 slope was installed at one end of the flume to resemble a plane beach. The swash of a solitary wave was chosen as the archetypical swash event. This was because: (i) the generation solitary waves can be well controlled in the laboratory; (ii) solitary waves generate a single swash event that is very repeatable. Waves on beaches also often resemble solitary waves since they are finite amplitude long waves. As well as a single solitary wave, limited cases of successive solitary waves were also generated to study the influence of one swash event on the following incident wave.
Typically, a swash event begins with a collapse of the broken wave near the shoreline and then the swash ‘tongue’ climbs the slope. The shear plate sensor was used to measure the bed bed shear stress in different locations within the swash zone. It was found that the bed shear stress is the highest in the vicinity of the swash tip, i.e., the moving shoreline and this is where the sediment experiences the largest forces. This high value is usually difficult to predict using standard models that estimate the bed shear stress from the mean, steady velocity. Behind the swash tip, the bed shear stress was found to be very low until the flow has changed direction and was rushing back down the slope. Typically, the bed shear stress under the swash tip is higher in magnitude than the bed shear stress during the downrush. Thus, the swash is said to asymmetric. The asymmetry reduces further up the slope and the values of the bed shear stress also drop in magnitude, being highest near wave collapse. The findings from this study are being written up into a paper that is currently under preparation.