My work primarily involves the the movement of sediment by waves and currents on a process scale (the size of ripples and smaller), and on a regional scale (the size of estuaries). The work includes computer modelling, experiments and fieldwork.
1. Hydrodynamics and sand transport modelling above flat and rippled beds
The process of sand transport by waves and currents in coastal seas is crucially dependant on the interface between the sand at the bottom and the water above it. This work involves the use of computer models to mimic sand and water movement above this interface and laboratory experiments to validate these models. Different strategies are used depending on whether the bed is rippled (corresponding to moderate flow conditions where the sand grains hop along the bottom) or flat (corresponding to more intense flow conditions where whole interface moves as a sheet) and also the composition of the bed material. This work is important in understanding sand transport because it determines how sand gets into the flow and which direction it will go. This work has been carried out in the EU
(02-05) Projects and the
EPSRCSANTOSS (05-09) Project.
This is an example of the output from a simple program I have written in MATLAB using Stokes second-order wave theory, which produces a movie of particle orbits under a wave. This is a progressive wave over 5 wave cylces and includes the effect of friction through
mass transport. I am also interested in the shape of free-stream velocity profiles near the bed under shoaling waves (see Malarkey and Davies, 2012b)
In the case of wave flow over steep ripples, the near-bed oscillating flow results in the periodic shedding of vortices from either side of the ripple, which completely dominates hydrodynamics and sediment sediment transport. The animation below is a result from a computer model I developed for my PhD.
It shows vortcity contours (blue -anticlockwise/ red -clockwise) in response to the horizontal oscillatory motion associated with a wave as shown schematically by the arrow. For more information see Malarkey and Davies (2003,2004); van der Werf et al. (2009) and Malarkey et al. (2015b)
Traditionally the behaviour of bedforms has been examined with non-cohesive 'clean' sand. As part of the
COHBED project (12-15) we have been considering the effect of biological cohesion, as a result of extra polymeric substances (EPS) secreted by micro-organisms, and physical cohesion as a result of clays. This work which is a collaboration between
has involved laboratory experiments and a field campaign in the Dee Estuary. At Bangor and Hull we have conducted laboratory experiments using Xanthan Gum as a proxy for EPS and kaolin (china clay) to represent the physical cohesion. These experiments have produced some quite strong effects (see Baas et al., 2013; Malarkey et al., 2015a; Schindler et al., 2015; Parsons et al., 2016). The animation below is an example from the Bangor flume using time-lapse photography. An initially flat, non-cohesive, sandy bed is subjected to a constant current (depth-averaged flow is 40 cm/s, water depth is 25 cm and the median grain size 0.140 mm). The flow is from left to right. The ripples migrate in the current direction, they start off small and regularly spaced (because they are straight crested) but become more varied as they grow and get more linguoid.
Whereas bedforms and ripples indicate relatively less energetic flow conditions I have also considered more active sheet-flow conditions where the whole water sediment interface moves en-mass. For more information see Malarkey et al. (2003, 2009) and Malarkey and Davies (2005).
2. Larger scale sediment transport modelling
My work involves not only the behaviour of sand transport processes at this local scale but also how to go about representing these processes at larger more practical scales. This work has focussed in particular on the Dee Estuary, but also includes work on other estuaries (Howlett, et al., 2015). Dee Estuary work
3. Other work Vacillation in the Langmuir circulation
Langmuir circulation (Lc)
was discovered by
. Lc is an important mixing process in the upper ocean. It consists of counter-rotating vortices within cells in the mixed layer generated by the wind, with axes aligned in the wind direction. These counter rotating cells cause windrows of flotsam and seaweed to build up along lines of convergence on the water surface. Here we use a line vortex description of Lc to explain vacillations in the intensity of Lc observed by
When the vortex representing the circulation is displaced away from the stationary center it precesses around the cell causing a vacillation in intensity on the surface as shown in the animation (for more details see Malarkey and Thorpe, 2016).
Flow / Llif
In collaboration with two local artists
Lindsey Colbourne and
we made an analogy between flow and marbles in a Synthesis
funded project entitled 'Flow / Llif' which was shown at
Pontio on 3rd July 2016. The event was a great success (check out Lindsey and Lisa's webpages for more details). This has led to an on-going collaboration including involvement at the Troelli exhibtion, LLif 10/08/2017 at the National Eisteddfod, Anglesey an interactive installation at Pontio (13/09/2017-11/01/2018), see movie below and at the 'soft opening' of M-Sparc, Bangor University's science park building (24/03/2018). The science of flow, which was presented at these events, is available to download in both english