Ph.D. Research Profile: Arash Moghaddam
Sediment transport mechanisms in pocket beach environments
Contact details
Room: 306
Phone: +64 3 364 2987 ext.
Fax: +64 3 364 2907
Email: arash.moghad@pg.canterbury.ac.nz
Mail: Department of Geography,
University of Canterbury,
Private Bag 4800,
Christchurch 8140, New Zealand
Research Overview:
About 80% of coastal areas worldwide are formed along the fringes of mountains and exposed rocks, resulting in embayed and pocket beaches (Ojeda and Guillen, 2008). Despite such ubiquity, our knowledge of the nearshore currents and sediment transport mechanisms operating in these specific environments is very poor compared to our understanding of these processes on open coast beaches (Dehouck et al., 2009). Over the last few decades much literature has focussed on the stability of pocket beach shorelines (Dehouck et al., 2009; Klein et at., 2002b) but few studies have quantified the nearshore processes influencing this stability (Ojeda and Guillen, 2008). Further, artificial pocket beaches are a common feature of built coastal environments such as port cities and the creation of pocket beach morphologies has recently been recommended as a solution to erosion problems for some coastal areas (Ojeda and Guillen, 2008). This research will seek to quantify the nearshore currents and sediment transport mechanisms operating in two example pocket beach environments on Banks Peninsula (Figure 1), and to extrapolate the findings from these case studies in order to build on our understanding of pocket beach systems in general.
Figure 1 Examples of pocket beaches enclosed by rocky headlands on Banks Peninsula, Canterbury, New Zealand. a) Okains Bay, b) Lavericks Bay
The process environments of pocket beaches differ in fundamental ways from those of open coast beaches due to (a) the limits placed on their wave climate by their confined geometries, and (b) the potentially greater influence of wind and tide processes on pocket beach sediment transport (Dehouck at al., 2009). By tacking the theoretical challenges involved in quantifying sediment transport in pocket beaches, this research will show how different types of wave event (low to high energy) influence this sediment transport; explore the role of wind and tides in controlling the sediment transport; and explore the behaviour of nearshore currents during different wave and wind conditions in pocket beach environments.
The research programme is divided into three phases: firstly, our understanding of nearshore currents under winds, tides and waves will be improved. Since wave-generated currents inside the surf zone of pocket beaches are not as strong as those of open beaches with similar offshore wave conditions, it is assumed that wind and tides may moderate the nearshore circulations of pocket beaches (Dehouck at al., 2009; Hegge, 1996). We will explore the correlation between winds, tides and waves, and measured nearshore currents using the array of instruments outlined in Figure 2. Nearshore currents will be analysed to determine the cross-shore and longshore currents inside pocket beaches, the offshore/onshore directed currents close to headlands inside the pocket beaches, bed return flow, and wind and tide generated longshore currents inside and outside of the pocket beaches. These field measurements will then be used to calibrate the Xbeach model. Figure 3 shows an initial example of the simulation of currents and suspended sediment transport with a theoretical 1.5 m swell wave height approaching the shoreline of Okains Bay at 45°.
Figure 2 All instruments at red points are going to be deployed for a duration of one year, except ADV&Sedimeter deployed few days before few storms lasting 10 days.
Figure 3 Modelled wave height (a), cross-shore mean velocity (b: red indicates onshore, blue indicates offshore), bed return flow (c), and cross-shore suspended sediment transport (d: red indicates onshore, blue indicates offshore) in Okains Bay generated using a one hour incoming-tide simulation of Xbeach
After examining and modelling the nearshore current processes, sediment transport mechanisms will be studied in the second phase of this research. Using the above field and modelling techniques, the pathways of sediment transport will be defined outside and inside the pocket beaches and in relation to the measured nearshore currents.
Finally, an attempt will be made to add nearshore current and sediment transport parameters to the existing morphodynamic classifications of pocket beaches (e.g. Short 1999), which are based on the length of headlands and size of pocket beaches, wave heights, and wave periods so that results can be used to evaluate the predominant factors in controlling sediment transport in different types of pocket beach environment.
References:
Dehouck, A., Dupuis, H., Sénéchal, N., 2009. Pocket beaches hydrodynamics: The example of four macrotidal beaches, Brittany, France. Marine Geology 266, 1-17.
Hegge, B., Eliot, I., Hsu, J., 1996. Sheltered sandy beaches of southwestern Australia. Journal of Coastal Research 12, 748-760.
Klein, A.H.F., Benedet, L., Schumacher, D.H., 2002b. Short-term beach rotation processes in distinct headland bay beach system. Journal of Coastal Research 18, 442-458.
Ojeda, E., Guillen, J., 2008. Shoreline dynamics and beach rotation of artificial embayed beaches. Marine Geology, 253, 51-62.
Short, A.D., 1999. Handbook of beach and shoreface morphodynamics. Chichester: Wiley, New York, pp. 230-249.
Supervisors:
Dr Deirdre Hart,
Dr Peyman Zawar-Reza
Mr Derek Todd