Ph.D. Research Profile: Iain Dawe
Longshore Sediment Transport in Mixed Sand and Gravel Shorelines
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Hi, my name is Iain Dawe and I am half way through the long and challenging job of doing a PhD. I am studying the process of longshore sediment transport in both lake and open-coast mixed sand and gravel shorelines, with the aim of developing a working numerical model that can be used by resource managers and engineers to more accurately predict longshore sediment transport rates. Longshore sediment transport is the process whereby sands and gravels are moved along the shore by wave and current action.
A mixed sand and gravel beach is composed of both sand and gravel in roughly equal proportions as distinct from either pure sand or gravel beaches. On a global scale they are relatively rare, but in New Zealand they are a common feature of the coastline. Mixed sand and gravel beaches were recognised as being somewhat different from other beach types as early as the 1920's, but most of the mixed sand and gravel beach research dates from 1965 onwards, much of it coming from the Department of Geography, University of Canterbury. Because of their uncommonness on a global scale and their prevalence on the Canterbury coast, the knowledge concerning them has had to be 'home grown'.
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Many of these mixed sand and gravel shorelines are actively eroding, threatening property (e.g. Oamaru and Ashburton), public amenities (e.g. Kaikoura), and in some cases whole industrial estates (e.g. Washdyke, Timaru). In most of these situations the causes of the erosion are only partly or poorly understood, but it is recognised that longshore sediment transport plays a important role in the problem. One of the difficulties faced when investigating mixed sand and gravel beaches, is having to rely on sediment transport models developed for use on sandy coasts. These models usually require input of data obtained from sensitive instruments placed in the breaker zone. Placing such delicate instruments in the breaker zone of a mixed sand and gravel beach would have them destroyed almost immediately. These beaches are very high energy environments and pose a real challenge both physically and academically for any person wanting to figure out how they operate. This often requires a bit of lateral thinking and kiwi ingenuity in building instruments rugged enough to survive the battering they receive by gravel and waves. Thus, because of the limited understanding surrounding these beaches, and the erosion problems facing many small communities, mixed sand and gravel coastlines have a lot of research potential for both coastal resource management and scientific reasons.
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I am currently half way through my field research programme which is based at Lake Coleridge in the Mid-Canterbury high country. Lake Coleridge is a 33km2 glacial lake formed by the retreat of a glacier that occupied the Rakaia River valley around 12 000 years ago. Lake Coleridge is an ideal place to develop a longshore sediment transport model for a number of reasons. At 110km west-northwest of Christchurch, it is relatively close with good all year round road access. The Lake is 18km long and 3.5km wide at its maximum and contains around 45km of shoreline, much of which is composed of mixed sand and gravel. It is orientated northwest-southeast and subsequently is exposed to strong north-westerly winds that blow down the Wilberforce valley. This ensures that many parts of the shoreline experience an oblique wave approach and as a result, longshore sediment transport. This wind can generate waves of up to 1.5m. Thus, it is small enough not to have a swell wave environment but big enough to get large waves by which to develop a longshore sediment transport model. The reason I am studying the process of longshore gravel transport in a Lake is because the wave environment is far safer to work in for both myself and the instruments, than a large open coast mixed sand and gravel beach. A small lake environment can be viewed as a large scale laboratory wave tank and it is in this manner to which I am approaching the work. However, in saying this Lake Coleridge was chosen because it is situated in a valley aligned to the prevailing winds, in particular the North Westerly and the Southerly. Lake Coleridge can be likened to a wind tunnel!
In the field I have a combination of instruments set up to measure both waves and currents. I have an S4ADW set up in the zone of deeper water to measure the deep water wave height and lake bed currents. To back up this data I have a wave capacitance gauge to measure the surface water elevation at 10hz (0.1s). In the shoaling and breaker zone I have two Marsh McBirney current meters aligned normal to the shore sampling at 2hz (0.5s). One to measure currents in the shoaling zone and the other to measure currents in the swash zone. The wave gauge and the Marsh McBirney current meters are connected to a Campbell 23x data logger which executes the sampling programme for 18min on the hour every hour for as long as I am in the field. In combination with this I collect transported sediment in a series of traps. The traps are aligned normal to the shore - one in the swash zone and the other in the shoaling zone. A large trap is set to collect bedload transport and a number of smaller traps are set vertically above this to collect suspended particles. The traps consist of a stainless steel mouth connected to a polyester bag (streamer) which allows water through, but traps sediment. The aim of this is to correlate transport rates with the wave and current conditions. In addition I take measurements of foreshore slope angles and wind conditions. Wind speed and direction data is collected on a weather station set up at one of my field sites, using a Campbell 21x data logger. I have found these small data loggers to be rugged and reliable and would recommend them to anyone working in often inhospitable environments where you need your equipment to perform day and night regardless of the conditions.
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I am also interested in the mixed beaches of the West Coast of the South Island. Particularly, why many of them are eroding and the dynamics of a two part mixed beach system that has a steep gravel storm ridge at the upper foreshore and a sandy gently sloping lower foreshore and nearshore.
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In August of this year I spoke at the International Conference on Tidal Dynamics and Environment in Hangzhou, China (8-13 August, 2002) where I presented a paper in conjunction with Martin Single entitled "Rollability: A Method for Assessing Sediment Transport Pathways in Tidal Inlets". I was assisted by a grant from the Coastal and Ocean Engineering Conference Fund. In April 2000 I spoke at the International Coastal Symposium in Rotorua, New Zealand about the mixed sand and gravel beaches of Kaikoura. This was from work based on my MSc thesis.
When I'm not studying shorelines, where better to relax than down at the beach or to do a spot of trout fishing or canoeing in one of the many South Island high country lakes. I just can't seem to get away from them…
Publications
Dawe, I.N. & Single, M.B. (2002), Rollability: A technique for assessing sediment transport pathways in tidal inlets. In Prep.
Dawe, I.N. (2001), Sediment patterns on a mixed and gravel beach, Kaikoura, New Zealand. In Healey, T.R. (ed), ICS 2000, Challenges for the 21st Century in Coastal Science, Engineering and Environment. Journal of Coastal Research - Special Issue, 34, 267-277.
Technical Reports
- Dawe, I.N. & Single, M.B. (2001), Lake Monowai Beach Profile Monitoring Network: An Assessment of Shoreline Change, February 2001. A Report to Trust Power Limited. Land and Water Studies (International) Ltd. Christchurch, New Zealand, 38pp.
- Dawe, I.N., Hemmingsen, M. & Kirk, R.M. (2000), Lake Monowai Beach Profile Monitoring Network, 1st Re-Survey May 20, 2000. A Report to Trust Power Limited. Land and Water Studies (International) Ltd. Christchurch, New Zealand, 21pp.
- Dawe, I.N. & Hemmingsen, M. (1999), Catamaran Wake from "Fiordland Express" on the Shoreline of Lake Te Anau. A Report to Fiordland Travel, Te Anau. Land and Water Studies (International) Ltd. Christchurch, New Zealand.
- Dawe, I.N. & Single, M.B. (2001), A Coastal Hazard Assessment of 51a West End Road, Ohope. A Report to Whakatane District Council. Land and Water Studies (International) Ltd. Christchurch, New Zealand, 11pp.
Awards/Prizes
2002 Awarded travel and conference expenses to Tidalite 2002, Hangzhou, China from the Coastal & Ocean Engineering Conference Fund, University of Canterbury.
2000 University of Canterbury Doctoral Scholarship.
2000 Awarded travel and conference expenses to ICS 2000, Rotorua, New Zealand from the Coastal & Ocean Engineering Conference Fund, University of Canterbury.
1996 R.W. Morris Prize for outstanding coastal research.
1996 Gleneagles Trophy for top presentation of a coastal research paper.
Supervisor
Professor R.M. Kirk,
Deputy Vice Chancellor,
University of Canterbury
Dr Jon Allan,
Coastal Geomorphologist,
Oregon Department of Geology and Mineral Industries, USA