By Alison Ballance
Until 6-8,000 or so years ago, Wairewa/Lake Forsyth, on the Canterbury Coast just south of Banks Peninsula, was open to the coast and filled with seawater. Then as Kaitorete Spit began to form, the lake became partially blocked, and connected to the sea by just an outlet river. Over the last few years this outlet has become blocked, meaning what was once brackish water has become freshwater. This has affected the plants and animals of the lake. As well, the lake is now prone to flooding over the nearby road. The lake outlet is occasionally opened using bulldozers, but blocks again within a few hours or days.
Crile Doscher, from the Faculty of Environment, Society and Design at Lincoln University, has been trying to understand the coastal processes that are taking place along this stretch of coast, to work out if it might be possible to engineer a groin or breakwater that would keep the outlet open.
Kaitorete Spit is made from rounded boulders and sand washed down the many braided rivers draining from the Southern Alps and running across the Canterbury Plains. This sediment is then moved along the coast by longshore drift.
To begin with Crile created a GIS or Geographic Information System model of the outlet and coast.
“GIS allows you to create different layers of information that reflect some characteristic about the surface of the earth,” says Crile. “So in this case what we’re really interested in is the shape of the seafloor. And that’s the most important thing because you’ve got interactions between the waves and the underwater bathymetry which affects movement of the sediment.”
Crile surveyed the stretch of coast between Birdlings Flat and the outlet by boat, collecting depth, location and sidescan sonar information on the sea floor. Within GIS he added these and layers of information such as aerial photos, which showed him the profile of the coast, and highlighted details such as large ripples in the sediment running parallel to the shore. These ripples stopped near the outlet, where there appears to be a bare rock platform at the base of the vertical cliffs.
Once he knew the profile of the shore, Crile created a 1:50 scale model of the coast in the Water Lab, using sand in lieu of boulders. The large triangular model has a sandy shore along one side, cliffs along another and a wave generator (a large metal plate that rocks back and forth to set up waves in the water). A piece of plastic spouting represents the lake outlet, while bricks are used to simulate groins and breakwaters.
“With the model, if we get the dynamics right then we can play around with different scenarios,” says Crile. “What if we put a 20 metre groin perpendicular from shore, what if we make it 40 metres, what if we put a wingwall on the end. And we just test these different scenarios to see what’s going to make the difference.”
Modelling options like this in the lab is much cheaper and faster than trying to build different options in real life. A half hour simulation equates to several days in real time, and the results have been very revealing. It seems the only option that might prevent the outlet blocking up would have to be so large and expensive that it’s not really a possibility.
The wave generator creates waves that wash onto the simulated shore and move the sediment around in ways that reflect real-life processes on the coast at Kaitorete Spit.