Local solution to slip control

The amount of slips being stabilised around the North Island by Dairy Flat company Erosion Control has increased 10-fold with this year’s weather events. As the name suggests, the company specialises in stabilisation of the land – whether that be permanent (on new subdivisions or alongside motorways) or temporarily shoring up slips on private or public land.

One way they do this is by applying a combination of grass seed and recycled newspaper (including old copies of Hibiscus Matters) supplied by Ōrewa Lions Club, as well as a product called Flexterra. The company’s Auckland operations manager, Troy de Jonge, says small slips have always been part of Erosion Control’s work, but they’ve been dealing with larger ones, and a lot more of them, since Cyclone Gabrielle. He says once a digger has done as much as possible to clear a slip, the grass mix sprayed on by Erosion Control quickly gets to work. The roots filter the water, preventing it building up behind the slip and pushing the soil forward to potentially slip again. This holds things in place until a more permanent solution can be engineered.

Locally, a lot of the slips the company assists with have been on private property in Stillwater and Whangaparāoa Peninsula. The focus recently has been on the areas worst affected by the heavy rain, such as Coromandel and Piha, where slips were preventing access.

“There we are stabilising over engineering works that have already been completed,” de Jonge says.

An important result of Erosion Control’s work, he says, is reducing dirty water runoff into waterways.

“For example, we have done a lot of stabilisation in new subdivisions like Millwater and Milldale. It ensures that any runoff is clean, which has a big impact on silt build up in our rivers and ocean.”