Case Study: Sleemans Slip. A Grass Spillway Solution, using the Five Fundamentals of Successful Rehabilitation

In June 2017, a one in 50-year storm event caused 1000m3 of material to fall into a water canal which is used to irrigate up 60,000ha of the Canterbury Plains. This material had to be removed and a grassed spillway installed to prevent any more erosion and stabilizing the slope during storm events. The solution was a result of a holistic approach using “The 5 fundamentals of successful rehabilitation”. The best form of erosion and sediment control is sustainable vegetation. Implementing the fundamentals lessens the risk of failure and the need to return to projects where vegetation has failed or not met compliance standards.

The first fundamental is understanding the soils we are attempting to rehabilitate. Taking soil samples to analyse the soil structure and deficiencies, and to understand what amelioration may be needed including adding back some much-needed organic matter. Soil testing also assists in specie selection as to what and will not grow in the soil we are rehabilitating.

The second fundamental is selecting the right species for the environment we are working in. This includes but is not limited to, what time of the year will rehabilitation take place, where in the world is the project, what is the long-term desired land use.

The third fundamental is selecting the appropriate method to apply the seed for vegetation establishment. This can be via hydraulic-mulches, drill or rip-seeding, tube stock planting and spreading. What processes can be used to make this selection and what the risks are for soil and erosion control (C-Factor ratings). In this case, it’s a hybrid system using a turf reinforcement mat combined with a hydraulically applied erosion control.

The fourth fundamental is making sure the sub-contractor is experienced in the selected application methods. There are tools available (application guides and rates), and techniques that can be followed to ensure the contractor is meeting the specification and the client is getting what they paid for. This is especially important at the quality and assurance stage of the project.

The fifth fundamental is following up inspections and maintenance of the project. When the client or contracted party should be inspecting the site and how frequent. What to look for, species diversity, invasive weeds and wash outs. When to schedule maintenance if required.