Erosion control

Located In the central area of New Plymonth, the London Terrace Stream flows out to Te Henui Stream. After several storm events, London Terrace stream suffered significant damage by erosion. Due to the level of damage, it was decided that the existing rock mattresses should be removed and replaced with a robust Concrete Mattress System. This will provide a long-term erosion protection to the stream bed minimizing environmental impacts. The Concrete Mattress was selected as the best alternative for this application due to the simplicity of installation and long-lasting. Several site-specific aspects such as flow velocities, interference from nearby vegetation, lifespan and layout also led to Concrete Mattress System. A Concrete Mattress (average thickness of 170 mm when filled) was selected by the designer as it was suitable for flow velocities of up to 9 m/sec and has been successfully installed on other projects for over 30 years. Additionally, the site had access restriction and was once again superior to other solutions because of the minimal requirements for installation.

Before the works commenced, the stream was carefully diverted past the repair area in a temporary culvert pipe to allow the stream to keep flowing and protect the environment. The existing rock mattress was removed, and the subgrade was then carefully trimmed in a shallow ‘V’ cross section ready for the Concrete Mattress to be laid. The unfilled Concrete mattress was manually laid on the subgrade and filled using an in-line concrete pump at some distance from the road as there was no access into the site for concrete trucks without significantly disturbing the surrounding vegetation.

The concrete mat is a robust erosion control solution that has a long service life. The excess water can escape and reduce the pressure. Because the concrete could be pumped remotely into the concrete mats, the surrounding vegetation was not damaged. No use of heavy machinery was needed because the concrete mats could be laid by hand.