Bio-Filtration Garden Forebay Catchpit

A unique (and award-winning) shift in effective pre-treatment and management of gross pollutants.

Improving the stormwater quality of Ōtāhuhu was one of the key goals of the Ōtāhuhu Town Centre upgrade project. A number of bio-filtration gardens have been installed along the newly-refurbished streets to promote a healthy environment. The project made a unique shift in effective gross pollutant pre-treatment and management.

During the design phase, the client (Auckland Council) challenged the design team to develop an improved rain garden forebay design that would filter the bulk of floating rubbish and sediment out of the incoming water from the road before it entered the rain garden, thus reducing the maintenance requirements.

The design team explored various options and collaborated with Auckland Council’s Healthy Waters, Council maintenance teams and Mana Whenua design representatives. A catchpit type forebay design emerged from the collaboration. It is unique and the first prototype to be installed in Auckland.

Water enters the catchpit via a precast concrete kerb entry unit and flows under a plastic baffle within a concrete catch pit; trapping floating rubbish and allowing sediment to settle. The catchpit is secured with a hinged steel lid and will be maintained by opening the lid and the use of a vacuum track. This involves less maintenance time, and is more effective than collecting rubbish once it is within the raingarden.

Precast concrete was used for the catch pit structure; and Corten steel was used for the cover and kerb lintel. Precast concrete was chosen for its durability and low maintenance requirements. These precast units were paired the Corten steel covers for safety and to conceal the rubbish and water. The tinted concrete finishes and Corten steel of the units blend well into the material palette of the overall Ōtāhuhu Town Centre Upgrade, in which a range of Firth concrete mixes and finishes were used.

From Day 1 of installation the units have been effective in their job of catching rubbish and sediment. The ongoing success of these units will continue to be monitored with Healthy Waters to see if they can be implemented elsewhere. The catchpit concrete mould was made in steel to ensure the ability for Auckland Council to produce more quantities in the future.

For further information please contact Yoko Tanaka, Alex Smith or Mark Lewis