During the design phase, the client 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 of its kind 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 catchpit; 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 using a vacuum track. This involves less overall maintenance and is more effective than collecting rubbish once it is within the rain garden.

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 with 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.

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 of steel to ensure the ability for Auckland Council to produce more quantities in the future.