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Groundwork

01.4 / Ecology / Hydrology

Kaituna Bioswale & Basin

Six hundred hectares of stormwater, made a garden.

Location
Bay of Plenty Basin
Year
2022
Scope
6.4 HA Watershed Parkland
Area
6.4 ha
Procedural site model — 39° / seed 27.9

Infiltration parkland and flood-mitigation landscape sculpturing civic storm paths.

The brief

Every heavy rain event sent 40 million litres of untreated runoff from the Kaituna plateau into the harbour in under 90 minutes. The council had a treatment wetland on the drawing board. It had no visitors. We proposed the opposite.

  1. Broad planted swale stepping through parkland after heavy rain
    01The upper swale in flood — designed to be wet for 40 minutes a year.
  2. Reed bed holding water at the basin outfall
    02The outfall reed bed, which is also the most visited seat in the reserve.
  3. Dense wet-tolerant planting mass reading as a continuous meadow
    03Carex and rūrū in the wettest third of the section.

[ Approach ]

Four phases on the ground

  1. 01

    Read the land

    We mapped every overflow path the catchment had used since 1987, including the ones that only run during a 1-in-20 year storm. The bioswale follows four of them instead of one engineered channel.

  2. 02

    Sketch the possibilities

    Sections were tested against a real storm hydrograph rather than the 10-year average. The 1:6 side slopes that met the code were rejected in favour of 1:4 swales that slow the water to a wadeable depth.

  3. 03

    Grow the proposal

    Terraces step down the fall line, each one a planted shelf and a place to sit. The section is designed so that every bed stays within its optimal moisture band for most of its length, rather than drying out to bare gravel.

  4. 04

    Steward the delivery

    A two-year maintenance contract was written before construction, not after — including a community weeding roster and a gravel replacement cycle tied to infiltration testing.

Outcome

The reserve now holds and filters 96% of annual runoff volume that previously reached the harbour untreated, and peak discharge has dropped 41%. It is the most visited open space in the district.

Runoff volume filtered
96% annually
Peak discharge
−41%
First flush captured
1.9 ha of wetland
Visitors per week
3,400

Materials

  • Salvaged basalt gabion baskets
  • Scoria drainage strata
  • Coir log weir stacks
  • Carex / rūrū / mānuka planting matrix

Credits

  • Lead designer — Isla Vance
  • Hydrology — Callum Reid
  • Gabion and weir detailing — Tomas Lindqvist
Client
Western Bay of Plenty Regional Council
Collaborators
NIWA, Kaituna Community Board

Site model / plan

The model above is generated from this drawing's height field, which is shared between the shader and the CPU so planting lands exactly on the surface.

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