Concept Design

Forbury Park Siphonic Storm Resilience

Test whether Forbury wetland storage can be prepared before forecast rain, then transferred through a siphonic line that requires no electrical power before, during or after a storm event.

  • Concept-design decision
  • RIF/Kānoa funding pathway
  • Not a lodged application
Decision statusConcept design not yet commissioned. No council, funder, engineer, mana whenua or regulator endorsement is implied.
Concept site addressForbury Park, 146 Victoria Road, St Kilda, Dunedin[6]
Site

Forbury Park, 146 Victoria Road, St Kilda, Dunedin

Concept question

Can siphonic storm transfer make Forbury wetland storage materially more useful during storm events?

Storm readiness

The concept tests whether Forbury wetland water levels can be lowered before forecast rain, creating extra live storage before peak storm inflow arrives.

No electrical dependency

Once primed, the Water Siphon transfers water without a motorised pump, PLC, standby generator or external electrical supply.

Commissioning decision

Decide whether to commission concept design before committing to funding application, consent or build decisions.

Next evidence

Survey, hydraulic design, consenting pathway, cost basis, ownership, maintenance and monitoring still need confirmation.

Funding path

Use a commissioned concept design to support a stronger RIF/Kānoa application.

System

How Storm-Ready Siphonic Transfer Happens

The concept design should test wetland storm preparation: lowering the standing water level ahead of forecast rain, holding that level with a controlled riser, then transferring stormwater through the sealed siphonic line while usable head remains. The transfer system is hydraulic, not an electrically powered pump station.

Rendered 1000 mm Water Siphon manifold on a coastal grass site near houses and dunes.
Hardware sequenceIntake and wet well

Forbury stores event water first. A controlled intake or wet well connects that storage to the closed pipe.

  1. Intake and wet well

    Forbury stores event water first. A controlled intake or wet well connects that storage to the closed pipe.[1][2][3]

  2. Siphonic primer, no electrical supply

    The siphonic primer purges trapped air and maintains vacuum without a motorised pump, PLC or external electrical supply. Once primed, the system can keep transferring water through storm conditions while usable head remains.

  3. Vacuum and crest

    At Forbury, the local high point is the Saint Clair Beach dune line, screened inside the system's about 8 m practical crest envelope. Crest height is a vacuum feasibility check, not the driving head.

  4. Head differential

    Flow is driven by the water-surface difference between wetland storage and discharge. More difference means more available head and a higher flow rate; pipe slope is not the driver.

  5. Measured transfer

    The concept design should define how any future monitored installation would log flow, tide, wetland level, tailwater, salinity, turbidity, primer status and interruptions before rollout claims.[4][5][3]

Proof

Morelands Farm and other sites.

Morelands shows the Water Siphon operating in wet New Zealand ground. Forbury is the urban stormwater concept site where the transfer idea can be tested before any funding or build commitment.

Water Siphon product visual in a grass field with the siphon manifold and HDPE pipe visible.
Main site product visual. Not an as-built Morelands or Forbury installation.

Field proof, urban test

The farm result supports commissioning concept design. Forbury still needs its own survey, modelling, consenting and water-quality evidence before any monitored pilot or build decision.[7]

resting groundwater reduction
225mm
drain level reduction
400mm
reported response window
4days
  • Field result225 mm resting groundwater reduction reported within four days.
  • Drain response400 mm open-drain level reduction reported at Morelands.
  • Farm economics context76% reported first-year farm return, based on owner/operator economics.
Play Morelands case study

Forbury

Why Forbury is the right concept site.

It is public land, already part of the stormwater conversation, with the exact discharge constraint the concept design needs to test.

Aerial concept visual of Forbury Park reworked as an engineered wetland and storm detention pilot beside Ocean Beach.
01

Public land, public benefit

DCC owns the former racecourse and has publicly linked the site to stormwater, flood mitigation and climate adaptation options.[6][8]

  1. 01

    Public land, public benefit

    DCC owns the former racecourse and has publicly linked the site to stormwater, flood mitigation and climate adaptation options.[6][8]

  2. 02

    Wetlands are already on the table

    Waterways and wetlands remain a shortlisted adaptation future. The concept design tests whether a wetland-style asset can actively shed water during events.[8][2]

  3. 03

    The discharge gap becomes measurable

    DCC short-term options flagged the missing free-discharge point. The concept design should test closed-pipe transfer under measured head, tide and water-quality limits.[1][9]

  4. 04

    A commissioned stop/go design

    The decision case is conditional: commission concept design first, then use that evidence to decide whether a RIF/Kānoa application deserves wider support.[10][11][1]

  1. Public land, public benefitDCC owns the former racecourse and has publicly linked the site to stormwater, flood mitigation and climate adaptation options.
  2. Wetlands are already on the tableWaterways and wetlands remain a shortlisted adaptation future. The concept design tests whether a wetland-style asset can actively shed water during events.
  3. The discharge gap becomes measurableDCC short-term options flagged the missing free-discharge point. The concept design should test closed-pipe transfer under measured head, tide and water-quality limits.
  4. A commissioned stop/go designThe decision case is conditional: commission concept design first, then use that evidence to decide whether a RIF/Kānoa application deserves wider support.

Numbers

The Numbers are hard to ignore.

This screen asks the operational question: if Forbury starts a storm with wetland storage already prepared, how long would the siphonic line take to move the selected volume without electrical lift power?

Forbury scenario

First-pass transfer calculator.

This screen asks the operational question: if Forbury starts a storm with wetland storage already prepared, how long would the siphonic line take to move the selected volume without electrical lift power?

Engineering cross-section showing a caged concrete riser, Water Siphon manifold and submerged outfall to the oceanStorm water rises above the wetland operating level, spills into a lower wet-well intake, transfers through the buried trunk pipe to the crest manifold, then discharges below the slightly tidal coastal water surface. Delta h marks the water-surface difference that drives the flow.normal wetland levelΔh
Cross-section: wetland overflow drops into a closed pipe, the Siphonic System manifold maintains vacuum, and the pressurised line crests the dune to a submerged outfall. Δh between the two water surfaces drives the flow.
Storm event200 mm over 11.76 ha
200 mm
mm

Total storm depth applied over the Forbury footprint for this first-pass screen.

0 mm
mm

Depth held below the overflow level before water enters the riser. Clamped so it cannot exceed storm depth.

Water to move200 mm23.5 ML across the Forbury footprint
Concept hydraulic setup
1800 mm
mm

Nominal outside diameter for the screened PE pipe.

500 m
m

Approximate pipe run from Forbury storage to the coastal outfall screen.

2 m
m

Water-surface difference between stored water and discharge.

6 h
h

Static check only. The public result reports pipe-hours to move the selected storm water.

Screened flow4,083 L/s
Storm water to move23.5 ML
Moved in selected window88.2 ML
Time to move storm water1.6 h
Screening setup
Default pipe
OD1800 PE SDR17 / PN10
Default route
500 m Forbury to coastal outfall screen
Loss allowance
K = 6 for manifold, valves and transitions
Footprint
11.76 ha Forbury Park equivalent

This is a screening calculator only. The time shown is pipe-hours for the selected water volume; it is not rainfall duration, a flood model, QS estimate or consented discharge volume.

Open the engineering reference

Review path

What a reviewer should do next.

The next decision is practical and defensible: approve concept support to test wetland storm preparation, siphonic transfer without electrical power, and measured hydraulic performance under Forbury tide and head conditions.

Approve Concept supportRead engineering reference
  • No funder commitment, council endorsement or consent pathway is assumed.
  • The useful next decision is whether to commission concept design and the measured site data it requires.

Sources

Citations.

A compact public source record for the numbered citations used on this briefing page.

CitationsPublic source recordOpen the numbered source list used by the briefing.
  1. 1
    South Dunedin flood alleviation short-term options

    Dunedin City Council · 2025-01-28

  2. 2
    Next steps toward future shape of South Dunedin

    Otago Regional Council · 2025-09

  3. 3
  4. 4
    Making an application for resource consent

    Ministry for the Environment

  5. 5
  6. 6
  7. 7
  8. 8
    South Dunedin Future programme

    Dunedin City Council

  9. 9
  10. 10
  11. 11