Severe weather impacts and road closures
The following summaries outline how Council determined the preferred options for the Infrastructure Acceleration Fund (IAF) stormwater and wastewater upgrades, noting that both projects:
- have the objective of providing infrastructure capacity improvements of a scale to enable the construction of 3,520 houses in the Hutt Valley
- went through rigorous multi-criteria assessment of alternatives using expert scoring panels
- involved feedback from mana whenua
- prioritised strong flood and overflow performance, cost efficiency, and low construction risks
- aimed to reduce existing environmental and public health risks without triggering worse impacts elsewhere.
You can access the feasibility reports here:
As outlined in the reports, the preferred option offers the most balanced solutions for enabling growth, avoiding future flooding or sewer overflows, and staying within budget.
Summary - Stormwater upgrade
What options were considered?
Seven different options for pipe alignment and pump station location were developed to improve the management of floodwater, including features such as:
- widening the Opahu Stream channel itself
- building new underground stormwater pipes to relieve pressure/improve capacity of the existing Opahu Stream
- installing new pump stations at various locations (e.g., Eastern Hutt School, St Bernard’s College, Chilton St James School)
- storing floodwaters temporarily in open spaces like Hutt Rec Ground.
Which is the preferred option?
The preferred option includes:
- a gravity interceptor pipe to collect stormwater from Kings Crescent
- a pump station near Eastern Hutt School on the Opahu stream
- A pipe (rising main) to carry the water through Pretoria Street to a new outlet into the Hutt River (Te Awa Kairangi).
Why was this chosen?
This option was selected because it:
- offers the best balance of cost, effectiveness, and construction practicality
- is better able to handle the expected stormwater volumes from 3,500+ new homes
- avoids “fatal flaws” (e.g., poor flood performance, excessive environmental or cultural impacts)
- was considered more buildable and less risky than the others, particularly in terms of underground water risks and potential impact on the Waiwhetū aquifer.
Comparison table
Option | Key features | MCA Outcome |
1a: Stream improvements + Eastern Hutt School Pump Station | Upgrades stream channel and culverts, pump station at Eastern Hutt School | Not preferred – lower capacity, higher flood risk |
1b: Gravity interceptor pipe along Kings Crescent + Eastern Hutt School Pump Station | Adds new pipe interceptor and Pump Station near Eastern Hutt School | Preferred – good capacity, cost-effective, manageable risks |
2: St Bernard’s College Pump Station and rising main to outlet 35 | An intake from existing stormwater + pump station on the grounds of St. Bernard's College + rising main along Pretoria St | Similar to Option 4 but addresses a smaller area of flooding |
3a: Hutt Rec Ground Channel Storage | Open channel storage in Hutt Rec Ground | Not preferred – long disruptions caused by digging up park, and storage limitations |
3b: Hutt Rec Ground Pump Station | Pump station at Hutt Rec Ground, discharges via Woburn | Not preferred – cost and land constraints |
4: Chilton + Riddiford Dual Pump Station | Dual pump stations, complex network | Not preferred – more complex to build and maintain, with higher property impact |
5: Hutt Rec Northwest Pump Station with two stream intakes | Dual intakes to one pump station in Northwest Hutt Rec | Not preferred – more complex to build and maintain, with limited benefits |
Summary - Wastewater upgrade
What options were considered?
Five different options were shortlisted as possible solutions to a predicted lack of wastewater capacity and an impediment to future growth. These were chosen after carefully comparing costs, flooding and overflow reduction, cultural impacts, disruption, and resilience to earthquakes.
The options mostly involved:
- cutting and bypassing existing sewers (e.g., High Street or Pretoria Street)
- diverting wastewater to new pump stations at different locations (Ewan Bridge, Hutt Rec, Ariki Street)
- using pipes (called rising mains) to carry pumped wastewater to the existing Western Trunk Main (WTM) or Barber Grove Pump Station.
Which is the preferred option?
Option 2, located in Pretoria Street and connecting to the Western Hills Trunk Main, scored the highest in initial assessments. However, after further review, it became clear that this option introduced serious operational risks:
- the Western Hills Trunk Main already has issues during heavy rain
- it could cause overflows upstream at Silverstream or Ava Pump Station.
This prompted a redesign—called the “Western Trunk Main Option”—with improvements like:
- increasing pipe size
- adding a large storage tank (2 million litres)
- adding real-time controls for better system management.
Why was this chosen?
The updated version of Option 2 was ultimately chosen because it:
- offered the best balance of cost, performance, and construction simplicity
- significantly reduced future overflows
- was the most cost-effective solution
- met the future service standards (one overflow every 2 years)
- minimised disruptions and construction issues compared to other options.
Although Option 4 avoided the problematic Western Trunk Main, it was more expensive and didn’t reduce overflows as effectively.
Comparison table
Option | Key features | MCA Outcome |
1: High St to Ewan Bridge | Bypass along High St to Ewan Bridge | Not preferred – highest cost, significant disruption to High St, limited access for maintenance |
2: Pretoria St to Pump Station + WTM | Bypass along Pretoria St to Western Trunk Main | Preferred – best cost-benefit ration, manageable with upgrades |
3: Pretoria to North Hutt Rec Pump Station | Bypass to North Hutt Rec with Western Trunk Main discharge | Not preferred – similarities with Option 2 but with higher cost, lower resilience |
4: Pretoria to South Hutt Rec Pump Station | Bypass to South Hutt Rec, discharges to Barber Grove | Runner-up – avoids Western Trunk Main, good cultural alignment, but a much longer, costlier route with similar proximity to roads and homes |
5: Potomaru to Ariki + Ewan Bridge (Dual Pump Stations) | Two pump stations, one at Ariki, another at Ewan Bridge | Not preferred – more complex and costly, with two stations to build and maintain, with limited benefit |
For more information
- If you have any further questions, you can reach the project team at IAF@huttcity.govt.nz