Stormwater

Stormwater is rain which has run off hard surfaces such as roofs, roads and carparks. Stormwater is not treated, and the water ends up in streams and harbours.

The stormwater network includes a system of kerbs, channels and underground pipes that drain the rain off our land, roads and footpaths, and from our gutters into our streams, rivers and eventually out to sea. On the way, this rainwater picks up contaminants such as bacteria and heavy metals from the surfaces it runs over.

Stormwater and wastewater use two different pipe systems - wastewater pipes take the wastewater to a treatment plant while stormwater pipes take stormwater to the nearest waterway. Because of this, we all share the responsibility to keep the stormwater network clean and protect our region’s waterways and the environment.

Every day we undertake tasks that can impact on the stormwater network. Because of this, it is illegal for anything other than rainwater to go into stormwater drains.

Wellington Water is committed to helping protect our harbours, rivers and streams by reducing pollution from the stormwater network.

We're working on a long term strategy for stormwater management, that will guide us on our journey to improving water quality - find out about our proposed strategy.

Tips for managing a flooding event

Most of our stormwater network was designed and built years ago to be able to cope with storms of a five or 10 per cent annual likelihood. Many heavy rain events now can easily exceed that, and be highly localised – swamping the network and causing flooding.

Heavy rain overwhelm the stormwater and wastewater networks in a matter of moments. There are some things residents can do to protect their property and reduce the risk of flooding on their property.

Before the flood occurs, do this

Check the kerb and channel on the street, and clear it of debris.
If there’s a sump (street drain) near-by, ensure it’s clear.
If it’s safe to do so, check your guttering and downpipes to make sure they are clear
Keep known areas of overland flow or low areas on your property clear of any items
Often sheds and garages occupy low points – lift valuable items off the floor if you are worried about flooding.
It is always a good idea to have an emergency water supply and grab-and-go bag ready, including dry towels and sanitising gel.

Wellington Water and council roading crews are responsible for keeping sumps clear. Prior to known events, crews will check on known trouble spots and risk areas, ensuring stormwater grates, inlets and outlets are clear.

During the flood, do this

If your property or neighbourhood is being affected by flooding, there are a few things you should do

Ensure your personal safety – have your emergency grab-and-go bag ready in case things get worse.
Treat all floodwater as contaminated. Do not play in it, and wash your hands after coming into contact with it before touching food or your face.
If you have to go outside, be extremely careful. Overwhelmed stormwater networks could have hidden hazards.
Report flooding to your local council.

Once the flooding is over, do this

If your property has been affected by flooding, you may need to make an insurance claim. Take photos of damage if you can.

Report the facts of flooding to your council and/or Wellington Water. Information about flooding will be useful for flood investigations and future planning.
Note that council may not be able to do anything about the risk of future flooding in the short term
Stormwater upgrades are often very costly, as working on only a part of the network will not change its overall performance.
There is often not a lot of flexibility to bring forward major infrastructure, but the more that we know about actual flooding events and impacts, the better they can design and prioritise solutions in the future.
In some areas wastewater overflows are a common occurrence after rain events. This is caused by rain water over saturating the ground, causing stormwater to find its way into the wastewater network. To see if there has been a wastewater overflow in your area look on our network status map.

Things you can do that will make a big difference in helping us to protect the network and the environment:

Wash your car on the grass

Collect dog poo and throw it in the bin

Fix car leaks

Don’t pour hazardous substances into household or stormwater drains

Monitoring and testing

Closed-circuit television drain inspections

Houghton Bay leachate

Houghton Bay has a long history of discharges of leachate, which originates from the former Houghton Valley landfill and is carried to the bay via the stormwater system.

Historical causes

The closed landfill extends up the original valley floor from Cave Rd to Houghton Valley School and on up to Sinclair Park. It was opened in 1951 and was filled in two stages. Stage 1 was completed in 1963 and stage 2 in 1971. The land is currently used as playing fields.

Since its closure, landfill management methodologies have improved considerably. Current best practice includes catchment lining, layering, diverting water flows, and capping.

These practices were not used in the Houghton Valley landfill.

The result is that water percolating from rain and subsurface water picks up dissolved and suspended components from the biodegrading activity of anaerobic bacteria in the landfill. These components form 'leachate', which enters the stormwater network that lies beneath the landfill.

These conditions are exacerbated by field drains in place that capture groundwater from the landfill, and are connected to the stormwater system. This water then discharges onto Houghton Bay beach above the high water mark.

Investigations

This situation has been investigated a number of times since 1990, with various actions initiated.

An investigation was carried out in 2012 to determine the state of the environment in the receiving waters adjacent to the stormwater outfall. The investigation found the effects of the discharge in the receiving environment are minor. Mean concentrations of heavy metals from sediment samples and water quality samples show that these are below the possible negative effects for recreational purposes or health issues (Annual Compliance Report, August 2012).

The investigation also noted that leachate-contaminated stormwater discharges to the receiving waters only during extreme wet weather conditions (during dry weather, the flows are much lower and can be diverted into the wastewater network).

Based on that investigation, a detailed assessment was undertaken of management and treatment options to further “prevent or minimise” inputs of contaminants (leachate) being discharged into the stormwater culvert.

Managing the problem

The following management and treatment options are now in place:

Diverting the dry weather base flow into the wastewater network and control of leachate flow in the stormwater pipe (diversion) by a weir arrangement
Monthly monitoring of weir overflows and stormwater outlets
Removing encrustation growth in the stormwater pipe – flushing the pipeline on an annual basis or more frequently if required
Carrying out fortnightly inspections of the diversion structure

We have also installed a depth sensor in the dry weather leachate diversion manhole. Linked to our telemetry system to receive alerts, this gives us an early warning that maintenance of the diversion structure is required. This reduces the likelihood of leachate entering the outfall and allow speedier identification and resolution of any issues.

We have inspected sumps in the catchment to identify whether sump boxes have baffles to trap debris. Preventing debris entering the stormwater main reduces potential blockages of the diversion chamber.

In 2013 we carried out an investigation to identify an engineering solution for the odour issues that is being experienced around the Houghton Bay stormwater outfall. This investigation recommended installing an odour control chamber. The odour-reducing equipment has been installed and is operating. It is maintained monthly.

Collaborative effort

We've collaborated closely with stakeholders interested in public and environmental health issues relating to Houghton Bay, including Wellington City Council and Greater Wellington Regional Council. Our report on behalf of Wellington City Council on the Houghton Bay stormwater outfall consent to Greater Wellington Regional Council in August 2016 showed that conditions at the outfall fully complied with the current resource consent.

We appreciate that even occasional discharges to Houghton Bay are not ideal. However, we and our colleagues are doing what we can to deal with an historical situation, and to minimise future incidents and impacts.

With the passage of time and usage, drains can deteriorater as a result of many factors such as ground movement, construction defects and tree root invasion.

Because drains are largely hidden from view, Wellington Water uses closed-circuit television (CCTV) to get an accurate and cost effective survey of drains, pipes and sewers with a minimum of disruption, allowing the detection of problems and establishing the most practical options for repair.

The CCTV camera is inserted into the drain via a manhole or appropriate access point and pushed or powered through the pipe. As the camera travels along the pipe, the image is relayed to a monitor screen above ground. The operator will then log any structural defects and record the location of each defect along the pipe length.

A survey report is then provided which lists the type and location of any defects. This can be backed up with photographs or video records together with a site plan showing the drains location.

Common faults revealed by CCTV	Cause	Consequences
Tree roots in drains	Roots seeking moisture	Materials flowing through the drain will snag on the roots causing continual blockages. Roots dislodge or crack the drain, resulting in leakage and further damage
Fractured pipe	Pressure above ground	Cracks or fractures in drains leak water which destabilises the surrounding soil. This will result in further damage to the drain and even subsidence to buildings nearby
Collapsed drain	Several possible causes	Drain becomes unusable with high likelihood of frequent blockages, environmental damage and building subsidence

Global Stormwater Consent

We require resource consents to legally operate the stormwater network, but they also enable proactive and integrated planning and management to reduce contamination from the network, as we work towards giving effect to Te Mana o te Wai.

You can view these documents below:

Overall summary
Draft Global Stormwater Consent:

Stormwater Management Strategy (which helps inform the above consent)