In March this year Auckland city was hit with a deluge and the council’s stormwater network was put to the test. Alan Titchall reviews the council’s future flood prevention plans as its unitary development is put into effect.
You can’t blame climate change for Cyclone Debbie back in March. NIWA research shows that the northern region of the country is whacked by an average of a little over one storm of tropical origin each year. The severity of these storms depends on their location and on the phase of the El Nino / La Nina cycle.
Every year between December and April, storms from the tropics move south toward us and sometimes maintain enough violence to produce damaging winds, high seas and heavy rain.
Occasionally, tropical cyclone remnants re-intensify in the extratropics to become storms capable of inflicting loss of life and severe property damage.
Such memorable events include Tropical Cyclone Gisele in April 1968, which sank the interisland ferry Wahine with the loss of 51 lives. In March 1988 Cyclone Bola dumped over 900mm of rain and produced hurricane-force winds in our northern regions.
More recently, in December 1996, Cyclones Fergus and Drena brought torrential rain and storm-force winds to the North Island, triggering an exodus of summer vacationers from coastal resort areas.
The big picture
Three years after the merging of the former seven territorial authorities and one regional council under the Auckland Council, the city adopted a new Auckland-wide bylaw managing the public stormwater network across the city. It came into effect in November 2015.
Under a big picture scenario the total land area of 4900 square kilometres (including Hauraki Gulf Islands) has been divided into 10 watersheds based on coastal receiving environments – streams, rivers, lakes and estuaries (the Auckland region features two harbours and 11,117 square kilometres of ocean).
Each watershed has specific management objectives defined through a ‘whole of catchment’ approach that is split into three stages.
These are: Mapping the current state and key issues for each watershed (with the outcome determining the approach for stage two); identifying options for achieving the objectives for each watershed; and the development of watershed-specific action plans to work towards the objectives, limits and targets set for each watershed in stage two. This third stage is expected to be complete by 2020.
A range of tools is being use during each stage, including local and international data; community consultation; and scenario modelling.
Tasked with this job is Healthy Waters (formerly council’s stormwater department), which looks after Auckland City’s stormwater assets; and Watercare Services, which looks after wastewater and drinking water.
In the area where the storm and wastewater systems are still combined, they work together.
View from the top
Craig McElroy, the head of Healthy Waters, is optimistic about the council’s planned level of capital investment for future flood control under the Unitary Plan, and is quoted in the press as saying Healthy Waters’ asset management system has even been “internationally benchmarked as representing global best practice”.
Auckland’s piped network is designed to take medium-sized storm events, with sizing varying on legacy council design standards and their age. The rainfall in March exceeded the design capacity of this network in many areas of Auckland.
Andrew Chin, Healthy Waters strategy & resilience manager, answered a number of questions sent to the department.
He says the actual cause of flooding in many cases was attributed to restrictions in the overland flow-path network.
“In extreme rainfall it is the overland flow network that becomes critical. Poorly thought out modifications to flow paths such as landscaping that obstructs or diverts flow paths can create significant issues.
“Historically many areas of the city were designed to standards lower than today’s, but this was appropriate for the time. As development in the city has become more intense the higher proportion of impervious surface causes higher peak flows during storm events as water is less able to soak into the ground and runs off faster.
“Overland flow paths may also become more restricted as development has occurred too close to them and also within the flood plains of streams.
“In order to better manage flood risk and flood effects Auckland Council has made available modelled overland flow paths, flood prone areas and flood plains on our website through the GIS viewer.”
Unitary Plan and water flows
“We have also implemented a better rule framework within the Auckland Unitary Plan to better manage potential flooding effects on new development,” says Andrew. The flooding rules that must be considered in a resource consent application are outlined in section ‘E36 of the Unitary Plan – Natural hazards and flooding’.
Andrew says standards for new greenfield development and larger-scale brownfield redevelopment are higher than in the past, with developers expected to take an integrated stormwater management approach.
“A part of this approach is avoiding development within flood plains and avoiding piping streams to maintain the natural functioning of the flood plain.”
On private land, there are also over 2000 kilometres of urban watercourses in Auckland from concrete-lined channels to streams, he notes.
“Once again we take a risk-based approach to managing these areas. Private land owners are responsible for watercourses on their land and dumping of rubbish on stream banks poses a particular problem.
“The council recently passed a Stormwater Bylaw to enable more enforcement options in this area. Typically, enforcement is undertaken reactively following complaints from the public.”
The council is also considering grading watershed assets between grade one and five, with the latter on private land coming under stricter council control.
In addition to new regulations and strategies are large infrastructure projects already approved. These include the massive, 13-kilometre Central Interceptor tunnel and link sewers, which will convey wastewater from western and central suburbs to the Mangere Wastewater Treatment Plant.
It will free up capacity in the Orakei Interceptor and duplicate critical ageing sections of the Western Interceptor.
Up to 4.5 metres in diameter, this large pipe is expected to reduce the frequency and volume of overflows in its catchment area by 80 percent. Construction will begin in 2019 and finish in 2026.
“While the council can reduce some flooding effects with improved infrastructure, it isn’t feasible to resolve all of the flooding issues, which have arisen as a consequence of historical development practices,” Andrew cautions.
This article was first published in the July 2017 issue of NZ Local Government Magazine.