Parts of the Opotiki township have faced repeated flooding and sewer overflows for years. Opotiki District Council set up a specialist project governance group and made the bold decision to fund on-property repairs of private infrastructure. Peter Askey, from WSP Opus, updated delegates at the IPWEA NZ Conference in Rotorua.
Old leaky pipes, a small rating base, high groundwater levels and pressure for development. This is an all-too-common situation for many small communities, demanding an innovative approach to investigations and project design.
Opotiki township has 1350 domestic sewer connections. The reticulation dates from the 1950s and parts are in poor condition. Very high inflow and infiltration (I&I) results in a loss of service in wet weather and an expensive system to run.
To optimise a solution for the future wastewater reticulation, a comprehensive and staged investigation was undertaken. This involved groundwater monitoring, sewer flow monitoring, CCTV inspection and a pilot repair project targeting private sewers.
Results showed that rehabilitation of the existing system would be more cost effective than a complete replacement. The first year of a three-year programme of works involving repair of private sewers, pipe lining, manhole refurbishments and for the worst lines, replacements, is now underway.
There are two notable features of the project. First, the establishment of the Sewerage Upgrade Steering Group – a specialist project governance group plus independent peer reviewer – that met regularly to review progress and confirm direction for the project.
Second, after analysing alternative approaches, council decided to fund on-property repairs of private infrastructure. It is an approach that is working well for the Opotiki community. It would work well in other similar smaller communities but may not be applicable in larger centres with a more diverse range of property.
Deteriorated sewers, high groundwater levels, flat low-lying land and an under-capacity stormwater system. Together this mix makes for a sewerage reticulation which is expensive to run and provides a poor level of service. Opotiki township (with a population of just 8436 at the last census) in the Eastern Bay of Plenty has all these features to a large degree.
This is combined with a low rating base and a community that is not affluent. In 2013, the median income was $20,700 compared with a national median of $28,500. So, tackling these issues is a large ask for the Opotiki District Council (ODC).
Opotiki is on relatively flat land bounded by the Waioeka and Otara Rivers to the west and east. (See figure 1). The lowest parts of town are around 2m above mean sea level rising to 5-6m at the southern end of town.
Ground conditions are alluvial and estuarine flood plain, with interbedded silts, gravels and estuarine muds. Trenching conditions are variable, high groundwater is the norm and, in parts, water inflows are very large. The ground is prone to settlement and is in a seismically active area.
The town was reticulated in the 1950s. Five main pump stations deliver flow to the treatment system.
Stormwater is by a combination of piped reticulation and open drains feeding to one main floodgated outlet. When river levels are high all stormwater has to be pumped from the town basin.
Loss of service
The level of service provided by the wastewater system has been declining over a long period of time and may never have been especially good. Settlement has led to infiltration at joints and to a large number of dipped lines. Concrete work in manholes has deteriorated.
At the same time, further development and infill (albeit gradual) has placed more pressure on the stormwater system. Surface flooding in and around an annual rain event is widespread and may persist for two to three days.
By the early 2000s the issues around loss of service were becoming increasingly severe. Rainfalls of 100-200mm over 24 hours (a relatively common event in Eastern Bay of Plenty) were resulting in wastewater overflows and toilets not functioning, with council having to provide a large number of portaloos for residents.
In 2013, an overflow basin had to be built beside the oxidation pond to contain wet weather events. A number of pipes were known to be dipped from settlement and blockages were frequent. Maintenance was reactive.
Frequent and severe storms from 2004 to 2012 brought the situation to a head. Council engineering staff put considerable effort into looking at alternative sewer reticulations including vacuum sewer.
However, there was concern at the cost estimates and the potential for council to embark upon a path that would commit the community to a very large and uncertain capital expenditure.
So, council resolved to take a step back and invest in a staged approach to problem definition to ensure that decisions on replacement or renovation were made on a sound basis. At this point WSP Opus was engaged to manage the investigations.
Council set up a project governance group and appointed an independent peer reviewer. This Sewerage Upgrade Steering Group met regularly to review progress and confirm direction for the project. Recommendations from the Steering Group went to full council for acceptance.
The programme of work was approached with an open mind as to what the solution may be. Possible options included a rehabilitation of the existing reticulation, a complete replacement with a new technology, or some combination of the two. Both approaches were to be advanced in parallel until cost / serviceability considerations led the project to a point where a preferred option was identified.
Tasked with deriving a programme of investigations, WSP Opus identified several critical aspects of the problem that needed more definition:
• How was I&I influenced by groundwater levels and the adjoining rivers?
• What was the system response to rainfall, ie relative proportions of infiltration versus direct inflow?
• Where, and to what extent, was the stormwater ponding contributing to problems?
• What was the condition of the existing assets and their remaining life?
To address these questions a comprehensive programme of measurement was put in place:
• 8 shallow groundwater bores were installed around town (logged to council’s RICADO network);
• 9 sewer flow (ie level) meters were installed at strategic locations;
• 2 additional river level monitors were added; and
• Rainfall was based on the Bay of Plenty Regional Council gauge at the town wharf.
Groundwater and sewer flow measurement continued for around eight months to ensure that WSP Opus obtained a range of rainfall events sufficient to back analyse the I&I.
To define asset condition a selection of lines (around 15 percent), including known problem lines, were
The groundwater monitoring clearly demonstrated the strong linkage between rainfall, river level (including tidal river level) and groundwater level. The response of groundwater to rainfall was very fast with rises of over 1m in a few hours.
A two-pronged analysis of sewer flows was undertaken. Analysis of several storm events broke down the relative contributions of direct inflow, rain-dependent infiltration and the longer duration base infiltration. As expected, the direct inflow from rain was a large component of the flow.
To better understand the relationship of infiltration to groundwater level, an analysis was carried out to correlate pump station flows with groundwater level in dry conditions (ie excluding any record within 24 hours of rain).
It had been postulated that the sewer pipes have a large number of defects and so infiltration would be directly a function of the head of water on the pipe. However, the analysis showed quite a different response curve.
Essentially, pump station flow was relatively constant in a defined band until groundwater reached to within 1-1.3m of the ground surface, at which point there was a step change in inflow. This response was clearly evident in the Pump Station 3 catchment but also showed up in the other catchments as well.
This indicated an increase in “leakiness” of the system at shallow depth. The likely explanation for this was that was the response of the private infrastructure, which is generally above the water table except after heavy rain or in winter.
Results of the CCTV
The CCTV inspections of the first selection of pipes showed:
• Many sewers were heavily coated with fat; and
• Large quantities of sediment were present: consequently heavy cleaning was required.
Overall, the condition of the pipes was moderate. The pipes chosen for the initial CCTV inspection were based on known service issues. This was reflected in the general condition of those pipes, with a disproportionate number with dips, and service conditions.
While some pipes were significantly deteriorated to the point where relining or possibly even replacement was justified, a sizeable proportion still had substantial service lives remaining.
The project group considered the outcomes from the first round of investigations. At this stage, preliminary assessments of cost were also made for three main options. These were: rehabilitation of the existing assets; full replacements with a pressure sewer system; and full replacement with a vacuum sewer.
At this stage, the rehabilitation option was appearing significantly less costly to implement. However, it did have sizeable project risks (cost) as estimates were perforce based upon extrapolation from limited information.
It was decided to advance two further items of work:
• Carry out a trial “find and fix” project on private laterals in one subcatchment: Pump Station 3 with around 120 connections. Clearly, there were large issues with the private infrastructure but the extent of these was not well defined. Any solution to the overall I&I needed to tackle all three components: the public sewers, the private sewers and the stormwater.
• Extend the CCTV and cleaning programme to the balance of the town. The lines that had been selected were some of the worst and known problem areas. It would help estimating to know if these were representative of the reticulation as a whole.
Pump Station 3 Find and Fix
The approach taken with the trial project was unusual but very effective. Council resolved to fund the assessment and repairs as part of the overall project. There would be no direct cost to the property owner. There was some discussion at the working group about this as it could be construed as unfair in that people who had taken good care of their infrastructure were effectively subsidising those who had not.
However, it was also very clear that enforcing repairs under the Local Government Act would be a dollar- and time-hungry exercise.
Contracts were let to two local drain laying companies. They were tasked to:
• Inspect by CCTV from the gully traps to the main sewer;
• Repair any major defects to pipes;
• Raise / replace all gullies to within 150mm of flood level. The Pump Station 3 area is low and subject to surface stormwater ponding; and
• Disconnect any illegal stormwater connections (fixing illegal connections was not part of the project cost).
The philosophy was an 80:20 approach, ie fixing the bulk of the obvious defects that would be contributing large I&I flows, but not completely renewing the infrastructure.
The council began obtaining property owner agreement using local media (newspaper) and a blanket letter drop, canvassing property owners. Occupiers were also contacted initially, though the aim was to get owner approval to carry out the works.
The response to the letters (which included a form to be signed and returned, approving ‘free’ works on the property, sent to addresses from the rating database) was poor. So, council spent considerable time contacting property owners. This included door-to-door visits by a council warrant officer and Opus staff. By the end of the project, only two property owners had not given approval.
The second stage of notification was to advise property occupiers of upcoming works. This was carried out by letter drop, in stages, advising that contractors would be on site within a set timeframe, and including Opus contact numbers.
The team tried to contact occupiers face-to-face, to ensure that occupiers were aware of proposed works and to ask that dogs be tied up or isolated (approximately a third of properties).
The drainlayers were tasked with getting signoff from occupiers that they were satisfied with the standard of reinstatement. They also delivered letters to occupiers of properties where illegal stormwater connections had been rerouted away from the sewer.
These letters advised that an illegal connection had been removed from the sewer, and signified the owner’s awareness of the works and their responsibility to remediate the issue. Council’s intention was to carry out follow-up inspections at all properties where an illegal connection was identified.
Outcome from on-property trial
The main defects found were gully traps that were broken or too low. The number of illegal connections was not overly large (16 in total). These were mainly roof downpipes. There were also several cases where entire sections or paved driveways had been drained direct to sewer.
In large part, this is a symptom of the lack of effective stormwater drainage, leaving ponded water for long periods.
The cost of the repairs averaged out at $1,780 per lot. There was a wide range of costs from properties: from just minor gully repairs through to extensive lateral replacement. This cost average included the initial inspection and CCTV of the lines.
This validated the initial assumption that by the time a drainlaying crew was mobilised to a property to do an inspection, it would be more cost effective just to carry on and fix defects.
If council had not done the repairs, the cost to chase up property owners and re-establish the drainlaying crew would have been much more than $1800. And the team would still have been working through the repairs.
The find and fix project focused mainly on residential property. To assess whether the commercial area of town was in a similar condition, a sample of some 20 lots private drainage were inspected. No repairs to commercial property laterals
Stage 2 CCTV
Through 2017, the balance of the sewers in the town were cleaned and CCTVd. This allowed the project team to build a complete picture of the reticulation. The outcome was that the extent of relining and replacements required was less than as indicated by the initial inspections.
Rehabilitation vs replacement
Detailed estimates were then made for the rehabilitation and replacement options. These clearly showed a large cost advantage for rehabilitation at around two thirds of the cost.
Rehabilitation maximises the use of the existing assets, is well suited to staging, is not disruptive and could be adjusted to match council’s other budget priorities. The downside is that the town is still left with a large number of old assets, with less resilience to earthquake and surface flooding than a complete new system.
The steering group decided on the rehabilitation option.
The rehabilitation works are being spread over four financial years. Year one started in November 2017. Three main contracts are underway:
• On-property repairs to the balance of the town (to be spread over three years). The find and fix trial is being funded as a project cost by council for the work on private infrastructure.
• Pipe relining of a first tranche of high priority pipes.
• Manhole repairs. Manholes are being inspected and a range of interventions being applied depending upon the defects, Again, an 80:20 approach is being taken with an emphasis on the major direct inflow components.
Concurrent with the sewer rehabilitation, ODC is upgrading stormwater infrastructure. Projects include:
• New trunk mains;
• Additional storage basins;
• More pump capacity; and
• Stopbanking to divert rural water to the main Bay of Plenty Regional Council pump station at the south of the town.
Reducing ponding is key to improving the level of service provided by the wastewater reticulation. It is also very important to improving the overall quality of life in the residential area. Unfortunately, the stormwater works will not in themselves solve the overarching problem of the town’s location on low land between two major rivers with climate change causing more intense storms and higher sea levels.
The Opotiki wastewater project demonstrates the value of council being willing to invest upfront in the comprehensive investigations that are essential to fully understanding a problem. With the benefit of the resulting information fully-informed decisions can be made.
The consequences of options are better defined. The comprehensive information that was available to the project team, especially from the find and fix trial and the full CCTV, has allowed more robust estimating and minimised the potential for cost overruns.
The council funding of on-property repairs of all private infrastructure is a notable feature of the project and is a bold approach by council.
In terms of reductions in I&I it is early days for the project. Given the nature of I&I sources it will not be until substantial progress has been made on all three key components of the problem – public sewers, private sewers and stormwater – that large reductions will be seen.
• Peter Askey is a principal environmental engineer at WSP Opus. He is based in the firm’s Whakatane office. firstname.lastname@example.org