WairakeI Stream Correspondence with Tauranga City Council
Background:
The Wairakei Stream which flows into the Palm Beach Stormwater lakes and then out to sea at Harrisons Cut is the critical part of the stormwater drainage system to stop flooding in Papamoa. This requires knowledgeable maintenance and an engineering understanding of the design principles on which the stream and the stormwater lakes were designed to operate. There is a Resource Consent RC 63636 which covers this and we believe that TCC is not acting according to the Resource Consent. As more residentialareas are added into this catchment, the risk of serious flooding increases.
Correspondence to Date
8 June 2023 New Culverts under the Gravatt Road and Palm Beach Boulevard
9 June 2023 Reply from Anne Tolley Commissioner
23 June 2023 Timing of Culvert Construction/ TCC taking responsibility for any Flooding
Wairakei Stream Correspondence to TCC 8 June 2023
New Culverts under Roads
From: neighandfoal@gmail.com <neighandfoal@gmail.com>
Sent: Thursday, 8 June 2023 7:44 pm
To: Anne Tolley <Anne.Tolley@tauranga.govt.nz>; Bill Wasley <Bill.Wasley@tauranga.govt.nz>; Stephen Selwood <Stephen.Selwood@tauranga.govt.nz>; Shadrach Rolleston <Shadrach.Rolleston@tauranga.govt.nz>
Cc: Radleigh Cairns <Radleigh.Cairns@tauranga.govt.nz>;
Subject: Wairakei Stream Works and Increased Flood Risk
RE: The Planned v Real Completion Dates for the new culverts under Gravatt Road and Palm Beach Boulevard.
Thank you for your response (dated 18 April 2023) to my queries concerning Wairakei Stream stormwater culverts, even though I was somewhat intrigued that TCC saw fit to respond by way of LGOIMA.
The prime functions of the Wairakei Stream, including design aspects which are part of Resource Consent #63636, are to ensure that stormwater is to enter this stream and then flow to the ocean without causing flooding of adjacent properties.
This Resource Consent has been in place since before 2015 and includes a number of dates for sub-project completion, all aimed at completion of Stage I (but excluding Stage II Te Tumu) by the end of 2025.
All of this project must be completed by this date to ensure that flooding (to a maximum level of 4.6 m Ref Moturiki) will not happen after that date, and implying that until all the works are completed there remains an increased risk of flooding – based on modelling carried out as part of the back-up reporting for Resource Consent #63636.
My concern now is that, by your response to my queries, TCC will not be starting the project for the culvert under Gravatt Road until mid-2025, and the Palm Beach Boulevard culvert will not start until early 2024.
These later timings (than any previously advised and included in Annual Plans) are even more concerning in the light of the serious flooding which has occurred in Auckland, Nelson, Whanganui and other metropolitan areas since early 2023, most recently in Waihi Beach. Seemingly, the scale of flooding in many of these areas has been made worse by the scope of existing design and ongoing lack of improvement and maintenance of waterways.
From scheduling details of two major TCC local projects related to the Wairakei Steam area (Harrison’s Cut and the sewerage pipe immediately south of Fashion Island), I cannot see how these works can inspire confidence in TCC being able to complete the design and contract construction work for the two Wairakei Stream culverts before the end of 2025 – as required by RC 63636.
From the time that site work first started on Harrison’s Cut and the Sewerage pipe (by Fashion Island), it was more than 12 months before each was completed, and this period ignores any design work before site work commenced. In fact, work on the sewerage system is still ongoing. There are many other projects in the area which could be used to illustrate time delays from the original schedule, some creating major commercial issues such as roading along Cameron Road.
TCC has already had more than eight years to design the culverts at Gravatt Road and Palm Beach Boulevard, and seems to have only recently determined that a different cross-section was required – twin 1m tall by 2m wide – for Gravatt Road, resulting in a much larger project site area (estimated 130 metres long with the culverts taking up at least 5 metres laterally. This is no small project given the two roads and numerous trees which have to be designed around.
I don’t envy your culvert designers, and how TCC is going to placate motorists and residents in the area. Put simply, even six months to complete the project(s) is impossibly tight.
These “critical” culverts are required now to reduce the risk of flooding in the wider Papamoa area by the end of 2025 and TCC needs to start now, even bringing the end date of the projects forward from what has been most recently advised!
New Factors and Poor Maintenance increasing the Flooding Risk
a) Harrison’s Cut
Up until at least mid-2022, the whole of the “valley” (stretching from the weir and low-level bridge by the Palm Beach Pond to the culvert at Papamoa Beach Road) was regularly mown (without significant issues). There did not appear to be issues of boggy ground along this “valley” unless there were prolonged periods where the pond outlet grille was significantly blocked, causing the pond water level to remain high – until the grille was cleared. Provided the grille was clear, short-term flood discharges over the weir and along the “valley” did not cause the “valley” to remain boggy beyond a few days after the “valley” flow had stopped.
Since early 2023 the base of the Palm Beach Pond outlet “valley” has been almost continuously plagued with sodden, boggy ground which has not allowed the grass along this base to be mowed in its entirety. Several attempts to mow the long grass appear to have just resulted in deep wheel ruts as vehicles have obviously been bogged in the soft ground. The very boggy ground has just persisted, even though there has been little or no flood flow over-land, and down the “valley”.
What has changed since early 2023 appears to have been the result of clearance action for debris (logs, etc) deposited in the area between the inland side of the Papamoa Beach Road culvert and the end of the pond outlet pipe, about 20 metres south of the culvert. Logs were deposited in this area by the sea from both the end of January cyclone and the later cyclone Gabrielle. Logs were “craned” off site, but other attempts to clear the rubbish, vegetation and rocks in this area were obviously limited to scraping the rubbish to one side, essentially creating a very narrow stream bed – from what was previously an almost flat profile from one side of the valley to the other (before the weed growth) between the pond outlet pipe and the culvert.
As I have mentioned in previous communications, I believe that this channel restriction has been the principal cause of the local water level at the end of the pond outlet pipe rising (to more than half-way up the outlet pipe). Not only does this restrict the flow of water in the outlet pipe, but, because of the very small elevation difference between the water level in the Palm Beach Pond and the invert of the culvert at Papamoa Beach Road, the hydrology within the “valley” means that there will be very little ground-water flow through the (sandy) “soil” towards the sea. The outcome of this can only cause the ground to remain boggy and limit any attempts to mow the grass – as has been displayed for the last couple of months.
I repeat my previous recommendation that the 10 metres or so of ground between the end of the pond outlet pipe and the bottom of the culvert be urgently levelled to provide a broad, unrestricted flow path for the water. I would remind TCC that one of the requirements of Resource Consent 63636 is that flow passages along all of the Wairakei Stream must be kept clear so that flows are not unduly reduced.
b) Wairakei Stream bed
What I have also observed in areas east of Domain Road is that weeds continue to grow unabated within the Wairakei Stream boundaries, creating significant flow restrictions. This growth is contrary to requirements under Resource Consent 63636. My impression is that clearance of this most of this weed is now made even more difficult by the additional planting as specified in the Wairakei Landscape Plan.
On 7 June I observed that there was considerable weed on the grilles at both the Grant Place control weir and the pipe outlet grille in land of Harrison’s Cut. After clearance of such grilles, TCC needs to ensure that the material removed is taken off site so that it doesn’t get re-entrained at the time of the next heavy rainfall.
For the security from flooding of all properties adjacent to the Wairakei Stream, TCC needs to put in place a more frequent maintenance regime to ensure a relatively clear stream bed along its full length.
Kind regards
Noel Hall
June 9
Wairakei Stream Correspondence to TCC
Reply to Noel Hall re Wairakei Stream Works and Increased Flood Risk
Good afternoon Noel
Thanks for your further email regarding the culverts under Gravatt Road and Palm Beach Boulevard. I’ve noted your views and passed these on to the relevant staff to consider.
For the record, consent 63636 only requires the Wairakei landscape plan stage one to be completed by December 2025.
Completion timings for the upgraded culverts in the Catchment Management Plan are indicative only and are subject to change based on updated modelling data and levels of growth within existing and Stage 1 areas. Additionally, Council has to manage stormwater across the city within the budgets set-out in the LTP, which can require reprioritising of projects and timing changes for planned capital works.
Up until the summer of 2021/ 2022, Papamoa and the wider Tauranga area had experienced a number of years of drought, with average rainfall being well below average. NIWA has identified 2022 as being Tauranga’s 4th wettest year on record, with the summer of 2022/23 recording almost 300% the average seasonal rainfall - the wettest summer on record. Across the city (and much of the North Island) the ground is saturated and given the low-lying nature of the overland flow path at Harrisons Cut and its proximity to the ground water table, we are experiencing prolonged periods of bogginess, inhibiting contractors’ ability to mow. Flow through Harrisons Cut is not constrained and continues to enable the return to normal levels within the stream.
Kind regards
Anne
June 23
Wairakei Stream Correspondence with TCC 23 June 2023
to Anne, Bill, Stephen, Shadrach, Radleigh,
Good morning Anne,
I have two comments based on your latest response.
1 Timing of additional culvert construction
I accept that there are no specific date requirements within RC 63636 for completion of the additional culverts. However, my understanding of that Resource Consent and documentation associated with it is that the scope of its requirements related primarily to ensuring that, in the event of 1 in 100 year rainfall, there would be no flooding of properties within Papamoa above 4.6 m (ref Moturiki Datum). The requirements for additional culverts were based on modelling of such water flows, always allowing for the planned residential and commercial developments throughout Papamoa.
If TCC has carried out additional modelling which now shows that there is some other requirement for culverts, I would be delighted to receive a copy of the relevant report.
However, I note that the only recent change in RC 63636, approved by BOP Regional Council in 2023, is that the cross-sectional shape of the additional culverts (e.g. under Gravatt Road) is allowed to be amended, provided the flow capacity (i.e. cross-section area) remains the same. I have seen no information related to RC 63636 that additional modelling shows that any of the culverts previously sized for the expected flows may no longer be appropriate.
As you and I have mentioned in previous emails, many parts of New Zealand have experienced very high rainfalls resulting in localised flooding. Many of the related Councils have been found wanting as a result of poor maintenance of waterways or lack of action on flood protective works to allow such flood flows.
I note your comment that TCC appears to be quite happy to reprioritise other capital works, even to the extent that critical components of flood protection within Papamoa now appear to be unimportant. By this, TCC is obviously taking complete responsibility for any flooding which happens in Papamoa prior to all culverts (identified in RC 63636) being in place and functioning. I am sure that most residents of Papamoa would be horrified with this attitude, based on flooding in other parts of New Zealand and particularly noting the vigour with which TCC seems hell-bent on implementing the proposed new stadium and central Tauranga development and roading network.
2 Recent rainfall and lack of maintenance
I acknowledge that 2023 has had more rain days that any in the past decade, but, based on weather records from my own Met Station, rainfall in Papamoa is far less than any of the “significant” areas like Northland, West Auckland, Coromandel and Waihi Beach, and even notably less (by up to 30%) than the Tauranga (Tauranga Airport) records. For example, Tauranga Airport rainfall over the last seven days has totalled 157 mm, while my own local records show 132.5 mm.
It is worth noting that airports are typically sited in areas of higher average wind speeds than areas nearby, and this pattern of “more wind” brings with it the other components such as rainfall. One therefore can’t immediately take Tauranga records and confidently apply them to Papamoa as being the same. Papamoa’s weather could best be described as fickle.
For at least my part of Papamoa, I can’t agree, therefore, that grossly excessive rainfall is the major cause of the Harrison’s Cut “valley” being excessively boggy for most of 2023. As I have said before, the real reason for the excessive bogginess has been flow restrictions out of the Pond by both blockage of the outlet grille and local flow obstruction immediately downstream of the pipe end with the very narrow outlet channel (less than 1 metre wide) at that location.
One of the chief culprits of flow restriction between the pipe end and the box culvert under Papamoa Beach Road appears to have been caused by the dumping of a load of large rocks, primarily to fill in a small section of “overland” stream bed from the end of the “dry” bridge nearby to near the pipe end. Many of these rocks now form part of a higher (than previously) stream bed. This creates a flow restriction for both the water coming out of the pipe and water flowing by the “overland” path, meaning that local water level adjacent to the pipe outlet is higher than it should be.
These flow restrictions have resulted in the pond water level taking considerably more time to drop to its low operating level. When the pond level is held higher, water is able to flow through the (relatively porous) surface layer at the bottom of the “valley” for longer durations, causing the extended bogginess.
As I have said before, a digger needs to be used to remove much of the material between the pipe end and the box culvert to minimise flow restrictions in that location and stop flooding around the (underground) pipe outlet.
I have previously included figures and graphs to show the impact of such flow restrictions, and if any of your staff do not understand I suggest that they enrol in appropriate hydrology/hydraulics courses or re-acquaint themselves with such technology.
When I wrote my last email to you, it was after I had visited the pond outlet and noted that the pond level was up to point of over-topping the weir and the grille was significantly blocked. I was therefore surprised with your statement that water flow was not constrained. When I then visited the site after getting your reply, I found that there had been an (obviously hurried) clearance of the grille, but nowhere near complete clearance - the bottom 150 mm of the grille (and further below water level) was not cleared (with a lot of algae over the debris and surrounding area – very much “untouched”).
I was also disappointed that material removed from the grilled was scattered over the nearby grass – and not taken away as I had suggested in my email.
Can you please confirm if your staff or contractors were responsible for this grille clearance (between the date of my email and the date of your reply)?
Regards
Noel Hall