WRSAG Proposed Improvements

 

Proposed improvements for implementation into Whangamata CSDC 105667

 

The following proposed improvements have been prepared because the workshops have broken down and council staff are making decisions in respect to improvements without stakeholders seeing the cost to benefit or priority ranking.

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WRSAG fear the current proposed work is knee jerk to at least show community something is being done. We fear this could be more unjustified expenditure following more bad decisions. It may be possible that the 6 work scopes being tendered now are useful to our stormwater infrastructure BUT that is not the point. The outcome does not justify the means. Staff are making decisions yet again.

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The alleged $750,000 spend at Williamson Park in 2024 does at least provide a pipe through the weir BUT the cost is 10 times what is should have been and came with a Gross Pollutant Trap WRC stated won’t work and has been left as a muddy bowl. WGSAG want all stormwater in Williamson Park redirected to other areas to return the park for the enjoyment of all the community, not some toxic pond that erodes and pollutes our beach. WRSAG agreed to the pipe through the weir on the proviso within 3 years it will be removed when the catchment is redirected in stages to other discharge locations. It is likely some discharge must continue as there are topography (height for fall) issues. If catchment was reduced to half of existing, it could be possible for infiltration and only have the pipes working in extreme events. This will remove the current erosion track down the beach and protect shellfish and fish.

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The pipe duplication into Williamson Pond in 2020 at yet undisclosed cost (likely $500,000 plus) along with consultant fees is an outstandingly dumb decision. What has not been disclosed to the community is the pipe duplication was to increase the catchment into Williamson Pond by a factor of 7.  The pond didn’t cope with what was discharging into so discharging 7 times would have been catastrophic. It is obvious the decision-making process was flawed and cannot be relied upon in future.

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Consultants’ fees are likely closing in on $200,000 for the current modelling. This does not include modelling costs from 2018 to 2023, and the costs associated with the dozen odd reports the HAL report references. WRSAG are concerned council is using consultants as its defence if decisions turn sour. But consultants are wary of this and generally include disclaimers based on terms of engagement which are withheld from us.

When costs blow out council can hide these costs as we don’t have starting points or options of what could be done another way.

 

If projects don’t stack up staff cannot be blamed. There is no accountability which increases the need for transparency for community but concealment by council. Decisions must be open and made by full council meetings BUT only if our elected members have been correctly briefed.

WRSAG have gained a lot of information (through LGOIMA) and provided dozens of written reports requesting various solutions. (You can view them here later)

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Before we can debate decisions, we must agree on why our stormwater systems are not performing: It is not the 1%AEP that caused the greatest loss to Whangamata in 2023. Hale and Gabrielle passed through quickly but surface flooding continued for 6 months. This nearly crippled our businesses and caused loss of enjoyment to community.

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The following schedule of works will obviously attract much debate. This is our starting point.

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1. Extreme rain events cause a proportion of homes to flood. This is a regular thing with these same homes. HAL has identified 71 throughout Whangamata. The modelling numbers have NOT been defined – whether it is the same 71, the 300 odd from the Opus 2003 report, the 2023 HAL 400, the 2024 HAL 1900, or all 3500 properties TCDC chose to send letters.

 

REASON for flood: Every flooded floor WRSAG has surveyed has a floor level below the road or overland flow path. Uncontrolled surface water run-off finds its way to the low-lying land.

 

• Fill these properties. Council offer a subsidy of $10,000, free building consents, free inspections and up to $30,000 of interest bearing loans.

 

Scenario 1: Immediate benefit 71 properties wont flood meaning fewer emergency services call outs, less insurable losses, better well-being of 71 owners and/or tenants, no pipes required as water goes somewhere else.

 

Cost to rates is $710,000 plus staff costs for BC and inspections. Loan is repaid through rates or if sold. Does not affect TCDC credit rating. Cost is likely to be delayed over a minimum 10 years as owners consider development changes. Budget allowance $71,000 per year.

 

• The average cost to raise the land depends on the type of structure. If an average cost to lift and fill is $100,000 then the owner will be paying $90,000 to do that. The benefit to the owner is restored land value as hazard flag removed, reduced insurance costs and excesses, less disruption at each rain event and peace of mind sleeping at night or listening to the news. WRSAG are not valuers but can appreciate if land cannot be developed it has little if any true value. Current dry land values are in the vicinity of $1M so spending $100,000 to restore land value is a sound investment.

 

Scenario 2: Medium benefit to 400 properties identified in the 2018 HAL flood modelling. This is chosen as the likely floor levels that may flood - but we have not had the model parameters disclosed. We think it’s likely this is the 2%AEP being modelled. Once confirmed we can review costs and benefits. Same offer same outcomes.

Cost to rates is $4M – use 20 year timeframe so budget $200,000 per year.

 

Scenario 3: Longer term benefit to 1900 properties identified by HAL – we have not been advised if this is floor level flooding, but assumption is worst scenario. Reconfirm after modelling. Same offer same outcomes but over 50 year to coincide with likely redevelopment timeframes as properties age etc

Cost to rates is $19M – likely over 50 years so budget $380,000 per year.

 

• these are NOT stormwater drainage systems and only redirect water back to roads. Roads and overland flow paths are still required. Some piping maybe required but for different reasons. These are included later.

Major benefit to council is this policy would put a timing equation into the loss of property value. The timing is up to the owner. If it floods again before they make a decision councils defence would in part be owners contributory negligence by failing to take the necessary steps to prevent loss. Owners and council need to get their own legal advice.

 

2. Long duration rain events cause abnormally high water table and breakouts causing surface flooding.

 

REASON for flooding: The sand base absorbs half its depth in rain. Normally the water table is about 1.8M down meaning it can absorb 900mm of rain. The water table bleeds out into the ocean at about the rate of 200mm per month ie bleeds about 100mm of rain. When rainfall is greater per month than 100mm the water table level rises. In Hale and Gabrielle the water table basically filled leaving the next 6 months of winter rains creating surface water ponding and flooding.

 

Install DrainMOD system or variation to all roads without pipes or those that won’t get them for the next 50 years. Increase soakage devices in number and volume.

 

This involves: swales along all roads, swales to have soakage devices every say 50m and these soakage devices are all linked with deep pipes of about 250mm diameter that discharge into piped systems. In selected areas construct detention devices as bulk temporary storage systems

 

How this works:

• In first flush soakage devices store rainwater and infiltrate at defined rates.

• In prolonged rain events soakage devices become overwhelmed so don’t infiltrate sufficiently so excess water discharges in the pipes to the next soakage device

• In even longer rain events, if the pipe network is full preventing discharge from the DrainMOD pipes the swales will fill and detain water.

• As rainfall slows and the pipe network clears the DrainMOD pipe system connecting the soakage devices can then discharge into the roading pipe system and discharge to the outfalls.

• Swales will gradually drain, and surface water is gone.

Noted this will be a slow process and does not need to manage the ‘peak’ rainfall at once. The 1%AEP intensity will be stored in the soakage devices, interconnecting pipes and swales along roads. By draining the soakage devices the surrounding water table will lower and be capable of absorbing the next rain. In this way surface water durations will be shorter and less problematic.

 

Cost to rates estimate around 20-30 roads will need this system at approximately $1M each. Undertake a trial Road first to test methodology.

Allow $30M spread over 10 years so $3M per year. If completed over 20 years allow $1.5M per year. Completing within 20 years means infrastructure will be completed by 2044 ie 46 years ahead of target.

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There may be some requirement to extend existing pipe networks to join the DrainMOD. Undertake trial on Mary Rd along with concrete edging and swales – to determine best way for further roads. Trial at $1M. We won’t be able to test the overall operation until we get rainfall of at least 50mm overnight. DrainMOD can be supplemented with other solutions like detention basins and overland flow paths. Some may already exist.

 

3. Large pipe for Port Road.

Port Road commercial area is dependent on the piped network which was designed pre 1970 well before the 1%AEP was considered, before Port Road was raised, before the service lane was raised and before much of the development took place. Pipes are not coping. This area is a special case requiring roads to act as overland flow paths supplemented by pipes for smaller rain events.

 

REASON for flooding: These pipes are small, old and out of date. They cannot cope with the catchment and all the development.

 

• Install 1.2m or 1.5M diameter pipe from the service lane to Hunt Rd dunes. This pipe to become the central hub for all future pipe discharge and new pipe feeds. Hunt Rd dunes can become a wetland if required – noted the service stations and parking areas are the main sources of contamination.

This involves digging up the service lane and through to Hunt. Create overland flow path by lowering one street to drain to Ranfurly. Picks up Dianna and Barbara.

Cost to rates: Allow $4M for central pipe in year 2. Then over the next 10 years add five pipe extension to this hub. Allow $2M each. Total budget $14M

 

 

4. Redirect commercial area to Hetherington. Redirect about 4 pipe discharges from the commercial area into Heatherington. What cannot be redirected become the retreat zones. Affected commercial owners to set up their own retreat or lift policy. Ratepayers cannot be expected to support this initiative without a good business case.

 

5. Beach Road and nearby low-lying land. Council is not authorised by Government to build infrastructure where it could become overwhelmed by rising sea levels and inundation predictions. It is understood this is marked by a 3.6 Deg Temperature rise equating to a 600mm rise by the turn of this century. This puts most of lower Beach Rd and surrounding low-lying areas under water. Whilst building sea walls is an option the issue with walls is during extreme rain events and at peak tides pipes and overland flow paths cannot discharge so rainwater runoff will be trapped inside the wall. An affective way is to build the wall and fill behind it. Affected landowners need to come up with a plan without interference by outside politics of other wards. This should be a 100-year plan for the future enabling owners and future owners to work together to determine fate.

 

A good example of this is the Marina Dredgings dumped behind the Boat Chandlery. By default this land is well above sea level rise predictions. Pressure and a business case should be made to dump dredging from the clean up of Moanu Anu Anu and implementation of a local bylaw requiring any property having fill removed to be dumped here. No fill should be allowed to be removed from Whangamata. It is needed to keep in tune with rising sea levels and inundation by rivers.

 

6. There are a number of one-off projects required. Several areas of low-lying land will still need individual plans to mitigate flooding.