Stormwater Toolbox

The Stormwater Toolbox is an easy to understand guide for lay people to see what stormwater solutions are available. This is split into homes and public land.

Private Homes and Property:

Property owners can develop or change their properties to mitigate loss, harm or safety concerns. This is important to owners of low-lying land that could be subject to flooding.

BRANZ and NIWA have developed a guide for owners of existing homes affected by flooding.

https://niwa.co.nz/sites/default/files/tool_4.4_individual_house_benefit_cost_tool.pdf

This guide includes cost and benefit studies of various suggested remedial approaches. WRSAG supports owners taking positive actions to mitigate future flooding. The extent of action you take should take into account your long-term plans for the property you own. All options should be considered before significant decisions are made. Selling your current property to develop another that is less prone to flooding is also an option.

WRSAG preferred option is raising the ground level to above the crown of the road. The freeboard suggestions are 300mm above the crown to allow for road flood depth and wave action of vehicles.

It is noted the TCDC District Plan s453 has required 500mm freeboard above the flood levels for a 1%AEP since 2007 (no amendment from the 1999 version).

TCDC District Plan at amendment 2007

453 - Standards

Floor levels of all houses and all habitable rooms shall meet the following standards:

In areas covered by flood management plans:

(a) Primary overland flow areas: Not less than one metre above natural ground level;

(b) Secondary overland flow areas: Not less than 0.5 metres above natural ground level;

(d) Overland flow and ponding areas: Not less than one metre above natural ground level.

Floor levels of all houses and all habitable rooms for sites located in a flood hazard area shall meet the following standard;
Not less than 0.5 metres above predicted flood levels. Predicted flood levels are determined by reference to flood modelling, flooding history, a derived flood event, and existing flood protection measures. In defended areas, Floor levels must be 0.5 metres above the predicted flood level for a 1% flood event.

In defended areas, new houses must be set back a minimum of 20 metres from the base of any flood defence, unless an easement, or other legal instrument, for the purposes of access to and maintenance of the flood defence has been registered on the Computer Freehold Register or Certificate of Title.

In medium flood hazard areas, replacement houses or additions to houses must be designed with foundations that are open and allow the free passage of floodwaters to pass beneath them to ensure habitable areas are not subject to inundation and floodwaters are not diverted or displaced onto surrounding properties.

Thames-Coromandel District Plan » Section 5 - Rules Applicable to Activity Groupings » 510 - Housing Activities » 513 - Standards » 513.8 » 513.8.2

Floor levels of all houses and all habitable rooms for sites located in a flood hazard area shall meet the following standard;

Not less than 0.5 metres above predicted flood levels. Predicted flood levels are determined by reference to flood modelling, flooding history, a derived flood event, and existing flood protection measures. In defended areas, floor levels must be 0.5 metres above the predicted flood level for a 1% flood event.

The Building Code E1 Surface Water requires floor levels to be 150mm above the crown of the road, OR alternately, 150mm above ground with a 1:20 unimpeded slope away to a natural waterway.

The guide recommends providing subsidies to owners to raise ground levels. It is noted WBPDC has adopted this recommendation.

Public Land including Reserves and Roads.

Council is responsible to WRC for the management of Whangamata Stormwater in accordance with its Comprehensive Stormwater Discharge Consent CSDC 105667.

Council’s duty is to consider all methods of stormwater improvement to mitigate within reasonable cost benefit adverse effect of flooding to safety, damage and access. This includes providing clear advice to private property owners and installing and maintaining appropriate stormwater infrastructure.

WRSAG has prepared this ‘toolbox’ to assist residents to understand some of the options available to council to mitigate the adverse effects of flooding. These are not the only measures council may consider.

Toolbox diagram:

1.0 Curb and channel with pipes: Pipes are the primary means of managing surface water runoff from impervious surfaces. Design capacity is to meet the building code E1/AS1 being a 2%AEP. For Whangamata 2%AEP is 137mm rain per hour for a 10 minute duration.

There are 8 primary pipe systems the main ones being into Williamson Pond, Hetherington Road outfall and various others with outfalls along the business area into Moanu Anu Anu
Many of the discharge pipes become blocked by tidal and sea level rise as the pipes become submerged in high, spring and King tides.
The HAL report identified 17 pipe systems where diameters decreased towards the discharge meaning pipes have restricted flow
HAL DEM flood modelling discounted pipe systems with diameters less than 600mm as they are considered ‘blocked’ during storm events
Along walkway 6 water bubble ups occurred through catchpit open grates flooding Barbara Rd and properties in minor rain intensities. This indicates downstream blockages backed up rainwater and flooded higher up areas pipes are designed to drain.
During the drop-in sessions residents were told ‘pipes will never solve Whangamata’s flooding problems’. Pipes are needed for smaller rain events up to 2%AEP. Beyond that secondary flow paths are required to remove surplus water.

2.0 Larger soak pits and catchpits: Soakage devices are used throughout Whangamata on many roads and all private dwellings to dispose of roof and some road catchment. Soakage devices are required to detain and manage 100mm in a 24-hour period. This is the mandatory minimum performance requirement under the building code called the Acceptable Solution. This does not prevent TCDC from adopting a larger storage soakage device for both roads and private dwellings.

Whangamata (flats) subsoil is Holocene sands that absorb 50% of volume as water. That is 500mm of rain can be absorbed by 1m depth of sand.
The sand-based water table bleeds into the rivers and Ocean at the rate of 200mm per month.
The water table level below ground averaged between 0.7m to 2.5m deep during a water table monitoring program. This would indicate the water table can absorb between 350mm to 1.25m of rain before break-outs depending on starting levels.
Hale and Gabrielle in 2023 loaded the water table to capacity, followed by average monthly rainfall in excess of 400mm, at least twice what naturally drains away. This caused extensive break outs and long duration surface ponding in low lying pockets of land. TCDC claims Whangamata received up to 6m of rainwater during 2023 which is well in excess of the ability of the sand base to manage this level of ongoing rainfall.
The ongoing rain following Gabrielle meant the water table rose above the freeboard of private and public soakage devices preventing them from performing as infiltration rates dropped to negative or zero at best.
A modification of DRAIN-MOD was proposed similar to trials underway at Mahurangi Beach. This system incorporates overflow pipes linking soakage devices to ensure surrounding water levels can drain away via the pipe network slowly following rain events to leave soakage devices operational for further rain events. This also allows redistribution of fully loaded soakage devices to lower downstream ones not yet fully loaded. Delaying stormwater is an effective management tool.
TCDC Building Consent Approvals do not require bubble up chambers or any means for owners to maintain them eg removable hatches for cleaning out organics that reduce infiltration efficiency.
TCDC Road services provider is contracted to clean out all public soakage devices each May every year. This has not been managed. The consequence is lower infiltration rates resulting in run off (flooding) to neighbouring lower properties. This is called hydrologic trespass water.

3.0 Tidy up swales and turn roads into secondary overland flow paths: Due to the way the Holocene sands formed the sand dunes have relatively flat topography. To service the area with access, we formed roading in a criss cross manner cutting off what were the natural flow paths evolved over the centuries. Roading was formed by cut to fill meaning fill was moved from high to fill lower land to form the road pathway. This has left many properties below the level of the road crown. To drain surface water off roads they form crowns with curb and channels with fall to catchpits or soakage devices. To control rain run off, swales are created along each side of the roads or curbs. Subsequent development has caused:

Driveways being formed blocking the swales preventing the flow of surface water. These require changing.
Driveways being formed allowing water to escape the swales and enter properties as trespass water. These require relaying to continue the swale catchment
Crowns have been formed at Tee intersections blocking off the swale flows. These need to be altered to allow secondary flow towards the Ocean or rivers.
The lowest end of the roads requires a secondary overland flow path to a river, the Ocean or stored temporarily in a detention device.

Bellona after rain flooding is marshaled and controlled by the depth of swales in the road verges. Houses are well above even a 500mm depth of flooding.

Poorly formed driveway with fall down from curb and channel that will direct surface water onto private property and flood. The road verge is a council reserve which is required to marshal surface water into pipes not allow it to trespass onto private property.

4.0 Form detention basins and use the fill-to-fill low lying land: The greatest form of managing stormwater run-off is by redistribution. For Whangamata pipes remove rainwater to a waterway, soakage devices allow infiltration into dry sand, and storing rainwater temporarily in a detention basin until the pipes or sand can cope. We have a lot of reserves and open land to create detention basins. We also have a lot of lower lying land formed due to the roading cut to fill process. It is just a matter of topography. One of the logical uses of the modelling is to identify these if we don’t already know. If correctly overlaid onto LiDAR topography it will identify where land is required for this.

Cities like Auckland are purchasing properties for this purpose where insufficient land is available along stream beds and rivers.
Private sections have open spaces between built properties for water to pass.
It is acceptable for council to purchase some land to unlock detention devices
In forming detention devices, the fill can be used to lift lower lying land that is to be redeveloped.

5.0 Maintenance and Road sweeping: It is essential the infrastructure we do have is well maintained and kept operational. The current management is via contracts with the road service provider. However, this has not been enforced.

What we did notice immediately after the clean was soakage devices levels of water immediately dropped indicating the water table level was lower than thought
TCDC current policy to police maintenance is to wait for an RFS. Our issue with this is the need for an RFS is because the cesspits are blocked and fill with sand so don’t perform in the rain events they are designed for. By then it’s too late and we get flooding. Maintenance is to ensure the asset performs when needed, not waiting until it failed to then clean it.
TCDC has yet to provide WRSAG with these contracts and whether the contractors have been paid for work they didn’t do.

6.0 Reform lost overland pathways:

Many of the old valleys have been filled with roading or by property owners trying to get the best use of land.
Where flood depths are greatest it is of use to consider reforming the natural overland flow paths as a more cost-effective way than trying to pipe.

7.0 Filling low lying land: Land now identified as likely to be flooded will need some form of toolbox solution. The obvious choice to consider would be to fill it to a level that it is above the flood level so the use of the land for a permanent structure can be done with assurance it won’t flood.

8.0 Water Table Management: The current stormwater management plan is heavily reliant on water table levels being sufficiently low so private and public soakage devices can continue working. The Building Code E1 Surface Water has a requirement soakage devices must remain above the water table. The abnormal events f cyclone Hale, followed by Gabrielle and then 6 months of higher than normal rainfall filled the water table above soakage devices so many failed to perform. This means an obvious solution is a form of water table management. There are many such solutions including the trials at Mahurangi that uses a modification of the DrainMOD system. This functions by connecting soakage devices with smaller pipes so when a soakage device gets full the water is piped to the next and to the next and eventually to the piped systems. This all occurs underground and can operate slowly following rain events. It does not need to manage 1%AEP but can reduce the water table level so the next rain event has more dry sand to infiltrate via soakage devices as they will be ready.

9.0 Innovation is required: It is completely impossible to pipe all roads – the costs would be prohibitive, the disruption unacceptable and we don’t have sufficient gradient to install pipes of adequate diameter that can discharge into the Ocean or rivers at King tide and as sea levels rise.

Council has become paralyzed by the flood of reports it has commissioned. It will take more reports to make sense of all the recommendations and ‘limitations’ consultants include to meet their insurers terms of engagement.
What we need is a back-to-basics approach using the toolbox of solutions together so that each small improvement ‘incrementally mitigates flooding’. It is not a one thing fits all. There is no silver bullet. There is no one project.
Innovation by studying the WBPDC subsidy to property owners to fill low lying land. By lifting the lower-lying properties this means the levels of inverts for pipes can also be raised. Don’t try and pipe the lowest property. Fill it and then pipe it.
Make the roads effective secondary overland flow paths.
Consider cost benefits in each decision. For example, we have been told of the 3500 property owners with letters up to 1900 could have serious flooding. We have been told the consultants could be spending $40M and still not solving all the problems. If these were the only properties adversely affected that is an average of $21,000 per property to mitigate flooding. If council offered an incentive of say $10k per property we will have over $20M left for other things.
If we purchased 10 properties in low lying land for detention ponds that could save millions in pipes.
There is a substantial sand dune area off Hunt Road that already becomes marshlands during heavy rain. This would make an ideal detention basin for Port Rd run-off and perhaps include wetlands if road water does require cleaning.

References

E1 surface water amendment

E1/AS1 Appendix A Rainfall Intensities

40mm overnight rain on 11 July 2024 caused blockage or overload

Bubble up chambers required in E1/AS1 and verified using E1/VM1

TCDC has refused LGOIMA to see various service contracts but a reference to the May requirement to clean cesspits is made in the SMP. This is of concern to us as cleaning all approximate 400 catchpits amounts to at least $100k per year. TCDC won’t answer if the contractor has been paid but not provided the service.

Awareness of flooding has devalued low lying land - some will have little to no value if council applies RMA s31 and prevents development or use of that land.