Our stormwater system
Primarily rainwater either soaks into the ground, is channelled into pipes or flows over the ground to a river or Ocean.
Soaking into the ground is reasonably effective in our sandy soils. Clay soils are useless. Soakage device designs must cope with 100mm of rain in a 24-hour period by either storage volume or infiltration or combination of. Hale and Gabrielle filled our water table, followed by heavier than normal winter rains caused ongoing flooding as run-off water could not soak into the ground and had nowhere to go.
Pipes are a way to remove rainwater quickly. Some of our roads have pipes. None of our houses are connected. Pipes are costly so the Building Act ONLY requires councils to cater for a 2%AEP - for Whangamata this is rainfall at the rate of 137mm/hour for a 10-minute duration. The reason to limit to 2%AEP is to cater for anything larger means huge pipes, large curb and channel, more catchpits and discharges costing much more than the insurable ‘chance’ of 1/100 or 1/250 event occurring. This is a government setting for a stormwater plan.
Over the ground is for everything else that pipes and soakage cannot manage above the 2%AEP. The natural over the ground are valleys, rivers, creeks and streams that discharge to the Ocean. Over the millennia nature made these. As we developed the land with roading and urbanisation we irreversibly altered natures tracks. The first rule of stormwater design is you cannot develop the land around the rivers and creeks and stream beds so as to obstruct the natural water flow in flood, or cause a bigger run off over the ground than the natural valleys, creeks and rivers can handle. Our development has gone unchecked as we changed absorbent sandy dunes and grassed areas into impervious concrete, roofs and seal. According to WRC every square meter of converted land causes a 4-fold increase in runoff. When our pipes were installed, they did not have to cater for the projected global warming. Now they are too small.
Detention basins are depressions and low-lying land that collects surface water runoff that is not removed by pipes, soakage devices or overland flow paths. Surplus surface flooding finds these areas by gravity. If your house is built in one of these, you will have experienced flooding. There are rules council must follow to avoid this causing damage or loss to you. This includes stipulating floor level heights. Developed higher ground cannot flood lower ground. The most common detention basins are swales along our roads.
Pipes are the primary flow paths and over the ground the secondary flow paths. Soakage devices and detention basins supplement these. Council can use all of these in a stormwater master plan to mitigate damage to properties and protect life.
Global warming is going to affect rainfall intensity and sea level rise. Councils are now encouraged to factor a 1%AEP rain event along with projected sea level rise. This means the existing pipes and soakage devices must have greater capacity and be installed higher to discharge so they can function even at King tide level towards the year 2090. It also means the overland flow paths and detention basins need expanding.
The purpose of the flood modelling is to provide councils with the ‘what ifs’ so improvement options can be thoroughly examined by the model so as to provide the most cost benefit mitigation processes.
Apparently, we have 2 master plans already developed from the modelling but these are still withheld from us.
What has happened in Whangamata is development of our land, which includes subdivisions and second homes on sites, larger and bigger homes and wider roading has turned our roads into default overland flow paths once the pipes are full and soakage devices are full. When the surface runoff is not controlled it fills low-lying land which floods those houses built on these sites.
To protect owners the building code required floor levels to be higher than the roads crown by 150mm (old 6 inches). This means when the roads turn into an overland flow path and floods your low-lying section the house would still be above the flood level. When our roads were constructed they cut to fill and blocked the valleys the rainwater runoff would naturally run to rivers and ocean.
Council had a duty to prevent development of land until it had a stormwater master plan in place and infrastructure to prevent property damage. This infrastructure gets paid for out of reserve contributions and resource approvals. But it hasn’t. We are now in an infrastructure deficit and in danger of claims from property owners with low level floor heights.
The Building Act does allow owners to build below these guidelines but in doing so they must sign a waiver they cannot hold council liable if the property does flood. These waivers appear on the Certificate of Title. Council has not done this as far as WRSAG can determine.