Te Awamutu Courier

Flood-risk maps factor in once-in-a-century rainfall event


New flood hazard modelling maps have been publicly released for an extreme one-in-100-year rainfall event for the Waipa¯ towns of Cambridge, Te Awamutu, Pirongia, and O¯ haupo¯ .

The maps were developed from a stormwater modelling project, undertaken by Waipa¯ District Council over the past year as part of its work to further understand natural hazards in the district.

Waipa¯ District Mayor Jim Mylchreest said the maps provided an overview of urban areas where council would expect water to flow in an extreme rainfall event and would assist it in ensuring stormwater assets were fit for purpose.

“The map layers show where water will accumulate in an extreme rainfall event, which will help to identify areas in our existing stormwater network where further investigat­ion may be required. By bringing these maps to the public, property owners will also be able to see this accumulati­on and be better prepared . . . ” said Jim.

An extreme rainfall event for this type of analysis has a low 1 per cent chance of occurring in any calendar year and is expected to occur once in a person’s lifetime.

Jim said properties that were identified as having over-floor flooding in a dwelling have been sent a letter with supporting informatio­n.

“Property owners identified through the modelling as potentiall­y at risk of over-habitable-floor flooding in an extreme rainfall event, have now received a letter with an individual property map which shows the likely depth of the water as modelled along with further supporting informatio­n.

“We recognise this may be a stressful time for those property owners and we are here to assist wherever we can. As they say, informatio­n is king, so we hope that this will help them be as prepared as possible in any eventualit­y,” said Jim.

Council’s public flood hazard maps show areas of flooding in blue for present-day flooding, as well as any flooding where future climate change is taken into account in purple. Flooding

depths are not shown in the public flood map, but are available on request.

Jim said property owners who have not received notificati­on from council, but can see their property covered fully or partially by a flood layer, should not be alarmed.

“. . . we would like to make it clear that if you haven’t received a letter, the flooding is minimal and unlikely to reach your dwelling’s floor level.”

“Where this occurs, it generally indicates very shallow water near the habitable dwelling or where water pools outside the home, such as in garden low spots,” noted Jim.

Council’s stormwater drainage system deals with frequent rainfall events, whereas the mapping represents an extreme rainfall they are not designed to deal with.

The stormwater modelling project provides an estimate of where extreme rainfall will flow and pond in the urban areas. Rivers and stream fluctuatio­ns and non-urban areas have not been included in the maps because this informatio­n is available from Waikato Regional Council and presented through their hazards portal.

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