What are flood hazard maps?
Flood hazard maps show the likelihood of land being flooded in frequent and extreme rainfall and flooding events.
Wellington Water has completed computer modelling of the likely impacts of extreme rainfall on the urban areas of Wainuiomata.
The modelling takes into account recorded rainfall data and flood levels from past events, the contours of the land and the existing storm water network. It also factors in the predicted effects of climate change out to 2120. The modelling is based on current best practice and has been peer reviewed by independent experts.
The flood hazard maps, based on the computer models, show which areas are likely to experience flooding in one in 100 year floods and one in ten year flood events. They show which areas might be flooded in an extreme rainfall event and to what depth they may be flooded.
The maps show known stream corridors, areas of inundation greater than 50mm and significant flow paths that carry flood water overland. Additional evidence like photos, video and eyewitness accounts of past flooding events are used to fine tune the maps.
A technique called dynamic freeboard allowance has been incorporated into the maps. This is effectively a safety margin, taking into account contributors to flooding that are difficult to represent, such as storm water blockages or sedimentation, or vehicle-generated waves. The freeboard allowance builds in another 200mm of water to every depth level indicated on the maps, to take account of these factors.
Why were the maps created?
Council’s main aim in producing these maps is to protect lives and property by reducing the impacts of flooding through a better understanding flooding events. Council has a responsibility to ensure future developments avoid flood prone areas and do not make downstream flooding risk worse.
Flooding is one of the major natural hazards in our region, and Lower Hutt has a long history and recent experience of extreme weather.
The potential threat to life and the damage and disruption caused by flooding have serious impacts on residents, property owners and businesses.
Council is required to collect flood-related data and make it available to the public and for anyone looking to buy a property or develop land in affected areas. Any property with a history of flooding will have this information added to its Land Information Memorandum (LIM).
The modelling data also is vitally important for council when making decisions on stormwater investment and will be used for civil defence planning.
Who did the modelling and mapping?
Wellington Water is owned by the Wellington Region’s councils and manages drinking water, waste water and stormwater services on their behalf.
Wellington Water has a comprehensive programme of modelling and mapping stormwater flood risk across the entire Wellington Region. The maps have been developed to best practice modelling standards and have been peer reviewed by external independent experts.
Why are Greater Wellington Regional Council and Wellington Water both creating flood maps?
Hutt City Council is responsible for managing stormwater, including culverts and drains. Greater Wellington Regional Council (GWRC) is responsible for managing waterbodies like lakes, rivers and streams.
So Wellington Water, on behalf of Hutt City Council, is developing stormwater modelling and flood hazard maps for urban areas of the city. Stormwater is the rainfall that falls on land, roofs, roads and footpaths and is paths to drains, rivers and streams.
GRWC is undertaking flood hazard modelling of the Wainuiomata River catchment to update their 2012 maps showing areas likely to flood from the river.
What is a 100 year flood?
A one in 100 year flood is the result of an extreme rainfall event, and has a one per cent probability of happening in any given year. If we had rainfall records going back far enough, it would be a flood that occurs on average once every 100 years.
What are stream corridors, overland flow paths and inundation?
Stream corridors typically consist of a buffer of five metres either side of a stream’s centreline.
Overland flow paths are the routes that rainwater will naturally follow on the way to a permanent waterway, such as a stream or river. They are generally the lowest point on a property.
Inundation is another word for flooding and is depicted on the map as areas where flooding is likely to exceed 50mm.
How does climate change affect flooding and how is this accounted for?
Climate change is causing sea level rise and is predicted to bring much heavier rainfall than we currently experience.
More intense rainfall means a greater amount of rainwater going into low-lying areas, overland flow paths and streams and rivers. More intense rainfall can cause land to saturate faster and drainage systems to reach capacity earlier.
Sea level rise may also cause the water table to rise, making land saturate faster. It may also slow the rate that water can drain form the land and rivers into the sea.
To take climate change into account, Wellington Water has factored in a projected one metre sea level rise and a 20 per cent increase in rainfall intensity and volume.
What is Council doing to manage flood risk?
Council is currently planning for the forecast impacts of climate change and population growth on our water infrastructure out to the next 30 years.
It has set aside $62.8 million over the next 10 years for stormwater upgrades and improvements.
Council manages flood risk in a variety of ways. The stormwater network of pipes and pumps is designed to drain normal levels of rainfall away from properties, into waterways and on to the coast.
There are also some retention ponds and wetlands that catch a certain amount of flood water when the system is under pressure.
Historically, storm water networks were designed to carry away water during low to medium intensity rainfall events. When rainfall exceeds the pipe capacity, then water flows overland.
New buildings are required to include stormwater retention tanks to store water during high rainfall events to delay when stormwater enters rivers and streams.
Wellington Water’s modelling of flood hazards across Lower Hutt will provide crucial data for Hutt City Council for future urban planning to minimise risk, as well as for planning future stormwater and other infrastructure investment.
Will these maps affect my insurance coverage or premiums?
Each insurance company has its own methods of determining risk, the services they provide and what they charge for flood insurance. Some larger companies do their own modelling of flood risk.
Typically insurance companies are more concerned about regular flooding than rare events. Talk to your insurance company to get accurate advice.
Will these maps affect my property value?
Council cannot comment on what the effect on property values might be. Many areas of New Zealand are subject to natural hazards, and this does have some influence on people’s decisions on where they choose to live.
It’s important to note the maps do not create natural hazard risk, they simply identify where flood events are likely to occur. They help ensure future developments avoid flooding and that development doesn’t increase risk to nearby properties.
My property didn’t flood previously so why is it in a flood plain/overland flow path on the map?
The catchments mapped have a history of flooding. While it may not have flooded in recent memory, the modelling shows it is potentially prone to flooding.
The model includes forecasted changes to our local climate. Flood events will be more severe in the future as sea levels rise and we experience increased frequencies of damaging and disruptive weather patterns.