Stormwater information for private property

Water Services - Stormwater

Information for private property - flood plains

Definition of a flood plain

Areas of land adjacent to waterways that get inundated with flood waters during a flood event.

Description of what is shown on the map

Flood plains shown on the maps indicate the areas that are likely to be inundated in a major flood event that has a 1% chance of occurring or being exceeded in every year.

The flood sensitive areas shown on the map indicate areas of uncertainty beyond the flood plains that is within 500mm in elevation of the predicted flood level.

What it means to you as a property owner

Development or filling of land within the flood plain is not permitted
Habitable floor levels must be a minimum of 500mm above the flood plain, and non-habitable floor levels must be above the level of the flood plain.
Every property owner is legally required to accept stormwater run-off that would naturally flow onto their property and may not obstruct or divert flood waters as this may affect other properties
For developments on land subject to flooding, applications for building consent will be subject to provisions under the Building Act which may include refusal or conditions of consent.
Proposed District Plan change 24 is likely to make it more difficult to develop within flood plains.

What this means for existing buildings in the flood plain

The associated risk to each property will need to be assessed on a site-by-site basis. The level of risk will involve many factors, such as:

Velocity of flow
Depth of water
Level of the floor
Structure of the house
Soil conditions.

Freeboard

Freeboard is the height above the flood level to the floor level. A minimum freeboard of 500mm above the flood level is generally required for habitable floor spaces within the flood plain. This is a factor of safety that allows for variations caused by:

Waves
Localised variations in topography and vegetation
Uncertainties associated with a large number of technical assumptions built into the computer model.

Calculation of flood plains

The flood plains have been calculated using computer models of the stormwater system in a particular stormwater catchment, to calculate the levels the flood waters would rise to during a rainfall event that has a one per cent chance of occurring or being exceeded in every year.

The maps show the outline of the extent of the calculated flood plain based on the topography. The topography has been established using data derived from LIDAR (Light Detecting and Ranging survey) to create a Digital Terrain Model (DTM).

Limitations

While the flood plains have been developed using the best information available to council at the time, they still have the following limitations:

The flood plains have been developed over the past 15 years using a variety of computer packages and technical assumptions
Due to development in catchments and improved modelling techniques, the flood plains need to be reviewed from time to time
The map only shows the outline of areas that are likely to be inundated during a major storm event, and no indication is made of depth, velocity or degree of risk
Some physical obstructions to flows, such as buildings, walls and fences may not be accounted for
Recently developed land may have altered the topography due to earthworks, and therefore flood plains may be altered in these areas. The LIDAR survey was carried out in 2004
The map does not make allowance for landslips or movement of land, or obstructions (such as debris) that may result, and has not taken into account erosion
The flood sensitive areas shown on the map indicate areas of uncertainty beyond the flood plains where more detailed investigation is required in order to determine if they are or are not likely to flood in a major event.

Stormwater information

See how your property may be affected by flooding hazards.

Stormwater maps