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Flood prone areas (FPA) are potential ponding areas that may flood and commonly comprise of topographical depression areas. The areas can occur naturally or as a result of constructed features which act as embankments when stormwater outlets are blocked. Flood prone areas typically include depressions formed by road/railway/motorway embankments which were built across natural gullies. Some of the depressions in the Auckland region are also from volcanic activity or sand dune depressions in coastal areas. FPAs are a flooding hazard that require particular consideration within assessments carried out under the Auckland Unitary Plan E36.9 Special information requirements.
For Flood Prone Areas that can fill up in a 1% AEP rainfall event (large majority), the mapped extent is the area water will pond up to before starting to spill downstream (typically over a road), assuming the outlet is blocked. The FPA extent does not however consider the required flow depth and hydraulic head required above the spill level for water to flow downstream and therefore does not represent the potential maximum level (nor maximum ponding depth and stored volume) that may be experienced in a 1% AEP event.
For Flood Prone Areas that cannot fully fill in a 1% AEP rainfall event, the mapped extent is the area water will pond up to assuming the outlet is blocked before the rainfall event. In this case the FPA extents are based on the 1% AEP rainfall event in accordance with TP108 (1999), the maximum probable development (MPD) as defined in the Auckland Unitary Plan (Operative in Part) in the catchment and a 2.1-degree climate change scenario – termed future scenario.
The FPA layer was generated from Auckland Council LiDAR data, flown in 2016-2017 and available after processing in 2018. Flood Prone Areas were identified using GIS techniques and are not based on hydraulic modelling. The layer was updated in June 2021 and replaces the previous layer that was first published in 2013. The layer will be updated on an ad hoc basis to align with updates to Auckland Council’s LiDAR data.
Vertical Datum: NZVD1946
A summary of the fields (visible on Geomaps only) and their definitions is as follows:
Field Name / Alias | Description | |
FPA_ID / Flood prone area ID | Unique identifier of the flood prone area. | |
CanFill / Can fill in a 100yr ARI rainfall event | Can the depression area fill in a 100yr ARI future scenario rainfall event if the outlet is blocked - either Yes or No | |
CatchmentArea / Catchment Area (m2) | Catchment area upstream of the most downstream point in the flood prone area. | |
MinimumLevel / Minimum elevation (m RL) | Minimum identified (based on LiDAR) ground elevation in the flood prone area. | |
Spill Elevation / Spill elevation (m RL) | Lowest elevation where water will start to spill downstream. | |
MaxDepth / Spill ponding depth (m) | Maximum ponding depth before the flood prone area starts to spill (spill elevation minus minimum elevation). | |
VolumeM3 / Volume to spill elevation (m3) | Volume that can be stored in the flood prone area up to the spill elevation, assuming the outlet is blocked. | |
RainfallRequired / Rainfall required to fill flood prone area (mm) | Rainfall depth required in the catchment to fill up the flood prone area to the spill elevation. | |
RainfallFuture / Rainfall depth 100yr ARI future scenario (mm) | TP108 24hr 100yr ARI future scenario rainfall depth (existing rainfall depth multiplied by 16.8% to account for a 2.1 degree Celsius temperature rise due to climate change). | |
Level100y / Flood prone elevation in 100yr ARI event (m RL) | The elevation water will pond to in a 100yr ARI future scenario event assuming the outlet is blocked, and the flood prone area cannot fill. Where the flood prone area fills, specific calculation of this parameter is required to allow for the depth of the peak flow rate to leave the flood prone area, usually via weir flow. | |
Depth100y / Flood prone depth in 100yr ARI event (m) | Maximum water depth in the 100yr ARI future scenario rainfall event, assuming the outlet is blocked, and the flood prone area cannot fill (flood prone elevation minus minimum elevation). Where the flood prone area fills, specific calculation of this parameter is required to allow for the depth of the peak flow rate to leave the flood prone area, usually via weir flow. | |
Vol100yrm3 / Flood prone volume stored in 100yr ARI event (m3) | Volume stored in a 100yr ARI future scenario rainfall event assuming the outlet is blocked, and the flood prone area cannot fill. Where the flood prone area fills, specific calculation of this parameter is required to allow for the depth of the peak flow rate to leave the flood prone area, usually via weir flow. | |
SHAPE.STArea() / Max. flooded area in the 100yr ARI Future Scenario (m2) prior to spilling | Surface area of the flood prone area. If it can fill, it will be the surface area prior to spilling; if it cannot fill, it is the maximum extent of the flood prone area. | |