As well as modelling for flooding extent and depth, Council also models the speed and direction of flood water. This gives us important information about where flood waters flow during a heavy rain event. This is ‘overland flow path’ information. It is viewable on Council’s mapping website and is publicly available along with the rest of Council records.
An overland flow path is an above-ground flow of water that caters for overland flows in heavy rainfall events. Overland flow paths can be low-lying natural drainage paths, road corridors or more likely natural flow paths based upon topography that can convey these flows.
Existing overland flowpaths throughout the City are progressively being mapped as Council undertakes its wider Flood Hazard Mapping project. The overland flowpaths are derived using a GIS-based model which determines the speed and depth of these flows which generally occur within low points of the topography where water concentrates and flows over properties and roads.
The methodology to map overland flowpaths was established using data derived from LiDAR (Light Detecting and Ranging) survey to create a Digital Elevation Model (DEM). Although the mapped overland flow paths used the most up to date information and technology available at the time of mapping they do have the following limitations:
- Some physical obstructions to flows, such walls and fences are not accounted for and what is mapped is the flow that would occur if existing obstructions were not in place;
- The map is based on the topography at the time of the LiDAR survey and does not account for recent earthworks undertaken as part of development;
- Council only maps the speed of the water multiplied by its depth where the outcome is greater than 0.05 m2/s. This gives the Council an understanding of the width of the overland flowpaths and determines where the significant overland flowpaths are considered to be present within the City. Mapping Speed times Depth (SxD), known as the Velocity Depth Product is a very effective measure for defining hazard categories and determining conveyance of flood waters.
- Floor levels are not taken into account.
There are no regulatory controls provided for within the Operative Tauranga City Plan requiring protection of overland flowpaths at this point in time, however consideration will be given to this matter in the future. It is however important to keep these areas free from obstruction.
If regulatory control is to be proposed a change to Council's planning rules would be required and consultation with all affected landowners would occur prior to any changes. Consideration of identified overland flowpaths will be taken into account through the subdivision consent and building consent processes.
Last Reviewed: 28/03/2017