Overland flow paths are a common way of representing the predicted path of stormwater as it flows over the topography. Depression areas map the potential extent of stormwater ponding if the stormwater network is full or failure occurs within the network. Overland flow paths require a Digital Elevation Model (DEM) that is representative of the ground surface. The DEM is obtained from LiDAR, a remote sensing technology that measures distance by illuminating a target with a laser and analysing the reflected light. LiDAR is typically recorded from an aircraft where the laser picks up multiple levels (deemed returns); the last return is usually taken to represent the ground surface. This return does not include building elevations, and above ground features are removed so as to not influence the topographical path of overland flow or ponding in depression areas. In-house mapping of overland flow paths and depression areas carried out Sept/Oct 2021 by Whanganui District Council GIS Lead.

• Overland Flow Paths

Limitations: The following limitations apply to overland flow paths: • Only the elevation of the topography is considered, there are no allowances for underground reticulation or land use. For example, stormwater reticulation may convey overland flow paths beneath the ground surface, and impervious areas are more likely to generate overland flow paths for pervious areas of a similar size; • Overland flow paths do not map the entire width of the flow, only what is likely to be its deepest point; • Velocity, depths and flows are not included; and • Buildings are not incorporated. Limitations of depression area mapping include: • Depressions have been mapped to their maximum extent. In some instances, the depressions may have a volume that exceeds that generated by a design storm event (such as the 1% AEP). The extent of fill for each generated depression area could be calculated, using design storm parameters and the catchment area raster; • No underground infrastructure has been included in the assessment, culverts would provide relief for depression areas; and, • Depression areas that intersect the physical house (not just the land parcel) may have a maximum potential depth of less than 300 mm and therefore may not be of concern. This could be addressed by checking each property listed to identify whether or not the physical house intersects the mapped depression area.

Input: • 1x1m DEM derived from Airborne Laser Scanner survey conducted over the Whanganui urban area and Whangaehu River area of interest totalling approximately 443 km2. One of the deliverables from this survey is a Gridded ground surface (1 m separation grid) with hydro flattening and breaklines for water bodies, streams and rivers. The LiDAR survey was captured on the following dates: 8 September 2020, 2 December 2020, 4 January 2021, 27 January 2021, 3 February 2021 Output: • Three overland flow path series, generated from: o Contributing Area >= 1ha < 2ha o Contributing Area >= 2ha < 5ha o Contributing Area >= 5ha • Depression areas: o Depth <= 0.3m o Depth > 0.3m