Overland Flow Path and Depression Mapping for the Auckland Region

A significant proportion of flooding issues in an urban environment can be attributed to stormwater flowing overland at relatively low depths. Typically, in a catchment-wide study a hydraulic model is developed to identify potential flooding areas. However, these models are often limited in extent to main channels and lack the ability to predict the path of small magnitude overland flow.

This paper describes two different methodologies used in producing mapped overland flow paths undertaken for the Auckland region. The methodologies make use of a series of automated Geographic Information System (GIS) tools in ArcGIS 10.

The final output is more than 60,000km of mapped overland flow paths showing the probable routes of stormwater in a storm event and associated catchment area.

Over 40,000 topographical depression areas have also been mapped for the region. These are the potential extent of ponded water if the piped network was to fail. Valuable information can be attributed to each depression area including the spill level, number of buildings in the extent and potential storage volume.

These mapped datasets ultimately lead to better decision making in catchment planning, evaluating consents, development control, hydraulic modelling and operational activities.