Overland flow in the Urban Environment

Modelled overland flowpaths (OLFP’s) are being systematically assessed for their accuracy and impact as part of the Stormwater Catchment Management Planning process being conducted by North Shore City Council. Mitigation measures have been developed for properties at risk of habitable floor flooding.

This paper discusses the field property assessments which record both the effect of the OLFP on impacted property and any obstruction the property development has on the course of the OLFP.

The following key findings are discussed:

• That 49% of properties (land sections) and 14% of houses in North Shore City are impacted to some extent by modelled Overland Flow Paths.
• Approximately 6,650 of the 45,500 properties in the current study area have been individually inspected.
• Of these, approximately 700 (1.5% of properties within the current study area) have been assessed to have a risk of habitable floor flooding from the overland flow during a major storm.
• Roads act as a significant collector and conveyor of overland flow with the locations where stormwater spills off road reserves being a major factor contributing to flood risk for downstream properties.
• Building and site works design for properties is a major factor contributing to flood risk for properties.

This paper outlines the field data collection process, data management, detailed assessments and the integrated approach between Council Departments and affected residents to provide preventative and mitigation measures to minimise the risk of habitable floor flooding from overland flow.