Urban waves : secondary flow control challenges and responses in Hamilton

The predicted increased intensity and frequency of significant storm events, combined with drives for increased intensification of the existing built-up areas, means adequate freeboard and effective secondary flowpaths are going to be critical to avoid unnecessary costs and hardship associated with urban flooding. This paper reviews Hamilton City’s progress with the challenges associated with establishing and protecting secondary flowpaths over recent years from the legal, engineering, and consenting perspectives. The results of new flood hazard assessment are discussed as is how Council is incorporating this information into the Proposed District Plan. As the hazard plots cover over half of the City, this has involved some detailed consideration of appropriate responses. A related topic of setting acceptable freeboards for floor levels adjacent to flood areas is also explored considering the Building Code and New Zealand Standard 4404:2010 documents both offer an arbitrary 0.5m freeboard for habitable floors. An alternative approach based on modeling the hazard as represented by the catchment area, depth and speed of flow is presented. The paper will benefit local government representatives and be of interest to a wide spectrum of the industry.