CONTINUOUS SIMULATION MODELLING TO QUANTIFY STREAM EROSION AT CATCHMENT SCALE

A. Hay (Beca Ltd), H. Smith (Beca Ltd) 

ABSTRACT 

Housing intensification presents several significant challenges to the health of our urban waterways. If not appropriately managed, housing developments can lead to: degradation of the water quality, ecological health and cultural values of urban streams; reduced community amenity value; and increased flood risk to people and property. Of particular concern is changes in the hydrological characteristics of urban catchments due to intensification and impacts on bank erosion, sediment movement and channel morphology in urban streams. Quantifying the effects of future development, to design and consent appropriate mitigation methods is key. Porirua Development is a partnership between Kāinga Ora, Porirua City Council and Ngāti Toa to replace 2,000 existing homes with approximately 3,800 new homes over a 20–25 year period within the Kenepuru Stream catchment. Te Aranga Alliance is delivering some of the enabling and civil works for Porirua Development. The Kenepuru Stream is already subject to substantial erosion, and increased urban development has the potential to exacerbate this, if not appropriately mitigated. Building on existing assessment approaches alongside a coupled hydrological and hydraulic model, the potential effect of this urban development on stream erosion was assessed. The developed method was used to quantify the effect of urban development on Kenepuru Stream erosion and assess proposed mitigation methods. Methodology included: establishing representative critical reaches of the stream throughout the catchment, by reviewing geology, ground conditions and existing erosion; Identifying the threshold velocities / shear stresses at which erosion will begin in the critical reaches; Long time series / continuous simulation hydrological and hydraulic modelling, running 10 years of historic rainfall data, to consider the effects of development on Kenepuru Stream erosion and study the relationship between stream hydrology, hydraulics and erosion; and modelling and selection of proposed solutions to mitigate the effects.