Use of hydraulic modelling to aid decision making in the management of Oakley Creek

This paper presents how a hydrodynamic Modelling study can be used tomaximise the collective efforts from the stakeholders to enhance the protection of a riparianenvironment. A hydraulic model of a sensitive part of the Oakley Creek in Auckland, NewZealand, was developed to identify and assess the existing and possible future erosion,flooding and storm water related issues. The stakeholders’ were involved in all the stagesof the project.Oakley Creek has a catchment area of 1220 ha comprising of predominately residentialland on the Auckland Isthmus, and discharges into the Waitemata Harbour adjacent to theNorth Western motorway interchange at Waterview. The creek traverses the Auckland Cityareas of Mt Eden / Mt Roskill and Mt Albert / Avondale, that have jointly administeredactivities within the catchments. The creek has the highest waterfall within the Aucklandcity limits and is an important waterbody protected and cared for by the council, residents,visitors, and community groups alike. The stakeholders of the Oakley Creek the UnitecInstitute of Technology (UIT), the residents and students living in the area, the OakleyCreek interest groups, the ARC (Auckland Regional Council), Metrowater, and thecommunity boards in Avondale and Eden/Roskill. The outcome of the modelling study wasto address the resource issues by pin-pointing to the areas to prioritise channel revegetationefforts by the parties that restore the creek channel, and prevent it from furthererosion.In the study presented in this paper, a detailed survey of 1km of the creek was undertakento determine the size and shape of the cross sections. A hydrodynamic model based onthese data was then developed using MIKE11 software. The purpose of this model of thedownstream part of the Oakley creek was to identify locations of excessive velocity alongthe channel where enhanced restorative solutions could be applied to mitigate the impact offurther erosion. The developed model was initially calibrated for modest to high flowconditions. Simulations for extreme flow conditions then followed to highlight stretches ofthe creek where erosion was likely to remove significant volumes of the river bank, alluvialflood plane or even adjacent reserves.The paper describes the appropriate treatment methods for each location varying fromprotecting the stability of adjacent slopes using planting of vegetation in areas of concern,to placement of gabion baskets to protect the actual creek bank itself. The most valuableoutcome of the study was that the model results assisted the stakeholders determine whereto focus the limited resources so that they targeted vulnerable areas identified to be at mostrisk from erosion in extreme weather conditions. The paper shows the restorative workcarried out by the stakeholders in response to the predicted flow characteristics resultingfrom simulations under an extreme storm event.