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

Abstract

This paper presents how a hydrodynamic Modelling study can be used to maximise the collective efforts from the stakeholders to enhance the protection of a riparian environment. A hydraulic model of a sensitive part of the Oakley Creek in Auckland, New Zealand, 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 stages of the project.

Oakley Creek has a catchment area of 1220 ha comprising of predominately residential land on the Auckland Isthmus, and discharges into the Waitemata Harbour adjacent to the North Western motorway interchange at Waterview. The creek traverses the Auckland City areas of Mt Eden / Mt Roskill and Mt Albert / Avondale, that have jointly administered activities within the catchments. The creek has the highest waterfall within the Auckland city 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 Unitec Institute of Technology (UIT), the residents and students living in the area, the Oakley Creek interest groups, the ARC (Auckland Regional Council), Metrowater, and the community boards in Avondale and Eden/Roskill. The outcome of the modelling study was to address the resource issues by pin-pointing to the areas to prioritise channel revegetation efforts by the parties that restore the creek channel, and prevent it from further erosion.

In the study presented in this paper, a detailed survey of 1km of the creek was undertaken to determine the size and shape of the cross sections. A hydrodynamic model based on these data was then developed using MIKE11 software. The purpose of this model of the downstream part of the Oakley creek was to identify locations of excessive velocity along the channel where enhanced restorative solutions could be applied to mitigate the impact of further erosion. The developed model was initially calibrated for modest to high flow conditions. Simulations for extreme flow conditions then followed to highlight stretches of the creek where erosion was likely to remove significant volumes of the river bank, alluvial flood plane or even adjacent reserves.

The paper describes the appropriate treatment methods for each location varying from protecting 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 valuable outcome of the study was that the model results assisted the stakeholders determine where to focus the limited resources so that they targeted vulnerable areas identified to be at most risk from erosion in extreme weather conditions. The paper shows the restorative work carried out by the stakeholders in response to the predicted flow characteristics resulting from simulations under an extreme storm event.