Protecting Eden Park From Flooding During The 2011 Rugby World Cup

Posted on December 10, 2011 by Jes

Extended abstract presented at the 50th Jubilee New Zealand Hydrological Society 2011 Conference in Wellington.

Introduction

Eden Park has historically suffered from flooding during major storm events. This has ranged from serious flooding as in the historic Maori vs Springboks test match in 1956, to less serious flooding of playing grounds and corporate areas in 2008.  Flooding occurs as groundwater levels in the underlying basalt aquifer rise in response to infiltration in the upstream catchment.  Stormwater drainage in Eden Park itself is constrained by both high groundwater levels and an undersized stormwater network.

A solution to protect Eden Park from flooding in the event of high rainfall was required. Auckland Council and Beca carried out a feasibility study which examined potential solutions.  Shortlisted solutions included increased stormwater pipeline capacity or a novel groundwater abstraction scheme.  The feasibility assessment recommended the groundwater abstraction scheme be taken forward, being the lowest cost option that could be implemented in the time available.

The aim of the project is to draw down groundwater levels to enable surface water to infiltrate into the aquifer, even during large storm events when flooding would normally occur.  The scheme will be in operation before and during the 2011 Rugby World Cup, following which it will be operated to provide protection to other high profile events. 

This paper presents key experiences from the construction of the production wells, the groundwater modelling carried out and operational phases of the project. 

Methods

The project includes the development of three Production Wells in the basalt aquifer, the tests that were performed and the modelling that was carried out to set operational parameters.  Additional testing of the scheme was carried out prior to the 2011 Rugby World Cup  in order to simulate winter conditions and to calibrate performance in practice against the model results.

Construction of the scheme was complicated by the range of other works being carried out simultaneously at Eden Park to prepare for the 2011 Rugby World Cup.  The scheme consists of a range of civil, mechanical, electrical and control works to manage groundwater abstraction from the three Production Wells. A series of fully cored investigation bores were drilled to the base of the basalt and slug tests carried out in order to design the production wells.

Three-dimensional groundwater flow modelling (Visual Modflow Pro 2010) was used to evaluate the aquifer budget and assess the likely capacity of a 1 in 10 year design storm on Eden Park when abstracting up to 250 L/s from the 3 production wells. The modelling also allowed an assessment of the potential for ground settlement beyond the Park. The model extent covers an area of 4 km by 3 km and comprises 2 layers; the unconfined basalt aquifer and the underlying Tauranga Group and Waitemata Group aquitard. 

 Results

The results from the investigation drilling provided information about the base of the aquifer under and around Eden Park. Initially one production well was designed and constructed to allow a constant rate aquifer test to be carried out. This information enabled a simple groundwater model to be set up and calibrated, and then used in conjunction with the conceptual geological model and aquifer test results to identify the locations of two additional production wells.

Following installation and testing of the two new production wells, the groundwater model was refined and calibrated against known average static water levels for existing wells, rainfall records and corresponding water level time series data from individual aquifer tests undertaken in the production wells. The numerical groundwater model was then used to simulate the drawdown effects on the environment under different design storms and pumping scenarios.  

The modelled simulations were tested by operating the production wells individually and in combination under winter conditions. The combined aquifer test was run for 3 weeks followed by a recovery period. Water level monitoring was undertaken in the observation bores within Eden Park and in observation bores further away to check the radius of influence. The combined aquifer test confirmed the results of the modelling indicating that sufficient storage in the aquifer can be created by abstraction to accommodate the design storm.

Conclusion

• A detailed hydrogeological investigation was carried out in order to confirm the feasibility of creating storage in the aquifer that could cater for a 1 in 10 year design storm.
• 3 highly efficient production wells capable of abstracting a total of 250 L/s (26,000 m³/day) and a number of observation wells were installed to protect Eden Park from flooding under the 2011 Rugby World Cup.
• A 3-dimensional groundwater flow model was set up and calibrated to long term rainfall and water level time series, and constant rate aquifer tests. The calibrated model was then used to simulate the effects of a design storm in order to design an operation scheme.
• A long term operational consent for operation of the scheme was obtained from Auckland Council, the regulatory authority.  This permits the abstraction of a maximum annual quantity of groundwater. Controls are provided by not exceeding stated ‘summer minimum’ groundwater levels at observation boreholes at the boundary of Eden Park, together with on-going monitoring to demonstrate that settlement does not occur.
• The on-going operation of the scheme is under the control of Auckland Council and the scheme can be used to protect future high profile events at Eden Park from flooding.