Is Providing a Stormwater Level of Service for Significant Flood Events Really Realistic?

A significant flood event is a strong trigger for infrastructure improvement works, and many Councils find themselves making political and public statements around acceptable tolerance levels for flooding, especially in the wake of a significant event. Often these flood events are extreme, and are in excess of what would ordinarily be regarded as provision of a basic stormwater utility service. The distinction between the utility provision and defence against the flooding natural hazard is often blurry. The key question that is not often considered through these statements is whether the desired outcome (for this paper a level of service) is affordable, or even attainable.

In Tauranga the stormwater budget is subject to large expenditure spikes immediately following intense rainfall events which have resulted in flooding. Following the May 2005 flood event the Council initiated a flood recovery programme which resulted in the construction of major stormwater infrastructural projects in the parts of the City affected by that event, resulting in approximately $80M being spent over a four year period, with the intention of delivering a 50 year ARI level of service (where attainable).

Continued improvements to the stormwater network are required if the Council and community want to reduce the existing flood risk within previously affected areas and those potentially at risk from flooding, if it intends to reduce the risk through infrastructure provision alone. In 2009, the then Council, received a paper from staff outlining an additional $170M would be required to improve the current situation, however no financial analysis had been undertaken to determine the accuracy of that figure, nor had any modelling been undertaken to determine the extent of the flooding issues.

The key issue is in deciding on any approach will be couched within the cost to the community of intervening or not, and whether other approaches can be put in place to support an infrastructure program. To consider a consistent approach to stormwater a level of service (LoS) provides the ability for a framework of intervention, however this is strongly influenced by funding considerations given the City’s current debt levels.

A LoS also ensures the consistent delivery of the Councils message to the community and can result in an appropriate backstop for the Council in regard to intervention and to ensure staff are appropriately provided with a policy approach to work within and funding to be provided.

To test these strategic issues above, the Council embarked on the development of a stormwater project which considered the following:

• 2D modelling and flood hazard identification; 2015 Asia Pacific Stormwater Conference
• Development of an affordable and realistic level of service in relation to flood hazard management;
• Utilising policy/regulatory, educational and infrastructure opportunities to reduce flood risk; and
• Establishing priority areas and considering economic benefits and implications of future upgrades and other options;

The purpose of this was to seek to define an appropriate and affordable level of service coupled with other risk reduction techniques in an overall approach to flood risk management.

In this paper the above process is described. Further, this paper explores the issues surrounding the political environment of delivering stormwater improvement works and setting levels of service along with consideration of the options available to Councils to create community resilience to the current situation. The focus is a Tauranga example and is based upon the learnings that the Tauranga City Council has made to these strategic issues over the past year.