THE POTENTIAL EFFECT OF CLIMATE CHANGE ON STORMWATER TREATMENT

M. Murdock, M. Groves (WSP)

ABSTRACT

Forecasted rainfall changes related to anthropogenic influences on the climate are typically focused on the number of wet days, annual rainfall, and predictions around extreme rainfall. While it is important to understand the possible changes to water supply yields and the potential for increased flooding, these do not directly address how the water quality rainfall depth (D_WQ) or water quality intensity (I_WQ) might change and what the knock-on effect might be for the performance of existing stormwater treatment systems and the size and cost of future systems.

Guidance in this space within Aotearoa New Zealand for designers and decision makers is currently lacking. The water quality depth and intensity parameters are typically derived from a rainfall percentile value, or a percentage of annual run-off treated, and thus they may be influenced by changes to rainfall. While the design of peak flow conveyance and detention systems consider various relative concentration pathways (RCPs) and certain time horizons, these parameters when used for sizing treatment systems are static regardless of a system’s design life.

This paper explores how these parameters might change with respect to climate change for multiple locations across Aotearoa (i.e. Auckland, Whangarei, Gisborne, Christchurch, Dunedin, Westport, Queenstown, and Invercargill). Firstly, understanding whether there is a correlation between annual average rainfall (AAR) and annual average temperature, AAR and D_WQ, AAR and I_WQ, or D_WQ & I_WQ. Secondly, by applying the relationships derived from this initial analysis, understanding how D_WQ and I_WQ might change based on the climate model projections from NIWA’s downscaled climate model for Aotearoa & archived CMIP5 multi-model results.

Relating projected annual rainfall to water quality design parameters, our assessments of Auckland and Christchurch indicate that a statistically significant change to the water quality rainfall depth or intensity is not currently anticipated, even when considering RCP8.5 to 2090. For these locations it appears that these design parameters do not need to be adjusted for projected climate change until such time that more specific rainfall projections are available. This is consistent with NIWA’s Climate Change Projection for New Zealand 2018 report which states, “for a number of regions of New Zealand, there is no clear direction of precipitation change, even at 2090 under RCP8.5”. However, NIWA’s report also states, “the overall pattern in annual precipitation trend is for a reduction in the north and east of the North Island, and increases almost everywhere else, especially on South Island’s West Coast”.

Based on projected changes in precipitation (from NIWA’s regional model), locations such as Dunedin, Invercargill, Queenstown, and Westport may need to include upward adjustment to D_WQ & I_WQ parameters, whereas locations such as Whangarei and Gisborne will likely need downward adjustment over time. However, it is recommended that monitoring for observed changes in rainfall patterns is undertaken before implementing any changes, given the large uncertainties associated with rainfall projections (IPCC AR6 WGI Chapter 4, Page 556).

This paper contains a summary of anticipated climate change effects associated with the water quality depth and intensity parameters for all analysed locations. This will provide an indication of potential future changes to these parameters to inform robust decision making related to existing, and the sizing of new, treatment systems to better manage stormwater water quality.