Stormwater runoff from roads creates detrimental impacts downstream. This is a result of an increased level of stormwater runoff conveying contaminants to downstream receiving environments. The philosophy and focus of the stormwater treatment design for the Transmission Gully project was to reinstate or improve the natural vegetation and pre-development processes and conditions using Low Impact Development (LID) principles wherever possible. This LID process used primary and secondary treatment measures (“treatment train” approach) to achieve the desired level of treatment.
The performance of the proposed treatment measures has been evaluated using two alternative approaches. The first was an instantaneous peak design flow spreadsheet based on the New Zealand Transport Agency (NZTA) stormwater treatment standards and guidelines. The second is MUSIC modelling, which is a software tool adopted extensively in Australia to assess pollutant removal efficiencies based on long term rainfall-runoff analysis.
MUSIC modelling was applied to three segments of the proposed Transmission Gully road alignment. This enabled the comparison of the estimated long term treatment performance against the instantaneous peak spreadsheet results. Ten years of continuous rainfall time series data from two rainfall gauges near the Transmission Gully project were used in the MUSIC modelling. A range of sensitivity analyses were undertaken to quantify the potential variation in treatment performance results and thereby gain a level of confidence in the overall outcomes.