The media report - An investigation into infiltration basin performance in Christchurch

Author: L. Allan & M. Joseph

Source: Pattle Delamore Partners Ltd

Year: 2020

Abstract

Infiltration basins are a method of managing and treating stormwater from both industrial and residential developments. The infiltration of the first flush of stormwater through the infiltration media provides treatment via adsorption and filtration of contaminants. The treated stormwater then either discharges to ground or is collected in underdrains, where it is conveyed to reticulated networks, surface waterways, or soak pits. The overall performance of these basins relies on resilient design approaches, regular maintenance, and basin age. 

Infiltrations basins are a commonly used form of stormwater treatment in various parts of Christchurch and a number of basins have been in place for more than 15 years. The age and differing design approaches used for these basins results in variations in performance; both in terms of drainage time and contaminant removal. One source of variation is in infiltration media design, where some basins have a thick infiltration media on top of geotextile fabric and free-draining gravels whilst others only comprise of a thin layer of sandy soil above gravels. The discharge method also varies, where some basins discharge directly to ground whilst others must contain underdrains to collect the infiltrated stormwater. These variations mean that the likely treatment efficiencies and drainage times of these basins cannot be determined by a single guideline range of values. Directly testing and monitoring basin performance is therefore the most effective way to determine whether the basin is working as intended. 

This paper covers the investigations carried out at several infiltration basins with reported performance issues in Christchurch during 2019 and the key findings that were uncovered. Field work consisting of double ring infiltrometer testing, soil sampling, and inspections of the basin infrastructure was undertaken at each basin. These investigations were then used to determine the likely cause of the performance issues experienced by the basins and to recommend remediation options to improve basin performance.