Particle size and settling velocity distributions for the design of stormwater treatment devices in the Auckland region

Melanie Skeen, Mike Timperley, Rajika Jayaratne
Auckland Regional Council
Publication date:

The design of engineered devices based on particle settling and/or filtration for the effective removal of suspended solids and attached chemical contaminants from stormwater requires a clear understanding of the physical characteristics of the suspended solids. These physical characteristics can be described by the particle size distribution (PSD) for filtration devices and settling velocity distribution (SVD) for settling devices. These distributions are also essential for calculating the removal efficiencies of treatment devices for suspended solids.

The PSD of a stormwater sample describes the range of suspended solids particle sizes present usually as a number of discrete size ranges or bands and the proportions of particles (by number), particle area or particle volume present within each of these bands. The SVD describes the proportions of particles (by number), particle area or particle volume that have settling velocities within each of several velocity bands.

This report uses particle data from several studies undertaken within the Auckland region to derive design PSD and corresponding SVD for the suspended solids in urban networks, urban streams and construction site runoff.

The design's PSD are the medians of the PSD determined for suspended solids in the Auckland region. The urban network samples were collected at eight network sites as part of the Auckland City stormwater monitoring programme between 2001 and 2003. The urban stream samples were collected from two urban streams in Auckland City during the same monitoring programme. The construction site samples were collected at the inlets to five sediment retention ponds during 2007 and 2009 at two different catchments Nukumea and Silverdale North.

Urban network and stream samples were collected by automatic samplers interfaced to either stage or discharge loggers using either time-based or flow proportional sampling. Between 7 and 16 events were sampled at each site and between 65 and 185 samples for each site were analysed for PSD. The samples were analysed for PSD using a Galai WCIS-100 particle size analyser, a “time of flight” instrument.

Construction site samples were collected by automatics samplers at the inlets and outlets of the retention ponds. Samples were collected for seven rainfall events in the Nukumea catchment and three rainfall events in the Silverdale North catchment. The Nukumea pond samples were analysed by Galai WCIS-100 particle size analyser and the Silverdale North pond samples were analysed by Mastersizer 2000 particle size analyser.

Auckland Regional Council technical report, TR2010/006

Last updated: 2018-06-20