Gross pollutants are large pieces of debris typically, litter and vegetation, that are flushed through urban catchments and stormwater systems. Gross pollutants are recognised as a threat to both wildlife and aquatic habitats, look unpleasant, smell and attract vermin and their removal from urban waterways is an important issue in sustainability because of their detrimental impact on the environmental.

Dr Richard Brown of QUT is leading a team investigating improved designs for Stormwater Quality Improvement Devices (SQIDs) that will remove these pollutants from stormwater runoff.  To remove gross pollutants from stormwater, Dr Brown and his team needed to understand the flow field and fluid mechanics in the SQID. Visualisation and interpretation of large data sets was necessary to understand the flow of the water in order to optimise the performance of the SQIDs.

The principle operation of the SQID is shown in Figure 1. Under normal conditions the flow enters the main chamber (A) and flows through the main outlet screen. If this chamber is filled with litter and debris or the screens are clogged, stormwater enters the trap and its flow is forced to the overflow zone (B) exit. The experimental study considers the worst case scenario when the device is filled and clogged with litter and this condition is termed fully blocked.  The experiments consisted of 3D flow field measurements within the chamber of a half scaled model of the prototype SQID using an acoustic Doppler velocimeter (ADV).  The fully blocked conditions are simulated by replacing the screens with solid perspex walls, allowing the intrusive nature of the probe to measure the velocities of the water particles.

Data obtained from these experiments were then used to build a three dimensional stereo animated visualisation using the scientific visualisation program MATLAB on QUT’s visualisation software and facilities.  This allowed Dr Brown and his team to examine the complex flow structures of both the water in the SQID. Important flow structures were revealed, previously unclear in more conventional two dimensional flow analysis and visualisation approaches.  An example of the simulations is shown in Figure 2.

Figure 2: A velocity vector diagram of the flow into the SQID.

This study will involve a more comprehensive understanding of litter capture so that the SQID trapping efficiency can be matched with the local authority maintenance schedules.

Dr Richard Brown
School of Engineering Systems, QUT

Publications
Madhani, Jehangir T and Kwek, Leonard M. K and Brown, Richard J and Kelson, Neil and Frost, Ray L.W (2005) Investigating the flow field of a Stormwater Quality Improvement Device. In Proceedings Environmental Engineering & Sustainability (EES), pages pp. 1-6, Power House, Sydney

Written by T. Curtis, August 2006