RFI - Attachment F - Engineering Report

AUCKLAND CITY’S FIELD AND LABORATORY

TESTING OF STORMWATER CATCHPIT
FILTERS

Kevin Butler, Principal, Tonkin & Taylor Lid
Grant Ockleston, Manager Stormwater Projects, Auckland City Council
Mark Foster, Tonkin & Taylor Ltd

 

ABSTRACT

The paper documents the resulis from a field and laboratory testing programme undertaken by Auckland City
Council to compare the performance of commercial Catchpit Filter Systems (CFS). A catchpit filter takes the
form of a fine-mesh filter bag that is inserted inside a standard catchpit. The need for the testing arose from the
City’s plans to install several hundred CFS units if they met satisfactory field and laboratory performance
targets for capturing sediment washed off roads and paved surfaces.

Four suppliers volunteered CFS units for the field testing programme. Each was installed in a City street and
observations made over a five-month period covering: ease of fitting, sediment retention, maintenance needs.
rigidity/strength, ability to catch flows and the effects of litter/organics.

Laboratory testing was carried out on CFS units from two suppliers. The testing, carried ont at the University of
Auckland, sought to quantify sediment capture performance. It also sought to determine the head loss
charactcristics of the filter fabric to establish its potential to limit the hydraulic capacity and cause flow to

bypass the CFS unit. In addition, a catchpit without the CFS unit was tested. Testing was carried out for a range
of flow rates and sediment cencentrations.

In summary, for a composite “street sweep sediment” sample, the CFS units were found to capture between 789
to 97% of the sediments entering the catchpit.

KEYWORDS

catchpit filter systems, filter bag/fabric, performance, field observations, operation and maintenance,
laboratory testing, sediment capture, head loss.

1. INTRODUCTION

Auckland City lies on the isthmus between the Waitemata and Manukau harbours. The harbours and
surrounding waters are used extensively for trade and recreation, and contain a number of important ecosystems.
The treatment of stormwater runoff is essential if the harbour and its surrounding waters are to be preserved.

Studies have shown that by removing the sediment contained in stormwater, a large amount of the pollutants
will be removed (Dept. of Land and Water Conservation NSW). It is found that pollutants attach to the
sediments contained in stormwater. Traditional methods of stormwater treatment, such as sediment retention
ponds, swales, and infiltration trenches, work by removing the sediment from the stormwater runoff. They often
require large arcas of land to function effectively. Because of the limited availability and high cost of land in the
more developed parts of the City, retrofitting these large stormwater treatment devices is extremely difficult.

Catchpit Filter Systems (CFS) are essentially fine mesh bags that are inserted into street catchpits to capture
sediments washed off the road in wet weather. The use of CFS as a means of stormwater treatment does not
require the purchase of additional land, or the use of recreational reserves that the council may already control.