RFI - Attachment F - Engineering Report

existing catchpits. Alternatively, for new catchpits, changing the design of catchpits to increase sediment
storage volume could increase sediment capture volumes. Clearly, the addition of CFS within catchpits provide
a level of protection to accumulated sediments not provided in a standard catchpit.

4.2 ADDITIONAL BENEFIT OF CFS IN CATCHPITS

The test results show that CFS can increase sediment removal above that expected from a standard catchpit. The
additional benefit of these devices is primarily in the 500-100 micron particle range. The increase in the percent
removal efficiency by removing this particle range will vary depending on the constituents of the stormwater
runoff. In the tests using street sediments from the Oakley Creck catchment, the addition of a CFS to the
catchpit increased the overall sediment removal efficiency from 58% to almost 90%.

The removal of additional sediment may not be entirely due to sediment becoming trapped by the CFS filter
material. Neither of the CFS tested had pore sizes as small as 100 microns yet both products still managed to
remove particles below this size, Field trial observations found that the use of CFS resulted in less turbulence
within the catchpit. The reduced turbulence may provide more favourable conditions for sedimentation and
hence the removal of particles smaller than the filter bag perforations. Alternatively the particles may be
adhering to the filter bag. As discussed above a valuable outcome of the installation of CFS would be the
protection of accumulated sediment from washout during higher flows.

4.3 LABORATORY VS FIELD EFFICIENCY

It is worth noting that the above CFS sediment removal efficiencies are based on laboratory conditions. These
are the sediment removal efficiencies from stormwater runoff that the CFS unit receives. In a field situation not
all stormwater runoff is received by the CFS, This is due to many factors including:

* Poor installation resulting in stormwater bypassing the CFS and entering the catchpit directly.
« High flows bypassing the filter bag via the engineered overflow.

« Reduced filter capacity due to filling with sediment, increasing the incidence of engineered overflow.

4A FINDINGS

* Modified catchpits or catchpits with CFS could be a practical and economical form of sediment capture in
retrofit situations such as heavily built up urban areas.

¢ Existing standard catchpits are capable of high annual rates of sediment capture. Logically, the more
frequently they are cleaned, the greater the annual volume of sediment capture.

« At the time of this study, some CFS products compromised the hydraulic capacity of the catchpit. Clearly,
any CFS system must not in any way compromise the hydraulic design capacity of an existing catchpit.

¢ Most current off the shelf CFS units are designed to be gross pollutant traps and/or fuel oil traps which trap

only the coarser sediment particles. It appeared that most CFS products would be able to be modified to
focus on sediment capture.

© The field installation of CFS products needs to carefully specified and be well supervised, especially for
new entrants (o the CFS market.

5 ENVIRONMENTAL BENEFITS

There are many stormwater treatment systems that provide various single or combined levels of environmental
benefit. These systems include coarse liner taps, fuel oil removal systems, chemical traps, nutrient removal
systems, sediment traps and the like.

This study has focused on the sediment capture efficiency of CFS products. Test results have shown that an
appropriately maintained standard catchpit can remove the majority of sediment down to 500 microns in size.