RFI - Attachment F - Engineering Report

Figure 6 - Sediment Removal by the Catchpit Containing a Flogard CFS with a 400 jun mesh

 

 

 

 

 

 

 

 

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jet sme —-— epee ee
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ee FP or remeron ty
, Particle Size(uy
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2 i ' |+@—500 -1,000
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i O_ Streetsweep
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0 5 10 15 20

Flow Rate (l/s)

*

Figure 4 above shows the results in the standard catchpit without a CFS installed. Sediments larger than 500u.
were completely removed for flows up to 20 I/s. The removal of 100-500 particles decreases from 75% to
47% by increasing the flow from 0.5 I/s to 20 W/s, and only 18% of particles smaller than 100, are wapped by
the catchpit. Approximately 58% of the sediment collected from street sweepings were removed at 4 I/s.

Figures 5 and 6 show the laboratory performances of the Enviropod CFS and a Flogard CFS. The performance
of both CFS was quite similar. The Enviropod CFS sediment removal efficiency was slightly greater than the
Flogard for the finer sediment. These CFS remove essentially all the sediments >100y, while the removal of
sediments <100p is similar to the standard catchpit with no CFS. The sediment removal efficiencies of the
street sweep sediments at 4, 12 and 20 I/s are 97%, 89% and 88% respectively for the Enviropod CFS, and 86%,
79% and 78% respectively for the Flogard CFS. For the street sweep sediments the Enviropod CFS gave

approximately 10% greater removal than the Flogard CFS due to more abundant fine sediments present in the
street sweep sediments.

4 DISCUSSION
41 EFFECTIVENESS OF CATCHPITS

The laboratory results indicate that catchpits without CFS installed are effective at removing coarser sediments.
Results show they have the potential to remove nearly 100 percent of sediments greater than 500 microns in
size. Tests undertaken with road sediments collected from the Oakley Creek catchment demonstrated this related
to a sediment capture efficiency of 58%. It would be expected that these efficiencies would reduce as greater
volumes of sediment accumulate within the catchpit and washout of the sediment occurs due to disturbance
under high flows. Similarly, efficiencies could be maintained with regular cleaning of the catchpits.

At present catchpits in Auckland City are cleaned three times per year. A more regular cleaning frequency or
targeted cleaning of catchpits receiving high sediment loads may increase annual sediment capture volumes of