RFI - Attachment F - Engineering Report

Removal Loads and Volumes
The highest loads were removed from the urban sub-catchment during the study period.
However this sub-catchment had the largest catchment area (12.75 ha) and the largest
number of Enviropods (52).

Removal Loads exhibited seasonal variations, with the highest loads encountered in
autumn and during months of high rainfall

Removal loads were related to rainfall. An analysis of monthly removal loads showed
that it had a log normal relationship with monthly rainfall. The analysis also indicated
that there may also be a relationship with gross pollutant loads and peak intensity of
rainfall during the month.

The in-situ density of removal material by the Enviropods in the study was determined to
be 176kg/m3 (damp). This is lower than the values determined by the CRC for gross
pollutants. However the CRC values were determined from wet gross pollutants from a
CDS unit.

Approximately 60m3 of material was removed from the Blackbutt creek catchment
during the year. This equates too 3.5 m3/ha/yr. The volume of material entrained in
urban runoff is an important factor in determining the appropriate sizing of a stormwater
treatment device

Considering the in-situ density and the available storage in the Enviropods filters it is
suggested that all Enviropods be cleaned after approximately 100mm of rainfall. A rain
gauge could be mounted in the catchment to help determine cleaning frequency.

Particle Size Analysis
It was observed that there were a greater percentage of suspended solid size particles as
you travel down the catchment. Inversely there were a greater percentage of gross
pollutant size particles as you travel up the catchment. It is suggested that finer particles
are being transported overland by wind and runoff.

On average and across the catchment the retained material trapped by the enviropod had
the following composition. 56% suspended solids, 32% bed load and 12% gross
pollutants. There is a considerable amount of material being transported as bed load
which is not usually considered when evaluating loads transported in stormwater.

As previously stated loads were related to the rainfall in the catchment. However particle
size composition of the loads was not related to rainfall. It was thought that higher
rainfall would cause a greater percentage of larger particles as these would be mobilized
during heavier rainfall. It was observed that there was a seasonal variation in suspended
solids and gross pollutants which was more influenced by wind than rainfall. Spring and


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