RFI - Attachment F - Engineering Report

                                     Gross Pollutants Vs Peak Intensity

                           200
    Gross Pollutant Load




                           150
            (Kg)




                           100                                        y = 100.4Ln(x) - 45.189
                                                                            R2 = 0.6655
                           50

                            0
                                 0           2              4             6              8      10
                                              Peak Rainfall Intensity (mm/6 min)

                                       Figure 6: Gross Pollutants and Peak Intensity.

Removal Volumes
Allison at the CRC suggests gross pollutants have a mass density (wet) of 250 kg/m3.
However the Enviropod retains captured material in a dry (damp) state with voids
between retained particles. During the study the level of fullness and weight of each
Enviropod was recorded at each service. A type A1 Enviropod contains 0.142 m3 of
storage. During the study it was recorded that the weights of a full type A1 Enviropod
were between 9 and 32 kg. Assuming a conservative value of 25 kg this gives an in-situ
density of 176 kg/m3.

Equating this to the total load removed over the year (10.5 tonnes) this equates to 60m3
of material was removed from the Blackbutt creek catchment or 3.5 m3/ha/yr. The
volume of material entrained in urban runoff is a important factor in determining the
appropriate sizing of a stormwater treatment device

Using this in-situ density, the following volume curve is derived Figure 7. In Blackbutt
Creek there are 80 Enviropods with a total storage volume of 8.6 m3, installed over a
total catchment area of 17.2 hectare. This equates to approximately 0.5m3 of storage for
each hectare. Assuming an Enviropod should be cleaned when it is 2/3 full, the
Enviropods should be cleaned after approximately 100mm of rainfall. This analysis
could be carried out on each sub-catchment to determine the most accurate frequency or
even on individual Enviropods. Ideally a rain gauge could be mounted within the
catchment to aid determination of cleaning frequency.




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