RFI - Attachment F - Engineering Report

Seasonal Variation
The figures below examine the variation in composition of retained material throughout
the year. While loads are influenced by depth of rainfall, the composition of the load
does not seem to be effected by rainfall. It was though that higher rainfall would cause a
greater percentage of larger particles as these would be mobilized during heavier rainfall.
Figure 10 show the retained material had a higher percentage of gross pollutants in
October and December in the urban and commercial areas possibly this is because of
increase use of the shopping area in warmer months and Christmas. Conversely Figure
11 shows lower percentages of TSS during these months in these areas. It is assumed
that this phenomenon is caused by wind. Typically spring and summer months in Sydney
have stronger winds The stronger and more frequent winds in these months tend to move
suspended solids size particles from impermeable surfaces while generating more gross
pollutants for example ripping of leaves from trees. Figure 12 shows the bed load % of
the retained material is relatively consistent over all the months monitored.



                                     Seasonal Variation of Gross Pollutants
   % Gross Pollutants




                        50%                                                       250

                                                                                          Rainfall (mm)
                        40%                                                       200                     Rainfall
                        30%                                                       150                     Urban
                        20%                                                       100                     Comm
                        10%                                                       50                      Base
                         0%                                                       0
                                       ne
                               ril




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                              Ap




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                                      Ju




                                                                     m
                                             Au


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                                                       O

                                                               De




                                                  Month


                                       Figure 10: Season Variation of Gross Pollutants.




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