Request to Amend Application and Response to DSDIP's Information Request · MCU14/0009

3 Stormwater Quantity
The objective of this component of the study is to investigate the potential impact
that the proposed development will have on peak flows discharging downstream of
the site. Where any increases in peak flows occur, potential mitigation options will
be investigated in the form of a detention system.
Hydrologic modelling of the catchment has been carried out using WBNM software
developed jointly by the University of Wollongong, Rienco Consulting and Balance
R&D. The January 2007 version has been used for this investigation. A peak flow
rational method calibration has also been carried out.
Modelling has been carried out for the 1, 2, 5, 10, 20, 50 and 100 year ARI storm
events. The following scenarios have been investigated:
       1.      Existing Scenario
       2.      Developed Scenario
       3.      Mitigated Scenario

3.1    Existing Scenario
As the site is relatively small and draining to a central outlet the WBNM model for
the site has been represented as one catchment with an area of 0.164ha, which is
the site itself.
Catchment fraction impervious has been calculated based on aerial imagery data.
The existing scenario sub-catchment areas and fraction impervious a value for the
catchment is tabulated below in Table 6.
Table 6: WBNM Sub-Catchment Fraction Impervious Values – Existing Scenario
                                                                Existing Scenario
  WBNM Sub-Catchment                   Area (ha)
                                                                 Impervious (%)
            SUB1                        0.164                           69


The WBNM model has been calibrated to Rational Method derived peak flows at the
catchment outlet. Details of the Rational Method calibration are provided in Section
3.2.
The WBNM model has been calibrated by adjusting the WBNM catchment lag
parameter. The value of the catchment lag parameter adopted for the investigation
is 2.0, which is within the accepted range of values provided in the WBNM technical
reference manual. Rainfall loss rates adopted for this investigation are an initial and
continuing loss of 0mm and 2.5mm for pervious, and 0mm and 0mm for impervious.
WBNM modelling has been carried out for the range of standard design ARI events
ranging from 1 year up to 100 years. Modelling has also been carried out for the full
range of standard design event storm durations ranging from 15 minutes up to 3
hours. Peak flows tabulated in this report represent the peak values extracted from
this range of storm events.




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