Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
Proposed Residential Development at Doonella - Stage C


3.0 HYDROLOGICAL ANALYSIS
Hydrological analysis has been undertaken to determine peak flows along and at the downstream end of the
environmental corridor. The analysis involved the following:
      •   Identifying the appropriate study areas, external and internal site sub-catchments and land uses.
      •   Modelling of the existing catchment using the Watershed Bounded Network Model (WBNM2007), calibrated to
          Queensland Rational Method calculations and comparison to results from the JF&P Site Based Stormwater
          Management Plan (2008);
      •   Modelling of the post development catchment (developed scenario) and comparison to existing scenario flows
          to determine changes to peak flows;
      •   Design of two detention storages to mitigate the increase in peak flows in the environmental corridor at the
          eastern site boundary; and
      •   Modelling of the post development catchment with the proposed detention storages (mitigated scenario) to
          confirm peak flow increases at the site boundary have been mitigated.
Mitigated scenario peak flows have also been determined at various locations through the environmental corridor and
downstream of the site for hydraulic modelling purposes.
Hydrological analysis has been undertaken using the Watershed Bounded Network Model (WBNM2007). WBNM is an
advanced Windows based hydrology software for simulation of runoff from natural or partially developed urban
catchments. The WBNM software in structure is generally similar to that of RORB, however, it is based on more
detailed considerations of geomorphological relationships (AR&R). Details of the Theory used in WBNM can be found
on the following website; http://www.uow.edu.au/eng/cme/UOW068932.html.
Hydrological modelling was undertaken for the 2, 10, 20, 50 and 100 year ARI. Various standard storm durations
ranging from 5 minutes to 180 minutes were analysed for each ARI event.

3.1       Catchment and Land Use
The existing catchment to the downstream site boundary is 9.38ha in area and primarily encompasses dense bushland
with the only impervious surface being where Beckmans Drive crosses the upstream end of the catchment to the
south. For analysis in WBNM the existing site catchment has been broken up into four sub-catchments. Details of the
existing scenario sub-catchment areas and calculated fraction imperviousness is summarised in Table 3.1 below.
Refer to Drawing No. B09182.W-SK01 in Appendix D for the location and delineation of each existing scenario sub
catchment area.
                                 Table 3.1: Existing Scenario Sub-Catchment Details
                    Sub-Catchment                       Area (ha)              Fraction Impervious
                    A                                     2.42                         0.18
                    B                                     2.33                           0
                    C1                                    2.46                           0
                    C2                                    2.17                           0

Under post development conditions the catchment to the downstream site boundary is larger (10.95ha). This occurs
as a result of earthworks required to facilitate the proposed layout of roads and allotment. The change in land use for
the site (from dense bushland to roads, low density residential allotments and some retained open space) significantly
increase the amount of impervious surface within the catchment. Details of the developed scenario sub-catchment
areas and calculated fraction imperviousness is summarised in Table 3.2 below.



B09182.W-02A                                                                                                      Page 3