Attachment 4 - Stormwater Management Plan by O2 · MCU13/0251

6.9.            Hydraulic Modelling
The unsteady state, 1D hydrodynamic model XP-SWMM was used to analyse the flood behaviour of the site
with the proposed mitigation measures. Flood levels, discharge and velocity can be extracted from the
model as functions of time at required locations.

The key objective of the hydraulic modelling was to demonstrate that the post development peak discharge
from the site for all events up to and including the 100 year ARI storm event is less than or equal to the pre
development peak discharge.

6.9.1.          Mitigation Measures
For the purposes of this assessment, it has been assumed an underground detention tank will be provided
to maintain pre development discharges from the site. This detention system will require 2 chambers
separated by the baffle weir and staged outlet structure, to ensure the peak flow rates for all ARI events
can be maintained.

6.9.1.1.        Depth-Area Relationship
The depth-area relationship applied to the hydraulic model is summarised in Table 13 for the primary and
secondary chamber respectively. Further hydraulic modelling will be undertaken prior to Operational
Works approval in conjunction with detailed design.
Table 13 – Detention Tank Configuration

                     Primary Chamber                                     Secondary Chamber
                                            2                                                        2
         Depth (m)                 Area (m )                 Depth (m)                       Area (m )

            0                          90                        0                              10

            1.0                        90                        1.0                            10



6.9.1.2.        Outlet Structure
To ensure the pre development peak discharges are not exceeded for all ARI events, a staged outlet
structure has been designed for underground rainwater tank system.

The outlet structure for the detention provided in the underground rainwater tank consists of:

          4m wide baffle weir at a depth of 0.85m
          2 x 150mm diameter orifices at invert level of tank
          3 x 100mm diameter orifice at a depth of 0.65m

The outlet structure will be refined prior to the Operational Works approval when the final tank
configuration is known.

6.9.1.3.        Basin Performance and Detention Volumes
The performance of the detention tank and proposed outlet structure has been assessed by comparing the
mitigated post development peak flow rate with the pre development peak flow rate. A summary of this
assessment is shown below in Table 14 along with the storage volume required for each event.




STORMWATER MANAGEMENT PLAN – PEREGIAN VILLAGE MARKET                                                     Page 16

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