Updated SWMP

2. 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 from the site.

2.1.        Hydrologic Assessment
Hydrologic modelling of the catchments has been undertaken using WBNM software (Boyd et al, 2019),
developed jointly by the University of Wollongong, Rienco Consulting and Balance R&D. The 2019 version
has been used for this investigation which utilizes ARR2019 hydrology.

Modelling has been carried out for the 1, 2, 5, 10, 20, 50 and 100-year Average Recurrence Interval (ARI)
storm events for the following scenarios:

     1.   Existing Scenario (base case)
     2.   Developed Scenario
     3.   Mitigated Scenario

2.1.1.      Existing Scenario
Sub-catchments have been delineated using 1m contours sourced from the NSC catalogue, developed
using 2015 LiDAR and site survey. The fraction impervious values have been calculated using aerial imagery.
Figure 2-1 shows the existing catchment delineation with a summary of parameters shown in Table 2-1.




Figure 2-1: Existing Catchment Delineation



The existing drainage capacity of Pavilion Street is approximately equivalent to the 10% AEP storm event,
with flow greater than this overtopping the verge and entering the site at both 25 and 33 Pavilion Street. To
account for this, the peak flow for SUB1-Ex was determined for the 10%, 5%, 2% and 1% AEP. The portion
of flow greater than the 10% AEP was converted into a ratio, which was then applied to the catchment area
as detailed below in Table 2-1. This ensured that flows from SUB1-Ex greater than the 10% AEP were
routed through the site.




23-0167SMP01-V3 | Stormwater Management Plan | 33 Pavilion Street, Pomona                            Page 18