Approved plan - Site Based Stormwater Management Plan - Job Ref SC-8407 - Revision B - 24 March 2020

SC-8407 – 45 Maple Street, Cooroy                                                              7



4.0 Stormwater Quantity Management
4.1      Background
The subject site has an existing fraction impervious of 30%. The proposed works will increase this fraction
impervious to approximately 80%. The proposed Medical Centre will subsequently require on-site detention
to ensure non-worsening to the downstream drainage infrastructure. In preventing nuisance to the
downstream Balance Lot, a formal inter-allotment drainage system is proposed in accordance with QUDM.


The software package XP-STORM was used to model the site and calculate an on-site detention volume
as described in the following sections.


4.2      Methodology
XP-STORM V2017.2.2 has been used to model the site in its current and post-developed state. The
modelling has been undertaken to determine detail design parameters of on-site detention to replicate pre-
development peak flow rates. As the development falls within the ‘Commercial’ development category
(QUDM), the (10% AEP) storm event was adopted for sizing the minor design drainage system.


Events ranging up to and including the 1% AEP storm event were analysed to compare pre/post
development discharges ensuring ‘non-worsening’. The development modelling and analysis undertaken
was based on the information provided by the detailed site survey and preliminary architectural layouts.