Amended Stormwater Plan · MCU18/0019

SC7440 – Goodchap St, Noosa                                                                              10




4.0 Stormwater Quantity Management
4.1     Background
The proposed development has the potential to alter the existing hydrologic and hydraulic characteristics of
the subject site unless suitable mitigation is achieved via careful hydraulic design of the proposed
stormwater management drainage system. With urbanisation there is potential for sites to create or
exacerbate drainage problems to downstream areas and/or drainage infrastructure. The purpose of this
section of the SWMP is to determine the general requirements for bulk stormwater management for the
proposed development. The design shall ensure that rainfall event discharges are mitigated and conveyed
from the site in a manner that complies with Council’s Urban Stormwater Planning Guidelines and does not
result in a worsened drainage situation to downstream infrastructure, without causing a nuisance or
annoyance to residents/property owners.


4.2     Site Drainage Constraints and Opportunities
4.2.1   Drainage Constraints
The increase of impervious areas will impact on the existing downstream boundary condition compared to
pre-development peak flows. Therefore, a volume of detention will be required to limit flows to that of pre-
development.


4.2.2   Drainage Opportunities
The existing drainage in Val Crescent provides sufficient depth for connection to provide quality and
quantity objectives.


4.3     Modelling Overview
4.3.1   Methodology
The software package XP-STORM V2017.2, 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 ‘Urban
Residential’ development category (QUDM), the minor (10yr ARI) and major (100yr ARI) storm events were
primarily sized for underground and overland design flows respectively.


All events ranging from 2, 5, 10 20, 50 and up to the 100yr were analysed to compare pre/post
development discharges ensuring ‘non-worsening’ discharge. The development modelling and analysis
undertaken was based on the information provided by the detailed site survey and preliminary architectural
layouts. A Rational Method “check” was conducted and compared with the pre-development model to
confirm there were no gross errors, refer to Appendix E for Rational Method results.