Site Based Stormwater Management Plan

4.0 STORMWATER QUANTITY MANAGEMENT

4.1    Introduction

In accordance with the Noosa Plan (2020) stormwater flow discharging from the proposed
development is required to be consistent with the pre-development flow characteristics.
Peak flow rate was estimated using XPSTORM modelling software and validated using the
Rational Method in accordance with QUDM (2016).


A detention basin was proposed to address Acceptable Outcome AO2.3 of the Noosa Plan
(2020), which requires post-development stormwater flow discharge to be consistent with
pre-development characteristics. A 94.5m3 underground stormwater detention tank is
proposed and will mitigate post-development flow rates discharging from the site to be less
or equal to the pre-development case.




4.2    Hydrology

       4.2.1      Rational Method

Fraction impervious for the pre-development case was taken as 0.0 (Open space – QUDM
2016), and 0.95 (from architectural drawings) for the post-developed.


The Rational Method, IFD data was sourced from the Bureau of Meteorology, where the 1-
hour rainfall intensity (1I10) at Noosa for the 10% AEP = 74.2mm/h. The pre-developed
scenario C10 = 0.70 (fi = 0.00), and the post-developed scenario C10 = 0.89 (fi = 0.95).




       4.2.1      XPSTORM

The XPSTORM model for the pre-developed and post-developed cases were developed
using an iterative approach by varying the loss rates until the results compared with the
Rational Method (±20% - Model error).Table 4.1 presents the rainfall losses used in the
XPSTORM model.


        Catchment Area                        Initial Loss             Continuing Loss
               Pervious                           5 mm                    1 mm/hr
            Impervious                            0 mm                    0 mm/hr

                                           Table 4.1 Rainfall Losses




Site Based Stormwater Management Plan                                               Page 4 of 19
2123-SBSMP-01_Version 1.0 (October 2021)