MCU14/0106 & OPW15/0001 Appendix E - Stormwater Management Plan

Noosa Civic
Stormwater Management Plan



The previous assumption for the pervious loss rate is considered appropriate and was verified against a
Rational Method calculation. We are therefore of the opinion that there is no need to change this parameter.
We have, however, updated our modelling to include this change, to demonstrate that a suitable
arrangement can be achieved even if Council’s value is adopted.
The model was run for a range of storm durations, with the 1 hour duration producing the peak discharge.
Table 2-3        Adopted Hydrologic Parameters

Parameters                    Impervious Surface           Pervious Surface
Initial Loss (mm)                       0                          15
Continuous Loss(mm/hr)                  1                          2.5
Fraction Impervious (%)               100                           0
Manning’s n                          0.016                        0.060


To ensure that the peak flow calculated by the model was reasonable, the existing case model was validated
against a Rational Method calculation. The calculations completed in relation to the Rational Method are
presented in Appendix B. The Rational Method was found to predict peak flows of 1.03 m3/s in Catchment B
and 0.93 m3/s in Catchment C for the 100 year event. In comparison, the XP-SWMM model provided peak
flows estimates of 1.03 m3/s in Catchment B and 0.89 m3/s in Catchment C for the existing case. Given this
close level of agreement, it was considered acceptable to use the XP-SWMM model to calculate peak flows
for the existing and developed cases and to determine the attenuation strategy required to offset the
increase in peak runoff caused by development.

2.2.2            Hydraulic Model Details
A digital terrain model (DTM) of the study area was set up using LIDAR data supplied by the Department of
Natural Resources and Mines (DNRM). A site survey of the subject site was provided by Bennett and
Francis surveyors.
Cross-sections in the drainage corridor in the OS7 corridor were included as shown on Figures 2 and 3.
The pre-development and post-development case surface roughness conditions were represented by
appropriate values of the Manning’s ‘n’ roughness parameter. The adopted roughness values are listed in
Table 2-4.
Table 2-4        Manning’s ‘n’ Values

Surface Condition                       Manning’s
                                           ‘n’
              Existing and Developed Cases
Concrete Pipes                               0.014
Drainage Corridor                            0.025


The existing culverts, 4/1200x600 RCBC’s at 3.05mAHD under Hofmann Drive were included based on the
survey data undertaken by Bennett and Francis.
The flow hydrographs were derived from the hydrologic model component in XP-SWMM discussed in
Section 2.2.1. The downstream boundary conditions assumed inlet control conditions on the culverts.
It is proposed that on-site mitigation of local flows will be provided by:
    - Utilising depth in the car parks to a maximum of 300mm. For the purposes of the development
      application assessment, the levels were based on the preliminary engineering levels and can be seen
      in Table 2-5 to Table 2-9.
    - Reducing the capacity of the culverts under Hofmann Road to 4/1200x300 and raising the invert level
      from 3.05mAHD to 3.35mAHD whilst maintaining a 225 RCP at the original invert level for low flows;
      and

QIC Noosa Civic Pty Ltd                                  Version Final                           11 May 2015
J13041R2_Final SWMP.docx                             Commercial in Confidence                         Page 4