Storm Water management plan · MCU12/0133.03

Noosa Council documents


                  Bligh Tanner Consulting Engineers                                                                                         Page 16
                  Noosa Flexi Learning Centre Stormwater Management Plan
                  August 2012



                                           •     In the front of the site there is an area of landscape that will be revegetated, and this
                                                 area could be easily depressed to create 1 m³ of shallow flood detention (0.1m depth
                                                 over 10 m²)
                                           •     As additional airspace within rainwater tanks.

                  Further guidance will be sought from Council at Operational Works to confirm this
                  requirement. In any case, the proposed development layout is compatible with attenuating
                  flooding impacts downstream.


                  5.3 Frequent flow impacts downstream of the site
                  According to the Girraween Stormwater Management Plan (Ecological Engineering 2006),
                  the increased frequency of surface runoff is considered to be the primary threat to the wet
                  heath vegetation of that ecosystem.

                  The strategy to minimise impervious areas and to harvest and infiltrate stormwater is
                  consistent with preserving the hydrology of this receiving environment.

                  Pre and post development flow frequency curves are presented in Figure 7 below. It shows
                  that the strategy is effective in avoiding any increase in runoff for all rainfall events within the
                  simulation period (1973 – 1983). Exact replication of pre-developed flows is the ideal
                  outcome, and the results show modest reduction in flows. The following points are noted:
                       • The results are generated from MUSIC in the absence of any calibration data to
                           reflect the sandy soils of the site (nonetheless, the relative impact between pre and
                           post development would probably not change greatly)
                       • The reduction is modest (it is only apparent when plotted on a logarithmic scale)
                       • A reduction is preferable to an increase (the wet heath is at risk from losing its drying
                           phase), and there is already surplus wetting already occurring from other parts of the
                           catchment, so a small reduction from this site is desirable.


                                                100


                                                 10
                     Daily flow (m³/day)




                                                  1


                                                 0.1                                                   pre
                                                                                                       post
                                                0.01


                                               0.001
                                                       0%         20%           40%           60%             80%        100%
                                                                              Cumulative frequency

                  Figure 7: Comparison of pre and post development flow frequency showing that the
                  proposed development avoids increasing the wetness of the downstream ecosystem.




                                                                                                                          Print Date: 2 March 2020, 2:19 PM