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                                                                                                 CALIBRE CONSULTING




5         FREQUENT FLOW MANAGEMENT & WATERWAY STABILITY
          MANAGEMENT
5.1       FREQUENT FLOW MANAGEMENT
As a result of increase to impervious surface areas, piped drainage and lined overland flow paths, the development has
the potential to increase the number and frequency of surface runoff events which may result in a disturbance to
downstream aquatic ecosystems. This section discusses the strategy to manage frequent flows from the development
and analysis undertaken to demonstrate its compliance with current requirements.

The management of frequent flows is now regulated by the SEQ Regional Plan 2009-2031 Implementation Guidelines
No.7 - Water Sensitive Urban Design (2009). The design objective to manage frequent flows from urban development
requires:

   10mm of initial runoff from all impervious surfaces is captured and managed for development with less than or equal
    to 40% impervious surfaces; or
   15mm of initial runoff from all impervious surfaces is captured and managed for development with greater than 40%
    impervious surfaces.

For the development the total impervious surface is greater than 40% and therefore the first 15mm of runoff capture from
impervious surfaces is required. In-lieu of any local guideline the Moreton Bay Regional Council (MBRC) Frequent Flow
Management equation has been used to determine whether the temporary ponding provided within the proposed SQIDs
will achieve the Frequent Flow Management requirement. The results are shown in Table 5.1 below. Based on these
results, it can be seen that the frequent flow management objectives for the development will be achieved.

                             Table 5-1: Frequent Flow Management Objective Calculations


                       Design Component                Catchment A      Catchment B        Catchment C

               Frequent Flow Volume (V) (m3)                220               406               480
               Treatment Volume (Vs) (m3)                   135               680               400
               *3Vs+2/3 Vr+ Vx (m3)                         405              2040               1200
               Objective met? 3Vs+2/3 Vr+ Vx> V             Yes               Yes               Yes
              *Objective Formula Updated by MBRC (25th October 2012), values Vr and Vx are set to zero

5.2       WATERWAY STABILITY MANAGEMENT
The SEQ Regional Plan Guidelines No.7 (2009) for requirements of Waterway Stability Management, where the design
objective is to limit the post development-development peak 1 year ARI event discharge within the receiving waters to the
pre-development peak.

The previous initial stormwater quantity analysis in Report No. B07056-02A includes analysis of peak flows for a 1 year
ARI rainfall event in which developed peak flows have been mitigated back to existing conditions in all recurrence
intervals. Therefore the Waterway Stability Management objective has been previously met. The previous detention
strategy has been incorporated and therefore it is anticipated that the strategy will be able to comply with the Waterway
Stability Management with hydrological analysis to confirm.




B14158.W-01A | COOROY RETIREMENT COMMUNITY| PALM LAKE RESORTS                                                          | 10