Stormwater Management Report

7.3 Non-worsening of Peak Discharge
Using the ‘Rational Method’ as per QUDM, the pre-developed and post-developed site runoff calculations are listed in
the following table:

                                                                  Table 1 - Catchment Details

                 t c (min)                    5            ARI                  10 years                 20 years                    50 years                  100 years
            1
                I 10 (mm/hr)                 70.2          AEP                    10%                        5%                        2%                         1%
                                                    i
                   C y (=F y .C 10 ) <1                 I y (mm/hr)               215                       245                        285                       316
                       i
            Q y =(C y . I y .A)/3600 (l/s)                  Fy                     1                        1.05                       1.15                       1.2


    Catchment                Area (m 2 )      fi           C 10            Cy          Q y (l/s)      Cy         Q y (l/s)      Cy         Q y (l/s)      Cy         Q y (l/s)
     Pre Development           10540          0            0.7            0.7           440.6       0.735          527.2      0.805           671.7      0.84           777.1
    Post Development           10540         0.9           0.88          0.88           553.9       0.924          662.8         1            834.4        1            925.2
                Post development discharge increase                    Q 10% (post)     113.3      Q 5% (post)     135.6     Q 2% (post)      162.7    Q 1% (post)      148.0


Referring to the stormwater infrastructure design drawings, by Leddy Sergiacomi & Associates Pty Ltd (Ref. No.
                                                                                                            3
45113-C22), the existing 525∅ RCP has been designed to convey an expected stormwater discharge of 0.8m /s.

Table 1 shows that the developed site, if left unmitigated, is likely to result in an increase of up to 162.7L/s during the
Q 2% storm event and up to 148L/s during the Q 1% storm event. To mitigate any potential impact that this increase in
peak discharge runoff may have to the existing drainage infrastructure during the Q 2% and Q 1% storm events, on-site
detention and harvest & re-use tanks are proposed to be installed to collect all roof water from each building.

Roof water not detained for re-use will be discharged through restricted office plates, sized accordingly to reduce flows
to predevelopment levels

The proposed stormwater pipe network for the subject site is shown on the attached engineering drawings 18-0165
Sheets C03 & C04.




HR DESIGN                                                           Reference No: 18-0165A                                                              Page 6 of 14