Glen - REC13/0146 Flood Study BSM 30 April 2014. Email 2 Attachment 2

4.0     STORMWATER DISCHARGES TO No 2 PEARSONS ROAD

A catchment area of 2.49 hectares presently drains across the northern boundary of the site and onto No 2
Pearsons Road, as shown for the pre-development scenario in Figure 3. Local stormwater discharges onto
the adjoining property will be maintained at or below their existing rates by diverting part of the runoff
from the relevant catchment, directly across to the western tributary floodway within the site, as shown
for the post-development scenario in Figure 3.

The existing discharge rates to the adjoining property, as calculated by the Rational Method for the
partial-area of 2.43 hectares shown in Figure 3, are summarised in Table 4 below:

Table 4:        Rational Method Calculations Of Pre-Development Peak Flow Rates To No 2 Pearson Road
 C10          Tc                    I (mm/hr)                 A                          Q (m3/s)
             (min)    1 / 2 / 5 / 10 / 20 / 50 / 100         (Ha)      1 / 2 / 5 / 10 / 20 / 50 / 100
 0.72         24      65 / 73 / 98 / 116 / 130 / 152 / 166   2.43    0.25 / 0.30 / 0.45 / 0.56 / 0.66 / 0.85 / 0.97



In the post-development scenario, runoff from the 2.40 hectare post-development catchment (excluding a
0.018 hectare drainage corridor connecting to the adjoining property) will be collected at the sag in the
proposed road, from which flows up to Q5 will be diverted eastward by piping along the road, to the
western tributary channel. Flows in excess of Q5 at the sag will discharge overland, through the
abovementioned drainage corridor, to the adjoining property.

The Q5 diversion capacity, as calculated by the Rational Method for a partial area of 2.25 hectares, is
summarised in Table 5, below:

Table 5:        Rational Method Calculation Of Q5 Diversion Capacity
      C10            Tc (min)       I5 (mm/hr)            A (Ha)                        Q5 (m3/s)
      0.78              18              112                2.25         0.00278 x (0.95 x 0.78) x 112 x 2.25 = 0.52

The post-development discharge rates to the adjoining property up to Q5, being just the runoff from the
surface of the connecting drainage corridor, are calculated by the Rational Method as in Table 6 below.
These flow rates are all practically zero.

Table 6:        Rational Method Calculations Of Post-Development Flows Up To Q5 To No 2 Pearson Road
      C10              Tc               I (mm/hr)                     A                      Q (m3/s)
                      (min)            1 / 2 / 5                    (Ha)                   1 / 2 / 5
      0.90             10             97 / 109 / 147                0.018              0.003 / 0.004 / 0.006

The post-development discharge rates to the adjoining property from Q10 to Q100, as calculated by the
Rational Method for the partial area of 2.30 hectares shown in Figure 3, are summarised in Table 7,
below:

Table 7: Rational Method Calculations Of Post-Development Flows Q10 - Q100 To No 2 Pearson Road
      C10              Tc               I (mm/hr)                    A                      Q (m3/s)
                      (min)        10 / 20 / 50 / 100               (Ha)             10 / 20 / 50 / 100
      0.78             18         132 / 151 / 172 / 192             2.30            0.66 / 0.79 / 1.00 / 1.15

                                                                                       Less 0.52 m3/s Q5, =
                                                                                    0.14 / 0.27 / 0.48 / 0.63




      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                                             Page 6