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

3.0     STORMWATER DETENTION REQUIREMENT

As already noted in Section 1, the increased rates of stormwater runoff from the site are proposed to be
mitigated partly by conventional detention storage in two bio-basins and partly by increased floodway
storage on the site, with a requirement that the combined volume of those two storages should be at least
equal to the detention volume that would be required for a fully-conventional system.

The detention volume that would be required for a fully-conventional system has been determined by
compiling hydrologic models of four distinct parts of the proposed development platform, notated as
models A, B, C and D in Figure 2 of this report.

The attenuation objective for each of the four models was set by Rational Method calculations of the pre-
and post-development peak flow rates, as summarised in Tables 1 and 2 hereafter. The contributing areas
shown for Area A in Tables 1 and 2 are slightly less than the actual area of 2.49 hectare, because of
partial-area criticality (refer Figure 2 for partial-area limits).

Table 1:     Rational Method Calculations Of Pre-Development Peak Flow Rates
 Area     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
  A       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
  B       0.70    18     75 / 84 / 112 / 132 / 151 / 172 / 192    0.85   0.10 / 0.12 / 0.18 / 0.22 / 0.26 / 0.33 / 0.38
  C       0.70    20     71 / 80 / 106 / 127 / 142 / 165 / 180    1.85   0.20 / 0.24 / 0.36 / 0.46 / 0.54 / 0.68 / 0.78
  D       0.70    16     80 / 90 / 120 / 140 / 158 / 183 / 202    1.34   0.17 / 0.20 / 0.30 / 0.37 / 0.43 / 0.55 / 0.63

Table 2:     Rational Method Calculations Of Post-Development Peak Flow Rates
 Area     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
  A       0.78    18     75 / 84 / 112 / 132 / 151 / 172 / 192    2.34   0.30 / 0.36 / 0.54 / 0.67 / 0.80 / 1.00 / 1.17
  B       0.80    13     89 / 100 / 133 / 155 / 175 / 201 / 221   0.85   0.13 / 0.16 / 0.24 / 0.29 / 0.35 / 0.44 / 0.50
  C       0.80    14     86 / 96 / 129 / 150 / 169 / 194 / 214    1.85   0.28 / 0.34 / 0.50 / 0.62 / 0.73 / 0.92 / 1.06
  D       0.80    11     93 / 105 / 141 / 163 / 186 / 215 / 235   1.34   0.22 / 0.27 / 0.40 / 0.49 / 0.58 / 0.74 / 0.84

Version 3.5g of the URBS software was used as the basis for the hydrological models of Areas A to D.
Each of those areas was modelled as a sub-area network, using the subdivision lines indicated on Figure
2. Each of the four models was based on post-development conditions and was calibrated to closely
match its peak outflow rates to the Rational Method calculations of the post-development flow rates (as
in Table 2). A hypothetical detention basin was then added to the outlet of each model, with its storage-
discharge characteristics adjusted so as to reduce the peak outflow rates to the Rational Method
calculations of the pre-development flow rates (as in Table 1). The resultant storage requirements are
summarised in Table 3, below:

Table 3:     Required Detention Volumes (m3) For Fully-Conventional System
  Area       1 Yr     2 Yr     5 yr   10 Yr 20 Yr       50 yr 100 Yr
   A         130      170      245     290      355      445     520
   B          60       80      110     130      150      195     220
   C         130      175      260     300      360      425     495
   D          75      100      155     190      230      305     350
 Total       395      525      770     910     1095     1370     1585




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