BsmFinal_1

3.0     XPSTORM MODELLING – DEVELOPED SCENARIO

3.1     Model Adjustment For Development Changes

The XPStorm model of the developed scenario was compiled by making the appropriate development
changes to a copy of the existing scenario model. Those changes, mostly shown on Figure 5, comprised:

-      Altering the pervious/impervious assignments of those model subareas which contained part of the
       site area.

-      Adding the driveway access to Lots 2 and 3, with an associated northward shift of the creek channel
       between nodes N50/N51 to offset the intrusion of the driveway batter into the existing creek bed.

-      Addition of two sets of box culverts under the abovementioned driveway, between nodes N49/N50
       and nodes N51/N52. Given that most of the runoff to these culverts comes from Range Street and
       Memorial Avenue, which are mostly residential areas, these culverts were assumed in the design-
       case modelling to be unblocked.

-      Addition of a shallow waterbody around nodes N52/N53 to alleviate future grass maintenance
       issues (this waterbody would be the third waterbody on the site, being additional to two existing
       shallow waterbodies around node N45 and between nodes N46/N47.

-      Adding a driveway access to Lot 4, with a 100-year capacity box culvert and a causeway across the
       northern tributary.

3.2     Flood Impacts For Existing Climate

The XPStorm model of the developed scenario was firstly used to design the driveway culverts for
practically zero impact at the site’s upstream boundaries, assuming that all reaches of the floodway other
than the proposed waterbody will remain in their present condition. Effectively, this meant sizing the
culverts under the driveway accesses to Lots 2 and 3 for the minimum possible afflux at the site’s
southern boundary at node N48. Two 3 metre wide boxes at each of these crossings, 450 mm high
between nodes N49/N50 and 825 mm high between nodes N51/N52, with causeways barely covering the
culvert crowns, resulted in 0.01 metre afflux (or within the accepted model accuracy) across the range of
ARI at node N48.

The model was then used to size the culvert and causeway level for a 100 year access to Lot 4, resulting
in a 1200 x 450 box culvert, with a causeway level of 91.38.

The consequent affluxes at the site’s upstream boundaries are summarised in Table 2, below:

Table 2: Impacts On Flood Levels At Upstream Boundaries Of Site, For Existing Climate Regime
 ARI       Western Bdy, Node N44      Southern Bdy, Node N48         Northern Bdy, Node N58
 (yrs)    Exg       Dev      Afflux   Exg     Dev     Afflux        Exg        Dev      Afflux
  100    91.28     91.28      0.00   91.17   91.18      0.01       92.65      92.65      0.00
   50    91.22     91.22      0.00   91.12   91.13      0.01       92.62      92.62      0.00
   20    91.14     91.14      0.00   91.06   91.07      0.01       92.61      92.61      0.00
   10    91.02     91.03      0.01   90.95   90.96      0.01       92.58      92.58      0.00
   5     90.88     90.88      0.00   90.92   90.93      0.01       92.57      92.57      0.00
   2     90.76     90.76      0.00   90.88   90.89      0.01       92.55      92.55      0.00
   1     90.67     90.67      0.00   90.84   90.85      0.01       92.53      92.53      0.00

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      Brisbane Stormwater Management Pty Ltd Document File: ChurchSt.Doc