BsmFinal_1

2.0     XPSTORM MODELLING – EXISTING SCENARIO

2.1     Adopted Software

The hydrologic routing of runoff from the catchment surfaces and the hydraulic modelling of flow
profiles along the floodways have been undertaken by the XPStorm software, using Interface Version
9.52A and Engine Version 9.28 – a particularly “robust” version of that software.

2.2     Catchment Representation

The extent and arrangement of the XPStorm model are shown in Figures 1 and 2 on pages 6 and 7 of this
report, with further detail in Figure 5 on page 13. The model was initially compiled for another of Mr
Jones’ developments, on the upstream side of Pottery Street, which included only cursory floodway
details between Pottery Street and its end-point at Church Street. The model has been extended to the
downstream side of Rectory Street to provide adequate run-in for this study and the cursory details in the
downstream end of the initial model have been replaced by field-survey data for all of the cross-sections
in this area. The model has also been extended sideways to include the overflow in Red Street.

The overall area of the modelled catchment is 41.13 hectares, including the 1.19 hectare site.

The modelled catchment includes future land-use zones of Open Space / Conservation, Rural Settlement
and Detached Housing, as delineated on Figure 1. For the purposes of this study, all of those parts of the
catchment which are external to the site have been modelled as being already developed to their future
zoning, with appropriate impervious fractions in accordance with QUDM, in both the existing and
developed scenarios.

The software was configured to hydrologically route the runoff from each subarea by the SWMM Runoff
Non-Linear Reservoir Method. This allowed for each subarea to be represented as two subareas, firstly
representing the directly connected impervious area (roofs and road carriageways) and secondly
representing the grassed areas and indirectly connected impervious areas (garden sheds, pool surrounds,
footpaths, driveways). Directly connected impervious areas were broadly assumed to occupy about two
thirds of the total impervious area within each subarea.

Directly connected impervious areas forming the first subarea were assumed to have a flow path
equivalent to a 20 metre length of 15 degree pitched roof – this inherently allowed for about 5 metres of
flow length down an actual roof or across a 3 metre half-road width, with the remaining 15 or so metres
representing the equivalent formalised (kerb/pipe) flow time to the subarea outlet. Accordingly, the
equivalent width of each impervious area was specified to the model by dividing the area by 20 metres.

The equivalent width of the pervious and indirectly connected impervious areas within each of the second
subareas was specified to the model by dividing the area by the average flow length to the subarea outlet.

Manning’s n values for the catchment surfaces were set at 0.025 for the impervious areas (tiled /
corrugated roofs and bitumen) and 0.100 for the pervious areas (forest in the conservation areas and
grass/landscaping with fences and other obstructions in the developed areas).

2.3 Floodway Roughness

Various areas of the existing floodways upstream of the site maintained their Manning’s n values of
0.035 to 0.150 from the model compilation for the previous study. The value of 0.035 was generally


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      Brisbane Stormwater Management Pty Ltd Document File: ChurchSt.Doc