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

6.0     FLOOD PROFILES, PRE-DEVELOPMENT

Version 9.28 of the Extran software, as embedded in the XPStorm modelling suite, has been used to
convert the catchment runoff hydrographs to flood profiles along the floodways. The extent and detail of
the model are shown in Figure 3 of this report. Te model provides approximately three times the detail of
the Mike11 model which L&T used in 2002 to model these floodways. The model network also includes
a number of interconnecting links to provide for simulation of overflows between the two tributaries
where they are aligned in close proximity, downstream of the site.

This version of the Extran software includes a facility to specify different internodal distances for the left
overbank, channel and right overbank segments, which in turn provides for a reasonably accurate
calculation by the software of the internodal flood volumes up to the peak flood level profile. This feature
was vital for checking that the reshaping of the floodway in the post-development scenario achieves the
required stormwater detention volumes, over-and-above the existing floodway volumes. Accordingly,
this facility was invoked for this analysis and separate internodal distances were measured and applied to
each connecting link of the model network.

Field-surveyed spot heights, which can be seen in the background of Figure 6 were used to define the
cross-section shape of each of the links between adjoining nodes of the model. The Manning’s n values
of the channel and overbank sectors of the floodways were perceived by site inspection. The adopted
values ranged from 0.035 for open grassed paddock or park, to 0.150 in densely timbered areas.
Examples of the adopted values are shown in Figures 7and 8 of this report.

The starting point for the simulations was the tailwater level just downstream of the convergence of the
two tributaries. Initial runs of the model, based on a “normal-depth” criterion at node 39 at the
downstream end of the model, simulated a 100 year tailwater level substantially higher than L&T’s
predicted tailwater level at this location. The slope of the outfall reach, between nodes 38 and 39, was
therefore artificially steepened so as to induce a simulated tailwater level much closer to the L&T level.

Initial runs of the model, for ARI’s of 2 year and 100 year, and for storm durations from 20 minutes to 9
hours, revealed that the critical storm durations varied from 1.5 hours to 3 hours along these floodways.
Accordingly, all subsequent simulations included three storm durations: - namely, 1.5, 2 and 3 hours.

The set-up of the model resulted in simulated flood levels near the upstream and boundaries of the site
that, apart from node 23, were generally at or slightly above the co-located levels from L&T’s Mike 11
model, as indicated in Table 15 below. Overall, the consistency between the two models appears to be
quite good, despite the considerable difference in network density between the two models.

Table 15 : Comparison of BSM / CLT Flood Levels (m) In Vicinity Of Site Boundaries
 BSM Node CLT X-Sect           10 year         20 Year            50 Year           100 Year
       3       T3-811      101.61 / 101.38 101.65 / 101.54 101.72 / 101.69 101.81 / 101.71
      16      T3-1199       99.60 / 99.58   99.68 / 99.61       99.74 / 99.68     99.79 / 99.71
      23       T2-389      100.98 / 101.33 101.05 / 101.39 101.12 / 101.47 101.17 / 101.51
      29       T2-611       99.89 / 99.82   99.95 / 99.89      100.01 / 99.97 100.05 / 100.05

The predicted pre-development peak flood levels at each node of the model, for ARI’s from 1to 100
years, are summarised in Tables 16 to 18 on the following pages of this report, where they are compared
to the corresponding post-development flood levels. The corresponding 100 year pre-development flood
profile is shown on Figure 10, and the pre-development 100 year boundary is mapped on Figures 9 and
13.



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