smiths lake planning study volume 1: text - Great Lakes Council

FLOODING AND DRAINAGE 30
The AWACS study concluded that the elevated ocean levels for Old Bar are those listed in
Table 3-2. However, due to this study being completed in 1989, it did not account for the
Greenhouse Effect. Hence, a conservative value of 0.2m is added to the Old Bar values to
derive appropriate levels for planning purposes.
Average Recurrence
Interval (years)
Table 3-2 Elevated Ocean Levels
Old Bar Levels (mAHD)
(from AWACS, 1989)
Recommended Smiths
Lake Levels (mAHD)
20 1.9 2.1
50 2.0 2.2
100 2.1 2.3
During a cyclonic event that would result in such elevated ocean levels, it is conceivable that
ocean water would flow into Smiths Lake with little hindrance from the entrance. Hence, it
could be conservatively assumed that these ocean levels are a useful guide for extreme levels
within Smiths Lake.
In considering the combination of runoff into Smiths Lake adding to these levels, it needs to be
recognised that the resulting lake level will only be sufficient to maintain the hydraulic gradient
from the lake to the ocean for draining the catchment runoff. Given the relative efficiency of
the entrance (ie. compared with a river), it is unlikely that this level will be significantly higher
than that in the ocean.
It is also possible that a cyclone could result in significant erosion of the entrance. This would
allow penetration of ocean waves into Smiths Lake causing higher levels of inundation due to
wave setup on the east-facing shores. However, to quantify and confirm this phenomena is a
complex task requiring consideration of such issues as lake bathymetry, storm orientation and
entrance conditions. It is suggested that assessment of this issue could be included in a coastal
hazard study.
3.2.2 Creek Flooding
RAFTS-XP Modelling
As described in Section 3.1.1, there are a number of watercourses in the SLA. The flood flows
associated with these catchments were analysed using the runoff routing program RAFTS-XP.
Each of the major catchments was modelled using site characteristics such as area, slope, creek
shape, degree of urbanisation and vegetation cover.
1% AEP Design Flows
Using the design rainfall data presented in Section 3.1.2, design flood events were simulated
using the RAFTS-XP models of the catchments. The critical durations for the catchments were
in the order of one hour. The peak flows for the 1% AEP flood events are shown in Figure 3-2
for the major catchments.
SMITHS_LAKE_PLANNING_STUDY.DOC
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