2014-03-25 Planning & Organisation Committee Agenda Pages 81-167

PLANNING & ORGANISATION COMMITTEE MEETING AGENDA                                 25 MARCH 2014


Flooding, Stormwater Management and Constructed Waterbodies

Flood Impacts - Regional
To facilitate the proposed development, large areas of the site below the existing 100 year ARI
flood level are proposed to be filled with no compensatory floodplain storage being provided.
The main areas to be filled are within Precinct 2 where a 25,600m3 loss of flood storage is
proposed.

The applicant has modelled the potential impact of filling all flood affected residential areas,
limited development areas and rural use areas that have been predominately cleared. These
areas are quite limited in extent and the cumulative impact of filling of all of these areas is a
negligible increase (maximum of 5mm) in regional flood levels.

The Lake Weyba catchment has extensive floodplain storage because of the size of the lake and
the storage available above it. Also, a significant portion of the Lake Weyba catchment is
National Park which has no further development potential. Other areas are currently rural and
rural residential where, even though not zoned for intensive development, the current application
shows that this is no guarantee that it will not be considered for future urban development.

The loss of floodplain storage is undesirable, however, the regional flood impacts posed are
minimal.

Other Off-site Flood Impacts

The existing situation within Precinct 2 is that floodwaters slowly move over a wide and flat
waterway. The proposed filling and installation of culverts to the access road to a pocket of
Precinct 2 and Precinct 1, means that the height of floodwaters will rise upstream and
downstream. Velocity will also increase but generally remain low. Downstream of the filling is
land proposed to be dedicated as National Park, so increases in flood levels and minor increases
in velocity in this area would need to consider impacts on ecological values.

Immediately to the south of Precinct 2 is a large parcel of private land with a number of cabins. A
very minor increase in flood levels is predicted on the northern boundary of this land. The area
where flood levels slightly increase is fully vegetated and is a registered nature refuge. The
predicted impacts are considered minor but the impacts on the ecological values would need to
be further considered. However, if the culverts become fully blocked, the upstream properties
will experience increased water levels by up to 1.84 metres. The upstream catchment is heavily
vegetated so major blockage of the culvert is a possibility during extreme events. To address
this, consideration would need to be given to amending at least some of the crossings to a bridge
structure or culvert with a clear opening height greater than 3m and width greater than 5m to
avoid blockage.

Residual Flood Risk (Probable Maximum Flood)

Allotment levels are proposed to be set 500mm above the peak 1% AEP + Climate Change flood
levels. In terms of the residual flood risk, some of the proposed detached house areas will be
inundated during events greater than a 1% AEP event. However, people in these areas can
progressively move to higher ground and get to flood free urban areas and Walter Hay Drive
during probable maximum flood events. Areas of the site proposed for retirement village/aged
care are located above the probable maximum flood. In addition, trafficable access from Walter
Hay Drive is proposed to the retirement village/aged care sites during a Probable Maximum
Flood for emergency access.




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