ATTCH 3 - IR Response - Erosion, Coastal Processes and Stormwater (Dr Trevor Johnson)

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14 November 2017

In respect of Burgess Creek, a review of available historical photographs (see attached) shows that the
mouth of the creek has not altered position significantly over the period of assessment. While there
have been changes in discharge location on the beach itself, the alignment of the creek adjacent to the
subject property has not altered. There has been no effect on the subject site as a consequence of
these beach changes, primarily because the site is significantly separated in the vertical dimension from
the creek. It is considered that development on this site will have no effect on the physical processes
which affect the location of the mouth of the creek. Similarly, on the basis of historical data, there is no
likelihood that the creek will have an erosive impact on the property.

In relation to the fore dune system, it is proposed that the site should be separated from the coastal
dune system by appropriate fencing. Access to the beach from the site is expected to be restricted to
the existing path which runs north from the site across Burgess Creek. However, it is also recognised
that the dunes are in public land accessible to all from the beach, and that there is currently no legal
restriction on persons accessing the dune area.

In relation to item 9, MUSIC modelling was undertaken in respect of the proposed development of the
site. This modelling demonstrated that an area of infiltration of 100 m? on each house lot would be
effective in controlling stormwater runoff from the site. A brief report outlining the modelling is
contained in the attached Stormwater Management Plan, and the MUSIC model itself can be provided
to Council for assessment purposes.

A similar exercise was undertaken in respect of stormwater runoff quantity. Under existing conditions, it
is unlikely that any runoff is generated from the site, because the soil is clean beach sand with high
permeability. It was calculated that the provision of a small infiltration basin with an area of about 200
m? on each house lot would be sufficient to handle all runoff up to the 1% Annual Exceedance
Probability (AEP) design storm event. This sizing is based on a long-term permeability of only 1m per
hour. In comparison, the permeability of clean beach sand would be expected to be in excess of about
8 m per hour. Consequently, the sizing is considered to be highly conservative.

The basins will contain only beach sand and be vegetated with sparse dune grass for stability
purposes. The basins will be located along the eastern boundary of each lot, such that any overtopping
for extreme events will occur as shallow sheet flow into the beach dunes.

It is proposed that the bio-retention system and the detention basin be incorporated in the same
physical basin. Consequently, an allowance of 100 m? for stormwater management on each proposed
lot is sufficient to ensure compliance with the requirements of the SPP and Council’s planning scheme.

Item 10 deals with the stormwater disposal strategy for the site. This is outlined in the attached
Stormwater Management Plan. Effectively, all runoff from the site will be captured and redirected to
bio-retention and detention basins along the eastern boundary of each lot. No stormwater will be
discharged from the site for all storms with AEPs in excess of 1%.

In relation to item 11, stability analysis indicates that the disposal of stormwater in the manner proposed

will have no adverse impact on slope stability. No stormwater outlets are proposed into Burgess Creek,
and all runoff will be disposed of by infiltration.

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