2 David Low Way - Attachment 4 - Specific Engineering Assessments - Cardno 10-8-17

1.1.2 Rate of Long Term Erosion (R)

Gradual and imperceptible long term shoreline erosion may occur as the result of an imbalance in the
net sediment budget. This gradual net loss of sediments may occur as a result of the following
processes:

e Differential Longshore Sediment Flux: Whilst incident wave direction at the study site is likely
to fluctuate naturally about the shore-normal direction due to incomplete wave refraction at
times, if the mean wave direction differs slightly from shore-normal then the result will be a net
longshore flux of sediments. Differential longshore flux across the shoreline may result in either
a net supply or loss of sediment at any particular location.

e = Artificial Sand Mining or Beach Nourishment: Anthropogenic gains and losses can constitute a
significant component of the sediment budget when material is artificially added or removed
from the coastal cell.

e Sand Losses Due to Coastal Dunes: Another potentially significant process is that of aeolian
transport. Onshore winds can blow sand inland off the beach so it cannot be reached by wave
action. This results in dune growth and in some cases inland dune migration and a net loss of
sand from the present beach system. If there is not a matching supply of sand from the surf
zone or alongshore shoreline recession occurs.

This process was quantified from aerial photograph analysis as described in Section 1.2.1.

1.1.3 Short Term Storm Erosion (C)

In order to determine setback distances for planning as part of the coastal risk assessment framework,
it is necessary to understand the magnitude and frequency of storm related erosion, and the resulting
fluctuations of the beach face over time. The values for short-term erosion from design storms (C) on
the resort beach are described by the outcomes of the SBEACH modelling described in Section 1.2.2.

1.1.4 Shoreline Recession Due to Sea Level Rise (S)

There are several models available that relate shoreline retreat to an increase in local sea level. The
best known of these is The Bruun Rule (Bruun, 1962), which is a popular approach based on the
concept that an equilibrium beach profile is maintained during a sea level rise, but is translated up and
landward. Sediment is removed from the upper beach and dune during shoreline retreat (erosion) and
deposited onto the adjacent nearshore zone, thus maintaining both the original beach profile and
nearshore shallow water conditions.

The estimated sea level rise is based on the best information currently available, and the current value
adopted for calculating the erosion prone area is 0.8m by 2100 (relative to 1990 MSL). The horizontal
foreshore erosion due to sea level rise (S) was determined for the 83 years planning period (from
present day to 2100).

Based on data presented in State Development (2014) and the LIDAR based profiles, a nearshore
seabed slope of 1:30 between the estimated top of wave run-up at the shoreline and closure depth
(*10m), the shoreline recession arising from projected sea level rise has been estimated to be 30 x
0.8m = 23m.

1.1.5 Factor of Safety (F)

The calculation procedures adopted for erosion prone area width determinations are consistent with
current engineering practice in this field, but are subject to uncertainties and limitations. Therefore, the
QLD Coastal Hazard Guidelines stipulates that a factor of safety should be applied to these calculations.
For this purpose, the factor of safety (F) of 0.4 has been adopted by DEHP for use in this equation.
This means that design long term, storm and sea level rise erosion distances would be scaled up by
40%.