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

1 Assessment of Erosion Prone Area Widths

 

For the purposes of informing permissible development locations in the coastal zone of the Castaways
Beach Development site, the Queensland Coastal Hazard Guidelines provided by the Department of
Environment and Heritage Protection were used to determine the erosion prone areas widths (EPAW).
Appendix A provides the relevant definitions for this coastal management region.

1.1 General Discussion and EPAW Overview

The Queensland Coastal Hazard Guidelines (2015) outline a method for assessing erosion prone area
widths (EPAWs). These widths are intended to accommodate both short-term and long-term erosion
over specified planning periods. This erosion prone area width is a specified horizontal distance back
from the HAT water line, and is calculated by the following equation provided in the guidelines.

E =[(NxR)+C+S] x (14+F) + D

Where:-

E = erosion prone area width (metres).

N = planning period (years).

R = rate of long-term erosion (metres per year).

C = short-term erosion from the design storm (metres).

S = erosion due to sea level rise (metres).

F = factor of safety.

D = dune scarp component to allow for slumping of the erosion scarp (metres)

The Department of Environment and Heritage Protection (DEHP) has developed default EPAW setback
distances at many Queensland coastal sites. For planning purposes, these lines are established for
projected sea level rise (SLR) of 0.8m to 2100. At Castaways Beach this default setback distance (from
the 2100 HAT contour) is 175m. As part of development plan investigations, the owners at Castaways
Beach have engaged Cardno to investigate whether or not a more realistic EPAW line could be
developed. Discussions would then be held with DEHP to address such a requested change.

1.1.1 Planning Period (N)

The planning period for this erosion prone area width assessment was to 2100. This planning period
was deemed the most practical when taking into consideration the design life of the study site structures
and appropriate level of risk. This affects the sea level rise applied to present-day highest astronomical
tide (HAT).