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

Rates of Shoreline Progradation at the Castaways Beach Site

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Transect_ID | Rate of
Shoreline

Change

(m/year)
2 0.62
3 0.72
4 0.74
5 0.75
6 0.73
7 0.74
8 0.64

9 | Ost

10 | 0.45
co 0.33
12 0.29
13 0.27
14 0.14
15 0.12
16 0.06
17 0.02
18 0.24

 

Based on the outcomes of these analyses, which demonstrate a long term shoreline progradation rather
than long term erosion, the long term erosion effect has been set at zero for the purposes of the EPAW
calculation. The observed rates vary from about 0.7m/year in the south to 0.1m/year in the north.

1.2.2 Storm Erosion

The LIDAR survey data was used to develop two shore normal beach profiles for application in the
SBeach storm erosion model. These profiles are located near the one-third points along the Castaways
Beach shoreline. The beach profile included characteristic sediment particle sizes of 0.15 and 0.35mm,
based on typical beach sand sizes and the need to test sensitivity to particle size.

Storm erosion of the shoreline at the site was investigated using the SBEACH modelling system.
SBEACH was developed by the U.S. Army Corps of Engineers (USACE) to investigate storm induced
profile response on fine to medium grain sand beaches. It is an empirically based model that includes
wave shoaling, refraction, breaking, set-up and run-up. The model can simulate a temporally varying
wave-breaking point, which produces offshore bar migration. The model has been widely applied at
sites all over the world and has demonstrated reasonable levels of calibration.

An SBEACH analysis was undertaken for the selected profiles at the study site using the 100-years ARI
cases described in Table 1.2 as part of a 12 hours storm, peaking at 6 hours. Storm tide levels and
nearshore wave heights were determined from information presented in State Development (2014).
Based on these analyses at two shore normal profiles, the storm erosion component of the EPAW
calculation is 32m — including post-storm slumping.