Year

2016

Degree Name

BEnvSci Hons

ANZSRC / FoR Code

050299 Environmental Science and Management not elsewhere classified

Department

School of Earth & Environmental Science

Advisor(s)

Brian Jones

Abstract

Estuaries are environmentally, recreationally and economically valuable systems, however they are increasingly becoming the focus of human developments. Estuaries that are intermittently open to the ocean are particularly prone to entrance works (Morris and Turner, 2010). This study aimed to quantify certain physical and biological changes occurring at Lake Illawarra, a trained barrier estuary located on the south coast of New South Wales, Australia, following entrance training in 2007.

A combination of entrance morphology mapping using GIS and aerial photography for the years 2007-2016, volume difference calculations of hydrographic survey data collected in 2008 and 2016, the comparison of tidal and flow gauging data collected in 2008, 2012 and 2016, and sediment sampling were used to quantify the physical changes occurring within the entrance at Lake Illawarra. The current extent of mangroves within Lake Illawarra was also mapped.

Lateral erosion and bed scour was most severe downstream of the Windang Bridge along the Windang foreshore where lateral erosion exceeding 100 m occurred and scouring down to -7 m to -8 m was evident, both of which were driven by an eastward shift in the thalweg. Upstream of the Windang Bridge, reactivation of the flood-tidal delta saw the progradation of the delta into the lake by over 100 m at several locations, and vertical accretion at the delta lobes of up to 2 m to 3 m. Sediment analysis classified the entrance channel as an area dominated by marine tidal processes. Mangroves cover approximately 8781 m², and since juvenile trees and small trees account for 92% of mangrove extent, it is evident that the majority of mangroves established due to entrance training.

These results will assist in the formation of the Coastal Zone Management Plan (CZMP) currently being prepared for Lake Illawarra. The CZMP will need to consider shoreline protection and engineering options to address channel scouring and erosion, particularly at those areas threatening to undermine shoreline developments along the Windang foreshore and Judbowley Point. The encroachment of mangroves into saltmarsh areas, an endangered ecological community in NSW, will also need to be considered by the CZMP. Future studies should seek to quantify how the physical changes occurring at trained barrier estuaries are impacting the ecosystem as a whole.

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