Case study: Assessment of the entrance stability of the Lake Illawarra Estuary

Abstract

Prior to completion of entrance improvements in 2007, Lake Illawarra behaved as an ICOLL (intermittently closed and open lakes and lagoons) with entrance conditions dependent on prevailing conditions. Since construction of twin training breakwaters and dredging in 2007, the entrance has remained permanently open, with water level stations within the estuary recording an increasing tidal signal. This paper provides a case study describing methods to assess the stability of the entrance to Lake Illawarra through preliminary analysis of aerial photography, hydro-survey, continuous water level records, consecutive tidal gauging and Escoffier analysis. Regarding the case study of entrance stability, consideration is given to data for analysis, limitations of the analysis and its reliance on local or site specific empirical data, needs for ongoing monitoring and tuning to represent further data, potential implications for Lake Illawarra and on similar estuaries and possible management responses. The likely effects of expected sea level rise on stability of the entrance are also estimated based on the understood key physical processes. Since completion of the entrance works, the lake is characterised by an ’unstable scouring’ entrance as indicated by empirical analysis supported by lake water level and catchment rainfall data, tidal flow gauging, hydro-surveys and aerial photography. Ongoing monitoring and evaluation will determine further action that may be required.