7 FORESHORE EROSION AND STRUCTURES ASSESSMENT SMEC has assessed the severity of foreshore erosion and condition of existing foreshore structures through a series of field visits in late 2009. This section of the report presents summaries of erosion and foreshore erosion protection and stormwater structures on a site by site basis, assigning gradings of light, moderate or severe to the foreshore erosion, and condition assessments for seawalls as good, fair or poor. Management responses for each particular site are suggested. Locations of foreshore erosion and foreshore structure assessments are mapped and described in Appendix 2. Particular sites for management actions are prioritised based on a catchment wide basis, and on an LGA basis. Priorities for particular management actions at particular sites were based on a combination of cost and environmental benefit. 7.1 Introduction Some erosion occurs with various degrees of severity in various locations all along the Georges River foreshore. In general, most erosion occurs in the areas of the river underlain by Wianamatta Shales in the upper reaches while the lower reaches are located in sandstone or are highly urbanised and protected by seawalls. The study team visited the site over several days by land and boat to assess the severity of erosion and the seawall conditions along the Georges River foreshore. Foreshore erosion along the Georges River is generally due to factors such as boat waves, tidal undercutting, floods and stormwater runoff. Erosion was rated as being light, moderate or high/severe. Many seawalls have been constructed along the Georges River. Most of them are located in the lower reaches and all around Chipping Norton foreshore. Seawall conditions were rated as followed: Good: the seawall is quite new or intact; it is properly engineered and does not need any maintenance or only minor maintenance. Fair: the seawall is in relatively good condition but would need maintenance or upgrades Poor: Dumped materials, collapsed seawall or seawall in need of major maintenance and upgrades Locations of foreshore erosion are provided in Table A in Appendix 2. The seawall assessment is documented in Table B in Appendix 2. The numbered locations are indicated on the maps 1 to 45. The recommendation options refer to the option presented in Section 7.3. Key management options per council and for the Georges River are described in Section 7.4. Stormwater, GPTs and WSUD are described in Section 7.5 and in Table C in Appendix 2. The prioritisation was undertaken using a “cost x environmental gain” methodology. This means that the lower the cost and the higher the environmental benefit, the higher the priority. 7.2 Site Summaries 7.2.1 Liverpool Weir to Moore Lake Results of the assessment of the foreshore erosion and seawalls at this location are summarised below and in Figure 1 of Appendix 2. 7.2.1.1 Foreshore Erosion The main channel of the Georges River between Liverpool Weir and Moore Lake is not protected by seawall – except the area surrounding the Liverpool Weir – and is subject to erosion (Figure 7.1). The foreshore along the railway is highly-to-severely eroded as well as the north-western part of McMillan Park foreshore. Some light undercutting occurs on the foreshore opposite the park and moderate erosion occurs opposite the railway (location reference E1 to E6). Figure 7.1 – Severe erosion between Liverpool Weir and Moore Lake Recommendations Suggested management options for the erosion management are given in Table 7.1 below. Table 7.1 – Erosion management option between Liverpool and Moore Lake Possible Management Response (Options provided refer to Erosion Location LGA Priority Rating Options of Section 7.3.1 of the report) E1.Along the railway Levelling of the bank to reach a between Liverpool Weir Liverpool High stable slope and vegetation planting and Liverpool Hospital (Option E1) E2.Opposite the railway Levelling of the bank to reach a between Liverpool Weir Liverpool High stable slope and vegetation planting and Liverpool Hospital (Option E1) E3.Along the carpark east Stabilisation of the bank toe with Liverpool Medium of the railway small seawall (Option E2) Levelling of the bank to reach a E4.Directly east of the Liverpool High stable slope and vegetation planting carpark (Option E1) Levelling of the bank to reach a E5.Along McMillan Park Liverpool High stable slope and vegetation planting (Option E1) E6.Opposite McMillan Liverpool Medium/High Stabilisation of the bank toe with Possible Management Response (Options provided refer to Erosion Location LGA Priority Rating Options of Section 7.3.1 of the report) Park small seawall (Option E2) Vegetation planting (Option E3) 7.2.1.2 Seawall Assessment A part of Liverpool Weir is settling on its eastern half. There are only a few seawalls along the foreshore upstream of Chipping Norton. A small gabion seawall supported by sheet piles is located along the northern bank between McMillan Park and Gandangara Island (Figure 7.2). This seawall looks to be in relatively good condition but has been covered with invasive plants (location reference S1-2). Figure 7.2 – Gabions seawall between McMillan Park and Gandangara Island Recommendations Suggested management options for the seawall management are given in Table 7.2 below. Table 7.2 – Seawall management option between Liverpool and Moore Lake Priority Management Options (Options provided Location LGA Rating refer to Seawall Options of Section 7.3.2 of the report) Fix the settling part of the weir on its eastern half S1.Liverpool Weir Liverpool Medium Creation of more habitat in front of weir (Option S2) Sheet piles can be replaced by environmentally friendly rock seawall to allow S2.Along northern bank vegetation growing in the gap of rocks (Option between McMillan Park Liverpool Medium/High S5) and Gandangara Island Creation of more habitat in front of sheet piles seawall (Option S2) 7.2.2 Moore Lake Results of the assessment of the foreshore erosion and seawalls at this location are summarised below and in Figure 2 of Appendix 2. 7.2.2.1 Foreshore Erosion Moore Lake foreshore is in good condition and mostly well vegetated. Some light tidal undercutting occurs along the northern side of the lake and some moderate erosion occurs along the south-eastern side of Bulba Dibeen Island (Figure 7.3). The bank located directly east of Moore Lake entrance along Georges River main channel is moderately eroding (location reference E7 to E10). Figure 7.3 – Moderate undercutting at Bulba Dibeen Island Recommendations Suggested management options for the erosion management are given in Table 7.3 below. Table 7.3 – Erosion management option at Moore Lake Possible Management Response (Options provided refer to Location LGA Priority Rating Erosion Options of Section 7.3.1 of the report) Levelling of the bank to reach a stable slope and vegetation planting E7.Northern bank of Lake Liverpool Medium (Option E1) Moore Stabilisation of the bank toe with small seawall (Option E2) Levelling of the bank to reach a E8.South-eastern side of stable slope and vegetation planting Liverpool Medium Bulba Dibeen Island (Option E1) Stabilisation of the bank with small Possible Management Response (Options provided refer to Location LGA Priority Rating Erosion Options of Section 7.3.1 of the report) rocks (Option E8) Stabilisation of the bank toe with small seawall (Option E2) E9.North of Bridges Road Vegetation planting where gaps in Liverpool High Wharf vegetation along bank (Option E3) E10.Directly east of Lake Stabilisation by further vegetation Moore entrance along the Liverpool Low/Medium planting(Option E3) Georges River No action 7.2.2.2 Seawall Assessment Some rock protections have been built to protect the bridge crossing Lake Moore entrance (location reference S3). Recommendations Suggested management options for the seawall management are given in Table 7.4 below. Table 7.4 – Seawall management option at Moore Lake Priority Management Options (Options provided Location LGA Rating refer to Seawall Options of Section 7.3.2 of the report) S3.Bridge crossing Lake Liverpool N/A No specific action required Moore entrance 7.2.3 Moore Lake to Governor Macquarie Bridge Results of the assessment of the foreshore erosion and seawalls at this location are summarised below and in Figure 3 of Appendix 2. 7.2.3.1 Foreshore Erosion Ngamba Island located in the Georges River east of Lake Moore entrance and upstream of the River bend is subject to light tidal undercutting. The Georges River embankments surrounding this island suffer from light to moderate erosion. The area downstream of this island but upstream of the private dwellings located along the river on the eastern bank within the bend is subject to high erosion (Figure 7.4). Many of the private dwellings have their own seawall and some undercutting is noticeable between the private seawalls (location reference E11 to E16). Between the bend of the river and the Cabramatta Creek entrance, some erosion is visible on both side of the river (location reference E17 to E20). Figure 7.4 – Erosion east of Chauvel Park Recommendations Suggested management options for the erosion management are given in Table 7.5 below. Table 7.5 – Erosion management option between Moore Lake and Governor Macquarie Bridge Possible Management Response (Options Priority Location LGA provided refer to Erosion Options of Rating Section 7.3.1 of the report) Stabilisation of the bank toe with small E11.Ngamba Island Liverpool Low seawall (Option E2) Sand nourishment (Option