Seawall Repairs FAQ — Lake Macquarie

Start with the wall itself: how many metres are damaged, how high it is, what it is built from and whether machinery can get to it. Location matters as well. Lake Macquarie's 2023 Coastal Management Program describes Swansea Channel frontages taking tidal currents and, nearer the entrance, ocean swell, while erosion on the rest of the lake is driven mainly by wind waves. If the work needs a development application, budget for the reports council asks for, which can include a Visual Impact Statement and an Aquatic Habitat Assessment by a suitably qualified consultant. Get an engineer's assessment before you commit to a repair.

Ask council before any work starts. In the Lake Macquarie City Council area, the Development Control Plan 2014 (Part 9) requires a Visual Impact Statement with every foreshore application. Any development below mean high water, which the DCP fixes at +0.132 m AHD, also needs an Aquatic Habitat Assessment by a suitably qualified consultant, prepared to Council's Flora and Fauna Guidelines and NSW DPI Fisheries fish habitat policy. Also find out who manages the land on the water side of your boundary. The 2023 Coastal Management Program says councils, NPWS and Crown Lands manage about 100 km of the lake's roughly 170 km of foreshore.

Much of the damage happens out of sight, in the fill behind the wall. Moving water draws fine backfill out through gaps and cracks, a void opens behind the face, and the wall looks sound until it leans, bulges or drops. What drives that loss depends on where the wall is. On the lake itself, Council's 2013 foreshore guideline named wind-generated waves, boat waves and wind-generated currents as the main erosion processes, with a tidal range of only about 0.2 m. In Swansea Channel, the 2023 Coastal Management Program records that faster tidal flow and scour upstream of Swansea Bridge have caused foreshore protection works to fail.

Treat it as one. A lean or a bulge, or a hole opening up in the ground behind the wall, is a sign the fill holding it up has probably washed out, and a wall in that state can go without much warning. Lake Macquarie has seen how suddenly erosion can take a structure. The 2023 Coastal Management Program records that erosion caused the sudden collapse of the Pelican Marina complex in February 2016, taking the marina buildings, a restaurant, a boat brokerage business and a residential apartment. Keep people, cars and machinery off the ground behind the wall, and have an engineer look at it before any work starts.

Council's 2013 Foreshore Stabilisation and Rehabilitation Guideline said a sloped rock rubble revetment on the lake should have a 1V:2H lake-side face (one metre up for every two across) and be more than one rock thick, with rock of 200 to 400 mm effective in most situations. It required a geotextile or gravel drainage layer between natural ground and the rock, and sound, durable, angular stone with its longest side no more than three times its shortest. That guideline has since been replaced by the 2024 Foreshore and Waterway Development Guidelines, so have your engineer check the 2024 version. A vertical concrete or block wall needs weep holes and drainage behind it, or the backfill loss starts again.

A patch can buy time while you organise a proper repair. Sandbags in a scour hole or fresh mortar in open joints can lower the risk of a sudden collapse, but neither replaces lost backfill. For a rebuild, ask the engineer what water level to design for. Lake Macquarie's 2023 Coastal Management Program reports that Council's Waterway Flooding and Tidal Inundation Policy set a year-2100 flood planning level of 2.82 m AHD for habitable buildings with a year-2100 asset life. That is the projected 2100 1-in-100-year lake flood level of 2.32 m AHD, including 0.9 m of sea level rise, plus 0.5 m of freeboard. It is a building level, not a wall rule.