Setting the scene for saltmarsh return at Windermere Bay (Tas)
NGT commenced a saltmarsh recovery project at Windermere Bay on the River Derwent (north Hobart) in July 2024. The Derwent Estuary Program (DEP) engaged us to undertake an assessment and develop a restoration plan for an area of saltmarsh at Windermere Bay that had been subject to infilling during the mid-1970s, resulting in a 50% loss of saltmarsh at the site.
Over the past seven months we’ve been slowly untangling the secrets of the area and attempting to find answers to pertinent questions such as:
- What is the extent and depth of the infill? And, is it contaminated?
- How has infilling changed the hydrology of the site?
- What volume of material would we need to remove to reinstate connectivity with tidal inflows and stimulate spontaneous saltmarsh recovery?
These questions helped us to formulate our monitoring, and investigation plans and ultimately inform the restoration objectives and activities.
Monitoring included collection of baseline water quality (salinity), water level and vegetation data. Water level and salinity in the saltmarsh responded to both high tides and high rainfall events (rainfall event late August). In the short period of data collection shown below, we can see that water level reached 1.2 m AHD (above mean sea level: related to rainfall event), but more regularly reaches 0.8-0.9 m AHD due to the influence of high tides.
Vegetation monitoring transects showed strong zonation in relation to elevation and landform, typical of wetlands and saltmarshes. All transects typically traversed saline rushland and saline herbland in the lower, central marsh areas, and cleared exotic grassland upslope. Above 0.9 m AHD (above mean sea level) exotic grasses dominated. Below this level, saltmarsh persisted with sea rush (Juncus kraussii) occurring along the foreshore, giving way to beaded glasswort (Sarcocornia quinquefolia), with common associates such as swampweed (Selliera radicans) and creeping brookweed (Samolus repens) in slightly more elevated areas. Investigations included taking a number of soil cores to characterise infill and determine the original land surface, and soil sampling to confirm infill was not contaminated.
All the monitoring and investigation results were drawn upon to formulate a restoration plan. The restoration objective for the site is to ‘Maintain, protect and enhance ecological diversity, extent and condition of the saltmarsh by reducing the impact of historic infill and changed landform on water and salinity regimes and the associated distribution and vigour of saltmarsh vegetation communities.’ This will be achieved by restoring as far as practicable, natural landform and hydrology. All existing areas of infill will be removed to reinstate natural landform and encourage saltmarsh extension and improvement in condition through increased hydrological connectivity. Excavated areas will tie into existing saltmarsh borders in the east and extend that elevation westward to the edge of the excavation area, with a gradient of approximately 0.2 m over this distance (e.g. from 0.7 – 0.9 m AHD). The western edge of the excavated area will be tapered to tie into existing ground levels and avoid creating a ‘step’. To align with Glenorchy City Council’s scheduled boardwalk works the saltmarsh restoration will take part in two stages. Stage one will remove areas of infill beneath and east of the boardwalk alignment. Once the boardwalk has been constructed, stage 2 will remove the remaining areas of infill in the west.
A field day was held at Windermere Bay by DEP and NGT on 16th February 2025, to provide an opportunity for stakeholders, local community and interested others to come along and learn about the saltmarsh, plans for its restoration and the new boardwalk development. Stage 1 works are scheduled to commence late February and we expect all works to be completed by the end of April 2025. We look forward to completing the works and observing saltmarsh recovery at this urban site over the years to come. We are grateful for playing a small part in helping to reverse ecohydrological change at important remnant saltmarsh areas on the foreshores of the Derwent Estuary.
This project is being delivered by NGT in partnership with the Derwent Estuary Program, with funding provided by the Australian Government’s Urban Rivers and Catchments Program
