Lavenders Bridge Replacement – Investigation and Design

Like most structures, Lavenders Bridge has a usable life span after which continued use will increasingly face higher maintenance costs, potential for reduced load capacity and weight restrictions and ongoing engineering assessments to ensure it’s safe and efficient use.

Whilst it may appear to be a concrete bridge on top, look underneath and you will see that Lavenders Bridge is a timber bridge. Typically, Council would expect a timber bridge to last for approximately 60 years. At 33 years old, Lavenders Bridge has reached roughly half its useful life, meaning it is likely that replacement will need to be considered within the next 20-30 years. 

While refurbishment of Lavenders Bridge is technically possible, the bridge's design presents inherent challenges. The design includes a concrete deck poured around the top portion of timber girders. This composite design is excellent at carrying traffic but can make major rehabilitation works complex and costly.

Any refurbishment would likely require substantial structural works and ongoing traffic disruptions, significantly impacting the community. Furthermore, Council does not expect refurbishment to provide a substantial extension of the bridge's overall service life nor is it considered a cost-effective long-term solution.

While no set time frame for the bridge’s replacement is set, Council is undertaking this project now as part of its long-term planning for essential community infrastructure.

Through the availability of grant funding, Council has an opportunity to investigate future bridge options, determine a preferred alignment and develop a detailed design for the bridge's eventual replacement. Completing this work in advance places Council in a strong position to respond to future funding opportunities and ensures planning is well progressed when replacement is ultimately required.

It also ensures that council is prepared in the case that the bridge faces serious structural damage from a major flood or weather event.

Construction of a new bridge offline to the existing will significantly reduce impacts on the community by allowing continued use of the existing bridge during construction, maintaining access for residents, businesses and emergency services. This approach minimises the need for extensive traffic management, detours and temporary closures, reducing disruption to daily travel and maintaining connectivity throughout the construction period.

A new bridge also provides substantial safety and accessibility benefits by meeting modern design standards. It can incorporate dedicated pedestrian and cycle facilities, contemporary bridge barriers and crash protection systems, wider traffic lanes, improved sight distances and safer road approaches. These features enhance safety, connectivity and accessibility for all users and support current and future transport needs.

From an asset management perspective, a new bridge typically requires less ongoing maintenance and offers greater reliability than an ageing timber structure. This reduces the likelihood of unplanned closures, emergency repairs and operational restrictions, while providing a longer service life for the community. A modern bridge can also deliver improved resilience to extreme weather events and changing environmental conditions and reduce flood water surcharge from the bridge structure.

The $365,000 project funding was secured by council under the NSW Flood Recovery and Resilience Grant Program administered by the NSW Department of Climate Change, Energy, the Environment and Water. The grant program aims to reduce the impacts of flooding and flood-related liability on communities and helps provide funding for the investigation, design and construction of new or upgraded flood mitigation infrastructure.

The proposed bridge height has been determined through flood modelling, engineering assessments, the requirements of State and Federal funding programs and practical consideration.

As a small regional council with a limited rate-base, the delivery of major infrastructure projects often sit beyond the financial capacity of Councils budgets. As such, Council is reliant on external funding from State and Federal Government Grant programs. Successful grant applications for bridge replacement projects depend heavily on projects that improve flood resilience, reduce long-term maintenance costs, increase asset life, and improve access and safety for all users. It is expected that without meeting these criteria external funding would be difficult to access.

The natural geography of the Bellinger River Valley and local climatic conditions result in relatively frequent and sharp rises in water level. This, coupled with impacts of climatic change expected indicates that the proposed deck height of approximately 10.0 metres AHD would, by 2100, provide flood immunity for a 5% AEP event (roughly a 1 in 20 year flood event). Lowering the proposed bridge height by just 2 metres would reduce performance to around a 50% AEP event (1 in 2 year event), resulting in a significant reduction in flood performance. This compares with the current bridge height of 4.7m AHD, sitting well below the 100% (or 1 in 1 year event) 2100 flood level.

Furthermore, modelling indicates that flood surcharge from the bridge structure itself is greatest when water levels approach the height of the bridge deck. A lower bridge would interact with floodwaters more frequently, resulting in increased flood impacts and more frequent effects on nearby properties.

For these reasons, the proposed height represents a balance between flood resilience, long-term value, community access, and the requirements needed to secure future external funding.

The riverside parklands are a valued community asset and an important destination for both residents and visitors. While construction of a new bridge will require temporary access arrangements and some impacts within portions of the parklands, Council recognises the importance of preserving and enhancing the long-term amenity of this area. As the project progresses, there will be opportunities to consider how the bridge design can be integrated with the surrounding parklands and riverside environment to support community use and enjoyment.

With any eventual bridge construction still some time away, Council has the opportunity to undertake longer-term planning for the riverside precinct and ensure parkland, recreational and amenity outcomes are appropriately considered alongside the bridge project. Community input will continue to play an important role in shaping future plans for the riverside parklands and helping to guide how this important public space evolves over time.

With a minimum 100 year life-span, a new concrete bridge would provide a durable, long-term crossing designed to serve the community well into the future. Compared with a timber bridge, a concrete structure is better suited to a riverine environment, offering greater resistance to moisture, weathering, decay/corrosion and other forms of deterioration. This results in significantly lower maintenance and rehabilitation needs and lower whole-of-life costs over the life of the asset, allowing allocation of Council budgets to other important community needs.

The longer spans achievable with a concrete bridge can also reduce the number of piers and structural elements within the river channel and floodplain. A more streamlined bridge form allows floodwaters to pass more freely, reducing obstructions to flow, minimising debris accumulation and lowering flood surcharge impacts. Fewer in-stream supports also reduce the likelihood of structural damage from flood-borne debris and flood loading, improving the bridge's resilience during major flood events.

From an asset management perspective, a concrete bridge would require fewer major repairs and rehabilitation works over its lifespan, providing greater certainty for long-term maintenance planning and budgeting. Reduced maintenance activities also mean fewer disruptions to road users and the broader community, resulting in a safer, more reliable and cost-effective crossing for the community into the future.

A pedestrian-only cable-stayed or suspension bridge would still be a substantial, structurally complex and costly piece of infrastructure. While it would improve pedestrian and cyclist connectivity, it would not address the ongoing flood vulnerability, maintenance burden or eventual replacement of the existing vehicle bridge. Vehicle access would continue to rely on the existing low-level crossing, meaning the community could face the cost of maintaining two bridge assets while still requiring a future vehicle bridge upgrade or replacement.

The community would also forgo the benefits of a higher-level vehicle crossing, including improved flood resilience, reduced maintenance demands and more reliable access during flood events. Funding opportunities may also be more difficult to secure where substantial investment is directed towards a pedestrian-only structure while the primary vehicle crossing still requires future replacement.

Option 2 was identified as the preferred option because it provides a strong balance of performance, affordability and long-term value for the community. By utilising the existing Lavenders Bridge crossing location, it minimises bridge span lengths and embankment requirements, resulting in lower construction and whole-of-life costs. The option also maintains the existing traffic arrangement and connection between the northern and southern parts of Bellingen, avoiding significant changes to the surrounding road network and reducing impacts on nearby residents and road users from a more remote alignment.

From a technical perspective, Option 2 performs well in both accessibility and flood immunity outcomes, providing a more reliable crossing during flood events while maintaining connectivity for vehicles, pedestrians and cyclists. The existing crossing location is also well suited to a major bridge structure due to the relatively narrow floodplain width, allowing a more efficient bridge and embankment design. In addition, the option performs favourably in flood impact assessments by largely maintaining existing valley contours and minimising changes to flood behaviour across the broader floodplain.

The project is proposed to include a wide, accessible shared path designed to meet current standards for pedestrians, cyclists and people using mobility aids. The new crossing will provide improved access for people using wheelchairs, mobility scooters and prams, making it safer and more inclusive for all users. By replacing the steep approaches associated with the existing bridge, the new structure will offer gentler gradients and a more comfortable crossing, making it easier for people of all ages and abilities to walk, ride and access the town centre.