A hydrology report on the proposed Richmond Bridge and bypass warns that the project could worsen flooding in parts of the Hawkesbury region, potentially affecting some properties and threatening the long-term resilience of the infrastructure itself. The findings highlight both the challenges of building on Australia’s most flood-prone floodplain and the need for further refinement in the project’s design.

The assessment highlights that while the new bridge and bypass are designed to remain usable during a 1-in-20-year flood, some properties in the area could experience increased flooding. The report notes that during a 1-in-50-year flood, up to 15 properties may face water level rises of up to 83mm. These impacts, though described as minor, underscore the vulnerability of the floodplain to development.

The report states that while the bridge and bypass are designed to remain trafficable during a 1-in-20-year flood event (5% Annual Exceedance Probability (AEP) flood), with 300mm of freeboard for flood immunity, some homes and properties may experience increased flooding. At least 15 properties could see water levels rise by up to 83mm during a 1-in-50-year flood event (2% AEP), and isolated locations could face greater impacts exceeding 50mm. The report acknowledges these increases as "relatively minor" but stresses the need for refinements to the design to mitigate these effects, especially as floor-level surveys for impacted dwellings have yet to be conducted.

Adding to the complexity, the report outlines how climate change threatens to heighten flood risks over time. By 2090, increased rainfall intensity could lead to flood levels rising by as much as 1.9 metres across the floodplain in a 5% AEP event under certain scenarios. These changes could cause overtopping of the bypass in multiple locations, with water depths reaching up to 1.2 metres and exacerbating flood impacts on properties. 

While the new bridge and bypass aim to provide improved flood immunity for the road during moderate flooding events, the report reveals a delicate trade-off: “A number of dwellings have afflux impacts of typically 35mm” during certain floods, and “15 properties may have up to 83mm impact” in a 2% Annual Exceedance Probability (AEP) flood. Without mitigation, the report warns these impacts could extend to habitable dwellings, pending further floor level surveys.

“The exact number of affected properties will need to be confirmed with floor level survey and site inspections in future design stages,” the report said.

Adding to the concerns, the report highlights the gaps in the flood modelling used to inform the design. The analysis was limited to a single design storm event for each AEP, potentially overlooking other storm scenarios that could pose greater risks. As the report states: “There may be other storm events which are more critical for velocity with lower tailwater levels which may become evident during detailed design when the Hawkesbury-Nepean River Flood Study model and additional storm events become available. Interim INSW model results indicate higher velocities through the floodplain than determined through the NRBS2 assessment. This indicates there may be another critical event for velocity that results in higher velocity changes through structures which may require mitigation.”

The report also identifies higher flood velocities than previously assessed, which could lead to increased erosion and downstream impacts. “The interim model results indicate higher velocities through the floodplain,” the report states, warning that these could become critical during extreme events, requiring additional mitigation measures.

The report paints a sobering picture of the project’s vulnerability to climate change, which is expected to intensify flood risks in the region. By 2090, projected increases in rainfall intensity could cause flood levels to rise by up to 1.9 metres during a 5% AEP flood, leading to significant overtopping of the bypass road. In some locations, floodwaters are expected to reach depths of 1.2 metres, potentially rendering the bypass impassable. The report cautions that climate change poses a risk to the long-term viability of the project and notes that the current design does not adequately account for this.

Efforts to minimise the project’s impact on nearby properties have focused on carefully sizing embankments and culverts to balance flood immunity with preserving natural water flow. However, even with these measures, the report acknowledges that afflux - where floodwaters back up due to structural obstructions - is inevitable. Flood modelling indicates impacts on Western Sydney University land, particularly near the eastern relocated pond, with the report suggesting that optimised detention storage could help reduce downstream impacts, including those at Hobartville. Adjustments to the road alignment near Hill Avenue are also recommended to improve flood immunity and ensure better connectivity during emergencies. 

The report acknowledges the trade-offs inherent in the design of the new Richmond Bridge and bypass, noting that some afflux is unavoidable due to the reduced waterway area caused by bridge piers and deck submersion during major floods. However, it concludes that the benefits of the project - such as increased flood immunity and improved connectivity during larger floods- outweigh the” relatively minor” impacts on flood levels. The report states that there is not expected to be any significant increase in flood damages to properties already subject to inundation and argues that the additional cost of further minimising these impacts would outweigh the benefit of only minor reductions in afflux.

“Some amount of afflux is unavoidable with a new bridge structure due to the reductions in waterway area due to piers and bridge deck (for events where the bridge deck becomes submerged). The additional cost to the proposal in attempts to further reduce the flood level impacts would outweigh the benefit of minor reductions in afflux achievable,” it said.