20 March 2021

With such a big catchment, floodwaters from the mountains flow down the Manning channel with

great force, especially in the constrained channel of the Manning above Wingham, then spread out across the estuary where the energy is dispersed but inundation of low-lying land becomes more of a problem.

MidCoast Council’s approach to flood management is set out in the Manning River Floodplain

Risk Management Study and Plan (BMT WBM 2019). The plan covers the low-lying floodplain area downstream of Wingham. The study addresses both mainstream flooding of the Manning River and the impact of climate change in the form of increased rainfall intensities and sea level rise for the following scenarios: • Predicted increased rainfall intensity: modelled 10% and 30% increased rainfall • Sea Level Rise (SLR): +0.28 m by 2050; and +0.98 m by 2100. The Floodplain Risk Management Study (FRMS) has derived an appropriate plan of measures and strategies to manage present and future flood risk in accordance with the NSW Government Floodplain Development Manual. These include flood modification measures, property modification measures, risk modification measures and emergency measures (e.g. evacuation, sandbagging). MidCoast Council works closely with the NSW State Emergency Service to establish flood triggers for emergency situations. The FRMS has also identified a Flood Planning Area for the Manning River floodplain. Development of land within the Flood Planning Area is restricted and controlled by Council due to the hazard of flooding. In defining the Flood Planning Area in the MidCoast LGA, Council has considered a future flood scenario that has accounted for climate change in the form of increased rainfall and sea level rise in a combined riverine flooding and high tail water scenario to the 1% Annual Exceedance Probability (AEP, 1 in 100-year event) plus a freeboard of 500mm. The Manning River Floodplain Risk Management Study and Plan is therefore the appropriate management tool for flooding within the geographical extent of this ECMP. Related Issues: erosion and sedimentation; climate change Stakeholders: MidCoast Council; NSW State Emergency Service; Adapt NSW; Department of Planning, Industry and Environment, residents, farms and businesses on the floodplain.

Figure 33: The Manning River floodplain, 20 March 2021

Photo: Evan Vale

6.10.2 Coastal Inundation

Coastal inundation is the flooding of coastal

management areas by ocean waters, occurring “when a combination of marine and atmospheric processes raises ocean water levels above normal elevations and inundates low-lying areas” (Coastal Management Manual 2018).

Coastal inundation on the NSW coast is most

often associated with east-coast lows (Heimhuber et al 2019). It is typically a short-term event with waters receding to normal conditions. Coastal inundation from storm surge in the Manning River estuary is caused by various processes including low barometric pressure, strong onshore winds, high tides, and trapped coastal waves.

Coastal inundation in the Manning generally affects the open coast and low-lying areas near

the entrance, such as Harrington and Manning Point which are outside our Planning Area. For the Manning River ECMP Planning Area, threats associated with coastal inundation include rising groundwater, impacts on coastal wetlands and vegetation, inland estuary flooding and damage to riverbanks and infrastructure.

The Manning River Floodplain Management Study (BMT WBM 2019) considers flood events driven by both catchment and oceanic processes, addressing the current probability and future scenarios under climate change as shown above. Storm surge is factored into tail-water levels.

In general, coastal inundation causes more

frequent nuisance flooding while riverine flooding is less frequent but causes more damage. Impacts include reduced efficiency of stormwater infrastructure and increased groundwater levels. Modelling in the MRFMS (2019) shows that in storm events, the impact of water across land from high rainfall and riverine flooding will be more significant than the impact of coastal inundation. The flood mitigation, planning and emergency response measures set out in the MRFMS and Plan (2019) should therefore provide an effective management approach to coastal inundation. For more information see Section 10.

6.10.3 Tidal Inundation

Tidal Inundation or nuisance flooding is defined as “the inundation of land by tidal action under average meteorological conditions” (Coastal Management Manual 2018). It causes short term nuisance flooding in low-lying coastal areas.

Tidal inundation is mapped and managed through the Manning River Floodplain Risk Management

Study and Plan (BMT WBM 2019), using the High High-Water Spring tidal signature provided in the Flood Risk Management Guide (DECCW 205) for locations south of Crowdy Head as the ocean water level boundary. Tidal inundation impacts on infrastructure such as roads, housing and stormwater systems are addressed in the Climate Change section of this document and the Climate Change Issue Paper provided as a supporting document.