Raising the Lismore levee under investigation
RAISING the Lismore CBD levee is one of four options to be investigated to help protect the town against flooding in the future.
Lismore's Floodplain Management Committee met last week to discuss the findings of a Floodplain Risk Management Study, which identified four options for further exploration by Engeny Water Management - raising the CBD levee, raising the South Lismore levee, extending the South Lismore levee and excavating Wilson River bends.
After the 2017 flood, Rous County Council engaged consultants to undertake the study as the first stage of a review of Lismore's ongoing strategy for managing flood.
A report tabled at the committee meeting looked at a variety of structural mitigation works that would protect homes and businesses in Lismore in the event of major flooding.
Option 1 looks at the impact of raising the CBD levee
This option looks at the impact of raising the CBD levee, which measures 12.2m AHD at the northern end and 11.4m AHD at the southern end, by 200-400mm.
The report shows this new levee height would not have been overtopped by the 2017 flood
but would have been breached by major floods in 1974, 1954, 1889 and 1880.
Raising the levee height was expected to offer flood protection to a large portion of the CBD if the levee was not overtopped, but an extension of the levee wall length was required to fully protect the CBD from the Wilsons River.
The CBD would still be impacted by local water sources and the higher levee wall could raise flood levels by up to 90mm in South Lismore and North Lismore, as well as increasing flood levels in Boatharbour and approaching Tucki Tucki.
In the event of a flood that overtopped the levee, the raised height would at least offer more time to evacuate and would reduce flood levels in the CBD.
Option 2 looks at the impact of raising the South Lismore levee
This option looks to raise the South Lismore levee, which varies in height from 12.4m AHD at the northern end to 11.8m AHD near Hollingworth Ck, by between 200-500mm.
It would reduce flood levels in South Lismore by 40-400 and offer full protection of properties west of Caniaba St behind the levee in the event of a flood that did not overtop the new levee height.
Properties on the south bank of Leycester Ck and North Lismore could experience a 15mm rise of flood water.
Work would be needed on the Bruxner Hwy, which acts as an hydraulic control, to provide relief to properties impacted west of the highway.
In a flood that overtopped the levee, reductions in flood level would extend to Gundurimba and in South Lismore a drop of 30-190mm would offer the potential of more time to evacuate.
Flood water levels in North Lismore would rise by 20mm.
Option 4 looks at extending South Lismore levee
This option would extend the South Lismore levee, with heights of between 12.2-14m AHD, terminating at Crown St.
The extension would offer protection to isolated properties in South Lismore, to varying effect in major flooding.
Option 6 looks at excavating Wilson River bends
This option looks at excavating the river bend north of Bunnings in South Lismore by approximately 1-3m to 5.6m AHD.
It looks at excavating the river bend south of Bunnings by approximately 1-3m to 4.9m AHD.
This work would result in decreased flooding in South and North Lismore as well as reducing flood levels through to Boatharbour.
It would require work on the Bruxner Hwy and would increase flood levels south of Three Chain Rd.
Once the study is complete the committee will look at updating the Floodplain Risk Management Plan.