Raising one Lismore spillway by 750 millimetres would keep the CBD dry in the floods that come most often. The model also says where the water goes instead

What the committee was asked to do

The Flood Risk Management Committee is a council committee of three councillors and community members, with Rous County Council in the room. Its 30 September meeting was an extra one, added by the committee itself in July. The first item asked it to note BMT's Browns Creek Levee Spillway Raising Feasibility Study, and then to:

Endorses the preferred option to raise the Browns Creek Spillway by 750 mm to RL 11.70 m AHD, subject to detailed design, approvals and funding requirements

and to support adding the project to the implementation schedule of the 2024 Lismore Flood Risk Management Plan, with timing subject to funding. A fourth part Recommends targeted community consultation be undertaken with potentially impacted landowners and communities before the measure goes into the plan.

None of that is a decision to build. The committee recommends to council, and the agenda says the preferred option is conditional on design, approvals and money. Whether the committee endorsed it on the day is not yet public: the minutes had not been published when we checked on 4 October, and no October meeting papers were listed on the council's meetings portal.

Why a spillway, and why this one

Lismore's CBD levee, built to its current shape in 2005, has two deliberate low points, at Browns Creek and at Gasworks Creek. They are there so that when the Wilsons River rises above them, water spills over in a controlled way rather than pouring over the whole wall at once. Browns Creek is the upstream one and, at 10.95 metres AHD, the lower one, which the council's report describes as the problem: This means the Browns Creek spillway commences overtopping prematurely during a flood compared with the Gasworks Creek Spillway.

The opening is practical. Council is already replacing the Browns Creek pump station, work the NSW Reconstruction Authority lists under its $22.56 million combined pumps and pump stations project for Lismore, and rebuilding part of the spillway with it is the chance to set it higher. BMT's report records that Since construction in 2005, the CBD levee has been overtopped by Wilsons River floodwater on three occasions: March 2017 at 11.59 metres, February 2022 at 14.36 metres and March 2022 at 11.40 metres. Two of those three peaks sit below the proposed new crest.

Five heights, eight floods, one recommendation

BMT ran five spillway heights through eight floods: five design events (10, 5, 1 and 0.2 per cent AEP, plus the 1 per cent with 2090 climate change) and three historic ones. The council's staff recommend Option 3, the full-length raise to 11.70 metres, because based on BMT's results it can achieve the maximum net benefit in terms of reducing property damages with the existing levee whilst maintaining the spillway function at Browns Creek. Option 5, a raise to the full 12-metre levee height, does slightly better on the numbers but removes the spillway altogether, which the report does not want.

BMT, Table 6.1, counts including buyback properties. Net figures: BMT warns that A degree of caution needs to be applied when interpreting Table 6.1 because some buildings dry in the base case become wet under the options.

On the CBD side the result is stark. For Option 3, the whole of Lismore CBD is now dry in the 10% AEP event as the raised spillway (at 11.7mAHD) is higher than the 10% AEP flood level in the Wilsons River. In the 5 per cent event the Dawson Street gauge in the CBD peaks 2.85 metres lower. Overtopping at Molesworth Street starts 6.75 hours later than it does now, and two low points on the CBD evacuation routes, Conway and Keen Streets, do not flood at all in that event. Overtopping still occurs under all options but the reduction in overtopping volume delays the rate at which the CBD basin fills resulting in delays in inundation of evacuation routes.

What it does not do

This is the sentence to hold onto, in BMT's words: the larger floods, meaning the 1 per cent, the 1 per cent with 2090 climate change, the 0.2 per cent and February 2022, show no notable change in flood behaviour under any modelled spillway option. At those heights the river is well over the whole levee and a spillway crest is irrelevant. The February 2022 row in the table above does not move. A town that has just lived through the record flood is being offered a measure for the floods it has every few years, not that one.

Where the water goes instead

A levee that lets less water into the CBD keeps more of it in the river. In the 5 per cent AEP event BMT finds increases of up to 0.08 metres in peak level elsewhere, and These increases extend across urban parts of North and South Lismore. The council's report puts the choice plainly:

any choice of option will be a trade-off between the significant benefits to the CBD area and the more minor but spatially extensive increases in peak level elsewhere due to the loss of flood storage as a result of the spillway raising options.

The property-level count makes it concrete. In the 5 per cent AEP event under Option 3, ten properties go from dry to wet above floor level and 243 see increases of 50 to 100 millimetres, against 252 that go from wet to dry and 198 whose levels fall by more than half a metre. Those figures exclude buyback properties. In the 10 per cent event the pattern is sharper: 296 properties dry out, 4 get wet for the first time, and 108 rise by 10 to 50 millimetres.

Who is on the losing side is also stated. The economic model finds there are properties which are worse off (i.e. they have a higher AAD) as a result of each option, and These properties are primarily located in North and South Lismore where flood impact mapping shows increases in peak water levels. Separately, when BMT checked the twenty properties that already carry the highest annual flood damage in Lismore, all were commercial and The majority (16) of the properties are located in North Lismore, outside the levee. The parts of town that flood most often now are the parts that would take a little more. BMT says that Further investigation should be undertaken for properties identified as experiencing increased flood damage if the spillway raising options are progressed, and the committee recommendation's call for consultation with potentially impacted landowners is the same point made from the other side.

Our reading, and it is an opinion on the figures above: a few centimetres is small against a flood that comes over a windowsill, and BMT is right that the net benefit is large. But the people asked to absorb it are not a random sample of Lismore. Consultation with them is in the recommendation; what they can do with the result is not.

The money

The raise is cheap by flood-infrastructure standards. Council's quantity surveyors put Option 3 at $1,286,642 in capital cost, and BMT notes Maintenance and operational costs are not available. Against that, the NSW flood-damage tool estimates Lismore's average annual flood damage, excluding buyback properties, falls from $45.27 million to $31.47 million, a benefit of $13.8 million a year. Over 50 years at the Treasury's 5 per cent discount rate the benefit-cost ratio is 196.

Two things temper that. Commercial buildings carry the damage: In events more frequent than the 1% AEP event, commercial damages account for more than 80% of all flood damage, and BMT had no size or value data for them, so it used the tool's averages. Rerun with every commercial building set to small and low-to-medium value, the ratio falls to 74. At a 7 per cent discount rate it is 148. Either way it clears any reasonable bar, and the report's limitations are explicit: The assessment does not take into account allowances for future climate change, it models river flooding only, and Local rainfall may exacerbate flooding conditions from that modelled.

One line in the document control is worth recording. BMT's disclaimer states that Some of the content of this document may have been generated using the assistance of Artificial Intelligence (AI). It does not say which content. We have no reason to think it affects the modelling, which uses the council's existing Wilsons River model, but a reader relying on the report should know the sentence is there, as they should know the report says No third party may rely on the contents of this report.

Molesworth Street is part of the levee now

A detail that will matter at the design stage. To make any of the five options work in the model, the raised crest had to be carried across Molesworth Street to high ground, otherwise water simply goes round the end. BMT: As this report is a hydraulic assessment only, the feasibility of raising this road or constructing a flood gate across the road would need further assessment. Options 2 and 4, which shortened the spillway, were dropped from the costing because it was identified that maintaining the full length of the spillways was preferred by Council to minimise the impacts on flood heights.

The rest of a dense agenda

Four other items sat behind the spillway on 30 September, and together they show how much of Lismore's flood future is in motion at once.

A new flood planning benchmark. Council is rewriting its flood development controls and asked the committee which flood to design to. Today The current planning benchmark utilises the 1% Annual Exceedance Probability (AEP) flood event with climate change projections to 2090 and a 0.5 metre freeboard; the options put to the committee include the 0.2 per cent event and the recent floods themselves. Whatever is chosen sets floor levels for every new building on the floodplain.

Detention basins, recalculated. At the committee's request Rous County Council re-estimated three upstream detention sites proposed by community member Graham Askey. Lower Terania Creek came out roughly as proposed; the report says the estimated storage volumes for the Lower Leycester Creek and Lower Wilson Creek sites were significantly lower than originally proposed: about 50 gigalitres against 300 proposed at Leycester, about 40.7 against 200 at Wilson. Rous attaches a long caveat that its figures are desktop estimates which must not be relied upon for decision-making purposes.

The Richmond River study has no plan yet. The big catchment-wide question, whether the CSIRO's upstream detention bundles are feasible, now belongs to the Reconstruction Authority's Richmond River Technical Feasibility Study. Council staff told the committee that Neither a project plan or communication plan are available for the Technical Feasibility Study at the current point in time. The Authority's own page says a consultancy will be appointed in late 2026 and more information is expected towards the end of the year.

Buybacks before more buybacks. The plan's measures to extend voluntary purchase and house raising beyond the Resilient Homes Program are parked: each is to be considered once RHP funding for the Lismore LGA concludes. The Authority's progress page, updated 30 September, puts the program at 993 buyback offers made, 910 accepted and 853 settlements completed across the Northern Rivers as at 31 August 2026. And council and the Authority are now asking what to do with the land: a consultation on the future use of more than 70 hectares of buyback land across four Lismore precincts opened on 28 September and runs to 1 November 2026, with the page stating that It is important to note that the land will not be used for new housing.

One small item has already happened. Two flood-warning sites installed last year at Rocky Creek Dam and Ballina Street Bridge were onboarded by the Bureau of Meteorology on 17 September 2026 and their rainfall and river height data is now publicly available through the Bureau's website, though they are not yet part of the Bureau's formal forecasting service.

What to watch

The committee's minutes, which will show whether the 750 millimetre option was endorsed and on what vote. Then the recommendation's arrival at a council meeting, where the decision actually sits. Then, if it proceeds, the consultation with the North and South Lismore landowners whose water the model says will rise.

Sources

The working. AI read the documents and drafted this story, under rules people set and answer for, working from the primary sources below. How we work.

1. Lismore City Council, Agenda, Lismore Flood Risk Management Committee, 30 September 2026 (PDF, 31 pages, downloaded and read 4 October 2026): the four-part recommendation at item 4.1 and the staff reasoning for the 750 mm option, including the premature-overtopping sentence and the trade-off passage; item 4.2 on the Interim Defined Flood Event and the current 1% AEP 2090 benchmark with 0.5 metre freeboard; item 4.3, Rous County Council's recalculation of the three detention sites with its figures and caveat; item 4.4 on the Richmond River Technical Feasibility Study and the absence of a project or communication plan; item 4.5, the plan implementation table including the PM02 and PM03 timing and the Bureau of Meteorology onboarding of the two warning sites on 17 September 2026; and the 29 July 2026 minutes recording the committee's membership and its resolution to add the 30 September meeting.
2. Lismore City Council, Attachments excluded from the agenda, Flood Risk Management Committee, 30 September 2026 (PDF, 142 pages, downloaded and read 4 October 2026), containing BMT, Browns Creek Levee Spillway Raising Feasibility: Flood Impact Assessment and Cost Benefit Assessment, version 01, 17 September 2026. Source for the five options and eight events, the three overtopping dates and heights, Table 5.8 (the 2.85 metre reduction at the Dawson Street gauge and up to 0.08 metre increases), Table 5.13 (the 6.75 hour delay and the two unflooded evacuation points), Table 6.1 (building counts above floor), Table 6.5 (impacted-property counts for Option 3), Tables 6.8 and 6.10 (average annual damage and benefit), Table 6.15 (costs), Tables 6.16 to 6.19 (benefit-cost ratios of 196, 148 and 74), the top-20 property check, the Molesworth Street passage, the climate-change and local-rainfall limitations, the AI-assistance and third-party disclaimer sentences.
3. NSW Reconstruction Authority, Northern Rivers: Our progress (web page, last updated 30 September 2026, read 4 October 2026): Resilient Homes Program figures as at 31 August 2026 (993 offers made, 910 accepted, 853 settlements completed), and the Lismore City Council entry under the Northern Rivers Recovery and Resilience Program listing combined upgrades to pumps and pump stations at $22,563,809.
4. NSW Reconstruction Authority, Richmond River Technical Feasibility Study (web page, last updated 3 September 2026, read 4 October 2026): the study's scope, the advisory group membership, and the statements that a consultancy will be appointed in late 2026 and more information is expected towards the end of 2026.
5. Lismore City Council, Have your say on the future use of buyback land (Your Say Lismore page, last updated 29 September 2026, read 4 October 2026): the four precincts, the more than 70 hectares, the statement that the land will not be used for new housing, and the engagement window of 28 September to 1 November 2026.

Live in North or South Lismore, sit on the committee, or read BMT's tables differently? Tell us and we will check it against the council's own papers and log the outcome.