title: Inverleigh Flood Mitigation Assessment council: golden-plains state: vic category: constraint classification: MINOR status: unknown last_compiled: 2026-05-30 source_docs:
- Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf
- Council Meeting Agenda 22.04.2025.pdf
Inverleigh Flood Mitigation Assessment
The Inverleigh Flood Mitigation Assessment is a local drainage-risk investigation for the northern Inverleigh low-density residential area around Common Road, Faulkner Road, King Road, Argyle Park Court and Gregory Drive. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.7) Its practical planning significance is not township-wide land supply, but the design, legal and implementation choice between moving floodwater through road reserves, private drainage easements, or a receiving waterway. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.62-65)
Background
Golden Plains Shire Council previously commissioned Water Technology to undertake flood and drainage investigations for the area, with reports issued in 2021 and updated in 2023 after a significant November 2022 rainfall event. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.7) The Water Technology work found that the local drainage network had limited capacity because of limited system capacity, lack of maintenance and flat grades, and that the network had been exceeded often enough to inundate private properties and in some instances flood above dwelling floor level. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.7)
The Loetis assessment was commissioned to test preferred mitigation options in more detail, including feasibility, functional design, quantities, cost estimates and staging. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.8-9) The assessment area is in northern Inverleigh, centred on a shallow north-south natural depression through Gregory Drive, Argyle Park Court, Faulkner Road and King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.9) The area is zoned Low Density Residential and the report states it is not subject to drainage-related planning overlays. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.9)
The wider planning scheme context is more complicated than the project-area overlay position alone. (Source: Council Meeting Agenda 22.04.2025.pdf, p.8) Council’s April 2025 agenda states that Amendment C80gpla introduced Flood Overlay and Land Subject to Inundation Overlay controls to Inverleigh in September 2019, but an administrative error removed associated schedules across the shire when the amendment was gazetted. (Source: Council Meeting Agenda 22.04.2025.pdf, p.8) That means the mitigation assessment should be read as a local works feasibility study, not as a complete planning-controls review for all Inverleigh flood risk. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.9; Source: Council Meeting Agenda 22.04.2025.pdf, p.8)
Analysis
The Core Mechanism: The Problem Is Not Only Storage, It Is Where Water Is Sent
The assessment shows that the binding issue is the direction and conveyance of stormwater through a developed low-density residential catchment, not simply the absence of one larger basin. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.7, 62-65) Earlier minor works and maintenance had only partly reduced flood risk and had limited further capacity to reduce flooding because of topography and the existing residential layout. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.7)
This matters because every main option changes who receives floodwater. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.17-18, 35-36, 44-45) Option 1 lowers road levels so overland flows are redirected toward King Road, which improves conditions for some properties but increases flood levels and extents downstream on private land. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.15-18, 50-52) Option 3a captures flows into a large pipe along Faulkner Road and discharges them toward the existing waterway near Common Road, which reduces the private-land afflux issue but requires easement acquisition and has blockage risk. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.43-45, 52-57, 63-65)
The report’s most important planning implication is therefore a liability and consent question rather than a narrow engineering preference. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.64-65) Loetis concluded that Option 1 scored better overall than Option 3a, but the report expressly excluded the legal implications of increased flood extents and depths on downstream private property and the net public impact of the alternative arrangements. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.64-65)
Option 1: Lowering Argyle Park Court, Faulkner Road and King Road
Option 1 lowers the intersection of Argyle Park Court and Faulkner Road to direct overland flows into King Road and reduce or prevent flows overtopping Faulkner Road into private property. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.13) The Water Technology concept used about 300 mm of lowering at the Argyle Park Court and King Road intersection, increasing to about 700 mm at the Faulkner Road and King Road intersection. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.15)
The construction footprint is material for a local road project. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.16-17) The report estimates about 2,000 square metres of road pavement would need partial or full removal and replacement, with likely driveway impacts at 44 and 45 Faulkner Road and lesser impacts at 56 Faulkner Road and 3 King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.17) The works would also likely require relocation or lowering of electrical, communications and water services at the Faulkner Road and King Road intersection. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.16)
The main benefit of Option 1 is deliverability. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.21, 63-64) It requires no easement or reserve acquisition, requires none to minimal vegetation removal, has an estimated cost of 1,330,950 excluding GST and easement acquisition, and has a delivery timeframe of less than 12 months in the comparison table. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.63-64) The earlier preliminary estimate for Option 1 was 1,464,039 excluding GST, before the later functional-design cost comparison. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.18)
The main drawback is downstream redistribution. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.17-18, 50-52, 64-65) The modelling showed no flows overtopping the southeastern Faulkner Road footpath under the Option 1 design, but also showed increased flow depths and extents along King Road and downstream properties. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.49-51) Loetis considered that most increases for King Road fronting properties could likely be resolved through detail design of culverts and swales, but residual increases on downstream Lot 3 TP668795 on Common Road would not be possible to design out. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.50-51)
Options 2, 2a and 2b: Basin Enlargement Is Constrained by Land, Levels and Vegetation
Option 2 proposed combining the existing Manna Gum Estate Stage 2 basin at 120 Gregory Drive and the Barrabool Views Estate Stage 2 basin at 130 Gregory Drive into one enlarged basin. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.22) The report records nominal design volumes of 760 cubic metres for the 120 Gregory Drive basin and 1,245 cubic metres for the 130 Gregory Drive basin, while also noting that Water Technology estimated about 3,400 cubic metres for the existing basins from LiDAR-based surface volumes. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.22)
The basin option is constrained by the floor level of the dwelling at 120 Gregory Drive, the incoming drain levels behind 102 and 110 Gregory Drive, and the need for freeboard. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.23, 27) To reach about 6,000 cubic metres of storage at the assumed levels, Loetis estimated a basin footprint of about 6,500 square metres and an easement area of about 10,000 square metres, compared with the existing collective easement area of about 5,450 square metres. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.27) The report therefore found that meaningful basin enlargement would require significant additional easement or reserve acquisition. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.27-29)
Option 2 was estimated at $1,230,000 excluding GST and excluding acquisition costs, but Loetis concluded that minor basin outlet improvements were unlikely to produce significant downstream impact for the construction cost. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.28-29) Option 2 was not taken forward into the later functional design phases. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.62-63)
Option 2a added a western outfall from the Gregory basin system toward Common Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.30) Its alignment crossed multiple private properties, areas with Environmental Significance Overlay Schedule 4 associated with Clover Glycine protection, drainage easements, mapped waterway sections and areas without existing easements. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.31-35) The report estimated Option 2a at $1,780,000 excluding GST, excluding easement or reserve acquisition costs, tree clearing, permits and offsets. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.36)
Loetis found Option 2a was not feasible because it would require significant easement acquisition and would significantly affect existing vegetation. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.39) The report identified Option 2b, a piped route along Argyle Park Court and Savage Drive, as a technically more attractive alignment because it could stay largely in council-controlled road reserves and reserves. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.36-39) Option 2b had an indicative cost of 2,040,000 excluding GST, or 1,290,000 if Gregory basin works were removed, with a further $280,000 estimated if the full-size pipe continued all the way to the Savage Drive waterway. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.36-37) Despite those merits, Option 2b was not taken forward into the later functional design phases. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.62-63)
Option 3a: Higher Cost, Lower Private-Land Afflux, More Approval Dependency
Option 3a would capture flows near Argyle Park Court and Faulkner Road, convey them southwest along Faulkner Road in an underground pipe, and discharge them into the existing waterway behind 119 and 135 Common Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.43) The design also includes lifting the low southeastern side of the Faulkner Road footpath to prevent overtopping during large rainfall events. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.40, 52-57)
The functional modelling found that a 1,500 mm diameter pipe would be required along Faulkner Road, reducing to twin 1,200 mm pipes at the Argyle Park Court inlets. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.52) The likely maximum pipe depth near the Common Road crossing is about 2.5 m, and Loetis considered it likely able to pass beneath existing underground services without relocation, although protection works may still be needed during construction. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.52)
Option 3a is more expensive than Option 1. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.63) The final comparison table gives Option 3a a cost of 1,728,780 excluding GST and easement acquisition, compared with 1,330,950 for Option 1. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.63) The earlier preliminary estimate for Option 3a was $1,728,000 excluding GST and excluding acquisition, tree clearing, permit and offset costs. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.45)
The reason Option 3a remains a serious alternative is that it sends water to the receiving waterway rather than increasing flood depths across downstream private land outside drainage easements. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.44-45, 55-56, 64-65) The model showed increased depths of about 20 to 30 mm in the waterway parallel with Common Road, but the inundation extent did not appear to alter and the affected area was within an existing drainage easement with existing flooding greater than 1 m and away from dwellings. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.55)
The planning trade-off is that Option 3a needs a private easement or reserve acquisition to connect Common Road to the waterway. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.43-44, 63-64) Loetis assessed its delivery timeframe as 6 to 18 months, depending on the easement acquisition process, compared with less than 12 months for Option 1. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64) Option 3a also carries a risk of blockage and reduced effectiveness in events larger than the design event, whereas the comparison table records no equivalent blockage risk for Option 1. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64)
Residual Flooding at King Road Is a Separate Local-Catchment Problem
Both Option 1 and Option 3a prevent external catchment water entering properties from Faulkner Road, but both leave residual flooding around 14, 16 and 22 King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.57-59, 64-65) Further model interrogation found that these residual flows originate from the local catchment within the area bounded by King Road, Faulkner Road and Common Road, rather than from upstream catchment flows. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.57)
This is a material implementation issue because road-reserve works alone are not expected to fully resolve flood exposure for those dwellings. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.57-59) Loetis concluded that at least part of the residual mitigation works would need to be undertaken within private properties. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.57) Concept swale works within 16, 22 and 30 King Road were modelled, and the report states that residual flooding around 14 and 22 King Road was significantly alleviated, while flood impacts remained around 16 King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.59)
The cost of the private-property swale drainage works was not fully estimated, but Loetis anticipated the drainage works themselves would be well below $50,000, excluding possible costs associated with vegetation, existing assets and final alignments. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.59) The report also states that additional options such as pumped systems may need investigation to obtain mapped flood immunity for 16 King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.59)
Decision Logic
The weighted assessment gave Option 1 a total score of 18 and Option 3a a total score of 22, with lower scores indicating more favourable outcomes. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64) The report states that Option 1 scores better, but not significantly better, than Option 3a. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64)
The scoring reveals the real decision structure. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64) Option 1 scores strongly on cost, no easement acquisition and delivery timeliness, but poorly on increased flood extents and depths on private property. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64) Option 3a scores poorly on cost, easement acquisition and blockage risk, but strongly on limiting increased flood extents and depths to the existing waterway. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.64)
From a planning perspective, the unresolved threshold question is whether Council is prepared to accept a lower-cost, quicker option that creates residual downstream private-property afflux, or a higher-cost option that is slower and needs land acquisition but better confines downstream impacts to a waterway context. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.63-65)
Current Status
The Loetis Rev03 report was issued on 6 June 2024 and represents completion of Phase 3 of the feasibility, functional design and drainage strategy assessment scope. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.1, 9) The source set does not include a council resolution selecting Option 1, Option 3a or any hybrid option. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.64-65)
Dependencies
- Blocks: A preferred local flood-mitigation works package cannot proceed to detailed design, approvals, procurement and construction until Council resolves the legal and public-impact implications of the downstream afflux question. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.64-65)
- Blocked by: Option 1 is blocked by legal advice on increased flood depths and extents on impacted private properties, while Option 3a is blocked by easement or reserve acquisition for the Common Road to waterway connection. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.18, 44, 63-65)
- Informed by: The assessment relies on Water Technology’s 2021 Inverleigh Drainage Assessment, the 2023 Inverleigh Investigation for Manna Gum Estate, LiDAR surface information, subdivision design plans, BYDA service information, site inspection and council officer advice. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.11)
- Implements: The works would implement a local flood-risk reduction response for properties affected by repeated drainage exceedance in northern Inverleigh. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, p.7)
- Conflicts with: Option 1 creates a potential conflict between lower-cost public works delivery and increased flood depths or extents on downstream private land. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.50-51, 64-65)
Cross-Jurisdictional Links
The source material does not identify cross-council infrastructure dependencies for the Inverleigh works. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.7-11) It does identify Barwon Water and Powercor assets in relevant road reserves and easements, which creates utility coordination requirements for any detailed design and construction package. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.11, 16, 41-44)
The broader flood-planning context involves the Corangamite Catchment Management Authority as floodplain management authority for flood advice in Golden Plains Shire, as shown in the April 2025 council agenda for the Teesdale flood amendment. (Source: Council Meeting Agenda 22.04.2025.pdf, pp.6-7) The Inverleigh mitigation source set does not include a CCMA decision, referral response or approval pathway for the proposed Inverleigh works. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.7-11)
Gaps in This Analysis
The primary gap is the absence of the Water Technology reports that underpin the Loetis assessment. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.7, 11) Without the 2021 Inverleigh Drainage Assessment and the 2023 Manna Gum Estate investigation, this page cannot independently verify the base flood model, design-event assumptions, model calibration, property-by-property flood-depth changes or the original mitigation option screening. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.7, 11, 57-59)
A second gap is the absence of legal advice on downstream afflux and private-property liability. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.18, 36, 45, 64-65) This is not a minor supporting document because the report’s preferred engineering option depends on whether increased flood depths and extents on downstream private land are legally and publicly acceptable. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.64-65)
A third gap is the absence of a council decision after the June 2024 Rev03 report. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.1, 64-65) The current source set supports analysis of the engineering assessment, but not confirmation of the adopted option, funding status, delivery program, land acquisition status or detailed design commencement. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.62-65)
A fourth gap is the absence of feature survey and floor-level survey for the private-property residual flooding around 14, 16 and 22 King Road. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.57-59) Loetis states that the modelling in that local area is at or beyond the accuracy available from existing LiDAR data, especially because post-2022 local earthworks, bunds, levees and open drains are not reflected in the modelled surface. (Source: Att 08.12 Inverleigh Flood Mitigation Assessment - V03.pdf, pp.57-58)