Concrete on Flood Prone Land: What Hawkesbury Owners Should Check Before Pouring

Plenty of Hawkesbury acreage sits on the floodplain, and plenty of it still needs a shed slab, a hardstand or a long driveway. Floodwater changes how that work is approved and how it should be built. Raising the ground with fill is restricted, exempt earthworks are off the table, and the concrete has to cope with water running over and around it. This guide explains how to find out whether your land is flood affected, what council’s flood rules say about fill, and what to sort out before you ask for a quote.

Why flooding changes a concrete job in the Hawkesbury

The NSW Reconstruction Authority’s Hawkesbury-Nepean River flood study overview says the valley has the highest flood risk in NSW, if not Australia. The study estimates a flood with a 1 in 100 chance each year would reach 17.3 metres at Windsor. The 1867 flood reached 19.7 metres there, which the Authority notes was nearly 6 metres higher than the July 2022 flood. The overview describes a bathtub effect that causes rapid, deep and widespread flooding.

Concrete itself copes with water better than most materials. The Hawkesbury-Nepean guidance on building in flood prone areas describes a slab on ground as generally undamaged by immersion. It also warns that a slab can suffer scouring and undermining, and that foundation soils can be eroded under slabs and footings. The risk is in the ground under and around the concrete, so that is where the planning goes.

How to find out if your land is flood affected

Council holds most of what you need. Get it before anyone prices the job.

A survey matters more on flood land than anywhere else. Every rule below is measured against levels, and an eyeballed level is no use to a certifier.

What council’s flood rules say about fill and earthworks

Under clause 5.21 of the Hawkesbury Local Environmental Plan 2012, consent can’t be granted for development in the flood planning area unless council is satisfied it is compatible with how floodwater behaves on the land and won’t worsen flooding for other properties, among other tests. Council applies that through its Flood Policy 2020 and a schedule of flood related development controls. The schedule is where the limits on fill sit.

• Hazard categories H1 and H2. Imported fill and excavation are not permitted, other than a balance of cut and fill to create a level building platform or driveway access, up to 1 metre of cut or 1 metre of fill. Fill up to 0.3 metres is allowed for a slab on ground within a drop edge beam, at or above the flood planning level.
• Hazard categories H3 to H6. Fill and excavation are not permitted, other than for environmental protection works, bank restoration or boat ramps.
• Floodways and flow paths. Development is not permitted, other than open style fencing, or farm buildings and other structures ancillary to agriculture where it is shown they won’t increase flood effects elsewhere.

The quick route is also closed. Council’s earthworks page says that if land is subject to flooding, you can’t carry out exempt earthworks under the state Codes SEPP, which excludes flood control lots from its earthworks exemption. On most flood-affected blocks, the cut, fill and levelling under a slab goes through an approval first.

Designing a slab or driveway to sit in floodwater

Once the approval path is clear, the design follows the same few rules.

1. Work from the surveyed flood planning level. Floor levels and fill depths are set against it, so the survey comes before the set-out.
2. Keep cut and fill in balance. The NSW Flood Risk Management Manual notes that when flow paths are blocked by development or fill, water can find new paths, and those can run across a neighbour’s land.
3. Use flood compatible materials. Council’s schedule requires them for buildings and structures in the flood area. Its flood compatible materials guide lists a reinforced concrete slab as suitable flooring and says the foundation must not be prone to failure from erosion, such as when located on fill.
4. Get engineering where council asks for it. The schedule says farm buildings in hazard categories H5 and H6 need engineering design or certification showing they can withstand flooding.
5. Plan the way out. Where access is below the flood planning level, the schedule requires a site flood emergency response plan.

For a machinery shed or hay shed slab, that usually means a level set from the survey, edges detailed to resist scour, and an apron that sheds water instead of holding it. A long rural driveway crossing low ground needs the same thinking at every dip.

What to have sorted before the quote

A concreter can only price flood land properly once the levels and the approval path are known. Without them, the quote is a guess about how much fill is allowed and how high the slab has to sit.

On acreage jobs across the Hawkesbury, the earthworks and the concrete are priced together, because on flood land one decides the other. Council’s schedule may have been updated since the version linked here, so confirm the current controls with council before you commit to a design.

Have these ready

• Flood Advice for the property
• Section 10.7 planning certificate
• Survey with ground levels
• Hazard category for the work area
• Approval path confirmed with council
• Shed plans and any engineering