Flood risk on the Clarence: what a shipping container will and will not do

The Clarence is a system, not a river

The catchment gathers off the ranges behind Coutts Crossing and Glenreagh, comes past Grafton and South Grafton, then spreads across a very large area of low country. Down at Ulmarra, Maclean, Yamba and Iluka it is wide, slow and tidal, so what happens there depends on rain that fell days earlier a long way inland, and on the tide when the peak arrives. Two properties twenty minutes apart can behave nothing like each other, which is why we ask for an address rather than giving a general answer.

The events people measure against are January 2013, when ex-Tropical Cyclone Oswald put the river up and Grafton was told to get out, and the wet run through 2022. Most landholders on the flats know exactly what those did on their own ground, to the fence post. That beats anything we can tell you from a yard.

What the watertight claim is actually about

Cargo-worthy units leave here checked wind and watertight, and the phrase has a narrow meaning worth understanding before you lean on it. The check is unglamorous: somebody climbs in, pulls the doors to, lets their eyes adjust, and looks for pinholes along the roof line, down the corner posts and around the door aperture. Anything showing light is repaired or rejected.

The claim covers weather from overhead — rain, hail, spray driven sideways off a paddock. It covers nothing else and was never meant to. As-is units are a separate matter: they are not sold watertight at all, they are sold exactly as they stand, and we tell you what is wrong with one before you buy rather than after.

Where rising water gets in

A flood works upward, and the underside of a container is where its sealing is weakest. The door gaskets are made to shed rain running down a shut door; they sit at the base of the aperture, they are not a submersible gasket, and water reaching that height goes past them. Most 20ft boxes also have forklift pockets through the base, which are open channels by design. None of that is a defect. It is what the thing was built for.

Once water is inside, the problem changes character. A shed lets it run out the way it came in; a welded steel box holds onto it, so the contents stay submerged far longer than the peak outside lasted.

Steel floats, and that is the serious part

Try to push a sealed 20ft under and it fights you. The shell weighs a couple of tonnes; the volume inside would hold something like thirty tonnes of water if it filled, and long before that the box is buoyant. Empty, with the doors shut, it lifts off its blocks in less depth than people expect, and the current then picks the destination.

That is a hazard well beyond the owner. Several tonnes of loose steel crossing a flooded cane paddock takes out fencing, jams a culvert, or finishes up in someone else's crop. On ground that goes under, a container needs load in it — weight is the only thing keeping it where you put it.

The three decisions that are genuinely yours

Where it stands. A container is not tied to the ground the way a slab is, so the spot is a choice. On most river-flat properties there is a rise, an old levee bank or a house pad well clear of the paddock. Choosing that over the convenient corner beside the shed costs nothing on the day and is expensive to undo later.

What is inside it. Load holds it down and shelving keeps things up. Rack the contents so the bottom 300mm carries only what you could stand to replace, and keep documents and electronics high or somewhere else entirely.

Whether a truck can still reach it. People often buy assuming they will shift the box if a big one is coming. Be careful with that on this river: the low roads and causeways across the flats close well ahead of most house pads, so the country in between may be cut before a peak is forecast. Treat relocation as a bonus, not the plan — and do not fence, plant or pave it in.

Siting on cane and cattle flats

The ground down here holds water under the surface long after the top has dried and looks trafficable. Block it up on the four corner castings on something that spreads the load — hardwood sleepers or concrete pads, not bricks pressed into mud. On soft cattle country a footing that is fine in August will have sunk by March, and a box gone out of square has doors that no longer meet, which quietly costs you the seal you paid for. Set it with the door end on the high side, off the table drains and the flow lines across the paddock.

What we will not claim

We will not say steel keeps a river out, because it does not — and be wary of anyone who puts the word "proof" near a container and rising water. What we will do is name the grade that actually seals against weather, tell you what to stand the box on, and point at the part of your block that gives it the best odds. If what you describe needs an engineer or Clarence Valley Council instead of a steel box, we will say so.

Come and look at one

Our yard is at 1a Reid St, South Grafton, which is not the norm in this trade — much container selling on this coast is a website and a mobile pointed at a depot hundreds of kilometres away. You can drive in, open a set of doors and stand inside with them pulled to. If you cannot get here, we will send photographs of the actual unit on request, before delivery. Ring (02) 5616 9099, give us the address, and tell us what the ground does in a wet year.