Containers and flood risk on the Clarence — what steel can and cannot do

Watertight means rain, not river

When we say a cargo-worthy container is checked wind and watertight, we mean a specific thing: the roof does not leak, the corners do not let daylight through, and the door seals seat well enough to keep weather out. It is a test done from the inside with the doors shut, looking for light.

All of that is about water arriving from above. Floodwater does not arrive from above. It rises, and it finds the bottom of the door seals, which are the lowest and least sealed part of the whole box. Water gets in there. Once it is in, a container is very good at holding it — which is exactly the wrong property to have. A shed drains. A container becomes a tank.

An empty container floats

This surprises people every time. A 20ft container weighs around 2.2 tonnes and displaces about 33 cubic metres. Empty, with the doors shut, it is buoyant. In deep enough water it will lift off its blocks and travel, and where it stops is not your decision.

This is not theoretical on this river. Containers moved in 2022. If a container is on ground that can go under, it needs weight in it, or it needs to be somewhere the water will not reach, or both.

So what is a container actually good for here?

Three real things, and they are worth having.

Height. A container sits on blocks, and you decide how high those blocks are and where they go. Gear inside a container on the highest firm point of a property is above the sort of water that puts 300mm through a shed floor. That is the common event, and it is the one a container handles well.

Mobility. This is the underrated one. A container is a box, not a building. Given warning and a truck, it can be lifted and moved to higher ground — which is a genuine option a shed does not have. The catch is access: if you have built a fence, a garden or a slab around it, or the paddock is already too wet to drive on, the option disappears exactly when you need it. If you are siting a container on ground that floods, leave a truck a way in and keep it clear.

Dry lockable space afterwards. When the water goes down and the shed is a write-off, having somewhere steel and dry and lockable to put tools, salvaged gear and the things you are trying to protect from a second problem is worth a great deal. This is the use we hear about most from people who went through 2022.

Practical siting on a flood-prone block

Put it on the highest firm ground you have, not the most convenient. Block it up properly on the four corner castings — higher blocks are not the same as safe, but 400mm of clearance costs almost nothing and covers a lot of minor events. Keep an approach open so a truck can reach it. Do not store the irreplaceable things at floor level inside; use shelving and keep the bottom 300mm for things you can afford to lose.

And think about what you would actually do with 24 hours of warning. If the answer is "move it", make sure moving it is possible. If the answer is "leave it", put weight in it.

What we will not tell you

We will not tell you a container keeps floodwater out, because it does not. Anyone who does is either not thinking about it or is hoping you are not. What we will tell you honestly is which grade seals properly against rain, how high to block it, and where on your place it is most likely to stay dry — and we would rather have that conversation than sell you something on a promise that will not hold.

Ring us and describe the block. If you want to look at how a unit is set up before you decide, the yard is at 1a Reid St, South Grafton.