Coastal squeeze (nowhere to retreat)

Walk to the back edge of a tidal marsh on this coast and, more and more often, you come to a wall. It might be an earth levee thrown up to keep a cane paddock dry, a road on its embankment, the bund around a canal estate, or just the first row of houses. Whatever its costume, it does not move. In front of the marsh, the sea rises. The wetland is caught between the two, a vice with no way out, and that is coastal squeeze.

To see why a rising sea should be fatal to a marsh, rather than merely inconvenient, you have to remember that the tidal bands are not fixed lines painted on the mud. Seagrass, mangrove and saltmarsh each sit at the height on the tide where they can just bear the salt and the soaking, and it is the sea, not the land, that sets that height. Raise the sea, and every band’s proper height rises with it. On a natural shore the response is undramatic. The saltmarsh climbs onto ground that used to be dry, the mangrove follows the saltmarsh, the seagrass follows the mangrove, and the whole system walks quietly inland, losing nothing. Given room, an estuary can rise with the sea for as long as the sea cares to rise.

That is the reassuring half of the story, and it is true. It is also the half that gets a developed coast into trouble, because it is easy to stop there and conclude that a rising sea simply relocates a wetland, nudging it a few metres up the slope, no harm done. The trap is buried in the phrase given room.

Migration only works where there is somewhere to migrate to, and a levee, a road embankment or a canal wall deletes that somewhere. Pressed forward by the water and stopped dead by the barrier, the bands have no slope left to climb. They compress against the wall, the saltmarsh crushed into an ever-narrower ribbon, and then, as the water keeps coming, they drown in place. The marsh is not relocated. It is extinguished, band by band, against a line somebody drew decades ago for a paddock or a boat mooring.

And much of this coast’s low country now sits behind lines like those. The habit that built them is the same misreading that treated the mangrove as wasteland for two centuries. Through the mid-to-late twentieth century the fashionable thing to do with a stretch of “useless” coastal wetland was to dredge a network of canals through it, pile the spoil to lift the ground between, and sell the result as prime waterfront housing, every block with a mooring at the bottom of the garden. Those are the canal estates that now ring the Mooloolah River and cluster at Caloundra. Where cane came instead of houses, the low floodplain was diked and drained behind bunds and levees on the same confident assumption: that a drained swamp is an improved one. Either way the outcome, read against sea-level rise, is a hard edge sitting exactly where the marsh will one day most need a soft one.

Fencing off a marsh and leaving it alone, then, is not enough on a rising coast. You can protect every square metre the wetland stands on today and still lose the wetland, because the ground it will need tomorrow is already under a road. The most useful thing you can do for a squeezed marsh is often the opposite of building a better wall: give the escape room back. Set the levee back from the water, or lower it, so the country behind can take the tide again and the bands have somewhere to climb as the sea comes up. It is the thinking behind letting the tide return to long-drained floodplain, the logic the Maroochy’s Blue Heart is built on. For two hundred years the work was to keep the sea out. The work now, in the right places, is to choose where to let it back in.