Because of an ancient maritime by-law, a bale of straw is to hang over Wandsworth Bridge as scaffolding is erected over the middle span of the bridge to facilitate the next phase of repairs and corrosion control.
The repairs, which are set to continue until October 2021, will affect the height clearance for boats. This has enacted a most unusual rule which requires a bale of straw to hang from the side fo the bridge.
This is to warn boats that plan to pass under it that the height clearance is lowered and helps to avoid potential damaging collisions with the bridge.
The story behind the Port of London Authority by-law is that it comes from historic maritime London when straw was transported to and from London via boat.
The story goes that if you could see straw on the underside of the bridge, the tide was too high to safely pass through, and that idea has endured.
In more modern times the bale is not technically needed except to keep within the law, as there are technological means to signal that the clearance has been reduced on a bridge, as well as detecting tide height.
At night, a system of lights will also be used to highlight the danger to boats travelling under, as the bale of straw itself will be practically invisible to boats travelling on the Thames.
The remaining work takes place on the underside of the bridge, reducing disruption to traffic, and includes concrete and steel repairs and corrosion protection, as well as a repaint.