Troubled waters

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Crossing a bridge can be an exciting moment in a journey along a path. Think of the fun children have playing Pooh sticks! Planning, designing and building the bridge can be equally exciting – and sometimes comes with unwelcome challenges!

Watercourses can change rapidly after heavy rain and the force of the floodwater, even in a small burn, can have dramatic effects. Planning and designing the bridge and its surroundings so they will cope with flood conditions is vital, because mistakes can be expensive once it has been built and installed.

The problem and the damage...

Choosing the location for the bridge is an important first step. Ideally it should be on a straight stretch of river, where the banks are not subject to erosion in a flood. At Oatridge College, where the site choices were constrained by the path line and by ground conditions on either side, the bridge had to be installed on a bend. Not long after the bridge was completed, a flash flood brought immense quantities of water down the burn. The fast flowing water backed up behind the bridge and then flooded over the ground on the north side, washing away the path. The pressure built up as more and more water came downstream, and the river bank itself was eventually washed away. The bridge managed to survive!

The repairs - willow spiling to reinforce the river bank...

Going 'back to the drawing board' has meant reinforcing the river bank using a traditional green engineering technique called spiling. Living willow stems, known as 'withies', have been planted into the bank and woven between stakes, like a basket, along the damaged bank to prevent further erosion from any future flooding.

The repairs - a stone pitched path to provide a durable surface and to sustain future flooding...

The path was rebuilt using an upland path technique called stone pitching, and lowered so that water can flow across the hard surface without causing damage.

Find out more...

You will find more technical information about designing bridges in  Path Bridges Guide: sections 6.1 and 6.2, about surveying the site and estimating the likely flood level, are particularly relevant.

Detail drawing - Stone pitched path

If you like the look of the stone pitched path at Oatridge College, as a potential path surface for your project, download the standard detail drawing here  Stone Pitched Path (Lowland Style) - Standard Detail Drawing & Specification Details.

Perhaps the most important thing to understand about bridges is that getting them right can be a complex process, sometimes even more so than developing the path itself. Finding the right solution can require a lot of knowledge and experience, and you may well need an engineer who has worked with path bridges to help you.