Flood arch

Bywell Bridge, two of the river spans with a flood arch beyond. The level of flooding can be seen by the driftwood caught above the cutwater

A flood arch is a small supplemental arch bridge provided alongside a main bridge. It provides extra capacity for floodwater.[1]

The space beneath a flood arch is normally dry and often carries a towpath or similar. In some cases it borders on the shallow edge of a river, but this does not carry substantial flow in normal conditions. A bridge with multiple arches across a flowing river would instead be termed a viaduct.

For some bridges, flood arches were added after the first bridge had been constructed, often after initial flooding.[2]

Pant-y-Goitre Bridge with both pierced spandrels and a flood arch

References

1. "Load-carrying capacity of flooded masonry arch bridges, by Author Colin Smith and Matthew Gilbert of The University of Sheffield on Researchgate".
2. Crow, Alan (1995). "Bigsweir Bridge". Bridges on the River Wye. Lapridge Publications. pp. 144–145. ISBN 0-9518589-9-8.