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A flood arch is a small supplemental 
A flood arch is a small supplemental arch bridge
An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its structural load, loads partially into a horizontal thrust restrained by the abutments at either si ...
provided alongside a main bridge. It provides extra capacity for floodwater
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant concern in agriculture, civi ...
.
The space beneath a flood arch is normally dry and often carries a towpath
A towpath is a road or trail on the bank of a river, canal, or other inland waterway. The purpose of a towpath is to allow a land vehicle, Working animal, beasts of burden, or a team of human pullers to tow a boat, often a barge. This mod ...
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
A viaduct is a specific type of bridge that consists of a series of arches, piers or columns supporting a long elevated railway or road. Typically a viaduct connects two points of roughly equal elevation, allowing direct overpass across a wide ...
.
For some bridges, flood arches were added after the first bridge had been constructed, often after initial flooding.
References
Arches and vaults Bridge components {{bridge-type-stub