Some historic stone arch bridges and stone arch culverts were built with waterway that is simply insufficient. Some stone arch bridges have a high enough “hump” over the bridge proper so that, during floods, water actually flows over the road around the bridge rather than over the bridge proper, thereby relieving the structure of some pressure. While this is not, perhaps, an ideal situation, it does tend to be easier on the bridge.
Some stone arch bridges simply are too small, with the result that they tend to become submerged rather frequently, and periodically damaged. While the worst offenders in this regard are usually long gone, surviving stone arch bridges that are too small often need some special attention in order to remain serviceable.
Maintenance Items for Constricted Arch Bridges
For a stone arch bridge that is generally sufficient save during massive floods, the best method of keeping the bridge from being damaged is to keep it maintained. For a mortared structure, the mortar joints should be kept in reasonably good condition in order to keep stones tight. Loose stones are rather vulnerable to being knocked out during flooding. Other high-maintenance items are the spandrel walls, approaches and wingwalls. All of these retaining walls are vulnerable to washouts, usually leading to loss of roadway and even damage to the main portion of the bridge. Bulging walls are the first sign of a problem, but can often be rectified with tie-rods. Signs of scour, of course, need to be dealt with promptly. Significant debris accumulation against a bridge also needs to be removed as soon as possible.
Capping retaining walls of any sort with concrete tends to help stabilize these sections of a stone arch bridge. Scour is an especial threat for a stone arch bridge that is too small, and scour aprons often end up being a necessity. One excellent solution for bridges that go underwater periodically is to replace the original fill with concrete and/or pour a concrete surface on top of the whole bridge. This solid concrete barrier prevents the roadway from being washed out, and provides a solid weight over the arch, which helps stabilize it during flooding.
Sometimes if the waterway is too constricted, it may be necessary to add an additional span of some sort to a bridge. While not a very historic treatment, adding an additional span to a stone arch bridge does not need to look glaringly out of place. A small culvert or other span can often be placed somewhere in the approach as opposed to directly against the arch of the bridge, thereby retaining the historic appearance of the main portion of the bridge.
Replacing Stones Knocked Loose
Sometimes, despite our best efforts, a stone arch bridge is damaged during flooding. Usually, this damage seems to consist of partial failure of a spandrel wall, and extensive loss of any stone curbing. Often, after the flood, the missing stones can be fished out of the water and restacked. However, where stones are missing new ones can be cut to fit, if care is taken to size them correctly.
Historic photos of the bridge before the damage can be invaluable for rebuilding the bridge as authentically as possible. However, for stone arch bridges that are not considered particularly historic, concrete is often used to rebuild missing sections of stonework.