Scour is a never-ending threat to any bridge, stone or otherwise. Scour is essentially the erosion of the streambed under the bridge. Erosion is a constant process within any stream. However, when water hits an obstruction, such as a bridge pier, the erosion process becomes stepped up significantly. It is possible for scour to erode away material to the base of and even under a bridge pier, causing collapse. Preventing scour is an important part of preserving historic bridges.
Why Scour is Such a Threat for Historic Bridges
Though scour is always a problem for bridges, modern bridges typically do not face serious scour concerns. The reason for this is that modern bridges cope with scour by being built with deep foundations. For modern bridges to be threatened by scour, a massive amount of erosion would have to occur, and, frankly, it is probable that the bridge will be replaced for other reasons before that amount of erosion has time to occur. On the other hand, stone bridges, and even some early concrete bridges, often have shallow foundations. In Kansas, the rule was to place the bridge on bedrock, which, conveniently, was usually the streambed proper, little to no excavating needed. The trouble is even bedrock is not impervious to water; limestone, so common in Kansas, is somewhat water-soluble, and eventually will be eroded away. This, by the way, can cause problems largely specific to limestone bridges. In many places, stone arch bridges are not even sitting on bedrock, having incredibly shallow foundations (a foot or two deep is not unheard of) set in mere soil! The long and short of it is that historic bridges tend to have vulnerable foundations.
Why Stone Arch Bridges are Particularly Sensitive to Scour
The scour problem is even worse with stone arch bridges than concrete bridges. The reason for this is rather simple. Whereas a concrete bridge is, more or less, one solid piece, a stone arch bridge is composed of many discrete blocks. Undermining of a portion of a bridge will cause settling to occur as the stones above the affected area fall into the resultant “scour hole.” A concrete bridge can “span” a scour hole much better by merit of being a solid piece. Of course, there is a flip side to this; namely, in case of scour trouble, a stone bridge tends to show signs of distress early on, giving early notice of problems. A concrete bridge, on the other hand, may give out suddenly when unable to withstand the strain of spanning a portion of the structure over a scour hole.
Signs of Scour
Scour can be quite subtle. A bridge that looks perfectly secure may not be, having been undermined. The reason for this is that scour is usually only active during high flows of water. As the flow goes down, sediments settle, filling in the scour hole with soft silt, which will be washed up again the next time the flow is heavy. For this reason, deposits of silt are a suspicious sign. Obviously, if, when the water is low, it is clear that parts of the bridge foundation are suspended over nothing there is a scour problem. Another sign of serious scour problems is settling or even partial collapse of the structure. In the end, bridges, especially ones with shallow foundations, need inspected periodically by professionals to ensure scour is not occurring, in order to catch and check the trouble before something serious happens.
Preventing Scour: The Advantages of Scour Aprons
“An once of prevention is worth a pound of cure” applies rather keenly to scour. Scour tends to be surprisingly simple to prevent in most cases, yet, if left unchecked, can cause all manner of expensive damage. Scour aprons are a highly simple and effective means of preventing scour. The scour apron (alternatively called the “collar” or, at least when applied to piers, “pontoon”) is a very simple barrier that surrounds and extends out a short way from a pier or abutment. These aprons are usually made of concrete, which is poured into a form set up around the pier and abutments of a bridge to form a complete, solid barrier around the lower parts of the bridge. Stone scour aprons are not unheard of. The point of these aprons is to prevent water from directly eroding the bridge foundations.
To be sure, the aprons can be undermined, but this is much more difficult for water to do, as the scour aprons present a relatively low profile. Furthermore, the scour aprons would have to be undermined a long way or fail completely before the bridge they are protecting would become threatened. Scour aprons, with their simplicity and effectiveness, are a highly successful way of protecting historic bridges. Much simpler and much less expensive than underpinning and more localized than streambed paving, scour aprons apply scour protection where it is needed the most.
Interestingly, in the southern Kansas area, almost all the stone arch bridges and many old concrete bridges feature scour aprons, which still successfully perform their job. As a testimony to the long-term effectiveness of concrete scour aprons, the double-arch Andes Bridge in Cowley County, Kansas, features a large scour apron around the pier which, according the National Register of Historic Places nomination form for this bridge, was put there in the 1920s, and is still doing its job.