Two Benefits of Cutwaters

Pudden Bridge

An interesting feature of many multi-span stone arch bridges is cutwaters — protrusions of stone attached to piers that improve the hydraulic properties of the bridge. Cutwaters not only help to prevent scour, they also prevent the accumulation of debris against the piers.

Turkey Creek Bridge
A very small amount of debris accumulated against a blunt pier. Blunt piers tend to gather debris, but adding a triangular point to the pier tends to slice piles of debris and guide it under the arches.

Benefits of Cutwaters: Debris Prevention

Bridges without cutwaters on the upstream faces of the piers tend to have serious debris-gathering tendencies. Trees floating down the stream hit the pier and get stuck. A cutwater tends to encourage floating objects to slide under the arches rather than smacking into the pier and staying there. In Kansas cutwaters were primarily used for this reason. The Dunkard Mill Bridge on the Walnut River near Arkansas City had cutwaters added after the bridge was completed, as the debris problem was worse than originally anticipated.

Benefits of Cutwaters: Improved Bridge Hydraulics

Blunt piers tend to create enormous vortexes when the water is up. This can undermine the bridge, leading to collapse. A triangular upstream cutwater smoothly slices the water, allowing it to flow under the bridge in a more controlled fashion. Downstream cutwaters are also useful for the same reason — they allow the streams flowing under the arches to rejoin smoothly, rather than forming a whirlpool against the pier. The idea is to keep the water flowing smoothly and gently, minimizing the effect the bridge has on the stream.

Though this bridge has cutwaters, the concrete aprons around the piers are blunt, creating a bit of a disturbance at lower water levels. When this picture was taken, the water was flowing quite rapidly under the bridge, causing a continual haunting, gurgling splash as it slammed into the concrete aprons. Had the aprons been poured triangularly to follow the shape of the cutwaters, the flow would have been much smoother. As it happens, however, at the low water levels blunt piers do not matter too much. It is when things really get moving that the cutwater is useful. At high water levels, the cutwaters on this bridge are still perfectly functional. Note, though, that at the far end of the bridge a pile of trees has begun, showing that cutwaters are not foolproof.

Cutwater Shapes

Stone arch bridges typically have triangular cutwaters. These triangles slice the water neatly, allowing for smooth flow. However, the tip of the triangular cutwater represents something of a weak point, as it can be chipped by high-velocity debris.

Occasionally stone bridges were constructed with a rectangular cutwater. Quite frankly, a rectangular cutwater probably does not significantly improve the hydraulics of the bridge, though scour will more likely occur away from the bridge proper. However, the cutwaters themselves may get undermined. Rectangular cutwaters downstream of a bridge quite possibly are really there as buttresses, helping to brace the pier against the current.

Circular cutwaters seem to be the best shape, as they deflect debris, allow the water to flow under the bridge fairly smoothly, and yet have no obviously vulnerable surfaces. Most modern highway bridges rest on circular piers that act as cutwaters.