6.11 Modeling Arch Bridges in the U.K.
The United Kingdom has a long history of bridge building and still has many river bridges that are centuries old. These are typically masonry arch bridges, but often multiple arches were needed to span a river, as these arches had limited spans for a given height. Later, brick was used instead of masonry, and relatively large spans could then be achieved with the appropriate temporary works. Finally, recent works often add further arches for flood relief, and these are frequently designed to be in keeping with the original arches.
However, many of these brick and, especially, masonry arches may never have been truly parabolic in the first place. As a result, over the years many have settled unevenly, creating slightly asymmetrical soffit (underside) shapes, with a different shape to the adjacent arch. Additionally, many arches have had their inverts lowered to improve drainage and the standard of flood protection, but this has often been carried out within the original width, resulting in a narrower section at the new invert level, which widens out at the level of the original bridge invert. These unique shapes make it impossible for the modeler to use any standard geometric shape, and so the model must be constructed to represent each individual arch. Fortunately, HEC-RAS makes this task very easy-high-chord and low-chord levels can be entered at varying stations. Being able to view the shape being constructed in the model is a real advantage-it is immediately obvious if an entered level is not quite right.
The importance of the levels of the top chord should not be overlooked; HEC-RAS will treat this as a weir, thus representing a flood that has flow passing through the arches as well as over the top of the bridge. Beware of entering the top chord by using roadway levels because these bridges usually have a solid parapet, which will not allow water to flow over the road until the parapet itself is overtopped.
Many brick arch bridges have a relatively flat soffit, which can cause problems with conveyance variations. In one example, where a blockage downstream was being modeled, the higher water level at the bridge created a slightly increased cross-sectional area, but a substantially increased wetted perimeter, even though the soffit was still clear of the water. Thus the increase in water level caused by the downstream blockage was higher upstream of the bridge than immediately downstream.
Most arch bridges that are modeled are existing bridges, but occasionally a new bridge is required to have the appearance of these old bridges. The restrictions attached for the construction of one such bridge was that the afflux (increase in water level caused by the new bridge) immediately upstream of the bridge should not exceed 70 mm (28 in.); and at a distance of 1.2 km (0.7 mi) upstream of the bridge (at the limit of the land owned by the potential bridge owner), the afflux should be zero. In this case, the soffits of the three new arches were clear of the flood level (important to allow floating debris to pass through), but the bridge approaches were inundated and acted as weirs. These approaches had to be reduced slightly in level to allow the requirements on afflux to be achieved.
Credit: Andrew Pepper
