Evaluation of the Extent of Backwater Effects of Bridge Piers

Introduction... The construction or renovation of bridges may require placement of bridge piers in the channel or floodplain of natural waterways. These piers will obstruct the flow and cause an increase in water levels upstream of the bridge for subcritical flows. The increase in the water level is called the backwater. The amount of backwater caused by piers depends mainly on their geometric shape, their position in the stream, the flow rate, and the amount of channel blockage. Investigation of how piers influence channel obstruction and hydraulic efficiency is an important issue in bridge design. Furthermore, it has been postulated that the hydraulic effects of the piers are localized and dissipate quickly in the upstream direction. Part of this project was to investigate this postulate. For subcritical channel flow, which is the type of flow that exists in most rivers, the rise in the water level due to bridge piers and abutments is usually assumed to occur where the flow contraction begins upstream of the bridge. This distance upstream of the bridge is approximately equal to the average encroachment distance of the roadway embankment into the channel. The hydraulic effects of bridge piers on backwater profiles have traditionally been included in the overall backwater effects of a roadway crossing of a stream. The National Flood Insurance Program, which is administered by the Federal Emergency Management Agency (FEMA), requires permits for channel improvements and floodway map revisions for any encroachment into a designated floodway. FEMA considers bridge piers in a floodway to be an encroachment, so regulations effectively allow no backwater due to the piers without a map revision. The map review, while both time-consuming and expensive, can also include the possibility of purchasing flood easements, yielding another construction-related cost. This research project has attempted to evaluate the water level change due to bridge piers and to study the nature of the variation of the water surface upstream of the piers. The following three objectives were addressed in this research: 1. Evaluate the drag coefficient of bridge piers to obtain a better understanding of scaling relationships between laboratory and prototype conditions; 2. Compare the experimental results to the results of previous studies and, where appropriate, develop relationships between the backwater and the Froude number (and possibly other factors); and 3. Study the nature of the water level variation upstream of the piers. To accomplish these objectives, two series of experiments were performed using a large physical model. The first series evaluated the drag coefficient, while the second series focused on water level variation. Only TxDOT’s Type A (Yarnell’s Type I) flows were considered in this project. Type A flows are those for which the water level is low enough that the flow does not impinge on the superstructure and remains subcritical in the contracted region. Much research has been undertaken on backwater effects from channel obstructions, and a few studies related specifically to bridge piers. The earliest study that was found in the literature was published in 1852, so it is clear that the backwater effects of bridge piers have been a concern for at least 150 years. Subsequent studies were done in the early part of the 20th century. The results that are most widely used were published by Yarnell in 1936, namely