Bridge Hydraulics

The consequences of a bridge failure can range from the unexpected need for maintenance, to the complete loss of the structure — to the loss of life. Hydraulic bridge failures are those caused by the uncontrolled interaction of water or ice on a bridge structure. Before the 1970s several hydraulic bridge failures happened across Canada.

Many of these failures were due to scour undermining the bridge pier foundations. Mostly, however, the failures were due to inadequate or poorly placed hydraulic openings.

The Canadian Highway Bridge Design Code sets out the design loading of a bridge structure, including dead, live, earthquake and ice loadings. But how is the hydraulic loading of that same bridge determined?

In the late 1960s the Roads and Transportation Association of Canada, now the Transportation Association of Canada (TAC), undertook a project to analyze the common mechanism of bridge hydraulic failure and provide a guide to help designers size hydraulic openings. The resulting book, The Guide to Bridge Hydraulics, first published in 1973 and revised in 2004, provides information not only on how large to make a hydraulic opening, but also where to put that opening. The guide is available through the Canadian Standards Association.

Where to put the bridge opening may seem self-evident — where the road crosses the river. However, a successful design will place the opening at the most advantageous location to ensure that the crossing requires little maintenance. Doing so often means having to realign the roadway. Bridge structures are now designed for a 75-year service life, so the hydraulic opening should also be designed for the same service life if not longer. It is worth the effort to find the optimum crossing location.

Designing a hydraulic opening and placing it at a location that will ensure a long service life is a multi-disciplinary exercise. Besides having a basic grounding in hydrotechical engineering (also referred to as river engineering or water resources engineering), it is essential that the designer has sufficient knowledge in the disciplines of geotechnical and structural engineering, roadway geometric design and biology in order to arrive at an optimized hydraulic design.

The design exercise is an iterative optimization process that identifies all constraints at the crossing site, accommodates most of them and compromises those that can be. Often this process is undertaken by a team of specialists, but ideally it is done by a single specialist. Unfortunately it is not a discrete discipline that is taught at our universities.

The types of constraint that should be identified include: the roadway geometrics, any slope stability or bearing capacity problems, environmental issues such as critical fish spawning habitat, restrictions on placement of piers in and around the water, the skew angle of the crossing and, of course, hydrotechnical constraints.

Where to place the hydraulic opening may be a decision dictated by the design high water and high ice levels, the river’s lateral mobility (shifting course, or widening during high flow), or scour related issues (undermining of the foundation). During the preliminary engineering phase it is often the hydrotechnical engineer that leads the design process rather than the structural or roadway engineer.

Smoky River Bridge

Scour Failure

In the summer of 2001 a pier on a resource road bridge across the Smoky River in west central Alberta was undermined and it failed. Subsequent analysis showed that the spread footing pier was constructed in a deep cohesionless sand bed, so when a significant flood occurred the approaching high velocity flow concentrated near the bed, causing a large scour hole that eventually undermined the footing. The pier rotated into the resulting hole.

The decision to use a spread footing foundation was based on economics since the bridge was only meant to be in service for 10 years; it failed in its 25th year of service.

By accepting the risk of using a spread footing in a scour susceptible river, a small amount of capital funding was saved, but the cost of replacing the pier far outweighed that saving. The cost difference between a mass concrete spread footing and a driven steel pile or large diameter caisson foundation that would be much less susceptible to scour is negligible compared to the risk. Several transportation departments in Canada have policies against the use of spread footings in active rivers to avoid this situation.

Calculating maximum scour depth and providing scour countermeasures can be time consuming and it can be risky to assign parameters to the many unknowns required for modeling. The best approach is to avoid the problem altogether by placing piers outside the main channel and using deep pile foundations.

James River Washout

A hydraulic opening can survive for many years before being tested, but when heavy runoff occurs shortcomings in the design can become obvious very quickly. In the summer of 2005 significant rainfall occurred in the Rocky Mountain foothills of Alberta, resulting in rapidly rising streams and numerous washouts. The James River bridge was designed to handle the largest flow on record, but its hydraulic capacity was compromised when drift (trees and other debris) accumulated on one side of the opening, forcing water over the highway, which eventually washed out the roadway fill.

Drift is a major consideration in sizing a hydraulic opening and a factor in deciding on span length and where piers are to be placed. If a stream can carry a significant amount of drift, the opening must be designed to accommodate it. The superstructure should be given sufficient freeboard to pass drift at the design high water level, piers should be placed outside the thalweg (deepest part of the river), and the minimum span should be wider than the longest tree being washed downstream.

In remote areas without stream flow gauges there may be little data available on which to base the hydraulic design of a bridge opening. Whether data is available or not, the channel morphology will always be a guide as to what water level and velocity to design for. Reading the channel is a skill that takes years of field experience to master and can be invaluable in arriving at a successful bridge hydraulic design.cce

Marcel Chichak, P.Eng. is a senior water resources engineer with AECOM in Edmonton. This April he is presenting a short course through the Canadian Society for Civil Engineering entitled “Introduction to the Guide to Bridge Hydraulics,” in support of the CSA S6 Bridge Design Code. For locations across Canada, visit www.canadianconsultinengineer.com/events, or www.csce.ca