Alaska Section, American Water Resources Association

Michael R. Lilly, Alaska Section AWRA Northern-Region Director

I would like to invite you to the December Brown-Bag presentation by Bob Burrows. Bob's talk should be of interest to hydrologists and engineers working in Arctic and sub-Arctic surface-water erosional projects. The deteriation of the nations bridges is a major concern both in Alaska and the lower "48".

December 1996 Alaska Section AWRA
Norther Region Brown Bag Lunch Meeting Presentation

Robert L. Burrows, U.S. Geological Survey.
"Estimating Scour at Bridges with Historical Data"

Failures of bridges over water are caused primarily by scour and channel instability. Over the past several years the Federal Highway Administration has mandated that all states evaluate bridges over water for potential failure. These evaluations have ranged from subjective risk rankings based on site visits to computer modeling using field-surveyed input data.

In Alaska, the U. S. Geological Survey, Water Resources Division, has been conducting evaluations of channel stability and scour at bridges in cooperation with the Alaska Department of Transportation and Public Facilities. Initial analyses were done by utilizing the water surface profile model, WSPRO to model the hydraulics at the bridge in order to calculate potential scour. Instead of visiting each site and surveying the channel geometry and streambed slope to provide the input for WSPRO, all input data were taken from the as-built plans, bridge inspection notes, maps, photographs, and USGS streamflow data where available. In some instances it was necessary to make estimates of some input data. Generally only one cross-section was available, and other sections needed in WSPRO were replicated up and downstream by the model. Once a successful, and apparently hydraulically sound model run was made, the 100 year and 500 year flood flows were run by the model and the results used to compute contraction scour and pier scour at each bridge site.

A selected number of bridges that appeared to be scour critical were field surveyed. The new data were then used to make a new WSPRO model run and scour calculations were redone. The results of the updated calculations were compared with the original as well as the actual measured scour depths apparent on site. This methodology provides a means of estimating potential scour without costly field surveys and provides an objective method for screening bridge sites for scour susceptibility.