QUANTIFYING ROCK SCOUR AT MONTEZUMA CREEK BRIDGE, SR-262, SAN JUAN COUNTY, UTAH

The SR-262 Bridge was built in 1955 across a channel that had been blasted through stratified Jurassic sandstone and claystone to permit a shorter bridge than would have been required for the natural channel of Montezuma Creek which the highway crossed on an embankment with a culvert. The bridge was founded on a ledge of hard sandstone; the constructed channel created a knickpoint approximately 30 m downstream from the bridge that was on the order of 2 m high. The unmodified knickpoint migrated upstream in response to fluvial erosion that created a plunge pool in claystone that undermined slabs of harder sandstone. The knickpoint migrated past the bridge in about 2000 and concrete walls were constructed in 2004 below the bridge foundations to keep erosion from widening the channel further. In 2008, the knickpoint was approximately 5 m upstream of the upstream edge of the bridge. Sandstone at the crest of the knickpoint was sculpted by abrasion and pits present on the downstream sides of small rises in the abraded channel form must have been caused by implosion of cavitation bubbles. Erosion of the sandstone by cavitation appeared to be minor compared to plunge-pool erosion in weaker, thinly bedded and fractured claystone, leading to undermining of sandstone slabs as the dominant form of erosion.

The Montezuma Creek Bridge was one of five bridge sites evaluated as part of a National Cooperative Highway Research Program project to develop guidelines for evaluating scour at bridge foundations on rock (NCHRP Project 24-29). This three-year project is scheduled to be completed in 2009.