May 3rd, 2012
STOPPING SCOURPreserve Infrastructure, by Catherine Higgins.
UDOT has recently surveyed bridges over waterways with unknown foundations and identified which is at risk for scour.
Over time, water can excavate soil and rocks from around bridge piers, piles and abutments causing bridge scour and putting the structure at risk for premature failure. Scour can happen gradually on structures with constant slow moving water or quickly during a flood event.
Bridge inspectors check for scour along with other structural features of bridges on regular inspections that occur within every 24 months. For bridges over waterways, inspectors look at the bridge structure, orientation, geomorphic conditions , the type of rock or soil near the abutments, the location of sediment carried by the water, and the angle, magnitude and duration of the flow. Inspectors also check piers or piles for evidence of scour holes (where water has excavated soil from around structures) or corrosion on structural elements.
Inspectors take detailed notes about the location of potential or actual problem areas. Close regular monitoring is a way to “keep our finger on the pulse of the bridge,” explains UDOT Central Hydraulics Engineer Denis Stuhff.
After inspection, a Plan of Action is developed for each bridge that includes management strategies and countermeasures for keeping the bridge safe for the traveling public before, during and after a flooding event. Sometimes permanent structural countermeasures are taken; however the most common countermeasure is the addition of strategically placed riprap.
Placing riprap is an economically sound and effective approach that allows UDOT to address the potential of scour at all bridges rather than just a few since pinpointing which bridges will be vulnerable from year to year is not an exact science. “At any time, any bridge over a waterway may have a flood,” says Stuhff who uses a gambling metaphor to explain.
“Catchment areas in Utah are like one big casino. We put our hydrology quarter in our hydrology slot machine once a year and we pull the handle… somewhere in the state, it’s paying off.”