collapses and, in worst-case scenarios, motorist fatalities.
To limit the damaging effects of scour, engineers follow different design standards for bridges over water.
“ We actually design our bridges, at least in the modern era, to withstand a certain amount of scour so that they’ re safe under moderate to fairly extreme hydraulic conditions,” said Will deRosset, a hydraulic engineer at Ayres Associates.
Implementing scour countermeasures provides additional reassurance.
“ You can place countermeasures that are very erosion-resistant near your bridge foundation features to prevent, limit, or disrupt the action that causes erosion,” deRosset said.“ For example, it is very common to see a blanket of very large, angular rock around bridge abutments. That’ s called riprap, and it’ s an erosion-resistant layer that helps protect the underlying parent material into which your bridge is founded.”
A nationwide handle on scour
Ayres Associates’ engineers teach courses in scour throughout the country and have been integrally involved in developing and updating the state of practice for prediction of bridge scour.
Scour countermeasures explained
Imagine standing barefoot on a beach as waves wash up and around your legs. After a cycle or two, your feet will start to sink into the soft sand, and the power of the moving water is quickly evident.
Add a beach towel to this scenario, and you’ ll no longer sink into the sand. That layer between your feet and the loose, erodible material beneath serves as a filter and prevents the sand from shifting.
Hydraulic engineers commonly use filter systems( as our simplistic beach example loosely illustrates)
“ Quote” underneath the rugged erosion-resistant“ riprap” rocks placed to provide protection from scour – or the removal of sediment from the area around bridge piers and abutments. The filter provides separation from the sediment below and helps armor the bridge foundation during a flood – when it’ s most susceptible to scour.
“ In broad classification, there are two kinds of countermeasures,” explained Paul Clopper, director of applied technology at Ayres Associates who has co-authored numerous publications on scour countermeasures for the Federal Highway Administration and other agencies.“ You could protect the existing structure against the flood with hydraulic countermeasures, or you can strengthen the existing structure with additional structural measures, making it stronger through construction methods.”
Besides riprap, other countermeasures include making channel improvements to stabilize and control flow patterns and designing weirs, spurs, or guide banks to redirect the flow into the main channel and keep it from attacking the streambank.
September 2009 Publication No. FHWA-NHI-09-111
Hydraulic Engineering Circular No. 23
Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Volume 1
Another option is to retrofit the bridge with additional structural measures such as deeper piles.
“ Of course, you can’ t design a countermeasure properly unless you know what the forces are during a flood that are going to act on that countermeasure and try to destroy it. You’ ve got to know your river – how steep it is, how fast and deep it flows,” Clopper said.“ Hydraulic modeling helps us predict and plan for the physical forces of that rushing water. For example, in a 100-year flood, we can map the hot spots where the forces are severe, and we need to protect these areas.”
– Jennifer Schmidt
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