Trends Summer 2017 | Page 14

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