Hydraulic Bridge Scour Analyses and Countermeasure Design
Clark County Public Works

VAI has provided Clark County Public Works a broad range of environmental services for roadway and bridge improvement projects since 1999 through several on-call contracts. As part of an on-call contract, VAI has provided bridge scour and vulnerability assessment and analysis of over 30 bridges to Clark County Public Works (CCPW). VAI provided an initial Phase 1 Analysis on 11 of the bridges and ranked their vulnerability to scour and erosion. This assessment included researching the design of the bridge structure, evaluating the upstream and downstream conditions for indicators of scour and stream instability, and evaluated the capacity of each bridge under flood conditions. From this initial assessment, several bridges were identified as needing further assessment and a Phase 2 analysis. This Phase 2 analysis included an evaluation of hydrological and hydraulic conditions, an estimate of theoretical scour potential, and rip-rap sizing. VAI then assisted CCPW with prioritization of all county scour critical bridges (over 30) throughout the County. This analysis included researching historical bridge documentation and brief field reviews. VAI staff utilized methodologies developed by the FHWA to perform all of these analyses and prepared reports documenting our findings.

Recent bridge scour analysis and design services provided by VAI to CCPW are described as follows.

Washougal River Bridge No. 1404
Phase 1 Bridge Scour Review that included reviewing historical bridge documentation, evaluation the upstream and downstream conditions for indicators of scour and stream instability, evaluating the capacity of the bridge under flood conditions, and limited hydraulic analysis of the bridge.

Felida Bridge
Abutment and contraction scour were the main concerns at this bridge. VAI assisted CCPW with scour countermeasure design which included rip-rap placement at bridge abutments. The project was constructed during the summer of 2009.

JC Ward Bridge
Pier scour and the potential for channel approach migration are the main concerns at this bridge. VAI assisted CCPW with scour countermeasure designs and construction documents, biological assessment, and wetland delineations. The design included bio-engineered bank treatments, wood habitat structures, and rip-rap placement. The project was constructed fall 2011. See Appendix for example scour countermeasure drawings.

Pleasant Valley Bridge
Pier scour and the potential for channel approach migration are the main concerns at this bridge. VAI assisted CCPW with preliminary scour countermeasure designs, construction documents, and a biological assessment. The design included bio-engineered bank treatments, wood habitat structures, and rip-rap placement. The project will be constructed during the summer of 2012.