A Proposed Risk-Based Screening Strategy for Bridges Potentially Affected by Rock Scour

Abstract

Based on assessment of rock scour at 15 bridges across West Virginia a risk-based screening strategy is proposed that accounts for both mode of scour and stream power. The proposed strategy is a result of research conducted to 1) characterize the hydraulic scour of rock at representative bridge sites in West Virginia with varying geological and hydrologic conditions; 2) assess the applicability of a recently published rock scour prediction technique for abrasion of degradable rocks from NCHRP Report 717 to the types of rock and scour conditions at the representative bridge locations; and 3) identify strategies to better characterize scour of rock at existing and proposed bridge locations. Steps in the research included bridge-site selection; field inspection; determination of scour mode (i.e., quarrying/plucking or abrasion) and magnitude; rock core sampling; laboratory analysis and testing; and hydrologic and hydraulic evaluation and modeling. The abrasion-scour-mode test described in NCHRP 717 for degradable rock that erodes gradually over time was applied to samples from all bridge sites; however, all but one bridge site in the West Virginia study consisted of durable, fractured rock that eroded by quarrying/plucking of rock blocks. A proposed risk-based screening strategy emerged from the research: bridges would be placed into one of three tiers: Tier I shows evidence of long-term channel stability and requires no further action in assessing rock scour; Tier II requires consideration of abrasion as the only mode of scour; and Tier III requires consideration of quarrying/plucking as the primary or significant secondary mode of scour.