Based on an 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 rock scour prediction techniques from National Cooperative Research Program (NCHRP) Report 717 to the types of rock and scour conditions at the bridge locations; and 3) identify strategies to better characterize scour of rock at 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 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. However, all but one bridge site in the West Virginia study included durable, fractured rock that eroded by quarrying/plucking of rock blocks. Based on the results of this study, a risk-based screening strategy is proposed for bridges potentially affected by rock scour. This screening strategy includes 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.