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NCHRP 24-47 [Final]
Revised Clear-Water and Live-Bed Contraction Scour Analysis
| Project Data |
| Funds: |
$500,000 |
| Research Agency: |
Ayres Associates |
| Principal Investigator: |
Peter Lagasse |
| Effective Date: |
10/6/2016 |
| Completion Date: |
4/6/2019 |
| Comments: |
Completed. Published as Research Report NCHRP 971 and WOD 294 Training Manual. |
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BACKGROUND
Current guidance in Hydraulic Engineering Circular No. 18 (HEC-18), "Evaluating Scour at Bridges," (Arneson et al. 2012) provides equations for estimating contraction scour. Existing equations are based on sediment transport theory using approaches developed by Laursen, 1960 (live-bed contraction scour) and Laursen, 1963 (clear-water contraction scour). Both equations assume that the scour is due solely to the contraction effect and that local effects are negligible (i.e., that the contraction is hydraulically "long"). Depending on the ratio of the length of contraction L to the approach channel width b1, (L/b1) channel contractions are designated as long or short. In a short contraction, local scour occurs throughout the contracted section as a result of large-scale turbulent flow structures created at the entrance to the contraction, and the total scour is the result of both the contraction and local effects. Analysis of existing laboratory data sets conducted under NCHRP Project 24-34, "Risk-Based Approach for Bridge Scour Prediction" revealed that the clear-water contraction scour equation does not envelope the observed data as a design equation. Rather, it is a predictive equation which is seen to under predict observed scour relatively frequently compared to pier and abutment scour equations. No laboratory data sets of live-bed contraction scour were identified during NCHRP Project 24-34. NCHRP Project 24-34 found that all of the previous studies suffered from a flaw in the experimental design, as none actually measured the depth of flow y0 in the contracted section before scour began to occur. Therefore, this value had to be estimated in order to determine the depth of scour. In addition, a number of laboratory studies did not directly measure the depth of scour using bed elevation measurements. Instead, the assumption was made that y0 was equal to y1 (the depth of flow in the approach section upstream of the contraction). This assumption ignores the hydraulic drawdown effect in the contraction which occurs during subcritical flow (particularly in a bridge reach). A contraction scour design approach for various bed materials is needed. Separating hydraulic erosion forces from the soil erosion resistance is needed to help determine a more realistic scour depth.
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