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ABSTRACT
Estimation of local scour depth at a bridge pier must be accurate and precise for the safety assessment of pier foundation. Many deterministic scour depth prediction models for bridge piers in sands and gravels have already been reported in the literature. However, the various types of uncertainties may be involved in the deterministic scour depth prediction models and thus affect the accuracy of the scour depth. To account for such uncertainties, a reliability analysis method is generally adopted. A sizable amount of laboratory and field data on scour has been procured in the present study to assess the performance of existing scour prediction models for gravel beds and develop a probabilistic scour prediction model using a First-Order Reliability Method (FORM) based on the object-oriented constraints optimization spreadsheet algorithm for the most appropriate selected deterministic scour prediction model. A new relationship of the silt factor has also been developed to estimate the scour depth in gravel bed as per Indian Practice. A safety factor based on the target reliability index has also been proposed in this study.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Notations
| Notations | = | Description |
| ds | = | Maximum local scour depth (m) |
| D | = | Diameter of the cylindrical pier (m) |
| KI | = | Coefficient for flow intensity |
| Ky | = | Coefficient for flow depth |
| Kd | = | Coefficient for sediment size |
| Ks | = | Coefficient for shape of the pier |
| Kθ | = | Coefficient for angle of attack of flow |
| U | = | Approaching velocity of flow (m/sec) |
| Ua | = | Limiting armour velocity (m/sec) |
| Uc | = | Critical velocity of flow (m/sec) |
| u*c | = | Critical shear velocity (m/sec) |
| d50 | = | Mean sediment size (m) |
| y | = | Approaching flow depth (m) |
| L | = | Length of the pier (m) |
| θ | = | Angle of attack of the flow (degree) |
| be | = | Effective width of the pier. (m) |
| q | = | Design flood discharge intensity (m2/sec) |
| f | = | Silt factor |
| dmm | = | Weighted mean diameter of sediment (mm) |
| b | = | Width of pier (m) |
| g | = | Acceleration due to gravity (m2/sec) |
| σg | = | Sediment gradation |
| K1 | = | Correction factor for pier nose shape |
| K2 | = | Correction factor for angle of attack of flow |
| K3 | = | Correction factor for bed condition |
| H | = | Hager number |
| ρ | = | Density of water (Kg/m3) |
| ρs | = | Sediment density (Kg/m3) |
| Fd50 | = | Densimetric Froude number |
| Kby | = | Flow depth–pier diameter factor |
| Kσ | = | Sediment uniformity/non-uniformity factor |
| Pf | = | Probability of failure |
| g(x) | = | Limit state |
| C | = | Covariance matrix also |
| R | = | Correlation matrix |
| F | = | Failure region of limit-state function |
| df | = | Depth of foundation (m) |
| λ | = | Model correction factor |
| β | = | Reliability index |
| βT | = | Target reliability index |
| SF | = | Safety factor |
| αi | = | Sensitivity factor |