Traditional practice of bridge local scour estimation relies upon the use of analytical models such as the one specified in Hydraulic Engineering Circulars, HEC-18 and HEC 20 (Arneson et al., 2012). Models such as HEC-18 were however developed based on data collected mainly from flume testing on sand. The data used for HEC-18 model development were mainly for narrow pier erosion in sand (scour depth/pier width>1.4) per Benedict and Knight (2017). Yet the model is applied in practice to intermediate and wide pier cases as well. In addition, the materials classified as “soils” include sand, and/or silt, and/or clay with a grain size distribution that can yield a bed soil behavior that may not be captured by a single parameter, such as D50. Approaches such as the HEC-18 model also lump the flow channel and bridge hydraulic and geometrical parameters with the bed erosion resistance parameters in one equation. While such an approach is simple to use, there is consensus in literature that it yields overly conservative scour estimates. On a fundamental level, the magnitude of erosion and scour can be assessed through knowledge of the flow-induced shear stress, the soil’s erodibility parameters, which include the critical shear stress (τc), co-efficient of erodibility (α'), and m, which is “an exponent defining the functional variation of the soil erosion rate with the flow-induced shear stress.” This approach is fundamentally implemented in the FHWA Hydraulic Toolbox and adopted by the NextScour Program. In parallel, geotechnical site investigation by the North Carolina DOT commonly involves the performance of SPT, and the retrieval of soil samples for characterization of physical and engineering properties. As such, there is an opportunity to obtain the site-specific erodibility (τc, α', and m) through linking such parameters with the geotechnical data for a rational assessment of site-specific scour magnitude, accounting for variability of channel-bed soil layers with depth. The objectives of the study proposed herein are: (a) to provide a means of estimating the magnitude of sediment erodibility parameters through correlation with physical and engineering parameters obtained from site geotechnical investigation; and (b) to provide correlations between flow parameters and the flow-induced shear stress to facilitate the computation of scour and erosion magnitude for piers, abutments, and embankments. A main guidance of the proposed research is relying on data traditionally obtained during geotechnical investigation including the soils’ engineering and physica properties and the blow count per foot “N-value” obtained from the Standard Penetration Test (SPT). The research will advance the assessment of scour at the following fronts: i. Correlation of the erodibility parameters (τc, α', and m) with the soils’ engineering and physical properties that include, for example, grain size distribution, Atterberg limits, percent clay content, the blow count per foot “Nvalue” obtained from the SPT and the associate correlation with strength parameters. ii. Perform geological reconnaissance and monitor and collect data on hydrodynamic and morphodynamic response at bridge sites for which geotechnical investigation data are available. Such data will be synthetized and used in numerical modeling to assess the magnitude of the applied shear stresses (τ) under various flood events, and the corresponding temporal and spatial evolvement of scour at the bridge location at these sites. Specifically, characterization of τ will be performed for various flood recurrence periods and flow domain configuration including skewness, changing the flow angle of attack, flow constrictions, type of structure including pier classification, embankment, or abutment. The estimation of τ is needed for estimating the magnitude of scour. iii. Utilizing the site geological and geotechnical data and the results from the numerical simulations for the development of guidance for estimating the erodibility parameters. Then, perform comparative study of the scour magnitude obtained of the rational approach (implemented in the FHWA Hydraulic Toolbox) and the advanced numerical modeling. iv. Development of a plan and guidance policy for NCDOT implementation of the research findings in concert with the FHWA next generation scour program, NextScour.
