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I. PROBLEM NUMBER To be assigned by NCHRP staff. II. PROBLEM TITLE Develop a Formula for Determining Scour Depth around Structures in Gravel-bed Rivers III. RESEARCH PROBLEM STATEMENT In the current state of engineering practice, there is a recognized lack of formulas for predicting the scour depth around hydraulic structures, such as bridge piers, abutments and barbs, in gravel-bed rivers. The majority of scour depth prediction formulas presently in use have been developed for sand-bed rivers, and as such do not account for fundamental processes and characteristics that are unique in gravel-bed rivers (e.g., Raikar and Dey, 2005; Holnbeck, 2011). For instance, the inhomogeneity of gravel particles promotes particle interlocking, armor development. At the same time, unique turbulent structures result from the interaction of the non-homogeneous material with the approach flow (Papanicolaou et al., forthcoming). Even after correcting their scour depth predictions using empirically derived coefficients to account for these processes, the traditional scour prediction formulas originating from sand-bed rivers still overpredict scour depths in gravel-bed rivers. Recent insights into gravel-bed river dynamics along with advances in sensing technologies, such as RFIDs and Ultrasonic Transducer Arrays, make have improved understanding of these gravel-bed river processes and for the first time allow the development of a scour prediction formula that directly accounts for their effects (Tsakiris et al., 2016). IV. LITERATURE SEARCH SUMMARY The majority of the formulas used in current engineering practice for predicting scour depth around hydraulic structures have been developed for sand-bed rivers, which are characterized by near-uniform bed material (e.g., Dey and Raikar, 2005; Ettema et al., 2011; Guo et al., 2012; Manes and Brocchini, 2015). As a result, parameters such the inhomogeneity of bed material have been excluded from their formulations, and the empirical coefficients appearing in these formulas have been derived from laboratory experiments conducted with near uniform sand-sized sediment. Due to these limitations, when applied in gravel-bed rivers, where the more inhomogeneous material exhibits different mobility compared to uniform sand, these formulas significantly overestimate the scour depth (e.g., Dey and Raikar, 2005; Holnbeck, 2011). Recognizing this shortcoming of current scour prediction formulas, researchers have attempted to include the effects of gravel particle inhomogeneity using empirically derived correction coefficients. However, the failure of existing scour prediction formulas to fundamentally account for the unique characteristics of gravel-bed rivers in their formulation, translates to large scour depth prediction errors, even after their correction. Scour around hydraulic structures is the outcome of the dynamic interaction between the approach turbulent flow, the hydraulic structure and the bed sediment (e.g., Guo et al., 2012; Manes and Brocchini, 2015). In gravel-bed rivers, the interaction of the inhomogeneous, large gravel particles with the approach turbulent flow favors the generation of coherent turbulent structures near the boundary. In addition to amplifying the Page 1 of 4 shear stress applied by the flow onto the bed, these structures create a highly variable bed shear stress field thereby increasing the gravel bed mobility (e.g., Tsakiris et al., 2014; Sarkar et al., 2016). On the contrary, the larger and less mobile gravel particles offer hiding effects to the smaller, more mobile gravel particles, therefore reducing the overall gravel bed mobility. Furthermore, as the smaller gravel particles are entrained during competent flows, the remaining larger gravel particles create an armor pavement on the gravel bed river surface, thereby reducing the probability of scour occurrence (e.g., Guo et al., 2012). Despite the recent progress made in understanding the role that the aforementioned processes, unique in gravel-bed rivers play in scour around hydraulic structures (e.g., Dey and Raikar, 2005; Guo et al., 2012; Manes and Brocchini, 2015), a formula that comprehensively accounts for these processes still remains to be developed. V. RESEARCH OBJECTIVE The objectives of this research project are: 1. To investigate the effects of gravel interlocking and of turbulent structures, such as the horseshoe and wake vortices, generated in gravel-bed rivers on scour around hydraulic structures. 2. To develop a physically-based formula for predicting scour depth around hydraulic structures in gravel-bed rivers that accounts for the sediment and flow conditions in the gravel bed rivers. The research tasks for each study phase are synoptically presented in the following: PHASE 1: Literature Review and Experimental Investigation. Task 1: Perform critical review of the literature for identifying the processes and related variables, which govern scour in gravel-bed rivers. Task 2: Develop flume experiments representing typical sediment and flow conditions encountered in the field. Task 3: Perform the designed experiments, where two types of measurements will be conducted. First, the turbulent flow structures and second, the scour hole geometry. Task 4: Develop a formula for predicting scour depth around hydraulic structures that will be specifically applicable in gravel-bed rivers. The new formula will include the role of particle inhomogeneity, as well as the effects of the turbulent flow structures that develop from the interaction of the approach flow with the hydraulic structure and the gravel bed. PHASE 2: 2D Numerical Modeling and Dissemination of the Results Task 5: Establish a 2D numerical model of the experimental setup. The 2D model will be calibrated and validated using the measurements performed during Phase 1 of the project. In addition, the scour formula developed from the experimental measurements of Phase 1 will be implemented in the 2D model. Task 6: The established 2D numerical model will be used to examine the effects of gravel interlocking and turbulence structures for various configurations of the selected hydraulic structures, such as spacing, and angle relative to the flow direction. Page 2 of 4 Task 7: The findings of this study will be documented in a final report, which will document the new scour depth prediction formula, detail its development method, range of applicability, as well as present the data input requirements for its application and make recommendations for using it in numerical models. Task 8: The study results will be disseminated and the knowledge gained from this project will be made available to DOT engineers. Remote training webinars will be organized and training videos will be made available. The outcomes of this project will also be published in high-impact, peer-reviewed engineering journals and presented in engineering conferences at the national and international levels. VI. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD Recommended Funding: $600,000 Research Period: 30 months VII. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION The development of a scour prediction formula that has been developed specifically for gravel-bed rivers is of immediate importance and use to DOT and practicing engineers. Because of the lack of formulas for predicting the scour depth around hydraulic structures in gravel-bed rivers, DOT and practicing engineers resort to using formulas, which have been developed for sand-bed rivers and subsequently correcting their predictions with empirically derived coefficients. These formulas, cannot fundamentally provide accurate predictions for the scour depth in gravel-bed rivers and even their corrected estimates are riddled with uncertainty (Papanicolaou et al., forthcoming). Adoption of a scour prediction formula that is specifically developed to account scour processes in gravel-bed rivers will be highly valuable for DOT and practicing engineers. Such a formula will be the product of this research and will for the first time account for scour processes in gravel-bed rivers and will provide more accurate predictions of the scour depth. This enhanced accuracy can, in turn, reduce the cost for the design and construction of hydraulic structures, by reducing the hours required for translating scour predictions with sand-bed formulas to gravel bed rivers. The new equations will allow for a reduction in the depth of scour in gravel-bed rivers as opposed to using sand-bed equations, which will reduce infrastructure foundation cost and environmental impacts such as unnecessary excavation and materials for constructing hydraulic structures. VIII. PERSON(S) DEVELOPING THE PROBLEM 1. Casey Kramer Principal Engineer Northwest Hydraulic Consultants, Inc. 711 Capitol Way South Suite 607 Olympia, WA 98501 2. Prof. Thanos Papanicolaou University of Tennessee Page 3 of 4 Professor and Henry Goodrich Chair of Excellence in Civil and Environmental Engineering Department of Civil and Environmental Engineering 412 John D. Tickle Engineering Building Knoxville, TN 37996 IX. PROBLEM MONITOR To be assigned by AASHTO TCHH. X. REFERENCES Ettema, R., Constantinescu, G., and Melville, B. (2011). NCHRP Web-Only Document 175. Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions, Transportation Research Board, Washington, DC. Guo, J., Suaznabar, O., Shan, H., and Shen, J. (2012). Pier scour in clear-water conditions with non-uniform bed materials (No. FHWA-HRT-12-022). Holnbeck, S. R. (2011). Investigation of pier scour in coarse-bed streams in Montana, 2001 through 2007 (No. Scientific Investigations Report 2011–5107). US Department of the Interior, US Geological Survey. Kothyari, U. C., Hager, W. H., and Oliveto, G. (2007). Generalized approach for clearwater scour at bridge foundation elements. J. Hydraul. Eng., 133(11), 1229-1240. Manes, C., and Brocchini, M. (2015). Local scour around structures and the phenomenology of turbulence. J. Fluid Mech., 779, 309-324. Papanicolaou, A.N., Bressan, F., Fox, J.F., Kjos, L., Ettema, R. and Kramer, C. (2016). Scour around a barb in a gravel-bed stream: Effects of submergence J. Hydraul. Eng., (in preparation). Papanicolaou, A.N., and Tsakiris, A.G. (2016) “Boulder effects on turbulence and bedload transport.” In: Gravel-bed Rivers: Processes and Disasters: Tsutsumi, D. and Laronne, J.B. (eds.), John Wiley & Sons, Chichester, West Sussex, UK. Sarkar, S., Papanicolaou, A.N., and Dey, S. (2016). Turbulence in gravel-bed stream with an array of large gravel obstacles. J. Hydraul. Eng., DOI: 10.1061/(ASCE)HY.19437900.0001191, 04016052. Raikar, R. V., and Dey, S. (2005). Clear-water scour at bridge piers in fine and medium gravel beds. Can. J. Civil Eng., 32(4), 775-781. Tsakiris, A.G., Papanicolaou, A.N., Hajimirzaie, S.M., and Buchholz, J.H.J. (2014). Influence of collective boulder array on the surrounding time-averaged and turbulent flow fields.J. Mountain Sci., 11(6):1420-1428. doi:10.1007/s11629-014-3055-8 Page 4 of 4