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Doc Date:12/11/2001 • III IIIIIIIIIIIII III sss TECHNICAL RELEASE No. 25, CHAPTER 6. APPENDIX A. Stream Armor Design Concepts 6-94 Appendix A Purpose This appendix (1) explains the underlying physical processes affecting armoring, (2) describes different SCS-approved math models available, and (3) presents an example illustrating one way to estimate armoring. 'Ihe various math models for critical and recommended allowable tractive stress discussed in this appendix are accepted in the engineering profes- sion; they differ mainly in choice of a safety factor, scope of applica- tion, or both. 7t+o different math models for recommended allowable trac- five stress are used in SCS. They differ solely in their safety factors. The armor designer is free to select the most applicable model. Actual transverse tractive stress of each situation must be determined through a hydraulic analysis. The example in this appendix uses a sim- plistic model to determine the hydraulic radius. In real situations, actual cross-sectional geometry and, possibly, precise water surface profile calculations are required. However, this requirement does not invalidate the concepts illustrated by the example. Fhysical Processes Armoring is a well-known natural phenomenon. Furthermore, its important features already are used in sore engineering structures, for a:ample, riprap. Armoring is sometimes called hydraulic sorting. It is a limit- ing or special case of sediment transport. It has been studied by vari- ous scientists over the years, (e.g., A. Shields, A. Strickler, E. Lane, H. Einstein, and others). Understanding the primary principles of armor- ing is still developing and is leading to various math models and pro- cedures for field application. • Armoring is the result of the dynamic interaction of unsteady fluid flow and a mobile bed composed generally o_' a broad range of discrete parti- cles.