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Rigid linings are generally quite smooth, so that they have a high capacity for a given cross sectional area due to low hydraulic resistance, and thus produce a high flow velocity. When properly designed and constructed, rigid linings will prevent erosion in steep or difficult channels where other linings cannot be used. They may also be used in areas where the channel width is restricted, since steep sidewall slopes may be constructed. So long as the rigid lining is intact, the underlying soil is completely protected upon construction of the lining. However, rigid linings also have a number of inherent disadvantages. They are expensive to construct and maintain, have an unnatural appearance, prevent or reduce natural infiltration, and contribute to high velocities and scour at the downstream end of the lining unless roughness elements are added to slow the flow. Many rigid linings are destroyed due to flow undercutting the lining, channel headcutting, or hydrostatic pressure behind the channel walls or floor. Rigid linings will be discussed briefly as related to the flexible lining materials. However, the hydraulic design of rigid linings is covered in detail in Hydraulic Deiign Series No.3, "Design Charts for Open Channel Flow." (1)1 The continued use of Hydraulic Engineering Circular (HEC) No. 11, "Use of Riprap for Bank Protection" (2), for the design of dumped stone riprap channel linings should be discouraged since the methods of this circular were based on more recent information. This recommendation is based on a detailed evaluation and comparison of the two methods. However, HEC No. 11 contains information and details on other rigid linings such as hand-placed riprap, sacked concrete and grouted riprap. Hand-placed riprap is considered to be a rigid lining since it cannot accommodate even minor movement of the surface it protects (2). 1/ Numbers in parenthesis refer to references in Appendix A. 2 e ,t . e . ; e