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External hydraulic pressure is the force water applies to slope protection materials. The term describing this effect is hydraulic pressure, since it names the load water exerts on the material interfaces. This pressure can push against mats, geosynthetics, or concrete facing, potentially causing displacement, separation, or loss of contact with the slope if the protection isn’t designed to resist it. Runoff describes water flowing over the surface, erosion is the removal of material due to water movement, and hydraulic failure is the actual failure event under hydraulic loading—outcomes of the pressure, not the term for the pressure itself. Understanding this helps engineers specify how thick, anchored, and durable the slope protection must be to withstand the hydraulic pressure acting on it.