Dirtwork term
Riprap
also called: rock armor, rip rap
Loose stone placed on a slope or shoreline to absorb wave or storm energy and prevent erosion.
Riprap is broken stone, usually angular limestone or sandstone, sized so each piece is heavy enough to stay put when water hits it. Placed on a bank, a culvert outlet, a pond spillway, or a shoreline, it absorbs the energy that would otherwise wash the soil away.
Stone size is matched to expected water velocity: 4-8 inch for low-flow drainage ditches, 8-18 inch for culvert outlets, 18-inch-plus for lakefront shoreline on Cedar Creek or Lake Fork. A geotextile fabric layer underneath keeps the underlying soil from washing out through the gaps between stones.
Riprap fails when undersized (stones move in the first big rain), when no fabric is used (the bank erodes from underneath), or when placed on too steep a slope (stones tumble). Done right, it's the longest-lasting erosion fix in the toolbox.
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Related terms
Other words that come up alongside this one
Erosion Blanket
Biodegradable mat pinned to a slope to hold seed, soil, and moisture in place while vegetation establishes.
Geotextile
Woven or non-woven synthetic fabric that lets water through but holds soil back.
Culvert
Pipe or box buried under a road, driveway, or fill that lets water pass from one side to the other.
Headwall
Concrete or rock structure at the inlet or outlet of a culvert that retains the embankment and directs flow.
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