page_title: Drop structure
section_title: Grouted sloping boulder
hierarchical_section_title: Drop structure / Typical designs / Grouted sloping boulder
caption_attribution_description: English: Drop structure on Trabuco Creek after a rain.
context_page_description: A drop structure, also known as a grade control, sill, or weir, is a manmade structure, typically small and built on minor streams, or as part of a dam's spillway, to pass water to a lower elevation while controlling the energy and velocity of the water as it passes over. Unlike most dams, drop structures are usually not built for water impoundment, diversion or raising the water level. Mostly built on watercourses with steep gradients, they serve other purposes such as water oxygenation and erosion prevention.
context_section_description: A grouted sloping boulder drop structure is the most versatile of drop structures. Able to accommodate both a broad floodplain or a narrow channel, they can also handle many different drop heights. Heights of these structures usually range from 1 foot (0.30 m) to 10 feet (3.0 m). These structures are built by creating a slope of riprap, which consists of large boulders or less commonly, blocks of concrete. These are then cemented together ("grouted") to form the drop structure. Another less common type of drop structure, the sculpted sloping boulder drop, is derived from this. The sculpted sloping boulder drop is used to create a more natural appearance to the drop structure. Both of these structures also tend to suffer from downstream erosion.
caption: This grouted sloping boulder drop on Trabuco Creek in California is almost entirely covered by floodwater.
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