Gabion Application 3 - Gabion Weirs / Drop Structures
Gabion weir, or drop structure, its prime objective is to determine a hydraulic profile where bed-load transportation and siltation are in equilibrium. Introducing weirs reduces the bed slope and therefore transportation.
Gabion weirs are robust, flexible and easy to install, and are used as both primary and counter-weirs on larger installations. Reno Mattresses are used to line stilling or energy dissipation basins if required.
Weirs are classified into three types, according to the shape of the downstream face at the centre of flow: vertical weirs, stepped weirs and sloped weirs.
The vertical drop structure or weir is the most basic type, and is most often used for small weirs in a channel system controlling only a reach of the stream.
The stepped weir has no essential differences from the vertical type, but the water flowing over the weir dissipates a part of its energy at each step, therefore, requiring protection of the horizontal surface of each step. This structure should only be used for small weirs where the values of discharge for unit width are limited in any case it should be avoided where a heavy bed load is carried which could case damage to the mesh on the steps.
The sloped weir is used for large weirs, and when the height of the structure ranges from 25' to 50', the requirements of greater stability and improved hydraulic behavior dictate this weir type. Sloped weirs are generally found in training schemes on major rivers carrying light bed loads and in which the soils are fine granular material having a poor bearing capacity. Where a heavy bed load of shingle and boulders is carried, the crest, slope and stilling pool must be protected from abrasion by concrete cap.
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