Flow and Energy Dissipation over Stepped Gabion Weirs

Gabions are a common construction material in the African Sahel and elsewhere, especially for spillways. Stepped gabion spillways are structurally stable, resistant to water loads, and efficient energy dissipators, but there are no clear design rules to date. One‐fifth‐scale model tests representing 3–5 m high standard prototypes discharging up to $3m^3\cdot s^{-1}\cdot m^{-1}$ enabled us to establish quantified energy‐dissipation criteria and efficient stilling‐basin design rules, saving 10–30% on basin length as compared with current methods. The different types of flow over the stepped face are also described and analyzed: isolated nappe flow, partial nappe flow, and skimming flow. From a technical point of view, stepped gabion spillways can withstand flows up to $3m^3\cdot s^{-1}\cdot m^{-1}$ without great damage, if setting of gabions complies faithfully with the code of practice. Some advice is given on preventing gabion deformation.