ABSTRACT
Very low head hydropower (VLH) projects, typically defined as having a head of less than 3.2 m,* has generated recent interest for two main reasons. One is that they are environmentally attractive because they can be developed without constructing dams, and the newer turbine designs minimize impacts on fish. Specific characteristics of VLH turbines that reduce fish injury and mortality include (a) relatively larger diameter runner allows fish to keep away from the hub or blade end gaps; (b) lower speed of the runner reduces the impact of blade strikes; (c) smaller pressure gradient between the inlet and outlet of runner reduces pressure-related injury; (d) lower incidence of cavitation reduces associated mortality; (e) smaller shear stresses, which result from rapid changes in velocity over short distances, lowers mortality.†
The second reason VLH sites have become interesting is that most financially attractive, higher head hydropower sites have either been developed already or are prohibited from development for environmental reasons, particularly in OECD countries. See Chapter 15 for environmental considerations, increasingly resulting in limited availability of sites for developing new hydropower projects.
