Rack Length in Bottom Intake Systems

Bottom intake systems, made by racks longitudinally disposed in the flow direction, are classically used to derive mean flows in continuous mountain rivers with intensive sediment transport. They are known as Caucasian or Tyrolean Intakes. Nowadays, works are also being addressed to study these intakes in order to derive flash floods from ephemeral rivers in semiarid zones to collect runoff and improve the availability of resources. In the design of bottom intake systems, the length of rack is an important variable. This article includes several formulations that come from experimental works in laboratory and in prototype. The objective is to complement and update those works, including some empirical expressions proposed and experimental works developed since 2010 at the Universidad Politécnica de Cartagena. Rack length values for different flow rates are presented in dimensionless form (L/h – required rack length over critical depth) for the spacing area ratio over total area (void fraction m) of values 0.12, 0.28 and 0.60. These lengths present important differences depending on the conditions for which they were obtained, for example type of bars (circular or flat), spacing between the bars or the slope adopted by the rack. Values of the dimensionless rack length considering obstruction factors are also analysed. The Froude similitude criterion has been considered to obtain prototype values from the laboratory experimental conditions.