Sizes the trash rack of a Tyrolean (bottom) intake for a target diversion discharge in a mountain stream. Implements the classical design equation Q = c · μ · b · L · √(2gh) with rack-shape coefficient c, bar-shape discharge coefficient μ, and inclination factor k tabulated per Mosonyi. Adjust inputs to compute required rack length, water capture ratio, and rack-bar geometry.
For engineering reference
The classical Frank–Mosonyi formulation treats the rack as a submerged weir with discharge dependent on initial water depth, void fraction, bar shape, and rack inclination:
Per Mosonyi's tabulation, k decreases from 1.000 at β = 0° to roughly 0.75 at β = 36°. The calculator linearly interpolates between tabulated values. Typical Tyrolean weirs are inclined 15°–30° to promote self-cleaning by gravity-driven sediment transport along the rack.
Bar shape strongly affects intake efficiency. μ ≈ 0.50 for sharp-edged
rectangular bars rises to ~0.95 for streamlined oval profiles. Round-edged
rectangular bars (the standard fabricated profile) sit at roughly 0.65 and
are the default here. These coefficients trace to Noseda's experiments and are reported
in Mosonyi's High-Head Power Plants.
The hydraulic rack length is multiplied by SF = 1.5 to 2.0 to allow for
clogging from cobbles, leaves, branches, or organic debris — particularly important in
glacier-fed and forested catchments. Without this margin, design discharge is not delivered
whenever the rack partially blocks. Drobir, Kienberger and Krouzecky (1999) recommend the upper
end of this range for unattended remote installations.
This is a first-pass design tool, not a substitute for engineering judgement or physical modelling. It does not compute: sediment-transport effects on rack capacity, the collecting gallery longitudinal profile, energy dissipation downstream, structural design of the bars and supports, scour protection at the toe, ice and frazil-ice handling, or the hydraulic behaviour at flood discharges substantially exceeding the design value. For projects above roughly 5 m³/s diversion or in highly variable catchments, supplement with CFD (FLOW-3D, OpenFOAM) or a physical model (TU Vienna, EPFL-LCH, University of Innsbruck).
The canal width B is shown for reference as B = L · cos(β), per Mosonyi's guideline that the canal should approximately match the projected horizontal length of the rack. The canal depth should be set so a freeboard of at least 0.10–0.20 m is maintained at design flow.