Riprap & Launching Apron Design – Quick Tool

Sizes riprap stone and launching aprons for stilling basin outfalls and guide bund toes using velocity‑based Izbash and shear‑based Shields concepts. Also gives a simple rule‑based launching width and depth. SI units: m, s, kg/m³, Pa.

1 Calculations – Flow & Loading

Provide local flow conditions downstream of the stilling basin or along the guide bund. You can design based on velocity (Izbash) and/or bed shear (Shields).
Typically taken just above the riprap layer. If unknown, use stilling basin / model results.
Depth over the riprap or at the toe section, used for shear estimation if τ not given.
Used to estimate bed shear τ ≈ ρw g h S when τ is not entered explicitly.
If provided, this overrides τ = ρw g h S for the shear‑based design.

2 Rock & Launching Geometry

Define stone properties, design safety factor and simple rules for thickness, launching depth and width.
Typical granite / hard rock ≈ 2600–2700 kg/m³.
Applied on velocity/shear demand. SF ≈ 1.2–1.5 is common for riprap stability.
Empirical coefficient in Vc = KIz √[(ρs/ρw − 1) g d].
For coarse gravel / small rock, θc ≈ 0.04–0.06 is typical.
Optional: size you intend to use. The tool will report a stability factor vs required size.
Typical riprap layer thickness ≈ 1.5–2.0 D50.
Trial thickness of the apron before launching. Compared to Tmin.
Simple rule: launch depth often taken as 1.5–3 times stone size.
Simple geometry rule scaling width to launch depth (e.g. slope ≈ 1V:1.5H).
From your scour tool (Lacey / HEC‑18 etc.). Launch depth will not be less than Ds.
All methods are computed; this selection decides which D50 is adopted for apron sizing.