Calculator Inputs
Example Data Table
| Case | Unit | Method | Key Inputs | Estimated D50 | Estimated Thickness |
|---|---|---|---|---|---|
| A | Metric | Velocity | V=1.8 m/s, Ss=2.65, K=1.20, FS=1.30 | ≈ 217 mm | ≈ 434 mm |
| B | Metric | Shear | y=3.0 m, S=0.003, Ss=2.65, θc=0.047, FS=1.30 | ≈ 151 mm | ≈ 302 mm |
| C | Imperial | Velocity | V=6.0 ft/s, Ss=2.65, K=1.20, FS=1.30 | ≈ 8.8 in | ≈ 17.6 in |
Formula Used
- D50 = median rock size
- V = design velocity
- Ss = specific gravity of stone
- K = coefficient (shape/placement)
- FS = safety factor
- τ = bed shear stress
- R ≈ depth for wide channels
- S = energy slope
- θc = critical Shields parameter
How to Use This Calculator
- Select a unit system and keep inputs consistent.
- Choose a sizing method to match your hydraulic basis.
- Enter velocity, depth, slope, stone gravity, and safety factor.
- For velocity sizing, pick a rock-shape preset or custom coefficient.
- For shear sizing, review the critical Shields parameter value.
- Click Calculate to view results above the form.
- Download CSV or PDF to attach to submittals.
Professional Guide to Riprap Sizing for Scour Protection
1) Why riprap matters at scour locations
Scour removes supporting soil around abutments, piers, outfalls, and channel transitions. Well-sized rock armor adds weight and roughness that resists particle pickup, reduces near-bed velocity, and stabilizes the bed surface. Project records often show that undersized rock fails first at edges and around appurtenances where turbulence is concentrated.
2) Inputs that drive rock size the most
Velocity, depth, and energy slope are the dominant hydraulic drivers. For rock properties, specific gravity is critical: common quarry stone is typically 2.60–2.75, while lighter stone requires larger size for the same stability. This calculator also applies a safety factor, normally 1.2–1.5 for routine conditions and higher when access, inspection, or consequence of failure is severe.
3) Velocity method data and typical ranges
The velocity method uses a coefficient to represent rock shape and placement. Angular rock is commonly modeled with K≈1.20, while rounded rock can be nearer K≈0.86. If your hydraulic model reports 1.5–3.0 m/s in the near-bed zone, median sizes often fall in the 100–300 mm range, depending on stone density and safety factor.
4) Shear method data and typical ranges
The shear method estimates bed shear stress using τ = ρ·g·R·S, where R is commonly approximated by depth for wide channels. A frequently used critical Shields parameter is θc≈0.047 for turbulent flow. For depths of 1–4 m and slopes of 0.0005–0.005, the resulting sizes often align with well-graded riprap classes used for river training works.
5) Thickness and placement guidance
A practical rock layer thickness is typically 2.0×D50 for dumped placement, with thicker layers used where bedding irregularity or construction tolerance is limited. This tool lets you select a factor from 1.5–4.0. In many field specifications, a two-layer placement and a stable toe are emphasized to prevent edge unraveling during high flows.
6) Gradation and durability checks
Gradation controls interlock and void ratio. A broad band with Dmax around 1.5×D50 and Dmin around 0.5×D50 supports packing and reduces selective transport. Durability is equally important: soundness loss, abrasion resistance, and angularity affect long-term performance, especially where bedload impacts occur.
7) Scour geometry and edge protection
Sizing alone is not enough; the layout must address likely scour geometry. A stable toe, adequate apron width, and tie-ins to existing banks reduce flanking. At piers and abutments, local vortices can exceed section-average conditions, so designers often adopt conservative sizing or add site multipliers consistent with agency guidance.
8) Documentation for submittals and reviews
Exported CSV and PDF results help capture assumptions, units, and chosen method. Include the hydraulic basis (model, event, and location), stone source properties, safety factor rationale, and placement details. Clear documentation improves constructability reviews, supports QA inspection, and reduces change orders during high-flow seasons.
FAQs
1) Which method should I use for design?
Use the method aligned with your hydraulic output. If you have reliable depth and slope, shear sizing is consistent. If velocity is better defined near the bed, velocity sizing is practical. Compare both and adopt conservative results.
2) What safety factor is common for riprap?
Many designs use FS between 1.2 and 1.5 for typical sites. Increase it when turbulence is high, inspection is difficult, or failure consequences are significant. Always follow local standards and project specifications.
3) Does stone shape change the required size?
Yes. Angular rock interlocks better and is often represented with a higher coefficient than rounded rock. Rounded material may require larger median size for similar stability, particularly in high-velocity zones.
4) How thick should the riprap layer be?
A common starting point is thickness t≈2.0×D50 for dumped rock. Use higher factors where grade tolerance is poor, where bedding is uneven, or where strong turbulence may cause rocking and displacement.
5) What does the “equivalent stone weight” represent?
It is a simplified estimate based on a spherical equivalent size at D50 and the stone density. It helps relate the design size to common stone classes, but it does not replace quarry gradation reports.
6) Can I use this for pier or abutment local scour?
You can use it as a baseline, but local scour features can demand larger rock and thicker placement. Apply site multipliers or agency guidance for piers, abutments, bends, and contractions, and confirm with hydraulic study outputs.
7) What should I include in a construction submittal?
Provide the design event, hydraulic inputs, method selection, safety factor basis, target D50 and thickness, gradation limits, stone durability properties, and placement details. Attach the exported results for clear traceability.