Ultimate Riprap Sizing Calculator

Isbash (1936) · USACE Maynord (1988) · Hudson (1974) · Professional river bank protection & slope stability design

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📐 What is Riprap and Why Proper Sizing Matters?

Riprap is a layer of large stones used to protect river banks, slopes, and bridge abutments from erosion. The correct stone size (D50) ensures stability against flowing water, waves, and ice. Oversizing wastes money, undersizing leads to failure. This calculator uses three internationally accepted methods – Isbash (for steep slopes), USACE EM 1110-2-1601, Maynord (FHWA HEC-23), and Hudson (1974) for wave/current conditions. All methods include a slope correction factor (K₁) based on the bank angle and rock repose angle (φ=40°).

📌 The tool automatically applies a safety factor (SF) and recommends a final D50 – use the AI recommendation for design.

⚙️ Live Riprap Calculator

Flow Velocity m/s

Local velocity at bank toe (m/s or ft/s)

Flow Depth m

Design water depth

Bank Side Slope (H:V)

e.g. 2 → 1:2 slope (26.6°)

Rock Specific Gravity (Ss)

Safety Factor (SF)

Wave Height Hs (m) – for Hudson

Required for Hudson method

Output Unit

Method

Company Logo URL (optional)

Company Name

Batch Processing (ZIP → PDFs)

📈 Calculation History & Trend

📚 How to Use – Step by Step

Enter your site data – flow velocity, water depth, bank slope (H:V), rock density and safety factor. For Hudson, also enter wave height.
Select output unit – metres, feet, or inches.
Choose the method – Isbash (steep slopes), USACE (general), or Hudson (wave/current).
Click "Compute Stone Size" – the table shows results from all three formulas, plus an AI recommendation.
Download a PDF report – optionally add your company logo and name.
Batch process multiple designs – upload a ZIP containing CSV files (each with columns: Velocity, Depth, SlopeH, Ss, SF, Wave).
Track your history – all calculations are saved locally; you can clear or export them as CSV.
Share on LinkedIn – click the button below to share your result with a pre‑defined message and join the monthly lucky draw for a free consultation.

📐 Example Calculation

Input: Velocity = 2.5 m/s, Depth = 3.5 m, Slope 1:2, Ss=2.65, SF=1.2, Wave=0.5 m.
Isbash result: D50 ≈ 0.12 m (120 mm)
USACE result: D50 ≈ 0.14 m (140 mm)
Hudson result: D50 ≈ 0.08 m (80 mm)
AI recommended: 0.14 m (140 mm) – safe for most rivers.

For steep slopes (>1:1.5) the slope correction factor (K₁) significantly increases the required stone size – the calculator automatically accounts for this.

❓ Frequently Asked Questions

❓ Which formula should I trust?

Isbash is best for very steep banks, USACE for general applications, Maynord for shear‑stress based design. Hudson is used for wave/current conditions. The AI recommendation takes the conservative (largest) value when Froude number >0.8, otherwise a weighted average.

❓ What is the slope correction factor K₁?

K₁ = √(1 – sin²θ / sin²φ) where θ = bank slope angle, φ = rock repose angle (40°). Steeper slope → smaller K₁ → larger required stone size.

❓ Can I use this for coastal riprap?

Yes, the Hudson method is specifically designed for wave/current conditions. For coastal applications, also consider wave run‑up and overtopping.

❓ Is the tool free?

Absolutely free. No registration required. Batch processing supports up to 100 CSV files per ZIP.

🏆 Share your result & win

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Riprap Sizing Calculator – Isbash, USACE, Maynord | Bank Protection