What Is Rip Rap? The Riprap Definition
Rip rap (also written as riprap or rip-rap) is a permanent or temporary layer of large, angular rocks or stones placed along embankments, streambeds, shorelines, or channels to prevent erosion caused by moving water. The rocks are piled together—either loose or grouted with mortar—to create a protective armor that absorbs and deflects the energy of flowing water.
Loose riprap is excellent for water drainage and allows natural vegetation to establish through the voids. Grouted riprap provides additional structural support but sacrifices drainage. Riprap is widely used in civil engineering applications including river bank protection, highway embankments, dam slopes, coastal shoreline stabilization, and open channel lining.
How Do We Calculate Rip Rap Rock Size?
Selecting the correct rock size is critical for riprap stability. Rocks that are too small will be swept away by the current, while oversized rocks increase project cost unnecessarily. The most common method for calculating the minimum required rock size is the Isbash equation:
Where:
- D₅₀ — median diameter of the riprap rock (50% of rocks are smaller than this size), in feet or meters
- V — average channel or embankment flow velocity (ft/s or m/s)
- g — gravitational acceleration: 32.174 ft/s² (US Customary) or 9.806 m/s² (Metric)
- C — Isbash stability coefficient:
- C = 0.86 for highly turbulent flow (e.g., direct wave impact, high-velocity jets)
- C = 1.20 for low-turbulence flow (e.g., channel linings, mild embankments)
- S — specific gravity of the rock (dimensionless ratio of rock density to water density). Typical values: granite ≈ 2.65, limestone ≈ 2.50–2.70, basalt ≈ 2.90–3.00
Sample Calculation of Rip Rap Rock Size
Example (US Imperial): A channel experiences a flow velocity of 5 ft/s with high turbulence. The available rock is granite (S = 2.65).
D₅₀ = 5² / (2 × 32.174 × 0.86² × (2.65 − 1))
D₅₀ = 25 / (2 × 32.174 × 0.7396 × 1.65)
D₅₀ = 25 / 78.52
D₅₀ ≈ 0.318 ft ≈ 3.82 inches
Example (Metric): Same scenario at 1.52 m/s high turbulence, granite (S = 2.65).
D₅₀ = 1.52² / (2 × 9.806 × 0.86² × 1.65)
D₅₀ = 2.310 / 23.93
D₅₀ ≈ 0.0965 m ≈ 9.65 cm
How Much Rip Rap Do I Need?
After determining the required rock size, you need to estimate the total volume and weight of riprap for your project:
- Measure the area to be protected: length × width (of the embankment face or channel bottom)
- Determine layer thickness: typically 1.5 to 2 times the D₅₀ diameter. For Class I riprap (D₅₀ ≈ 6 in), a layer of 12–18 inches is common
- Calculate volume: Volume = Length × Width × Thickness
- Calculate weight: Weight = Volume × bulk density. Loose riprap has a typical bulk density of about 100 lb/ft³ (1,600 kg/m³)
Our calculator performs all these steps automatically. Simply enter the embankment dimensions and layer thickness to get volume in cubic yards or cubic meters, and weight in short tons or metric tonnes.
Riprap Size Chart
The table below shows the standard riprap classification used in most US and international projects:
| Class | D₅₀ (in) | D₅₀ (cm) | Typical Use |
|---|---|---|---|
| Class 0 | < 4 | < 10 | Light erosion control, low-velocity runoff, garden features |
| Class I | 4–8 | 10–20 | Low-velocity channels (< 4 ft/s), small ditches, moderate embankments |
| Class II | 8–12 | 20–30 | Medium-velocity channels (4–7 ft/s), stream bank protection |
| Class III | 12–18 | 30–45 | High-velocity channels (7–10 ft/s), highway culvert protection |
| Class IV | 18–24 | 45–60 | Very high velocity (> 10 ft/s), bridge scour protection, dam outlets |
| Class V | > 24 | > 60 | Extreme conditions, tidal protection, large river embankments |
How to Use Our Rip Rap Calculator
- Choose your unit system: US Imperial (ft, in, lb, short ton) or Metric (m, cm, kg, tonne)
- Enter channel velocity: The average flow speed at the embankment face (ft/s or m/s)
- Select turbulence level: Highly turbulent (C = 0.86) or low turbulence (C = 1.20). Use high turbulence for direct wave impact, cascades, or irregular flow
- Enter rock specific gravity: Default is 2.65 for granite. Use 2.50–2.70 for limestone, 2.90–3.00 for basalt
- Optionally enter area dimensions: Length, width, and layer thickness to calculate volume and weight
- Optionally enter unit price: To get a total material cost estimate in your preferred currency
- Click Calculate to instantly see the required D₅₀ diameter, rock class, volume, weight, and cost
Want to Learn More?
For deeper study of riprap design, refer to the following engineering standards:
- ASCE/EWRI 36-01 — Sedimentation Engineering
- USACE EM 1110-2-1601 — Hydraulic Design of Flood Control Channels
- HEC-11 (FHWA) — Design of Riprap Revetment
- NCHRP Report 568 — Riprap Design Criteria, Recommended Specifications, and Quality Control
FAQs
- What is the Isbash constant C?
- The Isbash stability coefficient accounts for turbulence. C = 0.86 is used where flow is highly turbulent (wave impact, irregular beds). C = 1.20 applies to smooth, well-developed flow profiles. Using a lower C requires larger rocks, providing a safety margin for turbulent conditions.
- What is specific gravity and why doesn't it have units?
- Specific gravity (S) is the ratio of a material's density to the density of water (1,000 kg/m³ or 62.4 lb/ft³). Since both densities are in the same units, the units cancel out, leaving a dimensionless number. Granite has S ≈ 2.65 meaning it is 2.65 times denser than water.
- What layer thickness should I use for riprap?
- The recommended minimum layer thickness is 1.5 × D₅₀ for rounded stones and 1.0 × D₅₀ for angular stones. For practical construction, most engineers specify a minimum of 12 inches (30 cm) or 2–3 times the D₅₀, whichever is greater.
- Can I use riprap on slopes?
- Yes. Riprap is commonly used on slopes up to 1V:1.5H (33.7°). For steeper slopes, anchored riprap or grouted riprap may be required. The Isbash equation assumes a horizontal or mildly inclined placement; a slope correction factor may be needed for steep embankments.
- What is grouted riprap?
- Grouted riprap has mortar or concrete injected into the voids between rocks, bonding them together. It provides greater resistance to high-velocity flows and wave impact but eliminates natural drainage. It is used in high-energy environments such as dam spillways and tidal zones.
- What is the typical cost of riprap?
- Riprap costs vary by region, rock type, and transport distance. In the US, typical prices range from $25–$75 per short ton for Class I–II stone. Larger classes (III–V) requiring quarry blasting cost $50–$150+ per ton. Use our cost calculator to estimate your project budget in any world currency.