Design durable banks with confident riprap thickness choices. Use velocity inputs and practical thickness limits. Download results, review examples, and build better protection fast.
| Velocity (m/s) | Flow type | SG | Safety factor | Computed D50 (mm) | Thickness (mm) using 2×D50 |
|---|---|---|---|---|---|
| 1.50 | High turbulence | 2.65 | 1.20 | 94 | 188 |
| 2.00 | High turbulence | 2.65 | 1.20 | 167 | 334 |
| 2.50 | Low turbulence | 2.70 | 1.30 | 206 | 412 |
Example values are illustrative; verify with your agency standards and site conditions.
If you use the D100 option, the calculator estimates D100 as a ratio times D50, then applies max(2×D50, D100).
For final design, confirm hydraulic conditions, filter criteria, gradation, and local specifications.
Riprap stability is mainly controlled by near-bed velocity, turbulence, and the submerged weight of rock. Higher velocities increase required stone size rapidly because the governing term scales with velocity squared. Turbulence reduces stability by intensifying lift and fluctuating shear, so the calculator’s turbulence choice adjusts the sizing constant. Stone specific gravity improves resistance by increasing submerged unit weight and reducing particle motion. Acceleration around bends and structures can raise effective design velocity.
The calculator converts your selected units to meters per second, then computes a median stone diameter (D50) using a velocity-based relationship. A safety factor is applied by increasing D50 directly, which provides a simple way to include uncertainty in hydraulics, gradation, and placement quality. Use D50 as a gradation anchor for quarry selection, then confirm that available rock classes can meet the target size and durability.
After D50 is obtained, thickness is determined by a practical rule. Many specifications target a minimum layer thickness near two times D50 to ensure interlock and adequate cover. The optional D100 check adds conservatism by ensuring large stones can fit within the layer without creating voided zones. The calculator also flags unusually thin or thick layers relative to D50 to support constructability reviews.
For estimating, volume is computed from plan area times thickness and then increased by a waste factor to reflect trimming, settlement, and placement losses. If area is not entered, length and width are used to estimate the footprint. Unit weight converts volume into mass for hauling and pricing. Cost can be expressed per tonne, per cubic meter, or per cubic yard, matching common supplier quotes and bid formats.
Field performance depends on more than stone size. Confirm that the subgrade is prepared, the filter layer or geotextile is compatible, and the riprap is placed to the specified thickness without segregation. Check for adequate toe embedment and transitions near structures. After construction, verify thickness using spot probes or survey sections and compare installed gradation against delivery tickets and inspection samples.
Use the design velocity at the protected surface, ideally near-bed or depth-averaged at the bank or channel zone where riprap will sit. If you only have average section velocity, add conservatism with a higher safety factor.
It changes the sizing constant used in the velocity-to-stone relationship. High turbulence produces a larger required D50 than low turbulence, reflecting stronger fluctuations that can dislodge rock even when average velocity is the same.
Choose higher values when hydraulics are uncertain, geometry is complex, rock quality varies, or consequences of failure are high. Typical preliminary design ranges from about 1.1 to 1.5, depending on agency practice.
A thicker layer supports interlock, reduces exposure of the filter, and helps resist local scour and rearrangement. Using about two times D50 also improves constructability because the rock can be placed without creating a single-stone layer.
Use it when your specification requires checking that larger stones fit within the layer, or when gradation is broad. The calculator estimates D100 from a ratio, then ensures thickness is not governed by an undersized layer.
The estimate uses a placed unit weight and a waste factor, which together approximate typical voids and handling losses. For detailed takeoffs, verify your placed density from supplier data and adjust waste based on placement method and access constraints.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.