Sediment Fall Velocity Calculator

Formula used

How to use this calculator

1. Enter sediment diameter based on sieve or lab data.
2. Confirm particle density (2650 kg/m³ is common mineral soil).
3. Set water temperature so viscosity is estimated realistically.
4. Choose Auto unless you must enforce a specific method.
5. Enter settling depth to estimate how long particles need.
6. Press Calculate; review velocity, Reynolds number, and regime.
7. Use CSV or PDF exports for submittals and reports.

What fall velocity represents on site

Fall velocity is the downward settling speed of an individual grain moving through still water. In drainage controls, it acts as a performance benchmark: faster settling means shorter detention is needed. Use the reported velocity to compare against basin overflow rates, skimmer drawdown, and dewatering discharge expectations.

Typical grain sizes and expected behavior

Common size bands help interpret results: clay is typically <0.004 mm, silt is about 0.004–0.063 mm, and sand is about 0.063–2.0 mm. For the same density, velocity increases roughly with the square of diameter in laminar settling, so small changes in fines can drive large capture differences.

Why water temperature matters

Water viscosity decreases as temperature rises, so warm water lets grains settle faster. As a practical reference, dynamic viscosity is roughly 1.8 mPa·s near 0 °C and about 1.0 mPa·s near 20 °C. If you plan winter operations, use a lower temperature to avoid overly optimistic settling predictions.

Stokes range and Reynolds checks

Stokes settling assumes a laminar wake and works best when the particle Reynolds number is well below 1. This calculator reports Reynolds number and labels regimes: laminar (<1), transitional (1–500), and turbulent (>500). If Reynolds rises, drag behavior changes and Stokes can underpredict settling time.

Using Ferguson–Church for mixed sediments

The Ferguson–Church approach bridges laminar and turbulent ranges and is suited for mixed-site gradations from fine silt to coarse sand and small gravel. Default coefficients (C1 = 18, C2 = 1) are widely used. If field calibration shows slower settling, modestly increasing C1 can improve fit for angular grains.

Settling time and basin sizing

Settling time is computed as t = depth / w_s. For example, at 1.0 m depth and 0.01 m/s velocity, time is 100 s (about 1.7 min). For very fine silt at 0.0005 m/s, time becomes 2000 s (about 33 min), which often exceeds short storm drawdown windows.

Linking results to overflow rate

A common design check is comparing surface overflow rate to settling velocity: when the upward hydraulic loading is less than w_s, particles can be removed efficiently under ideal plug-flow conditions. Convert quickly: 0.01 m/s equals 0.6 m/min and 36 m/hr. Use conservative allowances for short-circuiting and inlet turbulence.

Field factors that change performance

Real basins rarely match still-water assumptions. Turbulence, flocculation, salinity, and high solids concentration can alter effective settling. In construction controls, applying a safety factor of 1.5–3 on detention time is common, especially when targeting fines. Verify by observation during commissioning, and adjust skimmer settings or baffles to reduce short-circuiting.

FAQs

1) What inputs are required for a reliable estimate?

Enter grain diameter, particle density, water temperature, and fluid density. Add settling depth if you need time. Use measured gradation data where possible; visual estimates can be off by an order of magnitude for fines.

2) When should I keep the method on Auto?

Auto is best for general design because it uses laminar settling only when Reynolds is clearly low. For mixed sands and silts, Auto typically selects the generalized method and avoids optimistic results.

3) Can this help size a sediment basin?

Yes. Use fall velocity to compare against surface overflow rate and to estimate settling time for a given depth. Then confirm detention time, drawdown method, and baffle layout to limit short-circuiting.

4) How do I use viscosity override correctly?

Check “Override viscosity” and enter either dynamic viscosity (Pa·s) or kinematic viscosity (m²/s). The tool will compute the missing value using fluid density. If both are blank, it falls back to temperature estimation.

5) What does Reynolds number mean in this report?

Reynolds number indicates the settling flow regime around the grain. Low values suggest laminar drag behavior; higher values indicate transitional or turbulent wakes. It is a quick validity check for whether laminar assumptions are reasonable.

6) Why are very fine particles showing tiny velocities?

Velocity decreases rapidly with diameter, especially for clay-size material. Fines can also remain suspended due to electrochemical effects and turbulence. In practice, polymers, flocculation, or extended detention are often needed for meaningful removal.

7) Are the results conservative for construction controls?

They represent still-water, isolated-particle behavior, which can be optimistic when basins are turbulent. For planning, apply safety factors, consider baffles, and validate with site observations. Treat the output as a baseline for comparison.