Sediment Transport Calculation Particle Size Velocity

Use this premium calculator to estimate settling velocity, particle Reynolds number, critical shear stress, shear velocity, and a practical threshold mean flow velocity for natural sediment grains. It is designed for river engineering, stormwater design, channel stability reviews, dredging studies, and geomorphology workflows.

Interactive Sediment Transport Calculator

Expert Guide to Sediment Transport Calculation, Particle Size, and Velocity

Sediment transport calculation is one of the most important tasks in fluvial hydraulics, geomorphology, stormwater engineering, channel design, reservoir management, and coastal science. Engineers and scientists often need a practical way to answer a few basic questions: how fast does a particle settle, what flow velocity is needed to begin moving it, and under what conditions is the sediment likely to remain in suspension rather than deposit on the bed? The interaction among particle size, particle density, fluid density, viscosity, and local flow velocity controls the outcome.

This calculator focuses on two highly useful ideas. First, it estimates settling velocity, which is the downward terminal speed of a grain falling through water. Second, it estimates the critical motion threshold through a Shields-based approach, producing critical shear stress, shear velocity, and an approximate section-average flow velocity at which a particle may begin to move. While no simplified calculator can replace a full sediment transport model, these metrics are extremely useful for screening, design checks, and field interpretation.

Why particle size matters so much

Particle size is the master variable in many sediment transport problems. Fine silt and clay can remain suspended for long distances because their settling velocities are extremely low. Sand grains settle much faster and therefore require turbulence to stay aloft. Gravel and cobbles settle rapidly and usually move as bed load during energetic flow events. As particle diameter increases, the grain generally becomes harder to keep in suspension, but it may also become easier to mobilize than cohesive clay because sand and gravel do not have the electrochemical cohesion that fine sediments exhibit.

In practice, sediment transport mode is often grouped into wash load, suspended load, and bed load. Fine grains tend to occupy the first two categories, while coarser grains are more commonly transported as bed load unless turbulence is intense.

Core concepts used in the calculator

• Settling velocity: estimated with the Ferguson-Church style relation, which performs reasonably across fine to coarse noncohesive grains.
• Submerged specific gravity: based on the ratio between sediment density and fluid density.
• Particle Reynolds number: a diagnostic value used to classify the settling regime.
• Critical shear stress: estimated from the Shields criterion, a classic approach for initiation of motion.
• Critical shear velocity: derived from bed shear stress and fluid density.
• Approximate critical mean velocity: estimated from a logarithmic velocity profile using flow depth and roughness proportional to grain size.

How settling velocity is interpreted

Settling velocity can be thought of as a simple competition against upward mixing by turbulence. If the vertical turbulent fluctuations in the flow are large enough compared with the settling speed, the grain may remain suspended. If turbulence weakens or the flow enters a backwater, pond, floodplain, or reservoir zone, particles with higher settling velocity deposit first. That is why coarser sediment is typically found nearer to sediment sources or in high-energy channels, while finer material travels farther downstream.

For very small particles in laminar settling, Stokes’ law can be used. However, many natural grains in channels do not settle strictly in the Stokes regime. The Ferguson-Church relation is useful because it smoothly spans finer and coarser grains and captures the effect of increased drag at larger particle Reynolds numbers. That makes it a strong choice for a practical field calculator.

What the critical velocity result means

The critical velocity displayed by this tool is not an exact universal threshold. Instead, it is an engineering estimate derived from the critical Shields parameter and a roughness-based logarithmic velocity profile. In real rivers and canals, initiation of motion is influenced by hiding and exposure effects, bed sorting, bed forms, grain shape, packing, armoring, cohesion, vegetation, and unsteadiness of flow. Still, the threshold estimate is valuable because it tells you whether your current or design velocity is comfortably below, near, or above the expected entrainment range.

Typical sediment classes and indicative settling behavior

Sediment class	Typical size range	Indicative settling velocity in water	Common transport behavior
Clay	< 0.004 mm	Often less than 0.001 m/s for dispersed particles	Mostly wash load or persistent suspension; cohesion dominates behavior
Silt	0.004 to 0.0625 mm	About 0.001 to 0.01 m/s depending on size and flocculation	Suspended load common in low to moderate energy flows
Fine sand	0.0625 to 0.25 mm	About 0.005 to 0.03 m/s	Easily suspended in turbulent flow; deposits in lower energy zones
Medium sand	0.25 to 0.5 mm	About 0.03 to 0.07 m/s	Moves as suspended and bed material load depending on turbulence
Coarse sand	0.5 to 2 mm	About 0.07 to 0.25 m/s	Often bed load with intermittent suspension during energetic events
Gravel	2 to 64 mm	Usually greater than 0.25 m/s and rising rapidly with size	Primarily bed load; requires higher boundary shear to mobilize

The ranges above are general and depend on grain shape, density, water temperature, and whether fines are flocculated. For example, natural quartz sand at 20°C behaves differently from heavy mineral particles or biologically flocculated estuarine sediment. This is why it is good practice to treat any single settling velocity as a representative estimate rather than a fixed truth.

Useful reference statistics for hydraulic design screening

Reference quantity	Common value	Practical design implication
Fresh water density at about 20°C	Approximately 998 to 1000 kg/m³	Small density changes modestly affect computed settling and threshold values
Kinematic viscosity of fresh water at about 20°C	Approximately 1.0 × 10^-6 m²/s	Warmer water lowers viscosity and generally increases settling velocity
Quartz sediment density	Approximately 2650 kg/m³	A common default for sands and gravels in river applications
Typical critical Shields parameter for noncohesive grains	About 0.03 to 0.06	Lower values predict easier motion; higher values are more conservative
Von Karman constant used in log-law profiles	Approximately 0.41	Used to convert shear velocity into an estimated section mean threshold velocity

Step-by-step method for evaluating sediment movement

1. Identify representative grain size. For a mixed bed, use a characteristic size such as d50, d35, or d84 depending on the design objective.
2. Enter sediment and fluid properties. Quartz in fresh water is a reasonable default for many channels.
3. Use a realistic flow depth. The estimated critical mean velocity depends on depth through the log profile.
4. Compare mean flow velocity with critical mean velocity. If the actual velocity is above the threshold estimate, entrainment becomes more likely.
5. Compare mean flow velocity and turbulence conditions with settling velocity. If turbulence is strong relative to settling, suspension is more likely.
6. Interpret within site context. Bed forms, armoring, cohesion, vegetation, and hydrograph timing can shift the true response.

How grain shape changes the answer

Rounded grains usually settle a bit faster than angular grains because they experience somewhat lower drag for a given nominal diameter. Angular grains can also interlock on the bed, which may increase resistance to movement in some contexts. In natural systems, however, shape is only one of many factors. Sorting, hiding and exposure, local roughness, and biologic effects can be equally important. The grain-type selector in the calculator changes the settling equation coefficients to provide a practical sensitivity check.

When this calculator works best

• Noncohesive sand and gravel analysis
• Screening studies for channel stability
• Stormwater conveyance checks
• Reservoir or basin deposition estimates
• Preliminary dredging and sedimentation assessments

When you need a more advanced model

• Cohesive clay and silt with flocculation
• Tidal, density-stratified, or saline systems
• Strongly unsteady flood waves
• Mixed-size armoring and hiding exposure problems
• Detailed morphodynamic forecasting

Important limitations

A calculator like this gives engineering approximations, not exact predictions. It does not directly compute bed-load transport rates in kilograms per second or cubic meters per year, nor does it resolve time-varying turbulence spectra. It also treats the bed as a representative roughness field rather than a dynamically evolving surface. For design work with serious safety, environmental, or regulatory consequences, use the results here as a first-pass estimate and confirm them with site data, field calibration, or a specialized sediment transport model.

Authoritative sources for further study

For readers who want standards, datasets, and research-backed methods, these sources are excellent starting points:

• U.S. Geological Survey publications on sediment transport and fluvial processes
• U.S. Army Corps of Engineers Hydrologic Engineering Center resources
• University of Colorado Community Surface Dynamics Modeling System

Practical engineering takeaway

If you remember only one thing, remember this: particle size controls both how quickly a sediment grain falls out of the flow and how much hydraulic force is needed to move it. Fine sediment is easy to keep in suspension but may be difficult to predict because cohesion and flocculation matter. Coarse sediment settles quickly and usually moves near the bed, but it requires larger boundary stresses to initiate motion. By comparing settling velocity, critical shear, and actual flow velocity, you gain a fast, defensible picture of whether deposition, suspension, or bed motion is the dominant behavior in your system.

That combination of metrics is exactly why sediment transport calculation remains central to river restoration, bridge scour assessment, culvert design, detention basin performance, reservoir life estimates, environmental permitting, and flood resilience planning. A thoughtful interpretation of particle size and velocity can prevent erosion failures, reduce maintenance costs, improve habitat outcomes, and support better hydraulic design from the first concept sketch to final engineering review.