Sedimentation in Reservoirs & Streams Notes

Last reviewed 1 Oct 2026 · 6 min read

Why sedimentation matters

Soil eroded from catchments is carried by streams and deposited in reservoirs, tanks, canals and river beds, which reduces storage capacity, shortens project life, raises flood levels, and clogs canals and turbines. Reservoir siltation is a major concern in India because many catchments are heavily eroded.

Sediment sources and yield

Sources: sheet and rill erosion, gullies and ravines, stream-bank erosion, landslides, construction and mining.
Gross erosion $E$ (from the catchment, e.g., from USLE) is greater than the sediment actually reaching the outlet — much is re-deposited on slopes and in channels.

FormulaSediment yield and delivery ratio

Sediment delivery ratio: $S D R = \frac{Y}{E}$ , where $Y$ = sediment yield at the point of interest and $E$ = gross erosion in the catchment.

SDR decreases as the catchment area increases (small catchments deliver a high fraction; large ones store much sediment in valleys); typical values range from about 0.1 to 0.8 and depend on slope, drainage density, land use and soil.

Sediment yield is expressed in t/km²/year (or m³/km²/year or ha-m/100 km²/year) and is estimated from sediment gauging, reservoir surveys, empirical formulas (Khosla, Dendy–Bolton), USLE with SDR, or GIS models.

Sediment transport

Load	Description
Suspended load	Fine particles carried in the body of the flow by turbulence (silt, clay, fine sand) — the main portion in most Indian rivers
Bed load	Coarse particles moving by rolling, sliding and saltation near the bed — about 5–25 % of the total
Wash load	Very fine (< 0.063 mm) particles that stay in suspension and are not in the bed material
Total load	Suspended + bed load

Initiation of motion

Grains begin to move when the bed shear stress $τ_{0} = γ R S$ exceeds the critical shear stress $τ_{c}$ ; the Shields parameter $τ_{*} = \frac{τ _{0}}{( γ _{s} - γ ) d}$ has a critical value of about 0.05 for coarse sediment (about 0.047 in Meyer-Peter and Müller's formula).
Bed-load formulas: Meyer-Peter–Müller, Einstein, Duboys, Shields; suspended load by the Rouse equation; total load by Engelund–Hansen or Ackers–White.

Settling velocity (Stokes' law)

For fine particles in laminar settling (particle Reynolds number below about 1):

v_{s} = \frac{g d ^{2} ( ρ _{s} - ρ )}{18 μ}

Worked ExampleExample — settling velocity

A silt particle of diameter 0.05 mm ( $d = 5 \times 1 0^{- 5}$ m), density $ρ_{s} = 2650$ kg/m³, settles in water ( $ρ = 1000$ kg/m³, $μ = 1.0 \times 1 0^{- 3}$ Pa·s):

$v_{s} = \frac{9.81 \times ( 5 \times 1 0 ^{- 5} ) ^{2} \times 1650}{18 \times 1 0 ^{- 3}} = \frac{9.81 \times 2.5 \times 1 0 ^{- 9} \times 1650}{0.018} = \frac{4.05 \times 1 0 ^{- 5}}{0.018} = 2.25 \times 1 0^{- 3}$ m/s $= 2.2 mm/s$ . It takes about $\frac{3}{0.00225} \approx 1330$ s (22 min) to settle through 3 m of still water; a clay particle of 0.002 mm settles about 600 times slower ( $d^{2}$ effect), which is why clay remains in suspension.

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