Seagrass Canopy Sediment Trapping

Meadow Structure

Shoot density 300 /m²

Canopy height 30 cm

Flow Conditions

Free-stream current 8 cm/s

Suspended Material

Particle diameter 80 µm

Live Readouts

Canopy flow attenuation Uc/U0—

Trapping efficiency—

Captured organic carbon (session)0.0 mg

How it works

Seagrass shoots act as a submerged canopy: their combined drag slows the water moving through the meadow far below the free-stream speed above it. Following the canopy-flow momentum balance (Nepf, 1999), the in-canopy velocity U_c relative to the free-stream speed U₀ depends on the shoots' frontal-area density:

a = n·d                (frontal area per bed area, m⁻¹)
Uc / U0 = 1 / √(1 + Cd·a·h)      (canopy attenuation)
Kz,canopy ≈ Kz,above · (Uc/U0)  (turbulence suppressed in-canopy)

n is shoot density, d the blade width, h the canopy height and Cd≈1 a drag coefficient. Denser, taller meadows attenuate flow more and quiet the near-bed turbulence, which is exactly what lets suspended organic particles settle out instead of being swept back into the water column.

Each particle falls under gravity at its Stokes settling velocity, perturbed by a turbulent random walk whose intensity depends on local Kz:

ws = 2r²(ρs − ρw)g / (9μ)     (Stokes settling velocity)
Δy = (−ws + η·√(2·Kz·Δt))·Δt,   η ~ N(0,1)

A particle that reaches the bed while inside the meadow is captured and its carbon is added to the sediment; one that drifts past the meadow's downstream edge before settling escapes. This settling-and-trapping process — canopy attenuation lowering turbulence and raising residence time for suspended matter — is the physical mechanism cited in seagrass blue-carbon literature (e.g. Gacia & Duarte, 2001; Hendriks et al., 2008) for why vegetated meadows bury far more organic carbon than bare sediment nearby. The animation runs at roughly 40× real time so settling — normally millimetres per hour for fine organic floc — is visible.

Shoot density / canopy height — set a and h, driving the attenuation ratio Uc/U0.
Free-stream current — sets U0, the speed above the canopy.
Particle diameter — sets the Stokes settling velocity ws (fine floc vs. coarser sediment).

Seagrass canopy Suspended organic particles

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