Mangrove Root Sediment Trap

Tidal Flow

Root (pneumatophore) density 120 stems/m²

Free-stream current U₀ 0.25 m/s

Sediment grain size 125 µm (fine sand)

Live Readouts

Velocity at canopy exit

— % of U₀

Settling velocity

— mm/s

Capture efficiency (trapped / released)

—

How it works

Emergent roots act as a drag-dominated porous obstruction. For a flow entering a canopy of cylindrical stems of diameter d at density n (stems per m²), the frontal area per unit volume is a = n·d, and the streamwise velocity decays with penetration depth x into the root matrix following a drag-attenuation model:

U(x) = U₀ · exp(−λx),   λ = ½·Cd·a
Cd ≈ 1.0 (cylinder drag coefficient), d = 0.03 m

Each sediment grain settles under gravity at its Stokes terminal velocity (valid for small, low-Reynolds-number particles such as silts and fine sands):

v_s = (2/9) · (ρs − ρw) · g · r² / μ
ρs = 2650 kg/m³ (quartz), ρw = 1000 kg/m³, μ = 1.1×10⁻³ Pa·s

A grain is advected downstream by the local U(x) while settling toward the bed at v_s. Where roots have slowed the current enough that the local flow can no longer keep the grain in suspension, it deposits and joins the bed — this is exactly the trapping mechanism by which real mangrove pneumatophores and prop roots baffle tidal currents and accrete sediment, raising and stabilizing the substrate around them.

Root density — raises a = n·d, steepening the velocity decay λ and shrinking the distance needed to slow the current below the resuspension threshold.
Free-stream current — faster water carries grains further into (or through) the root field before they can settle.
Grain size — larger grains have much higher v_s (∝ r²) and drop out almost immediately; fine silts stay suspended far longer and are only trapped where roots are dense.

Pneumatophore roots Suspended sediment Deposited / bed

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