Bioswale Flow & Sediment Trapping Simulator

Storm Inflow

Inflow discharge Q₀ 18 L/s

Longitudinal slope S 1.5%

Swale Design

Vegetation density 50%

Sediment grain size d 120 μm

Live Readouts

Inlet velocity

— m/s

Inlet depth

— cm

Outflow Q(L)

— L/s

Sediment trapped

—%

How it works

Flow down the vegetated channel is governed by Manning's equation, solved for normal depth h at every station x along the swale (rectangular approximation, bottom width b = 0.6 m):

Q(x) = (1/n) · A(x) · R(x)^(2/3) · S^(1/2)
A = b·h,  P = b + 2h,  R = A/P

n is the Manning roughness coefficient, higher for denser vegetation (mowed grass n≈0.035, dense wetland-style planting n≈0.24). Root structure also drives soil infiltration, so discharge decays exponentially down the channel:

Q(x) = Q0 · e^(−k·x),   k ∝ vegetation density

Each sediment grain settles under gravity at its Stokes settling velocity:

v_s = (2/9) · (ρ_s − ρ_w) · g · r² / μ

Classical ideal-settling theory says a particle is captured once it has time to sink through the local water depth before it exits: the fraction removed per unit length rises when v_s/h is large — i.e. fine clay barely settles, coarse sand drops out almost immediately. The 3D scene animates individual grains sinking to the bed (turning brown) once trapped, or escaping past the outlet if they don't.

Inflow Q0 — how much stormwater runoff enters the swale.
Slope — steeper channels flow faster and trap less.
Vegetation density — raises roughness (slows flow, deepens water) and raises infiltration.
Grain size — bigger grains settle faster and are trapped almost immediately; fine silt/clay mostly escapes.
Flush — releases a dense pulse of grains at the inlet, like the first flush of a storm.

Sediment in suspension Trapped on the bed

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