Drainage formula (Water Overlay)

Drainage passive: First the flow capacity is calculated

$Q_{p,t}=\Delta t\cdot q_{t}$

$Q_{b}=A_{d}\cdot f_{s}\cdot {\frac {(w_{g}-w_{w})\cdot A_{w}}{A_{d}\cdot f_{s}+A_{w}}}$

if w_w > w_g then:

$Q_{max}=A_{w}\cdot (w_{w}-z_{w})$

else:

$Q_{max}=A_{d}\cdot (w_{d}-z_{d})\cdot f_{s}$

$Q_{d}=min(Q_{p,t},Q_{b},Q_{max})$

Drainage active:

Where:

w_{g}

is the ground water level in meters

w_{w}

is the water level in the waterway

z_{d}

is the drainage datum height in meters.

A_{d}

is the drainage area size in square meters.

A_{w}

is the waterway area size in square meters.

z_{w}

is the max waterway height.

f_{s}

is the average storage percentage of the ground above the drainage.

Q_{b}

is the amount transfered for a balanced ground and surface water level

Q_{max}

is the amount available in the ground above the drainage.

Q_{p_{t}}

is the drainage capacity at time t

Q_{d}

is the actual drained volume at time t

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