# Drainage formula (Water Overlay)

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Depending on whether the drainage is passive or active, the formula for either passive or active drainage is used.

## Drainage passive

First the flow capacity is calculated

if then:

else:

Where:

- is the DRAINAGE_Q of the Drainage at time t.
- is the computational timestep.
- is the ground water level, above the drainage, in meters.
- is the water level in the waterway.
- is the drainage datum height in meters.
- is the max waterway height.
- is the drainage area size in square meters.
- is the waterway area size in square meters.
- is the average storage percentage of the ground above the drainage.
- is the possible amount of water drained, or pumped back when negative, based on the DRAINAGE_Q of the Drainage and timestep .
- is the amount transferred for a balanced ground and surface water level.
- is the amount of water available in the ground above the drainage.
- is the amount of water available in the waterway.
- is the actual drained, or infiltrated backwards, volume at time t.

## Drainage active

*Case 1: Active Draining:*

If a positive DRAINAGE_Q is defined:

If an overflow threshold is defined as well:

The actual water pumped out of the drainage system is calculated. If any of the terms are undefined, they are not included.

*Case 2: Active Reverse Draining:*

If a negative DRAINAGE_Q is defined:

If an overflow threshold is defined as well:

The actual water pumped into the drainage system is calculated. If any of the terms are undefined, they are not included.

Where:

- is the DRAINAGE_Q of the Drainage at time t.
- is the computational timestep.
- is the possible amount of water drained, or pumped back when negative, based on the DRAINAGE_Q of the Drainage and timestep <math<\Delta t</math>.
- is the ground water level, above the drainage, in meters.
- is the water level in the waterway.
- is the drainage datum height in meters.
- is the waterway's datum height.
- is the average storage percentage of the ground above the drainage.
- is the amount of water available in the ground above the drainage.
- is the overflow threshold in meters at time t.
- is the possible amount of water at time t, that can be added to the waterway until the overflow threshold is reached.
- is the amount of water available in the waterway.
- is the actual drained (or pumped) volume at time t.

## Notes

- A negative DRAINAGE_Q can potentially raise the ground water level at the drainage so much that it reaches the surface above the drainage.

## Related

The following topics are related to this formula.