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Open drain

Calculates the water flow in an open drain according to Manning.

Open drain

Input Result
Mannings number M= m1/3/s
Wet cross-sectional area Av=
Hydraulic radius Rh=
Average velocity v=
Slope I= % %
Flow q=



The flow that flows in the drain pipe or alternatively in the gutter.

Mannings number

Manning's numbers describe how rough the gutter is, less roughness results in greater Manning's number, which results in lower flow losses. Examples of Manning's figures can be seen in the table below.

Type of gutter Mannings tal
Rough concrete 50-70
Planed wood and ground concrete 70-90
Smooth plastic and the like 100-120
Strongly planted throughout the cross section 10-20
Overgrown bottom and overgrown slope 20-30
Overgrown bottom and slopes 30-40
Newly dug trench in clay 40-50
Canal through blasted mountain 25-30
Natural waterways 10-40

Hydraulic radius

Refers to the ratio between the wet cross-sectional area and the wet circumference of the drain.

Average velocity

Refers to the average velocity of water in the current drain.

Wet cross-sectional area

Cross-sectional area on the wet part of the drain.


Refers to the slope of the drain.

Drain bottom width

Refers to the width of the bottom of the current drain.

Natural water depth

Refers to the water depth in the gutter that flows unaffected by disturbances both upstream and downstream.

Slope angle

Refers to the slope of the slopes of the current drain.


Mannings number: Mannings number