Flow rate to rainfall

[Jmondo Junior Member Find all posts View Profile Close Jmondo]

Hello,

I work in sustainible urban drainage and have come across a conversion that I cannot work out.

There is a geotextile specifed that has a porosity of 80 l/m2.s which I need converted into mm/hr?

I have found an image that claims this same geotextile has a value of 4500mm/hr when referring to its hydraulic performance.

Any help would be gratefully received.

[JohnS Moderator Find all posts View Profile Close JohnS]

Hello,

I work in sustainible urban drainage and have come across a conversion that I cannot work out.

There is a geotextile specifed that has a porosity of 80 l/m2.s which I need converted into mm/hr?

I have found an image that claims this same geotextile has a value of 4500mm/hr when referring to its hydraulic performance.

Any help would be gratefully received.

1 mm of rain falling on 1 m² of area is 1 L of runoff water. 80 l/m2.s = 80 mm/s. There are 3600 s in 1 h. However, after flowing through the fabric, where does the water go? No soil can handle that.

80 mm/s is 288 m/h which seems incredible and hard to believe.

[Jmondo Junior Member Find all posts View Profile Close Jmondo]

Hi John,

Thanks for the response. Would you mind showing the workings for conversion please? If you could break it down for my learning?

The water once passed through the geotextile is stored in the sub structure. It is important to remove the water rapidly from the surface to reduce the run off. The infiltration rate of the ground will dictate how quickly the storm water leaves the sub structure.

I guess that the high value is brought about by the thickness of the geotextile which is about 1mm.

Also does 80 mm/(m²•s) mean 80l per m² per second?

Thanks

[JohnS Moderator Find all posts View Profile Close JohnS]

Hi John,

Thanks for the response. Would you mind showing the workings for conversion please? If you could break it down for my learning?

The water once passed through the geotextile is stored in the sub structure. It is important to remove the water rapidly from the surface to reduce the run off. The infiltration rate of the ground will dictate how quickly the storm water leaves the sub structure.

I guess that the high value is brought about by the thickness of the geotextile which is about 1mm.

Also does 80 mm/(m²•s) mean 80l per m² per second?

Thanks

If a depth of rain of 1 mm falls on 1 m², that is 0.001 m x 1 m x 1m, that is 0.001 m³, but there are 1000 liters in 1 cubic meter, so 1 L/m² = 1 mm .
Your 80 l/m2.s is 80 mm/s. There are 3600 seconds in an hour so 80 mm/s x 3600 s/h = 288 000 mm/h. As 1000 mm = 1 m, 288 m/h.

As that is radically different from 4500 mm/h (4.5 m/h), either the two claims are measured under significantly different conditions or one is incorrect. Some further investigation is required.

I did have an error in the original, in that I should have replaced the liter in the numerator by 1 mm x 1 m², and that would have cleared the m² in the denominator.

[Jmondo Junior Member Find all posts View Profile Close Jmondo]

Hi John,

Thanks for showing the workings. I was also arriving at 28800mm but thought it was too high. That's what stumped me to be honest.

Upon further investigation I have been told that the amount of head specified for the testing can be altered. Thismight explain the rapid movement.