hydraulic gradient for infiltration in trenches and wrapped pipes

[dpa]

I have a paper which discusses the rational method and the calculations for sizing trench or perforated pipe infiltration systems. In the paper the hydraulic gradient is defined as the (average hydraulic head + distance to the water table)/distance to the water table. I am confused about what the average hydraulic head represents in this case. Is it the depth of water inside the trench? Is it the distance from the bottom of the trench to the water table or is it related to the elevation of the highest catch basin? There is an example problem but the hydraulic gradient is just given as 1.75 with no explanation of how it was determined. Working backwards the average hydraulic head would have to be 4.5 feet but in the problem the site is flat and the water table is 6 feet below the surface.

Thanks,
DPA

 

[LHA]

Hydraulic Gradient is the difference in hydraulic head over a horizontal distance. I'm not sure how the equation you refer to is derived. It really only applies to water tables of a constant thickness/depth, not the situation where you have a water table, then unsaturated soil, then a volume of water in a trench.

If you are trying to find out how fast a trench will dewater, you need permeability (as a flow rate). Permeability can be approximated by soil type, but practically must be measured with percolation testing.

 

[dpa]

Hi lha

Now you see why I am confused. This particular defiition of hydraulic gradient is not something I saw in school.


Thanks,
DPA