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Hi,
It is possible to design a pressurised system to fall with the gradient of the site but this type of design should come with a health warning.
I suggest you do what you can to make the infiltrative surface level. It provides much better distribution and avoids the problem of the distribution piping draining after each dose at the lowest point in the distribution system. If you were to design the system to fall with the site grade you would need to change the orifice diameters and/or the distance between the orifices to apply a more even distribution. However, because the different rates of discharge from orifices and the different distances between orifices, the distribution is not as good as working with a level system.
Another issue is what happens to the water after it reaches the natural soil. If the soil has a lower permeability than the fill, water will pond at the fill/soil interface and begin to move laterally with the slope. If the mound is inline with the slope, the water can begin to accumulate as it moves down and cause surface breakouts. You really want to align the mound parallel to the surface contour. However, it sounds like this isn't possible. You might see if you can break the system up so you can use shorter, but more runs.
Also, if the mound is run downslope with the infiltrative surface level, the depth of fill will increase down slope. You will probably need to compact the fill material with water to stabilize the fill or the fill will subside more where the fill is deep than where it is shallow.
If you are forced to align the mound such that it is built downslope, you should locate the wet well below the lowest point in the distribution system so that you are always pumping up hill. You should also allow the forcemain to drain back to the wet well. By operating the system this way, after a dose the piping will quickly drain. You will want to use small diameter lateral piping but larger manifold and forcemain piping. Also if you allow the rising main to drain back to the wet well make sure you take account for this volume when calculating a dose volume to pressurise the system ever time the pump comes on.
I hope this is some help to you. Sometimes you have few options so you will need to think about how the water will react to the distribution system and try to mitigate any issues the best you can.
Needless to say if you are using a mound, the depth of fill will increase as you move down slope.
If you need a pressure design completed please leave me know and I’ll do one up for you.
Tim
(respect to Dick for help with this answer, cant beat a bit of team work and a combined onsite work experience of 67+ years). |
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