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I need to move water from the back yard to the front yard, to combat the monsoon season, in which a 50 gallon container in the backyard can fill up in 1-2 minutes. I bought some 4" piping, and will have it collect water in a drain in the higher back yard, and divert it in a half-circle around the house to the lower front yard.

I'm finding that due to the length of the pipes, about 150 feet, it is difficult to make it continuously downhill. There already is a slight slope of 1" per 10 feet horizontally. Is there any issue if the pipe is not perfectly sloped, and in places goes slightly higher than before, so long as all of the pipe is lower than the entry drain?

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    Can you create a swale instead?
    – Platinum Goose
    Aug 19, 2020 at 2:50

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It's true that, so long as the exit is lower than the entrance, water will find its way through the pipe even if there are low points in the path. However, as others have noted, debris could accumulate in those low points. So the answer to your specific questions "does it need to be continuously sloped" is a squishy "yes.. unless you're willing to install mitigations."

One mitigation you could consider is one or more sump pits/traps/catch basins along the path. Here's a sketch from Dejana Industries:

catch basin diagram

This construction is specifically designed to catch debris in storm water systems. That's not what you're after, so we can modify the design a bit.

Run pipe with a continuous slope at a sufficiently steep grade so that debris will carry along well. When you reach a point that the pipe is getting too deep in the ground, install a catch basin there. Take an exit pipe out of the pit at a level higher than the inlet and proceed to the destination.

During a storm water will fill the catch basin. Eventually the water level will rise high enough to flow out the exit pipe. Some debris will settle in the basin; other debris will float up and flow out the exit.

Construct the basin to be leaky so that after the storm the water remaining in the basin slowly drains away. That'll minimize the chance of breeding mosquitos or odors. From time to time you'll have to clean out the debris from the bottom of the basin, but it's better to have it there than in some unknown and inaccessible section of pipe buried in the yard!

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  • The first sentence could perhaps be clarified - most of the water will find its way through the pipe, but unless the flow is strong enough to generate a siphon, water will remain in the low points in the pipe. And why do you want the catch basin outlet to be higher than the inlet, which is opposite from what's shown the image? This will result in the inlet starting to back up before the catch basin begins to drain.
    – Nuclear Hoagie
    Aug 19, 2020 at 13:52
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    @NuclearWang Yes, I think that was the point. Will that result in debris getting stuck in the inlet pipe though? I wonder whether the flow will be faster through the backed-up inlet pipe, than through a straight pipe with a shallower gradient. Basically you're putting a U-bend in the pipe but leaving plenty of space at the bottom of the U.
    – user253751
    Aug 19, 2020 at 17:18
  • @NuclearWang Yes: with the outlet higher than the inlet the catch basin will back up before it drains. Consider OP's pipe in which a section rises instead of falling. Mentally compress that section of rising pipe so that the same rise is accomplished in a run of just a foot or two. Then stretch out the walls of the pipe in that little section. Keep mentally stretching until it morphs into a the shape of a rectangular box. The utility of the catch basin is simply this: the low point (and the section of rise) is made open/accessible so whatever debris accumulates in it may be removed easily.
    – Greg Hill
    Aug 19, 2020 at 17:28
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    Could the design be improved by having the outlet from the catch basin start with a pipe whose entrance is below the inlet, but which then extends upward to a height above the inlet, and has its outlet below the height of the catch basin's inlet Water would back up in the inlet until it reached the top of the outlet pipe, but once that happened, water would flow through the output pipe until the level fell below the level of its input or output (whichever was lower), which would in turn be below the level of the pipe feeding the catch basin.
    – supercat
    Aug 19, 2020 at 20:34
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    @supercat That's a clever idea: make the outlet act as a siphon. I do like it, but I'm not sure it would succeed -- purging the air from the outlet pipe sufficient to start a siphon could be difficult to accomplish in practice. Even if the siphon part didn't work, your suggestion does allow floating debris like leaves to be trapped in the catch basin rather than continue flowing down through the system, and that's a good thing too.
    – Greg Hill
    Aug 19, 2020 at 21:01
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If you are in a climate where the water can freeze it is a definite problem.

If not, there is still the potential for debris and sediment to settle in the low sections and eventually clog the pipe.

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    "monsoon season" generally doesn't happen in areas with freezing temperatures. 50 gallons of water in 1-2 minutes is truly "monsoon".
    – FreeMan
    Aug 18, 2020 at 16:28
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    @FreeMan, that's not true at all. For example, the southwest US has a vigorous monsoon season (with true monsoons) and most altitudes freeze regularly in winder. As a second example, climbing season in the Himalaya is the gap between winter and monsoons.
    – Reid
    Aug 18, 2020 at 23:10
  • @Reid the word "Generally" implies in most, but not all cases
    – FreeMan
    Aug 19, 2020 at 11:03
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    A 1000 square foot roof produces 55 gallons with 0.09" of rain. Yes, that's a lot for 1–2 minutes, but it certainly passes the smell test. I don't know how anyone could have a correctly set up rain barrel and not fill it regularly, even in low-precipitation areas.
    – Reid
    Aug 19, 2020 at 19:17
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    Also, why so much griping and moaning about insignificant details of the question? Who cares whether it's true monsoons? Who cares whether the OP's estimate of 1–2 minutes per 55 gallons is correct? Nit-picking like this serves no purpose and is cruel.
    – Reid
    Aug 19, 2020 at 19:19
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Your fix is to make like a Roman Aqueduct and support the lower "dips" so that the pipe runs at a more-consistent downward angle.

This will also make the water flow faster, flushing possible sediments without a chance to block up.

Aim for a straight run of pipe with the same drop over distance, rather than a specific angle. Start by supporting the pipe with wood or stone and once its well-aligned, consider more permanent solutions like concrete.

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    This may mean an exposed pipe, of course. That's probably not desirable.
    – isherwood
    Aug 19, 2020 at 12:46
  • If OP buried their pipe but it still goes up again, then it was poor prep-work for the install. The trench should have been graded properly. If the pipe is on the surface its a relatively accessible fix.
    – Criggie
    Aug 19, 2020 at 19:47
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Two problems can be encountered without continuous slope:

  • Ice can form, damaging the pipe. Obviously this is climate-dependent.
  • Silt can accumulate. When water sits stagnant, solids settle to the bottom and can harden. Eventually the pipe becomes clogged.

Otherwise, siphon effect and gravity will keep the water draining as you would hope.

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  • This will be especially prevalent in monsoon areas where there’s lots of rain for months and then it’s dry for months - plenty of time for water to sit.
    – Tim
    Aug 18, 2020 at 22:41
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One idea is to elevate the 50-gallon container in the backyard that is initially catching the water. Instead of setting the 50-gallon container on the ground, you can use pavers or other sturdy materials to create a platform on which the container will rest. Elevating the container itself will elevate the container's drain hole, which may give you enough elevation so that the piping to the front yard will run downhill.

Note: 50 gallons of water weighs 400 pounds, so whatever you set the container on needs to be really sturdy.

My rain barrel is only 35 gallons, but you can see that I have elevated it about 10-12 inches:

rain barrel painted with bluebonnets

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  • "I bought some 4" piping, and will have it collect water in a drain in the higher back yard" strongly suggests that the OP wants to avoid their yard flooding. Mentioning a barrel is intended to describe the significant rainfall in a short period of time, not the volume they want to empty or the location they are emptying from.
    – Nij
    Aug 21, 2020 at 11:45

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