Gravity systems are simple but very complex at the same time. Having lived on spring water that was fed by gravity for over 50 years, I have some experience in making these systems work and easy to maintain. I hope that my simple overview will help you design, build, and enjoy a gravity-fed system, too.
There are four basic elements to a gravity water system: source, intake, sediment removal, and storage. Of course, you may have to deal with some troubleshooting down the road as well.
The source can be any supply of free water. Spring, creeks, lakes, rivers or even collected rainwater are all viable sources. However, each has advantages and disadvantages to a homeowner. For instance, if you use flowing water, will a flood or mudslide possibly destroy your intake? First on your to-do list is to identify all water sources available to you. Even swampy areas can be a source, so do not be afraid to list everything. The next step is to have the water tested at the county from each source for pollutants. One common mistake beginners make is to assume that all natural water is clean water. Upstream cattle or wildlife populations, farmer field runoff, and so forth can all contribute toxic waste into water that can be hazardous to your health. Have the water tested and keep track of the pollutants found.
The amount of water (usually measured in gallons per minute) is a somewhat useless statistic. The only thing that matters is that the average daily flow is greater than your average daily need. Your storage tank will even out the difference on days your use exceeds your input and refill when the reverse is true.
Your next step is to pick your top two water sources. I pick the top two water sources using a scoring system; each source is rated based upon the following attributes to obtain each source’s total score. The top two highest scores at the two preferred sources. The movement score ranges from -3 for stagnant water to +3 for rapidly moving water. The pollution score ranges from -5 for water unsafe to drink to +5 for water that is cleaner than bottled water. (Note: Some pollutants will completely eliminate a source from consideration, such as mercury or arsenic at high levels.) The height score ranges from 0 for water that is level with its usage point to +5 for water that is 200 feet higher than its usage point. The volume score ranges from -2 for a flow that is a quarter of a garden hose to +5 for one that is a full 3-inch pipe flow.
Distance from the usage point is not really a significant consideration. Reliable, safe, plentiful drinking water for the long term is always worth the effort to lay your transport pipe.
Now that we have our scoring system, let’s take my personal gravity system as an example. It is a natural spring, with a seasonal creek addition, that is about 1/4 mile from my home and 230 feet higher than my house. It is a moving water source and would score a 0 on my scale above. Pollution levels actually tested cleaner than bottled water at the county. The spring comes out of a clay bank, so a +5 there. I get an additional +5 for the intake height (The height translates to water pressure at the usage point.) The volume ranks a +2, as it is enough to fill a 1-inch pipe. So, my total score is 12.
My father’s system is similar but is only 40 feet higher than the house and does not test as clean. (It is from an small open creek, but it is safe to drink.) So, the score for his system is 0 (movement) + 0 (pollution) + 1 (height) + 3 (volume) = 4 total score. By comparing water sources this way, it is easy to find your top one or two sources by score. The higher the score, the better.
Now that we have selected a source of water for our system, we need to devise a way to collect it and get it into our system. Options here include creating small dams, laying a pipe in the creek bottom, placing a pipe attached to the bottom of a float (great for large streams/rivers), or placing a screened box in standing water. Collection can be easy as even the most inexperienced plumber knows that water flows downhill. The problem is you probably do not desire to capture water bugs, salamanders, small fish, moss, leaves and other items inside your water system.
The easiest system to maintain is a graduated screening system. For my father’s system, I use a chicken wire fence across the stream several feet upstream. This keeps out leaves and large debris. At the pool where the intake is, we have a rigid wire mesh small animal cage. Inside the cage we stuff pond filter material, which is cheap and easy to obtain at most hardware stores, to prevent smaller debris from entering the pipe. One key item to remember here is that you are trying to get rid of particulates, not suspended items, so no “chunky” water, just “dirty” water gets through.
You should also build a simple shelter that will divert any falling material downstream from the intake point. You want to keep this as simple as possible, as you will probably need to repair it every other year if it is under tree cover, because falling branches and trees can cause a lot of damage. In open country, this shelter will provide shade, since sunlight causes algae to grow, and it will prevent animals from direct access to your water intake.
This is actually done in multiple areas. We already eliminated macro debris at the intake. The pipe should now flow into a sediment trap. Sediment traps are simple. It is just a place where the water pressure drops significantly and the water resides for enough time to allow sediment to collect at the bottom.
To design your removal system, collect a glass or jar of water after “muddying” the waters at the intake. Set the glass/jar down and see how long it takes for the material to collect on the bottom of the glass. The longer it takes, the larger your sediment trap needs to be. On my father’s system, we use a 55-gallon drum on its side. Water flows in on the top side. Once the barrel is full, the water flows out a connection at the top of the downhill end. A plug at the bottom of the downhill end allows us to drain the collected debris, periodically. I have also used a pipe system. Essentially, this is where a one-inch pipe T’s into a 2-inch pipe about two feet from the bottom. This two inch pipe must fill to a height of four feet to continue the flow. This pressure drop (due to the pipe size change) and the “fill” factor of movement, allows the sediment to collect on the bottom of the 2-inch pipe, where it terminates in a valve for sediment drainage. Anything will work, as long as you remember the key elements– pressure reduction, time, fill-action flow, and drain capability.
If you want to use a filter, you will need to install that near the usage point, where you have sufficient water pressure. I have reverse osmosis systems under my sinks and a fine grain filter on the mainline before it enters the house. You will need to replace filters more frequently than the manufacturer recommends, as they are usually rated for treated/city water rather than runoff.
You will need storage of the collected water. My system has a pool behind an artificial dam at the intake point that holds about 400 gallons of water. I don’t run the risk of running short of water as my source is a permanent spring that has been in use for over 100 years without ever drying up. Previously, my father’s system used a water tank from a steam locomotive that held about 500 gallons. After years of rust, sediment filling between the baffles, and discovering albino crawdads living inside, we switched him to a 1100 gallon polyethylene storage tank this year.
For best results, your tank should contain a minimum of a one-week water supply and preferably at least a month-long supply. Right now you will probably not need that much, but what about after an event? You may need to shut your water intake off for several weeks to prevent ash, fallout, or other contaminants from polluting your storage. What about repairs? You should have supplies on hand, and right now it is no big deal to run down and get another 100 feet of poly pipe and various connectors, but what about after an event? You may have to get very inventive, and it may take more than a few days to find usable replacement parts. Keep a solid supply of parts for whatever system you build. I keep an extra roll of each size polypipe I use and three five-gallon buckets of connectors, valves, and ring clamps.
Contamination. Do your homework in advance. Find out exactly how much bleach is necessary to “clean” the water in your storage tank. Know what “cleaners” are necessary for each suspected contaminant and make sure they will not interact with the composition of your pipe or storage tank. If you are in doubt, make sure you have a way to drain your storage and allow it to refill while you ration water. We had to do this on my father’s system when an inconsiderate hunter used our intake point to clean a deer and left all the organs in the intake pond. We know the hunter was aware of where he was because he dug up the pipe to move it out of his way while he worked and then laid it back in the pond when he was done! If you have solar power near your intake, I know of one person who built an enclosure over their spring and installed multiple submersible UV water purification systems designed for fish tanks. (He found four of these were cheaper than one designed for home use.) It’s your water system, so redundant and varied methods of filtration are recommended, since there is no silver bullet to safe water. However, make sure you have the appropriate systems installed for whatever you found when you had the water tested.
Air locks in the pipe. Hopefully your pipe is downhill all the way to your usage point. Unfortunately, many times it is not. In those cases, you will have a section of pipe that is higher than the rest. This can result in “air locks” preventing water flow. The solution is to install an air release. You do this by installing a “T joint” at the highest point of the rise. Attach a pipe to the “T” and run it up a pole or tree. The top of the pipe must be higher than the intake point if it is a closed system, with no open storage between intake and usage or just several feet higher than the rise if the water flows free into some point lower. You should also install a corner or inverted “J” pipe (think upside down snorkel) to prevent debris from falling into the pipe. Downhill from this point, install a valve. This allows you to back the water up, forcing the air lock into the vertical pipe.
Tree roots. Trees grow and so do their roots. They can “squeeze” or bend pipe to the point where it breaks/ruptures. So have spare pipe on hand and try to stay at least six inches away from all tree roots. Cutting the roots is a bad idea, as this weakens the tree, and if it falls over, it can completely uproot your water line.
Freezing. An open flow system (water continuously flows downhill and excess water runs off at your storage tank) usually only needs to be buried about six inches deep for an inch and a half pipe. During very cold weather, let all your faucets trickle, to keep water moving through the system. Know your local ground temperature! An easy way to do this is to check the internal temperature of any nearby mine shafts or caves. If not, your local U.S. Geological Survey office should have this information. When in doubt, bury your pipe more than one foot deep, but keeping the water flowing is still the best way to keep a pipe from freezing.
Blockage. Sometimes debris (mud, moss, leaves) gets into your pipe and inhibits or prevents proper water flow. You should have a diversion faucet every few hundred feet of pipe. This allows you to drain the pipe at full force near the blockage. It can take several attempts of flow, stop, flow, stop before the plug material comes out. However, it is less work to sit on a hillside at a faucet and play the on/off game than it is to dig up several hundred feet of pipe to find the blockage. If you designed your intake correctly, anything that gets into the pipe should easily exit via the outlet valve. Salamanders are my personal albatross, as they periodically find their way into the pipe past by filters.
Pipe. I prefer poly pipe, as it is easy to install and maintain. Others I know use iron (installed in the 1940s and still working), galvanized, PVC pipe, and copper piping. For me, copper is too expensive and hard to work with. Iron, copper, and galvanized are highly susceptible to freezing issues and can suffer from corrosion. PVC is a nightmare to repair, as it is hard to clean and get the glue to work when it is wet, so I avoid it. Poly pipe I can repair at 2 am with minimal light in less than an hour.
Squatters. If you build a shelter over your intake, you will most likely eventually run into squatters. Squatters are unwanted wildlife that can make life interesting. I have chased out a bobcat, seen cougar signs, found hornet/yellowjacket nests, and had wood rats move in. Any wildlife in your area, that might appreciate a shelter, may move in on you. You should make sure you can see inside your shelter from a distance on your normal approach. Chicken wire fencing or similar may keep out some interlopers, but shelter is shelter, and eventually you will have something move in; just be prepared.