Gravity Flow Water Systems

Welcome! This website is the culmination of my time designing and building aqueducts in the Peace Corps. I worked in rural Panama. The designs and construction were gravity-fed systems and had no pumps.

This website is meant to be general reference. There are certainly better ways of doing things and your feedback is appreciated. In addition, there may be a design or construction method that is better suited to your area.

The site is divided into three sections: design, construction, and maintenance. There are also other resources out there that I've started to link from the resources section.

A good starting place on gravity-flow water systems is to read this overview.

Working With Concrete

Concrete is a mix of cement, sand, and gravel (aggregate), and water.  A typical concrete ratio is 1:2:4. One part cement to two parts sand to four parts aggregate.


Cement
Cement is time-sensitive.  Be sure to store the cement in a dry location, protected from rain and potential flooding.  Store the cement in plastic bags, stacking vertically to get as much of the cement off the ground as possible.  The cement can pull moisture from the ground.  Older cement can still be used, but more cement is needed for the same equivalent strength.  This is outlined in the gravity flow handbook.

Sand
Avoid using beach sand when making concrete.  The shape of the grains of sand are rounder and reduce the overall strength of the concrete.  Instead, look for river sand clean of debris/silt or purchase sand from a local distributor.  Be careful storing sand as a heavy rain can wash it away.  Store on tarps or in sacks with an adequate cover.

Water
Use freshwater as saltwater can affect the curing of the concrete.  The appropriate amount of water is such that the concrete is smooth and plastic.  It should not appear runny nor so dry that it crumbles.


After mixing concrete, always cover the concrete with a plastic sheet to slow the curing process.  This will help strengthen the concrete.  Keep the concrete moist by adding water for the first 12 hours.

Maintaining Intake Structures

Cleaning the intake structure should be a monthly part of the maintenance program due to the level of debris that can collect in the intake.

Turn off the valve leaving the intake structure to avoid any sediment entering the system while the intake is being cleaned. The intake filter should be removed and cleaned with a plastic brush to remove any silt. The cleanout on the intake should be opened and drained. Remove any excess debris.

Building Intake Structures

Time Required~5 days depending on design

Materials List

Qty	Item	Notes
	sack of cement	Dependent on final design
	20’ length of 3/8” rebar	Dependent on final design
	16ga wire	For tying rebar. Dependent on design. Sold by the pound
	5-gallon buckets of sand	For concrete mix 1:2:4. Dependent on design.
	5-gallon buckets of gravel	For concrete mix 1:2:4. Dependent on design
2	shovels	For mixing concrete
1	hacksaw	For cutting rebar, PVC
1	hammer	For staking form
2	metal trowel	For concrete work
2	5-gallon bucket	For measuring, storing water for concrete mix
1	3' of 2” or 3” tube	Dependent on final design. Will be cast into intake structure
	wood boards	For formwork.  Dependent on final design

Construction
Construction will vary source to source.  The photos shown below are for a spring with an impermeable clay layer.  Consult A Handbook of Gravity Flow Water Systems for design ideas and construction techniques for other types of catchment.

	This photo series shows the construction of a spring. The spring was excavated down to the impermeable clay layer. All the sand and debris is removed in the area where a catchment wall is poured. Create a dam to divert water away from the spring during construction. If clay is accessible, a thick wall of clay will divert the water.
	After clearing debris when the form will be placed, measure and cut formwork.
	This photo shows the complete form with the 2” pipe running through the form. The tube will be cast into the concrete. The concrete is also reinforced with 3/8” rebar.
	Screens can be placed at the intake to reduce amount of debris entering the intake box.
	A PVC tube with a series of slits cut into the tube makes a cheap filter. This will reduce particulate entering the system to the size of a hacksaw slit.

Here is a photo from a stream catchment intake structure.  The higher wall is to reduce sediment entering the system.

Designing Intake Structures

Why is it Needed
The intake structure is it all begins. This is the water source for the community. You can have multiple sources feeding into the same aqueduct system. Typically, the intake structure is built into a small spring or creek, but could also represent a dam for a larger stream or river. Since this is the water the community will be drinking, make sure that the watershed (200+ meters in all directions from the intake point) is free of contamination (livestock, other villages, etc.).

Design
The design varies from source to source. For a spring, look at either a dam or intake box. For a stream, a modified dam structure can be used. Design ideas are located in the gravity-flow handbook. Consult with an engineer or someone who has experience designing and building intake structures prior to construction.

Make sure that the source is free of contamination.

Conduct a census of the community.  Figure out a good growth rate multiplier to find the population at the end of the life of the aqueduct.  Usually a system is designed for thirty years.  What is the expected population thirty years from now?  Use WHO’s 30 gallons/day/person, which is generous, to calculate the total water usage of the future community. 

Measure the flow of the intake structure during the dry season to find the minimum daily amount of water coming from that source.  If the flow does not produce enough to meet the future demand of the community, another source will be needed in addition or in place of the original source.

Maintaining Storage Tanks

The tank should be cleaned in a quarterly maintenance program. The valve to the community should be closed to avoid sediment flowing into the system during cleaning. Open the cleanout valve and drain the tank. The tank should be scrubbed with plastic brushes. A plastic broom can also be used.

Inspect the tank for organic growth. If there is any growth such as algae, the algae should be removed, the tank thoroughly scrubbed in that area and painted. If the tank does not have a smooth concrete surface (inside and outside) the tank should be spackled with a concrete mix (See concrete section of website). The outside tank should be painted as well. If the tank does not have paint, clean the outside of the tank thoroughly. An outdoor high gloss paint should be used.

Building Storage Tanks

In general, you should work with an engineer or someone who has experience designing and building storage tanks. 

Here are some photos of the construction of a octagonal deposit tank built by a fellow volunteer.  While the shape of the deposit tank can vary, the following photos are a great overview of formwork and the construction process.

	Clear the construction area of debris.  Make sure the area is level.
	Pour the foundation slab.  Dig away along the perimeter of the foundation to create a “key” where the slab will be slightly thicker.
	Pour the foundation slab with your rebar that will be used to reinforce the sides of the tank.  Using rebar, you can tie a horizontal piece to align/true the vertical pieces of rebar exiting the foundation.
	Use posts to stake the formwork.  Make sure the posts are strong enough and embedded deep enough to keep formwork in place under the pressure of the concrete poured into the form.
	Internal supports can be used to align formwork and ensure that the formwork stays in place during the first pour.
	Using boards as shown helps keep even spacing of your form to ensure an even wall thickness throughout the structure.
	The formwork from the first pour can be reused in a “lift” for the second pour.  The second pour should be made as soon as the first pour sets (1-2 days) so that the concrete from the second pour bonds to the concrete of the first pour.
	Use posts to keep the form elevated. Sand bags can be used to anchor the posts in place.
	Add a coat of plaster mix (1 part cement to 3 parts sand) to the tank walls.  A smooth coat on the inside and outside will reduce future organic growth on the walls.
	Setup the formwork table for the ceiling concrete slab.
	Posts are used to support the table form and are removed after the ceiling slab has set.
	Rebar layout to tie the slab to the rest of the tank structure and provide reinforcement to span the unsupported distance of the ceiling.  A form is used to provide an access door to the inside of the tank for future cleaning and maintenance.

Here are some photos of additional tank features worth considering.

	Here is an example of a built in ladder and hand hold as part of the tank structure.
	The tank should have an access hole from the top for cleaning and maintenance. The weight of the lid alone should deter children from tampering with the system
	The tank should have an inlet, overflow, exhaust vent, outlet, and cleanout (not shown). Note: This tank contains two chambers so there is redundant pipework. This tank also needs to be cleaned and painted.

Designing Storage Tanks

Why is it Needed
Some communities do not need storage tanks because the flow from the source is sufficient all year long. The storage tank is used for temporary water storage. In many cases, community members think that storage tanks are for the drier season. It is important to show that storage will only last for a day or two and that a much larger tank would be needed to store months worth of water. In many cases, the tank serves to buffer daily water usage. The tank fills during the night when water usage is low and empties during the day.

Design
The tank site should be a flat area free of debris.  If the tank is located on a hillside, create drainage trenches around the future tank area to avoid future erosion from rain runoff.

Each tank is designed on a site-by-site basis.  An engineer should be consulted for approval of the final design.

To size a tank, you will need to create a daily water usage chart of the community. Look at daily water usage by conducting surveys in a few representative homes.  What are the daily activities and how would that affect water usage (cooking, washing, etc.)?  Create a chart that shows approximate water usage based on the hour of the day.  Look at what peak water usage will be.

Use WHO’s 30 gallon/day/person (generous) to calculate total water usage across the community.  Conduct a census of the community to look at total population.  Consider using a multiplier to base the design on a future population.

Measure the flow during the dry season of your water source.  Calculate gallons/day.  Create spreadsheet that shows daily demand for community minus the input from your source.  For peak demand, there will need to be X number of gallons to meet future community needs.  This should be the approximate size of the tank.  Double-check to make sure tank will fill at night to meet those needs during dry season.

A more detailed description of tank sizing and potential designs is located in the fabulous book, A Handbook of Gravity Flow Water Systems, Thomas D. Jordan Jr.

Maintaining Bridge Crossings

Hopefully, you will not need to maintain the bridge crossing as most are elevated over water or high in the air where maintenance is difficult.

If there is a leak with the tube, this may be due to an insufficient bridge structure causing the tube to buckle and fail. If funds are available, take this opportunity to bring the bridge crossing up to snuff.

You will need to shutdown the water and disassemble the bridge. Bridge construction takes 1-2 days, so take this into account when planning bridge construction on existing systems. Make sure all materials are ready before starting construction.

Building Bridge Crossings

Time Required
1-2 Days for Anchor-Post Bridge crossing
2-4 Hours for tree-to-tree crossings

Materials List

Qty	Item	Notes
n/a *	3/8” Turnbuckle Hardware	For adding tension in cable. Truner brand recommended if available. Try to avoid cheap pot metal variety found in most hardware stores.
n/a *	3/8” steel cable	For elevating tube
n/a *	5/16” Hardware clips	For clipping cable to trees, other hardware
n/a *	Nylon string, wire, or thin PVC rings from larger piece of PVC	For attaching tube to cable
1	Hacksaw	For cutting cable, rebar for anchor-post crossing, PVC tube, galvanized pipe. Wire cutters also work well to cut cable
1	Pliers	For tightening nuts on hardware clips, bending wire when attaching tube to cable, tying rebar
n/a *	3/8” thimbles	For looping cable around rebar on anchor-post crossings
n/a *	3/8” rebar	For protection against UV rays
n/a *	Exterior paint	For protection against UV rays
n/a *	sack of cement
1	20’ length of 3/8” rebar	Will only use about 15’. Usually comes in 20’ lengths
1	5’ length of 16ga wire	For attaching tube to cable, tying rebar. Sold by the pound
n/a *	1-1/2” Galvanized pipe	For anchor-post bridge crossings. Usually comes in 20' lengths. Can sometimes be cut to length at store.
2	level	For plumbing post in anchor-post bridge
1	Duct tape	For attaching level to post
1	Large stick	For stirring concrete during pour to remove air pockets
n/a *	5-gallon buckets of sand	For concrete mix 1:2:4, For anchor and posts
n/a *	5-gallon buckets of gravel	For concrete mix 1:2:4, For anchor and posts
2	shovels	For mixing concrete for anchor-post bridge
1	can PVC cement	For any tube work
1	metal trowel	For concrete work on anchor-post bridge
2	5-gallon bucket	For measuring, storing water for concrete mix

* Quantity dependent on final design

Construction
Because maintaining a crossing can be a lot of work after the fact, initial construction is important. Be sure that anchor points will last the life of the system, that the tree, boulder, or post location makes sense.

Make sure that the tube is elevated high enough above any water crossings to avoid debris in a flood. Ask community members about flood conditions to determine how robust the bridge crossing should be. Be careful of future erosion. Make sure that the anchors and posts are setback enough from the ravine.

Make sure that tubes are painted or in a larger tube sheath to protect against UV rays.

If the tube leaves the ground or enters the ground as part of the bridge crossing, encase the tube in a larger tube sheath to protect against potential machete cuts in the future undergrowth or livestock knocking against the tubes.

Anchor and Post Bridges

	This is the finished rebar for an anchor. The cage will be placed in the upper portion of the concrete anchor so that the U-shaped piece sticks out of the finished anchor. The bridge cable will connect to the U piece.
	Pour concrete into the formed hole. Use a level to make sure the post stays straight during construction. Tamp down concrete while pouring the form. See working with concrete section of the website
	Earth is mounded around the post. Sticks are placed on the elevated mound so that a cover can be placed over the whole post structure during curing.
	Use a level to make sure the post is level in the concrete. Cover the concrete during curing. See working with concrete section at the end of this manual.

Using Trees to Elevate Tubes

	Note: There are several ways of attaching to trees. The method shown in the following photos shows hardware clips. Volunteers have encountered problems with these clips cutting the plastic sheathing on cables when put under tension. If this is the case, in lieu of clips, the cable can be tied in a knot around the tree by making a loop and running around the tree and back through the loop.
	Loop cable around the tree and slide on hardware clip. Tighten nuts extremely well using pliers.
	Leave about 12” of cable from the hardware clip around the tree to the end of the cable. Run cable through turnbuckle hardware. Create loop at end of cable by doubling back. The eye of the turnbuckle should be in this loop. Tighten loop with hardware clip.
	Loosen bolts on turnbuckle as shown in the picture. Create loop in new piece of cable using hardware clip. Attach new cable to hook on turnbuckle hardware.
	Repeat above attachment method on other side of bridge if attaching to another tree. For smaller crossings (< 20 feet), one turnbuckle should be sufficient. For larger crossings, the extra turnbuckle will help add more tension in the cable. For crossings with two turnbuckles, measure out distance of cable to connect to other side of bridge. Pull on cable to add tension before deciding where to cut the cable.
	Paint any tubes that will be exposed to UV rays.
	Attach tube to cable using nylon string or wire. This solution could use some work. Nylon string tends to degrade in the sun. Wire will rust. If you're able to find plastic coated metal or can make thin PVC rings from a larger diameter PVC tube, this is the preferred choice for attachment. Make sure attachments are located about every three feet and are have some slack since the cable will move when put under tension.
	Elevate cable and tube. Attach cable as needed using hooks of turnbuckle hardware. Tighten bolts in turnbuckle hardware as needed. The tube should be straight with very little sag in the middle of the crossing.

Using Rocks to Anchor

To the left is a picture of a potential rock anchor. This was taken from an existing system, so there are no installation instructions at this time.

Other Considerations

	If your bridge crossing involves a transition from the ground to being elevated, sheath the transition point in about 4' of larger tubing. This will help protect the inner tube from machetes or animals that may come in contact with where the tube enters the ground.
	Since you'll be shutting off the water for bridge maintenance or construction, the bridge may also be a location to install a shutoff valve and/or cleanout valve as shown in the picture to the left.

Designing Bridge Crossings

Why is it Needed
Most aqueducts have streams to cross or small ravines to avoid. At some point, there may be a good reason to elevate the tubes. All the bridge designs covered in this website are a variation on the same theme – tubes are attached to a suspended, taught cable that is anchored to something. In places where you can anchor to existing boulders or trees, this is preferable to creating posts and anchors because it means less concrete, which can sometimes present logistical issues in remote sites.

Design
The construction section focuses mostly on bridge crossings using trees and making anchored posts.  If you have access to a hammer drill and there is an existing rock that can be used, this is an easy option.  However, this is usually a rare occurrence.

Design resources are hard to find for this type of application. If you are a Peace Corps volunteer, consult your APCD as he/she may have some available software (excel spreadsheet) or reference documentation.

In general, these crossings do not experience high loads (no one is walking on them), so the important part is that the posts/anchors are properly sized given the type of soil.  For a softer soil prone to flooding, the post should be embedded in concrete at least three feet and the anchor should have a volume of about 1.5 ft3.  For rockier, more stable soil, the post and anchor can be downsized.  Review the final design with an engineer if available.

If you are using rocks or large trees as anchors, the cable alone should be sufficient for bridge crossings up to 60 feet.

The handbook for gravity flow discusses bridge crossings and shows a few design examples.  Though I haven't used them, Agua Para La Vida has a few resources on designing bridge crossings including a user guide and spreadsheet (Spanish).