Water Distribution Networks

The installation of a water distribution system is often a priority in emergency responses where people have been displaced. Distribution through pipes and tapstands will be much cheaper and provide a better service than water trucking. It is possible, with careful design and installation, to quickly install a water network that will last for a very long time. On the other hand, errors in design or installation are very hard to diagnose and correct once the system is installed.

Humanitarian settings are often quite different from typical water distribution networks that might be developed for other urban or rural areas.

It is important to have a good understanding of community expectations for how the water distribution system will function before commencing the design of a network.

A water distribution network will typically consist of one or more water tanks, a pipe network, and water distribution points. Each of these topics is dealt with below.

Design of Distribution Systems

Any water network will need some engineering design. Depending on the complexity of the situation this could be some simple calculations on a spreadsheet, or a design based on a network model using EPANET or similar computer software. Often the skills to carry out this work will be found, or can be developed, within a typical WASH team. For complex situations expertise may be required from outside the team.

Although computer tools are a great aid in designing water networks, it is still ultimately up to the designer to make the design decisions. The choices of input parameters used are going to be crucial to the success of the network, and there are many different ways to lay out a network – some will be better than others. It is crucial to have a good understanding of hydraulics and water network design before learning to use computer software.

There are lots of resources available online to learn more about the design of water distribution networks. Santiago Arnalich has made a high quality series of books and videos which cover both the principles of water network design, as well as the use of EPANET. Whilst the books must be paid for, some are made available online for free.

A Handbook of Gravity-Flow Water Systems gives a complete explanation of how to design and construct a water supply system. It is now dated, and focusses exclusively on Nepal, but still contains useful explanations and design examples.

It can be useful to roughly size pipes quickly in an emergency situation, and for this it can be useful to have an app available. Whilst there are many apps available for this task, one such simple app for this is Colebrook. (We would welcome suggestions for better apps).

Two simple actions which will help to prevent problems in design are:

1. Check your design. Always get somebody else to check your design. Also, be sure to think about the results from a computer model - do the pipe sizes that the model suggests make sense and seem similar to pipe sizes you have used before?
2. Always use inside diameters in calculations. Plastic pipes are normally identified by their outside diameter (e.g. 63, 90, 110, 160 etc). However, it is the inside diameter that is important for calculating the flow. This will vary depending on the pressure rating of the pipe.

Ensuring Equitable Supply

Very often in humanitarian situations the amount of water distributed depends on the amount of water supplied into the system, often determined as a 'ration' per person per day. This is as opposed to a 'normal' water network where the amount of water supplied depends on the demand (i.e. users can take an almost unlimited amount of water from the system, and the total amount of water distributed depends only on how much water people take).

Within 'rationed' water networks, effort must be put into the design and management to ensure there is an equitable supply of water to all users. The smaller the ration of water is, the more difficult this becomes to do.

Problems associated with rationed networks include: queueing at tapstands; the need to control water usage at tapstands; difficulties achieving equitable flow rates at all tapstands; very high peak demands as all taps are opened at once; and general dissatisfaction from the users.

The best solution is, if at all possible, to plan to supply more water. The more water there is available, the easier it will be to ensure that everybody has enough. Not only is this beneficial for the users, but in many situations the ongoing cost of providing extra water is very low, whilst the cost of trying to manage and maintain an equitable water distribution in a heavily rationed network can be significant. If household connections are a possibility, rationed networks are not a good option.

Where rationed water networks are the only option, the equity can be improved by the following:

• Create water distribution zones. Lay out the network in zones that are can be operated independently, operated by valves. Plan for each zone to have its own water meter too. This will make it easier to ensure each part of the camp has a fair share of water. It will also help with pressure control. By opening and closing each zone on rotation it will also reduce the peak flow in the network. Multiple zones can be served from one tank, although in some situations it makes more sense that each zone has its own distribution tank too.
• Ensure consistent, low, pressures across the network. Do not place the distribution tanks at a higher elevation than necessary. Ensure head loss is low in distribution mains (as a rule of thumb, around 5 metres head loss per kilometre of pipe). This should prevent large pressure, and therefore flow, differences between tapstands. It also gives flexibility if the layout of the network needs to change.
• Ensure there are ways of controlling the flow rate to the tapstands. If the flow to taps is uncontrolled, it will result in some tapstands (those lower down or closer to the tank) getting much more water than planned for, and consequently other tapstands will get less or no water. Ideally plan to use multiple methods to control the flow: small diameter pipe (for example ¾") within the tapstand construction, and valves to control flow. Note that valves will likely need to be adjusted then concreted or otherwise secured in place to prevent adjustment - typically the residents around a tap will want to adjust it to get more water (fair enough, but this results in less water being available for others).
• Do not add too many taps. This increases the maximum flow rate, without providing better service to the population. It reduces the distribution time and therefore makes it more difficult to balance the supply between all taps. Rather than adding lots of taps in one place, it will give a better service level to have more collection points, with only one or two taps at each location. Side benefits of this are reducing the maintenance cost associated with replacing taps, and making it easier to manage wasted water.

Water Distribution Tanks

Water is stored in tanks before being distributed through the water network. This storage can have multiple purposes, the most important of which is to accommodate differences between the rate of supply of water from the treatment plant or borehole, and the rate of distribution to the water network.

Water tanks can come in many shapes, sizes and materials. From plastic roto tanks, various metal tanks, concrete tanks, elevated tanks. Each has advantages and disadvantages and is suitable in different situations.

In the 1980s, Oxfam recognised that finding large tanks that can be installed quickly in emergency situations was difficult. This is still the case today. In response to this the famous ‘Oxfam Tank’ kit was developed, for quickly and cheaply providing large volumes of water storage. It is often seen in emergency responses throughout the world. This and other water storage solutions are available to any humanitarian organisation, and can be ordered from the supply centre website. The supply centre have also created a series of instructional videos on how to set up an Oxfam tank:

Installation of Pipes

Pipes are buried in almost all situations. This is an easy way to ensure that they are properly supported and protected.

HDPE pipes are now available throughout the world. HDPE pipes do not require the same level of trench preparation before installation. This means that installing a pipe network is now easier than ever, but it is still essential to properly plan and execute the installation of the pipes to ensure the network functions properly afterwards.

When planning the installation of the water network ensure that:

• Pipes should be buried a minimum of 0.8m (1.5 in sub-zero temperatures). When properly buried plastic pipes are unlikely to be damaged, and as such will last for a long time.
• Include appropriate wash out and air purging points.
• When crossing underneath a road or track, install a concrete slab above the pipe to protect it.

During the installation of the pipes it is essential that:

• All pipes are clean at the time of installation. If there is any detritus (stones, pebbles, rubbish) in the pipes when installed then no amount of flushing the network or using washouts will ever remove it - it will permanently reduce the flow in the pipes, leading to the system not working as designed, without it ever being really possible to work out why.
• All open ends are securely closed at all times when there is nobody on site. This is to avoid anything entering the pipe network - whether animals looking for shelter, or stones thrown in by children!
• Joints are made securely. See butt welding advice below.

Quality of HDPE Butt Welds

Butt welding is the most common type of joining method for HDPE pipes. It is simple, strong, and does not require any fittings. 'Manual' butt welding machines are very common. A manual welding machine is also available from the Supply Centre

An explanation of how the machine works is in the following video:

These types of machine are best suited to low-pressure networks. For higher pressure pipes (particularly pumping mains that may also experience water hammer), it is highly recommended to use automatic or semi-automatic welding machines. This is to avoid ongoing problems with bursts at poor quality joints. Where these machines are not available and cannot be imported within the necessary time frame, it is at least necessary to have a fixed procedure for the welding. The procedure should be established so that the welder clearly knows the timing and pressures required according to some standards.

Water Collection Points

The design, installation, and maintenance of the water collection points is a very important part of the whole water network. Typically these are tapstands with multiple taps, but in some circumstances, individual taps are more appropriate. In some cultures household connections are expected. Where water is to be paid for water kiosks are sometimes created. Pre-paid communal water dispensers are an emerging technology that can also be used.

Typical issues that need to be considered in the design of collection points are:

• Prevention of standing water. Normally there will be large quantities of waste water from a tapstand, provision should be made to direct this into drainage systems or (only if the soil conditions allow) into a soakaway pit.
• The taps should allow the normal water containers used to fit underneath. The taps should be no more than 10cm (4”) higher than the tallest locally used water container to prevent excessive wastage.
• Ensure that provision is made for people with limited mobility to access water.
• Users should be consulted on tap stand locations. Try to ensure the tap stands are located safely and strategically (near to schools, health and feeding centres etc.) In some cultures, tapstands that are too public or visible from the road will not be used by women.

The acceptable distance to collection points will vary significantly with different cultures, from several hundred meters down to 20-30 metres, or even household connections. In any case, more collection points with fewer taps at each point will generally provide a better user experience.

When running tapstands in a rationed network (see the design section of this page), it is very important to actively control the flow rate at each tap. This cannot be left to community volunteers, as there will be significant community pressure at each tapstand to increase the flow at their tapstand. Normally this will involve locking valves, or even concreting over the valve after fixing the flow!

The expected flow rate at each tap should be acceptable to the users, match with the designed flow rates in the network, and match with the number of people using each tap and how much the water ration is. An example set of figures is below (do not use these figures - always investigate and find figures appropriate to each situation).

Flow rate at tap:               10    litres / minute
                                 0.17 litres / second
Desired distribution time:       4    hours
Water ration per person:        20    litres / person / day
-----------------------------------------------------------
Total distribution per tap:  2,400    litres / day
People per tap:                120    person per tap

Pre-paid Communal Water Dispensers

In some places that we work, pre-paid water dispensers may be an appropriate solution for improved financial sustainability and equality of access to water. Although ‘pre-paid’ is the industry term for this type of water dispenser, they can be configured so that users receive free allowances for water. They are also commonly referred to as ‘Water ATMs’, due to their similarity in appearance and operation to a regular cash-dispensing ATM.

Pre-paid communal water dispenser interventions can contribute to:

• Financial sustainability of water supply provision
• Equity in access to water by all

Applications in humanitarian settings include:

• In protracted crisis, for example in camp settings, transitioning to long-term operation, in which water is eventually paid for by users. Dispensers could allow for improved financial transparency.
• Where pre-paid dispensers are already installed as part of normal operation of the system. In this case during a short-term acute crisis, free allocations of water can be given to users for a limited time.
• In contexts with water rationing, dispensers could potentially be used to equitably ration the available water.

Oxfam have written a technical brief to provide an introduction to this relatively new but very topical technology. It discusses what pre-paid water dispensers are, their expected advantages and disadvantages in different uses, some of the issues which must be considered if a programme is to implement them, and features to consider when selecting which technology to use. It is particularly focused on the usefulness of pre-paid water dispensers to humanitarian teams.

Technical Brief 27: Pre-paid Communal Water Dispensers

The Oxfam Water Distribution Manual

The Oxfam Water Distribution Manual contains practical advice for creating an emergency water distribution system using kits available from the Oxfam Supply Centre. It was written at a time when few items were available for local procurement. It is now outdated and makes reference to some kits which no longer exist. It is kept here as a reference as it still contains some useful information.

Oxfam Water Distribution Manual