What is head difference?

Head difference is the difference in height of a water level between two places, or between two points of a drain or sewer. In Dutch it is called verval. At a lock or weir, the head difference is the difference between the water on the upstream side and the downstream side. With the tides, it is the difference in height between high and low water at the same place (the tidal range). For pipes, it is usually called the fall and is often expressed relatively: as a percentage or in millimetres per metre.

In everyday Dutch, the same word also means the deterioration of a building through neglected maintenance: a building in decay.

Calculating with fall

• 0.5% = 5 mm per metre. Over a pipe of 8 m, that is a 40 mm difference in height.
• 1:100 = 1% = 10 mm per metre.
• Rule of thumb: difference in height = length x percentage / 100.

Head difference, hydraulic gradient and slope

• Head difference: the difference in height itself, absolute (in m) or relative.
• Hydraulic gradient: the slope of the water surface or groundwater level, the head difference divided by the distance. A term from hydraulic engineering and geohydrology.
• Slope (fall): the gradient you deliberately build into a roof, floor, gutter or pipe so that water runs away.

Application

• Indoor drainage and house connection: a drain must have enough fall to carry water and waste with it. Too little fall causes blockages. In the Netherlands, keep to the values in NEN 3215 and the instructions of the manufacturer and municipality.
• Hydraulic engineering: the head difference at a weir or lock determines the forces on the gates and the amount of energy a hydroelectric power station can generate.
• Historical: Roman aqueducts had a very even, small fall, so that the water flowed at a constant speed.

Tips for DIYers

• Measure with a spirit level or laser and set out the fall with pegs and string lines before laying the pipe.
• No back-fall: a spot where the pipe rises briefly holds water and waste.
• Support the pipe at fixed intervals, so it does not sag and lose its fall locally.

Related terms

• Hydraulic gradient
• Slope (fall)
• Sewer
• Lock
• Aqueduct

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.