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In a hydrostatic example (first figure), where the hydraulic head is constant, there is no flow. However, if there is a difference in hydraulic head from the top to bottom due to draining from the bottom (second figure), the water will flow downward, due to the difference in head, also called the hydraulic gradient.
This diagram illustrates the three different heads (hydraulic, pressure and elevation) for hydrostatic (no flow) conditions. The yellow rectangle represents a sand and water filled tube or column with its top open to the atmosphere and its bottom drained by a pipe or tube (blue line).
A head's location varies with the height of the water level against the dam. Since there is only an extremely low flow within the reservoir so no water level gradient, the head can be clearly seen: where the farthest watercourse discharges into the reservoir.
Hydraulic head (red line) gradients actually cause groundwater to flow (from high head to low head, down in this case). Pressure head (blue line) is zero at the top of the column, as designated by the inverted triangle and horizontal lines (showing the water table). Elevation head (green line) always increases 1:1 with elevation.
Energy in pipe flow is expressed as head and is defined by the Bernoulli equation. In order to conceptualize head along the course of flow within a pipe, diagrams often contain a hydraulic grade line (HGL). Pipe flow is subject to frictional losses as defined by the Darcy-Weisbach formula.
Hydrostatic head is also used as a measure of the waterproofing of a fabric, commonly in clothing and equipment used for outdoor recreation.It is measured as a length (typically millimetres), representing the maximum height of a vertical column of water that could be placed on top of the fabric before water started seeping through the weave.
The method consists of filling the flow area with stream and equipotential lines, which are everywhere perpendicular to each other, making a curvilinear grid.Typically there are two surfaces (boundaries) which are at constant values of potential or hydraulic head (upstream and downstream ends), and the other surfaces are no-flow boundaries (i.e., impermeable; for example the bottom of the dam ...
Hydraulic head is composed of pressure head (ψ) and elevation head (z). The head gradient is the change in hydraulic head per length of flowpath, and appears in Darcy's law as being proportional to the discharge.