Misplaced Pages

#561438

28-763: The Devonport Leat is a leat in Devon constructed in the 1790s to carry fresh drinking water from the high ground of Dartmoor to the expanding dockyards at Plymouth Dock (which was renamed as Devonport, Devon on 1 January 1824). It is fed by five Dartmoor rivers: the West Dart , the Cowsic, the Hart Tor Brook, the River Meavy and the Blackabrook (this last apparently was the first portion to supply Plymouth Dock). Dartmoor granite

56-531: A dye works or other industrial plant, and provision of drinking water to a farm or household or as a catchment cut-off to improve the yield of a reservoir . According to the Oxford English Dictionary , leat is cognate with let in the sense of "allow to pass through". Other names for the same thing include fleam (probably a leat supplying water to a mill that did not have a millpool). In parts of northern England, for example around Sheffield ,

84-414: 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 . Even though it is convention to use gauge pressure in the calculation of hydraulic head, it

112-470: A 400 m deep piezometer, with an elevation of 1000 m, and a depth to water of 100 m: z = 600 m, ψ = 300 m, and h = 900 m. The pressure head can be expressed as: ψ = P γ = P ρ g {\displaystyle \psi ={\frac {P}{\gamma }}={\frac {P}{\rho g}}} where P {\displaystyle P} is the gauge pressure (Force per unit area, often Pa or psi), The pressure head

140-596: A certain RPM can be read from its Q-H curve (flow vs. height). Head is useful in specifying centrifugal pumps because their pumping characteristics tend to be independent of the fluid's density. There are generally four types of head: After free falling through a height h {\displaystyle h} in a vacuum from an initial velocity of 0, a mass will have reached a speed v = 2 g h {\displaystyle v={\sqrt {{2g}{h}}}} where g {\displaystyle g}

168-407: A fluid in a gravitational field is equal to ρg where ρ is the density of the fluid, and g is the gravitational acceleration . On Earth, additional height of fresh water adds a static pressure of about 9.8 kPa per meter (0.098 bar/m) or 0.433 psi per foot of water column height. The static head of a pump is the maximum height (pressure) it can deliver. The capability of the pump at

196-459: A short distance to the north of Wistman's Wood at an altitude of over 410 metres (1,350 ft) and twice passes close to Two Bridges (following the contours up the Cowsic valley in between) before heading towards Princetown . Its water supply now ends up in Burrator Reservoir . It follows a meandering path across the moor, carefully selected by engineers to follow the natural contours of

224-859: Is a vector gradient between two or more hydraulic head measurements over the length of the flow path. For groundwater , it is also called the Darcy slope , since it determines the quantity of a Darcy flux or discharge. It also has applications in open-channel flow where it is also known as stream gradient and can be used to determine whether a reach is gaining or losing energy. A dimensionless hydraulic gradient can be calculated between two points with known head values as: i = d h d l = h 2 − h 1 l e n g t h {\displaystyle i={\frac {dh}{dl}}={\frac {h_{2}-h_{1}}{\mathrm {length} }}} where The hydraulic gradient can be expressed in vector notation, using

252-530: Is a direct one, an increase in atmospheric pressure is an increase in load on the water in the aquifer, which increases the depth to water (lowers the water level elevation). Pascal first qualitatively observed these effects in the 17th century, and they were more rigorously described by the soil physicist Edgar Buckingham (working for the United States Department of Agriculture (USDA)) using air flow models in 1907. In any real moving fluid, energy

280-445: Is dependent on the density of water, which can vary depending on both the temperature and chemical composition ( salinity , in particular). This means that the hydraulic head calculation is dependent on the density of the water within the piezometer. If one or more hydraulic head measurements are to be compared, they need to be standardized, usually to their fresh water head , which can be calculated as: where The hydraulic gradient

308-498: Is dissipated due to friction ; turbulence dissipates even more energy for high Reynolds number flows. This dissipation, called head loss , is divided into two main categories, "major losses" associated with energy loss per length of pipe, and "minor losses" associated with bends, fittings, valves, etc. The most common equation used to calculate major head losses is the Darcy–Weisbach equation . Older, more empirical approaches are

SECTION 10

#1732885071562

336-508: Is more correct to use absolute pressure (gauge pressure + atmospheric pressure ), since this is truly what drives groundwater flow. Often detailed observations of barometric pressure are not available at each well through time, so this is often disregarded (contributing to large errors at locations where hydraulic gradients are low or the angle between wells is acute.) The effects of changes in atmospheric pressure upon water levels observed in wells has been known for many years. The effect

364-399: Is the acceleration due to gravity. Rearranged as a head : h = v 2 2 g . {\displaystyle h={\frac {v^{2}}{2g}}.} The term v 2 2 g {\displaystyle {\frac {v^{2}}{2g}}} is called the velocity head , expressed as a length measurement. In a flowing fluid, it represents the energy of

392-569: The Romans , and can still be seen at many sites, such as the Dolaucothi goldmines. They used the aqueducts to prospect for ores by sluicing away the overburden of soil to reveal the bedrock in a method known as hushing . They could then attack the ore veins by fire-setting , quench with water from a tank above the workings, and remove the debris with waves of water, a method still used in hydraulic mining . The water supply could then be used for washing

420-983: The del operator . This requires a hydraulic head field , which can be practically obtained only from numerical models, such as MODFLOW for groundwater or standard step or HEC-RAS for open channels. In Cartesian coordinates , this can be expressed as: ∇ h = ( ∂ h ∂ x , ∂ h ∂ y , ∂ h ∂ z ) = ∂ h ∂ x i + ∂ h ∂ y j + ∂ h ∂ z k {\displaystyle \nabla h=\left({\frac {\partial h}{\partial x}},{\frac {\partial h}{\partial y}},{\frac {\partial h}{\partial z}}\right)={\frac {\partial h}{\partial x}}\mathbf {i} +{\frac {\partial h}{\partial y}}\mathbf {j} +{\frac {\partial h}{\partial z}}\mathbf {k} } This vector describes

448-547: The contour to the reservoir. Such leats are common around reservoirs in the uplands of Wales. Leats were built to work lead , tin and silver ores in mining areas of Wales , Cornwall , Devon , the Pennines and the Leadhills / Wanlockhead area of Southern Scotland from the 17th century onwards. They were used to supply water for hushing mineral deposits, washing ore and powering mills. Leats were also used extensively by

476-503: The direction of the groundwater flow, where negative values indicate flow along the dimension, and zero indicates 'no flow'. As with any other example in physics, energy must flow from high to low, which is why the flow is in the negative gradient. This vector can be used in conjunction with Darcy's law and a tensor of hydraulic conductivity to determine the flux of water in three dimensions. The distribution of hydraulic head through an aquifer determines where groundwater will flow. In

504-415: The equivalent word is goit . In southern England, a leat used to supply water for water-meadow irrigation is often called a carrier , top carrier , or main . Leats generally start some distance (a few hundred metres/yards, or perhaps several miles/kilometres) above the mill or other destination, where an offtake or sluice gate diverts a proportion of the water from a river or stream . A weir in

532-592: The fluid due to its bulk motion. The total hydraulic head of a fluid is composed of pressure head and elevation head . The pressure head is the equivalent gauge pressure of a column of water at the base of the piezometer, and the elevation head is the relative potential energy in terms of an elevation. The head equation , a simplified form of the Bernoulli principle for incompressible fluids, can be expressed as: h = ψ + z {\displaystyle h=\psi +z} where In an example with

560-418: The height of an equivalent static column of that fluid. From Bernoulli's principle , the total energy at a given point in a fluid is the kinetic energy associated with the speed of flow of the fluid, plus energy from static pressure in the fluid, plus energy from the height of the fluid relative to an arbitrary datum . Head is expressed in units of distance such as meters or feet. The force per unit volume on

588-483: The land. Leat A leat ( / ˈ l iː t / ; also lete or leet , or millstream ) is the name, common in the south and west of England and in Wales , for an artificial watercourse or aqueduct dug into the ground, especially one supplying water to a watermill or its mill pond . Other common uses for leats include delivery of water for hydraulic mining and mineral concentration , for irrigation , to serve

SECTION 20

#1732885071562

616-433: The leat and the main stream is great enough to provide a useful head of water – several metres (perhaps 5 to 15 feet) for a watermill, or a metre or less (perhaps one to four feet) for the controlled irrigation of a water-meadow . Leats are used to increase the yield of a reservoir by trapping streams in nearby catchments by means of a contour leat. This captures part or all of the stream flow and transports it along

644-528: The moor are marked on the 1:50000 and 1:25000 Ordnance Survey maps, such as that serving the now-defunct Vitifer mine near the Warren House Inn . Notable leats include: Hydraulic head It is usually measured as a liquid surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer . In an aquifer , it can be calculated from the depth to water in a piezometric well (a specialized water well ), and given information of

672-412: The ore after crushing by simple machines also driven by water. The Romans also used them for supplying water to the bath-houses or thermae and to drive vertical water-wheels . There are many leats on Dartmoor , mostly constructed to provide power for mining activities, although some were also sources of drinking water. The courses of many Dartmoor leats may still be followed. Many such leats on

700-416: The piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. The hydraulic head can be used to determine a hydraulic gradient between two or more points. In fluid dynamics , head is a concept that relates the energy in an incompressible fluid to

728-471: The rest goes into the Burrator Reservoir which provides most of the water supply of Plymouth. For part of the route to Dousland the pipes follow the route of the disused Yelverton to Princetown Railway. Before the piped supply to Dousland was installed, the water was used for a hydroelectric turbine near Yelverton Reservoir and fed by a 12-inch-diameter (300 mm) pipe. The Devonport Leat begins

756-477: The source stream often serves to provide a reservoir of water adequate for diversion. The leat then runs along the edge or side of the valley, at a shallower slope than the main stream. The gradient, together with the quality of the wetted surface of the leat, determines the flow rate . The flow rate may be calculated using the Manning formula . By the time it arrives at the water mill the difference in levels between

784-405: Was used to construct the water channel, as well as a small aqueduct and a tunnel. It was originally designed to carry water all the way to Plymouth Dock, a total distance of 27 miles (43 km), but has since been shortened and the operational part of the leat now stops near the Burrator Reservoir dam. Some of the water goes through underground pipes to the water treatment works at Dousland ;

#561438