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Irrigation Crossing Structures

• Water way + Road way

• Culvert

• Bridge

• Water way + Water way

• Syphon

• Aqueduct

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Open channel

RoadOpen channel

Tail escape

•Retaining walls

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Design Requirements

Hydraulic Design• Cross section Shape

• Number of vents

• Cross section dimensions

• Velocity inside the vents

• Heading up or total losses

Structural Design• Material of construction

• Thickness of the cross section

• Loads on the vents (D.L., L.L., Water pressure, Earth pressure,………)

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Culvert

1- IntroductionA culvert is an artificial water passage under a road or

rail way.

It is designed to convey water from one place to another. It is usually used for shorter spans (less than 6.0 m) which bridges can be used for longer water way.

The culvert consists essentially of a conveyance part (closed conduit) under the embankment fill with protection works at its entrance and exit.

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The main types of culverts are

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The culvert should usually follow the natural stream alignment and its gradient, in order to minimize head losses and erosion.

However, it is sometimes more economical to place the culvert perpendicular to the highway with certain acceptable changes in the stream alignment.

The design in the culvert will obey flow in closed conduit not open channel flow.

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2- Construction shapes and Materials2.1 The common shapes of culvert include round,

square, rectangular, egg shaped and arch shaped.

2.2 The construction materials include masonry, concrete, plastic and steel.

2.3 The culvert may consists of one or several vents.

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3- Hydraulic CalculationsCross section Shape and dimensions, Number of vents, Velocity

inside the vents, Heading up or total losses

The culvert is a simple structure but its hydraulic calculation may be complex due to the large number of variables which affect its flow.

These variables include upstream and down stream, water levels, inlet condition and size, shape, length, slope and roughness of the culvert material.

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Q= f( H up, H down, D , S , L , n , G, SL ) Q= discharge passes through the culvertH up= upstream water depth above the culvert

invertH down= down stream water depth above the

culvert invert D = Maximum vertical dimension of the culvertS = Maximum horizontal dimension of the culvert

vent (culvert span)L= culvert lengthSl= culvert longitudinal slopen= friction resistance factorG= the geometry of the inlet١٣

Design Precautions

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� The structure alignment should follow the stream channel or perpendicular to the embankment to minimize head losses and erosion.

� The mean velocity through the structure is taken from 2 to 3 velocity of the open channel or (1 to 2 m/s).

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� The maximum allowable heading up is 20 cm to reduce back water effect on the upstream controlling structures.

� Provide a minimum of 20 to 30 cm submergence at the lip entrance and exit of the structure to ensure full flow condition.

• If the submergency condition is not satisfied open channel bed can be lowered by not more than 50 cm with

a slope 5 :1

• Provide earth fill cover not less than 75 cm above the structure deck.

• The width of the vents in box type is selected equal or bigger than the height.

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� Scour protection using pitching for distance (2- 3 ) d should be provided at the outlet.

� A trash rack (screen) should be provided at the upstream side of the structure.

� Berm level not less than 50 cm above the H.W.L. must be achieved in the structure area.