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Runoff Measuring Devices




  • Unites of measurement of water:

    Water is measured under two conditions –at rest and in motion water

    Water at rest:

    [Water in reservoirs ponds soil, tank] it is measured in units of volume such as liter, cubic meters, hectare centimeter and hectare –meter

    Water in motion:

    [water flowing in rivers canal ,pipelines ,field channels ]it is expressed in rate of flow such as liters pre second liters pre hour ,cubic meters pre second hectare centimeter pre hr and hectare –meter pre day
     
                          1
    1) One liter = ———      cubic meter
                       1000

    2) 1 ha –cm =100 meter cube   =1, 00, 000 liter

    3) 1 ha m =10000 meter cube =10 million liter

    Measuring Device:

    In field most commonly used devices for measuring water are
    1. Weirs
    2. Pre shall flumes
    3. Orifices
    4. Meter gates
     
    In these devices ,the react of flow is measured directly by making reading on a scale which is part of instrument and computing the discharge rate from standard formulae .choice for the use of one or others devise depends on the expected flow rates site conditions.

    Weir:

    Weirs are used to measure the flow of runoff; an irrigation channel or discharged of a well or channel outlet at the source.

    A weir is a notch of regular form through which the water stream is made to flow. A weir consists of a weir wall of concrete, timber or metal. Weir may be built as stationary structure or they may be made portable. The notch may be rectangular, trapezoidal or triangular.

    It is desirable to install the weir at a point where there is dropped in elevation of channel bed.

    Terminology:

    Weir pond:

    Portion of channel immediately upstream from the weir

    Weir crest:

    Bottom of weir notch

    Head:

    Depth of water flowing over the weir crest measured at some point in weir pond.

    Sharp crested weir:

    A weir having thin edged crest such that over flowing sheet of water has the minimum surface contact with crest.

    End contraction:

    Horizontal distance from end of weir crests to side of weir pond.

    Bottom contraction:

    Vertical distance from weir crests to bottom of weir pond.

    Weir scale or gauge:

    The scale fastened on the side of air or a stake in weir pond to measure the head on weir crest.

    Nape:

    The sheet of water which over flow a weir

    Weirs:

    1. Sharp Crested:

    A. Rectangular weir:
    B. Cipoletfe weir 
    C.V-notch weir

    2. Broad crested.

    a. Rectangular Weirs:

    It takes its name from the shape of notch. They are used to measure comparatively large discharges.

    b .Cipoletfe Weir:

    It is contracted trapezoidal weir in which each side of notch has a slope of 1 horizontal to 4 vertical. It is named after its inventor “Cesare Cipolletti” an Italian engineer. It is used to measure medium discharges.

    c. V- Notch Weir:

    The 90o V- notch weirs are commonly used to measure small and medium size streams. The advantage of V- notch weir is its ability to measure small flows accurately.

    Source

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