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AS 3778.4.6-2007 pdf free download

AS 3778.4.6-2007 pdf free download.Measurement of water flow in open channels
Part 4.6: Measurement using flow gauging structures Flat-V weirs.
6.2.2 Approach channel If the flow in the approach channel is disturbed by irregularities in the boundary, e.g. large boulders or rock outcrops, or by a bend, sluice gate or other feature which causes asymmetry of discharge across the channel, the accuracy of gauging may be significantly affected, The flow in the approach channel shall have a symmetrical velocity distribution (see annex A.) This can be achieved by providing a long, straight approach channel of uniform cross-section. A minimum required length of straight approach channel shall be five times the width of the water surface at maximum flow, provided flow does not enter the approach channel with high velocity via a sharp bend or angled sluice gate.
NOTE I This figure refers to the distance upstream of the head-measuring position.
NOTE 2 A greater length of unifo-m approach channel is desirable if it can be readily provided. In a natural channel where it is uneconomic to line the bed and banks with concrete for this distance, and if where the width between the vertical walls of the lined approach to the weir is less than the approach width of the natural channel, the banks shall be profiled to give a smooth transition from the approach channel width to the width between the vertical side walls. The unlined channel upstream of the contraction shall nevertheless conform to and Vertical side walls constructed to effect a narrowing of the natural channel shall be symmetrically aligned with the centre line of the channel and curved to a radius not less than 2 Hm as shown in Figure 1. The tangent point of this radius nearest to the weir crest shall be at least 11m upstream of the head measurement section. The height of the side walls shall be chosen to contain the design maximum discharge. In a channel where the flow is free from floating and suspended debris, good approach conditions can also be provided by suitably placed baffles formed of vertical laths. No baffle shall be nearer to the point at which the head is measured than 10 times the maximum upstream head. Under certain conditions, a hydraulic jump may occur upstream of the measuring structure, for example if the approach channel is steep. Provided the wave created by the hydraulic jump is at a distance upstream of no less than 20 times the maximum upstream depth, flow measurement is feasible, subject to confirmation that an even velocity distribution exists at the gauging station. Conditions in the approach channel can be verified by inspection or measurement for which several methods are available such as current meters. floats, velocity rods, or concentrations of dye, the last being useful in checking conditions at the bottom of the channel. A complete and quantitative assessment of velocity distribution can be made by means of a current meter, The velocity distribution shall comply with the requirements of annex A, clause A.5.AS 3778.4.6-2007 pdf free download.

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