ISO 8368:2019 pdf download – Hydrometric determinations – Flow measurements in open channels using structures – Guidelines for selection of structure
1 Scope
This document gives guidelines for selecting a particular type of flow measuring structure for measuring liquid flow in an open channel. It describes how the individual structures function in simple non-technical terms, and sets out the factors and parameters to take into account in order to make an informed decision on which type of structure to use. Values of the relevant parameters describing the limitations and uncertainty involved in the use of these structures are given in this document. More definitive details of a particular type of structure are given in the individual standards listed in Table 1, which cover each type of structure.
5.1 General
In general, all flow measuring structures are subject to certain requirements irrespective of their form and operation. These are given below.
a) All these structures operate on the basis that they create a unique relationship between water depth at the critical section that lies within the structure and the flow passing through it down the channel. Therefore, it is necessary to observe the depth of water in the channel at specific points to derive a value of channel flow. Depending on the type of structure, these depth observation points can be:
1) in the approach channel;
2) at or near the crest of the weir or the critical section of the flume;
3) in some cases, the downstream end of the channel in which the structure is located; water depth measurements can be taken by hand using a suitable depth sounding instrument, by eye using a gauge board fixed at the specific point, or continuously using a water level recorder or pressure transducer and data logger set at the specific point.
b) The location of any structure should be such that it is free from the effects of backwater from downstream influences, such as a confluence with another watercourse. High backwater causes non-modularity or drowning of the structure with a corresponding loss of measurement accuracy. Typically, non-modularity occurs in weirs when the downstream level exceeds between 0,66 and 0,75 of the upstream level (measured relative to the crest of the weir). For some forms of weir, notably those with a triangular profile, it is possible to derive an adjustment factor to correct for the level of non-modularity by using the ratio of downstream level to a head within the separation pocket formed downstream of the crest. With the Parshall flume, a submerged calibration can be derived. However, with all forms of weir and flume, this has not been particularly successful in sediment laden streams, and, hence, for field observations, the avoidance of backwater effects is recommended.
c) Recent environmental legislation in many countries has required the installation of fish passes at weirs on fish migratory water courses.
d) When a flow measuring weir is located in a watercourse that carries a high silt load, the structure can be affected by siltation behind the crest, to an extent dependant on its configuration. In general, the greater the afflux created by the structure, the greater the build-up of sedimentation behind the crest. For example, a vertical thin plate weir creates a greater trap for sedimentation than a triangular profile weir. Alternatively, flumes tend to self-flush to greater degree especially at higher flow rates. This factor needs to be considered when a structure is being selected and designed. However, whatever type of structure is selected, it may be prudent to include a by-pass at the time of construction. This should have the capacity to take all the discharge in the watercourse at periods of low flow. The by-pass facilitates the clearance of any silt build-up without adverse environmental impacts during the silt removal process.