Broadly speaking, there are four different types of structures, each with its own particular function:

3 The selection of structures 3.1 Introduction In selecting a suitable structure to measure or regulate the flow rate in open channels, all demands that will be made upon the structure should be listed. For discharge measuring and regulating structures, hydraulic performance is fundamental to the selection, although other criteria such as construction cost and standardization of structures may tip the balance in favour of another device. The hydraulic dimensions of the discharge measuring or regulating structures described in the following chapters are standardized. The material from which the device is constructed, however, can vary from wood to brick-work, concrete, polyester, metal, or any other suitable material. The selection of the material depends on such criteria as the availability and cost of local material and labour, the life-time of the structure, pre-fabrication etc. Constructional details are not given in this book except for those steel parts whose construction can influence the hydraulic performance of the structure. Although the cost of construction and maintenance is an important criterion in the selection of structures, the ease with which a discharge can be measured or regulated is frequently more important since this will reduce the cost of operation. This factor can be of particular significance in irrigation schemes, where one ditchrider or gatesman has to control and adjust IO to 20 or more structures daily. Here, ease of operation is labour saving and ensures a more efficient distribution of water over the irrigated area. Although other criteria will come into play in the final selection of a discharge measuring or regulating structure, the remarks in this chapter will be limited to a selection based solely on hydraulic criteria. 3.2 Demands made upon a structure I 3.2.1 Function of the structure Broadly speaking, there are four different types of structures, each with its own particular function: - discharge measuring structure; - discharge regulating structure; - flow divider; - flow totalizer; Discharge measuring structure The function of such a structure is to enable the flow rate through the channel in which it is placed to be determined. If the structure is not required to fulfil any other function, such as water-level control, it will have no movable parts. Discharge measurement structures can be found in natural streams and drainage canals, and 87

also in hydraulic laboratories or in industries where flow rates need to be measured. All flumes and fixed weirs are typical examples of discharge measurement structures. Discharge regulating structure These structures are frequently found in irrigation canals where, as well as having a discharge measuring function, they also serve to regulate the flow and so distribute the water over the irrigated area. Discharge regulating structures can be used when water is drawn from a reservoir or when a canal is to be split up into two or more branches. A discharge regulating structure is equipped with movable parts. If the structure is a weir, its crest will be movable in a vertical direction; if an orifice (gate) is utilized, the area of the opening will be variable. Almost all weirs and orifices can be used as discharge regulating structures. In this context it is curious to note that in many irrigation canal systems, the discharge is regulated and measured by two structures placed in line in the same canal. The first structure is usually a discharge regulating gate and the second, downstream of the first, is a discharge measuring flume. It would seem to be a waste of money to build two such structures, when one would suffice. Moreover, the use of two structures requires a larger loss of head to operate within the modular flow range than if only one is used. Another even more serious disadvantage is that setting the required discharge with two structures is a more time consuming and complicated procedure than if a single regulating structure is used. Obviously, such procedures do not contribute to the efficient management of the available water. Flow divider It may happen that in an irrigated area we are only interested in the percentage distribution of the incoming flow into two or more branch canals. This percentage distribution can be achieved by constructing a group of weirs all having the same crest level but with different control widths. If the percentage distribution has to vary with the flow rate in the undivided canal, the crest level of the weirs may differ or the control sections may have different shapes. Sometimes the required percentage distribution of flow over two canals has to vary while the incoming flow remains constant. This problem can be solved by using a movable partition (or divisor) board which is adjusted and locked in place above a fixed weir crest (see Section 9.1). Although a flow divider needs no head measurement device to fulfil its function, a staff gage placed in the undivided canal can give additional information on the flow rate, if this is required by the project management. Flow totalizer If we want to know the volume of water passing a particular section in a given period, we can find this by using a flow totalizer. Such information will be required, for instance, if a farmer is charged for the volume of water he diverts from the irrigation 88

canal system, or if an industry is charged for the volume of effluent it discharges into a stream. The two flow totalizers treated in this book both have a rotating part and a revolution counter which can be fitted with an additional counter or hand to indicate the instantaneous flow rate. 3.2.2 Required fall of energy head to obtain modular flow Flumes and weirs The available head and the required head at the discharge measuring site influence both the type and the shape of the structure that will be selected. For weirs and flumes, the minimum required head AH to operate in the modular flow range can be expressed as a fraction of the upstream energy head H, or as (HI - H,)/H,. This ratio can also be written as 1 - H,/H,, the last term of which describes the limit of the modular flow range, i.e., the modular limit (see also Section 1.15). The modular limit is defined as the value of submergence ratio H,/HI at which the real discharge deviates by 1 % from the discharge calculated by the head-discharge equation. We can compare the required fall over weirs of equal width by considering their respective modular limits. The modular limit of weirs and flumes depends basically on the degree of streamline curvature at the control section and on the reduction of losses of kinetic energy if any, in the downstream expansion. Broad-crested weirs and long-throated flumes, which have straight and parallel streamlines at their control section and where part of the kinetic energy is recovered, can obtain a modular limit as high as H,/HI = 0.95. As mentioned in Chapter 1, the discharge coefficient of a weir increases if the streamline curvature at the control section increases. At the same time, however, a rising tailwater level tends to reduce the degree of streamline curvature, and thus reduces the discharge. Consequently we can state that the modular limit of a weir or flume will be lower as the streamlines are more strongly curved under normal operation. The extreme examples are the rectangular sharp-crested weir and the Cipoletti weir, where the tailwater level must remain at least 0.05 m below crest level, so that streamline curvature at the control section will not be affected. Modular limits are given for each structure and are summarized in Section 3.3. The available head and the required head over a structure are determining factors for the crest elevation, width and shape of the control section, and for the shape of the downstream expansion of a discharge measurement structure. This can be shown by the following example. Suppose a 0.457 m (1.5 ft) wide Parshall flume is to be placed in a trapezoidal concrete-lined farm ditch with 1-to-1.5 side slopes, a bottom width of 0.50 m, and its crest at ditch bottom level. In the ditch the depth-discharge relationship is controlled by its roughness, geometry, and slope. If we use the Manning equation, v = l/n R2I3 so.s, with a value of n = 0.014 and s = 0.002, we obtain a satisfactory idea of the tailwater depth in the ditch. Tailwater depth data are shown in Figure 3.1, together with the head-discharge curve of the Parshall flume and its 70% submergence line (modular limit). An examination of the 70% submergence curve and the stage-discharge curve shows 89

Figure 3. I Stage-discharge curves for 1.5 ft Parshall flume and for a concrete-lined ditch. Flume crest coincides with ditch bottom that submerged flow will occur at all discharges below 0.325 m3/s, when the flume crest coincides with the ditch bottom. Figure 3.1 clearly shows that if a design engineer only checks the modularity of a device at maximum stage, he may unknowingly introduce submerged flow conditions at lower stages. The reason for this phenomenon is to be found in the depth-discharge relationships of ditch and of control section. In the given example, a measuring structure with a rectangular control section and a discharge proportional to about the 1.5 power of upstream head is used in a trapezoïdal channel which has a flow rate proportional to a greater power of water depth than 1.5. The average ditch discharge is proportional to y2'.*. On log-log paper the depth-discharge curve (ditch) has a flatter slope than the head-discharge curve of the flume (see Figure 3.1). To avoid submerged flow conditions, the percentage submergence line of the measuring device in this log-log presentation must be to the left of the channel discharge curve throughout the anticipated range of discharges. The coefficient of roughness, n, will depend on the nature of the surface of the downstream channel. For conservative design the roughness coefficient should be maximized when evaluating tailwater depths. Various steps can be taken to avoid submergence of a discharge measuring device. These are: The 1.5 ft Parshall flume of Figure 3.1 can be raised 0.03 m above ditch bottom. The stage-discharge curve of the flume in terms of ha + 0.03 m plots as a curve shown 90

in Figure 3.2. The corresponding 70% submergence curve plots to the left of the stagedischarge curve of the ditch. The 1.5 ft Parshall flume of Figure 3.1 can be replaced either by a flume which requires more head for the same discharge, thus with a rating curve that plots more to the left on log-log paper, or by a flume which has a higher modular limit than 70%. A flat-bottom long-throated flume with 0.45 m wide control and 1 to 6 downstream expansion will be suitable. It must be recognized that the two previous solutions with a Parshall flume require a loss of head of at least 0.31 m at the maximum discharge capacity of the flume, being Q = 0.65 m3/s (see Figure 3.2). If this head loss exceeds the available head, the design engineer must select a structure with a discharge proportional to an equal or greater power of head than the power of the depth yz of the ditch. For example, he may select a flat-bottom, long-throated flume with a trapezoïdal control section and a gradual downstream expansion. Such a flume can be designed in such a way that at Q = 0.65 m3/s an upstream head h, = 0.53 m and a modular limit of about 0.85 occur resulting in a required head loss of only 0.08 m. He could also use a longthroated flume with a (truncated) triangular, parabolic, or semi-circular control section (Bos 1985). Figure 3.2 Stage-discharge curves for flume and ditch of Figure 3.1, but flume crest 0.03 m above ditch bottom 91

Orifices At the upstream side of free flowing orifices or undershot gates, the upper edge of the opening must be submerged to a depth which is at least equal to the height of the opening. At the downstream side the water level should be sufficiently low so as not to submerge the jet (see Chapter 8). For this reason free flowing orifices, especially at low flows, require high head losses and are less commonly used than submerged orifices. The accuracy of a discharge measurement obtained with a submerged orifice depends on the accuracy with which the differential head over the orifice can be measured. Depending on the method by which this is done and the required accuracy of the discharge measurement, a minimum fall can be calculated with the aid of Annex 2. In general, we do not recommend the use of differential heads of less than 0.10 m. 3.2.3 Range of discharges to be measured The flow rate in an open channel tends to vary with time. The range between Qmin and Qmax through which the flow should be measured strongly depends on the nature of the channel in which the structure is placed. Irrigation canals, for example, have a considerably narrower range of discharges than do natural streams. The anticipated range of discharges to be measured may be classified by the ratio Y = Qmax/Qmin (3-1) From the limits of application of several weirs, a maximum attainable y-value can be calculated. Taking the example of the round-nosed horizontal broad-crested weir (Section 4. l), the limits of application indicate that HI/L can range between 0.05 and 0.50 m. As a result we obtain a maximum value of y which is This illustrates that whenever the ratio y = Qmax/Qmin exceeds about 35 the horizontal broad-crested weir described in Section 4. I cannot be used. Weirs or flumes that utilize a larger range of head, or which have a head-discharge relationship proportional to a power of head greater than 1.5, or both, can be used in channels where y = Qm,,/Qmin exceeds 35. The following example shows how the y-value, in combination with the available upstream channel water depth y,, influences the choice of a control section. The process of selection is as follows: Find a suitable flume and weir for Qmin = 0.015 m3/s -+ y = 200 Qma, = 3.00m3/s Y1 = h, + pi Q 0.80m The flume is to be placed in an existing trapezoidal channel with a 4 m wide bottom and 1-to-2 side slopes. At maximum water depth y, = 0.80 m, the Froude number in the approach channel is Fr = v,/(ga,/b,) ~ = 0.27. It is noted that for Fr < 0.50 the water. surface will be sufficiently stable. 92

From the relatively high y-value of 200 we can conclude that the control section of the structure should be narrower at minimum stage than at maximum stage. Meeting the requirements of this example are control sections with a narrow bottomed trapezium, or a triangular or truncated triangular shape. Because of the limited available width we select a truncated triangular control section of which two solutions are illustra ted below. Triangular profile flat-v weir (Figure 3.3) According to Section 6.4.2 the basic head-discharge equation of this weir reads 4 B Q = C,C, -(2g)0.52 [h:. -(h,- Hb)2. ] (3-3) 15 H b in which the term (he- Hb)2.5 should be deleted if he is less than H,. If we use the l-to-2/1-to-5 weir profile and a 1-to-10 cross slope, the minimum channel discharge can be measured at the minimum required head, since Q at 0.06 m head is 4 4.0 Q0,06 = 0.66 x 1 x -(2g)0.5 x -(0.06-0.0008)2.5 15 0.20 Q0.06 = 0.0133 m3/s Another restriction for the application of this type is the ratio h,/p,, which should not exceed 3.0. The required width of the weir can be found by trial and error: Since y, = h, + p, < 0.80 m, the maximum head over the weir crest h, max = 0.60 m when p, = 0.20 m. Using a width B, of 4 m, we find for the discharge capacity at h, = 0.60 m (for C, see Fig. 6.10) 4 4.00 Q0.60 = 0.66 x 1.155 x E(2g)O. x x [(0.60-0.0008)2~5-(0.60-0.0008-0.20)2~5] 0.20 40.60 = 3.205 m3/s This shows that the full discharge range can be measured with the selected weir. Long-throated flume with truncated triangular control (see Fig. 3.3) According to Section 7.1.2, the head-discharge relationships for this flume read Triangular profile welr 8~:4.00 I Long throated flume BE: 3.00 I Figure 3.3 Two examples of suitable control sections 93