3.2 Structures in Small Lined Canals

Transcription

1 Weirs with a partition board - The flow divisor consists essentially of a broadcrested weir with rectangular control section and a partition board downstream from the crest. The partition board may be fixed or movable depending on the need to change the division of flow over the canal branches. The advantages of combining the measuring and regulation functions into one structure are: (1) less hydraulic head is needed; (2) a multipurpose structure usually is cheaper to construct than the two separate structures (one for regulation and one for measurement) it replaces; and (3) the operation of one structure is less timeconsuming. Owing to the latter, gatemen or ditchriders tend to distribute irrigation water more accurately, and a higher efficiency of irrigation water use can be attained. 3.2 Structures in Small Lined Canals In the past, flow measurements with flumes and weirs were costly, of variable accuracy, and difficult to apply in field situations. Major problems included the requirement of reshaping the canals to accommodate a limited assortment of calibrated devices, as is the case for Parshall flumes, and the requirement for relatively large water surface drops associated with sharp-crested weirs. Not the least of these problems was the general inability to control installation errors and readout errors for an accurate, convenient, and reliable head reading. Many of these problems are significantly reduced by the use of long-throated flumes and the hydraulically related broad-crested weirs provided in this book (Figure 3.2). Figure 3.2 Broad-crested weir operating at design discharge (Arizona). 62 Flume and Weir Qpes and Construction Methods

2 1 The construction of a small broad-crested weir in a concrete-lined (irrigation) canal is relatively easy and straightforward. The existing canal lining can serve as: 1) approach channel; 2) part of the converging transition, throat, and the diverging transition; and 3) tailwater channel. If no energy head needs to be recovered, an abrupt diverging transition is sufficient, and the weir has only two sections that need to be constructed: a throat section with a length L, and a converging transition with a length equal to three times the sill height, or 3pl (Figure 3.3). As was shown in Figure 1.3, the contraction of the canal can be from the sides, from the bottom, or both. Most lined canals are irrigation canals with a ratio of maximum to minimum flows, Qmar/Qmin, below 5. Since broad-crested weirs (bottom contractions) can satisfy this range of flows and are generally much easier and less costly to construct, they are recommended for this application. Design procedures, standard sizes (defined in terms of crest width, b,), and rating tables are discussed in Section The constructed crest width, b,, should be as close to the values in the rating tables as the design accuracy of the measurement, because a 1 % error in b, will produce about a 1% error in discharge (see Table 2.5). Although the sill height pi is important in selecting the weir because it controls the water surface elevation and thus the modular limit and freeboard relationships, its precise vertical dimension is not at all critical to the weir head-discharge relationship. For example, pi can vary by *lo% before causing noticeable calibration changes. Also, the crest length L can be adjusted 3=10% without significant effect. The ramp length of 3pl is also approximate and its purpose is to convey the water smoothly to the weir crest. Thus the ramp may be straight or nt for establishing gage zem L/C to L/3 from end, on weir recorder r--1-4-i- I I X K 4 SECTION ALONG CENTER LINE CROSS SECTION Figure 3.3 Broad-crested weir in a concrete lined canal. Chapter 3 63

3 gently curved, but straight is usually more readily constructed. These liberal tolerances should not be an excuse for sloppy or poor construction, but they can make construction quick and easy. More accurate rating tables can be developed by measuring the exact structure dimensions and using the computer program of Chapter Cast-in-Place Structures Trapezoidal channels Following site selection and the design of a structure, cadin-place construction is straightforward. If the canal has construction joints, the structure should be placed so that the gage location is at least 0.5 m downstream from the joint. If that is difficult, locate the gage slightly upstream from the joint and position the sill at distance (La + Lb) downstream as listed in the rating tables and shown in Figure 3.4. If possible, construction joints in the area of the sill should be avoided. Construction joints occurring in the ramp section are acceptable. If a construction joint is located between the weir sill and gage, care should be exercised to ensure that no vertical movement occurs. Otherwise, the zero-reference (sill-reference) of the gage may not remain reliable. The suggested construction sequence is In a workshop, make two forms from galvanized angle iron to the cross sectional shape of the control section, with the edge on one angle (not the flat side) making the shape. These edges will be used as the forms for screeding and troweling the concrete. For a flume with bottom plus side contraction, these forms shape the bottom and sides of the control. For weirs with bottom contraction only, the top of the angle irons have a length of b, since the sides of the canal function as sides of the control. gaging station control section J I I I I L I, /PI approach channel %onverging throat diverging transition " tailwater transition channel Figure 3.4 Illustration of terms. ~- 64 Flume and Weir Types and Construction Methods i

4

5

6 Figure 3.8 Structure with rectangular control section in lined trapezoidal canal ramp and crest from being forced downstream. Thus we recommend that the structure be firmly anchored to the floor or the walls. Otherwise, the construction sequence is the same as for the trapezoidal channels. In some cases, it may be desirable to narrow the structure. For trapezoidal channels, we recommend constructing the sill and then narrowing the crest. For rectangular structures, it is easier to narrow the structure with block or brickwork, and then add the sill, if needed. Culverts are an attractive site to construct a measuring flume in otherwise unlined channels, since the foundation for the structure is already in place. If the flume or weir is placed near the outlet of the culvert, additional scour protection may be needed (see Chapter 7). Other channel shapes There are a variety of other lined channel cross-section shapes, including semi-circular, U-shaped, circular (e.g., culverts), parabolic, triangular, etc. Where sloping sidewalls exist, construction procedures are essentially the same as for trapezoidal channels. If the sidewalls are vertical, then the construction considerations for rectangular channels are appropriate. The main objective is to build a sill that is level in both directions and has a cross-section that is smooth and uniform throughout the throat Prefabricated concrete structures In principle, all of the cast-in-place concrete weirs of Section can be precast as reinforced concrete beams and slabs. The locally available equipment for transporting Chapter 3 67

7

8

9

10

11

12 suitable for the sizes mentioned. The angles should be about 80 mm (6 in.) on each leg, and have a length equal to the weir width. The downward pointing leg of each angle end is then bent so that it will match the canal side slope z and serve as a tab for securing the sill to the canal walls. The ramp is then cut to match the sill width, and a tab is included to secure it to the sill edge. The tab is bent on a 3: 1 angle so that the sill and ramp smoothly intersect without overlapping. Like prefabricated concrete weirs, local materials, available tools, labor skills, and type of use will somewhat control whether the weirs are bolted together or spot welded, and painted or otherwise coated for rust protection. The weir shown in Figure 3.14 uses three galvanized, 50 x 76 x 3 mm (2 x 3 x '/* in.) steel angles available commercially. The sill top was 16 gage galvanized steel sheet (about in. or 1.6 mm thick). Also, the two ramp sections are shown with some stiffening members made from the galvanized sheet. Because the edge of the ramp rests on the canal wall and is fixed to the sill beam, very little additional stiffness is needed. In practice, water pressure holds all parts in place and the canal fastenings are needed only to prevent wind damage when the canal is empty and to prevent accidental movement by people servicing the canal or by animals. Simple structural analysis should be made to determine the expected deflection of the sill during use. Figure 3.15 shows the ramp section for a large temporary metal weir being hoisted into place. Figure 3.15 The upstream ramp of a 3 m wide (IO ft) weir is hoisted into place after the sill has been installed (Arizona). Chapter 3 73

13

14

15 _= Static pressure tube \ f i-.".. - _ - T i Ff i pjq... j....- W... :D : 3D _ Layout 112 of minor and major axis on ruler as shown here p(dm)/2 = D Y I+ D'2+jx TOP VIEW Y ENDVIEW " to establish outline of ellipse b sliding points x and y along respective axis as indicated. Figure 3.17 Layout scheme for portable long-throated measurement structures in partially full circular conduits. 16 Flume and Weir opes and Construction Methods

16

17 cup0 required height flow * PVC pipe end co I 300," $ LL2 rubber seal (a) goo 1L7 Y ' UNFOLDED SILL COVER UnfddPd sheet o sill reference point tor zerdng point mgo I I r I L L/2 converging sectm [ramp).l thmd spctmhlltopl,,~ diverginp I 1- sectlo" a FRONT VIEW SILL COVER AND sill cover sheet 2 1 oluminum tii ips (b) 3x100 mm rubber sheeting Figure 3.20 Construction drawings for portable weir 78 Flume and Weir Types and Construction Methods

18 Figure 3.20 (continued), Chapter 3 79

19 k, I 930 >I oluminum pipe golv. reducer galv. nipple I 020u FRONT ASSEMBLY BOTTOM FRAME Figure 3.20 (continued). The weight of the welded version shown is about 10 kg (25 Ib). Leakage past the weir is controlled by rubber sealing strips made of 3.2-mm (1/2-in.) thick sheeting cut about O. 10 m (4 in.) wide. The seal is fastened to the sides of the ramp and to the lower front angle of the framework by aluminum strips and pop rivets. It is helpful if, during construction, the rubber sheeting is stretched slightly so that it warps strongly upward, especially across the bottom of the weir. Water pressure then causes a good seal against the canal walls and bottom. Insufficient stretching may allow the rubber to turn under the weir frame during placement, negating its effectiveness. Translocation of the point gage and stilling-well cup to a position above the point of zero registration makes portable weirs insensitive to minor problems of leveling in all directions and eliminates the need for lateral transfers of elevations by surveying techniques (Figure 3.21). Thus careful leveling is not necessary, just gross adjustments readily observed by the eye (see also Section 4.10). The upstream water level can be sensed by attaching flexible tubing from the cup to a sensing pipe. The sensing pipe can either be attached to the portable flume (and retractable as shown here), or be separate (see also Sections 4.9 and 4. IO). For structures with side walls, as in Figure 3.22, a side-wall tap replaces the sensing pipe. The temporary structures described in Section can often be used as portable structures. To accomplish this requires a portable head detection method, which is described in Section Flume and Weir Types and Construction Methods

20