Contents preface (VII) introduction 1—37
Table 13.1 Salient details of some barrages (3)
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(i) From the stage-discharge curve at the weir site, the high flood level (HFL) for the design flood (usually 50- to 100-year frequency flood) discharge is determined. (ii) The level of the downstream total energy line (TEL) is determined adding the veloc-ity head to the HFL. The velocity of flow is computed using the Lacey’s regime equa-tion for velocity. (iii) The permissible afflux is added to the level of the downstream TEL to obtain the level of the upstream TEL. (iv) The discharge intensity q is determined by dividing the design flood discharge by the width of clear waterway. (v) Using the relation,
the height of TEL, above the weir crest, k is determined. Here, k (in meters) is the total head with respect to the weir crest. In Eq. (13.1), the value of C depends on many factors, such as the head over the weir crest, shape and width of the crest, the crest height over the upstream floor, and the roughness of the crest surface. It is, therefore, advisable to estimate C by the use of model studies if the values based on prototype observations based on similar structures are not available (2). For broad-crested weirs having crest width more than 2.5 times the head over the crest, C may be taken as 1.71. For the crest whose width is less than 2.5 times the head over the crest, the value of C is taken as 1.84. (vi) The level of the weir crest is obtained by subtracing k from the level of the upstream TEL. (vii) After fixing the weir crest level, length and suitable number of weir bays are de-cided. The total discharge capacity of the weir and undersluice bays is worked out using the following discharge equation which takes into consideration the reduction in width of flow on account of end contractions.
Here, L is the overall waterway, H the head over the crest, n the number of end contractions, and K is a coefficient which ranges from 0.01 to 0.1 depending upon the shape of the abutment and the pier nose. The exact value of the coefficient C depends on several factors, viz., head over the crest, the shape and size of the crest, the height of the crest over the upstream floor, and the roughness of the crest surface. Use of model studies is suggested for the estimation of C. In the absence of such studies, C can be assumed as 1.71 in SI units (4). (viii) The height of shutters or gates will be equal to the difference between the pond level and the level of the weir crest. For weirs without shutters (or gates), the crest level should, obviously, be at the required pond level. For weirs with falling shutters, the crest level should not be lower than 2 m below the pond level as the maximum height of the falling shutters is normally limited to 2m. If the crest level so fixed causes too much of afflux, the waterway of weir may be suitably increased. For barrages too, the crest level is similarly determined by the head required to pass the design flood at the desired afflux. It is desirable that the crest (of the barrage) and the upstream floor levels of the undersluices be kept at the lowest bed level of the deep channel of the river as far as practicable. The upstream floor level of the remaining bays should be kept normally 0.5 to 1.0 m above the upstream floor level of the undersluice bays or the general river bed level. 444 IRRIGATION AND WATER RESOURCES ENGINEERING As a result of the construction of weir across a river, the downstream bed levels will be lowered due to degradation (or retrogression) and, hence, the downstream HFL will also be lowered. The lowering of water levels due to retrogression on the downstream increases the exit gradient. Retrogression is relatively more in alluvial rivers carrying fine sediment and having steep slope. When the proposed weir is sited downstream of a dam, the retrogression increases. Retrogression should always be considered for the design of the downstream floor and the downstream protection works. During high floods, the river water carries lot of sediment which reduces the extent of retrogression and, hence, lowering of HFL is only marginal - of the order of about 0.3 to 0.5 m. But, during low floods the river water downstream of the weir is relatively clear and may increase retrogression thereby lowering the HFL by an appreciable amount ranging from 1.25 to 2.25 m, depending upon the amount of sediment in the river bed and the bed slope of the river (2). At the design flood in an alluvial river, the reduction in river stages due to retrogression may be considered to vary from 0.3 to 0.5 m depending upon whether the river is shallow or confined during floods (2). For other discharges, the effect of retrogression may be obtained by the variation of retrogressed flood levels with the flood discharges. The downstream TEL is lower than the upstream TEL by an amount HL which is equal to the sum of the assumed afflux and the estimated retrogression. During the first few years of weir construction, the sediment-carrying capacity of the river decreases due to ponding up of water upstream of the weir. This results in continuous deposition of the sediment upstream of the weir and the bed level rises. Ultimately, the bed slope regains its original slope and the afflux extends still further upstream of the backwater profile. The marginal bunds, constructed to take care of the rise in water level in the backwater region, will, then, have to be extended further upstream. A stage is thus reached when the upstream pond takes no more sediment. Since the offtaking canal withdraws relatively sediment-free water, the sediment is now carried downstream of the weir with reduced water discharge (and, hence, reduced sediment-carrying capacity). Therefore, the sediment will start depositing on the downstream side and raise the bed levels lowered due to initial degradation. Sometimes, the bed levels can rise even beyond the original bed levels. Yüklə 18,33 Mb. Dostları ilə paylaş:
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