Later investigations by Khosla indicated that the intermediate pile is ineffective if its length is shorter than that of the outer piles. However, there is some local redistribution of uplift pressure. Khosla, Dr. Bose and Dr. Taylor come with different results that led to the following interim conclusions.
There is a simple standard form for composite sections: for all cases, it is assumed that the thickness of the floor is negligible. Case-I: - Impervious floor with an downstream Pile. Exit Gradient For the floor to safe against piping, the exit gradient should be less than the safe gradient for the soil. Table Safe exit gradient for different soils S. Correction Coefficients The uplift pressures obtained from the superposition of the individual forms are to be corrected because of the individual pressures have been obtained based on the following assumptions: 1.
The floor has negligible thickness, 2. There is only one pile line, and 3. The floor is horizontal. As the above assumptions are not satisfied, the following corrections have to be applied. Correction for floor thickness, 2. Correction for mutual interference of piles, and 3. Correction for slope of the floor. Correction is positive for points in the rear or backwater and subtractive for points forward in the direction of flow. These corrections are further to be multiplied by the proportion of the horizontal length of the slope to the distance between the two piles lines in which the sloping floor is located.
The slope correction is applied only to those key points of pile line, which is fixed at the beginning or the end of slope. Table 3. In a hydraulic jump, there are six independent variables y1, V1, y2, V2, q and HL which can be interrelated by the above four equations.
In actual design q and HL are known. If so, the others can be obtained using the above equations. As the process is cumbersome Blench provide curves that relate q and HL with the specified energy Ef2 after the jump. If the hydraulic jump goes beyond the impervious floor, it may scour the river bed.
Thus the hydraulic jump will be formed on the glacis. The length of impervious floor can be reduced by providing appurtenances like chute blocks, basin blocks, and dentated sill. Determine conjugate depths y1 and y2 from Montague curve from the known values of Ef1 and Ef2. Profile before the Jump: - the water surface profile before the hydraulic jump can be determined with the help of Montague curve.
The following procedure is generally used. It may be noted that these values are for the supercritical stage. Profile after the Jump:-The water surface profile after the jump can be determined as follows; a. Select various points M, N, O etc downstream of point P. Determine the horizontal distances x of these points from point P. Join the water surface levels to obtain the post-jump profile. Weir failure due to scour can be prevented by extending the concrete or sheet pile cut-offs to a level sufficiently below the scour depth across the full width of the river.
Usually the maximum scour depth is taken as 1. These forces can be resisting and overturning. In order to be safe, the resisting force should be greater than the overturning forces. Weirs are with or without gates, whereas barrages are always gate controlled. This type of weir is suitable for any type of foundation.
Figure 4. It requires a very large quantity of stone. Florante Jr Poso Author. Add to cart. To prevent internal erosion and particle migration, control of seepage pressures and velocities. The percolation length seepage for a foundation can be determined by using various methods. There are number of methods available to analyze the problem on seepage and uplift pressure, and one of.
Based on Bligh's theory, that along the bottom contour of the structure, the water creeps, and the. These drops are flumed. The crest of the under-under sluice portion of the weir is kept at a lower level 1 1. These falls are Lsoping, economical and popular. Ogee falls with glaciz without raised crest 6. The cistern Roughness devices and the deflectors to deflect the high velocities.
It consists of masonry breast wall which is provided with adjustable crest shutter. Check for Scour depth Check for exit gradient Check for apron thickness due to uplift. It consists of a number of piers which divide the total width of the canal into a number of spans which are known as bays. But in case of a barrage, the. It is a simple fall broad crested weir with high raised sill, The nappe impinges into the water cushion below There is no hydraulic jump and the energy dissipation is brought about by the turbulent diffusion, as the high velocity jet enters the deep pool of water down stream.
Sheet piles are provided below the cut off walls. The muddy water flows towards the downstream through the scouring sluices. Down stream Energy Dissipation. The gates are operated form the top by suitable mechanical device. Trapezoidal Notch falls Required Basic Data: It is a low weir constructed at the end of cistern, working on the principle of horizontal impact for energy dissipation.
The platform below the baffle wall up to the deflector wall is known as the cistern. Registration Forgot your password? The sheet piles at the ends must go below the deepest anticipated scour level. For a discharge intensity q, the normal depth of scour R is given by Lacey's equation 4. For the design of sheet piles, it is just enough to take them down to the level obtained by measuring the normal depth of scour R, below the H.
Though sometimes, even 1. Length of Pucca Concrete Floor. The main turbulence of the hydraulic jump is generally confined to a length equal to five times the jump height. Hence, a pucca floor equal to or more than 5 y 2 - y 1 in length is provided after the lowest point of jump formation, i. The top width of the crest is generally kept as 2. Design of Protection Works. J:d ii Launching apron, as explained below. Value of x is generally taken as J. It generally consists of 1.
The openings between the blocks are filled with clean bajri. Details of dis loose projections. An 'inverted filter' invariably reduces the possibility of piping, as it allows free flow of seepage water through itself without allowing the foundation soils to be lifted upward. The filter, therefore, consists of layers of materials of increasing permeability from bottom to top. The gradation should be such that while it allows free flow of seepage water, the foundation material does not penetrate to clog the filter.
The design criteria to satisfy these conditions are discussed in chapter 20 on "Earth Dams". To prevent filter from dislocation under surface flow, concrete or masonry blocks are laid over the filter material.
After the inverted filter, the loose apron called 'launching apron' is provided for a length, generally equal to 1. Hence, Volume of stone per metre is given by 2. Since the volume of stone should. Different values oft have been recommended by different investigators. Blench has recommended t equal to 1. Spring has recommended t as about 0. Gales recommended t varying from l.
Blench' s recommendations are quite adoptive. B Upstream Loose Protections Just upstream of the concrete floor of the weir, block protection is provided. It generally consists of concrete blocks laid over packed stone, for a length equal to. Upstream of the blocks, a launching apron is provided in the same way as described for the downstream portion, except that the proper value of x should be chosen.
Toe walls are always generally constructed in between the 'filter' and the 'apron' as shown in Fig. Structural Design for the Weir Floor. The concrete floor is usually designed for the uplift pressures as a pure gravity section at each point.
In such cases, the floor may be designed as a reinforced concrete raft structure held down by the weight of the raft and piers.
A raft, in such cases, may be cheaper and more desirable as the thin section ofraft reduces deep excavations and ciewatering problems. Recently, Farraka Barrage, Narora Barrage, and Durgapur Barrage have been designed and constructed as reinforced raft structures. However, the treatment of this type of design is a simple structural problem.
Before we start with the actual design of a weir, let us first review the effects that are produced by the weir. This progressive lowering of the downstream levels is known as Retrogression ef downstream levels or retrogression. As soon as a weir is constructed, the water starts ponding on its upstream side, causing the water surface slope to flatten for some distance behind the weir.
This reduces the silt carrying capacity of the river in this reach, and consequently the silt deposition starts, i. S This causes the progressive lowering of the downstream bed levels. A provision must, therefore, be made for retrogression of dis bed, while designing a weir, as it shall lower the dis TEL and increase HL. Hence, if retrogression is not taken into account, it may lead to undermining of floor.
Figure showing the effects of weir construction on flood levds. The recovery of downstream bed levels, sometimes continues even beyond the original bed levels. This may lead to reduced control on silt regulation. Hence, sufficient margin must be provided between the canal full suppl " level and the pond level, so as to allow raising of the crest of the canal Hea?
Regulator, if found necessary, in future. Hence, the marginal bunds will have to be extended upstream as soon as the above effects come into picture. Since it happens after many years, it is economical to construct marginal afflux bunds only for the backwater. Factors'qoverning the Design of a Weir or a Barrage.
All these informations can be obtained from Topographical maps of the area and. Factors to be Decided. Besides Retrogression, certain other factors which have to be decided while designing a wei:r or a barrage are : i Crest Levels. They are described below : i Crest Levels.
It has been stated earlier that the weir consists of two parts : a The main weir section, called Weir Bay Section: b Undersluice Section. The UJJ. The undersluice crest is generally kept as near the bed level. It was defined earlier that the rise in the maximum flood level of the river upstream of the weir after construction is known as afflux.
It will govern the dynamic action downstream of the work as well as the depth and location of the hydraulic jump. By providing a higher afflux, tliiwaterway and, therefore, the length of the weir can be reduced, but it will increase the cost of training works and the risk of failure by outflanking.
However, in steep reaches with rocky bed, a higher value of afflux may be permitted. The waterway and afflux are correlated. If afflux is increased, waterway is reduced and vice-versa. It shall be seen that the cost pf works as a whole is minimum for a certain. Attempts should, therefore, be made to attain the most economical combination of these two factors. The pond level is the minimum water level required! The pond level is generally obtained by adding 1. This can be accomplished either by.
In modern design of a barrage, the entire ponding is done by gates which are opened during floods and the crest level of the undersluices is generally taken as the available river bed level in the deepest channel. No raised crest is thus generally provided for the undersluices. A raised crest is provided where possible,"as it improves the coefficient of discharge. H H If the head over the weir crest is more than 1.
Concentration factor. This factor is chosen arbitrarily. No concentration of flow is taken while designing protec. Stage Discharge curve for the river at the barrage site is given in Figr: Prepare a complete hydraulic design for the undersluice section as well as for other barrage bqy section, on the basis of Hydraulic jump theory and Khosla 's theory.
A safe exit gradient of may be assumed. Assume any o. The average bed level of the river iS given to be The crest level of other barrage bays may be kept 1. Let us keep it 1. Now let us provide a waterway approximately equal to 1. Now, let us check whether the maximum flood can pass thrqugh this waterway with the maximum permissible afflux of 1. Let us keep the width of the crest of other barrage bays portion as 2.
Hence, the assumed waterway and crest levels are in order. New head ii c velocity head required for this discharge intensity to pass. The conditions are shown in Fig. The dis HFL is depressed by 0. The dis water level when a discharge of 2, cumecsis passing can be found from the Stage-Discharge curve of the river, given in Fig.
It is found to be The new dis HFL shall be The values of q; HL, the water levels and energy levels for all these four cases are tabulated in Table Item without with without with No. Discharge intensity q Upstream water level Downstream water level Head lossHL I.
Ef2 from Plate No. Level at which jump will Y2 corresponding to Ep. Length of concrete floor Froude No. It can be seen from this Table The lowest level at which jump will form, from! The glacis is provided in 3 : 1 slope with a horizontal length of Let us n'ow calculate the total floor length required.
Before this is calculated, we shall have to decide the depth of dis cut off provided from scour considerations.
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