Design Of Sarda Type Fall

[Kylee Evancho]

Thedocument describes the design of a Sarda type fall. Key elements include a trapezoidal crest wall, cistern, impervious floor designed using Bligh's theory, downstream wings and protection works. An example problem is included where a Sarda fall is designed for a given canal reach with a discharge of 60 cumecs and drop of 1.5m. The crest wall, cistern, impervious floor length of 15m, downstream wings and protections works are designed and friction blocks are used as energy dissipators.Read less

This type of falls are constructed on Sarda canal in Uttar Pradesh. It is a fall with raised crest and with vertical impact. The soils in Sarda command comprised sandy stratum overlain by sandy-clay on which depth of cutting was to be kept minimum. This made it obligatory to provide number of falls with small drops. In Sarda type falls (q) discharge intensity varied from 1.6 to 3.5 cumec/m and drop varied from 0.6 to 2.5 m.

The height of crest above the upstream bed level is fixed in such a way that the depth of flow u/s of the fall is not affected. From the discharge formula mentioned above since Q is known value of H can be calculated.

The stability of body wall should be tested by usual procedure when the drops exceeding 1.5 m are to be designed. In the body wall drain holes may be provided at the u/s bed level to dry out the canal during closures for maintenance, etc.

The d/s floor should be made thick enough to resist uplift pressures. However, minimum thickness of 0.3 to 0.6 m (depending upon the size of the drop) of concrete under 35 cm of brick masonry may be provided on the d/s. On the u/s brick masonry is not necessary. The brick on the edge laid on the d/s impervious concrete floor provide additional strength and affords easy repairs to the floor.

Provision of other accessories like upstream wings, staggered blocks on the cistern floor, downstream wings, bed and side pitching is generally done on the basis of thumb rules. For big structures, however, actual design calculations may be done. For general arrangement see Fig. 19.13.

For small falls upto 14 cumec the upstream wings may be splayed at 1: 1. For higher discharges u/s wing walls are kept segmental with a radius equal to 6 H and continued thereafter tangentially merging into the banks. The wings may be embedded into the bank for about 1 m.

For the length of the cistern the d/s wing walls are kept vertical from the crest. Thereafter they are wasped or flared to a slope of 1: 1. An average splay of 1 in 3 for attaining the required slope is given to the top of the wings. The wings may be taken deep into the banks.

Staggered block of height dc should be provided at a distance of 1.0 dc to 1.5 dc from the d/s toe of the crest for clear falls. In case of submerged falls the blocks may be provided at the end of the cistern. A row of staggered cubical blocks of height equal to 0.1 to 0.13 of depth of water should invariably be provided at the end of the d/s impervious floor.

The d/s bed pitching with bricks 20 cm thick over 10 cm ballast is provided horizontally for a length of 6 m. Thereafter for lengths up to 5 to 15 m for falls varying from 0.75 to 1.5 m may be provided with down slope of 1 in 10. The side pitching with bricks on edge with 1: 1 slope is provided after the return-wing on the downstream. A toe wall should be provided between the bed pitching and the side pitching to provide a firm support to the latter.

Vertical falls should be full width falls, i.e., the width of the crest should be same as bed width of the canal because increased intensity of discharge due to fluming creates scour on the downstream.

Unlike vertical falls the glacis falls can be flumed when combined with bridge so as to economize in the cost. It k quite rational to select such (q) discharge per metre run of crest width which with the height of drop (HL) available gives value of total energy on the d/s (Ef2) equal to F.S. depth of the canal. (It can be read from Blench curves). It does not require deep cistern on d/s and avoids construction difficulty particularly when subsoil water level is high. The throat width may be rounded off to next half metre. The fluming thus calculated may not, however, exceed limits given below subject to the condition that overall width of fall crest is not more than bed width of the canal on the downstream.

(i) If the fall combines with it functions of a discharge meter as well, the side and bed approaches to the crest have necessarily to be gradual and smooth so as to avoid eddies and impact losses and to reduce concentration of flow.

(iii) Side walls in expansion may be flared out from vertical to 1: 1 if the earth fill behind is not problematic like black cotton soil. In such cases the side walls may be designed as vertical gravity walls.

(iv) Side protection consisting of 20 cm thick dry brick pitching for a length of 3 D2 should be provided. It should rest on a toe wall 1 brick thick and of depth equal to D2/2 subject to minimum of 0.5 m depth.

Friction blocks are found to be most effective. In case of flumed straight glacis falls (without baffle) four rows of friction blocks may be provided. they are staggered in plan. The u/s edge of the first row of the friction block is located at a distance of 5 times the height of the blocks (5 . h) from the toe of the glacis. The dimensions of the blocks may be as follows:

When glacis is provided with baffle only two rows of friction blocks is sufficient upto 2 m fall. The u/s edge of the first row may be located at 1/3 length of d/s expansion from the end of the cristern floor.

Sarda type fall is the combination of small-sized falls resulting in gradual energy dissipation of the water without hydraulic jump formation. Such combination is adopted for weaker strata as the depth of cutting is minimized. It is also economical to build to sarda type fall.

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Canal fall is a solid masonry structure which is constructed on the canal if the natural ground slope is steeper than the designed channel bed slope. If the difference in slope is smaller, a single fall can be constructed. If it is of higher then falls are constructed at regular suitable intervals.

The above two will decide the location of canal fall across canal. By understanding topographic condition we can provide the required type of fall which will give good results. At the same time, the provided falls is economical and more useful. So, economical calculation is also important. Unbalanced earth work on upstream and downstream result the project more uneconomical.

Ogee curve is the combination of convex and concave curves. So, Ogee fall consists of both convex and concave curves gradually. This gradual combination helps to provide smooth transition of flow and also reduce the impact. If the canal natural ground surface is suddenly changed to steeper slope, ogee fall is recommended for that canal. Stone pitching is provided in the upstream and downstream of the fall.

Rapid fall consists a long sloping glacis. It is constructed if the available natural ground surface is plane and long. For this, a bed of rubble masonry is provided and it is finished with cement mortar of 1:3 ratio. To maintain the slope of bed curtain walls are provided at both upstream and downstream. Rapid falls are high priced constructions.

As in the name itself, stepped fall consist vertical steps at gradual intervals. Stepped fall is the modification of rapid fall. It is suitable for the canal which has it upstream at very high level as compared to downstream. These two levels are connected by providing vertical steps or drops as shown in figure.

In case of trapezoidal notch falls, a high crested wall is built across the channel and trapezoidal notches are provided in that wall. Trapezoidal falls are very economical and suitable for low discharges. Now a days this type of falls are using widely because of their simplicity and popularity.

Well type falls are also called as syphon drop falls. In this case, an inlet well with pipe at its bottom is constructed in upstream. The pipe carries the water to downstream well or reservoir. If the discharge capacity is more than 0.29 cumecs then downstream well is preferred otherwise reservoir is suitable.

Simple vertical drop fall or sarda fall consists, single vertical drop which allows the upstream water to fall with sudden impact on downstream. The downstream acts like cushion for the upstream water and dissipate extra energy. This type of fall is tried in Sarda Canal UP (India) and therefore, it is also called Sarda Fall.

This is the modern type of construction, in which a raised crest is constructed across the canal and a gentle straight inclined surface is provided from raised crest to the downstream. The water coming from upstream crosses the raised crest and falls on inclined surface with sufficient energy dissipation.

Montage fall is similar to straight glacis fall but in this case the glacis is not straight. It is provided in parabolic shape to introduce the vertical component of velocity which improves the energy dissipation to more extent.

In this case, straight glacis fall is extended as baffle platform with baffle wall. This is suitable for any discharge. The baffle wall is constructed near the toe of the straight glacis at required distance in designed height. The main purpose of the baffle wall is to create hydraulic jump from straight glacis to baffle platform.

Water management is a critical aspect of modern infrastructure, especially within the construction industry where canal systems play a pivotal role. One key element in optimizing water flow within canals is the strategic implementation of canal falls. These solid masonry structures are designed to address the challenges posed by natural ground slopes and ensure a controlled descent of water. In this comprehensive exploration, we look into an extensive array of canal fall designs, examining their historical significance, contemporary applications, and the intricate considerations that engineers must navigate for effective water management.

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