Sarda Fall Design Pdf

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Thistype 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.

The document 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

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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The lowest stage of the Dhauliganga cascade will develop a head of slightly more than 300m and will have an installed capacity of 4x70MW. Water taken from Chirkila daily storage reservoir, about 4km upstream of the Sarda confluence, will pass along a 5.6km pressure tunnel to an underground power station on the western side of the Sarda valley. The project will generate 1285GWh annually, (90 per cent dependability). The construction programme sees the start of main civil works in 1999 and commissioning of the four generating units during late 2003 and early 2004.

Five-stage development of the Dhauliganga was many years ago the subject of master plan studies. In 1980, the National Hydroelectric Power Corporation (NHPC) carried out a feasibility study of the lowest project of the cascade, which was completed in 1985. Since then, NHPC has continued studies and has obtained clearance for project realisation from the Central Electricity Authority and the Public Investment Board. Work has recently started on the tender design.

The catchment of the proposed dam site at Chirkila is 1360km2, and is an extremely rugged area with elevation ranging from 1300m asl to more than 6000m. The average river slope is almost 50m/km but locally much steeper. Valley slopes are often unstable and landslides are common. This, combined with freeze/thaw in the spring and torrential rainfall during the monsoon, contribute to high erosion rates and sediment load.

The principal hydrological studies covered dependable 10-day mean flows, to compute energy production, construction diversion and spillway design floods, and sediment transport, to assess reservoir storage, flushing and desilting requirements. The mean flow of the Dhauliganga at Chirkila is 84m3/s, ranging from about 20m3/s in February to 200m3/s in the July-September monsoon. Suspended sediment is estimated at 1.40 million tons per year, or a mean concentration of 340g/m3, but may be disproportionally higher in wet years.

Review studies for the final design also have to include the potential for glacial lake outburst floods and whether these could exceed natural flood inflows and threaten the dam. Little information is currently available for this analysis but surveys are in progress based on satellite images of glaciers and moraine-dammed lakes in the upper catchment.

As regards geology, the dam site is a deep, asymmetrical gorge in sound but highly foliated pre-cambrian biotite gneiss bedrock, with extensive areas of slump debris upstream and downstream of the dam axis. Alluvial deposits (60m maximum thickness) fill the valley; these are fine-medium sand (10-4-10-5m/s), generally with boulders and gravel, but containing one lens solely of sand.

The 5.6km pressure headrace tunnel will pierce generally massive crystallines. The tunnel at its deepest is about 1000m below ground, but its final third is just 100-200m deep. The surge chamber, twin high pressure shafts and power caverns will be in strong, generally massive but locally-fractured biotite gneiss.

Completed site investigations include core drilling and grouting tests at the site, excavation of drifts into the dam abutments, surge tank area and power cavern area (with rock mechanics testing) and laboratory testing of materials.

The preferred dam site is characterised by the asymmetric valley with rock exposed on both banks above about El 1320m. The right flank is a steep high outcrop of bedrock, but on the left bank the alluvium forms a gently sloping terrace 150m wide. The overall width of the valley at dam crest level is about 270m.

The larger part of the dam foundation area will be the river channel alluvium, the upper and lower heterogeneous strata of which are separated by a distinct lens of medium to fine sand, of 3-20m thickness on the dam axis. This lens extends upstream and downstream and investigations of the longitudinal variation of its thickness are in progress.

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