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Canal Headwork - Types & Locations Advertisements Definition: Any hydraulic structure which supplies water to the off taking canal. Diversion head-work provides an obstruction across a river, so that the water level is raised and water is diverted to the channel at required level. The increasewater level helps the flow of water by gravity and results in increasing the commanded area and reducing the waterfluctuations in the river. Diversion head-work may serve as silt regulator into the channel. Due to the obstruction, the velocity of the river decreases and silt settles at the bed. Clear water with permissible percentage of silt is allowed to flow through the regulator into the channel. To prevent the direct transfer of flood water into the channel. Functions of a Headwork A headwork serves the following purposes A headwork raises the water level in the river It regulates the intake of water into the canal It also controls the entry of silt into the canal A headwork can also store water for small periods of time. Reduces fluctuations in the level of supply in river Types of Headworks 1. Storage headwork 2. Diversion headwork Component parts of Diversion Headwork Types of Diversion head works 1. Temporary: 2. Spurs Bunds 3. Permanent Components 1. Weir or Barrage 2. Divide Wall 3. Fish Ladder 4. Approach Canal 5. Silt prevention device 6. Canal head regulator 7. River training works Location of Headworks 1. Rocky Stage 2. Sub mountainous or boulder stage: boulder or gravel 3. Alluvial plan Rocky stage: River steep slope, high velocity Advantages: 1. Good foundation at shallow depth 2. Comparatively silt free water for turbines 3. High head for hydro-electric work Disadvantages: 1. Long ---- length of canal. In reach soil is good for agriculture. 2. More cross damage works 3. More falls (ground steep gradient - lined to permit high velocity) 4. Costly head regulator excluding shingle 5. Frequent repairs of the weirs. Sub mountainous or boulder stage: boulder or gravel Advantages: 1. Less training works 2. Suitable soil for irrigation available 3. Availability of construction material locally. 4. Falls can be utilized for power generation Disadvantages: 1. It has a strong sub-soil flow as a result 2. Reduce in storage and damage floor downstream 3. More percolation loss from canal 4. More x-drainage works 5. Less demand of water at head reaches (more idle length of canal) Alluvial plan: 1. x- section of river alluvial sand silt 2. Bed slope small, velocity gentle 3. No idle length of canal 4. less x- drainage works 5. Comparatively less sub soil flow Disadvantages: 1. Cost of headwork is more due to poor foundation 2. More river training works 3. Problem of silt in canal Causes of failure of Weirs & their Remedies Types & Components of Weirs Advertisements Common causes of failure of weirs include: Excessive and progressive downstream erosion, both from within the stream and through lateral erosion of the banks Erosion of inadequately protected abutments Hydraulic removal of fines and other support material from downstream protection (gabions and aprons) resulting in erosion of the apron protection Deterioration of the cutoff and subsequent loss of containment Additional aspects specific to concrete, rockfill or steel structures. The main causes are: 1. Piping Piping is caused by groundwater seeping out of the bank face. Grains are detached and entrained by the seepage flow and may be transported away from the bank face by surface runoff generated by the seepage, if there is sufficient volume of flow. The exit gradient of water seeping under the base of the weir at the downstream end may exceed a certain critical value of soil. As a result the surface soil starts boiling and is washed away by percolating water. The progressive erosion backwash at the upstream results in the formation of channel (pipe) underneath the floor of weir. Since there is always a differential head between upstream & downstream , water is constantly moving form upstream to downstream from under the base of weir. However, if the hydraulic gradient becomes big, greater than the critical value, then at the point of existance of water at the downstream end, it begins to dislodge the soil particles and carry them away. In due course , when this erosion continues, a sort of pipe or channel is formed within the floor through which more particles are transported downstream which can bring about failure of weir. Piping is especially likely in high banks backed by the valley side, a terrace, or some other high ground. In these locations the high head of water can cause large seepage pressures to occur. Evidence includes: Pronounced seep lines, especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bankassociated with seepage zones and layers; run-out deposits of eroded material on the lower bank. Remedies: Decrease Hydraulic gradient i.e. increase path of percolation by providing sufficient length of impervious floor Providing curtains or piles at both upstream and downstrea 2. Rupture of floor due to uplift: If the weight of the floor is insufficient to resist the uplift pressure, the floor may burst. This bursting of the floor reduces the effective length of the impervious floor, which will resulting increasing exit gradient, and can cause failure of the weir. Remedies: Providing impervious floor of sufficient length of appropriate thickness. Pile at upstream to reduce uplift pressure downstream 3. Rupture of floor due to suction caused by standing waves Hydraulic jump formed at the downstream of water Remedies: Additional thickness Floor thickness in one concrete mass 4. Scour on the upstream and downstream of the weir Scouring in Weirs Occurs due to contraction of natural water way. Remedies: Piles at greater depth than scour level Launching aprons: Stones of aprons may settle in the scour hole. Khosla &bligh After studying a lot of dam failure constructed based on Bligh’s theory, Khosla came out with the following; Following are some of the main points from Khosla's Theory From observation of Siphons designed on Bligh's theory, by actual measurement of pressure, with the help of pipes inserted in the floor of two of the siphons Does not show any relationship with pressure calculated on Bligh's theory. This led to the following provisional conclusions: Outer faces of end sheet piles were much more effective than the inner ones and the horizontal length of the floor. Intermediated piles of smaller length were ineffective except for local redistribution of pressure. Undermining of floor started from tail end. Related Pages It was absolutely essential to have a reasonably deep vertical cut off at the downstream end to prevent undermining. Khosla and his associates took into account the flow pattern below the impermeable base of hydraulic structure to calculate uplift pressure and Types of Weirs Weir loading rate Bligh's Creep Theory
