The weir is a solid obstruction put across the river to raise its water level and divert the water into the canal. If a weir also stores water for tiding over small periods of short supplies, it is called a storage weir. Barrage
The function of a barrage is similar to that of weir, but the heading up of water is effected by the gates alone. No solid obstruction is put across the river. The crest level in the barrage is kept at a low level. During the floods, the gates are raised to clear off the high flood level, enabling the high flood to pass downstream with maximum afflux. . When the flood recedes, the gates are lowered and the flow is obstructed, thus raising the water level to the upstream of the barrage. Due to this, there is less silting and better control over the levels. Q2: Write down the important components of barrage. Ans: (1) Divide wall
(2) inverted filter
(3) under sluices
(4) fish leader
(5) Flexible apron
(6) Sheet piles Q.3: Write down the purpose of barrage. Ans: (1) To allow proper silt control
(2) To provide better regulation than a weir
(3) To divert the required quantity of water from the river to the canals. Q.4: Write down the barrage site consideration. Ans: (1) Easy diversion of the river after construction.
(2) The site must have a good command over the area to be irrigated and must also be not too for distance to avoid long feeder channels, Q.5: Write down any two function of divide wall. Ans: Functions of the divide wall
1) The floor level of the under sluice or the pocket portion is generally kept lower than the floor level of the main weir. Hence, a divide wall is essential to separate The two “floors” This prevents the turbulent action.
2) If divide wall is not provided, currents approach the scouring sluices from all directions and their effectiveness is reduced. The divide wail helps in concentrating scouring action of the under sluices for washing out the silt deposited in the pocket by ensuring a straight approach through the pocket. Q.6: Define the under sluices. Ans:The under sluices or the scouring sluices maintain a deep channel in front of the head regulator and dispose off heavy silt and a part of food discharge on the d/s side of the barrage. Q.7: Write down the types of weir. Ans: Weirs are classified into two heads, depending upon the criterion of the design of their floors:
1. Gravity weirs
2. Non-gravity weirs Q.8: Write down the causes of failure of weir. Ans: Piping. Water seeps under the base of the weirs founded on permeable soils. When the flow lines emerge out at the d/s end of the impervious floor of the weir, the hydraulic gradient or the exit gradient may exceed a certain critical value for the soil. In that case, the surface soil starts boiling and is washed away by percolating water. With the removal of the surface soil, there is further concentration of flow lines into the resulting depression and still more soil is removed. This process of erosion thus progressively works backwards towards the upstream and results in the formation of a channel or a pipe underneath the floor of the weir, causing its failure. Q.9: Define the surface and subsurface flow of weir. Ans: We have already seen that the sub-surface flow or the foundation seepage may cause harm in two ways (i) piping and (ii) uplift. Following the damage to Khanki weir in 1895, a group of Anglo Indian Engineers under the leadership of Colonel J. Clibborn, Dean of the Roorkee College and J.S. Beresfored, inspector General of Irrigation, India, carried out experiments and confirmed the Dary’s law for seepage through granular soils. Later, in 1912. Bligh advanced a theory for the sub-surface flow and published ‘tin his book. The practical design of Irrigation Works’. In 1932, after analysing about 200 dams all over the world, Lane evolved his weighted creep theory. Scientific study of the sub-surface flow was, however, made by Pavlovsky (1922), and Khosla and his associates (1936). Q.10: Write down the difference between gravity and non gravity weir. Ans: A gravity weir is the one in which the uplift pressure due to the seepage of water below the floor is resisted entirely by the weight of floor. In the non gravity type, the floor thickness is kept relatively less, and the uplift pressure is largely resisted by the bending action of the reinforced concrete floor.
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