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Educational Basic Civil Engineering

Discussion in 'Education' started by Tazul Islam, Jun 15, 2016. Replies: 62 | Views: 4038

  1. Tazul Islam
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    Cross Drainage Works

    Definition:
    A cross drainage work is a structure carrying the discharge from a natural stream across a canal intercepting the stream.

    Canal comes across obstructions like rivers, natural drains and other canals.

    The various types of structures that are built to carry the canal water across the above mentioned obstructions or vice versa are called cross drainage works. There are many different factors involved in selection of a specific type of Cross drainage works and in selection of a suitable site for cross drainage works.

    It is generally a very costly item and should be avoided by

    • Diverting one stream into another.
    • Changing the alignment of the canal so that it crosses below the junction of two streams.
    Types of cross drainage works
    Depending upon levels and discharge, it may be of the following types:

    Cross drainage works carrying canal across the drainage:
    the structures that fall under this type are:
    [​IMG]

    1. An Aqueduct
    2. Siphon Aqueduct
    Aqueduct:
    When the HFL of the drain is sufficiently below the bottom of the canal such that the drainage water flows freely under gravity, the structure is known as Aqueduct.

    • In this, canal water is carried across the drainage in a trough supported on piers.
    • Bridge carrying water
    • Provided when sufficient level difference is available between the canal and natural and canal bed is sufficiently higher than HFL.
    [​IMG]
     
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    Last edited: Jul 24, 2016
  2. Tazul Islam
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    Tazul Islam Kazirhut Lover Member

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    Siphon Aqueduct:
    In case of the siphon Aqueduct, the HFL of the drain is much higher above the canal bed, and water runs under siphonic action through the Aqueduct barrels.

    The drain bed is generally depressed and provided with pucci floors, on the upstream side, the drainage bed may be joined to the pucca floor either by a vertical drop or by glacis of 3:1. The downstrean rising slope should not be steeper than 5:1. When the canal is passed over the drain, the canal remains open for inspection throughout and the damage caused by flood is rare. However during heavy floods, the foundations are succeptible to scour or the waterway of drain may get choked due to debris, tress etc.

    [​IMG]

    Cross drainage works carrying drainage over canal.
    The structures that fall under this type are:

    • Super passage
    • Canal siphon or called syphon only
    Super passage:
    • The hydraulic structure in which the drainage is passing over the irrigation canal is known as
      [​IMG]
      super passage. This structure is suitable when the bed level of drainage is above the flood surface level of the canal. The water of the canal passes clearly below the drainage
    • A super passage is similar to an aqueduct, except in this case the drain is over the canal.
    • The FSL of the canal is lower than the underside of the trough carrying drainage water. Thus, the canal water runs under the gravity.
    • Reverse of an aqueduct
    Canal Syphon:
    • If two canals cross each other and one of the canals is siphoned under the other, then the hydraulic structure at crossing is called “canal siphon”. For example, lower Jhelum canal is siphoned under the Rasul-Qadirabad (Punjab, Pakistan) link canal and the crossing structure is called “L.J.C siphon”
    • In case of
      [​IMG]
      siphon the FSL of the canal is much above the bed level of the drainage trough, so that the canal runs under the siphonic action.
    • The canal bed is lowered and a ramp is provided at the exit so that the trouble of silting is minimized.
    • Reverse of an aqueduct siphon
    • In the above two types, the inspection road cannot be provided along the canal and a separate bridge is required for roadway. For economy, the canal may be flumed but the drainage trough is never flumed.
     
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  3. Tazul Islam
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    Selection of suitable site for cross drainage works
    • The factors which affect the selection of suitable type of cross drainage works are:
    • Relative bed levels and water levels of canal and drainage
    • Size of the canal and drainage.
    • The following considerations are important
    • When the bed level of the canal is much above the HFL of the drainage, an aqueduct is the obvious choice.
    • When the bed level of the drain is well above FSL of canal, super passage is provided.
    • The necessary headway between the canal bed level and the drainage HFL can be increased by shifting the crossing to the downstream of drainage. If, however, it is not possible to change the canal alignment, a siphon aqueduct may be provided.
    • When canal bed level is much lower, but the FSL of canal is higher than the bed level of drainage, a canal siphon is preferred.
    • When the drainage and canal cross each other practically at same level, a level crossing may be preferred. This type of work is avoided as far as possible.


    Factors which influence the choice / Selection of Cross Drainage Works
    1. The considerations which govern the choice between aqueduct and siphon aqueduct are:
    2. Suitable canal alignment
    3. Suitable soil available for bank connections
    4. Nature of available foundations
    5. Permissible head loss in canal
    6. Availibility of funds
    Compared to an aqueduct a super passage is inferior and should be avoided whenever possible. Siphon aqueduct is preferred over siphon unless large drop in drainage bed is required.

    Classification of aqueduct and siphon aqueduct
    Depending upon the nature of the sides of the aqueduct or siphon aqueduct it may be classified under three headings:

    Type I:
    Sides of the aqueduct in earthen banks with complete earthen slopes. The length of culvert should be sufficient to accomodate both, water section of canal, as well as earthen banks of canal with aqueduct slope.

    Sides of the aqueduct in earthen banks, with other slopes supported by masonry wall. In this case, canal continues in its earthen section over the drainage but the outer slopes of the canal banks are replaced by retaining wall, reducing the length of drainage culvert.

    Type II:
    Sides of the aqueduct made of concrete or masonry. Its earthen section of the canal is discontinued and canal water is carried in masonry or concrete trough, canal is generally flumed in this section.
     
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  4. Tazul Islam
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    Effects of Chute Slope on Stilling Basin

    A factor which occasionally affects stilling basin operation is the slope of the chute upstream from the basin. The foregoing experimentation was sufficiently extensive to shed some light on this factor. The tests showed that the slope of chute upstream from the stilling basin was unimportant, as far as jump performance was concerned, provided the velocity distribution in the jet entering the jump was reasonably uniform.

    For steep chutes or short flat chutes, the velocity distribution can be considered normal. Difficulty is experienced, however, with long flat chutes where frictional resistance on the bottom and side walls is sufficient to produce a center velocity greatly exceeding that on the bottom or sides.

    When this occurs, greater activity results in the center of the stilling basin than at the sides, producing an asymmetrical jump with strong side eddies. This same effect is also witnessed when the angle of divergence of a chute is too great for the water to follow properly.

    In either case the surface of the jump is unusually rough and choppy and the position of the front of the jump is not always predictable. When long chutes precede a stilling basin the practice has been to make the upstream portion unusually flat, then increase the slope to 2:1, or that corresponding to the natural trajectory of the jet, immediately preceding the stilling basin.

    The most adverse condition has been observed where long canal chutes terminate in stilling basins. A definite improvement can be accomplished in future designs where long flat chutes are involved by utilizing the Type III basin. The baffle piers on the floor tend to alter the asymmetrical jet, resulting in an overall improvement in operation.
     
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  5. Tazul Islam
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    [​IMG]
    Typical Sloping Chute with baffle blocks (Pakistan)

    [​IMG]

    Sloping Chute
     
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  6. Tazul Islam
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    Recommendations
    The following rules have been devised for the design of the sloping aprons developed from the foregoing discussion:

    1. Determine an apron arrangement which will give the greatest economy for the maximum discharge condition. This is a governing factor and the only justification for using a sloping apron.
    2. Position the apron so that the front of the jump will form at the upstream end of the slope for the maximum. Several trials will usually be required before the slope and location of the apron are compatible with the hydraulic requirement. It may be necessary to raise or lower the apron, or change the original slope entirely.
      {loadposition articlemid}
    3. The length of the jump for maximum or partial flows can be obtained from Hydraulic charts based on experiments. The stilling basin apron is a decision for the designer. The average overall apron averages 60 percent of the length of jump for the maximum discharge condition. The apron may be lengthened or shortened, depending upon the quality of the rock in the riverbed and other local conditions. If the apron is set on loose material and the downstream channel is in poor condition, it may be advisable to make the total length of apron the same as the length of jump.
    4. With the apron designed properly for the maximum discharge condition, it should then be determined that the tail water depth and length of basin available for energy dissipation are sufficient. If the tail water depth is sufficient or in excess of the jump height for the intermediate discharges, the design is acceptable. If the tail water depth is deficient, it may then be necessary to try a different slope or reposition the sloping portion of the apron. It is not necessary that the front of the jump form at the upstream end of the sloping apron for partial flows.
    5. Horizontal and sloping aprons will perform equally well for high values of the Froude number if the proper tail water depth is provided.
    6. The slope of the chute upstream from a stilling basin has little effect on the hydraulic jump when the velocity distribution and depth of flow are reasonably uniform on entering the jump.
    7. A small solid triangular sill, placed at the end of the apron, is the only appurtenance needed in conjunction with the sloping apron. It serves to lift the flow as it leaves the apron and thus acts to control scour. Its dimensions are not critical; the most effective height is between O.O5D2 (D2= height after the jump) and O.10D2 and a slope of 3:1 to 2:1.
    8. The spillway should be designed to operate with as nearly symmetrical flow in the stilling basin as possible. (This applies to all stilling basins.) Asymmetry produces large horizontal eddies that can carry riverbed material on to the apron. This material, circulated by the eddies can abrade the apron and appurtenances in the basin at a very surprising rate. Eddies can also undermine wing walls and riprap. Asymmetrical operation is expensive operation, and operating personnel should be continually reminded of this fact.
    9. Where the discharge over high spillways exceeds 500 c.f.s. per foot of apron width, where there is any form of asymmetry involved and for the higher values of the Froude number where stilling basins become increasingly costly and the performance relatively less acceptable, a model study is advisable.
     
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  7. captcha
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    captcha Welknown Member Member

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    good share.
     
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  8. Tazul Islam
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    ধন্যবাদ মামা ।
     
  9. Tazul Islam
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    Requirements Of Small Hydro Power Project

    1.0 General Requirements Of Small hydro Power projects
    1. Durability
    All civil works must be durable and maintainable to 15 years, except in cases of specific components where rebuild is explicitly included and costed in the management plan.

    2. Optimization
    Civil works must be designed for adequate and safe performance at minimum expense.

    3. Flow closure
    Reliable methods of diverting flow from the silt basin, canal, forebay, and from the penstock, so that these components can be quickly emptied whenever required, must be included in the design, and must be tamper-proof. There must be at least two flow diversion devices if there is a canal included in the design.

    4. Maintenance and materials
    The use of high quality materials and construction techniques will result in less maintenance and repair work through the life of the scheme, whereas low-cost construction will require considerable maintenance. Both approaches are acceptable, though the first approach is recommended for most schemes of larger capacity. The management plan must make provision for high maintenance activity and cost in cases of low-cost civil works.

    5
    In all schemes concrete is recommended for the turbine base and for penstock anchor blocks; it can be used in medium-cost headworks, canals, and forebays to provide strength and erosion resistance in vulnerable areas such as the floor of the canal and forebay/silt basins, and the intake mouth.

    When visiting verify the above and in addition:

    6.
    Check for leaks from all civil constructions. These can quickly give rise to expensive damage and must be fully repaired before the scheme is put to use. Also repair cracks or any faulty work which could give rise to problems at a later date.

    7.
    A reliable and rapid shut-down or emptying method must be demonstrated for the silt basin, canal, forebay, and penstock. Close all gates, then progressively open each one to check that overflows work safely.

    [​IMG]
     
  10. Tazul Islam
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    2.0 Weir and intake Requirements Of micro hydro Power systems
    1. Natural weirs
    Particular attention must be paid to the choice of site for the intake. The river bed must not be in danger of deepening (due to scouring action) or in danger of changing course, such that an intake built to function with a natural weir would cease to function.

    2.
    During low river flow conditions (such as the 5-year drought), the intake must continue to draw water in accordance with consumer expectations for power supply. In schemes where seasonal drying out of the intake is a possibility, the management of the scheme must be planned to allow for this, and contracts with consumers and tariff agreements must allow for periods of power shut-down.

    3.
    The intake must be of a passive design; that is, it must function in 5- year flood conditions without any need for operators to make adjustmentsor work close to flooded area.

    4. If the weir and intake are a temporary structure, designed to be swept away high flow conditions, the seasonal replacement of this structure should be costed into the management plan, and provision made in consumer agreements for power shut down during the rebuild period.

    5.
    Intakes must be designed to operate reliably without clogging with sediment and debris. If a very low-cost design is adopted which is prone to clogging, provision must be made in the management plan to replace and upgrade as necessary. (An example is the use of a submerged pipe as an intake, which occasionally can work in some conditions, but will more often present problems and need to be replaced with a more reliable design).

    When visiting verify the above and in addition:

    6.
    Check that the intake is not in danger of clogging. If it is, upgrade the design.

    7.
    If a natural weir used, check for signs of bed scouring and changes of course. If gabions are used to define the river course, check that a natural bank is developing around them to provide stability as the gabion mesh disintegrates over time.

    3.0 Schemes without open canals
    1. If the penstock starts at the intake, the headworks should follow spillway and forebay tank design principles with respect to deflection of flood waters and prevention of stones/debris entering penstock and air entrainment. Desilting may be necessary as detailed. The penstock entry section must be secure against flood water and boulders/trees etc carried by flood water. The requirement for a reliable method of penstock closure and emptying, which can be activated during flood conditions, is particularly important.

    2. Air-traps
    If a closed canal or headrace is used, such as a pipe, it is required that:

    1. Either the pipe is laid without high points or low points
    2. or if there are high points (or potential high points resulting from settling), then these must be protected from air traps either by vent pipes rising to above static head level or by air release valves
    3. If there are low points these should be fitted with flush valves to allow removal of debris blocking the water flow
    4. Water will start to flow through by itself when it is diverted toward the system, without any need for priming.
     

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