Headrace Canal
Headrace is a canal or a pipe that conveys water from
intake to forebay. The high velocity in canal produces small cross-sections,
but also increased friction loss and accordingly decreased output. The
so-called maximum or minimum permissible velocity (or non-scouring or
non-silting velocity is significant only for unlined canals in earth. Canals in
permeable soil and sedimentary rocks should in general, be lined. The surface
rocks is often weathered and/or seriously jointed to a certain depth and
unlined canals may give rise to slope slides and decreased output due to
leakage. Therefore, power canals are often lined with various materials and a
reasonably high velocity is recommended to protect canals from silting or ice
trouble. The canal is designed for the design or maximum discharge of the
plant, but operates under much smaller discharges in the mean or dry season,
thus resulting in silting of canal, especially during the shutdown of power
plant. Also density and concentration of
suspended particles influence the deposition of silt. In particular, the
threshold velocity of fine sand or silt is greater or much greater then the silting
velocity and the re-established velocity after silting cannot pick up the
deposit again.
Therefore cross section designed for maximum discharge
of plant should be verified accordingly, and a reasonably high velocity is
preferred to prevent silting. A comparatively narrow canal is preferable to
wide one, because a shallow depth to the canal has a smaller hydraulic radius,
thus resulting in the deposition of silt.
Canal lining is provided for the functions such as to
prevent leakage from the canal, to properly raise the mean velocity beneficial
to protection against silting and plant growth, to prevent possible slides due
to leakage, in some cases to reduce friction loss and to reduce maintenance
cost.
The following types of lining are recommended for MHP
- Compaction of bed and both sides of canals in earth to a thickness of 0.2-0.3 m, depending on actual conditions.
- Lining with gravel, generally to 0.15 m thick. This method is especially suitable for canals in permeable soils, if gravel is available nearby, and has a good resistance to freezing. IN some cases, the gravel is laid with clay mortar or caulked with clay.
- Blanketing with a lime-clay mixture, carefully compacted, generally to 15-20 cm thick; the mix proportion (hydrated lime: clay or losses) is 1:4 – 1:7 by weight. Curing should be at least 14 days, with several additional spraying and re-compaction operations. This type of lining with a 14-day age can stand a velocity of several meters per second, but his is not suitable for very cold regions.
- Blanketing with a cement-soil mixture, carefully compacted, generally to 15-20 cm thick, with a mix proportion of 1:7 – 1:10 by weight.
- Lining with cement mortar masonry, generally to 20 cm thick, properly drained to avoid uplift or frost heaving.
- Concrete lining, precast or placed in situ, generally to 12-15 cm thick and properly drained.
- Geo-membranes, properly drained and protected from sunlight and damage or wearing.
· Types of Canal :
- Earth Canals: Constructed simply by excavating ground to the required shape.
- Stone Masonry in mud mortar canals: Compared to earth canals there will be less seepage from this type of canal. It can withstand high flow velocity.
- Stone masonry in cement mortar canals : It is least preferable in term of cost but seepage is minimal. Its is used where soil type is porous.
- Concrete canals : This type of canal is most expensive and virtually there will be no seepage. Generally HDPE pipes are more economic than concrete canals.
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