Headrace Canal
Headrace is a canal or a pipe that conveys water from the intake to the forebay.
The high velocity in the canal reduces the cross-section area but increases friction loss,
which in turn decreases output. The maximum or minimum permissible velocity
(non-scouring or non-silting velocity) is significant mainly for unlined canals in earth.
Canals in permeable soil or sedimentary rocks should generally be lined.
Unlined canals in weathered or jointed rock may cause slope slides or leakage, reducing efficiency.
Therefore, power canals are often lined with various materials, and maintaining a reasonably
high velocity helps prevent silting and ice formation.
The canal is designed for the maximum discharge capacity of the power plant but often operates
at lower discharges during dry seasons, which can result in silting — especially during
power plant shutdowns. The density and concentration of suspended particles also affect silt deposition.
The threshold velocity of fine sand or silt is usually much greater than the silting velocity,
and once deposits form, the flow may not be strong enough to remove them again.
Therefore, the cross-section designed for the maximum discharge should be verified, and a
reasonably high velocity is preferred to prevent silting. A narrow canal is better than a
wide one because shallower canals have a smaller hydraulic radius, leading to silt deposition.
Canal lining serves multiple purposes — to prevent leakage, to raise mean velocity (which helps
protect against silting and plant growth), to prevent slope failure due to leakage, and in some
cases, to reduce friction loss and maintenance costs.
Recommended Types of Canal Lining for Micro Hydropower (MHP)
- Compaction of bed and both sides of earth canals to a thickness of 0.2–0.3 m, depending on conditions.
- Lining with gravel (about 0.15 m thick), ideal for permeable soils. Gravel may be laid with clay mortar or caulked with clay.
- Blanketing with a lime-clay mixture (15–20 cm thick), with a mix ratio of 1:4 to 1:7 by weight. Requires 14-day curing with periodic spraying and re-compaction. Not suitable for very cold regions.
- Blanketing with a cement-soil mixture (15–20 cm thick), with a mix ratio of 1:7 to 1:10 by weight.
- Lining with cement mortar masonry (about 20 cm thick), with proper drainage to prevent uplift or frost heaving.
- Concrete lining, precast or in situ (12–15 cm thick), properly drained.
- Geo-membranes, properly drained and protected from sunlight and mechanical damage.
Types of Canals
- Earth Canals: Constructed by simply excavating the ground to the required shape.
- Stone Masonry in Mud Mortar Canals: Offer less seepage than earth canals and can withstand higher flow velocities.
- Stone Masonry in Cement Mortar Canals: More expensive but minimize seepage; suitable for porous soils.
- Concrete Canals: The most expensive type with virtually no seepage. In some cases, HDPE pipes are more economical alternatives.
