Types of Diversion Headworks
Any hydraulic structure which supplies water to the off-taking canal is called a headwork.
Types
- Storage Headwork: A storage headwork comprises the construction of a dam on the river. It stores water during the period of excess supplies and releases it when demand overtakes available supplies.
- Diversion Headwork: To divert required supply to canal from the river
- Temporary spurs or bunds- which are temporary and constructed every year after floods.
- Permanent weirs and barrages.
Diversion Headwork
Diversion headworks are generally constructed on the perennial rivers which have adequate flow throughout the year and, therefore, there is no necessity of creating a storage reservoir.
Purpose
- It raises the water level in the river so that the command area can be increased.
- It regulates the intake of water into the canal.
- It controls the silt entry into the canal.
- It reduces fluctuations in the level of supply in river.
- It stores water over small periods of short supplies.
Location of Headworks
- The torrential, rocky or mountainous stage: Very steep bed slope and high velocity.
- The sub-mountainous or boulder stage: Strong sub-soil flow & more percolation losses.
- Trough stage or alluvial plain: Bed slope is small and velocity is gentle.
- Delta stage.
Selection of Site for Diversion Head Works
- At the site, the river should be straight and narrow.
- The river banks should be well defined.
- The valuable land should not be submerged when the weir or barrage is constructed.
- The elevation of the site should be much higher than the area to be irrigated.
- The site should be easily accessible by roads or railways.
- The materials of construction should be available in vicinity of the site.
- The site should be not be far away from the command area of the project, to avoid transmission loss.
Component parts of a Diversion Headwork
- Weir or Barrage
- Divide wall or Divide Groyne
- Fish ladder
- Pocket or approach channel
- Scouring sluices
- Silt Preventing devices
- Canal Head Regulator
- River Training Works (Marginal Bunds and Guide Banks).
Weir
The weir is a hydraulic structure constructed 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 as storage weir. The main difference between a storage weir and dam is only in height and duration for which supply is stored.
Barrage
- Similar to weir, but the heading up of water is effected by the gates alone.
- No solid structure across river.
- The crest level is kept at a low level.
- Flood flow is controlled by gates.
- Less silting and better control over the water levels.
- Costlier than the weirs.
Divide Wall or Divide Groyne
- The divide wall is a masonry or concrete wall constructed at right angle to the axis of the weir.
- The divide wall extends on the upstream side beyond the beginning of the canal head regulator, and on the downstream side, it extends up to the end of the loose protection of the under-sluices.
Functions
- It separates the ‘under-sluices’ with lower crest level from the ‘weir proper’ with higher crest level.
- It helps in providing a comparatively less turbulent pocket near the canal head regulator, resulting in deposition of silt in this pocket and, thus, to help in the entry of silt-free water into the canal.
Fish Ladder
- It is usually located between the weir and divide wall.
- It is suited near the divide wall as there is always some water in the river section below the scour sluices.
- The baffles of an inclined trough with baffles with helps provided in them.
- The fish ladder should be designed to have a velocity of 3 m/s or less in the trough.
Scouring Sluices
They maintain a deep channel in front of head sluice and dispose off heavy silt and a part of flood discharge on the downstream side of the barrage or weir.
Function
- To preserve a clear and defined river channel approaching the regulator.
- To control the silt entry in to the canal.
- To help in passing low floods without dropping the shutters of main weir.
Silt Preventing Devices
The entry of silt into a canal, which takes off from a head works, can be reduced by constructed certain special works, called silt control works.
Types
- Silt Excluders
- Silt Ejectors
Head Sluice or Canal Head Regulator
A head regulator is structures constructed at the head of a canal off take from a reservoir behind a weir or a dam. It may consist of a number of spans separated by piers and operated by gates.
Functions
- To make the regulation of supply in the canal easy.
- To shut out river floods.
- To control the silt entry into the canal.
River Training Works
River training works are required near the weir or barrage in order to ensure a smooth and an axial flow of water when the length of a weir or barrage is smaller than the width of a river.
Types
- Guide banks
- Marginal bunds
- Spurs or Groynes
Guide Banks
When a barrage is constructed across a river which flows through the alluvial soil, the guide banks must be constructed on both the approaches to protect the structure from erosion.
Purpose
- It protects the barrage from the effect of scouring and erosion.
- It provides a straight approach towards the barrage.
- It controls the velocity of flow near the structure.
Marginal Bunds
The marginal bunds are earthen embankments which are constructed parallel to the river bank on one or both the banks according to the condition. The top width is generally 3m to 4m. The side slope on the river side is generally 1.5:1 and that on the country side is 2:1.
Purpose
- It retains the flood water or storage water within a specified section.
- It protects towns and villages from devastation during the heavy flood.
- It protects valuable agricultural lands.
Spurs or Groynes
These are temporary structures permeable in nature provided on the curve of a river protect the river bank from erosion.
- These are projected from the river bank towards the bed making angles 60-70 degree with the bank of the river.
- The length of the spurs depends on the width of the river and the sharpness of the curve.