Drop structures: need and types with sketches

Need for drop structures

The ground slope of the command is generally steeper than the permissible bed slope of the canal. If the canal followed the ground slope, its velocity would exceed the safe limit and it would scour and erode its bed. If the canal bed were kept at the permissible slope, it would run in deep cutting or lose command over the fields. A drop (fall) structure drops the water level abruptly and dissipates the surplus energy safely. It therefore:

1. Reduces the bed slope and the velocity, preventing erosion of the canal.
2. Limits cutting and filling to economical depths.
3. Maintains command (FSL above the field level).
4. Can also be used for power generation or discharge measurement.

Types of falls (with suitability)

1. Vertical drop fall (Sarda type)

 ~~~~~~~~~~|
           |   crest wall
 ______ ___|   drop
           |\___ cistern ___

Water falls vertically over a wall into a cistern in which a hydraulic jump forms. Suitable for drops up to about 1.5 m and for small and medium canals, and where the soil is good. Simple, cheap, easy to construct.

2. Glacis (sloping) fall

 ~~~~~~~~~~\
            \ glacis 2:1 to 3:1
             \__ jump on floor __

Water slides down a sloping glacis and forms a jump on the floor. Suitable for drops of 1.5 m to about 4 m and for large discharges.

3. Ogee fall

 ~~~~~~~~~~___
              ) ogee curve
              \__ cistern ___

Curved crest joined smoothly to the floor. It reduces the impact and shock; suitable for drops with high discharges.

4. Rapid fall A long sloping channel with the bed slope steeper than the normal; suitable where the ground slope is gentle and uniform, and the drop is large.

5. Stepped fall A series of small vertical drops in succession; suitable where the ground slope is steep and long and the drop is large.

6. Notch fall (rectangular or trapezoidal notch)

 ~~~~|  |~~~~   notches in the crest wall

The crest is divided into notches that are opened or closed, so the u/s depth is held constant over varying discharge. Suitable for small canals with varying supplies.

7. Montague type fall, Inglis (baffle) fall Montague has a parabolic glacis; Inglis fall has baffle wall and glacis; suitable for large drops and high discharges.

Choice depends on the drop height, the discharge, the soil type, and cost.