SCOURING OR UNDER SLUICES

• These are the openings in the weir.
• They are forme by depressing the weir.
• They have gates fitt in them.
• The scouring sluices are locat on the side of the weir on which head regulation of the canal, taking off from here, is locat.
• If two canals are taking off on either side of the river two sets of under sluices should be locat on either side of the weir.

Undersluices perform the following functions.

1. A clear well-defined approach channel is preserve in the river just U/S of the canal head regulator.
2. Silt entry into the off taking canals is controlled.
3. When sluices remain close for a few days, silt gets accumulat in the get just U/S of the sluices.
4. This can be easily scour to the D/S side by opening the under sluices from time to time.
5. Low floods can be passed through them without weir shutters being dropped.
6. During high floods, they provide greater waterway, and thus rise in the reservoir level can be restricted.
7. They help store a fair amount of receding floodwater.
8. The capacity of under sluices depends upon many factors.

However, it should be fixed base upon the following considerations.

1. Twice the rated discharge of the taking off canal.
2. 10 to 20% of the high flood discharge.
3. Winter floods should pass through them without the weir crest shutters being dropp.

Design of undersluices.

• In this case, the intensity of discharge per meter length, and crest level of the under sluices are different from the weir proper.
• Hence it is necessary to design them separately.
• The design of under sluices is done based on the principles of weir design.

Following considerations, however, should be made in the design.

1. The thickness and length of the impervious floor should be design in the same way as explained for weirs.
2. The crest level of under sluices should be about 1 m below the crest level of the head regulator of the canal, if no silt excluding device is provid in the river.
3. If silt excluding device, in form of silt excluder, has been provid in the river, the crest of head regulator should be about 2 m above the crest level of the under-sluices.
4. In the case of under sluices, the discharge is the more intensive, and hence more protective length on the D/S side is require.
5. The total length of the apron and Talus on the D/S side (l 3) should be as follows:

\[l_{3}= 27C\sqrt{\frac{H_{s}}{13}\times \frac{q}{75}} metres\]

where q is the intensity of discharge in cumecs/metre.

4. Length of impervious apron on U/S side of the sluice gate, should be as follows

\[l_{2}= 3.9C\sqrt{\frac{H_{s}}{13}}metres\]