A weir or a barrage is the most important component of a head-works. Therefore it is necessary here to understand the principles on which a weir is designed.
Following data is required to design a weir:
(a) Design Flood Discharge:
Maximum flood discharge data of the river recorded in past is collected. From it future flood discharge is anticipated.
(b) Stage Discharge Curve:
The levels attained during various seasons and also during the floods are collected. A graph is plotted between stage of the river and the corresponding discharge measured.
(c) Silt Data:
The amount of silt carried by the river is measured. The analysis of the silt is done to know its properties.
(d) F.S.L. and F.S:
F.S.L. and F.S. discharge of the off-taking canal.
(e) Cross Section of the River:
River cross sections at the weir site and also on the upstream and downstream of it are plotted.
(f) Amount of Afflux:
When the weir obstructs the flow in the river the water heads up on the upstream side of the weir. The amount by which it rises above the normal level is known as an afflux. The amount of afflux determines the height and the sections of the guide banks and protection bunds to be used for training the river. Normally afflux at design flood discharge it required in design.
(g) Pond Level:
The weir stores water on its upstream side in front of a head regulator to create a still pond. The canal gets water from this pond through the head regulator. It is essential to keep the level of the pond some 0.75 to 1 m above the F.S.L. of the canal. It provides sufficient working head. The pond level determines the crest level and height of the shutters to be provided.
(h) Waterway:
It is the width provided at the site for river water to flow. It is also the length of the weir. In practice waterway provided between the abutments of a weir may be calculated from Lacey’s regime perimeter formula.
Pw = 4.825 Q1/2
Here Pw, denotes the length of the weir between the abutments in metres.
Q is the design flood discharge in m3/sec.
To avoid the risk of overtopping of the training works the weir length is generally increased. This increase in weir length may be from 10 to 50 percent of Pw.
Design Procedure for Weirs:
On good foundation, following design criteria is found to be quite sufficient.
Dimensions of the Body Wall and the Crest of the Weir:
It is designed as a retaining wall to store water against it. Main two forces are water pressure and self-weight of the weir. Thus the design is similar in principle to that of a solid gravity dam. The resultant of water pressure (P) and self-weight (W) should remain within the middle-third of the weir base.
There should be sufficient width at the top for operating the shutters.
According to Bligh dimensions of the body wall are:
From Fig. 15.12, it is clear that B is the base width, “a” is top width and H is the height of the weir above its floor level in metres.
‘d’ is the height of the flood water above the weir crest or height of the shutter, whichever is more.
Whereas upstream curtain wall is generally kept 2 to 2.5 m deep, downstream curtain wall is kept deeper say 2.5 to 3 m.
The upstream apron length according to Bligh is 2.208√CH, where C is the creep coefficient and H is the head of water against the weir. The thickness is generally 0.3 m over 0.3 to 0.5 m foundation concrete. The only requirement of upstream apron is that it should be watertight.
Downstream apron length according to Bligh is 2.208 JUKI. The thickness of downstream apron should be sufficient to withstand uplift pressure below it.
It may be calculated from the formula:
where t is the thickness of apron in m and (H—h) is the residual uplift head at the point under consideration. Obviously H is the total head causing seepage and h is the head lost by creeping water in its path upto the point under consideration. ρ is specific gravity of the material used in the apron floor.