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Vertical drop weir is such a weir, whose D/S face is given steep batter. The U/S face is mostly vertical, but it can also be given batter like D/S face.
The design of such a weir consists of: 1. Fixing Various Elevations by Hydraulic Computations 2. Design of Weir Wall 3. Design of Impervious Floor of the Weir Wall 4. Design of Inverted Filter 5. Design of Launching Apron or Talus at D/S End 6. Design of Launching Apron or Talus at U/S End.
1. Fixing Various Elevations by Hydraulic Computations:
Before we actually perform any hydraulic calculation, we must know the following data:
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(a) Maximum likely flood discharge (Q).
(b) Level upto which water reaches during floods (HFL).
(c) Full supply level of the canal taking off from the river.
(d) Allowable rise in water level due to weir i.e. afflux.
(e) Lacey’s silt factor (f).
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Lacey’s silt factor for the soil, through which canal and river are running, is found out by formula f = 1.76 √ (mr), where mr is the mean particle diameter of silt in millimetres. Value off varies from 0.50 to as much as 1.50. Its value is less for finer silt and more for coarser silt or sand.
(i) Length of the water way, or in other words length of the weir (L) is found out from Lacey’s following regime formula –
L = 4.75 √Q
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Where L = Length of the weir in metres.
Q = Discharge in cumecs.
(ii) Find out the discharge per metre length (q) of the weir from relation q = Q/L.
(iii) Find out the scour depth by Lacey’s following formula –
(iv) Regime velocity (V) is found out as follows –
(v) Find out velocity head = V2 / 2G .
We will be referring term T.E.L. very frequently. This term denotes total energy line (T.E.L.).
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(vi) Level of D/S T.E.L. = D/S H.F.L. before weir construction + V2 / 2g.
(vii) Level of U/S T.E.L. = D/S T.E.L. + Afflux.
(viii) Level of U/S H.F.L. U/S T.E.L. – V2 / 2g
(ix) Crest level of the weir U/S T.E.L. – k.
Where
k is the height of U/S T.E.L. above the crest of the weir.
(x) Reservoir level at the back of weir = level of top of gates
= F.S.L. of canal + head loss through regulator.
Loss of head through regulator may be taken anything between 0.50 m to 1 m.
Height of gates = (g) = Level of top of gates – crest level.
(xi) Level of bottom of U/S pile U/S H.F.L. – 1.5 R.
(xii) Level of bottom of D/S pile D/S H.F.L. after retrogation – 2R.
2. Design of Weir Wall:
(i) Top Width:
Top width of the weir is usually denoted by a and bottom width by b.
Top width should be greater of the three values given as follows:
Where g is the height of shutter fitted over the crest of the wall.
(ii) Bottom Width:
(b) It is calculated by equating overturning moment (M0) to the resisting moments (Mr) taken about the outer third point of the base. There can be three states of flow –
(a) Water filled upto crest level or upto top of crest of gates if provided –
In this case
Equation M0 = Mr, b can be found out.
(b) Water passing over the crest and weir is submerged –
In this case
equating both, value of b can be found out.
(c) Water is passing over the weir crest but water level on D/L side is below the crest level –
This case is a typical case, as in it, there are two variables. Depth of water above crest on U/S side (d) and depth of water on D/S side (D), both are variable. For finding a suitable relation between d and D river gauging is necessary. According to SVK Pillai –
d = kD, where k is a constant.
Maximum value of overturning moment (M0) in this case is –
By equating M0 to resisting moment Mr, value of b can be found out
In all the three stages of flow, U/S face of the weir wall has been considered as vertical.
(d) The value of b can also be found out by the following formula
Where d = Depth of water above crest level.
H = Height of crest of the weir wall above the bed level.
3. Design of Impervious Floor of the Weir Wall:
The critical condition for maximum percolation will exist when water is filled upto top of the crest gates and there is no tail water level. In this case maximum percolation head is Hs.
Length of impervious floor (L) according to the Bligh’s theory is L = CHS.
If Khosla’s theory is to be used, length of horizontal impervious apron is found out by considering safe exist gradient GE.
Out of total length of impervious apron which is L in case of Bligh’s theory and b in case of Khosla’s theory, some minimum length has to be provided on D/S side. The remaining length of pucca apron is provided below the weir and U/S of the weir wall. Mr. Bligh gave following relations for floor lengths. He divided length of the floor in three length l1, l2 and l3.
4. Design of Inverted Filter:
An inverted filter is invariably provided in continuation at the end of D/S end of the impervious apron. Its main purpose is to relieve the pressure of seeping water. No design calculations are necessary. It consists of stone blocks 90 cm to 1 m thick placed over 50 to 75 cm thick layer of graded filter. If d2 is the depth of D/S cut-off below the D/S bed, the length of the filter should be at least 1.5 d2.
5. Design of Launching Apron or Talus at D/S End:
Immediately after D/S filter, launching apron of 2.5 d2 length is provided. Launching apron is also sometimes called talus. When talus is launched or sloped at 3:1 slope its thickness should be 90 cm to 1 m. Knowing the launched length i.e. inclined length and thickness, the volume of stones can be easily calculated. Thickness in horizontal position will be slightly more than in inclined position. Launching apron prevents undermining of the weir floors and protects D/S piles.
6. Design of Launching Apron or Talus at U/S End:
Talus is also provided U/ S of the U/S end of the impervious floor or apron. Horizontal length of this talus should be 2d1, where d1 is the depth of U/S pile. In launched position (3:1) its thickness like D/S talus should be 90 cm to 1 m.