Lecture 27
Protective Structures
I. Introduction
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Protective structures include energy dissipation and erosion control structures,
structures to divert excess water to prevent over-topping of canals, and others
In this context, protective structures are for the canals and other infrastructure,
and not for the protection of animals and people
For example, there should be a wasteway weir or other structure in a canal,
upstream of an in-line pumping plant -- the pump(s) could shut off unexpectedly,
possibly causing over-topping of the canal upstream
An inverted siphon could become clogged, or a landslide could block the flow in
the canal, also causing the canal to overflow on the upstream side
Of course, canal over-topping can also occur due to design deficiencies,
construction problems, and operational error
This means that spillway structures are generally designed to carry the full flow of
the canal
Side spillways sometimes have radial or vertical slide gates to facilitate
dewatering of the channel in emergencies, for sediment removal, and other
reasons
Some in-line gate structures in canals have fixed-crest side weirs to allow for
water to pass downstream in the event of an operational error
The flow from spillway structures is generally directed into a natural channel
which can safely carry the maximum spill rate away from the canal
II. Wasteway Weirs
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A wasteway weir is a sharp- or blunt-crested weir located along one bank of the
canal
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The weir may be designed to accept “stop logs” to allow for changes in the
operating level of the canal
The crest of the weir is equal to the maximum operating level of the canal, taking
into consideration that a higher water surface elevation will result in the canal for
the full design discharge
The wider the weir is, the less difference in water surface elevation between
impending spill and full-flow spill through the structure
Wasteway weirs are located at places where the canal would be most likely to
overtop in the event of an operational error or the clogging of a flow control
structure (e.g. cross regulator):
1. Upstream of an inverted siphon entrance
2. Upstream of a gate or weir structure
3. Upstream of a pumping station
III. Siphon Spillways
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As opposed to inverted siphons, siphon spillways operate under negative
pressures (below atmospheric)
Once a siphon spillway is primed, water will continue to flow through the structure
as long as the downstream water elevation is lower than that in the canal, or until
suction is broken by other means
Siphon spillways are generally more expensive to build than side spillway weirs,
but for the same discharge they require a much smaller width
Siphon spillways can discharge more water than a weir for a small increase in
upstream water surface elevation
As the water level in the canal increases, the siphon spillway acts like a weir
As the water level continues to increase, the siphon will become “primed” and
operate under full pipe flow conditions
The USBR design calls for a crest elevation of about 0.2 ft above the normal
water surface (or full supply level) in the canal
Note that the inlet to the siphon can serve as a sediment trap, requiring periodic
manual cleaning
Also note that it is impractical to change the FSL of the canal once the siphon
spillway is installed – with side spill weirs, you can always raise the level, if
needed
See Section 4-14 of the USBR Small Canal Structures book
Gary P. Merkley
294
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Air Vent for Breaking Suction
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An air vent is located just downstream of the top of the siphon spillway to break
suction when the upstream water level in the canal drops below a certain level
This location is about 15° (from vertical) downstream of the top of the siphon
pipe, as shown in Figure 4-17 of the USBR Small Canal Structures book
The upstream end of the vent is open at the normal water surface level (or FSL)
of the canal
A potential operating problem with this type of structure is that when the suction
is broken, the discharge will suddenly cease, and this can cause surges in the
canal
A pan can be attached to the upstream end of the vent to help prevent the vent
from acting as a siphon itself, possibly causing the water level in the canal to
drop below full supply level
The pan helps reduce the amount of water level fluctuation in the canal
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FSL
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2.00 ft
6.16 ft
FSL
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4.
ft
1.25:1
ft
297
Barrel
3.85 ft
30o
2
0.
2.00 ft
2.00 ft
15o
60o
3.0 ft
f
tof
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c
ll
wa
5.00 ft
road
12.00 ft
6.36 ft
10.00 ft
1:1
sill
1.5:1
8.30 ft
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0f
4.0
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all
w
f
tof
cu
high
water
Deflector at the Downstream Side of the Siphon
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A small angled deflector can be installed in the downstream end of the siphon to
help direct water up to the top of the barrel under non-full-flow conditions
This helps to mix air and water and cause the siphon to prime to full flow quicker
The roof of the siphon structure at the outlet should be above the expected
downstream water surface elevation
This helps to evacuate air from the siphon
IV. Design of a Siphon Spillway
This design example is adapted from an example given by the USBR (1978)
Given:
Suppose there is a canal with a design discharge of 120 cfs in which an in-line pump
station is used to lift the water up to a downstream reach. The canal is trapezoidal in
cross-section, with a base width of 8.0 ft, side slope of 1½:1 (H:V).
The canal is at an elevation of about 6000 ft above msl. The available head across
the siphon spillway, H, is 6.0 ft.
Solution:
(a) Preliminary Calculations According to USBR recommendations, use a ratio RCL/D
of 2.0. Assume an initial value of D = 2.0 ft. Then, the radius of the centerline of the
siphon is RCL = 4.0 ft, and the radius of the siphon invert is RC = 4.0 ft - ½(2.0) = 3.0
ft. The radius of the top of the siphon is RS = 4.0 ft + ½(2.0) = 5.0 ft.
(b) Full Pipe Discharge Estimate the full pipe discharge by assuming: (1) orifice flow;
and (2) a discharge coefficient of 0.65. This will give a flow rate per “unit width” of
the barrel:
q = CdD 2gH = (0.65)(2.0) 2(32.2)(6.0) = 25.6 cfs / ft
(1)
Note that this is an estimate, using an assumed Cd value, and D instead of area.
(c) Maximum Possible Discharge Now, estimate the discharge per unit width
according to the “vortex” equation, which takes into account the atmospheric
pressure available to “push” the water up over the invert of the siphon crest. The socalled vortex equation looks like this:
⎛R ⎞
qmax = RC 2g(0.7h) ln ⎜ S ⎟
⎝ RC ⎠
(2)
where h is the available atmospheric pressure head, based on the density of water;
and qmax is the flow rate per foot of barrel width (cfs/ft).
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The change in average atmospheric pressure with elevation can be approximated by
the following linear relationship:
Patm ≈ (33.9 − 0.00105Elev) / 2.31
(3)
where Patm is in psi and Elev is the elevation above mean sea level, in ft. For 6000-ft
of elevation, the atmospheric pressure is about 11.9 psi, or h = 27.6 ft of head
(water).
Then, qmax is equal to:
⎛ 5.0 ⎞
qmax = (3.0) 2g(0.7)(27.6) ln ⎜
⎟ = 54.1 cfs / ft
3.0
⎝
⎠
(4)
This means that the previously-calculated unit discharge of 25.6 cfs/ft (from the
orifice equation) is acceptable. If q were greater than qmax, it would have been
necessary to decrease H or change RCL.
(d) Barrel Width The width of the barrel is determined as:
b=
Q
120 cfs
=
= 4.7 ft
q 25.6 cfs / ft
(5)
This value could be rounded up to provide a margin of safety, but we will leave it at
4.7 ft (at least for now).
(e) Vent Diameter The diameter of the siphon breaker pipe, Dp, should be such that
the cross-sectional area is at least 1/24th of the cross-sectional area of the barrel
(according to USBR guidelines). This gives an area of (2.0)(4.7)/24 = 0.39 ft2. The
corresponding ID is 0.70 ft, or 8.5 inches. Thus, use whatever pipe size would be
closest to this diameter (perhaps 9-inch nominal size), noting that steel pipe is
usually used (for strength).
(f) Outlet Sill Height The height of the deflector sill at the outlet of the siphon is given
as 1.5D, or 3.0 ft in our case.
(g) Outlet Ceiling Height The ceiling of the outlet is defined as h2. Referring to Figure
4-17, this is given by:
h2 = 1.5D + Ecritical + 1.0 ft
(6)
where Ecritical is the specific energy for critical flow conditions, in feet. This is how the
dimensions are defined in the USBR design procedures.
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The width of the downstream pool will be the same as the barrel width, or 4.7 ft, and
the section will be rectangular. Critical depth for the design discharge is:
yc = 3
Q2
gb
2
=3
(120)2
2
(32.2)(4.7)
= 2.7 ft
(7)
The velocity is Q/A = 120/(2.7*4.7) = 9.5 fps, so the velocity head is:
V2
(9.5)2
=
= 1.4 ft
2g 2(32.2)
(8)
Then, Ecritical is 2.7 + 1.4 = 4.1 ft. And, h2 = 1.5(2.0) + 4.1 + 1.0 = 8.1 ft.
(h) Other Design Details The inlet structure from the canal can be designed with a
height along the side slope of 2D (minimum). The inlet structure should provide a
minimum submergence of 1.5hv + 0.5 ft, where hv is the velocity at the inlet, and the
inlet area is at least 2Db.
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