3 - Culvert Design Flashcards

(69 cards)

1
Q

What is a culvert?

A

A structure that channels water through an obstacle, typically in a road or embankment.

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2
Q

What do the culvert and soil bear the weight of?

A

The roadway and the vehicles.

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3
Q

What is the difference between a bridge and a culvert?

A

A bridge has a larger span, with its own abutments and bears its own weight plus that of the traffic.

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4
Q

In the US, if a structure has a span of over 20 feet, what is it considered as?

A

A bridge

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5
Q

What are 2 properties of pipe culverts?

A

They are circular, cheap and easy to install.

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6
Q

Why are box culverts popular in road design?

A

The shape provides a rigid structure that is appropriate in areas with poor soil conditions

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7
Q

What do culvert inlets determine?

A

The flow rate through the culvert

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8
Q

What is the ‘invert’ of a culvert?

A

The bottom level of the opening

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9
Q

What is the ‘soffit’ of the culvert?

A

The top level of the opening

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10
Q

What is the wing wall of a culvert?

A

The wall of the culvert.

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11
Q

Why is scour protection needed at the culvert exit?

A

Water can be travelling much faster than the inlet.

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12
Q

In order to avoid erosion in the culvert, what speed does the water need to be kept below?

A

1.2m/s

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13
Q

What is material failure?

A

Since circular culverts are made from corrugated steel tubes, corrosion can be a cause of structural failure.

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14
Q

What are the 3 failure modes of culverts?

A

Material failure, erosion, buoyancy

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15
Q

How can buoyancy cause failure of a culvert?

A

Air can get trapped in a full-flowing culvert, resulting in a buoyancy force and uplift on the culvert.

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16
Q

Blockages can reduce the ability of the culvert to convey water, what is a solution to this?

A

Using screens, to catch the debris before it enters the culver, and also keep children safe.

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17
Q

What are the 3 solutions when dealing with a bendy stream?

A

1 - Place culvert at most straight part
2 - Realign the channel
3 - Widen the culvert and add head walls

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18
Q

Why is full flow in a culvert (surcharge) not desirable?

A

Increased risk of blockage

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19
Q

If the hydraulic grade line (HGL) is above the soffit level, where is hydraulic pressure exerted?

A

On the culvert soffit

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20
Q

What is the total head, H1 of the energy grade line (EGL) ?

A

H1 = z1 + y1 + v^2 / 2g

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21
Q

What is the hydraulic grade line (HGL)?

A

z1 + y1

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22
Q

What does the froude number determine?

A

Wether the flow is subcritical, critical or supercritical

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23
Q

What is the equation of the Froude number?

A

Fr = v / root (g*y)

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24
Q

What happens when Fr > 1?

A

The flow is supercritcal and is characterised as rapid

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25
What happens when Fr < 1?
The flow is subcritical and characterised as tranquil
26
What happens when Fr = 1?
The flow is critical, and is transitioned between super and sub.
27
What is it called when flow jumps from supercritical to subcritical?
A hydraulic jump
28
What is a 'control section'?
When Fr =1?
29
What is the specific energy?
The energy relative to the channel bed and is the sum of the pressure and velocity heads. E = v^2 / 2g + y = (Q^2 / 2*g*B^2) * 1 / y^2 + y
30
At the critical depth, yc, what is the energy at?
A minimum
31
What is inlet control characterised by?
A flow control just inside the inlet
32
The barrel has a higher flow capacity than the inlet, what is the culvert performance determined by?
The headwater level and the inlet geometry.
33
Do hydraulic characteristics downstream of the control section play any role in the performance of the culvert?
No
34
If the inlet channel is much wider than the culvert barrel, what happens to the flow velocity inside the culvert?
It is increased.
35
If the barrel slope is much larger than the upstream slope, what may happen to the flow?
It may go supercritical.
36
What is the outlet control flow controlled by?
The culvert barrel, outlet or the open channel downstream
37
What will the difference between the headwater elevation and the tailwater elevation govern?
The discharge through the culvert.
38
What do frictional and form losses play a role in?
Outlet control
39
What are the 2 conditions a culvert will be designed for?
Free flow or surcharged flow
40
Under extreme conditions, what may happen to the embankment?
It may be overtopped, leading to a combination of culvert flow and weir flows
41
What is the 'freeboard' in a culvert?
The air gap between the water level and the soffit.
42
What does the freeboard prevent?
Debris from entering the culvert
43
What does the amount of freeboard depend on?
Culvert size, watercourse, type of debris, and the acceptable level of risk.
44
From an environmental viewpoint, why is it good to allow for some sediment to build at the bottom of the culvert?
It can shield the bottom culvert wall from erosion.
45
Why are culverts buried beneath the channel base?
To allow for some sediment build up
46
What do sediments change ( hint Liam)?
They can change the effective manning's n of the culvert.
47
If the barrel height is 0.45m - 1.2m, what should the freeboard and sediment take up in terms of volume?
A quarter each
48
If the barrel height is 1.2m - 1.8m, what should the freeboard and sediment take up in terms of space?80%
1/6th each
49
If the barrel height is greater than 1.8m, what percent of the culvert should be available for flow?
80%
50
What is Manning's formula for the flow rate, Q in an open channel?
Q = 1/ n * A * R^2/3 * S^1/2
51
What does manning's n depend on?
The rougness of the channel
52
In the headwater, culvert barrel and tailwater sections, what do we know?
1 - Flow rate 2 - Channel or barrel manning's n 3 - The channel or barrel cross-section shape 4 - The channel or barrel slope
53
For trapeziodal channels, a quintic solution is not possible, how do you therefore calculate Q?
Guess a value for y, and keep changing until a desired value for Q if found.
54
What is step 1 of culvert design?
Check for submergence by the tailwater at the design flow. If the tailwater is above both the inlet and outlet soffits, then full flow outlet control is likely If not, then the flow could either be inlet or outlet control
55
What is the second step of culvert design>?
Estimate normal flow depth and the froude number in the culvert barrel. If normal depth exceeds the barrel height, then full flow outlet control is likely If the normal depth is less than the barrel height, then: If the froude number in the barrel is supercritcal, inlet control is likely. If subcritical, outlet control is more likely.
56
While we show that the flow is subcritical in the barrel and in the tailwater section, we haven't accounted for losses in the culvert. Working downstream, what 5 losses do we take into account?
1 - Outlet head loss 2 - Frictional head losses in the barrel 3 - Bend losses 4 - Inlet head losses 5 - Screen losses.
57
By adding head losses to the tailwater height, what can we see?
Wether the water height at the inlet exceeds the soffit, or allows for acceptable freeboard.
58
In the equation of the head losses at the outlet, what is k0, v0, yt and vt?
k0 : An outlet loss coefficient v0 : Flow velocity at outlet yt : Flow velocity in tailwater section vt : flow veocity in tailwater section
59
What is the frictional head loss in the barrel given by?
hf = Sf * L
60
As the frictional slope is greater than the bed slope, what happens to the water depth as we move upstream in the barrel?
It increases
61
What does the change in depth yf equal?
Length * ( frictional slope - barrel slope )
62
Through reworking manning's equation, what is the equation of the frictional slope?
( n*Q / A * R ^ 2/3 ) ^2 Area and hydraulic radius calculated from the outlet.
63
What is the head loss, hi at the inlet given by?
hi = ki * ( vi^2 / 2g)
64
What is ki?
The inlet loss coefficient
65
What is the headwater depth the sum of>
The inlet depth and the inlet head loss
66
When designing culverts to run under full flow outlet control for a design flood flow, working at the downstream end of the culvert, what do we take into account?
Outlet head losses, firctional head losses in a full flow barrel and inlet losses.
67
For a full flow culvert design, what is the head loss at the outlet given by?
h0 = k0 * ( vb^2 - vt^2 ) / 2g
68
For full flow culvert design, what is the velocity in the barrel given by?
Velocity = Q / A
69
Under full flow conditions, what happens to the sediment in the inlet?
It gets flushed out