2 - Highway Drainage

Question 1

What are the main reasons for having highway drainage?

Answer 1

Driver safety and reducing degration of the road.

Question 2

What is the aim of highway drainage?

Answer 2

To remove water from the road as quickly as possible

Question 3

When does aquaplaning occur?

Answer 3

When there is standing water on the road.

Question 4

What happens as a result of aquaplaning?

Answer 4

Grip is lost and the car loses cnontact with the road surface, resulting in a loss of control.

Question 5

What are the 4 ways water can damage the road surface?

Answer 5

Seep in through cracks, seeps up from water table, sits on surface and causes failure as cars pass, degrades the soil structure.

Question 6

If water is not managed once it leaves the road surface, what can happen around the road?

Answer 6

The subsurface and soil can fail. Water needs to be taken away from the highway completely.

Question 7

What are the 2 ways that surace drainage is achieved by collecting surface water runoff?

Answer 7

Formal drainage (kerbs and gullies) and informal such as surface channels.

Question 8

What is the order of preference in which surface drainage shall discharge ?

Answer 8

1 - Ground
2 - Surface water body
3 - Surface water sewer
4 - Combined sewer

Question 9

What are ‘ditches’?

Answer 9

A roadside ditch, where the road’s camber encourages water to run into.

Question 10

What are ‘offlets’?

Answer 10

Areas of the road where water runs down, periodically spaced.

Question 11

What is the gradient of the road?

Answer 11

The slope in the longitudanal direction, the direction traffic is moving in.

Question 12

What is the camber of the road?

Answer 12

The transverse slope of the road surface, primarily provided for drainage.

Question 13

What is a typical camber for UK road?

Answer 13

2.5%

Question 14

Around bends the highway is often superelevated to allow cars to travel at higher speeds without skidding, what is the maximum permitted gradient?

Answer 14

5%

Question 15

What are gullies?

Answer 15

Periodically spaced next to the kerb to allow stormwater to drain into a seperate system. An assembly to recieve water for discharge into a drainage system.

Question 16

What is a grating?

Answer 16

The removable part of a gully that permits the passage of water to the gully.

Question 17

What is a gully top?

Answer 17

The part on top of a gully that is placed on the gully pot.

Question 18

What is a gully pot?

Answer 18

A device installed below the grating to collect settleable solids and prevent them from entering the drainage system.

Question 19

What are transverse bars?

Answer 19

Part of the grating which is at 90 degrees to the direction of flow.

Question 20

Why is the water level in gully pots maintained?

Answer 20

So that solids can settle and also to act as a water trap to prevent solids escaping.

Question 21

What are ‘beany blocks’?

Answer 21

A linear drainage technique, which periodically takes water to the main sewer.

Question 22

What 3 factors does manning’s equation consider?

Answer 22

The hydraulic radius, the slope and manning’s n

Question 23

What does mannings n consider?

Answer 23

The roughness of the channel.

Question 24

What is the hydraulic radius?

Answer 24

The ratio of the cross sectional area to the wetted perimeter, P

Question 25

Why do gullies not provide the best solution for steep gradients?

Answer 25

Too much water bypasses the gratings.

Question 26

What is the main principle of gully spacing design?

Answer 26

The flow width, B does not exceed 1m.

Question 27

Why is a flow width greater than 1m bad?

Answer 27

Danger to traffic and pedestrians.

Question 28

What is the efficiency of a gully?

Answer 28

A percentage of the ratio of water flow rate into the gully to the approaching flow rate.

Question 29

What is the effective width in gully design?

Answer 29

The catchment boundary.

Question 30

For a constant slope and gully spacing, what is the bypass flow equal to?

Answer 30

The same throughout.

Question 31

What is the time of concentation, tc?

Answer 31

The sum of ts, the time taken for water to fall from the furthest part of the catchment to the kerb, plus tg, the time taken to flow along the kerb to the gully pot.

Question 32

What value is usually recommedned for ts?

Answer 32

3 minutes

Question 33

What does the time of concentration have to lie between?

Answer 33

4 to 7 minutes

Question 34

If the time of concentration is less than 4 minutes, what should happen?

Answer 34

A new value for the rainfall intensity should be calculated, and hence a new value of the gully spacing Le.

Question 35

If the time of concentation is above 7 minutes, what should happen?

Answer 35

Decrease the spacing between the gullies.

Question 36

For gullies, what return period is used?

Answer 36

A 5 year return period, meaning that we should see the maximum width of 1m every 5 years.

Question 37

What is the water depth against the kerb?

Answer 37

H = B * camber slope

Question 38

What is the cross section area of flow, Af?

Answer 38

1/2 * BH

Question 39

Are manning's n values higher for concrete or asphalt?

Answer 39

Asphalt

Question 40

What happens to the value of manning's n when the surface gets worse in quality?

Answer 40

It increases.

Question 41

What is the flow collection efficiency, n of each grating?

Answer 41

100 - Gd* (Qf / H)

Question 42

What does Gd depend on?

Answer 42

The grating type

Question 43

What is the parameter m?

Answer 43

The grating performance factor

Question 44

What does the grating performance perameter depend on?

Answer 44

Whether the grating might be effected by debris build up or local road geometry.

Question 45

What is the flow enterning the grating when considering the parameter m?

Answer 45

m * n /100 * Qf

Question 46

To determine the grating type we need, what 3 factors do we need to consider?

Answer 46

The area, Ag of the smallest rectangle parallel to the kerb, the opening area as a percentage, and the coefficient Cb.

Question 47

What does the coefficient Cb depend on?

Answer 47

Whether the slots are transverse to the flow or not. Transverse = 1.75, all other configurations = 1.5.

Question 48

What is the equation of the grating parameter, G?

Answer 48

69Cb / A^3/4 * p^1/2

Question 49

Once you have a value for the grating parameter, Gd, what can you do?

Answer 49

Determine the type of grating.

Question 50

If the grating efficiency is less than 80%, what 2 solutions can be used?

Answer 50

A more efficient grating type could be used or reduce the maximum width below 1m.

Question 51

What are terminal gullies?

Answer 51

Those which no significant proportion of approaching flow is allowed to pass through the gully.

Question 52

Where are terminal gullies often positioned?

Answer 52

At the end of a road in order to "mop up" the bypass flow.

Question 53

What must the efficiency of a terminal gully be?

Answer 53

Above 95%

Question 54

What are surface water channels?

Answer 54

An alternative to the kerb gully arrangement, where flow is kept off the road and in the channel.

Question 55

What shape are surface water channels typically?

Answer 55

Triangular.

Question 56

Why is the max depth of a surface water channel 150mm?

Answer 56

Because if they were any deeper they would be a driving hazard.

Question 57

What should the longitudanal gradient of a surface water channel be?

Answer 57

The same as the area of pavement being drained.

Question 58

For surface water channels where surcharging is allowed, what are the 2 design criteria?

Answer 58

The channel will flow to a level y1, with a flow width Bd for a 1 year return period storm. The channel will overflow the roadway with a flow width of Bs to a height y3 where the channel is not overflowing (same height as the verge)

Question 59

At the upstream end of a surface water channel, what will the flow depth be?

Answer 59

Low

Question 60

Moving downstream a surface water channel, what happens?

Answer 60

More rainfall falls into the channel, and the depth will increase.

Question 61

In a surface water channel there will come a point where the channel depth gets to a point where the water just touches the edge of the road, what is this?

Answer 61

The design depth where manning's n can be used.

Question 62

Does the width Bd of a surface water channel include the width of the channel?

Answer 62

Yes, say the channel was 0.2m wide, the effective width would be 1.2m.ha

Question 63

What is y1 in the equation for calculating the length of the road drained in a surface water channel?

Answer 63

The distance between the bottom of the channel and the side of the road.

Question 64

If you know the length of the road to be drained, but don't know how deep to make the channel what should you do?

Answer 64

Use the equation for y1, which includes the hydraulic radisu factor.

Question 65

What is 'r'?

Answer 65

The hydraulic radius factor?

Question 66

What is the equation of the hydraulic radius factor?

Answer 66

(b1 + b2) / ( (root 1 + b1^2) + (root 1 + b2^2) )

Question 67

What is the upper limit of the storm duration for a surface water channel?

Answer 67

30 minutes

Question 68

What is the surcharge condition?

Answer 68

Water encroaches 1cm onto the road, increasing the depth and flow in the channel.

Question 69

The water in a channel needs to be drained away from the road, how is this achieved?

Answer 69

Through the use of incline double gratings at the end of the channel, or terminal gratings.

Question 70

What is the surcharge flow calculated from>

Answer 70

Qs = 1.575 * Qd * drainage length factor

Question 71

To determine the number of gratings required for a surface water channel, what do you first need to calculate?

Answer 71

Fd and Fs

Question 72

What efficiency should you aim for for gullies in channels for design flow and surcharge flow?

Answer 72

100% for design flow, and 80% for surcharge flow.