highway drainage

7/21/2019 Highway Drainage

1/25

Higher Diploma in Civil Engineering | CON4381 Highway Engineering

Topic 3 Highway Drainage | Page 3-1

3 Highway Drainage

Drainage infrastructure for a road project is planned and designed to provide a standard or level of

drainage immunity that conforms to good engineering practice and that also meets government

and community expectations. Modern highway drainage design should incorporate safety, good

appearance, control of pollutants, and economical maintenance. This may be accomplished with

flat sideslopes, board drainage channels, and liberal warping and rounding.

Highway drainage mainly concerned with the flow of surface water and subsurface water. The

principles of hydrology necessary for understanding rainfall as a water source are included.

Moreover, the fundamental design principles for surface and subsurface drainage facilities are

described in this chapter.

ROAD SURFACE DRAINAGE

Road surface drainage deals with the drainage of stormwater runoff from the road surface and the

surfaces adjacent to the road formation. Several elements can be used to intercept or capture

this runoff and facilitate its safe discharge to an appropriate receiving location. These include:

kerb and channel;

edge and median drainage;

table drains and blocks;

diversion drains and blocks;

batter drains;

catch drains and banks;

drainage pits; and

pipe networks

The first and last of the above list, i.e. kerb inlet and pipe networks, are of more relevance to the

road network in Hong Kong, and are the focuses of this chapter. In the following, hydrological

study will be discussed first to quantity the surface runoff and is followed by discussion on

hydraulic design.


2/25



A. Hydrological Study

Hydrological study can be described as the science which deals with the operations governing the

circulation of moisture in its various forms, above, on and beneath the earths surface. The

various phases of the hydrologic cycle are precipitation, surface runoff, infiltration, evaporation

and transpiration. The two main phases of the hydrologic cycle in which the highway engineer is

most interested are precipitation and runoff.

PRECIPITATION

Rainfall intensityis the amount of rainfall measured in mm at a specific location for a period oftime. The instantaneous rainfall intensity varies during a rainstorm, and it is thus more practical

to describe the average intensity within a specified time, commonly expressed in the unit of

mm/hr. The average intensity is inversely proportional to the length of storm (duration of

rainfall); i.e. the longer the rainfall, the smaller the average rainfall intensity since the

meteorological forces which cause a heavy rainfall in an area are also continually causing it to

move quickly to another area.

Given the same rainstorm duration, there can be different peak rainfall intensity (average over the

given duration) due to the rarity or severity of the storm, which is a measure of strength or

amount of rainfallof the occurrence of precipitation. The peak rainfall intensity must be higherfor a storm of a rarer occurrence. The extent of rarity is conventionally specified based on an

Average Recurrence Interval (ARI), which is defined as the average interval in years between

exceedances of a specified event (i.e. rainfall or discharge) and is written as ARI years. It is,for example, commonly referred to a frequency of once in 2, 5, 10, 20, 50, 100 or 200 years

despite the ARI is really a probability rather than an actual period between occurrences.

For engineering applications, it is common practice to present the extreme rainfall intensities as

intensity-duration-frequency (IDF) curves. Lam and Leung (1994), of the then Royal Observatory,

Hong Kong, used the Wisners formula to derive the IDF curves for rainstorm of duration not more

than 240 minutes for different return periods, which is expressed in the following form:

Precipitation

Infiltration

Surface Runoff

Transpiration

Evaporation

Hydrologic ycle

Water table


3/25



= (+ )

where

= extreme mean intensity in mm/hr

= duration of storm in minutes, , = storm constants calibrated from data, which can be made reference to,say, DSDs Stormwater Drainage Manual

Storm Constants for Different Return Periods(based on Gumbel Solution)(Table 3, Stormwater Drainage Manual (2013) published by DSD, HKSAR)

Return period T

(years)2 5 10 20 50 100 200 500 1000

548 573 603 639 687 722 766 822 855

5.2 4.6 4.4 4.3 4.2 4.1 4.1 4.1 4.0

0.51 0.47 0.44 0.43 0.42 0.41 0.39 0.39 0.39For longer rainstorm, a different approach in considering the depth of rain is adopted and one is

referred to other text including the DSDs Stormwater Drainage Manual.

Remarks are given to the constant need of updating the values IDF curves and constants as well as

the appropriate methodology in obtaining accurate extreme rainfall intensity. Wong and Mok

(2009) shows that the annual rainfall and the frequency of occurrence of heavy rain events have

increased during the period of 1885 to 2008 and the impact of climate change is being carried out by

the Hong Kong Observatory (Ginn et al, 2010). Other parties are also involved in the study ofrainstorm profile for practical use in Hong Kong, for example, the following IDF curves are being

proposed in Tang and Cheung (2011) in a GEO report:


4/25



RUNOFF

Rate of surface runoff () is the difference between the amount of rainfall during the time ofconcentration and the losses due to infiltration, evaporation, transpiration, interception and

storage. Major factors governing amount of runoff are as follows:

(a) Type and condition of the soil with respect to infiltration rainwater will infiltrate into

granular soil until the soil is saturated before the runoff flows on the surface.

(b) Kind and extent of cultivation and/or vegetation.

(c) Length and steepness of slopes.

(d) Number, arrangement, slope and condition of the natural and manmade drainage

channels in the catchment area.

(e) Irregularity of ground surface.

(f)

Size and shape of catchment.

(g) Temperature of air and water.

(h) Changes in land use.

The rate of surface runoff can be calculated by the Rational method, also known as the

Lloyd-Davies method. The Rational Method was used as far back as the mid-nineteenth century.

It continues to be the most commonly used rainfall-runoff analysis framework for design because

of its simplicity. It computes peak direct runoff instead of runoff hydrograph. The key concept of

this method is the assumption that uniform rainfall over time and space produces a steady peak

runoff after the water from all parts of the watershed has reached the runoff location considered.

The peak flow rate at a point of concern in the drainage system is computed by::-

= 3600 (litre/sec) or = 3.610(m3/sec)

where = maximum runoff (litre/sec) = design mean intensity of rainfall (mm/hr) = area of catchment (m2)

= runoff coefficient

Runoff coefficient() is the ratio of surface flow to the amount of rainfall and is mainly dependenton the impermeability of the surface. In general, the value of may be taken as 1.0, i.e. fullyimpermeable, for developed urban areas. In less developed areas, unpaved surfaces may be

given a value less than 1.0, but consideration should be given to possible future developmentand the possible saturation of soil with water before a rainstorm both of which will increase the

impermeability of the surface.

The runoff coefficient actually varies slightly with the rainfall intensity, as a matter of fact of

ponding effect as well as flow pattern of surface runoff. The following tables list out typicalvalues of of with respect to types of surface commonly encountered in Hong Kong and areapplicable for the more frequent storms (say 10-year and below). Less frequent storms of higher


5/25



intensity may require the use of different coefficients.

Character of surface Asphaltic 0.70 to 0.95

Concrete 0.80 to 0.95

Brick 0.70 to 0.85Lawns (heavy soil) - Flat 0.13 to 0.25

- Steep 0.25 to 0.35

Lawns (sandy soil) - Flat 0.05 to 0.15

- Steep 0.15 to 0.20

The runoff coefficient is a function of land use. If land use within the area is non-uniform, it is a

common practice to use an equivalent runoff coefficient computed by area-weighted averaging.

For a catchment consisting of

sub-catchments of areas

each with different runoff

coefficients , the peak runoff at the drainage outlet is given by the following expression: = =

where is the conversion factor corresponding to the adopted units.Due to the assumptions of homogeneity of rainfall and equilibrium conditions at the time of peak

flow, the Rational Method should not be used on areas larger than 1.5 km2without subdividing

the overall catchment into smaller catchments and including the effect of routing through

drainage channels. The same consideration shall also be applied when ground gradients vary

greatly within the catchment.

In a rainstorm, the instantaneous rainfall intensity varies with time and in general exhibits a

negative correlation with the duration of rainstorm, i.e. the instantaneous rainfall intensity would

gradually decrease with the duration. If the rainfall is more intense but of shorter duration not

all the catchment will contribute to the peak runoff; whereas if the rainfall is of longer duration

the average intensity over that duration will be less and the peak runoff will be less even though

the entire catchment contributes. For design purpose, the most intense rainfall that contributes

to the outflow will be that with a duration equal to the time of concentration

of the catchment

(which will be discussed shortly below). Therefore, is the duration used to select the designrainfall intensity from the intensity-duration-frequency (IDF) relationship (i.e. ) discussed in theprevious section.

The time of concentration () is the duration of rainfall commonly used in highway drainagedesign. It is defined either as (a) the time taken for water to flow from the most remote point on

the catchment to the outlet or point of interest; or (b) the time taken from the start of rainfall until

all of the catchment is simultaneously contributing to flow at the outlet or point of interest. The

significance of the time of concentration is that peak outflow will almost always result when the

entire catchment is contributing flow from rainfall on the catchment. The time of concentration

is generally made up of three components:

1. Overland flow time across natural or paved surfaces including retardance due to pondage


6/25



on the surface or behind obstructions;

2. Time of flow in natural and artificial channels; and

3. Time of flow in pipes.

The first two components are always considered for surface runoff across both natural terrains

and built areas; however, the third component is only considered where there is an urban

drainage system in place.

Time of Concentration in Surface Runoff

In natural catchments where surfaces are generally unpaved and surface water travels along

natural lines of flow, the time of concentration may be estimated from the following equation

which is a modified form of the Brandsby Williams equation: -

= 0.14465 .. where = time of concentration (minutes)

= area of catchment (m2) = average slope (m per 100 m) measured on the line of natural flow, from the summit

of the catchment to the point of design

= distance (on plan) measured on the line of natural flow between the design sectionand that point of the catchment from which water would take the longest time toreach the design section (m).

The average slope in a watershed can be calculated using the Average Basin Slopemethod or the

Channel Slope method. Once the slope is determined, the time can be found by the application of

the above equation or by using a nomograph.

Catchment

Area

Distance L

Average slope H

Design section

Drainage of natural catchment


7/25



Time of Concentration in Urban Drainage System

In urban catchments where surface water from paved surfaces, rooftops etc. is led directly to

established drainage channels or stormwater sewers, the time of concentration is the sum of all

three components as noted before. The first two components concern flow on open surface, and

the corresponding duration is termed as the entry time() to the urban drainage system, which isthen added by the time of flow() in the pipe system to give = + .Entry time () is the time required for a raindrop to flow from the most remote part of thecatchment area to the point of entry to a drainage system. It varies with the nature of surface

cover, surface gradient, spacing of inlets, method of collecting and discharging roof drainage, and

the rainfall intensity. Generally, inlet time of 3 to 10 minutes may be used for well-developed

urban areas, the lower figure being applicable to areas where water flows quickly to stormwater

drains through closely spaced inlets and the upper figure applicable to areas which are relatively

flat with widely spaced inlets. When conditions fit, one may use Brandsby Williams equation

with appropriate parameters as an approximation of .

()

()

()


8/25



The time of flow() is the time required for the water to flow from the most remote inlet to thedesign section in the drainage system. It may be estimated closely from the hydraulic properties

of the stormwater drain usually based on full-bore velocity, i.e. the pipe is running full of water.

It is common to estimate the pipe flow velocity using Colebrook-White equation which is

expressed in the following form in the DSDs Stormwater Drainage Manual (Table 12):

= 32 log 14.8+ 1.255

32which is readily applicable for full flow in a circular pipe when the hydraulic radius is equal toone quarter of the pipe diameter , i.e. = 4 .Lastly, the choice of design storm frequency

requires engineering judgment on the tradeoff

between the risk of flooding and cost. As notedbefore, it is expressed as the recurrence interval

or return period. The longer the returned

period, the higher the rainfall intensity and the

bigger the drainage costs in order to dispose of

the increase in runoff. However, the probability

of having a rainstorm of such severity or more is

at the same time smaller. It is therefore

necessary to consider the consequence of

flooding in order to determine what return

period should be used in the drainage design.

The following is a reproduction of Table 10 in Stormwater Drainage Manual HKSAR Planning, Design

and Management, which lists out recommended design return periods based on flood levels.

Intensively Used Agricultural Land 25 years

Village Drainage including Internal Drainage System under a Polder Scheme 10 years

Main Rural Catchment Drainage Channels 50 years

Urban Drainage Trunk Systems 200 years

Urban Drainage Branch Systems 50 years

Road Note 6 was first published by the Highways Department in 1983 providing methods for

drainage design on roads based on Transport Research Laboratory Reports Nos. LR277, LR602 and

CR2. The Note was later updated in 1994 (HyD, 1994) and is now superseded by Guidance Notes

on Road Pavement Drainage Design issued in 2010 (HyD, 2010). These Guidance Notes have

included the latest information and findings from extensive full scale testing carried out in Hong

Kong. HyD (2010) recommends a design return period of 1 in 50 years (with a minimum factor of

safety of 1.2) for the ultimate limit state and 2 per year for the serviceability limit state. The

rainfall duration is taken as 5 minutes, resulting in a design rainfall intensity of 270 mm/hr for the

ultimate limit state and 120 mm/hr for the serviceability limit state.


9/25



B. Hydraulic Design

Once the peak runoff has been determined for a particular catchment, the next step is to provide

a route for water to flow along from the highway to a suitable discharge point, known as outfall

which can be another drainage system, a natural watercourse, a nullah, or the sea. Hydraulic

designis the design of the drainage system to carry the runoff collected by gullies to the outfall

through stormwater sewer, channels, and culverts. The stormwater drainage system can be

divided into two types, stormwater sewer and open-channel. The stormwater drainage system

consists of collecting the surface runoff by a series of gullies and kerb weirs and carrying the water

through a network of underground pipes and manholes to the outfall.

GULLIES

A road gully is a waterway inlet designed to collect water which flows off the carriageway surface.

It consists of a gully pot (which acts as a trap for silt and small debris) connected by a pipe to an

underground pipe drain, and a steel frame fitted with a cover or grating which bridges the gully

pot. Normally, precast/preformed gully pots should be used instead of in-situ construction

except in very special cases where physical or other constraints do not allow their use. The


10/25



following are some of the advantages of using precast/preformed gullies:

a) easier to install and maintaining;

b) have a smooth internal finish which allows easy cleansing as debris tends to adhere to

rough in-situ concrete walls; and

c) where outfall trapping is required, it is simply the choice of a precast trapped gully pot

(it is extremely difficult to build an acceptable gully by in-situ construction)

GULLY POTS &CONNECTION

Untrapped gullies are preferred to trapped gullies because the latter is susceptible to choking.

The connection to the storm sewer should either be via a Y-junction connection or a manhole.

For illustration, to provide for the capacity of 240 mm/hr rainfall intensity for an area of 300 m2a

150 mm diameter gully connection with a hydraulic gradient of 2.6% will be sufficient. Otherwise,

a 225 mm diameter connection pipe will be required.

Gully Pots

DESIGN CONSIDERATION

The guidelines governing the design of road gullies in Hong Kong followed that of the then Road

Note 6 (first published in 1983 and updated in 1994) based on Transport Research LaboratoryReports in the UK, and is recently replaced by the Guidance Note No. 35 since May 2010 based on

extensive research and findings from local studies and physical tests. The design principles


11/25



covered here follow the current guidelines in this GN035. In brief, any design should be based on

the serviceability state considerations and checked for adequacy of the ultimate state conditions.

While the concept of the principles reflect the design philosophy, one should note specific figures,

e.g. rainfall intensity of certain ARI or flooded width, may be updated in accordance with the

prevailing climatic conditions and rainstorm profile.

Serviceability State Consideration

The design flooded width should represent a compromise between the need to restrict water

flowing on the carriageway to acceptable proportions to a reasonable level of cost efficiency.

The principle is to limit the likelihood of water flowing under the wheel paths of vehicles travelling

at high speed, and splashing over footways while travelling at low speed. Rainfall intensity of a

5-minute rainstorm of having a probability of occurrence of not more than 2 times per year is

considered for serviceability state design.

In general for flat and near flat Normal Roads, a design flooded width of 0.75 m under heavy

rainfall condition is adequate. This flooded width will imply that stormwater will just begin to

encroach into the wheel paths of vehicles, or would be restricted within the marginal strip, if

provided. A smaller flooded width is designed for steeper gradients to avoid any flow at a higher

velocity by-passing a particular gully but to add load to the next and subsequent gullies. The

maximum design gully spacing is also limited to 25 m in any case for the same reason.

Gully Gratings (Highways Department Standard Drawing H3105)


12/25



The design flooded width on the slow lane sides of expressways with 2.5 m hard shoulder can be

increased to 1.0 m under heavy rainfall conditions, which will ensure that there is no

encroachment onto the adjoining traffic lane. Again, there is a need to limit the flooded width

on expressways with moderate and steep gradients. In this respect, under no circumstances

should gully spacing exceed 25 m or drained area of gully be larger than 600 m2.

Ultimate State Consideration

The purpose of the ultimate state design is to prevent the occurrence of overtopping of the kerb

height by the kerbline flow, and hence flooding in the adjoining land or properties, even in

exceptionally heavy rainstorms. In this design standard, rainfall intensity of for a 5-min rainstorm

with a probability of occurrence of 1 in 50 years is considered in the ultimate state design.

During design the flow height is checked against the available kerb height . The kerbflow is mainly triangular in cross-section with crossfall

being the side slope. A factor of

safety of = 1.2 is recommended in the guidance notes. Therefore given an ultimate floodedwidth , a design is either acceptable if=

or the gully spacing is in practice reduced proportionately by

, where is theconversion factor to adjust between units.

In Hong Kong, the standard dropped kerb crossing has a kerb height of 125 mm. A kerb height of

150 mm can be used if necessary; otherwise the gully spacing should be adjusted if necessary.

Crossfall should be provided on all roads to drain stormwater to the kerb side channels. On

straight lengths of roads, crossfall is usually provided in the form of camber. On curves, crossfall

is usually provided through superelevation. The Transport Planning and Design Manual (TPDM)

suggests a standard crossfall of 2.5%. However, to facilitate surface drainage, a minimum

crossfall of 3% shall be provided for, except where required along transitions, where the

longitudinal gradient is 1% < < 5%.

GULLY SPACING DESIGN METHODOLOGY

Studies show that flows at differently sloped surface exhibit dissimilar hydraulic characteristics.

On a generally flat terrain, the flow is subcritical but that on a steeper gradient may be

supercritical in nature. The design should follow different approaches for these two flow regimes.

However a standardized methodology is adopted in HyDs guidance notes using two set of design

charts which values are also adjusted to enable the design using similar steps for gullies located at

the upstream crest, intermediate slope and terminal at a sag point. For such purposes, design

step set A is used for road with longitudinal gradient greater than 0.5% and set B

The design workflow for the gully spacing calculation and the key tables of design parameters in

GN035 are reproduced below for easy reference; one should refer to it for detailed discussions.


13/25




14/25



Table 3: Minimum Crossfalls Table 5: Reduction Factors for Gully Efficiency

Longitudinal Gradient Minimum Crossfall Type of Gully 1% or less OR 5% or more 3% GA1-450 0%

Between 1% and 5% 2.5% GA2-325 15%

Table 4: Roughness Coefficients for Different Types of Road Surface

Road Surface n

Concrete without flat channel 0.015

Concrete with flat channel 0.013

Bituminous Wearing Course 0.013

Precast block paving 0.015

Stone Mastic Asphalt (SMA) Wearing Course and Friction Course 0.016

Table 8: Minimum Rate of Provision of Overflow WeirsSection of Road Rate of Provision of Overflow Weirs

longitudinal gradient > 7% Every other gully

longitudinal gradient > 5% but not more than 7% Every third gully

longitudinal gradient between 0.5% and 5% inclusive No overflow weir

longitudinal gradient < 0.5% Every third gully

Sag points or blockage blackspots Every gully

Table 6: Reduction Factors for Blockage by Debris Table 9: Additional Gullies at Sag Points

Roads / Road Sections Catchment Area(m2) No. of Gullies at SagPointsExpressways < 600 3longitudinal gradient less than 0.5% & near sag

points15% 6001,999 4

longitudinal

gradient

0.5% or

more

near amenity area or rural area 10% 2,0003,999 5other sections 5% 4,0005,999 6

Normal Roads 6,0009,999 7longitudinal gradient less than 0.5% 20% 10,00014,999 8longitudinal

gradient

0.5% or

more

near sag points or blockage

blackspot, e.g. streets with

markets or hawkers

20%

15,00019,999 9

> 20,00010 for the first

20,000m2, plus one

for every extra 5,000

or less m2

near amenity area or rural area 20%

other sections 15%

In addition to the gully spacing calculations, GN035 also provide guidelines in other relevantaspects of road surface drainage which are of importance to a good design of the drainage system.

These include the allowance or provision for footway drainage, locating gullies at pedestrian


15/25



crossings, considerations of continuous drainage channel, flat channels and edge drains,

requirement and maintenance of gully pots, etc. In particular, gullies being the inlet to urban

drainage system, it is necessary to consider the overall capacity of outlet pipes of either a

gully-manhole system or simply that of a multiple gullies at certain locations such as sag points

highlighted in Table 9 of the guidance notes. As such, GN035 specifically states the need to check

the outlet pipe capacity against the required capacity to drain completely the design inflowsthrough these gully inlets : =

where I is the ultimate state intensity. Should it is necessary to provide an outlet pipe ofinconvenient diameter (e.g. diameter exceeding 300mm), the designer may wish to provide an

additional outlet pipe in the middle of the series so as to maintain using smaller diameter outlet

pipes.


16/25



MANHOLES

The functions of a manhole are as follows:-

1. As an inspection chamber to provide access for the maintenance of the drainage system,

2. As the head of the pipe run,

3. To accommodate a change of direction of the pipe,

4. To allow for the change of gradient or elevation, and

5. To facilitate the change of pipe size and/or type.

Once the locations of the gullies have been determined, the position of the manholes and the

underground pipe can be decided. Pipe lengths are generally laid straight between manholes, and

are usually arranged to drain by gravity. The distance between manholes based on the method of

maintenance is about 100 to 150 metres. In Hong Kong the table below is used.

Diameter of pipe (mm) Maximum intervals (m)

Smaller than 600 40

Between 6001050 80

Larger than 1050 120

As the size of a stormwater drain increases downstream, it is preferable to maintain the soffits at

the same levels at the manhole. This is to prevent the drain being surcharged by the backwater

effect when the downstream pipe is flowing full.


17/25



PIPES

Stormwater sewer pipes are generally of circular cross-section and can be made of concrete,

clayware, pitch fibre, plastic, or corrugated steel. The service conditions of a highway drain may

include external load due to earth pressures and surcharges imposed by the road itself and its

traffic, scour and wear due to the passage of suspended particles in the runoff water, and chemicalattack inside the pipe by de-icing salts and spillage in the road as well as outside the pipe by

aggressive chemicals such as acids and some sulphate present in the soil. The pipe should

therefore be either of a material which can withstand these conditions, or be protected from them.

In Hong Kong, concrete pipes are used in general and the available size is from 150 mm with a step

size of 75 mm up to 450 mm, and then with a step size of 150 mm up to 2500 mm diameter.

The material on which the pipe rests is known as its bed. Bedding materials in common use are

concrete (plain or reinforced), pea shingle (single sized granular aggregate of 14 mm or 20 mm

nominal size), sand, or the material previously excavated from the trench. This material may be

placed under the pipe only (bed), or may be extended up to half the pipe depth (bed and haunch),

or to completely cover the pipe (bed and surround). So that subsequent settlement of unbound

bedding and backfill materials may be avoided, it is important that these should be fully

compacted. Furthermore, the load required to produce failure of a pipe installed with bedding inthe ground is higher than that in a standard crushing test and its ratio is known as the bedding

factor. It varies with the type of bedding materials and method of construction. The designer

may therefore choose between the relative benefits of providing a strong bedding, and a weak

pipe, or vice versa.

Pipes are provided in units of between 900 mm and about 2 m depending on material and

diameter. The pipes are joined together and it is important that the joints are watertight. Some

of the different types of joints are shown below. It is possible to provide rigid joints between

pipes made of rigid materials but to do so can lead to the pipes being overstressed as a result of

ground movements after construction. Most rigid pipes are therefore provided with flexible

joints in order that a small amount of relative movement can be accommodated between one unit

and the next, and in cases where a rigid (concrete) pipe bedding is used it is important to ensure

Type of pipe bedding


18/25



that the required flexibility is maintained. This is achieved by providing movement joints in the

concrete bed at intervals of about five metres, placed to coincide with pipe joints. The

movement joints consist simply of a collar in fibreboard or similar compressible material, fitted

around the pipe prior to concreting and arranged to form a complete discontinuity in the concrete.

STORMWATER DRAINAGE SYSTEM DESIGN

The design of the stormwater sewer system is based on each individual section of a pipe run and is

an iterative process. A pipe run is the route in a drainage system along which the surface water is

carried from the most remote part to the outfall. In general, the slope of the pipe follows the

gradient of the road and the rate of runoff is calculated using the Rational method based on the

pipe running full. Some factors to be considered in the design of the storm sewer system:-

1. Construction costs increases with depth,

2.

The slope of the pipe follows the general gradient of the surface to minimize cost,3. The velocity of flow should be greater than 0.75 m/s to prevent silting up of the pipe,

4. The pipe should have sufficient cover to protect it against the loading at the surface,

5. Allowance should be made for the head loss at a manhole usually by means of having lower

invert elevation for the downstream pipe,

6. At a manhole, invert to invert connections are not favoured since they often prevent the full

capacity of the larger pipe being made available; the water cannot rise up to the soffit level

of the downstream pipe.

Pipe joints


19/25



Example of a storm drainage system design

Given the network of drainage system and the following information, design the corresponding

pipesize. Rigid pipes are used.

Runoff coefficient = 1.0

Frequency of storm = 1 in 5 years

Time of entry = 3 min

Minimum pipesize = 225 mm

Common nominal size of pipes used in Hong Kong

Materials Nominal size (diameter in mm)

Concrete pipes 150, 225, 300, 375, 450, 600, 750, 900, 1050, 1200, 1350, 1500, 1650, 1800

Vitrified clay pipes 100, 150, 200, 225, 300, 375, 400, 450 ,500, 600

Workings:

Stormwater Drainage Manual recommends a roughness value = 0.6mm for precast concretepipes for 80 to 100 years use.Highways Departments GN35 Guidance Notes on Road Pavement Drainage Design specifies the

typical value of kinematic viscosity of stormwater is 1 1 0m2/s.

Section No. Length (m) Gradient (%) Area CA(m2)

1.1 140 2 1200

1.2 200 2 1600

1.3 160 3 1300

2.1 120 2 1000

2.2 100 2 900

3.1 110 2 900

1.1

1.2

2.1

3.1

2.2

1.3

Storm Drainage Design Based on parameter values of

Runoff Coefficient = 1.0 & Time of entry = 3 min. Surface roughness, ks= 0.6 mm

Storm Frequency = 5 years & Minimum pipe size = 225 mm Kinematic visc osity, u = 0.000001m2/s

P ipe Length Gradient P ipe s ize Flow vel . Capacity te tf tc Intensi ty Area Runoff Remarks

No. (m) (%) (mm) (m/s) (l/s) (min.) (min.) (min.) (mm/h) (m2) (l/s)

1 .1 1 40 2 225 1 .85 7 3.7 3 3 1 .26 4.26 204.7 1 200 68.25 O.K.

2.1 1 20 2 225 1 .85 7 3.7 3 3 1 .08 4.08 206.8 1 000 5 7 .45 O.K.

3.1 1 1 0 2 225 1 .85 7 3.7 3 3 0.99 3.99 207 .9 900 51 .97 O.K.

1 .2 200 2 225 1 .85 7 3.7 3 4.26 1 .80 6.06 1 87 .2 2800 1 45 .5 9 Not O.K.

1 .2 200 2 300 2.23 1 57 .5 5 4.26 1 .50 5.7 5 1 89.8 2800 1 47 .62 O.K.

2.2 1 00 2 225 1 .85 7 3.7 3 4.08 0.90 4.98 1 97 .1 2800 1 53.30 Not O.K.

2.2 1 00 2 300 2.23 1 57 .5 5 4.08 0.7 5 4.83 1 98.6 2800 1 54.49 O.K.

1 .3 1 60 3 300 2.7 3 1 93.1 6 5.7 5 0.98 6.7 3 1 81 .8 6900 348.40 Not O.K.

1 .3 1 60 3 37 5 3.1 5 347 .66 5.7 5 0.85 6.60 1 82.8 6900 35 0.31 Not O.K.

1 .3 1 60 3 450 3.53 561 .57 5.7 5 0.7 6 6.51 1 83.5 6900 35 1 .69 O.K.


20/25



The pipe capacity can be calculated using the Colebrook-White equation assuming full of water.

The below table shows the matrix of both (i) velocity in m/s and (ii) capacity in L/s for typical pipe

size with surface roughness = 0.6mm and viscosity of stormwater = 1 10m2/s. Oneshould otherwise directly get these pipe flow properties using the Colebrook-White equation.


21/25



CULVERTS

A culvert can be defined as a conduit which conveys water through an embankment. A bridge

can also perform the same function. Usually, a bridge surface forms part of the pavement

whereas the top of the culvert is buried underneath it. Furthermore, very often, a culvert is

designed based on full bore whereas a bridge is normally designed with some headroom clearanceeither for boats or for floating debris. A culvert can be flexible such as corrugated steel pipes, or

rigid made of concrete, either precast or cast-in-situ, cast iron or vitrified clay. The shape of a

culvert can be rectangular, circular, elliptical or arch. In Hong Kong, the minimum internal size of

a concrete box culvert is 2.5 m by 2.5 m to facilitate the use of mechanical plant for maintenance.

Proper location is a prime prerequisite to the efficient and economical operation of a culvert in

order to keep the culvert sediment-free. A culvert is simply an enclosed channel which serves to

carry an open stream under a highway. If it is to be an efficient substitute for the open-ditch

section it must be placed so that the water has both a direct entrance and a direct exit. Thus a

culvert should be aligned as closely to the original stream channel as possible. If the stream

meanders and/or its location in the natural channel would require an inordinately long culvert,

some stream modification may be necessary.

New ditch

Ex. channel

Flow

VARIOUS METHODS OF LOCATING A CULVERT

The slope of a culvert should normally conform as closely as possible to the natural grade of the

stream which is usually the one which produces least silting or scouring. If the slope of the

culvert is greater than the natural slope of the stream, the increased velocity may cause scouring

of the stream at the outlet to the culvert. If the slope of the culvert is flatter than that of the

stream, silting is expected to occur and the culvert will eventually be blocked. On the other hand,

the silt carrying capacity of a stream varies as the square of its velocity. It is generally considered

S an Diameter S an Span

RiseRise

Retangular Circular Elliptical Arch

Rise


22/25



that culverts should be placed at a minimum slope of about 0.5% if significant sedimentation is to

be avoided. Culvert slopes can be used to arrest stream degradation, improve hydraulic

performance and reduce the length of the structure.

Change gradient

Paved

Depressed inlet

Head cut

Channel excavation

Degrading channel

Stream location

POSSIBLE CULVERT PROFILES


23/25



SUB-SOIL DRAINAGE

In spite of the main focus of this chapter on the road surface drainage, a brief discussion is given

here on sub-soil drainage which forms an integral part of the highway drainage provision.Sub-soil drainage deals with the drainage of water in the pavement structure underneath the

pavement surface. Subsurface or subsoil drains are required to intercept and drain excessive

moisture or groundwater flow in order to avoid premature pavement failures. These moisture

can be of the following forms:

Water that has permeated through cracks and joints in the pavement structure to the

underlying strata.

Water that has moved upward through the underlying soil strata as a result of capillary

action.

Water that exists in the natural ground below the water table, usually referred to as

ground water.


24/25



SUB-SOIL DRAINAGE SYSTEM

The function of a sub-soil drainage system is to collect and discharge water which may enter the

pavement structure through the surface course, surface cracks, granular shoulders or from the

subgrade. Sub-soil drainage prevents the build-up of moisture which could adversely affect the

strength and stability of the granular layers and subgrade. A sub-soil drainage system may

include sub-soil drains, open-graded drainage layer, and other pre-manufactured drainage systems

placed under a roadway to collect, remove and carry the water to the stormwater drainage system.

Subsurface drainage systems are usually classified into five general categories:

Longitudinal drains

Transverse drains

Horizontal drains

Drainage blankets

Well systems

The design of pavement is based on the certain moisture content of the soil in the field. If the

moisture content exceeds this amount, then the design conditions no longer apply and the

pavement may fail. Therefore, it is necessary to ensure that water is kept out of the pavement or

that if water enters the pavement, it is removed as safely and quickly as possible. An alternative

to this approach is to construct a pavement that can withstand the traffic load with excess water

pressure in the soil. This would be very expensive and as it is difficult to predict the stresses

developed in a pavement when water is present, the pavement so constructed may not be

adequate when subject to continuous traffic loads.

A Drainage layer (blanket)is a layer of highly permeable granular material which is placed beneath

the pavement structure where a road is constructed over spring or groundwater discharge area.

The blanket drain is sloped towards a ditch or subdrains installed at the edge of the road to

provide a positive outlet. Clear stone is used in this application. Most drainage blankets should

be sandwiched between geotextile to prevent (i) subgrade fines from moving upwards into the

blanket and (ii) subbase fines from moving downwards into the blanket. A herring bone or grid

pattern of subdrains achieves the same objective although the blanket drain provides more

uniform coverage and drainage capability.

Aparallel drainsystem consists of perforated or slotted pipes surrounded by aggregate placed in a

grid or herringbone pattern on the slope face. Alternatively, open-channels are used instead.

This is used to carry the water from the surface of the slope to prevent surface erosion.

Horizontal drainsare gravity draining perforated or slotted pipes wrapped in geotextile installed

into the face of a slope in order to lower the ground water table or to drain water from bearing

layers. The drains extend into the slope in a horizontal direction and can achieve significantly

greater lowering of the groundwater table when comparing with the other methods. They are

generally not successful in clay soils because of the low soil permeability. Where significant flow

from the drain is anticipated, soil which is exposed at the outlet of the drain should be protectedagainst erosion.


25/25


REFERENCES

Essential text

1.

Highways Department, Government of HKSAR. (2010). Guidance Notes on Road PavementDrainage Design RD/GN035 May 2010. Available from (last access on 7 August 2014)

http://www.hyd.gov.hk/en/publications_and_publicity/publications/technical_document/guid

ance_notes/

Reference texts

2. Drainage Services Department, Government of HKSAR. (2013). Stormwater Drainage

Manual with Eurocodes, 4th

Ed, May 2013. Available from (last access on 15 August 2014)

http://www.dsd.gov.hk/EN/Technical_Manuals/Technical_Manuals/index.html

3. Bransby Williams, G. (1922). Flood Discharge and the Dimensions of Spillways in India.

The Engineer, Vol. 121, September 1922, London, pp. 321-322.

4. Ginn, W.L., Lee, T.C., Chan, K.Y. (2010). Past and future changes in the climate of Hong Kong.

Acta Meteorological Sinica, Chinese Meteorological Society, 24(2), pp 163-175.

5. Lam, C.C., Leung, Y.K. (1994). Extreme Rainfall Statistics and Design Rainstorm Profiles at

Selected Locations in Hong Kong (Technical Note No. 86). Royal Observatory, Hong Kong, 89 p.

6. Tang, C.S.C., Cheung, S.P.Y. (2011). Frequency Analysis of Extreme Rainfall Values (GEO

Report No. 261). Geotechnical Engineering Office, 212p.

7. Wong, M.C., Mok, H.Y. (2009). Trends in Hong Kong Climate Parameters Relevant to

Engineering Design. The Hong Kong Institution of Engineers Civil Engineering Conference

2009 (in CD-ROM).

highway drainage

Documents