Uploaded by bpmcbean

Highway Drainage - A Basic Guide

Highway Drainage
A Practical Guide
Paul Jones
Lewes Consult
Highway drainage is a funny subject! Deceptively simple on the face of it
although like a lot of things, the devil is in the detail.
Since highway drainage runs from the start to the end, it impacts on almost
every aspect of the highway scheme itself, from what it will ultimately look like
to how much land-take is required, from what pollution control is required to
options for the pavement construction. Highway drainage takes up about 810% of the scheme capital cost but about 20% of the design fees due to all
the stakeholders it engages with and the complexity of each element.
Despite advances in engineering technology such as remote working, CAD
and GIS systems there is as great a need today as ever for engineers to pass
on their experience to others, especially through the use of practical examples
from the real world. I have been involved in many highway drainage schemes
during my career most recently the alterations to the A27/Lewes Road next to
the Amex football stadium and have worked as a consultant for Atkins, Motts
and Amey. It is my pleasure to pass on what I’ve learnt during my career and
hopefully convert at least one of you to the wonderful, exciting world of
highway drainage (!)…..
This Guide has been designed to discuss elemental solutions which, if
combined, can produce a holistic solution to highway drainage design.
The Basics.
Types of Road
Why remove rainwater from the carriageway?
Design Considerations
Transferring rainwater to storage/containment
Environmental Mitigations and Pollution control
Flow control
Discharge consents
Culvert design
Design standards and legislation
Climate Change and Highway Design
SuDS Guidance
The use of specialist design software
Asset Management/Inspection schedules
The Basics
Highway drainage design is the umbrella term used to describe the
steps involved in producing a coherent and consistent answer to the
need to drain both the carriageway and the surrounding catchment of
In only 1 hour, I cannot expect to cover this subject in anything other
than the most superficial level but I hope to give you a taster of the
scope and breath of the subject and how it impacts on other discipline,
both at the time of initial design and later on when inspections and
maintenance are required.
The likely format of this lecture will be:•
Run-through of this document (which, hopefully, you will have
A made up highway scheme to look at the design elements
Tutorial on any specific subjects of interest
Q & A on highway drainage issues
Hydraulics in Civil and Environmental
Engineering (written by two Brighton
lecturers) has been the mainstay of
water engineers for years (I still rely
on the 2nd Ed.)
While the subject of hydraulics is
steeped in academia, with research,
expressions to rival astrophysics (!)
the application is far more pragmatic.
Context-specific pragmatism is far
more relevant that engineering
perfection. In the real world, drainage
assets get driven over, ignored
maintenance-wise, experimented on
by junior engineers and the like.
Issues surrounding site safety and
structural integrity tend to win over
fragile or self-destructing designs.
What I’m saying here is really while the principles of hydraulics are
obviously really important to understand they must be balanced by the
need to provide a serviceable solution. It is with the above in mind that
I will be exploring aspects of Highway Drainage…….
Types of road
Not all roads are created equally. Expectations of both performance
and maintenance standards differ wildly with not just the public but also
with designers.
Major Highways (Motorways, trunk roads, etc)
Motorways are designed
to the Highway Agency’s
DMRB Standards which
include everything from
pollution risk to visibility
standards for vehicles
travelling at high speed.
Super-elevation of the
designers working closely
together to fit everything in.
Reactive, seasonal and planned maintenance is undertaken by Area
Teams (we are in Area 4 here) on a large scale using budgets derived
from national coffers. Contracts to manage each area are typically
tendered on a 5-year cycle, usually with 2-year extensions for good
Rural Roads
At the opposite extreme
from the motorway, the
rural road is usually a
boundaries strung together
to form a route.
responsibility of the local
manages ditch cleaning,
culverts and the like on an ad-hoc basis together with pot-hole repairs
and resurfacing when the original finally falls apart. Flooding is usually
the result of field run-off from unmanaged or nonexistent ditches or
unloved gulleys and pipework that don’t get cleaned out due to budget
cuts or “savings”! A common theme of most drainage engineers is the
lack of maintenance their output gets once in the ground.
The current epidemic of pot-holes following last winter’s heavy rain
should remind us of the importance of effective drainage on roads.
However, this is rarely the case. The year before it was frost heave.
Hey ho.
Roads in urban areas
Apart from new roads in residential
developments, roads in urban areas
have usually been there for some time
so from a drainage perspective the
focus is on maintenance and
upgrading existing systems as
opposed to anything new.
Recent trends in traffic management,
including elevating the carriageway to
the level of the kerbs, “shared
surfaces” and pedestrianisation of
town centres has had the unfortunate
repercussion of removing the only
barrier between rainfall run-off and
shops! In the recent past, Brighton,
Eastbourne, Uckfield and Lewes have
all suffered from flooding following high intensity/short duration or
“flash” storms and this phenomenon is only likely to increase due to
climate change.
Why remove rainwater from the carriageway?
Water has a number of unhelpful characteristics which impact on
highway performance.
It is a lubricant reducing the effectiveness of tyre grip on the
carriageway wearing surface which can increase stopping distances.
Spray from rainwater being thrown up by car tyres can reduce visibility
which can lead to delays in reacting to events on the carriageway.
Drag on car tyres from local rainwater ponding can alter the balance of
vehicles travelling at speed which can be alarming or cause skidding.
It is incompressible therefore standing water effectively acts as a jackhammer on the wearing course right through to the sub-base when
vehicles pass over head.
It expands when frozen pulling apart the carriageway construction
which then falls apart when it warms up
In extreme storms, rainwater can simply wash away roads on
embankment should the culvert become blocked or lack capacity.
All of the above simply highlight the need to remove rainfall from
carriageways as a matter of course.
Edge Drainage Assets
Mechanisms to remove rainfall from highway schemes are called edge
assets. Edge assets can include any/all of the following:•
Kerbs and gulleys with carrier drains
Gulleys and kerbs work together to transport and collect
rainfall run-off into the highway drainage system. Methods for
designing the spacing/locations are explored in the
Combined kerb drainage system (Beanie-block)
Proprietary drainage products which combine kerbs with a
flow and collection system. These products are very good
at collecting flows, especially in flat areas where a flowpath isn’t likely.
Drainage ditches
The historic method for flow collection, especially
where kerbs are not installed. Water simply runs off
the carriageway into the ditch. Management of these
is usually provided by the relevant local authority
highways team as are the connecting culverts,
headwalls etc.
French drains
Combined filter drains
These collect the run-off from the carriageway and the sub-base
together with any run-off from cutting embankments.
Grass swales
Each of these has advantages/disadvantages and typical suitabilities or
locations and each have design standards that can be used to design
Design Considerations
Horizontal Alignment
Modern roads typically are designed to have a cross-fall (that is, a
gradient from the centre to the edge) so that water can run off. When
the road reaches a bend, however, it tends to be super-elevated i.e. a
camber is added to help vehicles go round the bend. Here, the road
can either have an increased gradient towards the edge or actually go
the other way, that is towards the centre. I this case the collection and
drainage system need to be relocated to the centre. So far, so simple!
The problem lies at the “cross-over”; the stretch of road where the
carriageway is effectively flat. There is the risk that water will have
nowhere to go and simply lie across the carriageway. Fortunately, we
have one more tool, vertical alignment, to shift the water.
Vertical Alignment
Typically, highway schemes tend to be designed to balance out the
points of cut and fill and this is done by evening out the peaks and
troughs of the natural contours along its route. Ideally the low points
(LP) are being roughly coincident with those on the original site so the
highway discharge point will have somewhere to go. The high points
(HP) along the scheme define the edges of the individual catchments.
Highways in Cutting
Where the carriageway is to
be built below the natural site
contour the scheme is
described as being “in
increases to include the face
of the cutting and “hard strip”
and typically this run-off can
include a lot of silt so it’s usual to keep this flow separate from the
highway drainage although it will probably discharge at the same
location. It also means that the interceptors don’t get clogged up and
can be designed for just the hydrocarbon load based on the
carriageway area and traffic volumes.
Combined filter drains can typically accommodate both the highway
and the cutting as the grass top acts as a filter.
Depending on the original contours, another drain may be required to
intercept the flows at the top of the embankment. These drains are
called “cut-off” drains and, along with the drains at the top of
embankments are generally described as “secondary drainage”.
Some cuttings are deep enough to impact on the natural water-table. In
these cases, the stability of the entire carriageway structure will be
dependent on effective de-watering the entire area. How we do this is a
bit specialised for this course!
Highways on Embankments
Where the carriageway is to be built above the natural site contour the
scheme is described as being “on embankment”.
The catchment area will include not only the carriageway but also the
faces of the embankment and the “hard strip”; the last two typically
forming part of the “secondary drainage” system.
It is important to insure that the stability of the embankment is not
compromised by the choice of edge drainage. Combined filter drains
are difficult to construct at the top of large embankments as the
exposed “nib” can fall over during maintenance (that and the flows can
simply run down the bank rather than stay in the drain). It is more usual
to simply have carrier drains on embankments with kerbs and gulleys
which afford some control for vehicles.
Transferring rainwater to storage/containment
Once the rainfall runoff has been collected from the carriageway it
needs to be transported to one of the following:
Balancing pond - Balancing
ponds are designed in accordance
with suitable design standards
discharge to
Local stream or watercourse –
small flows can typically discharge
to whatever is available if no increase in flow rate is anticipated
Storage Tank – the use of oversized pipes, GRP or concrete tanks or
even grassed areas dedicated to storm storage can be used depending on
the location, volumes, etc.
Soakaway – for areas where the underground soil type can allow
rainwater to percolate into the sub-strata, the use of soakways is a
useful mechanism to recharge groundwater levels subject to pollution
risk assessments and controls upstream.
The need for storage
with the need for flow
control (see below)
together as part of the
Environmental Mitigations and Pollution control
While rainfall is typically
considered clean water in the
UK for the purposes of
pollution control, when it hits
highway areas and cars it
quickly becomes loaded with
any/all of the following:•
Hydrocarbons – from leaking fuels (diesel, petrol, etc) hydraulic fluids
Brake dusts – especially on junctions or roundabouts where heavy
deceleration takes place
Detritus from highway usage – rubbish, old food, dead loved-ones etc.
Road Traffic Accidents – from fire-foams, liquids from radiators,
breaking systems, pathogens, etc.
It is considered that the risk associated with any/all of the above is
proportional to traffic flows (including those generated from the
proposal itself) and the specific location involved.
Contemporary pollution control devices are split between full and bypass interceptors. The former contains 50mm of rainfall over the
catchment area while the latter contains up to 8mm on the basis that it
is this “first flush” which contains 90% of the pollution. (The rest is
considered diluted by the rest of the rainfall).
Full interceptor (BS EN 858-2:2003)
These are used where
pollution risk is considered
ramifications of a pollution
incident are considered
severe. (major junctions,
petrol stations, vehicle
washing areas)
By-pass interceptor
These are used where pollution risk is
considered low(ish) or where the
ramifications of a pollution incident are
considered nominal. (major roads
without junctions, car-parks, etc) only
the “first flush” is collected.
Silt removal
These devices are used increasingly for
pollution control since about 80% of
hydrocarbon pollutants stick to silt particles.
effectively reduces both BOD and COD
levels downstream.
As with all drainage assets, these devices
require cyclical maintenance and
inspections otherwise they quickly become
dilapidated and ineffective.
Environmental issues
The Environment Agency1 produces ground-water protection maps
where abstractions occur, called Aquifer Protection Zones
Principal Aquifers
These are layers of rock or drift deposits that have high intergranular and/or fracture
permeability - meaning they usually provide a high level of water storage. They may
support water supply and/or river base flow on a strategic scale. In most cases, principal
aquifers are aquifers previously designated as major aquifer.
Secondary Aquifers
These include a wide range of rock layers or drift deposits with an equally wide range of
water permeability and storage. Secondary aquifers are subdivided into two types:
Secondary A - permeable layers capable of supporting water supplies at a local rather
than strategic scale, and in some cases forming an important source of base flow to rivers.
These are generally aquifers formerly classified as minor aquifers
Secondary B - predominantly lower permeability layers which may store and yield limited
amounts of groundwater due to localised features such as fissures, thin permeable
horizons and weathering. These are generally the water-bearing parts of the former nonaquifers.
Secondary Undifferentiated - has been assigned in cases where it has not been possible
to attribute either category A or B to a rock type. In most cases, this means that the layer
in question has previously been designated as both minor and non-aquifer in different
locations due to the variable characteristics of the rock type
In these areas, the risk from highway pollutants would be a question of
judgement based on the traffic volumes, inherent risks (junctions and
the like), what’s there at the moment and/or any pollution mitigations
the scheme is proposing.
Understanding storm frequency
The concept of storm retunes of frequency is often fraught with danger
since hydraulic engineers seem to talk a different language to the rest
of the population!
The Wallingford procedure describes mechanisms for determining
rainfall severity based on location, soils type etc but the thing that
makes the biggest difference when looking at rainfall is return periods.
If you imagine all the storms that hit an area, say Brighton, then it
becomes apparent that the rally big ones happen less frequently that
the annoying small ones. If all these storms were place on a percentile
curve then it may look something like the one below (once it has been
“normalised” to fit a “bell-curve”).
From this graph, it becomes apparent that the 100-year events produce
a higher peak intensity than the 30-year storm. This is a simplified way
of describing what is a complex subject……
Flow control
When rainwater falls on fields several things happen:
The topsoil becomes wet, absorbing the first drops before ponding
locally around plant stems and low-spots etc.
Some soakage into the underlying soil occurs to feed roots
If the storm is severe enough, some run-off occurs on the surface
which can fill local ditches or follow the natural topography to streams
The cumulative affect of the storm profile and site conditions will
elongate the response thus reducing the peak flow rate.
All of the above take time which
impacts on the overall response
to any particular storm and is
describes as Green Field Runoff2.
By way of comparison, when
rainfall falls on carriageway, the
following occurs:•
The wearing course, being
relatively smooth, allows
the runoff to quickly enter
the drainage system where it is transported at speed to the discharge
There will be little attenuation and an extremely high peak runoff rate
for a short time (during which, the flows can be highly destructive!)
I’m sure you can see the
storm will produce a very
depending on whether it
lands on a field or a road.
So, flow control in the
typically seek to limit peak discharge rates to those of the equivalent
“Green Field” in order to reduce any risks of problems downstream
which will involve storage or containment on site.
Flow control devices typically take the form of:-
Extracted from Urban Drainage, Third Edition - David Butler & John Davies
Control weir – either Vee notch or flat
Weirs are useful to control flows, especially
where the range of flow depths is relatively
small. Vee notch weirs combine the need to
control flows during low flows as well as
during storms where the flow level obviously
Vortex control device – HydroBrake® or similar
The flow-depth relationship is key to most
hydraulic design but these clever devices use
the inertia from the flows to reduce the inherent
potential energy of the system to produce a
constant discharge within a controlled flow
depth span.
Orifice pipe – limiting flows by throttling down pipe diameter
Each of the above devices has advantages/disadvantage and each is
more suited to different locations. Critical to the design of flow control
devices is the relationship between flow depth and discharge rates.
Discharge consents
Along with flow control, it is usual to have to get discharge consent
from whoever owns/controls the watercourse the highway ultimately
discharges into.
This can be from several sources:-
Environment Agency
The generally EA manages main watercourses and can thus set
maximum flow rates and volumes for the majority of schemes in the
Likewise, where scheme cross floodplains, measures to mitigate the
loss in storage volume must be made upstream by extending what’s
Water Company
Drainage systems where roof drainage is a component of the overall
flows, i.e. from housing estates, commercial developments and the like,
can be adopted by Water Companies keen to attract new customers. In
this case, Sewers for Adoption (the new 7th Ed.) would be the design
principal used to satisfy their needs under Section 104 of the Water
Industry Act (1991).
Local Authority
Since 2012, local authorities have been given responsibility for
alterations to minor watercourses although this does not include flow
rates or pollution control measures. Their power is therefore limited to
managing ditch profiles and possible culvert issues.
Independent Drainage Board
Along with flow rates comes pollution levels and maybe specific
guidance regarding silt removal, “polishing” prior to discharge and/or
local improvements to the watercourse.
On large schemes it is usual to provide a mechanism to isolate the
highway drainage from the receiving watercourse (in the event of a
crash or similar).
Culvert design
Typically, highway schemes tend to even out the peaks and troughs of
the “vertical alignment” along its route with the “low points” being
roughly coincident with those on the original site.
Original site contours are typically a
function of the erosive action of the
landscape although many have dried
up or now run underground since
originally formed.
This means that it is usual to have to
provide culverts across valleys even
if they are “dry” so that the
embankment does not act as a dam.
Determining the maximum flow rate from a valley catchment can be
undertaken using many design principles; the DMRB, SIRIA and
FSR/FEH methodologies being the most common.
In any event, the practicalities of designing large pipes which cross
under carriageways are covered in the DMRB guidelines.
It is worth noting that large pipes can and often do come under the
auspices of the structures teams within the design scheme and they
deal with the headwalls, etc. In this case, we as drainage engineers
would really just specify levels, gradients and things like that.
Design standards and legislation
While the basic design of each element of highway drainage is covered
under DMRB standards, many also have their own special standards to
insure that they meet site-specific standards where not used for
Highways Agency schemes.
Design Manual for Roads and Bridges (DMRB) – Volume 4 – Geotechnics and Drainage –
Section 2 – Drainage (Parts 1-9)
Part 1
HA 78/96 – Design of Outfalls for Surface Water Channels
HA 39/98 – Edge of Pavement Details
HA 103/06 – Vegetative Treatment Systems for Highway Run-off
HA 106/04 – Drainage of Runoff from Natural Catchments
Part 2
TA 80/99 – Surface Drainage of Wide Carriageways
Part 3
HD 33/06 – Surface and Sub-surface Drainage Systems for Highways
HA 102/00 – Spacing of Road Gullies
HA 105/04 – Sumpless Gullies
Part 4
HA 79/97 – Edge pf Pavement Details for Porous Asphalt Surface Courses
HA 37/97 – Hydraulic Design of Road-Edge Surface Water Channels
HA 41/90 – Permeameter for Road Drainage Layers
HA 83/99 - Safety Aspects of Road Edge Drainage Features
HD 43/04 - Drainage Data Management System for Highways Agency
HA 217/08 - Alternative Filter Media and Surface Stabilisation Techniques for Combined Surface and
Sub-Surface Drains
HA 219/09 - Determination of Pipe Roughness and Assessment of Sediment Deposition to Aid
Pipeline Design
Part 5
HA 40/01 - Determination of Pipe Roughness and Assessment of Sediment Deposition to Aid
Pipeline Design
HA 104/09 - Chamber Tops and Gully Tops for Road Drainage and Services: Installation and
Part 6
HA 113/05 - Combined Channel and Pipe System for Surface Water Drainage
Part 7
HA 107/04 - Design of Outfall and Culvert Details
Part 8
HA 118/06 – Design of Soakaways
Part 9
HA119/06 - Grassed Surface Water Channels for Highway Runoff
CIRIA manuals are available for SuDS schemes, soakaways, pollution
The SUDS manual (C697)
Site handbook for the construction of SUDS (C698)
control devices and the like.
Other sources of design guidance (residential developments)
• BRE Digest 365 – Soakaway Design
• TTRL papers and guidance
• Local Authority planning guidance and Design Manuals
• Water Company guidance
• Housing development guidance
Storm returns
Highway drainage differs from other drainage design (i.e. housing
estates) insofar as the key performance indicators used reflect the
need to protect the carriageway construction as well as flood
prevention. DMRB describes these in details- suffice to say here, the
sub-base must remain drained to prevent rainwater forcing itself back
through the construction during heavy storms.
Climate Change and Highway Design
Highway drainage design follows DMRB standards which incorporate
Climate Change for extreme storms, storage requirements etc3.
Issues such as SuDS play their part, more from a pollution control and
landscaping standpoint than the realities of discharge rates and the
safety element of highway drainage is always paramount to any
environmental considerations (especially on major highways).
Ex Flood Estimation Handbook - Revitalised Flood Hydrograph (ReFH) Method
Climate change affects Total flows insofar as both the rainfall pattern
and the antecedent conditions on the ground become altered.
Development and Flood Risk states that peak rainfall
intensity is likely to increase by 30% by 2085 - Planning Policy
Statement 25
The Stern Review (2007) – Highlighted climate change as a serious
and urgent issue, in need of urgent action.
The Climate Change Act (2008) – widely regarded as providing a
robust framework for climate change adaptation. Although the HA is
not obliged to report under this act, as a major custodian of national
infrastructure it volunteers results to the UK climate change risk
Changes to winter and
summer rainfall
patterns (HA Climate
Change Risk Assessment)
indicate more extreme
rainfall patterns.
The revitalised FSR/FEH rainfall-run-off Method – Supplementary Report No 1, Thomas, Rodding & Kjeldson
Highways Agency - Climate Change Risk Assessment - August 2011
SuDS Guidance
A recent trend in highway schemes is to incorporate flow management,
pollution control and collection into a more organic solution which is
visually less intrusive than traditional separate methods.
While attractive these schemes are not without issues, the major one
being the costs associated with maintenance and asset management
against a backdrop of serviceability, vandalism and pressures on small
sites to provide adequate parking/recreational space.
SuDS schemes are often considered more landscape gardening than
robust engineering solution by many and the complexity of their design
elements which often needs fine tuning to balance the various flows
can be a hindrance to their popularity. They appear to have little
resilience to extreme storms and are to a large extent dependent on
site conditions/soil types, etc. to work.
Many SuDS solutions simply do not work in unsympathetic soil types
(clays) and in areas where the discharge point is a main watercourse
has little benefit over traditional drainage methods.
Mitigating highway or car-park schemes using SuDS has been likened
to putting lipstick on a pig,
“If you put lipstick on a pig you just end up with a pig wearing lipstick”
it doesn’t really matter what colour it is or how much you apply!!!!!
The use of specialist design software
While highway drainage schemes are relatively straightforward to
model when compared to things like housing estates or large urban
areas they do require computer modelling unless extremely small.
This is done to calculate peak flow rates, storage volumes and flow
control devices and are based on further calculations done using
computers to calculate peak “green field” run-offs.
Most if not all of the above now incorporate CAD plans into the
drainage design for locations of gulleys, inspection chambers, pipes,
headwalls and the like.
WinDES –
FEH – Flood Estimation Handbook – CEH Institute of Hydrology,
Wallingford. The manuals are convoluted the CD’s less so.
PMS – Another industry-popular software package which
incorporates both the highway alignment strings and drainage levels
etc. Thus, changes to the design feed through to the hydraulics
automatically. (not necessarily a good thing!)
Asset Management/Inspection schedules
It is a fact of professional life that drainage
systems tend to get ignored once built with
Councils or the HA happy to spend money
on other priorities while the road gets eaten
away from below!
Many of the flooding problems associated with even reasonable levels
of rainfall are the direct consequence of ineffective management of the
highway drainage assets.
Many of the mechanisms for accelerated pavement deterioration can
be directly laid at the door of ineffective management of the highway
drainage assets.
HAGDMS/HADDMS new functionality - 16th March 2012
The HAGDMS and HADDMS team are pleased to announce the following additions and changes to the
system. These items will be formally launched with documentation, webinars etc shortly but are
now available for use by Service Providers.
Geotechnical Incident Reporting - allows input of incidents associated with GAD observations,
input of information such as impact on the carriageway, and one-off and ongoing costs associated
with the incident. Existing GAD Editors have access to add and edit incidents and their costs,
and MAGLEs will see a new box on the main screen listing any "Preliminary" status incidents in
their Area(s) for approval.
Spills Register - allows locations and details of spillage incidents to be entered. This operates in
a similar way to the Flood Events Register. Each spill is assessed against the criteria in HD45 and
a Spill Severity Index is calculated. Existing Drainage Editors are able to add and edit spill details,
and these can be "closed" by the DLE once all required information has been entered.
The Flood Severity Index calculation methodology no longer takes into account the time of day
a flood occurred. The new definition is available for registered users to download from the
Downloads page. FSIs for all existing floods have been recalculated, which will mean that the FSI
for some flood events has increased. National FSI percentile values will be recalculated for the
March monthly drainage report, available in early April - please contact support if you require
these sooner.
New EA map layers have been added showing river catchment extents and waterbodies defined
under the Water Framework Directive. Waterbodies are shown with their current overall status
(indicated by the colour) and overall objective (displayed in a tooltip together with other details).
A number of minor improvements have also been carried out across other areas of the system to address
various user-reported issues.
EXOR – the Highways Agency and local authorities both use this
database to record surface-based highway assets and schedule
Typically, EXOR ignores assets
underground which may go some way to explain our current
Appendix A – The Wallingford Procedure
Wallingford Procedure (Wallingford)
The Wallingford Procedure runoff method (Wallingford) is a non-linear reservoir routing
method. This method can be simulated with any runoff time step though using the
appropriate time step is essential for simulating the correct reservoir routing. The
percentage of runoff is fixed throughout the storm. The Wallingford Procedure runoff
method routing equation is shown below:
Routing Coefficient (KR)
The Variable PR runoff method routing coefficient (KR) is typically input as a value 0.5 or
less when used in rural catchments.
Antecedent Wetness Condition (UCWI)
The Antecedent Wetness Condition (Urban Catchment Wetness Index - UCWI), which is a
composite of two antecedent wetness parameters and is given by:
UCWI = 125 +8API5 – SMD
Where API5" = 5-day antecedent precipitation index (mm)
SMD = soil moisture deficit (mm)
Alternatively, design values are provided by referring to average annual rainfall for a given
The WRAP SOIL percentages define the index of the water holding capacity of the soil
(0.15 tot 0.5), based on the FSR WRAP parameter obtained from FSR. The formula for
calculating the WRAP Soil index is shown below:
WRAP SOIL Index = (Soil 1 % * 0.15) + (Soil 2 % * 0.3) + (Soil 3 % * 0.4) + (Soil 4 % *
0.45) + (Soil 5 % * 0.5).
There are five possible WRAP SOIL percentages which can be input to define the subcatchment soil conditions. The WRAP SOIL percentages are input as fractions and the
sum of which must equal 100% (i.e.: Soil 1 % + Soil 2 %...+ Soil 5 % = 1.0). The W.R.A.P.
soil classifications are defined as follows:
1 (i) Well drained permeable sandy or loamy soils and shallower analogues over highly
permeable limestone, chalk, sandstone or related drifts.
(ii) Earthy peat soils drained by dikes and pumps
(iii) Less permeable loamy over clayey soils on plateaux adjacent to very permeable
soils in valleys.
2 (i) Very permeable soils with shallow ground-water
(ii) Permeable soils over rock or fragipan, commonly on slopes in western Britain
associated with smaller areas of less permeable wet soils
(iii) Moderately permeable soils, some with slowly permeable subsoils
3 (i) Relatively impermeable soils in boulder and sedimentary clays, and in alluvium,
especially in eastern England
(ii) Permeable soils with shallow ground-water in low lying areas
(iii) Mixed areas of permeable and impermeable soils, in approximately equal
4 Clayey, or loamy over clayey soils with an impermeable layer at shallow depth
5 Soils of the wet uplands, (i) with peaty or humose surface horizons and impermeable
layers at shallow depth, (ii) deep raw peat associated with gentle upland slopes or
basin sites, (iii) bare rock cliffs and screes and (iv) shallow, permeable rocky soils
on steep slopes
It is noted that while using the Wallingford Procedure runoff method the following subcatchment slope values are recommended:
Recommended Sub-Catchment Slope Values:
1.25% - mild slope
2.75% - medium slope
4.0% - steep slope
Appendix B – The Modified Rational Method
The Modified Rational method
This method was developed by H R Wallingford, and uses four hydrological constants to
determine rainfall intensity. These are:•
SAAR - the Standard Average Annual Rainfall, (in mm/yr);
M5-60 - The rainfall intensity sixty minutes into a five year storm;
r - the ratio between the M5-60 and the M5-2day storm intensity;
Soil - the rainfall acceptance potential of the soil
Each constant has fixed values for a specific location, and these are obtained from
hydrological maps.
Masterdrain includes images of these maps, in National Grid squares, by kind permission of
H R Wallingford. The program also contains a database of over 4000 UK locations with the
constants already extracted from the maps.
This method of calculating rainfall is an essential part of the program. A fuller explanation of
this method can be obtained from Vol. 1 of the 'Wallingford Procedure'.
Appendix C – Typical gulley design to HA102/00 (example)
Appendix D – Legislative framework for Highway Drainage
Highways Act (1980) – Part V, Sections 100-105 “Drainage of Highways”
Land Drainage Act (1991) – Schedule 4 “Schemes for small drainage works”
Planning Act (2008) – Part 3, Section 22 “Highways”, Sections 27/28 “Water” and
Section 29 “Wastewater”
Water Framework Directive EU – (2000)
Flood and Water Management Act (2010) – Schedule 3 , “SuDS”
Water Resources Act (1991)
Water Industry Act (1991) – Section 115 “Highway drainage and sewers”
Environment Act (1995)
New Roads and Street Works Act (1991) – Sections 89 & 148 “Sewers”
There are many more!