Highway Drainage A Practical Guide Paul Jones Lewes Consult Introduction 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. Index 1 The Basics. 2 Types of Road 3 Why remove rainwater from the carriageway? 4 Design Considerations 5 Transferring rainwater to storage/containment 6 Environmental Mitigations and Pollution control 7 Flow control 8 Discharge consents 9 Culvert design 10 Design standards and legislation 11 Climate Change and Highway Design 12 SuDS Guidance 13 The use of specialist design software 14 Asset Management/Inspection schedules Appendices 1 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 rain. 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 read) 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, experiments and formulaic 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……. 2 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. 2.1 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 surface, signage and traffic monitoring all require a team of 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 performance. 2.2 Rural Roads At the opposite extreme from the motorway, the rural road is usually a patchwork of field boundaries strung together to form a route. These are the responsibility of the local authority who usually 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. 2.3 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. 3 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. 3.1 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 Appendices. • 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 them. 4 Design Considerations 4.1 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. 4.2 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. 4.3 Highways in Cutting Where the carriageway is to be built below the natural site contour the scheme is described as being “in cutting”. The catchment area 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! 4.4 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. 5 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: 5.1 Balancing pond - Balancing ponds are designed in accordance with suitable design standards depending on the location, ownership and where they discharge to 5.2 Local stream or watercourse – small flows can typically discharge to whatever is available if no increase in flow rate is anticipated 5.3 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. 5.4 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 goes hand-in-hand with the need for flow control (see below) where the combination work together as part of the overall drainage solution. 6 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 etc. 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). 6.1 Full interceptor (BS EN 858-2:2003) These are used where pollution risk is considered large or where the ramifications of a pollution incident are considered severe. (major junctions, petrol stations, vehicle washing areas) 6.2 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. 6.3 Silt removal These devices are used increasingly for pollution control since about 80% of hydrocarbon pollutants stick to silt particles. Thus, removing these particulates 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. 6.4 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. 1 http://www.environment-agency.gov.uk/ 6.5 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…… 7 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 etc. 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 point. 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 difference. The same storm will produce a very different response depending on whether it lands on a field or a road. So, flow control in the context of highway drainage design will 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:- 2 Extracted from Urban Drainage, Third Edition - David Butler & John Davies 7.1 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 rises. 7.2 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. 7.3 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. 8 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:- 8.1 Environment Agency The generally EA manages main watercourses and can thus set maximum flow rates and volumes for the majority of schemes in the UK. Likewise, where scheme cross floodplains, measures to mitigate the loss in storage volume must be made upstream by extending what’s there. 8.2 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). 8.3 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. 8.4 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). 9 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 watercourse that shaped 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. 10 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 Maintenance 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 10.1 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. 11 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 assessment4. Changes to winter and summer rainfall patterns (HA Climate Change Risk Assessment) indicate more extreme rainfall patterns. 3 4 The revitalised FSR/FEH rainfall-run-off Method – Supplementary Report No 1, Thomas, Rodding & Kjeldson Highways Agency - Climate Change Risk Assessment - August 2011 12 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!!!!! 13 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. 13.1 WinDES – 13.2 FEH – Flood Estimation Handbook – CEH Institute of Hydrology, Wallingford. The manuals are convoluted the CD’s less so. 13.3 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!) 14 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. 14.1 HADDMS • 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. 14.2 EXOR – the Highways Agency and local authorities both use this database to record surface-based highway assets and schedule maintenance/inspections. Typically, EXOR ignores assets underground which may go some way to explain our current predicament! 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 location. WRAP SOIL (%) 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 proportions 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!