Roadway water management

Transcription

1 Technical guide Roadway water management October 2006 Translate August 2007

2 The Technical Department for Transport, Roads and Bridges Engineering and Road Safety (Service d'études techniques des routes et autoroutes - Sétra) is a technical department within the Ministry of Transport and Infrastructure. Its field of activities is the road, the transportation and the engineering structures. The Sétra supports the public owner The Sétra supplies State agencies and local communities (counties, large cities and urban communities) with informations, methodologies and tools suited to the specificities of the networks in order to: improve the projects quality; help with the asset management; define, apply and evaluate the public policies; guarantee the coherence of the road network and state of the art; put forward the public interests, in particular within the framework of European standardization; bring an expertise on complex projects. The Sétra, producer of the state of the art Within a very large scale, beyond the road and engineering structures, in the field of transport, intermodality, sustainable development, the Sétra: takes into account the needs of project owners and prime contractors, managers and operators; fosters the exchanges of experience; evaluates technical progress and the scientific results; develops knowledge and good practices through technical guides, softwares; contributes to the training and information of the technical community. The Sétra, a work in partnership The Sétra associates all the players of the French road community to its action: operational services; research organizations; Scientific and Technical Network (Réseau Scientifique et Technique de l'equipement RST), in particular the Public Works Regional Engineering Offices (Centres d'études techniques de l'equipement CETE), companies and professional organizations; motorway concessionary operators; other organizations such as French Rail Network Company (Réseau Ferré de France RFF) and French Waterways Network (Voies Navigables de France - VNF); Departments like the department for Ecology and Sustainable Development The Sétra regularly exchanges its experience and projects with its foreign counterparts, through bilateral co-operations, presentations in conferences and congresses, by welcoming delegations, through missions and expertises in other countries. It takes part in the European standardization commissions and many authorities and international working groups. The Sétra is an organization for technical approval, as an EOTA member (European Organisation for Technical Approvals).

3 Technical guide Roadway water management This document is the translation of the work "Assainissement routier" published in october 2006 under the reference 0632.

4 This technical guide on road improvements has been prepared by a working group set up by representatives of the Scientific and Technical Network of the Ministry of Infrastructure and consulting engineers. It was technically validated by (Sétra). Members of the working group: David Gaillard (Sétra), J. Ranchet (DREIF - LREP), Jean Béréterbide (CETE Sud-Ouest), Marc Valin (CETE Nord-Picardie), Jacques Hurtevent (CETE Méditerranée), Alain Costille (DDE (District-level Offices for Infrastructure) 95), Gilles Cartoux (DDE 58), René Marcaud (Sté SILENE), Alain Limandat (SEEE). Editing and layout were carried out by: Marie Odile Cavaillès (Sétra). Serge Criscione (DREIF - LREP), Jacques Hurtevent (CETE Méditerranée), Marc Valin (CETE Nord-Picardie), Bruno Van-Hauwaert (CETE Nord-Picardie) prepared the diagrams. On reading the guide the glossary, in appendix 4.3 gives the definitions of main special terms (marked with *) used; the abbreviations and symbols encountered in the text are clarified in appendix 4.4; bibliographic references: in the text, the numbers in brackets [ ] correspond to the documents listed in the bibliography in appendix 4.6. Collection «Les outils» Sétra 4 September 2007

5 Contents The Sétra supports the public owner...2 The Sétra, producer of the state of the art...2 The Sétra, a work in partnership...2 Contents... 5 Preamble Technical design of structures Re-establishment of natural flows (catchment areas < 100 km 2 ) General principles Determination of the project flow Design of hydraulic structures Maintenance and operation of the hydraulic structures Surface drainage of the platform General principles Nature and function of systems Choice of drainage structures and hydraulic design calculation Maintenance and operation of the structures [11] Road structure drainage Definition Who does what? Summary of the effects of water on the road Controlling road pollution Definition Who does what? Summary of risks and challenges Sequence of studies Preliminary studies Hydraulics* Protection of Water Resources Outline Preliminary Project (APS) studies Choice of scales for the plans of the routes studied Road surface drainage at the Outline Preliminary Project (APS) study stage Project studies Height setting constraints Validation of the general principles Water Law Dossiers (DLE) or water police [police de l eau] dossier At the project study stage At the Outline Preliminary Project (APS) level Study quality approach Notions of process and progression, of inputs, outputs and tasks Process Progression Required "inputs" and expected "outputs" Main tasks Issuing an order General principles...41 Collection «Les outils» Sétra 5 September 2007

6 Progression of studies Traceability of choices/decisions, archives Validating the production General technical appendices General elements of hydrology Rainfall data Numerical example of application for the calculation of a project flow of a natural catchment area Elements of general hydraulics On the theory of flows ABAC design charts for small hydraulic structures for re-establishing natural flows Constructional arrangements and protection of hydraulic structures Dimensioning of a hydraulic structure for re-establishing natural flows - application example Surface drainage of the platform - calculation method Surface drainage of the platform - hydraulic calculations - application examples Glossary Abbreviations and symbols Abbreviations (French) Symbols Table summarizing principle formulae Bibliography (non-exhaustive list) Technical documents: Regulatory texts: For information Collection «Les outils» Sétra 6 September 2007

7 Preamble Road construction hydrology covers the re-establishment of natural flows, the drainage of roadways, drainage and the control of road pollution. The recommendation on road improvements of 1982 dealt essentially with the first two fields. The collection of guides "Water and road construction" deals with the protection of water resources and aquatic environments in the context of road infrastructure. A working group made up of hydraulic experts was set up by Sétra to collect up-to-date knowledge in the field of hydrology and take account of environmental protection-related impacts. This work took shape in a guide in three main sections: the technical design of structures; studies; the quality procedure. General technical appendices give calculation examples and ABAC calculation charts to be used. It was devised to meet the needs and expectations of prime contractors. Its primary purpose is as a tool that assists in the design of drainage structures for new road projects and improvement planning studies on existing roads. This guide puts forward a methodological approach to the technical design of structures to re-establish natural run-offs, drain the platform and drain off road use generated pollution. It can also provide assistance in drawing up an improvement project and in applying the quality approach at the study level. In must be noted that this document deals only with the re-establishment of natural flows in small catchment areas (catchment area smaller than 100 square kilometers or so). For larger catchment areas or where there are specific hydraulic problems, a specialist must be consulted. It is for the project designer to co-ordinate the various aspects to be taken into account in the design of structures (road safety, signage, multi-functional structures etc.). The present document includes the aspects of maintenance, operation and management of structures at the project design stage. The chapters dealing with internal drainage and road pollution are summarized in the present document, as these topics are the subject of specific guides, one on drainage and the other on pollution management, to be published by Sétra at the same time as this one. Collection «Les outils» Sétra 7 September 2007

8 1 - Technical design of structures Roadway water management covers the following aspects: the re-establishment of natural flows, the collection and evacuation of surface water within the footprint of the road, the collection and evacuation of internal water i.e. internal drainage, the management of road pollution Re-establishment of natural flows (catchment areas < 100 km 2 ) The re-establishment of natural flows consists in ensuring the continuity of surface run-off in catchment areas through which the road passes. This re-establishment must be commensurate with local risks, conditions and requirements (flooding, erosion or sedimentation, durability of the infrastructure, safety of users and respect for the aquatic environment), which should be identified, and must be designed in accordance with the regulations in force. The road can present an obstacle to natural water flows and, conversely, these can cause damage to the road (see diagram No. 1). The hydraulic structures re-establishing natural flows must therefore be correctly sized to limit the risks of flooding and submersion of or damage to the road within acceptable limits, of flooding upstream of the road, of breaches in the road structure. We can distinguish three cases of interaction between a water course and the road: a part of the road line encroaches on the flood plane or high water channel of a significant water course; a specific study is required, which goes beyond the scope of the present document, the road line crosses a water course that is significant or poses specific hydraulic problems; here, too, a study by a specialist is required, the road line crosses a water course with a catchment area not exceeding some hundred square kilometers, with no particular challenges, which is the subject of the present chapter; above this limit, the study requires the intervention of specialists in hydrology, hydraulics and hydrogeomorphology. Initially the water concentration point was downstream of point A The realization of the infrastructure displaced this point upstream of point B Cours d'eau = water course Ouvrage hydraulique de rétablissement=hydraulic structure re-establishing flow Diagram No.1: displacement of the point of concentration of run-off Collection «Les outils» Sétra 8 September 2007

9 General principles The hydraulic re-establishment of natural flows is one of the greatest constraints on road projects, especially on the longitudinal section. Consequently, particular attention should be paid to it at the pilot project stage. Apart from the regulatory aspect, which requires checking, the various stages in determining the hydraulic structure to be installed are: the estimation of the project flow as a function of a recurrence interval and an exceptional flow, the dimensioning, selection and setting of the hydraulic structure (checks of upstream water level, flow speeds, free space, hydrological impact and, where required, the free movement of the icthyofauna). Choice of the recurrence interval (T).The recurrence interval, T, to be taken into account must, in each case, be the subject of an analysis setting the infrastructure investment costs against the consequences of an overflow for users, landowners adjacent to the road and the water course, road structures (local and temporary traffic perturbations and risk situations) and, finally, on the natural environment. In all cases, knowledge of the regulations and consultation with the water authorities (water police and the Mission Inter-Service de l Eau [Inter-department Water Mission] (MISE)) will be necessary. In the absence of this type of analysis, it is recommended to adopt the following values for recurrence intervals: under motorways: 100 years, under roads or restored communication links: 100 years, 50 years or even 25 years for catchment areas where floods are limited in time and subject to a low or zero incidence of overflow, depending on the case, roads and motorways in flood zones: the height of the infrastructure must take account of the risks and challenges connected with the flood zone. For each type of infrastructure, the run-off conditions and the effect of an exceptional flood level must be assessed. Upstream water level (H AM ) and speed of flow (Ve) in hydraulic structures The upstream water level must be compatible with the height setting of the infrastructure and the flood risk. In all cases, the upstream water level must not exceed 1.2 times the height of the structure for the project flow for structures with an opening of ð 2 m. The speeds must meet the following criteria with regard to the durability of the structures: concrete structures: 4 m/s, metal structures: 2.5 m/s => see appendix 4.6 [8]. To take account of the fish population, lower speeds must be demonstrated (approximate speed of 1 m/s). If it is impossible to satisfy these conditions, protective arrangements should be considered. Free space (TA) of the hydraulic structure The free space corresponds, strictly speaking, to the free height between the water line and the upper generatrix of the structure (see diagram No. 2). In our case, it is measured with respect to the notional water level ye + H AM 2 For an opening 2.00 m: to be assessed according to the nature of the catchment area. For an opening > 2.00 m: TA of 0.50 to 1.50 m. The fill ratio of the hydraulic structure for the project flow must not exceed Collection «Les outils» Sétra 9 September 2007

10 Diagram No. 2: free space of the hydraulic structure Collection «Les outils» Sétra 10 September 2007

11 Impact of the hydraulic structure The rise in the water line upstream of the hydrological structure relative to the existing situation and the speed of flow out of the structure are to be assessed with respect to the local risks and challenges. Open channel flow within the hydraulic structure must be ensured for the project flow. Free movement of the ichthyofauna The minimum water level with limited speed of flow must make adequate provision for the upstream migration of fish during low-water periods. It is often necessary to arrange for the base of the structure to reconstitute a natural river bed. Reference should be made to the works "facteurs biologiques à prendre en compte dans la conception des ouvrages de franchissement" ("biological factors to be taken into account in the design of bridging structures") [1] and "Passes à poissons : expertise, conception des ouvrages de franchissement" ("Fish-passes: appraisal, design of bridging structures") [2]. Location of the hydraulic structure In plan, the hydraulic structure is generally located on the axis of the low-level bed of the water course; its opening must be at least equal to that of the low level bed. It may, nevertheless, be necessary to straighten the natural course of flow beneath the infrastructure to achieve a more direct crossing. It is a question of establishing its feasibility in both environmental and regulatory respects. The continuity of flow must be respected and protection must be provided in areas susceptible to erosion. In longitudinal elevation, the setting of the hydraulic bridging structure is conditional on the natural topography of the terrain and the flow conditions (gradient of bed). So far as possible, the hydraulic structure should be set to follow the gradient of the natural bed of the water course. Evaluation of the project flow and the exceptional flow The following chapter sets out some simple methods that can be used to evaluate the project flows. The exceptional flow to be taken into account is at least equal to 1.5 times Q 100. An evaluation of its impact (with the hydraulic structure dimensioned for the project flow) on the safety of users, on the durability of the infrastructure and on the environment must be carried out with a view to assessing the measures to be taken Determination of the project flow (an application example of the calculation of the project is given in appendix 4.1.2) The project flow corresponds to the peak flow for a given recurrence interval, on the basis of which the dimensions of the hydraulic structure are determined. The calculation methods proposed below use the "rational" and "crupidex" formulae with a "transition" formula to make the link between the two. They are simple and can be applied to natural catchment areas. They were developed by experts for the realization of the Mediterranean TGV. The same applies for the run-off coefficient, the concentration time* and the transition formula. Other proven methods can also be applied. Whatever the chosen method, the results of project flow calculations for a natural catchment area are subject to uncertainties (precipitation values, complexity of phenomena etc.). An investigation in the field must be carried out to ensure that the calculation results are consistent with reality. Rational formula Range of validity Its range of validity is as follows: up to 1 km 2 in mainland France, except for the Mediterranean sea-board, Collection «Les outils» Sétra 11 September 2007

12 up to 10 km 2 on the Mediterranean sea-board (zone with rainfall intensities similar to the regions of PACA, Corsica, Languedoc Roussillon). Formula where: : recurrence interval project flow m 3 /s : run-off coefficient* weighted for the recurrence interval, T : rainfall intensity in mm/h for the recurrence interval* T during the concentration time* t C : total area of the catchment area in km 2. A J : partial area of the natural catchment area with coefficient C J in km 2 t c : concentration time* t c in minutes where L j : length of flow (in m) on a section where the speed of flow is V j (en m/s). The Montana coefficients, a and b are obtained by statistical adjustment from the water levels observed during a given time. The base data or the reconstituted Montana coefficients can be obtained from the weather service. Run-off coefficient, C 10 For T = 10 years (indicative values) (see table No. 1) Variability of the run-off coefficient* The values of the coefficients increases with the intensity of the precipitation but this variation differs with the degrees of permeability and retention of the ground making up the catchment area. Thus, a highly impermeable natural catchment area will have a high coefficient C (10) and this will increase little with the recurrence interval under consideration. Conversely, a highly permeable natural catchment area or one with a high retention capacity will have a run-off coefficient* of almost zero until a threshold is reached and then increase very rapidly and may reach values comparable with those for an impermeable catchment area. This behavior is characteristic of natural catchment areas with a threshold effect. The variability of the run-off coefficient is a function of the initial retention, P o of the natural catchment area: For C (10) < 0.8 on a P 0 in mm and P 10 = the 10-year daily rainfall in mm If C (10) 0.8, we generally take: P 0 = 0 and C (T) = C (10) Run-off coefficient C T for a recurrence interval, T > 10 years P (T) = daily rainfall of the recurrence interval, T Collection «Les outils» Sétra 12 September 2007

13 An application example of the variation of the run-off coefficient* of the rain is given in appendix Rainfall parameters These parameters (see appendix 4.1.1) can be obtained from Météo France (the French weather service). These are the Montana coefficients a (T) and b (T) of the rain i (T) = a (T) x t b c (T) with i in mm/h and t c in minutes Ten-year daily rainfall of the recurrence interval Daily rainfall of a given recurrence interval P T in mm. Vegetation cover Morphology Gradient % Sandy coarse terrain Alluvial terrain Clayey terrain Wooded almost flat undulating mountainous p < 5 5 p < p < 30 0,10 0,25 0,30 0,30 0,35 0,50 0,40 0,50 0,60 Grazing almost flat undulating mountainous p < 5 5 p < p < 30 0,10 0,15 0,22 0,30 0,36 0,42 0,40 0,55 0,60 Arable almost flat undulating mountainous p < 5 5 p < p < 30 0,30 0,40 0,52 0,50 0,60 0,72 0,60 0,70 0,82 Table No. 1: run-off coefficient for T = 10 years Collection «Les outils» Sétra 13 September 2007

14 Determination of the concentration time*, t c, for T = 10 years The determination of this parameter requires the evaluation of the speed of run-off of the water on the natural catchment area*. The run-off can be : very gentle: run-off in a surface layer (see diagram No. 3), characterized by run-off spread out over the natural catchment area or more rapid: concentrated run-off (see diagram No. 4), characterized by talwegs* and ravines fed by the valley slopes and by the low-water beds of the water courses. Diagram No. 3: run-off in a surface layer Diagram No. 4: concentrated run-off The speeds shown in tables 2 and 3 could be used. These values are determined from: V in m/s p in m/m V = k x p 1 / 2 x R h 2 / 3 (see appendix 4.2.1) Table No. 3 below was drawn up for k = 15 and R h =1, values generally accepted for pilot projects. Slope in m/m Speed in m/s Table No. 2: evaluation of the speed of run-off in a surface water layer Slope in m/m Speed in m/s Table No. 3: evaluation of the speed of concentrated water run-off Determination of the concentration time* for a recurrence interval > 10 years where: tc (T) : concentration time* for the ten-year recurrence interval, in minutes. Tc 10 : ten-year concentration time*, in minutes. P (T) : daily rainfall of recurrence interval T, in mm. P 10 : ten-year daily rainfall, in mm. P 0 : initial retention, in mm. Collection «Les outils» Sétra 14 September 2007

15 The value of the concentration time* is an approximate value, which depends, in part, on the precipitations and the morphology of the natural catchment area. In the interests of simplification, it is generally accepted that, for studies up to the pilot project stage, the empirical formulae in appendix 4.5 can be applied. Collection «Les outils» Sétra 15 September 2007

16 Crupedix formula This comes from the Ministry of Agriculture (Cemagref, 1980). Range of validity: from 10 km 2, except for the Mediterranean sea-board (50 km 2 ), and up to 100 km 2, formula valid only for the ten-year flow, the interval (Q/2-2Q) represents confidence interval with a probability of more than 80 % of including the calculated value. Formula: The flow where: Q 10 : ten-year flow, in m 3 /s, R : regional coefficient reflecting the aptitude for run-off P 10 : daily rainfall of ten-year recurrence interval, in mm S BV : area in km 2 Evaluation of the hundred-year flow from the ten-year flow of the Crupedix formula We obtain the hundred-year flow from the correlation: Q 100 = b. Q 10 a priori: 1.4 b 4 The parameter b depends on the area of the catchment area: up to 20 km 2, b is determined using the rational formula (calculation of Q 10 and Q 100 as if the rational formula were applicable), above 20 km 2, b is determined from data obtained from gauged water courses on catchment areas near the project. Failing this, b = 2 minimum. Choice of the parameter R The regional coefficient, R, is to be checked locally. If this is not possible (absence of gauged water courses in representative catchment areas near the project), the following coefficient values can be used: R = 0.2 for permeable areas (Champagne, Beauce), R = 1.5 to 1.8 for impermeable areas (Lorraine plateau, Vendée), R = 1 for intermediate areas. Evaluation of the flow, Q T, of recurrence interval*, T The evaluation of a flow of recurrence interval, T, between 10 and 100 years can be obtained from the following formula, assuming that the statistical distribution of the observed values follows Gumbel's law: where Δ Q = Q 100 Q 10 y = (-ln (-ln (1-1 T if T = 20 years y = 2.97 if T = 30 years y = 3.38 Transition formula This formula can be justified to the extent that the ten-year flow yielded by the rational formula can sometimes be twice that yielded by the Crupedix formula. The flow yielded by the transition formula is written: ))) Collection «Les outils» Sétra 16 September 2007

17 where: : project flow with recurrence interval T, : flow yielded by the rational formula, recurrence interval T, : flow yielded by the Crupedix formula, recurrence interval T, α, β : weighting coefficients α varies linearly from 1 to 0 as the surface area (S) increases from 1 to 10 km 2, from which: α = France except Mediterranean sea-board and β = 1 - α α varies linearly from 1 to 0 as the surface area (S) increases from 10 to 50 km 2 α = Mediterranean sea-board and β = 1 - α The ranges of applicability of the three formulae presented above are as follows: (see table No. 4). An application example is shown in appendix on a fictional natural catchment area. Area of catchment (in km 2 ) France except Mediterranean sea-board Rational formula Transition formula Crupedix formula Crupedix formula Mediterranean sea-board Rational formula Rational formula Transition formula Crupedix formula Table No. 4: ranges of applicability for each of the three formulae Collection «Les outils» Sétra 17 September 2007

18 Design of hydraulic structures Structures are generally classified in 5 families: circular ducts, box culverts*, arched culverts*, large arched structures and major structures. So far as possible, production stock items should be chosen rather than the more costly structures cast in place. Reinforced concrete structures, provided they are correctly designed and built with due care, are assured of excellent strength and longevity. The structural design of proposed structures is a task for civil engineering consultants. Factors influencing the choice of hydraulic structures In choosing the structure to be used, the longevity of the road, the safety of users, investment costs and methods for subsequent maintenance of the structure must always be borne in mind. The factors influencing the choice are: the quantity of the flow to be discharged, which fixes the flow cross-section and the type of structure, the hydraulic characteristics of the structure: roughness factor (K), funneling coefficient (K e ) creating a loss of head at the entry, shape of the flow section, the width of the bed; a unique structure, adapted to the flow to be discharged and the width of the bed of the water course is generally preferable to multiple structures, which increase head losses and obstruct the passage of floating bodies, the available height between the project height and the talweg*, the static and dynamic loads acting on the hydraulic structure, the foundation conditions of the structures, the speed and ease of implementation: production stock items supplied in transportable sections and assembled on site can be an interesting solution for reducing completion times and in cases where site access is difficult, the resistance to chemical agents, the shock resistance: massive structures withstand best the shocks and abrasion from solid materials carried by the current. Protection of hydraulic structures (see appendix 4.2.3) Consideration can be given to setting the invert* of the structure 0.30 m or more below the deepest part of the water course to allow the reconstitution of a natural bed in the structure (ascent of fish). The raised upstream level of flows and the increased speed of flow at the exit from the structure most frequently require protective measures both upstream and downstream of the structure. Any straightening of the course will require: continuity of flow, effective protection of the banks against changes of direction by durable techniques with a priority on vegetation based systems [10] "Protection des berges de cours d eau en techniques végétales [Protection of the banks of water courses by means of vegetation]". Reinforcement techniques using rock lining and gabions* should be used only on sections other wise heavily eroded by the current if there are significant risks to personal safety or high value-added assets, flows on steep gradients, p = 4%, pose special problems (determination of the upstream water level, speed in the structures etc.) that are not discussed in this guide. Calculating the structures Calculation of the structures can only be carried out after determining the constraints on natural flow up to at least 100 m downstream of the hydraulic structure. Furthermore, the openings of the hydraulic re-establishment structures are generally narrower than the current section of the stream or talweg* for reasons of cost. This narrowing is not without consequences for its operation, especially when discharging the peak flood flow. What must be borne in mind: Collection «Les outils» Sétra 18 September 2007

19 the structure must be able to discharge the flood quantity corresponding to the project flow* with an upstream water level* (H AM ) of the structure compatible with the setting of the project and the preservation of private property, the verification for an exceptional flow must be examined, in the context of the present guide, the upstream water level (H AM ) is confused with the total energy head line, the setting of the longitudinal section requires knowledge of the rise in the water line inherent in this narrowing of the flow; it is therefore necessary to determine the flow régime, the setting of the structure must not engender a hydraulic jump*, the calculation is carried out from downstream to upstream, i.e. the first thing to look for is the flow régime in the stream downstream of the hydraulic structure, the speed of flow must not exceed 4 m/s for concrete structures and 2.5 m/s for metal pipes. The method presented in the present guide is a simplified method (simplified Bernoulli theorem). It draws on the basic notions of hydraulics* (taking account of the flow régimes). The basic data required to understand the calculation method are shown in appendix The principle of the method consists in determining, in the first place, the flow régime downstream of the proposed structure in order to calculate the upstream water level, H AM, of the structure: if the flow is in fluvial régime*, the proposed structure must be set in fluvial régime (application of ABAC charts 1 to 5 see appendix 4.2.2), if the flow is in torrential régime*, the proposed structure can be set in fluvial régime* (application of ABAC charts 1 to 5 see appendix 4.2.2), The general relationship giving H AM is the following: where: y e = water level at the entry and immediately inside the hydraulic structure, in meters. V e = speed at the entry to the structure in meters per second under y e. K e = head loss coefficient at the entry to the hydraulic structure (function of the type of head). G = acceleration due to gravity in m/s 2 and V e = S EM = stream cross-section at the entry to the hydraulic structure under y e in m 2 A numerical application example is shown in appendix Maintenance and operation of the hydraulic structures Access to the hydraulic structures must take account of the operating constraints. An annual inspection and an inspection after a flood event are necessary in order to plan, if needed, maintenance of the structure and clearance of any silt. The minimum diameter of hydraulic structures under motorways is 800 mm. This dimension should, in all scenarios, be compatible with the managing body's maintenance capabilities. For 2 or 3-lane roads, this diameter may be reduced to 600 mm while still guaranteeing the operating conditions. Collection «Les outils» Sétra 19 September 2007

20 1.2 - Surface drainage of the platform This is the collection and evacuation of surface water within the footprint of the road. An essential component of the road project, the three objectives of the platform drainage are: the safety of users through the evacuation of water from the roadways and embankments, the longevity of the infrastructure, by collecting the water and evacuating it from the road, the management of road pollution. A badly designed network will lead to surface disorders (system overflows, floods etc.) and major structural disorders of the roadway in the medium term. These situations are aggravating factors for the safety of users and the integrity of the road. Furthermore, any road run-off transferred off the platform is not environmentally neutral General principles The environmental constraints (outfalls*, environmental vulnerability), the hydrogeology*, the engineering geology (nature of the ground) and the mapping of the project (high and low points, banked roadways) and the safety of users all come into the overall design of the systems. It is recommended to adopt the following principles: In questions of road design: in grazing profile, the longitudinal profile must be set so that the roadway and subgrade structures are embanked and platform run-off can be evacuated by gravity in the drainage system, avoid gradients less than 0.5 % as they may lead to water stagnating at changes of crossfall, avoid deep excavated sections (cuttings); these are often critical points for drainage and sometimes subject to drawdown of surface water, proscribe low points in cuttings. In questions of drainage: respect the criteria for the placing of structures with regard to the safety of users (see Guide sur le traitement des obstacles latéraux [Guide on dealing with lateral obstacles] [6]), adapt sealing of the collector structures to the requirement to protect water resources (see Guide sur le traitement de la pollution routière [Guide on the management of road pollution] [13]), equip the tops of cutting banks of longitudinal structures in the case of run-off in a natural catchment area (bank erosion and overload of the system at the foot of the bank), proscribe pumping stations (reversing or lifting stations) except in exceptional cases (costly installations, complex to operate and maintain), always try to keep water moving under gravity and on the surface, use as many discharge points as possible to avoid flow concentrations to be weighed against environmental considerations, study the possibility that infiltrations overflow (if this is consistent with the protection of water resources) and of downstream flows (water meadows*, intermediate holding basins etc.), dimension systems for rainfall with a recurrence interval of at least ten years (T = 10 years), check that the roadway will not be submerged for a recurrence interval of 25 years; on the contrary, this can be permitted for the shoulder for T = 25 years, avoid discharging into the road platform* drainage system any water from natural catchment areas or surface water layers, plan for a clad structure if the gradient is 1 % or if the speed of flow is likely to cause erosion (the critical gradient is often of the order of 3.5 %), in regions subject to frost, give the preference to concrete or masonry structures, turfed structures slow the downstream passage of flows, favor infiltration and tend to reduce pollution Nature and function of systems The drainage network must collect the run-off water from roads and their enclosing Collection «Les outils» Sétra 20 September 2007