STREETSCAPE GUIDANCE

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HOME INTRODUCTION PART A A vision for London’s streets PART B From strategy to delivery PART C New measures for new challenges PART D Balancing priorities PART F Appendix PART E Physical design and materials SECTION 6 Introduction SECTION 7 High quality footways SECTION 8 Carriageways SECTION 9 Crossings SECTION 10 Kerbside activity SECTION 11 Footway amenities SECTION 12 Safety and functionality SECTION 13 Street environment SECTION 14 Transport interchanges Streetscape Guidance [Part E – Physical design and materials] Street environment 284 Figure 277: A typical highway drainage system. There are features visible on the surface, such as covers and gratings as well as those buried and out of sight such as pipes and chambers Typically drainage systems on the road network will consist of kerbs, channels, road gullies, and drainage pipes. The profile of the road surface forms an important function of this type of drainage system by guiding water towards gullies, minimising aquaplaning and splashing and maximising the longevity of the pavement and its associated earthworks. Layout Within the highway there are multiple elements which may influence layout and material choices. These include available outfalls for new systems, surface profile and steepness of gradients, pedestrian desire lines, constraints resulting from the location of utility services, the likelihood and impact of systems becoming blocked by detritus, trees and street furniture. All of these elements have knock-on effects to drainage systems above and below ground, so will influence the drainage design. For example, road gullies should be sited to intercept surface water immediately before a pedestrian crossing point and drainage pipe runs should avoid tree root systems. Drainage systems need to have a minimal physical impact on the carriageway or footway. A well-designed and maintained road drainage system ensures: • Safe conditions in all weather • Minimal nuisance to pedestrians via splashing • Minimal environmental impact • Durability and robustness of the carriageway surface • Minimal disruptions caused during regular maintenance Where possible, drainage and possibly utility apparatus should be laid in ‘corridors’ in the footpath. This will improve ride quality and facilitate the future maintenance of the services with minimum impact on road closures. Refer to the NJUG Guidelines on the Positioning and Colour Coding of Underground Utilities’ Apparatus (2007) for further information. Cyclic maintenance activities are generally carried out without road space permits, hence thought needs to be given to positioning of gullies based on access and traffic flows. Design criteria The following is an overview of the requirements for typical drainage systems on the highway: • New or upgraded drainage relating to highways shall be designed in accordance with the Design Manual for Roads and Bridges, the Specification for Highway Works, and Sewers for Adoption • The principle adopted for all highway drainage should be the use of straight sections of pipe serving trapped gully pots. Pumping of surface water should be avoided • Where it is not possible to install a gully pot due to depth of construction, a catch pit connected to a gully is permissible • Designers should avoid the practice of connecting gully to gully as a means of transferring flows. Each gully pot should ideally have its own connection to the carrier pipe • In exceptional circumstances short lengths of highway rider sewer will be allowed • Preference should be given to flexible jointed pipes • All access chambers shall be designed and constructed in accordance with Sewers for Adoption 7 • Gully pots shall be constructed in accordance with the appropriate standard detail and be of high performance concrete
HOME INTRODUCTION PART A A vision for London’s streets PART B From strategy to delivery PART C New measures for new challenges PART D Balancing priorities PART E Physical design and materials SECTION 6 Introduction SECTION 7 High quality footways SECTION 8 Carriageways SECTION 9 Crossings SECTION 10 Kerbside activity SECTION 11 Footway amenities SECTION 12 Safety and functionality SECTION 13 Street environment PART F Appendix SECTION 14 Transport interchanges Streetscape Guidance [Part E – Physical design and materials] Street environment 285 • Gully covers shall be hinged, not lift out • Cyclic maintenance activities are generally carried out without road space permits, hence thought needs to be given to positioning of gullies based on access and traffic flows Trees and drainage systems Trees, like drainage, are an essential element of any good street. Care must be taken when working with planting and drainage infrastructure to ensure that one does not affect the other causing increased maintenance requirements. Tree roots are attracted to moisture leaving drainage infrastructure through defects, cracks and displaced joints. Slow flowing drainage and standing water at the surface may indicate a blockage due to root infiltration. If the defects are not detected and addressed by routine maintenance the roots will continue to grow, expand and exert pressure at the crack or joint where they entered the pipe. The force exerted by the root growth will over time break the pipe and may result in its total collapse, attracting a higher cost to bring the asset back to an acceptable level of service. Therefore all new drainage systems near existing or newly planted trees must be designed to prevent root infiltration. Topography of carriageway profiles The size and shape of a drainage area, as well as the length and gradient of the surface have an effect on the surface water runoff. CCTV surveys may be required to confirm whether existing drainage facilities are underperforming and whether new drainage facilities are required. Topographic surveys will need to be provided along with a localised gully catchment plan for any required remedial works as well as to support the positioning of new drainage assets. The combination of topographic surveys and local gully catchment plans can be modelled to show how effective the spacing of features such as gullies and carriageway profiles are and used to develop improved layouts to reduce ponding and aquaplaning. This analysis can be refined when looking at specific locations such a crossing points, turning circles and bus stops where there is a higher potential of ponding and nuisance. Cambers and falls Gradients at building frontages should prevent surface water from entering buildings, basements or station entrances. Surface water should be intercepted before bus stops, pedestrian crossings and junctions. Falls should be encouraged to direct surface water into gullies as collection points. Please refer to HD39/01 Footway Design of the DMRB for further information on the design of crossfalls and longitudinal gradient. Low point should be at the gully and not located at the crossing otherwise localised ponding will occur at the crossing. If the road cannot be profiled to create a gully low point additional/alternative drainage arrangements will have to be made. Materials and maintenance Regular maintenance can have a positive impact on the life of assets. For example, wellmaintained road drainage will reduce the risk of water-logged foundation layers resulting in improved ability to withstand loading and minimise reactive repairs or premature failures. It may be false economy to choose materials on cost alone as there may be high maintenance costs if proprietary products or special maintenance activity is necessary to maintain an asset. Key to an excellent drainage system is ensuring that the need for disruptive traffic management during maintenance is kept to a minimum. Therefore it can be cost effective to invest in high performance materials at the outset. Installation of ironworks and covers needs appropriate consideration and material specification. The following items need to be considered when installing a cover: • Potential loading and strength of materials • Manual handling limitations • Noise generation • Friction • Seating • Differential settlement • Milling when resurfacing is carried out
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STREETSCAPE GUIDANCE

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