Author: Christian Nea, senior engineer, environmental policy and compliance section, Transport Infrastructure Ireland.[1] In March 2015, the National Roads Authority1 (NRA) finished significantly revising its road drainage standards for major projects (refer to http://nrastandards.nra.ie/). The revision of the NRA drainage standards was precipitated by post-doctoral research, carried out under the NRA Research Fellowship Programme and mentored by the NRA Environment Unit. This research examined the impacts of national road drainage systems on both surface and ground water and concluded that the NRA drainage standards needed to be expanded to promote the use of sustainable drainage systems and to maximise environmental benefits. A report entitled Drainage Design for National Road Schemes – Sustainable Drainage Options (NRA, 2014) documents this research and provides useful background reading.

Summary of revisions

In relation to the revision of the NRA DMRB, the NRA significantly amended one document, NRA Addendum to HD 33/06 Surface and Sub-surface Drainage Systems for Highways, to create a new NRA HD 33/15 Drainage Systems for National Roads. It also added 13 new documents to the NRA DMRB. The NRA also made significant changes to the drainage-related aspects of the NRA MCDRW.

Some notable changes to the NRA DMRB

NRA 1In March 2015, the NRA introduced NRA HD 33/15 Drainage Systems for National Roads, the core NRA DMRB drainage document that provides guidance on the selection of the types of surface and sub-surface drainage options for national roads projects. This new standard document significantly alters the design options available for verge-side edge and central reserve drainage, promoting Sustainable Drainage Systems (SuDS) whilst simultaneously reducing the scope to use non-sustainable options, where appropriate. For example, restrictions have been placed on the use of kerbed drainage systems within the verge-side edge. Such systems are now only permitted where there is a footway in the verge or where there are urban or other site-specific conditions. In contrast, restrictions on the use of over-the-edge drainage, a SuDS, have been relaxed with such options now being permitted in embankments over 1.5m in height. Previously, over-the-edge drainage was only permitted on embankments between 1.5m and 6m in height, constructed of free-draining material. Furthering this promotion of SuDS, restrictions on the use of grassed surface water channels (GSWCs) have also been loosened. While the use of GSWCs was previously not allowed unless a departure from standard had been obtained, they are now permitted, albeit subject to certain conditions. Further, following a policy that is contrary to that adopted by Highways England (as manifest in its DMRB), the use of combined filter drains, also a SuDS, in cuttings is still ‘encouraged’ by the NRA. Whereas NRA HD 33/15 Drainage Systems for National Roads is undoubtedly the core NRA DMRB drainage document, NRA HD 45/15 Road Drainage and the Water Environment is arguably the second most important document. The introduction of this document is another manifestation of the promotion of sustainable drainage systems with the document explicitly providing that ‘SuDS should be considered in the first instance over conventional drainage systems.’ NRA HD 45/15 provides guidance on the assessment and management of impacts on the water environment. The document adopts the methods laid down in UK HD 45/09 for assessing:
  • impacts of routine runoff on surface waters; and
  • pollution risks from accidental spillages.
[caption id="attachment_27472" align="alignright" width="300"]Figure 1: Construction of grassed surface water channels Figure 1: Construction of grassed surface water channels[/caption] It also refers readers to the Office of Public Works’ website for information on assessing flood impacts. NRA HD 45/15 also describes a novel Groundwater Protection Response (GPR) which is a risk assessment and management tool, dealing with discharges of routine runoff to groundwater. This GPR was devised in collaboration with the Environmental Protection Agency and the Geological Survey of Ireland. The GPR determines whether or not an impermeable system is required, based on:
  • Aquifer Category;
  • Vulnerability Rating; and
  • Source Protection Area presence.
Regard is also had to the thickness of the unsaturated zone in this determination. The NRA also introduced another core standard, HA 33/15 Design of Earthworks Drainage, Network Drainage, Attenuation & Pollution Control. This document, unique to the NRA DMRB, has been introduced to assist in the design of: earthworks drainage; road network drainage; attenuation; and pollution control. This document provides design process flowcharts and worked examples on elements of drainage design. Further core documents include: NRA HD 83/15 Safety Aspect of Road Edge Drainage Features, which provides guidance on safety aspects of road edge drainage features; and NRA HD 139/15 Edge of Pavement Details, which provides guidance on the use of the various types of edge of pavement drainage details. [caption id="attachment_27474" align="alignright" width="300"]Figure 2: GPR matrix for the use of permeable drains in national roads Figure 2: GPR matrix for the use of permeable drains in national roads (click to enlarge)[/caption] In addition to the introduction of these core documents, the NRA also introduced peripheral standards that are to be consulted when dealing with particular design issues. For example, a significant number of standard documents, detailing the hydraulic and structural design of drainage elements, have been introduced. NRA HD 119/15 Grassed Surface Water Channels for Road Runoff contains further information on the structural and hydraulic design of GSWCs, a SuDS. The hydraulic design of various elements is also dealt with in: NRA HD 137/15 Hydraulic Design of Road-Edge Surface Water Channels; NRA HD 78/15 Design of Outlets for Surface Water Channels; and NRA HD 102/15 Spacing of Road Gullies.

Some notable changes to the NRA MCDRW

Series 500 – Drainage and Service Ducts of Volume 1 – NRA Specification for Roadworks has been revised, with the layout, organisation and cross-referencing being significantly improved. In a move to increase sustainability, there is now increased scope for the use of lightweight aggregates as bedding and filter material. [caption id="attachment_27475" align="alignright" width="300"]Figure 3: RCD/500/50 Rock Armour: Scour Protection Figure 3: RCD/500/50 Rock Armour: Scour Protection[/caption] Series 500 – Drainage and Ducts of Volume 4 – Road Construction Details has also been subject to significant revision. A new RCD/500/50, which illustrates ‘Rock Armour: Scour Protection’, has been introduced in an attempt to improve the quality and longevity of rock armour being constructed on national roads. A new RCD/500/53, which replaces RCD/500/19, 20 and 21, and illustrates ‘G.A. of Formed Headwalls 150 – 1800 Diameter Pipes’, explicitly prohibits rock armour/gabion headwalls and now requires most headwalls to be constructed of reinforced concrete (in situ or pre-cast). The drainage standards were also revised in order to reduce health and safety risks to those inspecting and maintaining drainage systems. For example, there have been significant changes to Road Construction Details (RCDs) dealing with ‘Access Chambers’ in order to reduce such risks, namely: reductions in plan areas; and the removal of steps, handholds and ladders to encourage working from surface and entry in accordance with health  and safety legislation.

Conclusion

In line with the recommendations of the NRA-funded post-doctoral research, the NRA drainage standards have been significantly revised and expanded to promote the use of sustainable drainage systems and to maximise environmental benefits. The drainage standards were also revised in order to reduce health and safety risks and improve engineering quality. For those interested in learning more about the NRA new drainage standards, a more detailed summary is contained in NRA Technical Bulletin Numbered 13 Revised Roads Drainage Standards, which is available at: http://nrastandards.nra.ie/road-design-construction-standards/technical-bulletins/nra-tb-13-revised-road-drainage-standards [1] Since the 1st of August, 2015, the National Roads Authority describes itself as Transport Infrastructure Ireland for operational purposes.