Celebrating 25 years of CIRIA culvert design guidance

Jeremy Benn, Director, JBA Consulting and Dr Amanda Kitchen, Technical Director, JBA Consulting - authors of the CIRIA Culvert Screens and Outfalls Manual (C786F) discuss the evolution and popularity of CIRIA culvert design guidance

Whilst many will remember 2022 for war in Ukraine, the easing of Covid-19 restrictions or turmoil in Downing Street, it also marked the 25th anniversary of publication of the best-selling CIRIA Culvert design guide (R168).  As the first UK good practice guide devoted to culvert design, it provided step-by-step guidance on hydraulic assessment and design of new culverts and topped the CIRIA bestseller list for many years.

In the noughties, increasing awareness of the potential impacts of culverts on the natural environment, geomorphological processes, flood risk and safety led to changes in industry practice and an accompanying need for updated guidance.  The Culvert design and operation guide (C689) (affectionately known as CDOG or “seadog”) replaced R168 in 2010.  The new guide encouraged designers to take a systems approach to culvert design, to consider a culvert as part of a wider water management system, and to avoid, reduce and/or mitigate for adverse impacts.  For the first time, environmental, geomorphological and legal considerations were covered, and the guide was published in both pdf format as well as hard copy.

In 2013, a supplementary note on blockage risks at screens (C720) was published, reporting on laboratory research undertaken by the Flood Risk Management Research Consortium.

Six years later, the Culvert, screen and outfall manual (C786F or “sisom”) replaced C689 to satisfy demand for a single manual that combined and updated advice on culverts, culvert screens and outfalls which had previously been split across different manuals.  Published in 2019, it provided expanded guidance on environmental considerations and updated the guidance on legal considerations (with separate sections for each of the four UK nations).

While the fundamental engineering aspects of culvert design have not changed much since the first manual 25 years ago, the need to consider the wider impacts of a culvert and to justify a culvert (and/or a screen) as a sustainable asset are now much clearer.

Size matters: as coverage has expanded, so inevitably has size.  Whilst R168 was 189 pages long and 10mm thick in print format, C689 was 342 pages and 17mm thick, and C786 is a whopping 427 pages and 30mm thick.  In defence of C786, it is smaller than the sum of its parts which collectively came to 519 pages.  The merging and consolidation of disparate guidance improves consistency, reduces reading time and (hopefully) generates productivity savings.