Kerb-break CFD Modelling to Improve Cycle Track Safety

Abstract

The prioritisation of active transport, as a means of improving the sustainability of our cities, has seen an uptake in separated cycleways. Separated cycleways provide physical separation from the road, generally through use of an island median, and create a safe and direct transport route for users. This separating median often includes kerb breaks spaced at frequent intervals, which allows stormwater flow to be drained from the road into the cycleway. But how much do we really know about the efficiency of kerb breaks as a primary drainage measure? Have drainage design standards kept up with the shift towards separated cycleway design layouts? It is noted that current drainage design standards are prepared for an on-road cyclist arrangement with a simple absolute limit of 0.5m flow width, irrespective of the width or use of a cycle lane. Separated cycle tracks are currently not given any specific flow width criteria, and as such Arup have taken a performance-based review of a typical cycle track layout and current cycle lane standards to propose flow width criteria across separated cycleway infrastructure. In addition, there is minimal guidance available on the inlet capture efficiency of kerb breaks and therefore the spacing of these kerb breaks from a flow management perspective. If not adequately designed, the consequence of uncaptured flows could present a safety risk to road and cycleway users. To fill this knowledge gap, Arup have undertaken computational fluid dynamics (CFD) modelling to provide a reasonable assessment of flow capture at kerb breaks. CFD models the kerb break and approach flow in 3 dimensions and uses first principals to assess how much flow will bypass and how much will be captured. The results can be used to create inlet capture charts, for the kerb breaks for use in traditional modelling software such as 12d or Drains.