3a.1.1 RECENT ADVANCES IN MODELLING FLOOD WATER LEVELS AT BRIDGES AND CULVERTS ROBERT LAMB, PETER MANTZ, SERTER ATABAY, JEREMY BENN JBA Consulting ANDREW PEPPER ATPEC Key Words: Afflux, bridges, culverts Abstract: Afflux at bridges and culverts can be a significant source of flood risk by causing elevated flood water levels. A wide range of methods are currently used to model afflux, but a review of current practice found that these are not always well understood and can be applied inappropriately. Some of the underlying assumptions and calibration data are not the most relevant for typical situations in the British Isles. This paper presents a summary of recent research to develop more consistent afflux methods and software for use in Flood Risk Management and related activities. INTRODUCTION What is afflux? Afflux is an increase in water level that can occur upstream of a structure, such as a bridge or culvert, that creates an obstruction in the flow. The afflux is illustrated in Figure 1 for a bridge structure located in a watercourse. The dashed line represents the normal water surface for the undisturbed watercourse. The solid line represents the water surface when the structure is present. Afflux is shown as the maximum increase of water level above normal depth in the undisturbed stream. Note that the afflux differs from the headloss across the structure, as the latter varies depending on the upstream and downstream locations of measurement. Figure 1. Side elevation at a bridge contraction