An analysis of the impact of trash screen design on debris related blockage at culvert inlets
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The construction of a culvert within a river channel alters the local hydraulic characteristics which often increases upstream water elevation as a result of the volume of water being constricted as it enters the culvert. This effect can be exacerbated if there is a build up of debris either at the inlet or trapped within the culvert. As a consequence culverts, especially those which are prone to becoming blocked, may considerably increase the potential for out of channel flows and therefore the risk of serious flooding. While trash screens may be fitted at a culvert inlet to prevent debris from entering, unless they are well designed and maintained they may increase the potential for flooding if they become blocked by trapping debris that would have passed unrestricted through the culvert. Guidelines for screen design focus mainly on ensuring sufficient screen area is provided to handle the expected debris load, while recommendations for individual screen elements, such as bar spacing, are generally based on anecdotal evidence and site specific environmental or safety concerns. However, many different trash screen configurations can influence blockage potential. To gain a better understanding of how blockage, and therefore any potential associated flood risk, of culvert trash screens is influenced by screen geometry and position, a Froude-scaled physical model was developed to facilitate assessment of the efficiency of different trash screen configurations. To minimize scaling issues related to complex geometry, and to ensure the focus of the research remained on the influence of screen design rather than on factors specific to the debris, wooden dowel was used to represent different debris lengths. Detailed analysis based on 147,000 debris passes is used to show that, as would be expected, potential for screen blockage by debris increases as the ratio of debris length to bar spacing increases. However, in addition, a key finding has been that the screen position relative to the zone of flow acceleration created as the flow is constricted on approach to the culvert inlet is a significant driving factor in the blocking potential of the screen. Detailed statistical analysis was used to define blockage potential in terms of all contributing factors. The derived model was used to develop end user focussed tools, a nomograph and an interactive spreadsheet, to aid estimation of blockage at a screen for a given geometry and position.