Author(s): Janice Blanc; Scott Arthur; Grant B. Wright
Linked Author(s):
Keywords: Trash rack; Culvert; Debris; Urban hydrology; Physical model
Abstract: 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 trapped debris. In response to this, UK guidelines for screen design focus mainly on ensuring sufficient screen area is provided to handle the expected debris load. However, many different aspects of trash screen configuration can influence blockage potential. To gain a better understanding of how blockage of culvert trash screens, and therefore any potential associated flood risk, 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 retain the focus on the influence of screen design and to minimize scaling issues related to complex geometry, wooden dowel was used to represent different lengths of simple geometry debris. Analysis based on 70, 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, the screen position relative to the zone of flow acceleration created as the flow is constricted on approach to the culvert inlet was found to be a significant driving factor in the blocking potential of the screen. Regression analysis was used to define blockage potential in terms of all contributing factors. The derived model was used to develop simple tools, a nomograph and an interactive spreadsheet, to aid estimation of blockage at a screen for a given geometry and position. Once further refined and fully validated these tools, and the underlying models, offer a mechanism to assess and therefore enhance the efficiency of screens at the design stage. In addition there may also be a role for these models in ranking existing screen installations as high or low risk of blockage which could contribute to optimization of screen maintenance programs.
Year: 2013