Physical modelling of low-cost modifications to the Crump Weir in order to improve fish passage : evelopment of favourable swimming conditions and investigation of the hydrometric effect

More than 350 Crump-type weirs (which are triangular in profile) form part of the Environment Agency's hydrometric network in rivers across England and Wales. These weirs operate as effective measurement structures and are useful over a large

flow range. However, they also act as barriers that impede the passage of many species of coarse fish within their natural habitat. The primary aim of this research project was to recommend modifcations to Crump weirs in order to improve fish passage, while still allowing the weirs to fulfll their hydrometric purpose in a reliable way. It was an additional requirement that any proposed solution(s) be both practical and achievable at low-cost. This is in contrast to conventional fish pass solutions, that tend to be expensive, are generally not hydrometrically rated, and most of which were not designed with coarse fish in mind. The method used was a model study conducted in the laboratory, which allowed for a great number of layouts to be trialled. Laboratory research combined with fish swimming data provides a basis for projecting successful fish ascents. Brimpton weir on the River Enborne was chosen as a suitable reference on which to base laboratory model tests. The preferred arrangement (termed a `rotated-V' layout) was found to be a series of baffles located on the downstream slope of the Crump weir. These baffles effectively act as weirs at low flows and roughness elements at high flows. Each baffle has a slot which helps to form a path of ascent for fish. The base closest to the crest was set at the same height as the crest, as this led to optimum low velocities in the slots on the downstream slope. Extensive testing revealed that the proposed solution results in a change in a weir's hydrometric characteristics. However, it was demonstrated that the deviation of the coefficient of discharge is predictable. Therefore, it allows for reliable flow measure- ment to be achieved (subject to a standardised calibration trial using volumetric flow measurement techniques). In addition, a detailed measurement and analysis of wa- ter velocities within the recommended solution strongly suggest that it substantially improves on the fish passage capability of a Crump weir.