Abstract:
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.
