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Permeability of small weirs for upstream fish passage
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Resumo(s)
River fragmentation is a serious threat to the sustainability of fish populations. Although far more
numerous than dams, the impact of small weirs on fish migration have received less attention, for
being considered permeable to fish movements. Nevertheless, the presence of small weirs may cause
successive losses of connectivity by partially or totally blocking migratory routes. Potamodromous
cyprinids are particularly impacted due to their need of undergoing seasonal upstream migrations. The
goal of this thesis was to assess the effects of small weirs, particularly broad-crested and low-head
ramped weirs, which are the most frequent designs in Iberian watercourses, on behaviour and
passage performance of potamodromous cyprinids. A field assessment was performed to understand
how these obstacles may affect, in an otherwise non-impacted stream, the movements of cyprinids. I
addition to this field study, laboratorial experiments were carried out in an indoor flume. In these
experiments, the influence of key hydraulic parameters – plunge pool depth, waterfall height, and weir
crest width, in the case of broad-crested weirs; ramp length, and slope, for low-head ramped weirs, on
passage performance of fish was tested across a range of discharges. Iberian barbel (Luciobarbus
bocagei) was selected as target species. Main results suggest that barbel may cease to migrate, when
suitable habitats exist between barriers. Regarding broad-crested weirs, it was found that i) fish
passage was inhibited by shallow plunge pool depths in association with high waterfall heights; ii)
passage behavior was dependent on combinations of plunge pool depths and waterfall heights; and iii)
the width of the crest influenced downstream passages but not upstream ones. For ramped weirs, i)
negotiation was conditioned by the increase of slope and ramp length; and ii) retrofitting ramped weirs
with natural substrates like cobbles may increase permeability to fish movements. These outcomes
are useful to identify potential migration obstacles and to help design more permeable structures to
fish movements
Descrição
Doutoramento em Restauro e Gestão Fluviais - Instituto Superior de Agronomia / Faculdade de Arquitetura / Instituto Superior Técnico. Universidade de Lisboa
Palavras-chave
ecohydraulics river connectivity potamodromous cyprinids species migratory movements small weirs
Contexto Educativo
Citação
Amaral, S.C.A.D. - Permeability of small weirs for upstream fish passage. Lisboa: ISA, 2020, 166 p.