Medium-term response of lowland river reaches to changes in upland land use.
The afforestation of upland areas in the UK has been the largest rural land use
change this century. As a consequence of afforestation the bed load yields of
upland catchments have been shown to increase substantially. The upland
catchment bed load is transported downstream, through the transfer zone to the
lowland reaches where it may enter storage in barforms, cause flow diversion
and cause channel instability. This study investigates the Afon Trannon, a river
in mid-Wales whose upland catchment was afforested between 1948 and 1978
and whose lowland channel is currently unstable. Historical rates of channel
change in the lowland channel have been quantified using aerial photogrammetry
in ERDAS Imagine GIS. Channel instability is shown to increase significantly
between 1963 and 1976, some 15 years after upland catchment afforestation.
However, upland catchment bed load yields are shown to be of low magnitude
(up to an estimated maximum of6.35 t km-2 yr") and incapable of producing the
high medium-term lowland channel change rates observed (up to 1.88 m yr"
between 1963 and 1976). Contemporary channel DTMs constructed from field
survey data have allowed the construction of a contemporary lowland channel
sediment budget. Local inputs of bed load from composite bank erosion are
shown to dominate in the budget and reaches of maximum instability are shown
to correspond with the location of these composite banks. Additionally. flood
magnitude and frequency are shown to have increased since 1988 from a
maximum stage of 1.50 m between 1969 and 1988 to a maximum stage of2.23
m between 1989 and 2000. A conceptual model is presented in which the
medium-term instability of the lowland Afon Trannon is suggested to be
triggered by local lowland bed aggradation as a result of elevated upland
catchment bed load yields and a risk assessment diagram provides advice to river
engineers and fluvial geomorphologists interested in assessing the potential
stability of lowland rivers whose upland catchments have been afforested.