Planform cyclicity in an unstable reach: complex fluvial response to environmental change

Abstract

Long-and short-term channel changes are documented and analysed for a historically unstable reach of the River Severn at Llandinam, mid-Wales. Long-term changes (the last 150 years), reconstructed from 10 archival sources, are characterized by channel planform switching between meandering (1836-1840 and 1948-1963) and braided (1884-1903 and 1975-present) phases. Short-term changes, monitored by detailed planform surveys over a 2·5 year period, showed smaller-scale channel adjustments involving channel switching, bar accretion and channel expansion. Phases of braiding at Llandinam have been triggered by extrinsic controls, primarily flooding, but intrinsic controls (floodplain sediments, planform evolution and channel gradient) have been influential in priming the reach prior to destabilization. Flow regulation on the River Severn since 1968 has partly frozen the planform of the contemporary braid zone. Management of channel planform adjustments, where environmental change is phased in over time, must be informed by a knowledge of the potential for triggered planform switches. In addition, the effects of environmental change on fluvial systems are often historically contingent upon the state of the channel at the time of impact.

Gravel extraction and planform change in a wandering gravel-bed river: The River Wear, Northern England

“…The use of historical plans, maps and aerial photographs to reconstruct changes in river channel planform is common practice (Werritty and Ferguson, 1980;Hooke, 1987;Passmore et al, 1993;Brewer and Lewin, 1998;Werritty and Leys, 2001). Channel planform reconstructions were derived from a range of historical maps and aerial photographs spanning 150 years (Table 1).…”

Temporal variations in fluvial processes on an active meandering river over a 20-year period

“…A few reaches have been monitored for both processes and morphological changes over periods of several years, e.g. Brasington et al (2000) and Brewer and Lewin (1998). Most of the research at timescales longer than a few years has involved use of maps and aerial photographs or periodic resurveys, providing snapshots of morphology in time from which data on change in epochs between dates of evidence can be derived.…”

Complex channel responses to changes in stream flow and sediment supply on the lower Duchesne River, Utah

“…The difficulty in predicting channel response arises because many degrees of freedom exist for channel adjustment, including changes in channel slope (Lane, 1955), channel geometry and planform (Schumm, 1969), bed texture (Lisle et al, 1993), and bed organization (Madej, 2001). The nature of the adjustments to a given perturbation in a given stream reach is contingent upon prior conditions (Brewer and Lewin, 1998), such that similar changes in inputs can produce different channel adjustments in different reaches (Carling, 1988).…”