Fluvial-tidal Transition Research Articles

Adjustments in river channel geometry associated with hydraulic discontinuities across the fluvial–tidal transition of a regulated river

Adjustments in river channel geometry associated with hydraulic discontinuities across the fluvial–tidal transition of a regulated river

Adjustments in river channel geometry associated with hydraulic discontinuities across the fluvial–tidal transition of a regulated river

Adjustments in river channel geometry associated with hydraulic discontinuities across the fluvial–tidal transition of a regulated river

Channel change on the River Dee meanders, 1946–1992, from the analysis of air photographs

Gurnell et al. (1994) identified spatial and temporal trends in the planform of the River Dee on the Welsh–English border over a 115 year period (1887–1992) by overlaying information primarily derived from historical maps within a GIS. This paper focuses on channel change within the same river reach over the last 50 years based on the analysis of information from six sets of air photographs within a GIS. The shorter time period of study has resulted in greater difficulty in identifying channel changes which exceed the errors incurred in information extraction from the photographs and registration to a common map base. However, consistency in the spatial and temporal trends identified from different sources and for different stretches within the study reach support interpretation of channel change. Air photographs contain more information that is relevant to the interpretation of processes of channel change than can be obtained from maps. For example, information on bank cover and the pattern of sediment deposition are explored. However, information obtained from air photographs requires careful handling if errors in data extraction and registration are to be minimized. The analysis of channel planform information from air photographs presented in this paper confirms some of the tentative conclusions previously drawn from the analysis of information derived from maps of the same river stretch. Additional information on downstream changes in the spatial extent and pattern of sediment deposition and on temporal trends in bank vegetation cover, that has been extracted from air photographs, provides further details of possible adjustments to flow regulation, indicates the importance of vegetation cover to bank sedimentation processes and illustrates a downstream change in the mode and distribution of sedimentation across the fluvial–tidal transition. © 1997 by John Wiley & Sons, Ltd.

Dynamics and facies model of a macrotidal sand‐bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)

ABSTRACTThe 40‐km‐long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high‐tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel‐shaped geometry is developed, and by erosional or non‐depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single‐channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary.Longitudinal progradation will produce a 20‐m‐thick, upward‐fining succession, the lower 1/2–2/3 of which will consist of cross‐bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb‐dominated portion of this unit will be finer grained than the flood‐dominated part, and will contain trough crossbedding produced by 3‐D megaripples; the flood‐dominated areas, by contrast, will consist mainly of compound cross‐bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb‐ and flood‐dominated areas) will create packages of ebb cross‐bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral‐accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper‐flow‐regime parallel lamination. The succession will be capped by a 4‐m‐thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.