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19. Deposition of climbing-ripple beds: a flume simulation.

Author

Ashley, Gail M., Southard, John B., and Boothroyd, Jon C.

Subjects
SEDIMENTATION & deposition, FLUMES, FACIES, STRATIGRAPHIC geology, SEDIMENTOLOGY, GEOLOGY
Abstract

Thirteen runs were made in a small recirculating flume to simulate the deposition of the climbing-ripple sequences commonly present in fine-grained facies of fluvial and deltaic deposits. These sequences consist of intergradational climbing-ripple cross laminae and draped laminae. The experiments were based on the assumption that stratification type depends mainly on near-bottom flow structure and uniform sediment fallout from an overloaded flow. Various combinations of curves of velocity versus time and of sediment feed versus time in runs lasting from 45 to 840 min were used in an exploratory program; conditions for each run were selected on the basis of experience in previous runs. The runs verified that Type A (erosional-stoss) climbing ripples are produced by aggradation rates that are small relative to ripple migration rate, and Type B (depositional-stoss) climbing ripples are produced by aggradation rates that are large relative to ripple migration rate. Draped lamination results from continued fallout of sediment from suspension after ripple migration ceases or almost ceases. Comparison of geometric details of the ripple stratification produced in the flume runs with that in natural sequences, supplemented by considerations on maximum and minimum migration rates of ripples, suggests times of no more than a few tens of hours for the deposition of the climbing-ripple portions of sequences 10-20 cm thick. Runs in which deposition of a 20cm sequence took more than 10 h produced such atypical features of ripple geometry as sharp crests and planar lee-side laminae. [ABSTRACT FROM AUTHOR]

Published
1982
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20. Bed Form Development and Distribution Pattern, Parker and Essex Estuaries, Massachusetts

Author

Boothroyd, Jon C. and Hubbard, Dennis K.

Subjects
River channels, Bed Form, Massachusetts, Tidal Cycle, Deltas, Ingenieurwissenschaften (620), Megaripple, Sand waves, Ripple, Estuaries
Abstract

Source: https://erdc-library.erdc.dren.mil/jspui/ Velocity, depth, temperature, grain size, and bed form scale and orientation were measured for complete tidal cycles at 50 stations in two New England estuaries. Scuba observation of bed form change and migration, fathometer profiles, and 700 bed form scale and orientation readings were also carried out. This investigation led to the recognition of a sequence of bed forms based on increasing "flow strength." Bed form type is governed by maximum flood and ebb velocities (Umax) attained at a given locality. Velocity asymmetry and duration are important in determining bed form morphology and amount of crossbedding bimodality. Froude number (Fr) shows good correlation with bed form type only in depths less than 2 meters. In the intertidal and shallow subtidal (2 meters MLW) areas, sand waves are the principal bed form. They show no dominant orientation. However, where Umax exceeds 80 centimeters per second, megaripples are superimposed on the sand wave form and crossbedding is complex.

Published
1974

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