Computational models for dune time-lag: Population structures and the effects of discharge pattern and coefficient of change

An earlier model for dune time-lag in periodically varying unidirectional flows (J.R.L. Allen, 1976) is modified so as to treat more realistically the stochastic behaviour of dunes and the ability of the individual to change in height during its life-span.The improved model shows that hydrograph shape could substantially influence dune behaviour in unsteady flows. For the same flow period and extreme discharge values, a reduction in the relative duration of the high-water stages causes an increase in the phase differences between dune dimensions and flow, and an increase in the dimensions averaged over the flow cycle as compared with the similarly averaged dimensions given no lag. The relative range of dimensions over the flow cycle is little affected.The time-dependent structure of the dune populations is explored using histograms of the instantaneous values of dune wavelength, height and age. At small values of the time ratio (ratio of characteristic dune life-span to flow period), the dunes invariably are unimodally distributed in wavelength, height and age, and the relative dispersion of these properties is small. At intermediate ratios, dune properties are bimodally distributed and moderately to highly dispersed over much of each flow cycle, usually the later part and sometimes the whole of the low-water phase. Dune properties are at all times highly dispersed and, broadly, unimodally distributed when the time ratio is large. There is a close correlation between the patterns in time of dune creation rate, population structure, the average values and relative dispersions of dune dimensions, and the phase difference evaluated for each instant.An increase of the coefficient of change of dune height decreases the equivalent phase difference for height, diminishes the calculated wavelength and height averaged over the flow cycle, but increases the ranges of height and wavelength. Increasing the coefficient has no apparent effect on the equivalent phase difference for wavelength.     

Sedimentology of the Old Red Sandstone (Siluro-Devonian) in the Clee Hills area,Shropshire, England

The sequence of approximately 1300 m is divided by a major unconformity (Middle Devonian) into the thick Lower Old Red Sandstone (Siluro-Devonian), resting disconformably on Ludlovian (Silurian) marine strata, and the much thinner Upper Old Red Sandstone (Upper Devonian) overlain by the Carboniferous.The Lower Old Red Sandstone commences with littoral sediments (Downton Castle Formation) followed by tidal mud-flat deposits (Temeside Formation) formed after a brief marine transgression. The predominant remainder of the sequence (Ledbury Formation, Ditton Group, Abdon Group, Woodbank Group), characterized by fining-upwards cyclothems, records the establishment during a marine regression of extensive and persistent alluvial plains. Prior to Ditton Group times, detritus came from relatively distant regionally metamorphosed rocks lying to the north or west of the Clee Hills. Subsequently, apparently as the result of river-capture or drainage-reversal consequent on the commencement of the final (mid-Devonian) phase of Caledonian movement, high-level crustal rocks closer at hand (largely Wales) replaced the metamorphics as the sources of sediment, the earlier Lower Old Red Sandstone itself being recycled. To judge from the calcretes preserved in the alluvial formations, the area lay near the Equator and experienced a relatively dry hot climate.The Upper Old Red Sandstone likewise reveals fining-upwards cyclothems. The overlying Carboniferous rocks evidence the renewed marine transgression of the area, after the removal of the effects of the mid-Devonian movements.     

Suspended‐sediment sources in an urban watershed,Northeast Branch Anacostia River,Maryland

Fine sediment sources were characterized by chemical composition in an urban watershed, the Northeast Branch Anacostia River, which drains to the Chesapeake Bay. Concentrations of 63 elements and two radionuclides were measured in possible land‐based sediment sources and suspended sediment collected from the water column at the watershed outlet during storm events. These tracer concentrations were used to determine the relative quantity of suspended sediment contributed by each source. Although this is an urbanized watershed, there was not a distinct urban signature that can be evaluated except for the contributions from road surfaces. We identified the sources of fine sediment by both physiographic province (Piedmont and Coastal Plain) and source locale (streambanks, upland and street residue) by using different sets of elemental tracers. The Piedmont contributed the majority of the fine sediment for seven of the eight measured storms. The streambanks contributed the greatest quantity of fine sediment when evaluated by source locale. Street residue contributed 13% of the total suspended sediment on average and was the source most concentrated in anthropogenically enriched elements. Combining results from the source locale and physiographic province analyses, most fine sediment in the Northeast Branch watershed is derived from streambanks that contain sediment eroded from the Piedmont physiographic province of the watershed. Sediment fingerprinting analyses are most useful when longer term evaluations of sediment erosion and storage are also available from streambank‐erosion measurements, sediment budget and other methods. Copyright © 2010 John Wiley & Sons, Ltd.