Least action principle,equilibrium states,iterative adjustment and the stability of alluvial channels

The energy based least action principle (LAP) has proven to be very successful for explaining natural phenomena in both classical and modern physics. This paper briefly reviews its historical development and details how, in three ways, it governs the behaviour and stability of alluvial rivers. First, the LAP embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology. Second, the variational approach underlying the LAP provides a more straightforward and simpler fuzzy‐object orientated method for solving river regime problems than do the various complex Newtonian formulations. Third, it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and/or channel geometry adjustments, with the degree of channel geometry adjustment quantified by the dimensionless numbers F for depth dominated adjustment and H for width/depth dominated adjustment. Different planforms are preferred at different energy levels, with H providing a quantitative measure of the flow's efficiency for moving sediment. In rivers with insufficient energy, the interactions of endogenous and exogenous factors are shown to be capable, in certain circumstances, of achieving a stationary equilibrium condition which acts as the attractor state. Importantly, this study describes how iterative changes enable systems to achieve such a stable equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.     

The influence of bank strength on channel geometry: an integrated analysis of some observations

Bank strength exerts a significant influence on river channel geometry, but quantification of this relationship has been limited to only a few specific circumstances. This is due to both the complex nature of bank strength and the difficulty in incorporating its influence in river channel geometry relations. In order to undertake an integrated analysis of wide-ranging field observations, this study applies a recently developed multivariate model of channel geometry. When the banks of a number of laterally stable streams are categorized on the basis of the bank sediment and vegetation, the multivariate model yields numerical indices of bank strength. Within the range of the data analysed, bank strength can produce a three-fold change in channel width and a two-fold change in depth corresponding to about a 1·6-fold change in cross-sectional area. © 1998 John Wiley & Sons, Ltd.     

British fluvial geomorphology in the 1980s: a review of recent reviews

FLUVIAL FORMS AND PROCESSES by D. Knighton. 218pp. Edward Arnold: London 1984 (ISBN 0 7131 6405 0) $A32.50 (limp).

BRITISH RIVERS edited by J. Lewin. 216pp. George Allen and Unwin: London 1981 (ISBN 0 04 551079 2) $A29.95 (limp).

RIVERS, FORM AND PROCESS IN ALLUVIAL CHANNELS by K. Richards. 358pp. Methuen: London 1982 (ISBN 0 416 74910 0) $A41.95 (limp).     

Saldanha Bay,South Africa III: new production and carrying capacity for bivalve aquaculture

Measurements of NH₄, NO₃, urea and HCO₃ uptake using 15N and 13C stable isotope tracers were undertaken in Saldanha Bay, South Africa, between January 2012 and January 2013. These studies provide the first direct measurements of N utilisation by the plankton in the bay. Primary production in the bay is driven predominantly by the advection of nutrients from the neighbouring shelf environment during upwelling events, with terrestrial and other sources providing minor inputs. New production (NO₃-based) was calculated from the f-ratio and total primary production and was used to provide estimates of potential carrying capacity for bivalve culture. Despite the apparent light limitation of NO3 uptake in the winter, the availability of NO₃ appeared to exert the major influence on new production throughout the year. In addition, new production was modulated by NH₄ availability as shown by the suppression of NO₃ uptake by concentrations higher than 1?1.5?mmol m^?3. The estimated areal new production of 0.60?g C m^?2 d^?1 yielded a bay-wide annual estimate of 9 811 t C ha^?1 y^?1, slightly higher than previous calculations based on physical models. It is estimated that the total annual production of mussels and oysters, respectively, for a 1 000-ha cultivation area is approximately 40 000–53 000 t y^?1 (mainly Mytilus galloprovincialis) and 4 600–6 000 t y^?1 (Crassotrea gigas). The combined total production figures constitute only 24–31% of the surplus new production. A combined harvestable carrying capacity of 74 000–82 000 t y^?1 can be calculated from this surplus. However, from a management and ecological perspective, bivalve culture should be limited to well below this theoretical maximum. Even with this constraint, there appears to be considerable scope for expansion of bivalve farming over the modest, present levels with little jeopardy to ecological integrity.     

Sedimentary record of Triassic intraplate extension in North China: evidence from the nonmarine NW Ordos Basin, Helan Shan and Zhuozi Shan

The Helan Shan and Zhuozi Shan of the NW Ordos basin, China, contain thick (up to 4 km) sequences of nonmarine Triassic strata. These rocks represent a major intraplate sedimentary basin, the paleogeography, tectonic setting and provenance of which are poorly understood and controversial. Studies of the sedimentary geology of the basin, supported by new palinspastic reconstruction of younger deformation, demonstrate that the basin filled from three sides by fluvial, lacustrine-deltaic and alluvial fan depositional systems. The basin forms a westward-thickening wedge that reaches its maximum thickness along the western margin of the Helan Shan and thins to a relatively constant 600–800 m east of the Zhuozi Shan. The stratigraphy of the basin is strongly asymmetric; alluvial fan strata are restricted to the extreme western margin of the basin and interfinger with axial fluvial deposits low in the section and deep lacustrine facies high in the section. Much of the eastern part of the basin is dominated by west-flowing meandering river and deltaic systems. Large structures of Triassic age have not been identified in the Helan Shan or Zhuozi Shan, but small Triassic normal faults have been documented in the western and central Helan Shan. These characteristics most strongly support an extensional origin for the Triassic basin in NW Ordos. The basin is interpreted to have been a north-trending half graben, bound along its western margin by an east-dipping normal fault, presently concealed beneath Quaternary cover west of the Helan Shan. The eastern margin, now found in the Zhuozi Shan, has simple ramp-margin geometry. Driving mechanisms for this extension are not obvious due to limited documentation of Triassic structure throughout the region, but probably relate to far-field stresses from the Qinling or Jinsha active margins interacting with the stable Ordos block.