A genetic classification of floodplains

Floodplains are formed by a complex interaction of fluvial processes but their character and evolution is essentially the product of stream power and sediment character. The relation between a stream's ability to entrain and transport sediment and the erosional resistance of floodplain alluvium that forms the channel boundary provides the basis for a genetic classification of floodplains. Three classes are recognised: (1) high-energy non-cohesive; (2) medium-energy non-cohesive; and (3) low-energy cohesive floodplains. Thirteen derivative orders and suborders, ranging from confined, coarse-grained, non-cohesive floodplains in high-energy environments to unconfined fine-grained cohesive floodplains in low-energy environments, are defined on the basis of nine factors (mostly floodplain forming processes). These factors result in distinctive geomorphological features (such as scroll bars or extensive backswamps) that distinguish each floodplain type in terms of genesis and resulting morphology. Finally, it is proposed that, because floodplains are derivatives of the parent stream system, substantial environmental change will result in the predictable transformation of one floodplain type to another over time.     

Saldanha Bay,South Africa II: estimating bay productivity

Saldanha Bay is a narrow-mouth bay on the west coast of South Africa linked to the southern Benguela upwelling system. Bay productivity was investigated by use of the conventional light-and-dark bottle oxygen method, and, for comparison, through assimilation of the stable isotope tracer ¹³C. Gross community production GCP and net community production NCP, as determined from the oxygen method, were respectively 2.6 and 2.4 times higher than estimates determined from the stable isotope method. Chlorophyll a (Chl a) concentrations increased with the onset of spring and well-defined subsurface maxima developed in association with increasingly stratified conditions (mean water column Chl a concentrations ranged from 5.4 to 31.5?mg m^?3 [mean 15.5?mg m^?3; SD 7.6]). A sharp decline in photosynthetic rates P* (GCP normalised to Chl a concentration) with depth was attributed to light limitation, as demonstrated by the high vertical attenuation coefficients for downward irradiance K_d, which varied from 0.29 to 0.70?m^?1 (mean 0.48?m^?1; SD 0.12). Productivity maxima were consequently near-surface despite the presence of deeper subsurface biomass maxima. The community compensation depth Z_cc, where gross community production balances respiratory carbon loss for the entire community, ranged from 2.9 to 9.2?m (mean 5.8?m; SD 2.2), and was typically shallower than the 1% light depth for PAR (photosynthetically available radiation), Z1%PAR, which is traditionally assumed to be the depth of the euphotic zone and which ranged from 6.6 to 15.9?m (mean 9?m; SD 2.6). Autotrophic communities, where organic matter is produced in excess of respiratory demand, were confined on average to the upper 5.8?m of the water column, and often excluded the bulk of the phytoplankton community, where light limitation is considered to lead to heterotrophic community metabolism. Estimates of integrated water column productivity ranged from 0.84 to 8.46?g C m–2 d^?1 (mean 3.35?g C m^?2 d^?1; SD 1.9).     

Comparison of thermoluminescence and radiocarbon age-determinations from late-pleistocene alluvial deposits near Sydney, Australia

Radiocarbon and thermoluminescence (TL) age-determinations have been obtained for a large Pleistocene alluvial terrace on the Nepean River near Sydney, New South Wales, Australia. The deposit was laid down by a braided river system prior to the last glacial maximum. Six thoroughly pretreated samples of charcoal and degraded wood buried within gravels at the base of the terrace yielded apparent ¹⁴C ages of 37,000–42,000 yr B.P. These compare favorably with four TL determinations that gave apparent ages of 41,000–47,000 yr B.P. for the same deposit. Adjustment of the ¹⁴C ages to take geomagnetic effects into account further improves the correlation between these two independent dating techniques. In addition, ¹⁴C and TL correctly identified a reworked portion of the fine-grained alluvial overburden as being substantially younger than the main body of the terrace. These results augur well for the usefulness of TL age determinations of certain alluvial deposits.     

Hydraulic geometry and maximum flow efficiency as products of the principle of least action

Basic flow relationships have previously been seen to be insufficient to explain the self‐adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self‐adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright © 2000 John Wiley & Sons, Ltd.     

Serious problems in using equations to estimate bedload yields for coastal rivers in NSW

Bedload yields have been calculated using eight bedload equations at a total of 11 gauging sites in four coastal river basins in New South Wales. Comparisons of yields calculated by each equation at each site show enormous variations. Furthermore, on the Manning River, where calculations could be made on the four main tributaries and compared to those from the trunk channel below their combined confluence, there was no recognisable continuity of results. For the following reasons, the use of bedload formulae on these rivers appears to be a futile exercise. Firstly, the formulae appear to be inherently unstable under natural field conditions. Secondly, application of the formulae must rely on extrapolated flow data, as actual flow measurements are rarely conducted at discharges that are more than a small fraction of largest discharges recorded at any site. Thirdly, formulae must be applied assuming an unlimited availability of bed material; yet the rivers studied here behave as ‘conveyor belts’ of considerable power but with very low and irregular rates of sediment feed. Finally, temporal step‐functional shifts in climate and flow regimes are shown to have an important impact on estimation of sediment yields. The implication of these results is that, until there is a carefully monitored scientific program of bedload measurement or estimates of reservoir sedimentation on the rivers of south eastern Australia, there can be no reliable evaluation of sediment yields from these rivers. As a result, the impact of gravel extraction, the dispersal of mine tailings, or the construction of dams can not be adequately assessed for this region, nor probably for the rest of Australia.     

Flow fields in tightly curving meander bends of low width‐depth ratio

Upland swamp channels with low width/depth ratios (w/d), armoured beds, minimal sediment loads, tightly curving bends and an absence of point bars provide a striking contrast to the flow characteristics of larger channels with higher w/d ratios. Two subsets of these bends were examined in relation to their patterns of cross‐stream flow relative to the channel boundary. The first, with mean w/d = 2·0 and gentle barforms, exhibited even velocity distributions at bend entrances but developed vertically stacked pairs of maximum velocity filaments (MVFs). Cross‐stream circulation increased with decreasing curvature before essentially ceasing in the tightest bend due to the conservation of angular momentum and reduced vertical velocity differentials; bed friction has more limited influence in narrow deep channels relative to bank friction. In the second subset of bends, with larger w/d (mean 4·8) and much steeper barforms, the MVFs were laterally paired and strongly helical flow was partly driven by the vertical confinement of flow due to large, stable barforms at the bend entrances. In one bend, the velocity profile became inverted immediately past the apex and caused helical flow to abruptly reverse. Point bars in relatively wide bedload channels appear to greatly distort secondary flow patterns. In narrow, deep, sediment‐starved channels, separation zones against the convex and/or the concave bank deliver the flow confinement that would otherwise be provided by point bars or concave‐bank benches. In these channels, separation zones are important for protecting both the channel bed and banks from scour. Three‐dimensional near bankfull flow fields are presented for one bend with a meander pool; inward shifting of the MVF and limited sediment supply are proposed as mechanisms for the development and maintenance of these features. These flow data in narrow and deep peatland channels demonstrate very different flow patterns and morphological characteristics relative to the more commonly studied wide, shallow channels with more abundant sediment. Copyright © 2009 John Wiley & Sons, Ltd.     

青海锡铁山铅锌矿床硫同位素地球化学研究——深源与海水硫的混合

锡铁山铅锌矿床发育较为完整的喷流沉积系统,包括管道相、近喷口相、远端沉积相及各种喷流沉积岩,并有后期改造作用形成的脉状铅锌矿体。本文通过喷流沉积系统各部位硫化物硫同位素的分析,不同部位硫化物硫同位素组成不同,且规律性变化。以黄铁矿分析结果为例,网脉状石英钠长岩δ34S=+0.8‰,代表供给系统的硫化物脉2.95‰,非层状矿体4.48‰,层状矿体3.25‰,炭质片岩为+6.26‰,后期改造型铅锌矿脉为+2.93‰。代表管道相的网脉状石英钠长岩黄铁矿具有深源(幔源)的硫同位素组成,而矿体或大理岩上盘炭质片岩具有海水硫来源的特点。矿体的硫介于二者之间,更靠近炭质片岩的硫化物同位素组成,其来源可能更多受海水硫酸盐的制约,即锡铁山矿床硫具有混合来源性质,主要是海水硫酸盐的还原,部分来源于深部卤水的供给。硫的还原方式以生物细菌还原为主。层状矿体中硫同位素组成由早至晚δ34S逐渐降低,表明层状矿体成矿作用过程中,发生了生物成因的H2S的大量加入。     

Fluvial transport as a natural luminescence sensitiser of quartz

The optically stimulated luminescence (OSL) sensitivity of quartz sampled from the bed of the Castlereagh River in inland New South Wales increases linearly with distance downstream, through both a proportional increase in the number of luminescent grains and increases in the sensitivity of individual grains. It is argued that downstream transport provides numerous opportunities for repeated irradiation and bleaching which combine to increase sensitivity of the quartz grains. Individual quartz grains collected from the uppermost sampling site on the Castlereagh River increase in sensitivity in response to repeated cycles of laboratory irradiation, heating and illumination, providing an explanatory analogue. Furthermore, initially non-luminescent grains are shown to be ‘switched on’ by this same laboratory treatment. We conclude that downstream increases in the luminescence sensitivity of quartz observed in the Castlereagh River are due to intrinsic changes within the quartz and not due to any macro changes in the grains, for example polishing, or abrasion and loss of non-luminescent grains. We also infer that the high OSL sensitivity of sedimentary quartz from Australia is due to the predominance of environments which provide numerous opportunities for repeated irradiation, illumination and heating. Observation of the change in luminescence sensitivity of quartz bedload has the potential to provide additional information on the nature of bedload transport processes. Data from the Castlereagh River indicate that the rate of bedload transport is approximately constant along the 325 km sampled reach.     

Flow transmission along an arid zone anastomosing river,cooper creek,australia

Cooper Creek is characterized by an exceptionally wide floodplain and a complex system of anastomosing channels which together can accommodate a large range of highly variable flows. In consequence flood frequency curves are unusually steep. With minimal tributary contribution over the two study lengths — Currareva-Nappa Merrie (> 400 km) and Nappa Merrie-Innamincka (32 km) — downstream output is largely controlled by upstream input, so that similar flow regimes and hydrograph forms can be maintained despite transmission losses and the variable activation of channels. Input-output relationships defined in terms of total flow volume and peak discharge suggest that, above a threshold flow of about 25% duration, transmission losses exceed 75% on average over the Currareva-Nappa Merrie length. The large difference in absorption between primary channels on the one hand and subsidiary channels and the floodplain surface on the other means that transmission losses vary non-linearly with stage. They are relatively low when flow is confined to the primary channels, but increase at higher stages as lesser channels and the floodplain are activated. Early levels do not recur until flows of < 0.5% duration are attained when output/input ratios increase rapidly with discharge. The influences of seasonality and background flow on output discharge are small relative to that of input discharge. Despite the long river distance, peak flows at Currareva and Nappa Merrie are well correlated. However, over the shorter Nappa Merrie-Innamincka length with its more confined system of channels, correlations are better and transmission losses less. They vary from 60% just above the threshold discharge to <10% at flows below 20% duration. Over this length outflow hydrographs can be effectively estimated by the three parameter Muskingum procedure, at least on an event by event basis, but further application must await additional records to understand fully how the parameters vary in an environment where streamflow is transmitted through a highly variable system of channels.     

Analytical models for velocity distributions in compound channels with emerged and submerged vegetated floodplains

The lateral distributions of depth-averaged velocity in open compound channels with emerged and submerged vegetated floodplains were analyzed based on the analytical solution of the depth-integrated Reynolds-Averaged Navier-Stokes equation with a term to account for the effects of vegetation. The three cases considered for open channels were two-stage rectangular channel with emerged vegetated floodplain, rectangular channel with submerged vegetated corner, and two-stage rectangular channel with submerged vegetated floodplain, respectively. To predict the depth-averaged velocity with submerged vegetated floodplains, we proposed a new method based on a two-layer approach where flow above and through the vegetation layer was described separately. Moreover, further experiments in the two-stage rectangular channel with submerged vegetated floodplain were carried out to verify the results. The analytical solutions of the cases indicated that the corresponding analytical depth-averaged velocity distributions agree well with the simulated and experimental prediction. The analytical solutions of the cases with theoretical foundation and without programming calculation were reasonable and applicable, which were more convenient than numerical simulations. The analytical solutions provided a way for future researches to solve the problems of submerged vegetation and discontinuous phenomenon of depth-averaged velocity at the stage point for compound channels. Understanding the hydraulics of flow in compound channels with vegetated floodplains is very important for supporting the management of fluvial processes.