首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 44 毫秒
1.
Prior to European settlement, the Upper Hunter River near Muswellbrook, New South Wales, was a passively meandering gravel‐bed river of moderate sinuosity and relatively uniform channel width. Analyses of floodplain sedimentology, archival records, parish maps and aerial photographs document marked spatial variability in the pattern of channel change since European settlement in the 1820s. Different types, rates and extents of change are reported for seven zones of adjustment along an 8 km study reach. This variable adjustment reflects imposed antecedent controls (buried terrace material and bedrock), which have significantly influenced local variability in river sensitivity to change, as well as contemporary morphodynamics and geomorphic complexity. Local variability in system responses to disturbance has important implications for future river management and rehabilitation. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

2.
In the twentieth century Polish Carpathian rivers were considerably modified by channelization and gravel mining, with significant detrimental effects to their ecological integrity, vertical stability of the streambeds and flood hazard to downstream river reaches. Restoration of the rivers is thus necessary to improve their ecological status and re‐establish geomorphic dynamic equilibrium conditions. Various approaches to defining hydromorphological reference conditions, proposed to date in river restoration literature, have serious deficiencies. In particular, environmental changes that took place in the catchments of Carpathian rivers during the twentieth century invalidate the historical state of the rivers as reference for their restoration. This is illustrated by a change from bar‐braided to island‐braided channel pattern that occurred in the past century in unmanaged sections of the Czarny Dunajec in response to a reduction in flow and sediment dynamics of the river. We indicate that reference conditions should be defined as those which exist or would exist under present environmental conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river which is to be restored. This assumption was tested through the evaluation of hydromorphological river quality of the Czarny Dunajec according to the European Standard EN‐14614. The evaluation confirmed a high‐status hydromorphological quality in an unmanaged channel section, which can thus be used as a reference for restoration of impacted river sections. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

3.
The properties of rivers and their catchments can be expressed by statistical relationships between geomorphic parameters. These statistical relationships may reveal some inherent differences in geomorphic evolution for different reaches or different order tributaries of a river basin. A case study was undertaken of the Yunnan reach of the Langcangjiang River. The catchment area, channel length and gradient of the first-, second- and third-order tributaries all with catchment areas larger than 100 km2 in the Yunnan reach were the main geomorphic parameters evaluated. The correlation between catchment area and channel length as well as between catchment area and channel gradient with respect to the total tributaries, different reach tributaries, and different order tributaries were revealed using statistical methods. In general, the channel length as a function of catchment area, was best expressed by a quadratic function where channel length increases with increasing catchment area (half parabola), while the channel gradient as a function of catchment area is best expressed by an exponential decay function. Comparison of the best-fit formulas revealed the following phenomena: the lower Yunnan reach tributaries and the first-order tributaries have a dominant effect on geomorphic parameters of the total tributaries. In addition, the statistical relationships indicate that the river geomorphic system in the upper and lower Yunnan reaches evolved differently. This study method used to differentiate river characteristics by determining statistical relationships between geomorphic parameters may be extended to other rivers and their catchments.  相似文献   

4.
How does river hydrology and morphology change due to tidal influence? We contend that this is a question of particular consequence to many earth surface disciplines, but one that has not been adequately addressed. Previous studies have relied on gradients in channel morphology and stratigraphy to infer energy regime of channels. However, in tidal rivers geomorphology influences the energy regime while the energy regime influences morphology; thus, geomorphic and stratigraphic patterns do not fully resolve the mechanisms which lead to change. We addressed this problem by comparing measurements of hydraulic energy and channel morphology along a tidal gradient to predictions of these characteristics in the absence of tides, and attributed the differences to tidal processes. Measurements of discharge, channel area, and energy dissipation (in kJ day–1) were made over a 24·8 hour period at four sites spanning the non‐tidal to tidal freshwater Newport River, NC. We then predicted those characteristics under non‐tidal conditions using hydraulic geometry relationships and literature values from coastal plain rivers. Discharge was enhanced more than 10‐fold by tide, and this tidal effect increased from upstream to downstream along the tidal gradient. Cross‐sectional area increased three‐fold due to tide. Energy dissipation measured in the upper tidal river was four‐fold lower than predicted to occur in the absence of tide because tides decreased average velocity and discharge. Energy dissipation measured downstream was similar to that predicted to occur without tides, although there was large uncertainty in predicted values downstream. While this limited dataset does not permit us to make broad generalizations for definitive models, it does provide a proof‐of‐concept for a new approach to addressing a critical problem at the interface of fluvial and coastal morphology. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

5.
A rising exposure to flood risk is a predicted consequence of increased development in vulnerable areas and an increase in the frequency of extreme weather events due to climate change. In the face of this challenge, a continued reliance on engineered at‐a‐point flood defences is seen as both unrealistic and undesirable. The contribution of ‘soft engineering’ solutions (e.g. riparian forests, wood in rivers) to integrated, catchment scale flood risk management has been demonstrated at small scales but not larger ones. In this study we use reduced complexity hydrological modelling to analyse the effects of land use and channel changes resulting from river restoration upon flood flows at the catchment scale. Results show short sections of river‐floodplain restoration using engineered logjams, typical of many current restoration schemes, have highly variable impacts on catchment‐scale flood peak magnitude and so need to be used with caution as a flood management solution. Forested floodplains have a more general impact upon flood hydrology, with areas in the middle and upper catchment tending to show reductions in peak magnitude at the catchment outflow. The most promising restoration scenarios for flood risk management are for riparian forest restoration at the sub‐catchment scale, representing 20–40% of the total catchment area, where reductions in peak magnitude of up to 19% are observed through de‐synchronization of the timings of sub‐catchment flood waves. Sub‐catchment floodplain forest restoration over 10–15% of total catchment area can lead to reductions in peak magnitude of 6% at 25 years post‐restoration. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

6.
Anticipating geomorphic tipping points requires that we learn from the past. Major geomorphic changes in coastal plain rivers of Texas resulting in river metamorphosis or regime shifts were identified and the major driving factors determined. Eleven such transformations – possible tipping points – were identified from contemporary observations, historical records, and Quaternary reconstructions. Two of the tipping points (between general aggrading and degrading valley states) are associated with reversals in a fundamental system control (sea‐level). One (stable or aggrading versus degrading channels) is associated with an abrupt change in sediment supply due to dam construction, and two others (changes from meandering to anastomosing channel patterns, and different anastomosis styles) are similarly related to changes in sediment supply and/or transport capacity, but with additional elements of historical contingency. Three tipping points are related to avulsions. One, from a regime dominated by re‐occupation of former channels to one dominated by progradation into flood basins, is driven by progressive long‐term filling of incised valleys. Another, nodal avulsions, is triggered by disturbances associated with tectonic uplift or listric faults. The third, avulsions and related valley metamorphosis in unfilled incised valleys, is due to fundamental dynamical instabilities within the fluvial system. This synthesis and analysis suggests that geomorphic tipping points are sometimes associated with general extrinsic or intrinsic (to the fluvial system) environmental change, independent of any disturbances or instabilities. Others are associated with natural (e.g. tectonic) or human (dams) disturbances, and still others with intrinsic geomorphic instabilities. This suggests future tipping points will be equally diverse with respect to their drivers and dynamics. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

7.
Mountainous river basins are one of the main sources of sediment. Over long time scales, sediment production is sustained by the persistent dissection of river basins, which is promoted by tectonic activity. The response or adjustment of rivers to forcing factors such as uplift is based on the concept of the graded river and a feedback mechanism between the incision and uplift. Although the development of graded rivers under natural circumstances has been discussed for a long time, knowledge about the transition of river basins under heterogeneous uplift is not enough. To understand the development of a river basin with a non‐uniform uplift rate, two simple cases are examined: landward and seaward tilting uplift, where the uplift rate varies linearly in space. For our study, laboratory experiments were conducted and the results were compared with those of natural river basins; two river basins in Yakushima Island were selected for this purpose. In both the laboratory and Yakushima, the longitudinal profile of the river basin under landward‐tilting uplift has a convex‐up zone and a specific knickpoint is formed at the upstream end of this zone. This knickpoint is inactive with respect to migration and incision owing to the insufficient cumulative uplift to the equilibrium state. It was also observed in both the experimental and natural cases that the profile of the river basin under seaward tilting is unlikely to have such a convex‐up zone in the long term, and will instead have a smooth concave profile. Therefore, the spatiotemporal pattern of dissection differs depending on the type of tilting uplift, which suggests that sediment production also varies in time and space according to the type of uplift.  相似文献   

8.
There is increasing demand for models that can accurately predict river temperature at the large spatial scales appropriate to river management. This paper combined summer water temperature data from a strategically designed, quality controlled network of 25 sites, with recently developed flexible spatial regression models, to understand and predict river temperature across a 3,000 km2 river catchment. Minimum, mean and maximum temperatures were modelled as a function of nine potential landscape covariates that represented proxies for heat and water exchange processes. Generalised additive models were used to allow for flexible responses. Spatial structure in the river network data (local spatial variation) was accounted for by including river network smoothers. Minimum and mean temperatures decreased with increasing elevation, riparian woodland and channel gradient. Maximum temperatures increased with channel width. There was greater between‐river and between‐reach variability in all temperature metrics in lower‐order rivers indicating that increased monitoring effort should be focussed at these smaller scales. The combination of strategic network design and recently developed spatial statistical approaches employed in this study have not been used in previous studies of river temperature. The resulting catchment scale temperature models provide a valuable quantitative tool for understanding and predicting river temperature variability at the catchment scales relevant to land use planning and fisheries management and provide a template for future studies.  相似文献   

9.
Applications of ergodic reasoning (or location for time substitution) aid efforts at environmental reconstruction and prediction, providing a useful tool to analyse and communicate stages of landscape evolution. Analysis of the historical range of behaviour and change that a river system has experienced can be used to interpret thresholds that have been breached, and underlying controls and/or triggers for adjustment and change. This information can be used to forecast future trajectories of adjustment and provide target conditions for management activities. This paper uses a case study from upper Wollombi Brook, New South Wales, Australia to demonstrate how ergodic reasoning can be used to assess river behaviour, change and responses to natural and human‐disturbances. The ‘river evolution diagram’ developed by Brierley and Fryirs (Geomorphology and River Management: Applications of the River Styles Framework. Blackwell Publishing: Oxford, 2005) is presented as a means for depicting the range of behaviour and evolutionary variability of this river. These approaches can be readily applied in other systems. Implications for approaches to analysis of river evolution and management are outlined. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

10.
Geomorphic effectiveness has been an influential concept in geomorphology since its introduction by Reds Wolman and John Miller in 1960. It provided a much needed framework to assess the significance of an event by comparing event magnitude to the resultant geomorphic effects. Initially, this concept was applied primarily in river channels, under the linear assumption that geomorphic responses to similarly sized flood events will be consistent. Numerous authors have since attempted to quantify a direct, proportional relationship between event magnitude and different forms of geomorphic response in a variety of geomorphic settings. In doing so, these investigations applied an array of metrics that were difficult to compare across different spatiotemporal scales, and physiographic and geomorphic environments. Critically, the emergence of other geomorphic concepts such as sensitivity, connectivity, thresholds, and recovery has shown that relationships between causes (events) and geomorphic effects (responses) are often complex and non‐linear. This paper disentangles the complex historical development of the geomorphic effectiveness concept and reviews the utility of various metrics for quantifying effectiveness. We propose that total energy (joules) is the most appropriate metric to use for quantifying the magnitude of disturbance events (cause) and volumetric sediment flux associated with landform modification is the most appropriate metric for quantifying geomorphic effects. While both metrics are difficult to quantify, they are the only ones which facilitate comparison across a range of spatiotemporal scales (comparability) in a variety of geomorphic environments (flexibility). The geomorphic effectiveness concept can continue to be useful provided that geomorphologists use flexible and comparable metrics. Today, geomorphologists are better prepared to consider the influence of non‐linear processes on determinations of geomorphic effectiveness, allowing investigators to not only determine if a disturbance event was effective but also to explain why or why not. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

11.
The sediment flux from a catchment is driven by tectonics and climate but is moderated by the geomorphic response of the landscape system to changes in these two boundary conditions. Consequently, catchment response time and the non‐linear behavior of landscapes in response to boundary condition change control the downstream propagation of climatic or tectonic perturbations from catchments to neighboring basins. In order to investigate the impact of catchment response time on sediment flux, we integrated a spatially‐lumped numerical model PaCMod, with new routines simulating the evolution of landscape morphology and erosion rates under tectonic and climatic forcing. We subsequently applied the model to reconstruct the sediment flux from a tectonically perturbed catchment in central Italy. Finally, we coupled our model to DeltaSim, a process‐response model simulating fluvio‐deltaic stratigraphy, and investigated the impact of catchment response time on stratigraphy, using both synthetic scenarios and a real world system (Fucino Basin, central Italy). Our results demonstrate that the differential response of geomorphic elements to tectonic and climatic changes induces a complex sediment flux signal, and produces characteristic stratigraphic architectures and shoreline trajectories. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

12.
Discharge and sediment load data for several stations along the Ganga River and its major tributaries in the western Ganga plains (WGP) for a period of ~30 years have been analysed to understand the hydrological characteristics and sediment dynamics. In terms of hydrology, the rivers are less flood‐prone than believed, exceeding bankfull discharges less frequently than the expected 1.5 year return interval. This has been attributed to the rivers of this region occupying incised valleys formed in the Late Quaternary period. Rivers draining the WGP are supply‐limited systems compared to those draining the eastern Ganga plains (EGP) which have been characterized as transport‐limited systems. We suggest that such geomorphic diversity as a function of spatial variability in precipitation regime and hinterland geology has existed for at least the Late Quaternary period and they in turn influence the modern day hydrology of the river systems in a significant way. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

13.
High Arctic river responses to changing hydroclimatic and landscape processes are poorly understood. In non‐glacierized basins, snowmelt and rainfall generate river discharge, which provides first order control over fluxes. Further factors include the seasonality of precipitation, seasonal active layer development, and permafrost disturbance. These controls were evaluated in terms of sedimentary and biogeochemical fluxes from paired catchments at Cape Bounty, Melville Island, Nunavut during 2006–2009. Results indicate that the source of runoff can be more important than the amount of runoff for sediment, solutes, and organic yields. Although the snowmelt period is typically the most important time for these yields, heavy late summer precipitation events can create disproportionately large yields. Rainfall increases yields because it hydrologically connects areas otherwise isolated. Inorganic solute yields from late summer rainfall are higher because the thick active layer maximizes hydrologic interactions with mineral soils and generates high solute concentrations. Results also indicate that while the catchments are broadly similar, subtle topographic differences result in important inter‐catchment differences in runoff and suspended and dissolved loads. The East watershed, which had less extensive permafrost disturbance, consistently had higher concentrations of dissolved solids. These higher dissolved fluxes cannot therefore be explained by thermokarst features, but rather by deeper active layer development, due to a greater proportion of south‐facing slopes. Although warm temperatures in 2007 led to extensive active layer disturbance in the West watershed, because the disturbances were largely hydrologically disconnected, the total disturbed area was small, and inter‐annual variability in discharge was high, there was no detectable response in dissolved loads to disturbances. Sediment availability increased after 2007, but yields have largely returned to pre‐disturbance levels. Results indicate that seasonality and frequency‐magnitude characteristics of projected increases in precipitation must be considered along with active layer changes to predict the fluvial sedimentary and biogeochemical response to regional climate change. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
Climate change is inevitably altering the hydrological regime of water bodies. The interest in changing behaviour of intermittent rivers is increasing in many countries. This research was focused on intermittent rivers (rivers which naturally, periodically cease to flow) in Lithuania. The purpose of this research was to provide an overview of flow intermittency phenomena according to available data in a historical period and to evaluate the impact of catchment geographical features and climate variability on zero-flow events. The calculated indices of flow intermittency showed that the selected rivers had very different flow regimes. The threshold for the separation of typically intermittent rivers from only occasionally intermittent ones was suggested. Multiple linear regression analysis defined the crucial role of catchment size and watercourse slope on the river cessation process in Lithuania. The applied non-parametric Wilcoxon–Mann–Whitney test revealed the significance of the relationship between precipitation (in June–September) and zero-flow duration. Flow intermittency phenomena in Lithuanian rivers were linked to a low-frequency teleconnection pattern (SCAND index). A methodology of estimating the relation between river intermittency and large-scale atmospheric circulation pattern (based on SCAND index) was created. The generated regression equations between flow intermittency indices and catchment characteristics might be useful for the estimation of zero-flows in ungauged river catchments. The main aspect of future investigations might be related to forecasting flow intermittency using modern hydrological models and climate scenarios as well as the defined relationships between zero-flow indices and physico-geographical features of river catchments.  相似文献   

15.
Confluence–diffluence units are key elements within many river networks, having a major impact upon the routing of flow and sediment, and hence upon channel change. Although much progress has been made in understanding river confluences, and increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry, most studies have been conducted in laboratory flumes or within small rivers with width:depth (aspect) ratios less than 50. This paper presents results of a field‐based study that details the bed morphology and 3D flow structure within a very large confluence–diffluence in the Río Paraná, Argentina, with a width:depth ratio of approximately 200. Flow within the confluence–diffluence is dominated largely by the bed roughness, in the form of sand dunes; coherent, channel‐scale, secondary flow cells, that have been identified as important aspects of the flow field within smaller channels, and assumed to be present within large rivers, are generally absent in this reach. This finding has profound implications for flow mixing rates, sediment transport rates and pathways, and thus the interpretation of confluence–diffluence morphology and sedimentology. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

16.
The HIRHAM regional climate model suggests an increase in temperature in Denmark of about 3 °C and an increase in mean annual precipitation of 6–7%, with a larger increase during winter and a decrease during summer between a control period 1961–1990 and scenario period 2071–2100. This change of climate will affect the suspended sediment transport in rivers, directly through erosion processes and increased river discharges and indirectly through changes in land use and land cover. Climate‐change‐induced changes in suspended sediment transport are modelled for five scenarios on the basis of modelled changes in land use/land cover for two Danish river catchments: the alluvial River Ansager and the non‐alluvial River Odense. Mean annual suspended sediment transport is modelled to increase by 17% in the alluvial river and by 27% in the non‐alluvial for steady‐state scenarios. Increases by about 9% in the alluvial river and 24% in the non‐alluvial river were determined for scenarios incorporating a prolonged growing season for catchment vegetation. Shortening of the growing season is found to have little influence on mean annual sediment transport. Mean monthly changes in suspended sediment transport between ? 26% and + 68% are found for comparable suspended sediment transport scenarios between the control and the scenario periods. The suspended sediment transport increases during winter months as a result of the increase in river discharge caused by the increase in precipitation, and decreases during summer and early autumn months. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

17.
  相似文献   

18.
We describe additions made to a multi‐size sediment routing model enabling it to simulate width adjustment simultaneously alongside bed aggradation/incision and fining/coarsening. The model is intended for use in single thread gravel‐bed rivers over annual to decadal timescales and for reach lengths of 1–10 km. It uses a split‐channel approach with separate calculations of flow and sediment transport in the left and right sides of the channel. Bank erosion is treated as a function of excess shear stress with bank accretion occurring when shear stress falls below a second, low, threshold. A curvature function redistributes shear stress to either side of the channel. We illustrate the model through applications to a 5·6‐km reach of the upper River Wharfe in northern England. The sediment routing component with default parameter values gives excellent agreement with field data on downstream fining and down‐reach reduction in bedload flux, and the width‐adjustment components with approximate calibration to match maximum observed rates of bank shifting give plausible patterns of local change. The approach may be useful for exploring interactions between sediment delivery, river management and channel change in upland settings. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

19.
Large wood tends to be deposited in specific geomorphic units within rivers. Nevertheless, predicting the spatial distribution of wood deposits once wood enters a river is still difficult because of the inherent complexity of its dynamics. In addition, the lack of long‐term observations or monitored sites has usually resulted in a rather incomplete understanding of the main factors controlling wood deposition under natural conditions. In this study, the deposition of large wood was investigated in the Czarny Dunajec River, Polish Carpathians, by linking numerical modelling and field observations so as to identify the main factors influencing wood retention in rivers. Results show that wood retention capacity is higher in unmanaged multi‐thread channels than in channelized, single‐thread reaches. We also identify preferential sites for wood deposition based on the probability of deposition under different flood scenarios, and observe different deposition patterns depending on the geomorphic configuration of the study reach. In addition, results indicate that wood is not always deposited in the geomorphic units with the highest roughness, except for low‐magnitude floods. We conclude that wood deposition is controlled by flood magnitude and the elevation of flooded surfaces in relation to the low‐flow water surface. In that sense, the elevation at which wood is deposited in rivers will differ between floods of different magnitude. Therefore, together with the morphology, flood magnitude represents the most significant control on wood deposition in mountain rivers wider than the height of riparian trees. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

20.
Climate change is one of the main drivers of river warming worldwide. However, the response of river temperature to climate change differs with the hydrology and landscape properties, making it difficult to generalize the strength and the direction, of river temperature trends across large spatial scales and various river types. Additionally, there is a lack of long‐term and large‐scale trend studies in Europe as well as globally. In this study, we investigated the long‐term (25 years; 132 sites) and the short‐term (10 years; 475 sites) river temperature trends, patterns and underlying drivers within the period 1985–2010 in seven river basins of Germany. The majority of the sites underwent significant river warming during 1985–2010 (mean warming trend: 0.03 °C year?1, SE = 0.003), with a faster warming observed during individual decades (1985–1995 and 2000–2010) within this period. Seasonal analyses showed that, while rivers warmed in all seasons, the fastest warming had occurred during summer. Among all the considered hydro‐climatological variables, air temperature change, which is a response to climate forcing, was the main driver of river temperature change because it had the strongest correlation with river temperature, irrespective of the period. Hydrological variables, such as average flow and baseflow, had a considerable influence on river temperature variability rather than on the overall trend direction. However, decreasing flow probably assisted in a faster river temperature increase in summer and in rivers in NE basins (such as the Elbe basin). The North Atlantic Oscillation Index had a greater significant influence on the winter river temperature variability than on the overall variability. Landscape and basin variables, such as altitude, ecoregion and catchment area, induced spatially variable river temperature trends via affecting the thermal sensitivity of rivers, with the rivers in large catchments and in lowland areas being most sensitive. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号