Physical and chemical characteristics of a metal-contaminated overbank deposit,west-central South Dakota,U.S.A.

A metal-contaminated overbank deposit in west-central South Dakota resulted from the discharge of a large volume of mine tailings into a river system between the late 1800s and 1977. The deposit along the Belle Fourche River is typically up to 2 m thick and extends about 90 m away from the channel along the insides of meander bends. The sediments contain above-background levels of copper, iron, manganese, zinc, and particularly arsenic, which is commonly two orders of magnitude above background level in the contaminated sediments. Carbonate minerals in the deposit limit the desorption of arsenic by preventing acid formation. Arsenic concentrations provide a measure of the dilution of mine tailings by uncontaminated sediment. The arsenic appears to have been transported and deposited as arsenopyrite, but is now at least partially associated with iron oxides and hydroxides. Within individual samples, arsenic concentration has an inverse relation with grain size that results from the more efficient accumulation of arsenic on the greater surface area of the smaller particles. Arsenic concentration is inversely related to the sample weight percent finer than 16 μm, however, as a consequence of the dilution of the contaminated sediments by uncontaminated sediment with a finer grain-size distribution. Dilution by uncontaminated sediment from tributaries cause arsenic concentrations to decrease by a factor of 3 along 100 km of floodplain. An influx at high streamflow of uncontaminated sediment from terraces and the premining floodplain as well as from tributaries causes arsenic concentrations in parts of the contaminated deposit that are farthest away from the channel to be two to three times less than arsenic concentrations in overbank sediment that is immediately adjacent to the channel.     

Channel change and the significance of floodplain stratigraphy: 1990 flood event,lower river Tay,Scotland

During February 1990 a flood event on the River Tay with an estimated recurrence interval of 70 years in its lower reaches caused extensive flooding and geomorphological change. The most extensive area of flooding and dramatic geomorphological change, the erosion of two new channels across the floodplain, was in the vicinity of the village of Caputh, Perthshire. The location of the eroded channels relates to the position of former river courses, identifiable by depressions in the floodplain surface and floodplain stratigraphy. The lower floodplain surface elevation and occurrence of gravel and sand deposits along the line of the former river courses caused the area to be more vulnerable to erosion and controlled the morphology of the eroded channels.     

River behaviour and Holocene alluviation: The River Severn at Welshpool,mid-Wales,U.K.

A combination of archaeological evidence, ¹⁴C dates, terrace mapping, heavy metal analysis, grain size analysis and historical maps is used in a detailed analysis of the alluvial history of the River Severn floodplain around Welshpool in mid-Wales, U.K. ‘Welshpool Gravels’ underlie a higher terrace surface up to 6–7 m above the present channel. They form a sequence of gravels at least 30 m in thickness. The upper surface is characterized by a series of braided palaeochannel patterns. These sediments were probably deposited at the end of the last glaciation as outwash, and are contemporaneous with other high, gravelly terrace deposits found in the Severn and other mid-Wales basins. Overlying the Welshpool Gravels on the contemporary floodplain are a variable thickness of finer sediments, the ‘Leighton Silts’. Morphological mapping and dating of two cut-offs to 2850 ± 60 a BP and 1190 ± 70 a BP indicates that a channel pattern similar to the present planform had formed by the mid to late Holocene. From this period, floodplain development has been dominated by a single-thread meandering channel with fine vertical sedimentation and limited lateral gravel accretion. Abandonment of extended lengths of channel formed by an avulsion mechanism is apparent. A combination of historical map data, ¹⁴C dates and the analysis for heavy metals in fine sediments, which were washed into the river system during mining, indicates that there has been at least 4 m of sedimentation since the early 17th century, but only in a central belt of varying width. Metal-rich waste, identified in the fine sediments of this zone of ‘Trehelig Silts’, indicates those areas which were most heavily sedimented during the peak of metalliferous mining in the 18th and 19th centuries. Although the near-channel margins appear to be superficially similar to the older floodplain, the spatial and vertical pattern of historic sedimentation is complex, and is not reflected in marked elevation differences. The division of sedimentation periods into these three broad time-spans (Late Quaternary Terraces, Late Holocene alluviation and avulsion, and the historical metal-mining period) shows that an apparently simple planar floodplain is in reality underlain by complex sedimentation units. Floodplain construction has involved the development of inset units, in cut-offs and adjacent to migrating channels, as well as the expected contrasts between in-channel and overbank environments. This has implications both for alluvial sedimentation modelling and for the identification of high-pollution zones on the floodplain. These cannot be predicted on the basis of simple ‘in-channel’ and ‘overbank’ environments given the historically complex evolution.     

On seasonal approach to flood frequency modelling. Part II: flood frequency analysis of Polish rivers

The annual peak flow series of Polish rivers are mixtures of summer and winter flows. As Part II of a sequence of two papers, practical aspects of applicability of seasonal approach to flood frequency analysis (FFA) of Polish rivers are discussed. Taking A Two‐Component Extreme Value (TCEV1) model as an example it was shown in the first part that regardless of estimation method, the seasonal approach can give profit in terms of upper quantile estimation accuracy that rises with the return period of the quantile and is the greatest for no seasonal variation. In this part, an assessment of annual maxima (AM) versus seasonal maxima (SM) approach to FFA was carried out with respect to seasonal and annual peak flow series of 38 Polish gauging stations. First, the assumption of mutual independence of the seasonal maxima has been tested. The smoothness of SM and AM empirical probability distribution functions was analysed and compared. The TCEV1 model with seasonally estimated parameters was found to be not appropriate for most Polish data as it considerably underrates the skewness of AM distributions and upper quantile values as well. Consequently, the discrepancies between the SM and AM estimates of TCEV1 are observed. Taking SM and TCEV1 distribution, the dominating season in AM series was confronted with predominant season for extreme floods. The key argument for presumptive superiority of SM approach that SM samples are more statistically homogeneous than AM samples has not been confirmed by the data. An analysis of fitness to SM and AM of Polish datasets made for seven distributions pointed to Pearson (3) distribution as the best for AM and Summer Maxima, whereas it was impossible to select a single best model for winter samples. In the multi‐model approach to FFA, the tree functions, i.e., Pe(3), CD3 and LN3, should be involved for both SM and AM. As the case study, Warsaw gauge on the Vistula River was selected. While most of AM elements are here from winter season, the prevailing majority of extreme annual floods are the summer maxima. The upper quantile estimates got by means of classical annual and two‐season methods happen to be fairly close; what's more they are nearly equal to the quantiles calculated just for the season of dominating extreme floods. Copyright © 2011 John Wiley & Sons, Ltd.     

Drainage basin characteristics,their selection,derivation and analysis for a flood study of the British isles

A large data bank of morphometric, cover and climatic data for over a thousand drainage basins of all sizes in the British Isles was set up by the N.E.R.C. Flood Study. The paper describes the selection of these ‘independent’ variables, their degree of correlation, their portrayal of regional variability in the British Isles and their success as predictors of flood variables. The internal adjustment of drainage basins is confirmed, as is the basic physical division of Britain.     

Numerical modelling of floodplain hydraulics and suspended sediment transport and deposition at the event scale in the middle river Elbe,Germany

The objective of this case study was to calibrate and verify detailed transport model of sediment in a 4‐kilometre stretch of the middle Elbe floodplains in Germany. The hydraulic RMA‐2 model and the SED2d‐WES sediment transport model were used. These models were calibrated and validated by detailed measurement of the surface water elevations, the velocities at six profiles, and the suspended sediment concentration and deposition (by means of 10 sediment traps). The flow was modelled for three steady‐state discharges. The surface water elevations were calculated to an accuracy of less than 5 cm compared to measurements. The differences between the calculated and measured velocities were with one exception smaller than 0.2 m/s (measured range 0.1…?1.0 m/s). An average sediment input of 35 g/(m² d) was calculated for the flood event studied. The highest calculated sedimentation rates of 700 g/(m² d) (dry density 90 kg/m³) occurred in quiescent zones and abandoned channels. Twenty‐five percent of the deposited sediment settled in the quiescent zones (which only account for 13% of the area). The most sensitive parameters of the sediment transport model were the settling velocity and critical shear stress. The modelling techniques used allowed sediment deposition on the floodplains of the Elbe to be realistically depicted.     

Hydrodynamics of floodplain wetlands in a chalk catchment: The River Lambourn, UK

Variations in floodplain channel water levels and valley floor groundwater levels (measured in piezometers and boreholes) are examined at selected points along the course of the River Lambourn, a chalk river in southern England. A local alluvial gravel aquifer in the valley bottom is associated with numerous small wetlands that extend over much of the river's perennial profile. Variations in hydraulic gradient between local borehole levels and/or floodplain channel water levels are described for three sites in the seasonal section of the channel at Bockhampton, East Garston and West Shefford. The results indicate that observed groundwater levels are closely associated with flows from discrete springs at the margins of the channel and floodplain. However, as the floodplain widens and the alluvial gravel aquifer increases in size, the gravel aquifer accounts for a substantial down-valley component of groundwater flow with a diffuse vertical water flux. In the lower catchment, the exchange of flows between the gravel aquifer and the river enables some attenuation of floodplain water-table variability, providing a stable hydrological regime for valley-bottom wetlands. Catchment controls upon the local, valley-bottom, wetland regime are demonstrated with the application of a simple groundwater model developed using MODFLOW. The model is used to simulate groundwater discharge to the river in the upper and lower catchment, in addition to the water level regime at selected points in the valley bottom in the lower catchment. The results demonstrate the importance of taking catchment-scale water flow into account when managing isolated wetlands in a permeable catchment.     

Wave-movement and water-quality variations during a controlled release from Kielder Reservoir, North Tyne River, U.K.

Data are presented to describe the at-a-station variations and downstream patterns of change, of flow and water quality during the passage of a controlled reservoir release along a short 10 km reach, immediatly below the dam. By removing the effects of runoff from diverse catchment sources, which characterise natural flow variations, reservoir releases are used to focus attention on the effects of channel characteristics. At each of four main sites, measurements were made at 4 min intervals for at least 4 h. The data illustrate the dominant effect of initial flow conditions, especially channel roughness, upon wave movement, suspended solids transport, and hydrochemical lags. Variations in the patterns of change appear to relate to spatially variable in-channel sources which can be particularly influential within such short study reaches.     

Use of a derived distribution approach for flood prediction in poorly gauged basins: A case study in Italy

A derived distribution approach is developed for flood prediction in poorly gauged basins. This couples information on the expected storm scaling, condensed into Depth Duration Frequency curves, with soil abstractions modeled using Soil Conservation Service Curve Number method and hydrological response through Nash’s Instantaneous Unit Hydrograph. A simplified framework is given to evaluate critical duration for flood design. Antecedent moisture condition distribution is included. The method is tested on 16 poorly gauged Mediterranean watersheds in Tyrrhenian Liguria, North Western Italy, belonging to a homogeneous hydrological regions. The derived flood distribution is compared to the regional one, currently adopted for flood design. The evaluation of Curve Number is critical for peak flood evaluation and needs to be carefully carried out. This can be done including local Annual Flood Series data in the estimation of the derived distribution, so gathering the greatest available information. However, Curve Number influence decreases for the highest return periods. When considerable return periods are required for flood design and few years of data are available, the derived distribution provides more accurate estimates than the approach based on single site distribution fitting. A strategy based on data availability for application of the approach is then given. The proposed methodology contributes to the ongoing discussion concerning PUB (Prediction in Ungauged Basins) decade of the IAHS association and can be used by researchers and practitioners for those sites where no flood data, or only a few, are available, provided precipitation data and land use information are at hand.     

Multivariate analysis of flood characteristics in a climate change context of the watershed of the Baskatong reservoir,Province of Québec,Canada

The analysis of the impact of climate change (CC) on flood peaks has been the subject of several studies. However, a flood is characterized not only by its peak, but also by other characteristics such as its volume and duration. Little effort has been directed towards the study of the impact of CC on these characteristics. The aim of the present study is to evaluate and compare flood characteristics in a CC context, in the watershed of the Baskatong reservoir (Province of Québec, Canada). Comparisons are based on observed flow data and simulated flow series obtained from hydrological models using meteorological data from a regional climate model for a reference period (1971–2000) and a future period (2041–2070). To this end, two hydrological models HSAMI and HYDROTEL are considered. Correlations, stationarity, change‐points, and the multivariate behaviour of flood series were studied. The results show that, at various levels, all flood characteristics could be affected by CC. Copyright © 2011 John Wiley & Sons, Ltd.     

Colonisation of epilithic diatom population of the River Taff, South Wales, U.K.

1. A light and scanning electron microscopic study of the structure of epilithic diatom communities was carried out at two sites on the River Taff, South Wales, an upstream unpolluted site (Site 1) and a downstream polluted site (Site 2). 2. Community structure at the micro-scale revealed that, although communities at both sites were similar in terms of species present (similarity indices ranging between 0.44 and 0.96), they were very different with respect to their physical structure. 3. The traditional approach in studying epilithic diatom communities ignores information relating to community structure. The combination of light and scanning electron microscopy proved to be most useful for studying the epilithic diatom communities. 4. The examination of community succession on stone substrata showed that communities were highly dynamic on a weekly time scale, which suggests that the “age” of a substratum is a very important determinant of the attaching communities. A change from a prostate, two-dimensional community structure to an erect, three-dimensional community structure, and a shift from the dominance of epilithic diatoms to the dominance of epiphytic diatoms were noted. Such a change was more noticed at the polluted downstream site, possibly due to a higher rate of surface pre-conditioning and biofilm production at this site than at the unpolluted upstream site.     

Water isotope characteristics of a flood: Brisbane River,Australia

Flooding associated with tropical storms can cause extreme perturbations in riverine and coastal ecosystems. Measuring isotope variability of tropical storm events can help investigate the impacts of flooding. We measured the water isotope composition (δD and δ¹⁸O) of rain and associated floodwater collected during two storms and subsequent major and minor flooding events in the subtropical coast of eastern Australia. Compared with baseline regional rainfall isotope values of ?15.0 ± 1.9‰ for δD and ?3.3 ± 0.2‰ for δ¹⁸O, floodwater had lower values with ?33.8 ± 2.5‰ δD and ?5.1 ± 0.4‰ δ¹⁸O for the major flood and ?29.4 ± 1.0‰ δD and ?4.6 ± 0.1‰ δ¹⁸O for the minor flood. The low isotope composition of the floodwater was associated with the transport of large quantities of suspended sediments, with sediment loads 30 to 70 times larger than during base flow conditions. Floods carried up to 35% of the annual phosphorus and up to 208% of the currently calculated average annual nitrogen load of the Brisbane River. The dramatic changes caused by a rapid increase in discharge from 2 to 2015 m³ s^?1 over 2 days in the major flood would have major consequences in riverine and coastal ecosystems of the region. These changes could potentially be traced using the isotope composition of the floodwaters. Copyright © 2015 John Wiley & Sons, Ltd.     

Changes to flood peaks of a mountain river: implications for analysis of the 2013 flood in the Upper Bow River,Canada

The mountain headwater Bow River at Banff, Alberta, Canada, was subject to a large flood in June 2013, over which considerable debate has ensued regarding its probability of occurrence. It is therefore instructive to consider what information long‐term streamflow discharge records provide about environmental change in the Upper Bow River basin above Banff. Though protected as part of Banff National Park, since 1885, the basin has experienced considerable climate and land cover changes, each of which has the potential to impact observations, and hence the interpretations of flood probability. The Bow River at Banff hydrometric station is one of Canada's longest‐operating reference hydrological basin network stations and so has great value for assessing changes in flow regime over time. Furthermore, the station measures a river that provides an extremely important water supply for Calgary and irrigation district downstream and so is of great interest for assessing regional water security. These records were examined for changes in several flood attributes and to determine whether flow changes may have been related to landscape change within the basin as caused by forest fires, conversion from grasslands to forest with fire suppression, and regional climate variations and/or trends. Floods in the Upper Bow River are generated by both snowmelt and rain‐on‐snow (ROS) events, the latter type which include flood events generated by spatially and temporally large storms such as occurred in 2013. The two types of floods also have different frequency characteristics. Snowmelt and ROS flood attributes were not correlated significantly with any climate index or with burned area except that snowmelt event duration correlated negatively to the Pacific Decadal Oscillation. While there is a significant negative trend in all floods over the past 100 years, when separated based on generating process, neither snowmelt floods nor large ROS floods associated with mesoscale storms show any trends over time. Despite extensive changes to the landscape of the basin and in within the climate system, the flood regime remains unchanged, something identified at smaller scales in the region but never at larger scales. Copyright © 2016 John Wiley & Sons, Ltd.     

Urban fluvial flood modelling using a two‐dimensional diffusion‐wave treatment,part 1: mesh resolution effects

High‐resolution data obtained from airborne remote sensing is increasing opportunities for representation of small‐scale structural elements (e.g. walls, buildings) in complex floodplain systems using two‐dimensional (2D) models of flood inundation. At the same time, 2D inundation models have been developed and shown to provide good predictions of flood inundation extent, with respect to both full solution of the depth‐averaged Navier–Stokes equations and simplified diffusion‐wave models. However, these models have yet to be applied extensively to urban areas. This paper applies a 2D raster‐based diffusion‐wave model to determine patterns of fluvial flood inundation in urban areas using high‐resolution topographic data and explores the effects of spatial resolution upon estimated inundation extent and flow routing process. Model response shows that even relatively small changes in model resolution have considerable effects on the predicted inundation extent and the timing of flood inundation. Timing sensitivity would be expected, given the relatively poor representation of inertial processes in a diffusion‐wave model. Sensitivity to inundation extent is more surprising, but is associated with: (1) the smoothing effect of mesh coarsening upon input topographical data; (2) poorer representation of both cell blockage and surface routing processes as the mesh is coarsened, where the flow routing is especially complex; and (3) the effects of (1) and (2) upon water levels and velocities, which in turn determine which parts of the floodplain the flow can actually travel to. It is shown that the combined effects of wetting and roughness parameters can compensate in part for a coarser mesh resolution. However, the coarser the resolution, the poorer the ability to control the inundation process, as these parameters not only affect the speed, but also the direction of wetting. Thus, high‐resolution data will need to be coupled to a more sophisticated representation of the inundation process in order to obtain effective predictions of flood inundation extent. This is explored in a companion paper. Copyright © 2005 John Wiley & Sons, Ltd.     

Hydrological analysis of the Upper Tiber River Basin,Central Italy: a watershed modelling approach

The quantification of the various components of hydrological processes in a watershed remains a challenging topic as the hydrological system is altered by internal and external drivers. Watershed models have become essential tools to understand the behaviour of a catchment under dynamic processes. In this study, a physically based watershed model called Soil Water Assessment Tool was used to understand the hydrologic behaviour of the Upper Tiber River Basin, Central Italy. The model was successfully calibrated and validated using observed weather and flow data for the period of 1963–1970 and 1971–1978, respectively. Eighteen parameters were evaluated, and the model showed high relative sensitivity to groundwater flow parameters than the surface flow parameters. An analysis of annual hydrological water balance was performed for the entire upper Tiber watershed and selected subbasins. The overall behaviour of the watershed was represented by three categories of parameters governing surface flow, subsurface flow and whole basin response. The base flow contribution has shown that 60% of the streamflow is from shallow aquifer in the subbasins. The model evaluation statistics that evaluate the agreement between the simulated and the observed streamflow at the outlet of a watershed and other three different subbasins has shown a coefficient of determination (R²) from 0.68 to 0.81 and a Nash–Sutcliffe efficiency (E_NS) between 0.51 and 0.8 for the validation period. The components of the hydrologic cycle showed variation for dry and wet periods within the watershed for the same parameter sets. On the basis of the calibrated parameters, the model can be used for the prediction of the impact of climate and land use changes and water resources planning and management. Copyright © 2012 John Wiley & Sons, Ltd.     

The two-component extreme value distribution for flood frequency analysis: Derivation of a new estimation method

The two component extreme value (TCEV) distribution has recently been shown to account for most of the characteristics of the real flood experience. A new method of parameter estimation for this distribution is derived using the principle of maximum entropy (POME). This method of parameter estimation is suitable for application in both the site-specific and regional cases and appears simpler than the maximum likelihood estimation method. Statistical properties of the regionalized estimation were evaluated using a Monte Carlo approach and compared with those of the maximum likelihood regional estimators.     

Stratigraphic signatures due to flood deposition near the Rhône River: Gulf of Lions,northwest Mediterranean Sea

Episodic flood delivery provides the bulk of the solid discharge for many small to moderate river systems, including the Rhône River in the northwestern Mediterranean. Several recent studies have demonstrated that the fate of this sediment depends on the coherence between river discharge and energetic ocean conditions. The deposition of flood sediment in the ocean can be confirmed by common signatures of episodic discharge events: presence of ⁷Be, physical stratification, and elevated clay content associated with low ²¹⁰Pb activities.     

Morphology and sedimentology of a tributary confluence bar in a regulated river: North Tyne,U.K.

The form and structure of a tributary confluence bar in a regulated channel are analysed and related to channel adjustment to the imposed flow regime. The bar is characterized by a tripartite structure with two facies, and by marked lateral coarsening, characteristics which relate to the changing hydraulic conditions within the narrowing channel form.