Extreme Discharge Research Articles

Experimental investigation of the axial discharging velocity of particles from rotary kilns

The axial discharging velocity of the particles was experimentally investigated at the discharge end of two rotary kilns with 400 and 250 mm internal diameter, respectively. Sand, glass beads and clinker were used as experimental materials. A physical method without using imaging technology was developed to measure the axial discharging velocity based on the discharge behavior. The mass flow and the rotational speed of kiln were changed in a wide range. The axial discharging velocity can be directly determined by the volume flow rate of solid bed divided by the whole cross section area of the bed at the kiln discharge end. The axial discharge velocity can be nominally expressed as the function of the axial transport velocity, if the kiln is fully loaded by the materials and the radial rotational velocity of the cylinder. Clinker among other two materials presents a higher difference of the extreme discharge velocities because of the wider particle size difference.

Improving catchment discharge predictions by inferring flow route contributions from a nested-scale monitoring and model setup

Abstract. Identifying effective measures to reduce nutrient loads of headwaters in lowland catchments requires a thorough understanding of flow routes of water and nutrients. In this paper we assess the value of nested-scale discharge and groundwater level measurements for the estimation of flow route volumes and for predictions of catchment discharge. In order to relate field-site measurements to the catchment-scale an upscaling approach is introduced that assumes that scale differences in flow route fluxes originate from differences in the relationship between groundwater storage and the spatial structure of the groundwater table. This relationship is characterized by the Groundwater Depth Distribution (GDD) curve that relates spatial variation in groundwater depths to the average groundwater depth. The GDD-curve was measured for a single field site (0.009 km2) and simple process descriptions were applied to relate groundwater levels to flow route discharges. This parsimonious model could accurately describe observed storage, tube drain discharge, overland flow and groundwater flow simultaneously with Nash-Sutcliff coefficients exceeding 0.8. A probabilistic Monte Carlo approach was applied to upscale field-site measurements to catchment scales by inferring scale-specific GDD-curves from the hydrographs of two nested catchments (0.4 and 6.5 km2). The estimated contribution of tube drain effluent (a dominant source for nitrates) decreased with increasing scale from 76–79% at the field-site to 34–61% and 25–50% for both catchment scales. These results were validated by demonstrating that a model conditioned on nested-scale measurements improves simulations of nitrate loads and predictions of extreme discharges during validation periods compared to a model that was conditioned on catchment discharge only.

Mechanisms of sediment flux and turbidity maintenance in the Delaware Estuary

[1] An observational study was conducted to identify mechanisms of suspended sediment flux and turbidity maintenance in the Delaware River estuary. From March through October 2005, instrumented moorings were deployed to obtain continuous measurements of currents and suspended sediment concentration at sites along the estuarine channel and on flanking subtidal flats. Data time series were analyzed to determine the relative influence of nontidal advection and tidal pumping on residual fluxes of sediment. Results indicate that the estuarine channel is a strongly advective transport environment with residual sediment fluxes driven mostly by gravitational circulation. Tidal pumping is a contributing process of residual sediment flux in the channel near the estuarine null point and turbidity maximum, though the magnitude and direction of pumping vary with river flow and resident sediment inventory in the upper estuary. Sediment pumping in the channel is driven by tidal asymmetries in velocity and particle settling and perhaps by tidal variations in internal mixing in the stratified lower estuary. In contrast to the estuarine channel, residual sediment fluxes over the subtidal flats are weak and dominated by tidal pumping. Landward advective fluxes of sediment in bottom waters of the lower estuarine channel are strongest during neap tides; during large spring tides sediment is mixed high in the water column and the advective flux reverses to seaward under the residual surface outflow. Despite these transient seaward fluxes, the estuary has an enormous capacity to buffer extreme freshwater discharges and suppress export of suspended sediment to Delaware Bay.

Nutrient and Sediment Concentrations and Corresponding Loads during the Historic June 2008 Flooding in Eastern Iowa

A combination of above-normal precipitation during the winter and spring of 2007-2008 and extensive rainfall during June 2008 led to severe flooding in many parts of the midwestern United States. This resulted in transport of substantial amounts of nutrients and sediment from Iowa basins into the Mississippi River. Water samples were collected from 31 sites on six large Iowa tributaries to the Mississippi River to characterize water quality and to quantify nutrient and sediment loads during this extreme discharge event. Each sample was analyzed for total nitrogen, dissolved nitrate plus nitrite nitrogen, dissolved ammonia as nitrogen, total phosphorus, orthophosphate, and suspended sediment. Concentrations measured near peak flow in June 2008 were compared with the corresponding mean concentrations from June 1979 to 2007 using a paired t test. While there was no consistent pattern in concentrations between historical samples and those from the 2008 flood, increased flow during the flood resulted in near-peak June 2008 flood daily loads that were statistically greater (p < 0.05) than the median June 1979 to 2007 daily loads for all constituents. Estimates of loads for the 16-d period during the flood were calculated for four major tributaries and totaled 4.95 x 10(7) kg of nitrogen (N) and 2.9 x 10(6) kg of phosphorus (P) leaving Iowa, which accounted for about 22 and 46% of the total average annual nutrient yield, respectively. This study demonstrates the importance of large flood events to the total annual nutrient load in both small streams and large rivers.

Sensitivity of river discharge to ENSO

El Niño Southern Oscillation (ENSO) has significant impacts on streamflows around the world. While many studies have assessed correlations, an assessment of the magnitude of this impact is lacking, and little is known of ENSO's impact on extreme discharges. We use a daily discharge dataset to provide a global assessment of the sensitivity of annual mean and flood discharges to ENSO, and a gridded climate dataset to assess the global impact of ENSO on precipitation and temperature. We find that, on average, for the stations studied ENSO has a greater impact on annual high‐flow events than on mean annual discharge, especially in the extra‐tropics. The quantification of ENSO impacts provides relevant information for water‐management, allowing the identification of problem areas and providing a basis for risk assessments.

Bayesian MCMC approach to regional flood frequency analyses involving extraordinary flood events at ungauged sites

summary This paper proposes a method for using major flash flood events occurred at ungauged catchments to reduce the uncertainties in estimating regional flood quantiles. The approach is based on standard regionalization methods assuming that the flood peak distribution rescaled by a site-dependent index flood is uniform within a homogeneous region. A likelihood formulation and a Bayesian Markov Chain Monte Carlo (MCMC) algorithm are used to infer the parameter values of the regional distributions. This statistical inference technique has been selected for its rigorousness – various hypotheses are explicitly formulated in the likelihood function, its flexibility as for the type of data that can be treated, and its ability to compute accurate estimates of the confidence intervals for the adjusted parameters and for the corresponding flood quantiles. The proposed method is applied to two data sets from Slovakia and the South of France that consist of series of annual peak discharges at gauged sites and estimated peak discharges of extreme flash flood events that have occurred at ungauged sites. The results suggest that the confidence intervals of the quantiles can be significantly narrowed down provided that the set of ungauged extremes is the result of a comprehensive sampling over the selected region. This remains valid, even if the uncertainties in the estimated ungauged extreme discharges are considered. The flood quantiles estimated by the proposed method are also consistent with the results of site specific flood frequency studies based on historic and paleoflood information.

Hydrological triggering of the seismicity around a salt diapir in Castellane, France

The subalpine belts in Provence (France) present a low level of background seismicity. However, Castellane and some surrounding villages in the middle of one of the most renown belts, the Castellane arc, have been damaged twice in a century by significant earthquakes, in 1855 and 1951. The macroseismic database acquired after these events suggests that the sources were moderate (M L ∼ 4.5) but very shallow (∼ 1 km). A recent instrumental microseismic catalogue further attests to the shallow seismicity in the area. A 58 year hydrological record of the Verdon river discharge allows us to test whether this seismicity is modulated by some transient local state-of-stress changes induced by large aquifer and artificial-lake loadings. We show that 41% of the extreme discharges (those with a probability of exceedance at 0.1%) are followed by at least one seismic event within a 7–28 day optimal interval and that this correlation, which represents 8% of the earthquakes, is not due to chance. We consider several natural and artificial hydrological mechanisms, including artificial reservoir and aquifer elastic loading and induced pore-pressure variations. We favour a model in which the local salt–gypsum domes respond to the aquifer forcing.

Changes in phytoplankton morpho-functional groups induced by extreme hydroclimatic events in the Middle Paraná River (Argentina)

The extreme hydrological fluctuations in the South American Parana River are unequivocally coupled with El Nino-Southern Oscillation’s (ENSO’s) global climatic phenomenon. In order to identify phytoplankton changes triggered by periods of extraordinary flood (November 1997–1998) and drought (November 1999–2000), the main channel of the Middle stretch was monthly analysed and compared with neutral hydrological conditions (November 1977–1978). These results show that water discharge plays a stirring role on phytoplankton dynamics, and that light availability acts as an important selective force. Phytoplankton transport, density, and biovolume were lower during El Nino and higher during La Nina (P < 0.01). A great development of small flagellate unicells belonging to morpho-functional groups Y (Cryptomonas spp.) and X2 (Chlamydomonas spp., Plagioselmis nannoplanctica and small Cryptophyceae) prevailed during both anomalous periods, accompanied by small centric diatoms and Skeletonema cf. potamos (D). This scenario significantly differs from those encountered during neutral conditions, where codon P (Aulacoseira granulata) dominated. The absence of these large filaments during drought and flood peaks could be explained by sedimentation processes in low waters, and by organisms’ retention by obstacles after water displacement from the plain in high waters periods; however, the disruption of the inocula from floodplain areas to the main flow may be the most important factor in both periods. The hydroclimatic anomalies provoked changes in the phytoplankton structure of the Parana River, favouring X2 and Y functional groups instead of the typical P and C. The interruption of the constant feedback between the main channel and the floodplain water bodies due to low connectivity during droughts and the homogeneity of environment during floods, govern phytoplankton dynamics in this large river. The rapid occurrence of community reorganization after the conclusion of these events and the persistence of certain variations lead to the hypothesis that the magnitude and duration of extreme water discharges will determine the intensity and duration of changes.

Estimating the out-of-band radiation flare levels for extreme ultraviolet lithography

For the commercialization of extreme ultraviolet lithography (EUVL), discharge or laser-produced, pulsed plasma light sources are being considered. These sources are known to emit into a broad range of wavelengths that are collectively referred to as out-of-band (OOB) radiation by lithographers. Multilayer EUV optics reflect OOB radiation emitted by the EUV sources onto the wafer plane, resulting in unwanted background exposure of the resist (flare) and reduced image contrast. The reflectivity of multilayer optics at the target wavelength of 13.5 nm is comparable to that of their reflectivity in the deep ultraviolet (DUV) and UV regions from 100 to 350 nm. The aromatic molecular backbones of many of the resists used for EUV are equally absorptive at specific DUV wavelengths as well. To study the effect of these wavelengths on imaging performance in a real system, we are in the process of integrating a DUV source into the Sematech Berkeley 0.3-NA microfield exposure tool (MET). We present the simulation-based imaging results predicting the potential impact of OOB based on known resist, mask, and multilayer conditions. It should be noted that because the projection optics work equally well as imaging optics at DUV wavelengths, OOB radiation cannot be treated simply as uniform background or DC flare.

Flood hazard in Europe in an ensemble of regional climate scenarios

We analyze changes in flood hazard in Europe by examining extreme discharge levels as simulated by the hydrological model LISFLOOD when driven by a multimodel ensemble of climate simulations. The ensemble consists of simulations from two regional climate models (RCMs), both run with boundary conditions from two global models, and for two scenarios of greenhouse gas emissions. In northeastern Europe, a general decrease in extreme river discharge is observed in the scenario period, suggesting a reduction in the hazard of extreme snowmelt floods. Elsewhere, we find a consistent tendency toward a higher flood hazard in the majority of the model experiments in several major European rivers. These changes can partly be attributed to large, decadal‐scale variability in the simulated climate and can be expected to occur naturally when comparing two 30‐year time periods, even without a change in greenhouse gas forcing. We furthermore find evidence for a considerable influence of especially the global model that is used to drive the RCMs. At the scale of individual river basins, using a different combination of climate models or assuming a different emissions scenario sometimes results in a very different or even opposite climate change signal in flood hazard. We therefore believe that a multimodel approach as adopted in the present paper provides the best way to address the various uncertainties in impact studies of hydrometeorological extremes. Probabilistic scenarios that consist of multiple realizations of the current and future climate state are indispensable to better identify the climate signal amidst large variability.

Effects of land use changes on streamflow generation in the Rhine basin

The hydrological regime of the Rhine basin is expected to shift from a combined snowmelt‐rainfall regime to a more rainfall‐dominated regime because of climate change, leading to more extreme flood peaks and low flows. Land use changes may reinforce the effects of this shift through urbanization or may counteract them through, for example, afforestation. In this study, we investigate the effect of projected land use change scenarios on river discharge. Sensitivity of mean and extreme discharge in the Rhine basin to land use changes is investigated at various spatial scales. The variable infiltration capacity (VIC) (version 4.0.5) model is used for hydrological modeling forced by a high‐resolution atmospheric data set spanning the period 1993–2003. The model is modified to allow for bare soil evaporation and canopy evapotranspiration simultaneously in sparsely vegetated areas, as this is more appropriate for simulating seasonal effects. All projected land use change scenarios lead to an increase in streamflow. The magnitude of the increase, however, varies among subbasins of different scales from about 2% in the upstream part of the Rhine (about 60,000 km2) to about 30% in the Lahn basin (about 7000 km2). Streamflow at the basin outlet proved rather insensitive to land use changes because over the entire basin affected areas are relatively small. Moreover, projected land use changes (urbanization and conversion of cropland into (semi)natural land or forest) have opposite effects. At smaller scales, however, the effects can be considerable.

Oscilação do nível de água e a co-oscilação da maré astronômica no baixo estuário do rio Paraíba do Sul, RJ

A variabilidade do nível da água no baixo estuário do rio Paraíba do Sul foi estudada para avaliar os efeitos da co-oscilação da maré astronômica, descarga fluvial e de eventos meteorológicos. Dados de nível da água foram registrados em intervalos horários para o período entre outubro de 2000 até fevereiro de 2003 em uma estação localizada próximo à desembocadura do estuário. Foi observado que durante os eventos de descarga fluvial baixa os níveis de água do estuário oscilam principalmente em função da co-oscilação da maré astronômica e eventualmente por marés meteorológicas. Durante períodos de descarga fluvial alta o nível da água de preamar no baixo estuário é dominado pela co-oscilação, enquanto que nas baixa-mares é influenciado pela descarga fluvial. Em períodos de descarga fluvial muito alta o estuário passa a receber a influência tanto da descarga fluvial quanto da maré astronômica nas oscilações do nível. Em períodos de baixa descarga quase não houve modificação na onda de maré, sendo observada pequena assimetria no sentido de vazante. Ainda em baixa descarga, eventos de maré meteorológica produzem distorções e assimetrias, com o tempo de subida menor que o de descida, gerando correntes mais fortes no sentido de enchente. Contudo, isto não pôde ser confirmado com a relação de fase entre M2 e M4, que sugeriu domínio de vazante. A descarga fluvial alta causou fortes distorções na onda de maré em períodos de quadratura e uma atenuação considerável nas amplitudes tanto de sizígia como de quadratura, com assimetrias no sentido de vazante corroboradas pela relação de fase entre M2 e M4.

Future risk of flooding: an analysis of changes in potential loss of life in South Holland (The Netherlands)

Abstract Potential loss of life is considered an important indicator of flood risk. We examine the future development of potential loss of life due to flooding for a major flood prone area in The Netherlands. The analysis is based on projections and spatial distribution of population under a high economic growth scenario and a loss of life model. Results show that the projected population growth in flood prone areas is higher than average in the Netherlands between 2000 and 2040. Due to this effect the potential number of fatalities is projected to increase by 68% on average for 10 different flood scenarios, not including impacts from climate change and sea level rise. Just sea level rise of 0.30 m leads to an average 20% increase in the number of fatalities. The combined impact of sea level rise and population growth leads to an estimated doubling in the potential number of fatalities. Taking into account increasing probability of flooding due to sea level rise and extreme river discharges, the expected number of fatalities could quadruple by 2040. The presented results give a conservative and upper bound estimate of the increase of the risk level when no preventive measures are undertaken. It is found that the consideration of the exact spatial distribution of population growth is essential for arriving at reliable estimates of future risk of flooding.

Distribution functions of extreme sea waves and river discharges

Some of the important elements to be considered by the designer in the field of water defenses and hydraulic structures include the determination of the maximum environmental loads such as maximum wave height, maximum water level and maximum river discharge at locations of interest. These maxima can be estimated by various statistical methods based on observations. The main point of interest is the behaviour of each method for predicting these extrema and its deviation from the true value. An overview of statistical methods is given to determine the extreme values of river and sea related variables. The method of Regional Frequency Analysis (RFA) is proposed to predict the occurrence probabilities of these extrema. The RFA intends to extend the dataset by pooling the data of neighboring locations and establishes the regional growth curve of a so-called “homogenous region”. An important aspect is the formation of the homogeneous regions because only for those regions data can be pooled. Statistical methods will also be presented to derive homogeneous clusters. Applications include the prediction of extreme river discharges in Northwest and Central Europe and extreme waves along the Dutch North Sea coasts.

Climate change impact on flood hazard in Europe: An assessment based on high‐resolution climate simulations

Global warming is generally expected to increase the magnitude and frequency of extreme precipitation events, which may lead to more intense and frequent river flooding. This work assesses the implications of climate change for future flood hazard in Europe. Regional climate simulations from the HIRHAM model with 12‐km horizontal resolution were used to drive the hydrological model LISFLOOD, and extreme value techniques were applied to the results to estimate the probability of extreme discharges. It was found that by the end of this century under the Special Report on Emission Scenarios (SRES) A2 emissions scenario in many European rivers extreme discharge levels may increase in magnitude and frequency. In several rivers, most notably in the west and parts of eastern Europe, the return period of what is currently a 100‐year flood may in the future decrease to 50 years or less. A considerable decrease in flood hazard was found in the northeast, where warmer winters and a shorter snow season reduce the magnitude of the spring snowmelt peak. Also in other rivers in central and southern Europe a decrease in extreme river flows was simulated. The results were compared with those obtained with two HIRHAM experiments at 50‐km resolution for the SRES A2 and B2 scenarios. Disagreements between the various model experiments indicate that the effect of the horizontal resolution of the regional climate model is comparable in magnitude to the greenhouse gas scenario. Also, the choice of extreme value distribution to estimate discharge extremes influences the results, especially for events with higher return periods.

Understanding the surface hydrology of the Lake Eyre Basin: Part 2—Streamflow

This is the second of two papers examining the surface hydrology of the Lake Eyre Basin (LEB) (1,140,000 km 2) in Australia. The streams are unregulated and are characterised by extreme discharge variation. The analyses reported cover only surface hydrology and include comparisons with arid zone catchments globally. The paper discusses spatial runoff and annual streamflow characteristics, flow duration and baseflow index (BFI) analyses, annual flood frequency analysis and flood transmission losses, a water balance study, wet and dry run length analysis and, finally, yield from hypothetical reservoirs located across the LEB. There are 12 conclusions listed at the end of the paper. We identify two highlights as follows: • The coefficient of variation of annual flows, Cv, varies from 0.98 to 2.62. Compared with 45 arid zone rivers world-wide excluding Australian rivers, the annual Cv of the LEB streams are approximately double the average variability found world-wide. • Large transmission losses occur as flood flows move down the middle reaches of the major river systems. The transmission losses vary non-linearly with flood size as a result of differing transmission efficiencies between primary channels and the floodplain, and varying floodplain utilisation.

How fast can a river flow over alluvium?

A new “vortex–drag” approach is proposed to compute the rate of dissipation of mean–motion energy in alluvial streams flowing over mobile bed forms. 1D–routing exercises strongly suggest that, for a given depth and energy–slope, there exists a certain flow velocity limit an alluvial stream is unlikely to exceed. This remarkable flow condition would actually correspond to a fundamental mode in terms of vortex Strouhal frequency (to which only higher harmonics can eventually be associated). Interestingly, the maximum possible velocity is not attained over a plane bed, but occurs when the wavy shape of the water–surface remains in phase with the sand wave. The model treats flow over sandy alluvium as a combination of boundary–attached and –detached flow features, both necessary to explain how well–marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water–sediment discharge and stream power conditions. Through the vortex–drag concept and its mathematical development, a lot is learned about the nature of the mutual interactions existing between the various alluvial processes: bedform development, alluvial channel resistance, sediment transport capacity and turbulence–damping. Bedform types occurring in the upper alluvial regime are at the center of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the inphase wave [IPW] flow configuration is associated with minimum possible drag, creating low–turbulence high–velocity conditions ideal to evacuate extreme river discharges. Occurrence of IPW conditions is therefore important in terms of flooding alleviation, but will develop only if the right type of sediment is maintained available on the stream bed. The boundary–attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographically–forced gravity waves occurring in non–shallow environments such as ripple fields. For the boundary–detached component of the model, we introduce a “control factor m” (m ≥ 1) which reflects a complex feedback–control loop process active in the separation cell immediately downstream of the bedform crest. Lift–up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting–based conceptual models of non–cohesive sediment transport.

A Bayesian approach to decision-making under uncertainty: An application to real-time forecasting in the river Rhine

Enhanced ability to forecast peak discharges remains the most relevant non-structural measure for flood protection. Extended forecasting lead times are desirable as they facilitate mitigating action and response in case of extreme discharges. Forecasts remain however affected by uncertainty as an exact prognosis of water levels is inherently impossible. Here, we implement a dedicated uncertainty processor, that can be used within operational flood forecasting systems.

Détection et prise en compte d’éventuels impacts du changement climatique sur les extrêmes hydrologiques en France

Climate change is widely considered as a reality by scientists. Nevertheless, impacts on hydrological extremes are more difficult to observe and to forecast. The aim of this thesis is to answer the following questions: How to detect changes in hydro-climatic series? What are the observed changes for extreme discharges in France? How to take into account possible changes in frequency analysis? These objectives refer to both local and regional scales. This paper describes the developments related to the third question. In a first step, the concept of return period is revisited in a non-stationary context. Frequency analysis methods are then updated in order to account for evolutions in time. This is achieved by modelling trends affecting the distribution parameters. Parameter estimation uses the Bayesian formalism, which is a convenient tool for quantifying the uncertainty related to the stationarity hypothesis. This approach can be generalized at the regional scale, by means of non-stationary regional models. Such models are more general than the model underlying the index flood method. However, results of such a regional analysis are affected by the spatial dependence existing between studied sites. Impacts of this dependence on quantile estimates are highlighted, and a first approach is proposed in order to explicitly model spatial dependence.

Toward flood risk prediction: a statistical approach using a 29-year river discharge simulation over Japan

A statistical approach that considers the bias and uncertainty of models is proposed for interpreting the simulated river discharge as a flood risk. A 29-year simulation was performed to estimate parameters of the Gumbel distribution for the probability of extreme discharge. The estimated discharge probability index (DPI) showed clear agreement with observed values. Even more strikingly, high DPI in the simulation corresponded to actual flood damage records. This indicates that the real-time simulation of the DPI could potentially provide flood warnings. This paper also suggests an application using the same statistical method for real-time flood risk prediction that overcomes the lack of sufficiently long simulation data through the use of a pre-existing long-term simulation to estimate statistical parameters. A preliminary flood risk prediction that used operational weather forecast data for 2003 and 2004 gave results similar to those of the 29-year simulation for the Typhoon Tokage (T0423) event on October 20th 2004, demonstrating the transferability of the technique to real-time prediction, which is differently biased.