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Return Periods (return + period)

Distribution by Scientific Domains

Engineering	55%
Life Sciences	18%
Earth and Environmental Science	14%

Selected Abstracts

Extreme value predictions based on nonstationary time series of wave data

ENVIRONMETRICS, Issue 1 2006
Christos N. Stefanakos
Abstract A new method for calculating return periods of various level values from nonstationary time series data is presented. The key idea of the method is a new definition of the return period, based on the MEan Number of Upcrossings of the level x* (MENU method). In the present article, the case of Gaussian periodically correlated time series is studied in detail. The whole procedure is numerically implemented and applied to synthetic wave data in order to test the stability of the method. Results obtained by using several variants of traditional methods (Gumbel's approach and the POT method) are also presented for comparison purposes. The results of the MENU method showed an extraordinary stability, in contrast to the wide variability of the traditional methods. The predictions obtained by means of the MENU method are lower than the traditional predictions. This is in accordance with the results of other methods that also take into account the dependence structure of the examined time series. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Effects of increased flow in the main stem of the River Rhine on the invertebrate communities of its tributaries

FRESHWATER BIOLOGY, Issue 1 2005
Melanie C. Beckmann
Summary 1. We hypothesised that increased flow in the main stem of the River Rhine would influence the invertebrate communities of its tributaries and therefore investigated the invertebrate fauna of six tributaries over 2 years. 2. We collected quantitative invertebrate samples at three sites in each tributary: in the tributary mouth (influenced by Rhine water whenever flow in the Rhine exceeded mean annual level), in the zone reached by average floods (return period 1.5 years) in the Rhine (average flood level sites), and immediately upstream of the range of extreme Rhine floods (reference sites). Samples were taken in spring, summer and autumn of each year, at different flow levels of the Rhine. We also compared substratum composition at the three sites. 3. Tributary mouth sites had the finest substratum, the lowest total invertebrate density and the lowest taxon richness. At average flood level and reference sites, these three parameters were similar. 4. Taxa known to prefer larger rivers were mostly confined to the tributary mouth sites, and species preferring upland streams dominated at the average flood level and reference sites. 5. Multivariate analyses confirmed the influence of the Rhine on the tributary mouth sites. Invasive invertebrate species, which usually appear only in the Rhine itself, were found at the tributary mouth sites but not further up in the tributaries. 6. Our study shows that increased flow in the main stem of the Rhine influenced substratum composition and invertebrate communities at the tributary mouth sites. These results imply that the relationship between the main stem of a river and its tributaries is not one-way (from tributary to main stem), but rather a two-way interaction. [source]

Development of design flood hydrographs using probability density functions

HYDROLOGICAL PROCESSES, Issue 4 2010
Niranjan Pramanik
Abstract Probability density functions (PDFs) are used to fit the shape of hydrographs and have been popularly used for the development of synthetic unit hydrographs by many hydrologists. Nevertheless, modelling the shapes of continuous stream flow hydrographs, which are probabilistic in nature, is rare. In the present study, a novel approach was followed to model the shape of stream flow hydrographs using PDF and subsequently to develop design flood hydrographs for various return periods. Four continuous PDFs, namely, two parameter Beta, Weibull, Gamma and Lognormal, were employed to fit the shape of the hydrographs of 22 years at a site of Brahmani River in eastern India. The shapes of the observed and PDF fitted hydrographs were compared and root mean square errors, error of peak discharge (EQP) and error of time to peak (ETP) were computed. The best-fitted shape and scale parameters of all PDFs were subjected to frequency analysis and the quartiles corresponding to 20-, 50-, 100- and 200-year were estimated. The estimated parameters of each return period were used to develop the flood hydrographs for 20-, 50-, 100- and 200-year return periods. The peak discharges of the developed design flood hydrographs were compared with the design discharges estimated from the frequency analysis of 22 years of annual peak discharges at that site. Lognormal-produced peak discharge was very close to the estimated design discharge in case of 20-year flood hydrograph. On the other hand, peak discharge obtained using the Weibull PDF had close agreement with the estimated design discharge obtained from frequency analysis in case of 50-, 100- and 200-year return periods. The ranking of the PDFs based on estimation of peak of design flood hydrograph for 50-, 100- and 200-year return periods was found to have the following order: Weibull > Beta > Lognormal > Gamma. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Frequency analysis for predicting 1% annual maximum water levels along Florida coast, US

HYDROLOGICAL PROCESSES, Issue 23 2008
Sudong Xu
Abstract In the Coastal Flood Insurance Study by the Federal Emergency Management Agency (FEMA, 2005), 1% annual maximum coastal water levels are used in coastal flood hazard mitigation and engineering design in coastal areas of USA. In this study, a frequency analysis method has been developed to provide more accurate predictions of 1% annual maximum water levels for the Florida coast waters. Using 82 and 94 years of annual maximum water level data at Pensacola and Fernandina, performances of traditional frequency analysis methods, including advanced method of Generalized Extreme Value distribution method, have been evaluated. Comparison with observations of annual maximum water levels with 83 and 95 years of return periods indicate that traditional methods are unable to provide satisfactory predictions of 1% annual maximum water levels to account for hurricane-induced extreme water levels. Based on the characteristics of annual maximum water level distribution of Pensacola and Fernandina stations, a new probability distribution method has been developed in this study. Comparison with observations indicates that the method presented in this study significantly improves the accuracy of predictions of 1% annual maximum water levels. For Fernandina station, predictions of extreme water level match well with the general trend of observations. With a correlation coefficient of 0·98, the error for the maximum observed extreme water level of 3·11 m (National Geodetic Vertical Datum) with 95 years of return period is 0·92%. For Pensacola station, the prediction error for the maximum observed extreme water level with a return period of 83 years is 5·5%, with a correlation value of 0·98. The frequency analysis has also been reasonably compared to the more costly Monte Carlo simulation method. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Long-term precipitation and slide activity in south-eastern Norway, autumn 2000

HYDROLOGICAL PROCESSES, Issue 4 2008
Christian Jaedicke
Abstract In autumn 2000 high amounts of precipitation caused a large number of landslides in south-eastern Norway. The precipitation reached more than 400% of the normal monthly values in November. In addition to the precipitation, groundwater monitoring showed unusual high levels. High air temperatures in the mountains caused the precipitation to fall as rain instead of snow, causing additional runoff in the catchments. In this paper, 90 landslides are presented that were studied and evaluated by the Norwegian Geotechnical Institute (NGI) in the counties effected by the slide activity. The landslide database serves as start point for an analysis of the connection between slide activity and precipitation. The analysis shows a high variability of the observed precipitation just a few days before the slides. This variability decreases significantly around 50 days prior to the slides when the results stabilize at about 400% of normal precipitation in a 50-day period. Comparison of these results to long time observations gives an estimate of return periods for the precipitation events. The 53 days accumulated precipitation has the highest return period of 132 years. This suggests that the high landslide activity in autumn 2000 in south-eastern Norway was caused by a long-term infiltration of large amounts of precipitation rather than high rainfall intensities during a short period. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Importance of soil surface characteristics on water erosion in a small grazed Sahelian catchment

HYDROLOGICAL PROCESSES, Issue 8 2003
H. Karambiri
Abstract This study concerns the problem of water erosion in the Sahel. Surface water and sediment yields (suspended matter and bedload) were monitored for 3 years (1998,2000) at the outlet of a small grazed catchment (1·4 ha) in the northern part of Burkina Faso. The catchment consists of about 64% sandy deposits (DRY soil surface type), which support most of the vegetation, and about 34% of crusted bare soils (ERO soil surface type). The annual solid-matter export is more than 90% suspended sediment, varying between 4·0 and 8·4 t ha,1. The bedload represents less than 10% of soil losses. In a single flood event (10 year return period), the sediment yield can reach 4·2 t ha,1. During the period studied, a small proportion (20 to 32%) of the floods was thus responsible for a large proportion (80%) of the solid transport. Seasonal variation of the suspended-matter content was also observed: high mean values (9 g l,1) in June, decreasing in July and stabilizing in August (between 2 and 4 g l,1). This behaviour may be a consequence of a reorganization of the soil surfaces that have been destroyed by trampling animals during the previous long dry season, vegetation growth (increase in the protecting effect of the herbaceous cover) and, to a lesser extent, particle-supply limitation (exhaustion of dust deposits during July). The particle-size distribution in the suspended matter collected at the catchment outlet is 60% made up of clay: fraction ,2 µ m. The contribution of this clay is maximum when the water rises and its kaolinite/quartz ratio is then close to that of the ERO-type surfaces. This indicates that these surfaces are the main source of clay within the catchment. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Multi-annual dry episodes in Australian climatic variability

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2009
B. G. Hunt
Abstract The output from a 10 000-year simulation with the CSIRO Mark 2 coupled global climatic model has been analysed to investigate the occurrence of multi-year dry episodes for three selected regions of Australia, specifically, the northeast, southeast and southwest of the continent. Results are presented for dry episodes lasting for 8 or more years. An episode is defined as a time interval having consecutive negative rainfall anomalies, but not necessarily a major drought, for each year of the episode. The hydrological consequences of such an episode can persist for over a century. Typically about 30 episodes are found over the 10 000 years of the simulation for each of the three regions. There is little synchronicity between the regions in the occurrence of the dry episodes. While there is an El Nino/Southern Oscillation (ENSO) influence associated with these episodes, it is not continuous over the duration of an episode. Composites of sea surface temperature anomalies over an episode highlight the limited presence of ENSO events. The occurrence of the dry episodes for all three regions is essentially random, with multi-centennial periods without an episode, and episodes at multi-decadal frequency at other times. Following a discussion of possible mechanistic influences, it is concluded that stochastic forcing is responsible for the occurrence of dry episodes. This implies that there is no predictability associated with the initiation, duration or termination of individual dry episodes. This also suggests that the 2000,2007 dry episodes occurring over much of Australia may not be caused by the greenhouse effect. Such an episode has a return period of between 200 and 300 years based on the mean frequency of occurrence in the present simulation. Copyright © 2008 Royal Meteorological Society [source]

Long-term trends in near-surface flow over the Baltic

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2003
S. C. Pryor
Abstract We report an analysis of trends in 850 hPa wind speed, as manifest in the NCEP,NCAR reanalysis fields, over the Baltic region during the latter half of the 20th century. The results indicate that annual mean wind speeds over the Baltic significantly increased over the period 1953,99 with the majority of the increase being associated with increases in the upper quartile of the wind speed distribution. Accordingly, much of the change is focused on the winter season. The trends in annual and seasonal mean wind speeds are greatest in relative and absolute sense in the southwest of the Baltic basin, where they are in excess of 0.25 m s,1 per decade for the annual mean. The extremes of the wind-speed distribution also increased by up to 5 m s,1 over the study period for the wind speed with a 50 year return period, again with the largest magnitude changes in the southwestern Baltic. These changes in wind speed are strongly linked to changes in the synoptic-scale circulation. The majority of the increase in wintertime wind speeds is attributable to an increase in westerly anticyclonic, westerly cyclonic and northwesterly cyclonic circulation types as manifest in the Grosswetterlagen catalogue, which are in turn related to the recent prevalence of the positive phase of the North Atlantic oscillation. Copyright © 2003 Royal Meteorological Society [source]

The potential role for economic instruments in drought management,

IRRIGATION AND DRAINAGE, Issue 4 2004
Stephen Merrett
changement du climat; gestion des sécheresses; irrigation; prix de l'eau Abstract Climate change in the twenty-first century will likely reduce the return period of drought events, indicating that drought management will be even more important in the future than it is already. In the case of England's Anglian region it is shown that two principal institutions are responsible for drought management,the Environment Agency and Anglian Water Services (AWS). The region has a fast-growing population of more than 5 million people, it has 58% of the most productive agricultural land in England and Wales, and in some summers irrigation can make up 50% of water use. An examination of the drought plans of the Agency and AWS demonstrates that in both cases the policy instruments that they deploy to manage drought are informational, infrastructural and regulatory. In neither case would they use water pricing as a management tool. Moreover, the government's drought plan guidelines for the water utilities make no reference to economic instruments of drought management nor do they suggest that utilities should review the economic impact on their customers of regulatory action. The principal issues that would arise if water charging were to be deployed as a drought management instrument are then reviewed. The paper concludes by proposing that national government should evaluate the feasibility, costs, benefits and risks of replacing the regulatory instruments of drought management by economic instruments. Copyright © 2004 John Wiley & Sons, Ltd. Avec les changements climatiques prévus au vingt-et-unième siècle les périodes de sècheresse deviendront probablement plus fréquentes et la gestion de ces sécheresses deviendra plus problématique. Cet article indique que dans la région Anglian de l'Angleterre deux institutions ont la responsabilité de cette gestion: "the Environment Agency" et "Anglian Water Services". Un examen des plans d'action pour la sécheresse de ces deux institutions montre que, dans les deux cas, les instruments qui seront déployés seront soit informationels soit infrastructurels soit règlementaires. Il n'y a pas dans ces plans une seule référence au prix de l'eau comme instrument de gestion. De même les conseils du gouvernement national pour la préparation de ces plans ignorent les instruments économiques. L'article conclut que le gouvernement doit réviser la faisabilité des instruments de gestion des sécheresses, et d'estimation de leurs coûts et bénéfices pour introduire davantage d'instruments économiques. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Optimization of water management in the RUT Irrigation District, Colombia,

IRRIGATION AND DRAINAGE, Issue 1 2004
Herman Depeweg
canaux d'irrigation; régulation de canaux; simulation des écoulements en canal Abstract The RUT Irrigation District is a flat polder area with an infrastructure for irrigation, drainage and flood protection. Water is pumped out for drainage, mainly during the wet season and is pumped in for irrigation during the two growing seasons. The RUT Irrigation District is one of the 16 districts that were transferred by the Colombian government to a water users' association. During this process the farmers agreed to take over the management and to give up governmental subsidies. It appeared that the farmers grossly underestimated the future energy costs and nowadays they complain about high costs for operation and maintenance of the pumping stations. In addition, the service provided is not adequate and the condition of the pumps is declining. The supplementary irrigation is based on the rainfall deficit, but a distinct criterion to supply water according to daily rainfall and cropping calendar does not exist. Hence, it is possible that more water than required is supplied to the area, affecting not only the pumping costs for irrigation but also for drainage. The present situation has been evaluated with a water balance at scheme level and an analysis of the measured groundwater fluctuations. Consecutively, a water balance at field level with an unsaturated groundwater flow model has been simulated to determine the irrigation requirements, yield reduction and drain flow based on a 20-year analysis. The water balance at field level and the one-, two- and three-day rainfall with a return period of 5 years resulted in design drain flows. Finally, these flows were used in hydrodynamic simulations to analyse the pumping requirements for optimal drainage. The simulations also indicated the inundated areas to be expected for different rainfall intensities and frequencies. Copyright © 2004 John Wiley & Sons, Ltd. Le district de RUT possède un système d'irrigation et de drainage avec une protection contre les inondations. L'eau est non seulement pompée pour assurer le drainage du polder durant la saison humide, mais aussi pour alimenter le système d'irrigation durant les deux saisons agricoles. Le district de RUT est l'un des seize districts transférés par le gouvernement colombien à une association d'usagers de l'eau. Durant ce transfert, les exploitants agricoles ont accepté de prendre en charge la gestion et d'abandonner les aides gouvernementales. Il est apparu alors que les exploitants agricoles se plaignent des couts élevés pour l'opération et la maintenance des stations de pompages. De plus, le service fourni n'est pas adéquat et les pompes se détériorent. L'irrigation de complément est fonction des précipitations, mais il n'existe pas de critère précis pour la fourniture d'eau en fonction de précipitations quotidiennes ou du calendr des récoltes. Il est ier possible qu'une quantité d'eau supérieure à celle requise soit apportée au système, affectant non seulement les exigences de pompage pour l'irrigation mais aussi pour le drainage. L'évaluation de la situation actuelle est basée sur un bilan hydrique à l'échelle de système et sur une analyse des fluctuations des eaux souterraines. Par la suite, un bilan hydrique a l'échelle de la campagne délivré par le model WASIM a été utilisé pour déterminer les exigences d'irrigation, la réduction des récoltes et le débit dans le drain basé sur une simulation de 20 ans. Enfin, le drain principal fut simulé avec DUFLOW pour analyser les caractéristiques du pompage pour le drainage utilisant le débit de drainage fourni par un bilan hydrique pour des durées deun, deux et trois jours de pluie fréquence de quinquennale. La simulation a aussi permis d'identifier les étendues des inondations pour différentes intensités et fréquences de pluie. Copyright © 2004 John Wiley & Sons, Ltd. [source]

A TRIANGULAR MODEL OF DIMENSIONLESS RUNOFF PRODUCING RAINFALL HYETOGRAPHS IN TEXAS,

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 4 2003
William H. Asquith
ABSTRACT: A synthetic triangular hyetograph for a large data base of Texas rainfall and runoff is needed. A hyetograph represents the temporal distribution of rainfall intensity at a point or over a watershed during a storm. Synthetic hyetographs are estimates of the expected time distribution for a design storm and principally are used in small watershed hydraulic structure design. A data base of more than 1,600 observed cumulative hyetographs that produced runoff from 91 small watersheds (generally less than about 50 km2) was used to provide statistical parameters for a simple triangular shaped hyetograph model. The model provides an estimate of the average hyetograph in dimensionless form for storm durations of 0 to 24 hours and 24 to 72 hours. As a result of this study, the authors concluded that the expected dimensionless cumulative hyetographs of 0 to 12 hour and 12 to 24 hour durations were sufficiently similar to be combined with minimal information loss. The analysis also suggests that dimensionless cumulative hyetographs are independent of the frequency level or return period of total storm depth and thus are readily used for many design applications. The two triangular hyetographs presented are intended to enhance small watershed design practice in applicable parts of Texas. [source]

A short history of muddy floods

LAND DEGRADATION AND DEVELOPMENT, Issue 4 2010
J. Boardman
Abstract The term ,muddy flood' has been used widely in the lowland, arable areas of western and central Europe to describe muddy runoff from arable fields that causes damage to property. There is some evidence that muddy floods are much more frequent in the last two decades than previously. It is clear though that there is very substantial under-reporting of the phenomena even in areas where they have been recognised for 20 years e.g. UK and France. Reconstructions based on questionnaires, news media and local authority records have had some success in historical analysis of muddy flood frequency but there is still a huge data deficiency. Records from some countries are woefully lacking e.g. Germany, Spain and Italy. Costs of muddy flooding are substantial especially in the loess belt of Belgium. The number of properties flooded in France suggests also that costs are high; similarly in England (UK) where costs for case studies are known but not for the country as a whole. There are two quite different solutions to the problem of muddy flooding. Protection can be provided by engineering devices: retention ponds, dams, trenches. This is an ,end of pipe' solution with severe cost implications and risks with regard to the design return period. Alternatively, land use change on relatively small areas of catchments, can be shown to be effective at reducing flood-risk hazard. A combination of the two has proved most effective at several sites in Europe. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Damage-based design with no repairs for multiple events and its sensitivity to seismicity model

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 3 2007
S. Das
Abstract Conventional design methodology for the earthquake-resistant structures is based on the concept of ensuring ,no collapse' during the most severe earthquake event. This methodology does not envisage the possibility of continuous damage accumulation during several not-so-severe earthquake events, as may be the case in the areas of moderate to high seismicity, particularly when it is economically infeasible to carry out repairs after damaging events. As a result, the structure may collapse or may necessitate large scale repairs much before the design life of the structure is over. This study considers the use of design force ratio (DFR) spectrum for taking an informed decision on the extent to which yield strength levels should be raised to avoid such a scenario. DFR spectrum gives the ratios by which the yield strength levels of single-degree-of-freedom oscillators of different initial periods should be increased in order to limit the total damage caused by all earthquake events during the lifetime to a specified level. The DFR spectra are compared for three different seismicity models in case of elasto-plastic oscillators: one corresponding to the exponential distribution for return periods of large events and the other two corresponding to the lognormal and Weibull distributions. It is shown through numerical study for a hypothetical seismic region that the use of simple exponential model may be acceptable only for small values of the seismic gap length. For moderately large to large seismic gap lengths, it may be conservative to use the lognormal model, while the Weibull model may be assumed for very large seismic gap lengths. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Probabilistic seismic demand analysis of controlled steel moment-resisting frame structures

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 12 2002
Luciana R. Barroso
Abstract This paper describes a proposed methodology, referred to as probabilistic seismic control analysis, for the development of probabilistic seismic demand curves for structures with supplemental control devices. The resulting curves may be used to determine the probability that any response measure, whether for a structure or control device, exceeds a pre-determined allowable limit. This procedure couples conventional probabilistic seismic hazard analysis with non-linear dynamic structural analyses to provide system specific information. This method is performed by evaluating the performance of specific controlled systems under seismic excitations using the SAC Phase II structures for the Los Angeles region, and three different control-systems: (i) base isolation; (ii) linear viscous brace dampers; and (iii) active tendon braces. The use of a probabilistic format allows for consideration of structural response over a range of seismic hazards. The resulting annual hazard curves provide a basis for comparison between the different control strategies. Results for these curves indicate that no single control strategy is the most effective at all hazard levels. For example, at low return periods the viscous system has the lowest drift demands. However, at higher return periods, the isolation system becomes the most effective strategy. Copyright © 2002 John Wiley & Sons, Ltd. [source]

On the construction of multivariate extreme value models via copulas

ENVIRONMETRICS, Issue 2 2010
F. Durante
Abstract Copulas represent a fundamental tool for constructing multivariate probability distributions. Exploiting recent theoretical developments concerning the construction of copulas, we outline several methods for generating multivariate extreme value (MEV) laws having a suitable number of parameters, a feature of great importance in applications. The corresponding random vectors can be efficiently simulated, and easily fitted to empirical data. The use of multivariate return periods for extreme events is also discussed. A practical illustration involving maxima sampled via a network of non-independent gauge stations is presented. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Extreme value predictions based on nonstationary time series of wave data

Local disturbance history affects patchiness of benthic river algae

FRESHWATER BIOLOGY, Issue 9 2003
Christoph D. Matthaei
Summary 1.,Recent research has shown that high-flow events in streams leave a small-scale mosaic of bed patches that have experienced scouring, sediment deposition (fill), or remained stable. Few studies have investigated if this ,local disturbance history' contributes to the patchy distribution of benthic organisms in streams and rivers. 2.,In the present research, we demonstrate that local disturbance history in a mid-sized river can have both short- and long-term effects on epilithic algae. Chains buried vertically in the substratum of the river bed (236 in a 800-m reach) indicated that two floods (return periods ,1 year) caused a mosaic of bed patches with different disturbance histories. Once after the first and twice after the second flood, we sampled epilithic algae (mainly diatoms) in replicate patches that had been scoured, filled, or remained stable during the respective event. Algal biomass and cell density per substratum area were determined. 3.,Three months after the first flood, algal biomass, total diatom density, diatom taxon richness, and densities of six of nine most common taxa were highest in fill patches. Six days after the second flood, biomass was highest in stable patches, indicating a refugium function of these patches. The refugium patches consisted of average-sized stones, in contrast to previous studies of flood refugia for benthic algae in which these refugia were always large and/or immobile substrata. Four weeks after the second flood, diatoms tended to be most abundant in scour patches. With one exception, these differences between patch types could not be attributed to differences in local near-bed current velocity or water depth. 4.,The effects of disturbance history were more complex than a simple refugium function of stable patches because algal patterns changed with time since the last disturbance, possibly depending on the successional state of the algal mats. [source]

Development of design flood hydrographs using probability density functions

Frequency analysis for predicting 1% annual maximum water levels along Florida coast, US

Effect of growing watershed imperviousness on hydrograph parameters and peak discharge

HYDROLOGICAL PROCESSES, Issue 13 2008
Huang-jia Huang
Abstract An increasing impervious area is quickly extending over the Wu-Tu watershed due to the endless demands of the people. Generally, impervious paving is a major result of urbanization and more recently has had the potential to produce more enormous flood disasters than those of the past. In this study, 40 available rainfall,runoff events were chosen to calibrate the applicable parameters of the models and to determine the relationships between the impervious surfaces and the calibrated parameters. Model inputs came from the outcomes of the block kriging method and the non-linear programming method. In the optimal process, the shuffled complex evolution method and three criteria were applied to compare the observed and simulated hydrographs. The tendencies of the variations of the parameters with their corresponding imperviousness were established through regression analysis. Ten cases were used to examine the established equations of the parameters and impervious covers. Finally, the design flood routines of various return periods were furnished through use of approaches containing a design storm, block kriging, the SCS model, and a rainfall-runoff model with established functional relationships. These simulated flood hydrographs were used to compare and understand the past, present, and future hydrological conditions of the watershed studied. In the research results, the time to peak of flood hydrographs for various storms was diminished approximately from 11 h to 6 h in different decrements, whereas peak flow increased respectively from 127 m3 s,1 to 629 m3 s,1 for different storm intensities. In addition, this study provides a design diagram for the peak flow ratio to help engineers and designers to construct hydraulic structures efficiently and prevent possible damage to human life and property. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Long-term precipitation and slide activity in south-eastern Norway, autumn 2000

Temporal and spatial variation of annual rainfall on the island of Crete, Greece

HYDROLOGICAL PROCESSES, Issue 10 2003
S. Naoum
Abstract Annual rainfall records from the island of Crete in Greece were used with the aid of a geographical information system (GIS) to study the temporal and spatial rainfall characteristics. The GIS was used to produce a digital elevation model, delineate watersheds and estimate the areal rainfall from a network of raingauges by using different interpolation schemes. The rainfall,elevation correlation was significant, suggesting an orographic type of precipitation for the island. The rainfall records for the majority of the stations were found to fit the normal distribution. Deviation from normal for the rest of the records was attributed to the wettest year of 1977,1978. The year 1989,1990 was the driest, and most rainfall records showed a decrease in rainfall over 30 years with higher negative rainfall gradients at the higher elevations. Frequency analysis of the rainfall records was used to estimate areal rainfall for the island of Crete and its main watersheds for return periods of 2, 5 and 10 years. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Analysis and objective mapping of extreme daily rainfall in Catalonia

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2007
M. Carmen Casas
Abstract The main objective of this study is to determine the maximum daily precipitation in Catalonia for several established return periods with a high spatial resolution. For this purpose, the maximum daily rainfall annual series from 145 pluviometric stations of the Instituto Nacional de Meteorología (INM) (Spanish Weather Service) in Catalonia have been analyzed. Using the L-moments method of Hosking, every series has been fitted by the extreme value distribution function of Gumbel. From this fitting, the maximum daily precipitation for each of the pluviometric stations corresponding to return periods between 2 and 500 years, have been determined. Applying the Cressman method, the spatial analysis of these values has been achieved. Monthly precipitation climatological data, obtained from the application of Geographic Information Systems (GIS) techniques, have been used as the initial field for the analysis. The maximum daily precipitation at 1 km2 spatial resolution on Catalonia has been objectively determined by the method employed, and structures with wavelength longer than approximately 35 km can be identified. The results show that places where the maximum daily precipitation values are expected are the zone of Guilleries in the Transversal Range, in the highest zones of the Catalan Pyrenees and Cape Creus zone at the northeastern end of Catalonia and in the south, around the Prelittoral Mountain Range between the Mountains of Prades and Montsià. A good fit between the distribution of minimum values and the driest Catalan areas has been found, the lowest values being on the western end of the Central Basin. Copyright © 2006 Royal Meteorological Society. [source]

Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

AFRICAN JOURNAL OF ECOLOGY, Issue 2 2000
H. C. Eckhardt
Changes in the cover and density of shrubs and trees were assessed from aerial photographs (1940, 1974 and 1998) as well as from fixed-point photographs taken in 1984 and 1996 in the Kruger National Park, South Africa. Woody cover (trees and shrubs combined) increased by 12% on granite substrates but decreased by 64% on basalt substrates over the past 58 years. Both these figures are expressed in terms of the initial values, respectively. The density of the large tree component of woody vegetation decreased on both substrates. Woody vegetation cover declined as fire return periods became shorter, but the relationship was weak. The increases in woody plant density and cover on granite are thought to be the result of decreased competition from grasses, which in turn is a result of overgrazing by wild herbivores whose numbers have been kept high through the provision of surface water. These effects were not seen on the relatively nutrient-rich basalts, where grasses can recover rapidly even after heavy grazing. The decline in overall woody cover on basalts is interpreted as a result of regular, short-interval prescribed burning over the past 40 years, while the universal decline in large trees seems to result from an interaction between regular, frequent fires and utilization by elephants. The implications for management are discussed. Résuné On a évalué les changements du couvert et de la densité des arbustes et des arbres à partir de photos aériennes (1940, 1974 et 1998) et de photos prises d'un endroit fixe en 1984 et en 1996 dans le Parc National Kruger, en Afrique du Sud. Le couvert boisé (arbustes et arbres pris ensemble) a augmenté de 12% sur les substrats granitiques mais diminué de 64% sur les substrats basaltiques, au cours des 58 dernières années. Ces deux chiffres sont exprimés en termes de valeurs initiales, respectivement. La densité de la composante de grands arbres de la végétation boisée a diminué sur les deux substrats. Le couvert végétal boisé a diminué alors que les périodes de récupération entre les feux raccourcissaient, mais la relation est faible. On pense que l'augmentation de la densité et du couvert boisés sur le granite est le résultat d'une diminution de la compétition exercée par les herbes qui elle, résulte d'un surpâturage des herbivores sauvages dont le nombre a été maintenu élevé par l'apport d'eau de surface. On n'a pas observé ces effets sur les basaltes relativement riches en nutriments, car les herbes peuvent y repousser rapidement même après un pâturage intense. On interprète le déclin du couvert boisé global sur le basalte comme le résultat des feux provoqués régulièrement et avec de brefs intervalles depuis 40 ans, alors que le déclin général des grands arbres semble être le résultat d'une interaction entre les feux, réguliers et fréquents, et la présence des éléphants. On discute de ce que cela implique pour la gestion. [source]

Flood risk analysis for determining optimal flood protection levels in urban river management

JOURNAL OF FLOOD RISK MANAGEMENT, Issue 3 2008
Masaru Morita
Abstract The objective of the paper is to present a specific risk-analysis method for the assessment of optimal flood protection levels in urban flood risk management using intensity,duration,frequency relationships. Risk herein is understood as the product of flood damage potential and its occurrence probability. The risk analysis is based on a geographic information system-based flood damage prediction model to calculate flood damage for design storms with different return periods. Estimation of the monetary damages for these design storms and their return periods is the prerequisite for quantifying flood risk based on an annual risk density curve. The risk-analysis method is applied to determine optimal flood protection levels for the Kanda River basin in Tokyo, Japan. It shows how two cost curves can be used: risk cost reduction curves and capital cost curves. [source]