Extreme Flood Discharges Research Articles

Changed Seasonality and Forcings of Peak Annual Flows in Ephemeral Channels at Flagstaff, Northern Arizona, USA

Flood variability associated with urbanization, ecological change, and climatic change is of increasing economic and social concern in and around Flagstaff, Arizona, where flood hydrology is influenced by a biannual precipitation regime and the relatively unique geologic setting at the edge of the San Francisco Volcanic Field on the southern edge of the Colorado Plateau. There has been limited long-term gauging of the ephemeral channels draining the developed lands and dry coniferous forests of the region, resulting in a spaciotemporal gap in observation-based assessments of large-scale flooding patterns. We present new data from over 10 years of flood monitoring using a crest stage gauge network, combined with other channel monitoring records from multiple agency sources, to assess inter-decadal patterns of flood change in the area, with a specific emphasis on examining how various controls and disturbances have altered the character and seasonality of peak annual flows. Methods of analysis included the following: using Fisher’s Exact Test to compare the seasonality of flooding between historic data spanning the 1970s and contemporary data obtained since 2010; summarizing GIS-based spatial data and meteorological timeseries to characterize study catchment conditions and changes between flood study periods; and relating spatiotemporal patterns of flood seasonality and occurrences of notably large floods with catchment characteristics and environmental changes. Our results show systematic patterns and changes in Flagstaff-area flood regimes that relate to geologic and topographic controls of the varied catchment systems, and in response to records of climate variations and local catchment disturbances, including urbanization and, especially, high-severity wildfire. For most catchments there has been a shift from predominantly late winter to spring snowmelt floods, or mixed seasonal flood regimes, towards monsoon-dominated flooding, patterns which may relate to observed local warming and precipitation changes. Post-wildfire flooding has produced extreme flood discharges which have likely exceeded historical estimates of flood magnitude over decade-long monitoring periods by one to two orders of magnitude. We advocate for continued monitoring and the expansion of local stream gauge networks to enable seasonal, magnitude-frequency trend analyses, improved climate and environmental change attribution, and to better inform the many planned and ongoing flood mitigation projects being undertaken in the increasingly developed Flagstaff region.

Numerical study on the effect of tangential intake on vortex dropshaft assessment using pressure distributions

ABSTRACT Dropshafts with tangential intakes are common structures in urban water drainage systems. The plunging and vortex flowpatterns are the typical flow regimes, which are mainly affected by the tangential intake design and flood discharge. However, thehydraulic flow transition between vortex and plunging flow patterns has received little attention, which may affect the dropshaftoperation during extreme flood discharge events. In this study, the effects of the flow discharge and contraction ratio on the pressuredistributions of vortex dropshafts are analysed based on a series of numerical simulations. The pressure distributions of the plungingand vortex flow patterns are compared; two hydraulic factors are used to distinguish rotational and non-rotational dropshaftoperations. For appropriate vortex dropshaft designs, the minimum to maximum wall pressure ratio should be greater than 0.1 at theintake cross-section and greater than 0.2 during the falling process. Based on this principle, flows in the annular dropshaft areclassified into non-rotation, transition, and rotation regimes. The results agree with the theoretical rotation intensity of the vortex flow,and an empirical relation is proposed. The present study is easy to implement and practically useful to assess the vortex dropshaftdesign for local water drainage systems.

The Coupling Effect of Flood Discharge and Storm Surge on Extreme Flood Stages: A Case Study in the Pearl River Delta, South China

The low-lying Pearl River Delta in South China is subject to severe flood threats due to watershed floods, sea level rise, and storm surges. It is still unknown to what extent and how far inland storm surges and sea level rise impact the extreme flood stages. This study investigated the coupling effect of flood discharge and storm surge on the extreme flood stages in the Pearl River Delta by using on site observations and simulations generated by the Hydrologic Engineering Center-River Analysis System model. The results show that flood discharges dominated the flood stages in the middle and upper Pearl River Delta, while the storm surges had maximum impact near the river mouth. The storm surges and flood stages showed a significant increase after 2002 in the Hengmen waterway. The design flood stages for the post-2002 period were 0.23–0.89 m higher than the pre-2002 ones at Hengmen at the six return periods from 5 to 200 years examined in this study. Their difference declined toward the upper waterway and reduced to zero about 23 km away from the Hengmen outlet. The coincidence of extreme flood discharges and storm surges further escalates the extreme flood stages in the lower 30 km of estuarine waterways. Our results quantify the severe threats due to sea level rise and intensified storm surges in the lower Pearl River Delta, and are significant for urban planning and designing and managing flood control facilities in the Pearl River Delta and in other coastal fluvial deltas.

Estimation of extreme floods by regional methods in Trois Rivières watershed - North West Algeria

The objective of this study is to estimate the extreme floods corresponding to different recurrence intervals in Trois Rivieres watershed, a large sub-basin of the Macta basin (NW-Algeria). Two regional models are used flow-duration-frequency (Qdf) modelling and gradex method. A methodology for modelling QdF curves based on the so-called continuous converging model has been applied. The second model is gradex method which is a rainfall-runoff probability approach to computing extreme flood discharges in a river. The QdF model gives a complete description of the flood dynamics of Trois Rivieres basin. Nevertheless, it is recommended to use this method with vigilance for the predetermination of severe floods for important return periods. However, the gradex method overestimates the extreme floods for different recurrence intervals. This study highlights that diverse methods should be employed for the regional analysis of extreme hydrological events.

Flood Frequency Analysis in Sabarmati River basin and Estimation of Peak Discharge under Climate Change Scenario

Due to climate change, there is an increased/decreased frequency of peak flood discharge in river and streams. The most important concern of planning is to safe passing of the extreme flood discharge influenced by extreme climatic changes. It is a concern for planning for the storage capacity to safely store extreme discharge/inflow of the river. In this paper, probability theories and statistics for flood frequency factor (K) are applied and based on the results; it is found that the Extreme value model results in to a best model for frequency factor K, as it is yielding the minimum relative error. Using the frequency factor, the flood frequency analysis for peak flood is carried out for climate change scenario/Advance scenario. The peak discharge in advance scenario is more as compare to the base line scenario at most of the stations except three stations located on the south-east of the Sabarmati river basin.

Changing properties of extreme flood in Xiliugou River Basin, China

Based on the annual maximum flood discharge sequence of the Longtouguai hydrological station in the Xiliugou River Basin from 1960 to 2015, the statistical probability characteristics of extreme flood discharge were analyzed by using generalized extreme value (GEV) distribution, generalized Pareto (GP) distribution, Pearson type III (P-III) distribution, general logistic (GLO), and Burr distribution. The parameters were estimated by the L-moments method, and the optimal distributions were selected by some of goodness-of-fit method. The results showed that the annual maximum flood discharge series decreased significantly with an abrupt change point around 2005. From the L-moments and goodness-of-fit test method, Burr distribution can better fit the annual maximum flood discharge before and after the abrupt change. Compared with the sequence before the change, the return period becomes larger and the probability of severe flood is smaller. The designed maximum flood peak discharge for the same return period of the entire sequence is smaller than that of the sequence before the change.

Forensic hydro-meteorological analysis of an extreme flash flood: The 2016-05-29 event in Braunsbach, SW Germany

The flash-flood in Braunsbach in the north-eastern part of Baden-Wuerttemberg/Germany was a particularly strong and concise event which took place during the floods in southern Germany at the end of May/early June 2016. This article presents a detailed analysis of the hydro-meteorological forcing and the hydrological consequences of this event. A specific approach, the “forensic hydrological analysis” was followed in order to include and combine retrospectively a variety of data from different disciplines. Such an approach investigates the origins, mechanisms and course of such natural events if possible in a “near real time” mode, in order to follow the most recent traces of the event. The results show that it was a very rare rainfall event with extreme intensities which, in combination with catchment properties, led to extreme runoff plus severe geomorphological hazards, i.e. great debris flows, which together resulted in immense damage in this small rural town Braunsbach. It was definitely a record-breaking event and greatly exceeded existing design guidelines for extreme flood discharge for this region, i.e. by a factor of about 10. Being such a rare or even unique event, it is not reliably feasible to put it into a crisp probabilistic context. However, one can conclude that a return period clearly above 100years can be assigned for all event components: rainfall, peak discharge and sediment transport. Due to the complex and interacting processes, no single flood cause or reason for the very high damage can be identified, since only the interplay and the cascading characteristics of those led to such an event. The roles of different human activities on the origin and/or intensification of such an extreme event are finally discussed.

Numerical Analysis of Flood modeling of upper Citarum River under Extreme Flood Condition

This paper focuses on how to approach the numerical method and computation to analyse flood parameters. Water level and flood discharge are the flood parameters solved by numerical methods approach. Numerical method performed on this paper for unsteady flow conditions have strengths and weaknesses, among others easily applied to the following cases in which the boundary irregular flow. The study area is in upper Citarum Watershed, Bandung, West Java. This paper uses computation approach with Force2 programming and HEC-RAS to solve the flow problem in upper Citarum River, to investigate and forecast extreme flood condition. Numerical analysis based on extreme flood events that have occurred in the upper Citarum watershed. The result of water level parameter modeling and extreme flood discharge compared with measurement data to analyse validation. The inundation area about flood that happened in 2010 is about 75.26 square kilometres. Comparing two-method show that the FEM analysis with Force2 programs has the best approach to validation data with Nash Index is 0.84 and HEC-RAS that is 0.76 for water level. For discharge data Nash Index obtained the result analysis use Force2 is 0.80 and with use HEC-RAS is 0.79.

The June 2013 flood in the Upper Danube Basin, and comparisons with the 2002, 1954 and 1899 floods

Abstract. The June 2013 flood in the Upper Danube Basin was one of the largest floods in the past two centuries. An atmospheric blocking situation produced precipitation exceeding 300 mm over four days at the northern rim of the Alps. The high precipitation, along with high antecedent soil moisture, gave rise to extreme flood discharges in a number of tributaries including the Tiroler Ache, Saalach, Salzach and Inn. Runoff coefficients ranged from 0.2 in the Bavarian lowlands to 0.6 in the Alpine areas in Austria. Snowfall at high altitudes (above about 1600 m a.s.l.) reduced the runoff volume produced. Precipitation was distributed over two blocks separated by a few hours, which resulted in a single peak, long-duration flood wave at the Inn and Danube. At the confluence of the Bavarian Danube and the Inn, the small time lag between the two flood waves exacerbated the downstream flood at the Danube. Because of the long duration and less inundation, there was less flood peak attenuation along the Austrian Danube reach than for the August 2002 flood. Maximum flood discharges of the Danube at Vienna were about 11 000 m3 s−1, as compared to 10 300, 9600 and 10 500 m3 s−1 in 2002, 1954 and 1899, respectively. This paper reviews the meteorological and hydrological characteristics of the event as compared to the 2002, 1954 and 1899 floods, and discusses the implications for hydrological research and flood risk management.

Evaluation of inundation areas resulting from the diversion of an extreme discharge towards the sea: case study in Tabasco, Mexico

AbstractThis investigation comprises the hydraulic characterisation of a river located in the Mexican State of Tabasco, including the performance of its flood plain under the action of an extreme river discharge. This is done through the combination of a high‐quality validation dataset, remote sensing information, and a standard 2D numerical model. The dataset was collected during an intensive field campaign that took place in August 2009. In particular, in situ measurements of river discharge, bathymetry, water level, and velocities through a whole tidal cycle are employed along with multi‐spectral satellite imagery. The purpose of this study is twofold. Firstly, the integrated approach comprising the combination of a 2D hydrodynamic model, high‐quality in situ measurements and satellite imagery reduce the uncertainty in the model parameterisation and results. Secondly, it is shown that freely available sources of information, such as the Shuttle Radar Topographic Mission (SRTM) data can be processed and utilized in 2D hydraulic models. This is particularly important in countries where high‐resolution elevation data is not yet available. It is demonstrated that the selected approach is useful when the study of possible consequences in a flood plain induced by an extreme flood discharge are sought. Copyright © 2011 John Wiley & Sons, Ltd.

Estimation des débits rares de crue pour les cours d’eau de la Région wallonne (Belgique)

The estimate of the extreme flood discharges of the rivers of Wallonia (Belgium) fits into the flood cartography project financed by the Walloon Region. This project, initiated in 2004, consists in the cartography of the flood hazard and of the damages risk due to floods by overflowing rivers. The flood hazard maps are established on the basis of hydraulic modelling which requires the estimate of extreme flood discharges.The source data are the average hourly discharges acquired by the hydrological measurement networks of the Walloon Region. Extreme flood discharges, corresponding to return periods of 25, 50 and 100 years, are calculated by adjustment of a statistical law to series of maximum annual discharges or to series of peaks-over-threshold, in the case of recent historical records of observations. The software HYFRAN used here was developed by the University of Quebec.The flood cartography is established on a scientific basis and is associated to urban regulations of corresponding areas. It will have a positive impact on the citizens by providing tools useful to treat request of permit, to target protection measures or installations, to make easier crisis management...

Palaeoflood hydrology: insight into rare events and extreme flood discharges

Les etudes paleohydrologiques permettent la reconstitution de chronologies de crue a partir des traces laissees par les sediments de crue ou l'erosion du courant. Ces marques geologiques donnent une indication sur le niveau atteint par des crues anciennes, sur des periodes de quelques siecles a plusieurs millenaires. La datation des sediments s'effectue a partir des materiaux presents dans les depots de crue (isotope 14 du carbone ou thermoluminescence de grains de mineraux). Les donnees paleohydrologiques peuvent ensuite etre exploitees d'un point de vue probabiliste, en considerant que les evenements recenses ont depasse un ou plusieurs seuils de niveau. Les differentes applications recentes de la paleohydrologie aux evenements de crue ont porte sur: (1) l'analyse frequentielle des crues; (2) la determination d'un niveau maximum atteint par les crues et sa coherence avec la Crue Maximale Probable (PMF); et (3) meilleure connaissance de la relation entre les crues extraordinaires et la variabilite climatique.

A geomorphologic and hydrologic evaluation of an extraordinary flood discharge estimate: Bronco Creek, Arizona

The Bronco Creek, Arizona flood of August 19, 1971, was an extraordinary and rare flash flood. The published peak discharge estimate of 2080 m3 s−1 makes it virtually the world's largest known rainfall‐generated flood to come from a 50‐km2 basin. Previous workers have suggested that the peak discharge was overestimated based on its unlikely hydrological and hydraulic implications. In this study we reevaluate the Bronco Creek flood discharge by modeling peak discharges using relict high‐water marks in bedrock canyon reaches near the mouths of the three major subbasins of the watershed. The new estimated peak discharge of 750 to 850 m3 s−1 is based on summing the subbasin estimates and correcting for omitted drainage area. Qualitative and quantitative analyses of site‐specific and regional hydrological, meteorological, and geomorphological information support a lower peak discharge. We assessed preflood and postflood channel characteristics at the site of the original estimate using historical aerial photographs. Our analysis indicates that much of the discrepancy between the estimates may be explained by significant morphologic changes in the channel caused by the flood. The original estimate was made in a wide, high‐gradient alluvial channel where the flood changed the channel pattern from braided with vegetated bars to straight, wide, and devoid of vegetation. It is also likely that net vertical scour occurred in the channel. Although our new estimate is much lower than the original estimate, it is consistent with the dominant trend in regional flood magnitude‐drainage area relationships. By integrating hydraulic modeling of peak discharges in stable channel reaches with assessment of ancillary hydrological, meteorological, and geomorphological information, this study presents a viable approach to assessing the accuracy of extreme flood estimates in general. It also highlights potential uncertainties in estimating extreme flood discharges in steep, alluvial channels.

Log Pearson III distribution — Is it applicable to flood frequency analysis of Australian streams?

An analysis using moment ratio diagrams of peak annual discharges of 172 Australian streams suggests that the log Pearson Type III distribution is appropriate for estimation of extreme flood discharges. The practice of setting to zero the coefficients of skewness of the logarithms of the peak discharges that are not statistically different from zero is questioned.