Frequency Of Extreme Floods Research Articles

Physically-based extreme flood frequency with stochastic storm transposition and paleoflood data on large watersheds

Summary Traditionally, deterministic flood procedures such as the Probable Maximum Flood have been used for critical infrastructure design. Some Federal agencies now use hydrologic risk analysis to assess potential impacts of extreme events on existing structures such as large dams. Extreme flood hazard estimates and distributions are needed for these efforts, with very low annual exceedance probabilities ( ⩽ 10 - 4 ) (return periods >10,000 years). An integrated data-modeling hydrologic hazard framework for physically-based extreme flood hazard estimation is presented. Key elements include: (1) a physically-based runoff model (TREX) coupled with a stochastic storm transposition technique; (2) hydrometeorological information from radar and an extreme storm catalog; and (3) streamflow and paleoflood data for independently testing and refining runoff model predictions at internal locations. This new approach requires full integration of collaborative work in hydrometeorology, flood hydrology and paleoflood hydrology. An application on the 12,000 km2 Arkansas River watershed in Colorado demonstrates that the size and location of extreme storms are critical factors in the analysis of basin-average rainfall frequency and flood peak distributions. Runoff model results are substantially improved by the availability and use of paleoflood nonexceedance data spanning the past 1000 years at critical watershed locations.

Historic flood marks and flood frequency analysis of the Danube River at Bratislava, Slovakia

Abstract In this paper we focused on the history of floods and extreme flood frequency analysis of the upper Danube River at Bratislava. Firstly, we briefly describe the flood marks found on the Danube River in the region of Bratislava, Slovakia, and provide an account of the floods’ consequences. Secondly, we analyzed the annual maximum discharge series for the period 1876-2012, including the most recent flood of June 2013. Thirdly, we compare the values of T-year design discharge computed with and without incorporating the historic floods (floods of the years 1501, 1682, and 1787 into the 138-year series of annual discharge peaks). There are unfortunately only a few historic flood marks preserved in Bratislava, but there are very important and old marks in neighbouring Hainburg and other Austrian cities upstream to Passau. The calculated T-year maximum discharge of the Danube at Bratislava for the period 1876-2010 without and with historic flood values have been compared. Our analysis showed that without incorporating the historic floods from the years 1501, 1682, and 1787 the 1000-year discharge calculated only with data from the instrumented period 1876- 2013 is 14,188 m3 s-1, and it is lower compared to the 1000-year discharge of 14,803 m3 s-1 when the three historic floods are included. In general, the T-year discharge is higher throughout the whole spectrum of T-year discharges (10, 20, 50, 100, 200, 500-year discharge) when the three historic floods are included. Incorporating historic floods into a time series of maximum annual discharge seems to exert a significant effect on the estimates of low probability floods. This has important implications for flood managements and estimation of flood design discharge.

Revisiting the Concepts of Return Period and Risk for Nonstationary Hydrologic Extreme Events

Current practice using probabilistic methods applied for designing hydraulic structures generally assume that extreme events are stationary. However, many studies in the past decades have shown that hydrological records exhibit some type of nonstationarity such as trends and shifts. Human intervention in river basins (e.g., urbanization), the effect of low-frequency climatic variability (e.g., Pacific Decadal Oscillation), and climate change due to increased greenhouse gasses in the atmosphere have been suggested to be the leading causes of changes in the hydrologic cycle of river basins in addition to changes in the magnitude and frequency of extreme floods and extreme sea levels. To tackle nonstationarity in hydrologic extremes, several approaches have been proposed in the literature such as frequency analysis, in which the parameters of a given model vary in accordance with time. The aim of this paper is to show that some basic concepts and methods used in designing flood-related hydraulic structures assuming a stationary world can be extended into a nonstationary frame- work. In particular, the concepts of return period and risk are formulated by extending the geometric distribution to allow for changing exceeding probabilities over time. Building on previous developments suggested in the statistical and climate change literature, the writers present a simple and unified framework to estimate the return period and risk for nonstationary hydrologic events along with examples and applications so that it can be accessible to a broad audience in the field. The applications demonstrate that the return period and risk estimates for nonstationary situations can be quite different than those corresponding to stationary conditions. They also suggest that the nonstationary analysis can be helpful in making an appropriate assessment of the risk of a hydraulic structure during the planned project-life. DOI: 10.1061/ (ASCE)HE.1943-5584.0000820. © 2014 American Society of Civil Engineers.

Eolian and fluvial sedimentation in the southwestern Sinai Mountains, Egypt: a record of flash floods during the late Pleistocene

Wadis emerging from the southwestern Sinai Mountains (Egypt) westwards to the Gulf of Suez are filled by >40 m thick late Pleistocene sediments, which have been subsequently incised to bedrock after the Last Glacial Maximum (LGM). Sedimentation and erosion resulted from changes in the basin's hydrological conditions caused by climate variations. Sediment characteristics indicate distinct processes ranging from high to low energy flow regimes. Airborne material is important as a sediment source. The fills are associated with alluvial fans at wadi mouths at the mountain fronts. Each alluvial fan is associated and physically correlated with the respective sediment fill in its contributing wadi. The alluvial fans have steep gradients and are only a few kilometers long or wide. The alluvial fans converge as they emerge from the adjacent valleys. According to optically stimulated luminescence dating, the initial sediment has an age of ∼45 ka and the sedimentation ends ∼19 ka, i.e., happened mainly during marine isotope stage (MIS) 3 and early MIS 2 formation and initial incision sometime during LGM. As the delivery of sediments in such a hyper-arid environment is by extreme floods, this study indicates an interval of intense fluvial activity, probably related to increased frequency of extreme floods in Southern Sinai. This potentially indicates a paleoclimatic change in this hyper-arid environment.

Research Spotlight: Rainfall variability increasing in northern Australia

Rainfall variability, including the frequency of extreme floods and droughts, is increasing in northeastern Australia, a new study shows. Rainfall in Queensland is variable from year to year, partly due to El Niño–Southern Oscillation (ENSO) events. As massive corals grow, their skeletons create yearly growth patterns, similar to tree rings, and record a wealth of proxy information about the corals' environment over several centuries. Flood events show up in nearshore annually banded coral skeletons as luminescent lines. (Paleoceanography, doi:10.1029/2010PA002050, 2011)

Rainfall-runoff modelling and palaeoflood hydrology applied to reconstruct centennial scale records of flooding and aquifer recharge in ungauged ephemeral rivers

Abstract. In this study we propose a multi-source data approach for quantifying long-term flooding and aquifer recharge in ungauged ephemeral rivers. The methodology is applied to the Buffels River, at 9000 km2 the largest ephemeral river in Namaqualand (NW South Africa), a region with scarce stream flow records limiting research investigating hydrological response to global change. Daily discharge and annual flood series (1965–2006) were estimated from a distributed rainfall-runoff hydrological model (TETIS) using rainfall gauge records located within the catchment. The model was calibrated and validated with data collected during a two year monitoring programme (2005–2006) at two stream flow stations, one each in the upper and lower reaches of the catchment. In addition to the modelled flow records, non-systematic flood data were reconstructed using both sedimentary and documentary evidence. The palaeoflood record identified at least 25 large floods during the last 700 yr; with the largest floods reaching a minimum discharge of 255 m3 s−1 (450 yr return period) in the upper basin, and 510 m3 s−1 (100 yr return period) in the lower catchment. Since AD 1925, the flood hydrology of the Buffels River has been characterised by a decrease in the magnitude and frequency of extreme floods, with palaeoflood discharges (period 1500–1921) five times greater than the largest modelled floods during the period 1965–2006. Large floods generated the highest hydrograph volumes, however their contribution to aquifer recharge is limited as this depends on other factors such as flood duration and storage capacity of the unsaturated zone prior to the flood. Floods having average return intervals of 5–10 yr (120–140 m3 s−1) and flowing for 12 days are able to fully saturate the Spektakel aquifer in the lower Buffels River basin. Alluvial aquifer storage capacity limiting potential recharge by the largest floods is a common problem in arid environments, with the largest infiltration volumes favoured by increasing depth to groundwater levels.

Climate change, flooding in South Asia and implications

South Asia is one of the most flood vulnerable regions in the world. Floods occur often in the region triggered by heavy monsoon precipitation and can cause enormous damages to lives, property, crops and infrastructure. The frequency of extreme floods is on the rise in Bangladesh, India and Pakistan. Past extreme floods fall within the range of climate variability but frequency, magnitude and extent flooding may increase in South Asia in future due to climate change. Flood risk is sensitive to different levels of warming. For example, in Bangladesh, analysis shows that most of the expected changes in flood depth and extent would occur between 0 and 2°C warming. The three major rivers Ganges, Brahmaputra and Meghna/Barak will play similar roles in future flooding regimes as they are doing presently. Increases in future flooding can cause extensive damage to rice crops in the monsoon. This may have implications for food security especially of poor women and children. Floods can also impact public health in the flood plains and in the coastal areas.

Paleohydrologic bounds and extreme flood frequency of the Upper Arkansas River, Colorado, USA

The Upper Arkansas River basin has experienced notable large floods, including the event of 2–6 June 1921 that devastated the city of Pueblo, Colorado. We investigated flood and paleoflood hydrology at strategic sites to determine the frequency and geographic extent of extreme floods within the basin for a dam safety application. Streamgage, historical, and paleoflood data were utilized to develop frequency curves at sites near Salida, Cotopaxi, Parkdale, and Pueblo. Soil/stratigraphic descriptions, radiocarbon dating, and hydraulic modeling were used to estimate paleoflood nonexceedance bounds at the four sites, which ranged from 400 to 2200 YBP for late Holocene surfaces to late Pleistocene surfaces near Cotopaxi. Peak-flow data are from lower-magnitude snowmelt runoff in May and June in the upper basin and from high-magnitude rainfall runoff from June to August in the lower basin. Flood frequency curves reflect this transition near Parkdale from snowmelt to extreme rainfall-runoff. For similar return periods, paleoflood peak discharges increase from about 480 m 3/s upstream at Loma Linda to about 4250 m 3/s downstream near Pueblo. This increase is attributed to the larger rainfall component derived from lower elevations between Loma Linda and Pueblo. Return periods for design floods at Pueblo Dam exceeded 10,000 years based on paleoflood frequency curve extrapolations.

Extreme floods and short-term hydroclimatological fluctuations in the hyper-arid Dead Sea region, Israel

The autumn Active Red Sea Trough (ARST) rainstorm in the southern Dead Sea region on 29 October 2004 was documented using calibrated radar images, some rainfall measurements, and slope-area and HEC-RAS peak discharge estimates in several tributaries. The extreme rainstorm had a maximum intensity of 175 mm h − 1 , a duration of 2 h, and rainfall amounts of up to 74 mm. The resulting flood in Nahal Zin (1400 km 2), which increased downstream up to 1280 m 3 s − 1 (probability of < 1%), was effectively contributed from the lower tributaries that constitute only 250 km 2 or 18% of the total drainage basin area. Specific peak discharges in small drainage basins (1–5 km 2) exceeded 50 m 3 s − 1 km − 2 , and for basins of 50 km 2 were up to 18 m 3 s − 1 km − 2 . Re-plotting of the flood envelope curve was necessary, as it was exceeded by several floods at all catchment sizes. A recent increase in the frequency of extreme floods has been recognized in flood studies in the Negev, northern Israel, and across the wider eastern Mediterranean basin. This increase is reflected in other palaeohydrological and palaeoclimatological evidence in the southern Levant such as the levels of the Dead Sea, but contradicts hydroclimatological trends in the western Mediterranean that are correlated to the NAO index, suggesting that these are quite different hydroclimatological environments.

Freshwater management and climate change adaptation: Experiences from the central Yangtze in China

The Yangtze is the largest river basin in China and home to over 400 million people. In recent history, and especially during 1950s–1970s, extensive lakes and floodplains were reclaimed as polders for agriculture and rural development. Consequently, the flood retention capacity was decreased, many lakes were disconnected from the main channel of the Yangtze by embankments and sluice gates, and eutrophication was common. It is anticipated that there will be a greater frequency of extreme floods and droughts in the basin according to climate change scenarios. WWF commenced a programme in 2002 in partnership with government agencies and local communities to reconnect three lakes (Zhangdu, Hong and Tian-e-zhou) in Hubei Province to the river by opening sluice gates seasonally and improving lake management. The resilience of the lake environment to climate change and the livelihoods of local people were enhanced. The measures assessed here highlight: (a) the need for adaptation programmes to concurrently improve livelihoods and reduce exposure to physical risks; (b) the need to build the capacity of people and institutions; and (c) the value of decentralized adaptation as compared with new infrastructure investments.

Climate informed flood frequency analysis and prediction in Montana using hierarchical Bayesian modeling

It is widely acknowledged that climate variability modifies the frequency spectrum of extreme hydrologic events. Traditional hydrological frequency analysis methods do not account for year to year shifts in flood risk distributions that arise due to changes in exogenous factors that affect the causal structure of flood risk. We use Hierarchical Bayesian Analysis to evaluate several factors that influence the frequency of extreme floods for a basin in Montana. Sea surface temperatures, predicted GCM precipitation, climate indices and snow pack depth are considered as potential predictors of flood risk. The parameters of the flood risk prediction model are estimated using a Markov Chain Monte Carlo algorithm. The predictors are compared in terms of the resulting posterior distributions of the parameters that are used to estimate flood frequency distributions. The analysis shows an approach for exploiting the link between climate scale indicators and annual maximum flood, providing impetus for developing seasonal forecasting of flood risk applications and dynamic flood risk management strategies

Power-law extreme flood frequency

Abstract Conventional Flood Frequency Analysis (FFA) has been criticized both for its questionable theoretical basis, and for its failure in extreme event prediction. An important research issue for FFA is the exploration of models that have theoretical/explanatory value as the first step towards more accurate predictive attempts. Self-similar approaches offer one such alternative, with a plausible theoretical basis in complexity theory that has demonstrable wide applicability across the geophysical sciences. This paper explores a simple self-similar approach to the prediction of extreme floods. Fifty river gauging records from the USA exhibiting an outlier event were studied. Fitting a simple power law (PL) relation to events with return period of 10 years or greater resulted in more accurate discharge and return period estimates for outlier events relative to the Log-Pearson III model. Similar success in predicting record events is reported for 12 long-term rainfall records from the UK. This empirical success is interpreted as evidence that self-similarity may well represent the underlying physical processes generating hydrological variables. These findings have important consequences for the prediction of extreme flood events; the PL model produces return period estimates that are far more conservative than conventional distributions.

The influence of recent environmental change on flooding and sediment fluxes in the Yorkshire Ouse basin

This study attempts to establish causes of historical variations in flood frequency and magnitude in a 119-year flood stage record at York, and assess its likely effects on fluxes of fine sediment and associated base metal contaminants. Climatic controls of flooding are evaluated through analysis of synoptic flood generation, and large-scale land use changes are also considered in terms of their influence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low flood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream fine sediment delivery would have been high. A further decline in flood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant fluxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high flood frequency and magnitude resulted in significantly enhanced sediment fluxes. Rates of flux declined between 1969 and 1977 owing to extremely low flood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude floods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high flood frequency and magnitude in the UK, and significantly enhanced sediment and contaminant metal flux. Analysis of synoptic flood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme floods and consequently increased sediment fluxes.