Article

Reducing uncertainty with flood frequency analysis: the contribution of palaeo- and historical flood information

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  • Water Technology
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Abstract

Using a combination of stream gauge, historical and palaeoflood records to extend extreme flood records has proven to be useful in improving flood frequency analysis (FFA). The approach has typically been applied in localities with long historical records and/or suitable river settings for palaeoflood reconstruction from slackwater deposits (SWDs). However, many regions around the world have neither extensive historical information nor bedrock gorges suitable for SWDs preservation and palaeoflood reconstruction. This study from subtropical Australia demonstrates that confined, semi-alluvial channels such as macrochannels provide relatively stable boundaries over the 1000-2000 year time period and the preserved SWDs enabled palaeoflood reconstruction and their incorporation into FFA. FFA for three sites in subtropical Australia with the integration of historical and palaeoflood data using Bayesian Inference methods showed a significant reduction in uncertainty associated with the estimated discharge of a flood quantile. Uncertainty associated with estimated discharge for the 1% Annual Exceedance Probability (AEP) flood is reduced by over 50%. In addition, sensitivity analysis of possible within-channel boundary changes shows that FFA is not significantly affected by any associated changes in channel capacity. Therefore, a greater range of channel types may be used for reliable palaeoflood reconstruction by evaluating the stability of inset alluvial units, thereby increasing the quantity of temporal data available for FFA. The reduction in uncertainty, particularly in the prediction of the ≤ 1% AEP design flood, will improve flood risk planning and management in regions with limited temporal flood data. This article is protected by copyright. All rights reserved.

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... Improvements to industry decision-making can only be facilitated by establishing the 'plausible ranges of climate change' 8 and the reduction in the uncertainty afforded by millennial-scale records 9 . The relatively short observational record-length (<100 years) available for hydrological modelling and water planning, is insufficient to capture variability around the extremes of floods and droughts [9][10][11][12][13][14] . ...
... Improvements to industry decision-making can only be facilitated by establishing the 'plausible ranges of climate change' 8 and the reduction in the uncertainty afforded by millennial-scale records 9 . The relatively short observational record-length (<100 years) available for hydrological modelling and water planning, is insufficient to capture variability around the extremes of floods and droughts [9][10][11][12][13][14] . Climate information also plays a key role in enabling the sort of 'smarter solutions' required of the industry, with several applications demonstrating the tangible benefits of incorporating palaeoclimate data into water management 13,[15][16][17] . ...
... Climate information also plays a key role in enabling the sort of 'smarter solutions' required of the industry, with several applications demonstrating the tangible benefits of incorporating palaeoclimate data into water management 13,[15][16][17] . Palaeoflood data, for example, is now routinely used to improve flood frequency analysis in several countries 9,18,19 and is especially valuable to 'stress test' infrastructure design to safeguard against dam overspill. ...
Article
Full-text available
Palaeoclimate data relating to hydroclimate variability over the past millennia have a vital contribution to make to the water sector globally. The water industry faces considerable challenges accessing climate data sets that extend beyond that of historical gauging stations. Without this, variability around the extremes of floods and droughts is unknown and stress-testing infrastructure design and water demands is challenging. User-friendly access to relevant palaeoclimate data is now essential, and importantly, an efficient process to determine which proxies are most relevant to a planning scenario, and geographic area of interest. This paper presents PalaeoWISE ( Palaeo climate Data for W ater I ndustry and Se curity Planning) a fully integrated, and quality-assured database of proxy data extracted from data repositories and publications collated in Linked Paleo Data (LiPD) format. We demonstrate the application of the database in Queensland, one of Australia’s most hydrologically extreme states. The database and resultant hydroclimate correlations provides both the scientific community, and water resource managers, with a valuable resource to better manage for future climate changes.
... Recent reviews (Baker, 2006(Baker, , 2013Benito and Díez-Herrero, 2015) have summarised the growing spatial coverage of SWD-PSI studies, and more recent work since include additional studies in Thailand (Ng et al., 2015), France (Dezileau et al., 2014), and China (e.g., Mao et al., 2015;Guo et al., 2017). While the increase in the application of SWDs across Asia has been rapid, studies in Australia remain very limited and geographically biased (Lam et al., 2017). Initial application of SWDs in arid and semiarid areas used clear contrasts in flood unit stratigraphy to help delineate individual flood units. ...
... They have been shown to have large channel capacities, with bank top capacities approaching a 50-year annual recurrence interval (ARI), and are laterally stable because of adjacent, highly resistant, clay-rich Pleistocene alluvium. Results over a range of timescales from post-European to Holocene indicate that lateral migration is limited and changes in cross-sectional area, even postextreme flood events are relatively minor (Croke et al., 2013;Fryirs et al., 2015;Thompson et al., 2016;Lam et al., 2017). A C C E P T E D M A N U S C R I P T ...
... Importantly, the macrochannel settings of SEQ fulfil the stable boundary conditions required to provide robust discharge estimates because they have experienced minimal channel adjustments over the past 200 years (Fryirs et al., 2015;Thompson et al., 2016) and even over the longer timescale of the Holocene (Croke et al., 2016a(Croke et al., , 2016bDaley et al., 2016). In a more detailed assessment of how changes in withinchannel sedimentation rates may affect predicted palaeoflood discharges, Lam et al. (2017) ACCEPTED MANUSCRIPT ...
Article
The application of palaeoflood hydrology in Australia has been limited since its initial introduction > 30 years ago. This study adopts a regional, field-based approach to sampling slackwater deposits in a subtropical setting in southeast Queensland beyond the traditional arid setting. We explore the potential and challenges of using sites outside the traditional physiographical setting of bedrock gorges. Over 30 flood units were identified across different physiographical settings using a range of criteria. Evidence of charcoal-rich layers and palaeosol development assisted in the identification and separation of distinct flood units. The OSL-dated flood units are relatively young with two-thirds of the samples being < 1000 years old. The elevation of all flood units have resulted in estimated minimum discharges greater than the 1% annual exceedance probability. Although these are in the same order of gauged flood magnitudes, > 80% of them classified as ‘extreme event’. This study opens up the renewed possibility of applying palaeoflood hydrology to more populated parts of Australia where the need for improved estimation of flood frequency and magnitude is now urgent in light of several extreme flood events. Preliminary contributions to improve the understanding between high magnitude floods and regional climatic drivers are also discussed. Recognised regional extreme floods generally coincide with La Niña and negative IPO phases, while tropical cyclones appear to be a key weather system in generating such large floods.
... FFA is used in flood mapping to estimate the floods design or quantiles (Condie and Lee 1982) for specified return periods or exceedance probabilities, based on a series of annual maximum discharge, and frequently using probability distributions (Parida et al. 1998). However, in order to reduce the limitation of the FAA due to uncertainties in the probability distribution model, availability of the data in length, and the uncertainties linked to the annual maximum discharge, different solutions have been proposed in previous works (Osorio and Reis 2016), In this context, Neppel et al. (2010) and Lam et al. (2017) included the historical events and palaeofloods to extend the observational period and reduce the uncertainties in design flood. Accordingly, Steinbakk et al. (2016) and Osorio and Reis (2016) combined the results derived from a Bayesian rating curve with FFA to study the influence of discharge uncertainty on flood design computing. ...
... A return period, known as a recurrence interval, is an estimation of the mean time between events such as flood or river discharge to occur (Wiltshire 1986). It is a statistical measurement usually established on historical data over a prolonged period (Lam et al. 2017). We addressed return periods for design floods of 5, 10, 50, and 100 years, selected according to the Algerian flood safety standards. ...
... It would also be useful to use this methodology to improve the flood hazard mapping in the case of the use of palaeoflood records. For instance, in hydrologic hazard studies the palaeoflood data are often used to extend the extreme flood data series and has been proved to be a useful tool to reduce the uncertainty associated to the design flood estimation (Lam et al. 2017). The bound uncertainty of the predicted flooded area is high, but it is a realistic representation of the reliability of flood risk evaluation based only on the rating curve uncertainty. ...
Article
The accuracy of flood control models depends on the set of annual maximum discharge used to estimate design flood via statistical flood frequency analysis (FFA). The uncertainties associated with the discharge time series from stage records are often ignored. Indeed, the uncertainty associated with discharge estimation is not addressed in many of the previous hydraulic risk analyses. In this study, we provide a quantitative approach to rigorously explore the effect that the rating curve uncertainty has on the design flood estimation and the flood hazard mapping. The town of Vieux-Ténès, Algeria, located near the mouth of the Allala River, was used as a case study. Despite the presence of concrete flood protection walls, several floods caused severe damage over the last decades in the town. Multisegment Bayesian rating curve, based on the Bayesian rating curve (BaRatin) method, was used to compute the rating curve uncertainty of the Allala hydrometric station, allowing for the creation of a new time series of annual maximum discharge for the 1973–2017 time period and the estimation of the design flood for different return periods by FFA. The Hydrologic Engineering Center’s river analysis system (HEC-RAS) was used to model the water levels for different locations based on steady flow analysis, using them to define flood-prone areas and an effective protection system. We found that estimations of the flooded area varied between −18% and 15% when assessing rating curve uncertainties. Results highlighted that the existing flood control system is not sufficient to protect the inner city against flood risks, especially in the lower-lying areas of the flooded area.
... The longest of these Queensland coral records (Lough, Lewis, and Cantin 2015) extends back to the mid seventeenth Century and indicates high flow events consistent with the evidence provided by a number of the slackwater deposit studies in Queensland (e.g. Wohl 1992a; Lam et al. 2017b; Table S1). Lough, Lewis, and Cantin (2015) was able to identify that flow in the Burdekin River and its variability has increased since the late nineteenth century, including more frequent wet extremes. ...
... The incorporation of both palaeoflood and palaeo-hydroclimate information into flood frequency and risk analysis can help improve our understanding of flood risk (e.g. Lam et al. 2017b;Zhang et al. 2018). ...
... It is also recognised that palaeoflood records can provide estimates of design floods directly, or help guide design inputs for rainfallbased procedures. Inclusion of palaeoflood records for southeastern Queensland (and several other rivers) in estimates of the 1% Annual Exceedance Probability flood resulted in more than a 50% reduction in uncertainty (Lam et al. 2017b; Table S2). Similarly, Engeland et al. (2018 and references therein), found that estimates of reliability and stability of flood design were improved by including documentary evidence extending back to 1604 CE in analyses. ...
Article
Extreme rainfall is projected to increase with climate change, but the impact of climate change on floods is uncertain. Infrastructure design based on information available from short gauged time series (typically ~30 – 80 years) may not take account of the full range of possible flood events, or be suitable for identifying non-stationarity. Australian palaeoflood and palaeo-hydroclimate records drawn from a wide variety of natural archives and documentary sources suggest that Australia has been subjected to larger flood events in the past; a pluvial period for eastern Australia in the eighteenth Century is particularly note-worthy. If the current infrastructure is inadequate for past floods, it is unlikely it will adequately mitigate future floods. We discuss how improved awareness, and incorporation, of palaeoflood records in risk estimates could help guide infrastructure planning and design, flood event prediction and inform flood mitigation policy. This is particularly relevant for Australia with its notoriously variable hydroclimate.
... 2. Paleofloods: Past maximum flood discharges can also be reconstructed using botanical analyses like trees [e.g. 2] or geological evidence like fluvial sediments [27,35,19,37]. ...
... They provide a rigorous framework for the treatment of non-systematic (i.e. floods exceeding a certain threshold) and imperfect information [32,25,30,19]. In particular, Bayesian methods are particularly relevant for the treatment of the different sources of uncertainty [36]. ...
Article
Accurate flood hazard assessments are crucial for adequate flood hazard mapping and hydraulic infrastructure design. The choice of an acceptable and cost-effective solution for such assessments depends upon the estimation of quantiles for different characteristics of floods, usually maximum discharges. However, gauge series usually have a limited time length and, thereby, quantile estimates associated to high return periods are subject to large uncertainties. To overcome this limitation, reconstructed flood series from historical, botanical or geological archives can be incorporated. In this study, we propose a novel approach that i) combines classic series of observations with paleodischarges (of the Rhône River) reconstructed from open lake sediments (Lake Bourget, Northwestern Alps, France) and ii) propagates uncertainties related to the reconstruction method during the estimation of extreme quantiles. A Bayesian approach is adopted in order to properly treat the non-systematic nature of the reconstructed flow data, as well as the uncertainties related to the reconstruction method. While this methodology has already been applied to reconstruct maximum discharges from historical documents, tree rings or fluvial sediments, similar applications need to be tested today on open lake sediments as they are one of the only archives that provide long and continuous paleoflood series. Reconstructed sediment volumes being subject to measurement errors, we evaluate and account for this uncertainty, along with the uncertainty related to the reconstruction method, the parametric uncertainty, and the rating-curve errors for systematic gauged flows by propagating these uncertainties through the modeling chain. Reconstructed maximum discharges appear to largely overcome values of observations, reaching values of approximately 2,600, 4,200, 2,450 and 2,500 m³/s in 1689, 1711, 1733 and 1737 respectively, which correspond to historically-known catastrophic floods. Extreme quantiles are estimated using direct measurements of maximum discharges (1853-2004) only and then combined to the sedimentary information (1650-2013). The comparison of the resulting estimates demonstrates the added value of the sedimentary information. In particular, the four historical catastrophic floods are very unlikely if only direct observations are considered for quantile estimations.
... Francés (2001) showed how to include paleoflood data in FFA using the Maximum Likelihood Estimation (MLE) method. Lam et al. (2017) integrated paleoflood data into FFA using Bayesian Inference methods, thereby showing a significant reduction in uncertainty for 100-yr flood estimation in subtropical Australia. Multiple studies have focused on assessing the contribution of PFH to FFA improvement by promoting the incorporation of paleoflood data. ...
... To ignore realities is to be "unscientific." PFH continues the exploratory imperative of what is most essential in a doing science of extreme flooding by making discoveries of viable and efficient data sources from real-world evidence, which can act as a "spotlight" for improving both FFA (Schendel and Thongwichian, 2017;Lam et al., 2017) and hydrological modeling Table 7 A comparison between flood frequency results using different methods on the lower Green River, Utah (in m 3 /s). The regional-regression based 1000-yr flood is not provided in Kenney et al. (2007). ...
Article
Through a comprehensive paleoflood hydrological investigation we document natural evidence for at least 27 high-magnitude paleofloods at six sites on the Lower Green River, Utah. Hydraulic analysis, using the Sedimentation and River Hydraulic-2D model (SRH-2D), shows that the responsible peak paleoflood discharges ranged between 500 and 7500 m³/s. At least 14 of these paleoflood discharge peaks exceed a level twice that of the maximum systematic record of gauged flows: 1929 m³/s. Geochronological analyses, employing optically stimulated luminescence (OSL) and radiocarbon dating techniques, demonstrate that these 14 largest paleoflood peaks occurred during the past 700 years. Integration of the paleoflood data into flood frequency analyses (FFA) reveals considerably higher values for the upper tails of the flood distribution than does a FFA based solely on the systematic gauged record, indicating that extreme floods are larger and more frequent than implied by the relatively short gauged record. Through examination of three approaches to extreme flood estimation – conventional FFA, probable maximum flood estimation (PMF), and paleoflood hydrology (PFH) – we show the significance of the natural evidence for advancing scientific understanding of extreme floods that naturally occur in the Colorado River system. We argue that this kind of scientific understanding is absolutely essential for achieving a credible evaluation of extreme flood risk in a watershed of immense importance to economic prosperity of the southwestern U.S.
... Strictly speaking, risk is related to the probability of an uncertain event happening and the consequences if it does happen, which can be defined as Risk = f (P, C), where P is the probability, C is the consequence. However, for many real-world problems, it is difficult to estimate the consequences of an uncertain event, especially when the pre-event evaluation is required (before the uncertain event happens) [34][35][36][37]. For instance, in real-time flood control operations, pre-evaluating the consequences of an operation strategy is very challenging because of the time limits and complexity of the problem. ...
Article
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The inherent uncertainty of inflow forecasts hinders the reservoir real-time optimal operation. This paper proposes a risk analysis model for reservoir real-time optimal operation using the scenario tree-based stochastic optimization method. We quantify the probability distribution of inflow forecast uncertainty by developing the relationship between two forecast accuracy metrics and the standard deviation of relative forecast error. An inflow scenario tree is generated via Monte Carlo simulation to represent the uncertain inflow forecasts. We establish a scenario tree-based stochastic optimization model to explicitly incorporate inflow forecast uncertainty into the stochastic optimization process. We develop a risk analysis model based on the principle of maximum entropy (POME) to evaluate the uncertainty propagation process from flood forecasts to optimal operation. We apply the proposed methodology to a flood control system in the Daduhe River Basin, China. In addition, numerical experiments are carried out to investigate the effect of two different forecast accuracy metrics and different forecast accuracy levels on reservoir optimal flood control operation as well as risk analysis. The results indicate that the proposed methods can provide decision-makers with valuable risk information for guiding reservoir real-time optimal operation and enable risk-informed decisions to be made with higher reliabilities.
... Nevertheless, the differential preservation and censoring of potential palaeoflood data (Lewin and Macklin, 2003) needs to be considered. Where stable depositional sites can be located and combined with high resolution dating, reliable flood magnitude estimates can be made (Leigh, 2018), with great potential to extend flood records and improve flood risk assessments by reducing uncertainty (Lam et al., 2017b). ...
Article
This study presents a new analytical framework for combining historical flood data derived from sedimentological records with instrumental river flow data to increase the reliability of flood risk assessments. Historical flood records were established for two catchments through re‐analysis of sedimentological records; the Nant Cwm‐du, a small, steep upland catchment in the Cambrian Mountains of Wales, and a piedmont reach of the River Severn in mid Wales. The proposed framework is based on maximum likelihood and least‐square estimation methods in combination with a Generalised Logistic distribution; this enables the sedimentological data to be combined effectively with existing instrumental river flow data. The results from this study are compared to results obtained using existing industry standard methods based solely on instrumental data. The comparison shows that inclusion of sedimentological data can have an important impact on flood risk estimates, and that the methods are sensitive to assumptions made in the conversion of the sedimentological records into flood flow data. As current industry standard methods for flood risk analysis are known to be highly uncertain, the ability to include additional evidence of past flood events derived from sedimentological records as demonstrated in this study can have a significant impact on flood risk assessments.
... The incorporation of paleoflood information has been shown to reduce uncertainty and add considerable value to flood frequency analysis in Australia(Lam, Thompson, Croke, Sharma, & Macklin, 2017). ...
Conference Paper
Full-text available
Design floods for most dams and levees typically have an annual exceedance probability (AEP) of 1:100 (1E-2) or less frequent. In the U.S., high hazard dams are designed to pass the Probable Maximum Flood (PMF), which typically has an AEP of 1:10,000 (1E-4) or less frequent. In order to reduce epistemic uncertainties in the estimated AEP for extreme floods, such as the PMF, it is important to incorporate as much hydrologic information into the frequency analysis as reasonably possible. This paper presents a Bayesian analysis framework, originally profiled by Viglione et al. (2013), for combining at-site flood data with temporal information on historic and paleofloods, spatial information on precipitation-frequency, and causal information on the flood processes. This framework is used to evaluate the flood hazard for Lookout Point Dam, which is a high priority dam located in the Willamette River Basin, upstream of Portland, Oregon. Flood frequency results are compared with those from the Expected Moments Algorithm (EMA). Both analysis methods produce similar results for typical censored data, such as historical floods; however, unlike the Bayesian analysis framework, EMA is not capable of incorporating the causal rainfall-runoff information in a formal, probabilistic manner. Consequently, the Bayesian method considered herein provides higher confidence in the fitted flood frequency curves and resulting reservoir stage-frequency curves to be used in dam and levee safety risk assessments.
... Since both the accuracy and the precision of frequency-magnitude relations can improve by incorporating historic floods (e.g. Parkes and Demeritt, 2016;Lam et al., 2017a;Schendel and Thongwichian, 2017;Bomers et al., 2019c;Reinders and Muñoz, 2021) Next to improving risk assessments, studies into past extreme floods (events that actually occurred instead of abstract extrapolations) can serve to raise public awareness. History shows that societal and political interest toward river floods mainly occurred directly following extreme events (e.g. ...
Thesis
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This thesis examines the largest floods of the Lower Rhine river over the past centuries to millennia (Late Holocene to historic time frame). Its central objective is to quantify these events. First, the thesis determines maximum water levels in the past based on sedimentological interpretation of archaeological stratigraphy and geographical survey of epigraphic marks. Next, it provides detailed reconstructions of the alluvial terrain of the Lower Rhine valley and delta, covering the early modern channel bathymetry restored from historic maps and the medieval floodplain topography restored from geomorphological analysis and interpolation. The past landscape context then serves to set up a palaeoflood hydraulic model that resolves the discharges of the largest floods by linking the simulation results to reconstructed water levels. The insights gained in this thesis may ultimately contribute to the design of future flood protection measures.
... Multi-proxies -sediment deposits and pre-observational flood information -have been used to reduce the uncertainty in flood frequency estimation 29,30 . Findings in this study are in line with 36 in demonstrating that including SST with tree rings for streamflow reconstruction improves above normal (Fig. 5) flows and in the process improves overall reconstruction skill (Figs SI-3, SI-4). ...
Article
Full-text available
Reconstructed annual streamflows using tree-ring chronologies provide useful information on moisture availability during the pre-historic period, but they have limitations in estimating high flows due to the upper bound on soil water holding capacity and trees' metabolic growth limits. We propose a hybrid approach that uses tree-ring chronologies and climatic indices for improving high flows in 301 basins whose annual streamflows are modulated by ENSO and/or PDO. The hybrid decomposition approach relies on separating the moisture supply into the basin as outside-the-region moisture and within-the-region moisture with the former being estimated by SST indices and the latter being estimated by tree-ring chronologies. Analyses over the 301 stations from coterminous US show that the proposed approach improves the high flows and improves the overall error in the reconstructed streamflows. Potential utility of the improved reconstructed annual streamflows with improved high flows is also discussed.
... where ̅ ( ) is the mean activity rate for the level of completeness (Kijko and Sellevoll, 1989), and Other examples of constructing a total likelihood function to combine different types of data are employed in earthquake, tsunami, and extreme flood analyses (e.g. Stedinger and Cohn 1986;Fernandes et al. 2010;Kijko et al. 2016;Lam et al. 2017, Smit et al. 2017). ...
Article
This study presents a method for estimating two area‐characteristic natural hazard recurrence parameters. The mean activity rate and the frequency–size power law exponent are estimated using Bayesian inference on combined empirical datasets that consist of prehistoric, historic, and instrumental information. The method provides for incompleteness, uncertainty in the event size determination, uncertainty associated with the parameters in the applied occurrence models, and the validity of event occurrences. This aleatory and epistemic uncertainty is introduced in the models through mixture distributions and weighted likelihood functions. The proposed methodology is demonstrated using a synthetic earthquake dataset and an observed tsunami dataset for Japan. The contribution of the different types of data, prior information, and the uncertainty is quantified. For the synthetic dataset, the introduction of model and event size uncertainties provides estimates quite close to the assumed true values, whereas the tsunami dataset shows that the long series of historic data influences the estimates of the recurrence parameters much more than the recent instrumental data. The conclusion of the study is that the proposed methodology provides a useful and adaptable tool for the probabilistic assessment of various types of natural hazards.
... Historical flood information, for example, flood marks, newspapers, oral history and paintings, could be additional valuable sources of extreme flood information (e.g. Lam et al., 2017;Wilhelm et al., 2019). An alternative is to execute a probabilistic analysis with recorded data in which potential flood events are identified and probabilities and hazards of these events quantified. ...
Article
Determining optimum flood protection levels is challenging in data-poor environments typical of developing countries. This study develops a risk-modelling approach to calculate current flood risks for a predominantly agricultural area in the Brahmani–Baitarani river basin in India. A combined hydrological–hydraulic model is developed together with a damage model to analyse the economic efficiency of different safety levels of embankments. The results show that more than 90% of the total flood risk would be controlled with embankments giving a protection level of once in 25 years. The study illustrates the feasibility of a relatively basic flood-risk analysis that can be applied at pre-feasibility level in a data-poor environment.
... Aside from the Frequentist methods described above, a number of Bayesian flood frequency methods have been available to further reduce the uncertainty of return level estimates. Combined with Markov Chain Monte Carlo (MCMC) algorithms, Bayesian methods have successfully been applied to instrumental discharge records around the world in both a parametric (Lam et al., 2017;Reis & Stedinger, 2005), and nonparametric setting (O'Connell, 2005). In addition, existing Bayesian algorithms offer the advantages of allowing for the incorporation of multiple types of paleo-data, such as paleohydrologic bounds and 2D model output (Kuczera, 1999;O'Connell et al., 2002). ...
Article
Full-text available
Hydrologists and engineers routinely use flood frequency analyses to compute flood probabilities for mitigation, infrastructure planning, and emergency management. Conventional flood frequency analyses—in which annual discharge maxima from a stream gage are fit to a statistical probability distribution—often encounter large uncertainties when estimating extreme flood levels. Most gage records span relatively short periods of time (<100 years), and thus the most extreme and infrequently occurring flood events tend to be poorly represented in instrumental data sets. Here, we demonstrate how a new generation of paleoflood records derived from floodplain sediments can be used to improve the accuracy and precision of extreme flood probability estimates along alluvial rivers. We use a series of simulation experiments to show that incorporating large numbers of paleoflood events in flood frequency analyses can significantly reduce the uncertainty of extreme flood estimates when the paleoflood data are sufficiently accurate and precise. Our results illustrate that robust paleoflood records can improve the shape parameter of flood frequency distributions, which determine the thickness of distribution tails, when as many as 50 paleoflood events are incorporated. We conclude by demonstrating how an alluvial paleoflood data set reduces uncertainty in a flood frequency analysis for a gage on the lower Mississippi River. Finally, we provide recommendations for how to incorporate paleoflood information into flood frequency analysis to improve the accuracy of extreme flood probabilities.
... Pooling methods that incorporate regional data can augment flood frequency analysis (Miller et al., 2013;Reed and Robson, 1999) but short time series are a persistent obstacle when calculating return periods of rare, high-magnitude floods (Macdonald and Sangster, 2017). As a result, non-conventional sources of flood data that extend beyond the instrumental record are increasingly sought (Lam et al., 2017). ...
Article
p>Concern is growing that climate change may amplify global flood risk but short hydrological data series hamper hazard assessment. Lake sediment reconstructions are capturing a fuller picture of rare, high-magnitude events but the UK has produced few lake palaeoflood records. We report the longest lake-derived flood reconstruction for the UK to date, a 1500-year record from Brotherswater, northwest England. Its catchment is well-suited physiographically to palaeoflood research, but its homogeneous, dark brown sediment matrix precludes visual identification of flood layers. Instead, an outlier detection routine applied to high-resolution particle size measurements showed a >90% match, in stratigraphic sequence, to measured high river flows. Our late-Holocene palaeoflood reconstruction reveals nine multi-decadal periods of more frequent flooding (510–630 CE, 890–960, 990–1080, 1470–1560, 1590–1620, 1650–1710, 1740–1770, 1830–1890 and 1920–2012), and these show a significant association with negative winter North Atlantic Oscillation (wNAO) phasing and some synchrony with solar minima. These flood-rich episodes also overlap with local and regional land-use intensification, which we propose has amplified the flood signal by creating a more efficient catchment sediment conveyor and more rapid hillslope-channel hydrological connectivity. Disentangling anthropogenic and climatic drivers is a challenge but anthropogenic landscape transformation should evidently not be underestimated in palaeoflood reconstructions. Our paper also demonstrates that flood histories can be extracted from the numerous lakes worldwide containing organic-rich, visually homogeneous sediments. This transformative evidence base should lead to more reliable assessments of flood frequency and risks to ecosystems and infrastructure.</p
... Zhang et al. (2017) concluded that generalized extreme value (GEV) was the best statistical distribution for 34 stations in the Pearl River Delta during a period of about 60 years. Lam et al. (2017) used PREC (Probabilistic Regional Envelope Curve) approach and spatial information on maximum floods and achieved considerable improvement on results of FFA, particularly for floods greater than 100year return periods. ...
Article
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The peak flow of extraordinary large floods that occur during a period of systematic record is a controversial problem for flood frequency analysis (FFA) using traditional methods. The present study suggests that such floods be treated as historic flood data even though their historical period is unknown. In this paper, the extraordinary large flood peak was first identified using statistical outlier tests and normal probability plots. FFA was then applied with and without the extraordinary large floods. In this step, two goodness-of-fit tests including mean absolute relative deviation and mean squared relative deviation were used to identify the best-fit probability distributions. Next, the generalized extreme value (GEV), three-parameter lognormal (LN3), log-Pearson type III (LP3), and Wakeby (WAK) probability distributions were used to incorporate and adjust the extraordinary large floods with other systematic data. Finally, procedures with and without historical adjustment were compared for the extraordinary large floods in terms of goodness-of-fit and flood return-period quantiles. The results of this comparison indicate that historical adjustment from an operational perspective was more viable than without adjustment procedure. Furthermore, the results without adjustment were unreasonable (subject to over- and under-estimation) and produced physically unrealistic estimates that were not compatible with the study area. The proposed approach substantially improved the probability estimation of rare floods for efficient design of hydraulic structures, risk analysis, and floodplain management.
... Pooling methods that incorporate regional data can augment flood frequency analysis (Miller et al., 2013;Reed and Robson, 1999) but short time series are a persistent obstacle when calculating return periods of rare, high-magnitude floods (Macdonald and Sangster, 2017). As a result, non-conventional sources of flood data that extend beyond the instrumental record are increasingly sought (Lam et al., 2017). ...
Preprint
Concern is growing that climate change may amplify global flood risk but short hydrological data series hamper hazard assessment. Lake sediment reconstructions are capturing a fuller picture of rare, high-magnitude events but the UK has produced few lake palaeoflood records. We report the longest lake-derived flood reconstruction for the UK to date, a 1500-year record from Brotherswater, northwest England. Its catchment is well-suited physiographically to palaeoflood research, but its homogeneous, dark brown sediment matrix precludes visual identification of flood layers. Instead, an outlier detection routine applied to high-resolution particle size measurements showed a >90% match, in stratigraphic sequence, to measured high river flows. Our late-Holocene palaeoflood reconstruction reveals nine multi-decadal periods of more frequent flooding (CE 510-630, 890-960, 990-1080, 1470-1560, 1590-1620, 1650-1710, 1740-1770, 1830-1890 and 1920-2012), and these show a significant association with negative winter North Atlantic Oscillation (wNAO) phasing and some synchrony with solar minima. These flood-rich episodes also overlap with local and regional land-use intensification, which we propose has amplified the flood signal by creating a more efficient catchment sediment conveyor and more rapid hillslope-channel hydrological connectivity. Disentangling anthropogenic and climatic drivers is a challenge but anthropogenic landscape transformation should evidently not be underestimated in palaeoflood reconstructions. Our paper also demonstrates that flood histories can be extracted from the numerous lakes worldwide containing organic-rich, visually homogeneous sediments. This transformative evidence base should lead to more reliable assessments of flood frequency and risks to ecosystems and infrastructure.
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An adequate characterization of extreme floods is key for the correct design of the infrastructures and for the flood risk estimation. However, the short length of the rainfall and flow data series along with the low probability of occurrence of this type of event cause that, to date, their adequate estimation still presents significant difficulties. This paper presents a methodology for the estimation of extreme floods based on the continuous generation of precipitation data series using weather generators and the integration of information of various types (systematic and non-systematic). The results obtained in the case study, Rambla de la Viuda, indicate that the joint use of continuous synthetic data series generated by a stochastic weather generator, a hydrological model and the integration of systematic and non-systematic information reduces the uncertainty in the estimation of extreme floods.
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Stochastic weather generators combined with hydrological models have been proposed for continuous synthetic simulation to estimate return periods of extreme floods. Yet, this approach relies upon the length and spatial distribution of the precipitation input data series, which often are scarce, especially in arid and semiarid regions. In this work, we present a new approach for the estimation of extreme floods based on the continuous synthetic simulation method supported with inputs of (a) a regional study of extreme precipitation to improve the calibration of the weather generator (GWEX), and (b) non-systematic flood information (i.e., historical information and/or palaeoflood records) for the validation of the generated discharges with a fully distributed hydrological model (TETIS). The results showed that this complementary information of extremes allowed for a more accurate implementation of both the weather generator and the hydrological model. This, in turn, improved the flood quantile estimates, especially for those associated with return periods higher than 50 years but also for higher quantiles (up to approximately 500 years). Therefore, it has been proved that continuous synthetic simulation studies focused on the estimation of extreme floods should incorporate a generalized representation of regional extreme rainfall and/or non-systematic flood data, particularly in regions with scarce hydrometeorological records.
Book
Cambridge Core - Geomorphology and Physical Geography - River Dynamics - by Bruce L. Rhoads
Chapter
This article reviews paleohydrologic techniques for estimating the magnitude and frequency of past floods using geological evidence. Quantitative paleoflood hydrology typically involves several activities: (1) identification and analysis of physical evidence of past floods, such as slackwater deposits, boulder bars, and tree scars; (2) geochronological studies to determine the ages of features left by past floods, commonly by radiocarbon and luminescence dating; (3) estimating peak discharges associated with flood evidence from hydraulic calculations; and (4) incorporating information on the number, magnitude and timing of large past floods into flood frequency estimates, thereby improving quantile estimates for large and rare floods.
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Many governments and organisations are currently aligning many aspects of their policies and practices to the Sustainable Development Goals (SDGs). Achieving the SDGs should increase social-ecological resilience to shocks like climate change and its impacts. Here, we consider the relationship amongst the three elements – the SDGs, social-ecological resilience and climate change – as a positive feedback loop. We argue that long-term memory encoded in historical, archaeological and related ‘palaeo-data’ is central to understanding each of these elements of the feedback loop, especially when long-term fluctuations are inherent in social-ecological systems and their responses to abrupt change. Yet, there is scant reference to the valuable contribution that can be made by these data from the past in the SDGs or their targets and indicators. The historical and archaeological records emphasise the importance of some key themes running through the SDGs including how diversity, inclusion, learning and innovation can reduce vulnerability to abrupt change, and the role of connectivity. Using paleo-data, we demonstrate how changes in the extent of water-related ecosystems as measured by indicator 6.6.1 may simply be related to natural hydroclimate variability, rather than reflecting actual progress towards Target 6.6. This highlights issues associated with using SDG indicator baselines predicated on short-term and very recent data only. Within the context of the contributions from long-term data to inform the positive feedback loop, we ask whether our current inability to substantively combat anthropogenic climate change threatens achieving both the SDGS and enhanced resilience to climate change itself. We argue that long-term records are central to understanding how and what will improve resilience and enhance our ability to both mitigate and adapt to climate change. However, for uptake of these data to occur, improved understanding of their quality and potential by policymakers and managers is required.
Article
The paper explore the idea that flood-related mortality from river over-bank flows in the SE Asian region could be reduced by incorporating evidence from the past to foster a better understanding of the realm of plausible flood regimes, and hopefully guide flood hazard management practices in the future.
Article
The purpose of this paper was to investigate the effect of the uncertainty of historical flood information on flood frequency analyses using a Bayesian approach. The uncertainty of the magnitude was described using variations in lower or upper bound values, but the return period was not changed. Two types of hydrologic data—synthetic data generated from the Pearson type three distribution population and observed peak discharge data from the Yichang hydrologic station located below the Three Gorges Dam on the Yangtze River, China—were used. For the measured and the synthetic data, when the magnitudes were underestimated and the errors increased, the accuracy of the historical flood samples decreased, the estimated values of the parameters and design floods decreased using the Bayesian and probability weighted methods, and the values using the Bayesian method were more sensitive to errors and more quickly decreased relative to the probability weighted method. In contrast, overestimated magnitudes did not improve the representativeness of the historical flood samples but merely appended their values with an increase in errors, and the estimated values were hardly affected by changes in the errors using the Bayesian approach. At the same time, the estimated values continued to increase using the probability weighted method and did not accord with the variations in the errors for the flood frequency analysis. In practice, when the return period could be set with a specific value using several historical or paleoflood events, accurately quantifying the peak values of historical floods with known information was difficult, and we overestimated their magnitudes within a specified interval using the existing approaches and available information to estimate their bounds. The estimated values of the parameters and design floods and their uncertainty were less affected when using the Bayesian approach.
Article
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The flow regime conditions of the Danube River are continually changing. These changes are the result of natural processes and anthropogenic activities. The territory of the Danube River Basin is one of the most flood-endangered regions in Europe and assessing the design discharges along the Danube channel is complicated by the different estimation methods that are applied in particular countries. For this reason, it is necessary to harmonize flood design value assessment methods. The long-term maximum annual discharge series of the Danube River and other rivers in the Danube basin were analyzed and used to estimate the flood design values. We used the Log-Pearson type III distribution, which is one of the most widely used theoretical probability distributions to estimate extremes. This distribution can be flexibly applied to extreme values depending on the skew coefficient. We also analyzed the effect of the inclusion and exclusion of the historical extremes in the processed dataset. The results show that the inclusion of historical floods and the regionalization of the Log-Pearson type III distribution skew parameter can change the design discharges.
Article
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This comprehensive field study applied paleoflood hydrology methods to estimate the frequency of lowprobability floods for the Tennessee River near Chattanooga, Tennessee. The study combined stratigraphic records of large, previously unrecorded floods with modern streamflow records and historical flood accounts. The overall approach was to (1) develop a flood chronology for the Tennessee River near Chattanooga using stratigraphic analyses and geochronology from multiple sites at multiple elevations in the study area; (2) estimate peak flow magnitudes associated with elevations of flood evidence using a one-dimensional hydraulic model; (3) combine the information obtained from steps 1 and 2 to develop a history of timing and magnitude of large floods in the study reach; and (4) use all available information (including paleoflood, gaged, and historical records of flooding) to estimate flood frequency using a standardized statistical approach for flood-frequency analysis. The stratigraphy, geochronology, and hydraulic modeling results from all paleoflood sites along the Tennessee River were distilled into an overall chronology of the number, timing, and magnitude of large unrecorded floods. In total, 30 sites were identified and the stratigraphy of 17 of those sites was closely examined, measured, and recorded. Floodfrequency analyses were done using the U.S. Geological Survey software program PeakFQ v7.2 that follows the Guidelines for Determining Flood Flow Frequency— Bulletin 17C. Resolving stratigraphic and chronologic information from all 17 sites yielded information for eight unique large floods in the last 3,500–4,000 years for the Tennessee River near Chattanooga. Two of these floods had discharges of 470,000 cubic feet per second (ft3 /s), slightly greater than the 1867 historical peak at the Chattanooga streamgage (459,000 ft3/s). One flood with a discharge of 1,100,000 ft3/s was substantially greater than any other flood on the Tennessee River during the last several thousand years. This large flood occurred only a few hundred years ago, likely in the mid-to-late 1600s. Two additional floods in the last 1,000 years had estimated magnitudes of about 420,000 and 400,000 ft3/s. The remaining three unique floods identified in the paleoflood record were much smaller (less than 240,000 ft3 /s) and occurred about 3,000–800 years ago. Flood frequency analyses show that the addition of paleoflood information markedly improves estimates of low probability floods—most clearly shown by substantial narrowing of the 95-percent confidence limits. For the most plausible flood scenario, the 95-percent confidence interval for the 1,000-year quantile estimate derived from incorporating the four most recent paleofloods is about 480,000–620,000 ft3/s compared to about 380,000–610,000 ft3/s for the gaged and historical record alone, a reduction in the uncertainty of the estimate by 38 percent. Similarly, uncertainty for all flood quantile estimates from 100 to 10,000 years was reduced by 22–44 percent by the addition of the paleoflood record to the flood-frequency analyses.
Technical Report
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Paleoflood studies are an effective means of providing specific information on the recurrence and magnitude of rare and large floods. Such information can be combined with systematic flood measurements to better assess the frequency of large floods. Paleoflood data also provide valuable information about the linkages among climate, land use, flood-hazard assessments, and channel morphology. This document summarizes methods and techniques for the preparation, gathering, evaluation, and interpretation of paleoflood information, including uncertainties, especially with respect to new statistical approaches available to efficiently use such data. We summarize best practices and strategies for assessing and mitigating uncertainties and provide guidelines on appropriate technical review of paleoflood analyses based on project goals and requirements.
Article
This paper provides an important archive of the largest paleoflood event along the Gold Gorge of the upper Hanjiang River, with a review of regional extraordinary flood magnitudes using historical, gauged, and paleoflood datasets. Paleoflood slackwater deposit (SWD) was identified using the sedimentological criteria and high-resolution physicochemical indexes. End-member model demonstrates the paleoflood SWD was dominated by sand component. Optical chronology indicates the paleoflood event was dated to 1000–900 a BP, corresponding to the North Song Dynasty (AD 960–1127). Paleoflood magnitudes derived from one-dimensional unsteady model vary between 23,000 and 34,000 m³/s. The paleoflood data provide a good representation of extraordinary flood event with a 10,000-years return period and greatly improve current flood frequency estimations. Moreover, the envelope curve for extraordinary floods was first established as Q = 50D0.65 and the paleoflood peak discharges are approximately 2–2.5 times of the largest gauged floods within the catchment. These results provide an upper limit of the maximum flood magnitudes along the upper Hanjiang River and also expand our understanding of rare and large magnitude floods in the middle Yangtze River valley.
Article
The aim of this study was to establish the source and provenance of sediments deposited in a large floodplain sink during extreme floods in the Lockyer Creek catchment, Australia, in 2011 and 2013. We place the sediment source patterns in context of the longer-term record to determine whether coarse-grained sediment sources (i.e., very fine sand to very coarse sand) and the spatio-temporal pattern of (dis)connectivity have changed over time. We do this by matching the geochemical properties and age structure of a sediment profile located in a downstream floodplain sink to the elemental composition of source sediments. One hundred and fifty-seven sediment samples from 20 sites across the catchment are analysed using X-ray fluorescence (XRF) spectrometry to compare the elemental ratio composition of the downstream floodplain sink to its source materials. We use Optically Stimulated Luminescence (OSL) dating to determine the age structure of the sediments in the floodplain sink. The northern tributaries and parts of the Lockyer River trunk stream are the primary sources of coarse sediment. These areas are connected (coupled) to the lower trunk stream and floodplain sediment sink. Southern tributaries are largely disconnected (decoupled) and supply little sediment to the floodplain sediment sink. This pattern of sediment source contribution has remained similar over the last 6.8 ka at least. Sediment sources as observed in the 2011 flood have predominated over the mid-late Holocene whilst those in the 2013 flood are rare.
Article
Banjir merupakan salah satu bencana alam yang sering terjadi di seluruh dunia, menyebabkan kerugian ekonomi, rusaknya infrastruktur bahkan sering menimbulkan korban nyawa. Oleh karena itu, mengatasi permasalahan banjir sangat penting untuk dilakukan. Tujuan makalah ini adalah untuk mensintesis permasalahan banjir dan menyusun kerangka kerja pengelolaan banjir secara holistik. Makalah ini mengulas dan mensintesis 124 jurnal yang diterbitkan antara tahun 2011 – 2021. Artikel jurnal yang diulas dikategorikan ke dalam tema pengaruh perubahan tutupan lahan terhadap hidrolgi (PL-H), perubahan penggunaan lahan (LULC), pengaruh klimatologi terhadap hidrologi (PK-H), mitigasi banjir (MB), ekonomi terkait banjir (Ek), evaluasi kejadian banjir (Ev), pengaruh perubahan lahan dan pengaruh klimatologi terhadap hidrologi (PL-PK-H), dan hidrologi (H). Tema hidrologi (H) merupakan topik dominan yang kami temukan dalam literatur sebanyak 30% selanjutnya 25% tema mitigasi banjir (MB) dan 17% tema pengaruh perubahan tutupan lahan terhadap hidrolgi (PL-H). Kontribusi dari makalah ini adalah menghasilkan kerangka kerja komprehensif yang perlu dilakukan dalam melakukan pengelolaan banjir.ABSTRACTFloods are one of the natural disasters that often occur around the world, causing economic losses, damage to infrastructure and often cause casualties. Therefore, overcoming the problem of flooding is very important to do. The purpose of this paper is to analyze flood problems and develop new flood management frameworks. The paper reviews and synthesizes 124 journals published between 2011 and 2021. The journal article reviewed is categorized into the themes of the influence of land cover changes on hydrolgi (PL-H), land use change (LULC), climatology influence on hydrology (PK-H), flood mitigation (MB), flood-related economy (Ek), evaluation of flood events (Ev), influence of land change and climatology influence on hydrology (PL-PK-H), and hydrology (H). The theme of hydrology (H) is the dominant topic that we found in the literature as much as 30% of the theme of flood mitigation (MB) and 17% of the theme of the influence of land cover changes on hydrolgi (PL-H). The contribution of this paper is to develop a comprehensive framework that needs to be done in conducting flood management.
Article
Banjir merupakan salah satu bencana alam yang sering terjadi di seluruh dunia, menyebabkan kerugian ekonomi, rusaknya infrastruktur bahkan sering menimbulkan korban nyawa. Oleh karena itu, mengatasi permasalahan banjir sangat penting untuk dilakukan. Tujuan makalah ini adalah untuk mensintesis permasalahan banjir dan menyusun kerangka kerja pengelolaan banjir secara holistik. Makalah ini mengulas dan mensintesis 124 jurnal yang diterbitkan antara tahun 2011 – 2021. Artikel jurnal yang diulas dikategorikan ke dalam tema pengaruh perubahan tutupan lahan terhadap hidrolgi (PL-H), perubahan penggunaan lahan (LULC), pengaruh klimatologi terhadap hidrologi (PK-H), mitigasi banjir (MB), ekonomi terkait banjir (Ek), evaluasi kejadian banjir (Ev), pengaruh perubahan lahan dan pengaruh klimatologi terhadap hidrologi (PL-PK-H), dan hidrologi (H). Tema hidrologi (H) merupakan topik dominan yang kami temukan dalam literatur sebanyak 30% selanjutnya 25% tema mitigasi banjir (MB) dan 17% tema pengaruh perubahan tutupan lahan terhadap hidrolgi (PL-H). Kontribusi dari makalah ini adalah menghasilkan kerangka kerja komprehensif yang perlu dilakukan dalam melakukan pengelolaan banjir.ABSTRACTFloods are one of the natural disasters that often occur around the world, causing economic losses, damage to infrastructure and often cause casualties. Therefore, overcoming the problem of flooding is very important to do. The purpose of this paper is to analyze flood problems and develop new flood management frameworks. The paper reviews and synthesizes 124 journals published between 2011 and 2021. The journal article reviewed is categorized into the themes of the influence of land cover changes on hydrolgi (PL-H), land use change (LULC), climatology influence on hydrology (PK-H), flood mitigation (MB), flood-related economy (Ek), evaluation of flood events (Ev), influence of land change and climatology influence on hydrology (PL-PK-H), and hydrology (H). The theme of hydrology (H) is the dominant topic that we found in the literature as much as 30% of the theme of flood mitigation (MB) and 17% of the theme of the influence of land cover changes on hydrolgi (PL-H). The contribution of this paper is to develop a comprehensive framework that needs to be done in conducting flood management.
Article
This chapter reviews the paleohydrologic techniques and approaches used to reconstruct the magnitude and frequency of past floods using geological evidence. Quantitative paleoflood hydrology typically leads to two phases of analysis: (1) documentation and assessment of flood physical evidence (paleostage indicators); and (2) relating identified flood evidence to flood discharge, based on hydraulic calculations. Most paleoflood studies rely on stratigraphic sequences of fine-grained flood sediments deposited in slackwater and eddy environments in bedrock rivers to enable calculation of robust palaeodischarge estimates for floods that occurred during recent centuries or millennia. Geochronological developments such as optically stimulated luminescence age dating, together with radiocarbon techniques, are key for structuring the paleoflood discharge data into different threshold levels that are exceeded by floodwaters over specific periods of time, providing the input data necessary for flood-frequency analysis. The value of paleoflood data is the potential to include physical evidence of rare floods and limits on their largest magnitude, with direct applications for different scientific and engineering problems, including: (1) risk assessments for critical structures such as nuclear facilities, dams, or bridges; (2) understanding of the recurrence of geomorphically effective flows; and (3) assessment of nonstationarity in the frequency of large floods due to climate, land-use, or other changes.
Article
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Better understanding of flood occurrences and long-term, floodplain planning, and flood risk assessment is achieved by integration of gauged, historical, and paleoflood data. The Ardèche River is ideal for this historical flood-paleoflood study because its historical flood levels record dates back as early as A.D. 587 and useful data date back to A.D. 1522, its systematic gauging record is over 100 years long, and the geologic and geomorphic settings are optimal for paleoflood studies. Three sites provide three different thresholds for flood stages and SWD accumulation. According to our one-dimensional (1-D) step-backwater calculations these three thresholds are 5200-5700 m3/s, 4900-5400 m3/s, and 3600-4000 m3/s recording 6, 9, and 19 large Holocene floods, respectively. Dating the deposits enabled a correlation with the historical record. These paleoflood studies indicate that there are long gaps in flood occurrences on the Ardèche River; the floods are not randomly distributed in time but are clustered. They also indicate that the recent nineteenth century floods were the largest at the millennial timescale.
Article
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Historical records are an important source of information on extreme and rare floods and fundamental to establish a reliable flood return frequency. The use of long historical records for flood frequency analysis brings in the question of flood stationarity, since climatic and land-use conditions can affect the relevance of past flooding as a predictor of future flooding. In this paper, a detailed 400 yr flood record from the Tagus River in Aranjuez (central Spain) was analysed under stationary and non-stationary flood frequency approaches, to assess their contribution within hazard studies. Historical flood records in Aranjuez were obtained from documents (Proceedings of the City Council, diaries, chronicles, memoirs, etc.), epigraphic marks, and indirect historical sources and reports. The water levels associated with different floods (derived from descriptions or epigraphic marks) were computed into discharge values using a one-dimensional hydraulic model. Secular variations in flood magnitude and frequency, found to respond to climate and environmental drivers, showed a good correlation between high values of historical flood discharges and a negative mode of the North Atlantic Oscillation (NAO) index. Over the systematic gauge record (1913–2008), an abrupt change on flood magnitude was produced in 1957 due to constructions of three major reservoirs in the Tagus headwaters (Bolarque, Entrepeñas and Buendia) controlling 80% of the watershed surface draining to Aranjuez. Two different models were used for the flood frequency analysis: (a) a stationary model estimating statistical distributions incorporating imprecise and categorical data based on maximum likelihood estimators, and (b) a time-varying model based on "generalized additive models for location, scale and shape" (GAMLSS) modelling, which incorporates external covariates related to climate variability (NAO index) and catchment hydrology factors (in this paper a reservoir index; RI). Flood frequency analysis using documentary data (plus gauged records) improved the estimates of the probabilities of rare floods (return intervals of 100 yr and higher). Under non-stationary modelling flood occurrence associated with an exceedance probability of 0.01 (i.e. return period of 100 yr) has changed over the last 500 yr due to decadal and multi-decadal variability of the NAO. Yet, frequency analysis under stationary models was successful in providing an average discharge around which value flood quantiles estimated by non-stationary models fluctuate through time.
Article
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The role of extreme events in shaping the Earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how semi-alluvial channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 168 km2 show that the predicted discharge for the 2011 event ranged from 415 to 933 m3 s−1, with unit stream power estimates of up to 1077 W m−2. Estimated entrainment relationships predict the mobility of the entire grain-size population, and field data suggest the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat lidar data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of coarse-grained alluvium with a strong positive relationship between catchment area and normalised erosion (R2 = 0.72–0.74). The extensive scouring in the 2011 flood decreased thalweg variance significantly removing previous step pools and other coarse-grained in-channel units, forming lengths of plane-bed (cobble) reach morphology. This was also accompanied by the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps. Post-flood field data indicate a slight increase in thalweg variance as a result of the smaller 2013 flood rebuilding the alluvial overprint with pool-riffle formation. However, the current form and distribution of channel morphological units does not conform to previous classifications of bedrock or headwater river systems. This variation in post-flood form indicates that in semi-alluvial systems extreme events are significant for re-setting the morphology of in-channel units and for exposing the underlying lithology to ongoing erosion.
Article
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In recent decades, the quantification of flood hydrological characteristics (peak discharge, hydrograph shape, and runoff volume) from documentary evidence has gained scientific recognition as a method to lengthen flood records of rare and extreme events. This paper describes the methodological evolution of quantitative historical hydrology under the influence of developments in hydraulics and statistics. In the 19th century, discharge calculations based on flood marks were the only source of hydrological data for engineering design, but were later left aside in favour of systematic gauge records and conventional hydrological procedures. In the last two decades, there has been growing scientific and public interest in understanding long-term patterns of rare floods, in maintaining the flood heritage and memory of extremes, and developing methods for deterministic and statistical application to different scientific and engineering problems. A compilation of 46 case studies across Europe with reconstructed discharges demonstrates that (1) in most cases present flood magnitudes are not unusual within the context of the last millennium, although recent floods may exceed past floods in some temperate European rivers (e.g. the Vltava and Po rivers); (2) the frequency of extreme floods has decreased since the 1950s, although some rivers (e.g. the Gardon and Ouse rivers) show a reactivation of rare events over the last two decades. There is a great potential for gaining understanding of individual extreme events based on a combined multiproxy approach (palaeoflood and documentary records) providing high-resolution time flood series and their environmental and climatic changes; and for developing non-systematic and non-stationary statistical models based on relations of past floods with external and internal covariates under natural low-frequency climate variability.
Article
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Extreme flood events have detrimental effects on society, the economy and the environment. Widespread flooding across South East Queensland in 2011 and 2013 resulted in the loss of lives and significant cost to the economy. In this region, flood risk planning and the use of traditional flood frequency analysis (FFA) to estimate both the magnitude and frequency of the 1-in-100 year flood is severely limited by short gauging station records. On average, these records are 42 years in Eastern Australia and many have a poor representation of extreme flood events. The major aim of this study is to test the application of an alternative method to estimate flood frequency in the form of the Probabilistic Regional Envelope Curve (PREC) approach which integrates additional spatial information of extreme flood events. In order to better define and constrain a working definition of an extreme flood, an Australian Envelope Curve is also produced from available gauging station data. Results indicate that the PREC method shows significant changes to the larger recurrence intervals (≥100 years) in gauges with either too few, or too many, extreme flood events. A decision making process is provided to ascertain when this method is preferable for FFA.
Article
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This paper describes a Bayesian statistical model for estimating flood frequency by combining uncertain annual maximum (AMAX) data from a river gauge with estimates of flood peak discharge from various historic sources that predate the period of instrument records. Such historic flood records promise to expand the time series data needed for reducing the uncertainty in return period estimates for extreme events, but the heterogeneity and uncertainty of historic records make them difficult to use alongside Flood Estimation Handbook and other standard methods for generating flood frequency curves from gauge data. Using the flow of the River Eden in Carlisle, Cumbria, UK as a case study, this paper develops a Bayesian model for combining historic flood estimates since 1800 with gauge data since 1967 to estimate the probability of low frequency flood events for the area taking account of uncertainty in the discharge estimates. Results show a reduction in 95% confidence intervals of roughly 50% for annual exceedance probabilities of less than 0.0133 (return periods over 75 years) compared to standard flood frequency estimation methods using solely systematic data. Sensitivity analysis shows the model is sensitive to 2 model parameters both of which are concerned with the historic (pre-systematic) period of the time series. This highlights the importance of adequate consideration of historic channel and floodplain changes or possible bias in estimates of historic flood discharges. The next steps required to roll out this Bayesian approach for operational flood frequency estimation at other sites is also discussed.
Article
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Reliable long-term flood forecasts are needed because floods, among environmental disasters worldwide, do most damage to lives and property, a problem that is likely to increase as climate changes. The objective of this paper is to critically examine scientific approaches to flood forecasting under deep uncertainty and ambiguity as input to flood policy, and to explore alternative approaches to the development of better forecasts along with the necessary organizational support. This therefore a paper on science policy Aleatory (i.e. frequentist) probability estimates have dominated the science, attached to which are irreducible uncertainties. The lower priority given to finding a physical theory of floods means that ambiguity is high, particularly in relation to choosing a probability density function for forecasting. The historical development of flood forecasting is analyzed within the Uncertainty-Ambiguity Matrix of Schrader, S. Riggs, W.M., and Smith, R.P. (1993). Choice over uncertainty and ambiguity in technical problem solving. Journal of Engineering and Technology Management, 10, 13–99 showing that considerable uncertainty and ambiguity exist and are likely to continue. The way forward appears to be a mix of: broadening the information input to forecasts by engaging many disciplines, Bayesian analyses of probabilities, scenario analyses of catastrophic floods based on all available evidence, and adaptive forecasting in the face of climate change.
Conference Paper
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A fundamental assumption in classic flood frequency analysis is that geomorphologic and climatic characteristics of a watershed are stable over time. This allows hydrologists to assume available data represent a single time-invariant population of extreme events. Increasingly, people are concerned about the impacts of climate change on the hydrologic cycle, and they doubt that the assumption of stationarity is valid. So the question is, what should be done? This paper evaluates different methods that can be used for flood frequency estimation that allow for climate change, including time-varying parameters that capture trends, using a limited short-term flood record or window, and adopting a safety factor (e.g. 30% increase) applied to the design flood estimators. A Monte Carlo re-sampling study is employed to estimate the 100-year-flood 50 years beyond the end of a 100-year flood record. Though small trends (e.g. +0.25%) in annual maximum series are not statistically detectable in most U.S. watersheds with a 100-year record, the 100-year flood will be significantly underestimated if such positive trends are neglected. When the trend magnitudes are known and employed to update log-Pearson type III (LP3) parameters, the 100-year flood estimators have the smallest log-space mean squared error (LMSE). Employing trends estimated from the record is an alternative for high trend magnitudes; such estimators have small biases but large variance. Use of a short-term flood record and safety factors only worked well for the low trend magnitudes such as 0 or 0.25%. Clearly, flood risk management in an uncertain world will be a challenge.
Article
Full-text available
In the last decades, the quantification of flood hydrological characteristics (peak discharge, hydrograph shape, and runoff volume) from documentary evidence has gained scientific recognition as a method to lengthen flood records of rare and extreme events. This paper describes the methodological evolution of the quantitative historical hydrology under the influence of developments in hydraulics and statistics. In the 19th century, discharge calculations based on flood marks was the only source of hydrological data for engineering design, but later was left aside on favour of systematic gauge records and conventional hydrological procedures. In the last two decades, there is growing scientific and public interest to understand long-term patterns of rare floods, maintain the flood heritage and memory of extremes, and to develop methods for deterministic and statistical application to different scientific and engineering problems. A compilation of 45 case studies across Europe with reconstructed discharges demonstrates that (1) in most cases present flood magnitudes are not unusual within the context of the last millennium, although recent floods may exceed past floods in some temperate European rivers (e.g. the Vltava and Po rivers), (2) frequency of extreme floods have decreased since the 1950s, although some rivers (e.g. the Gardon and Ouse rivers) show a reactivation of rare events over the last two decades. There is a great potential of gaining understanding of individual extreme events based on a combined multiproxy approach (palaeoflood and documentary records) providing high-resolution time flood series and their environmental and climatic changes; and to develop non-systematic and non-stationary statistical models based on relations of past floods with external and internal covariates under natural low-frequency climate variability.
Article
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At-site flood frequency analysis is a useful technique to estimate flood quantiles if reasonably long at-site flood record is available. In Australia, FLIKE software has been proposed for at-site flood frequency analysis. The advantage of FLIKE is that, for a given application, the user can compare a number of most commonly adopted probability distributions and parameter estimation methods relatively quickly using a windows interface. The new version of FLIKE has been incorporated with the multiple Grubbs and Beck test, which can identify multiple numbers of potentially influential low flows. This paper presents a case study considering ten catchments in eastern Australia, which compares two outlier identification tests (original Grubbs and Beck and multiple Grubbs and Beck tests) and two commonly applied probability distributions (Generalized Extreme Value (GEV) and Log Pearson type 3 (LP3)). The results show that the LP3 distribution with multiple Grubbs and Beck test provides more accurate flood quantile estimates than when LP3 distribution is used with the original Grubbs and Beck test. Between these two methods, the differences in flood quantile estimates have been found to be up to 61% for the ten study catchments. It has also been found that GEV distribution with L moments and LP3 distribution with the multiple Grubbs and Beck test provide quite similar results in most of the cases; however, a difference up to 38% has been noted for flood quantiles for annual exceedance probability (AEP) of 1 in 100 for one catchment. The methodology presented in this paper can be applied to other catchments/countries.
Article
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We present a new well dated Holocene record of environmental change from Little Llangothlin Lagoon in eastern Australia derived from aquatic plant macrofossils, macroscopic charcoal flux, and sediment stratigraphy from multiple cores. Little Llangothlin was an ephemeral freshwater wetland exhibiting frequent dry phases between 9800 and 9300 calendar years before present (cal. yr BP). There was a switch to a more positive water balance after 9300 cal. yr BP, and by 8000 cal. yr BP, there was a lake that persisted until 6100 cal. yr BP. The period between 6100 and 1000 cal. yr BP was much drier, and there is no evidence for a permanent lake during this period. The Little Llangothlin record provides evidence for a wet phase during the Early to Middle Holocene (9000–6000 cal. yr BP) from the boundary region between temperate and tropical influences in eastern Australia. We propose that generally enhanced circulation after 9000 cal. yr BP explains the pattern of increasing moisture at the site at this time. The later Holocene climate at the site is consistent with other sites in south east Australia with a switch to generally drier conditions after 6000 cal. yr BP.
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The Southern Hemisphere SHCal04 radiocarbon calibration curve has been updated with the addition of new data sets extending measurements to 2145 cal BP and including the ANSTO Younger Dryas Huon pine data set. Outside the range of measured data, the curve is based upon the ern Hemisphere data sets as presented in IntCal13, with an interhemi-spheric offset averaging 43 ± 23 yr modeled by an autoregressive process to represent the short-term correlations in the offset.
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The Southern Hemisphere SHCal04 radiocarbon calibration curve has been updated with the addition of new data sets extending measurements to 2145 cal BP and including the ANSTO Younger Dryas Huon pine data set. Outside the range of measured data, the curve is based upon the ern Hemisphere data sets as presented in IntCal13, with an interhemi-spheric offset averaging 43 ± 23 yr modeled by an autoregressive process to represent the short-term correlations in the offset.
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Historical hydrology can be defined as a research field occupying the interface between hydrology and history, with the objectives: to reconstruct temporal and spatial patterns of river flow and, in particular, extreme events (floods, ice phenomena, hydrological droughts) mainly for the period prior to the creation of national hydrological networks; and to investigate the vulnerability of past societies and economies to extreme hydrological events. It is a significant tool for the study of flood risk. Basic sources of documentary data on floods and methods of data collection and analysis are discussed. Research progress achieved in Europe in reconstructing past runoff conditions, hydrological and hydraulic analyses of historical floods, their meteorological causes, impacts and relation to climate change, as well as use of combined series of palaeofloods, instrumental and historical floods for reconstructing long-term flood records, is reviewed. Finally, the future research needs of historical hydrology are discussed.
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The authors propose a simplified approach to the calibration of radiocarbon dates. They use splines through the tree-ring data as calibration curves, thereby eliminating a large part of the statistical scatter of the actual data points. To express the age range, they transform the [plus minus]1 [sigma] and [plus minus]2 [sigma] values of the BP age to calendar dates and interpret them as the 68% and 95% confidence intervals. This approach by-passes the conceptual problems of the transfer of individual probability values from the radiocarbon to the calendar age. They have adapted software to make this calibration possible.
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[1] Better understanding of flood occurrences and long-term, floodplain planning, and flood risk assessment is achieved by integration of gauged, historical, and paleoflood data. The Ardèche River is ideal for this historical flood-paleoflood study because its historical flood levels record dates back as early as A.D. 587 and useful data date back to A.D. 1522, its systematic gauging record is over 100 years long, and the geologic and geomorphic settings are optimal for paleoflood studies. Three sites provide three different thresholds for flood stages and SWD accumulation. According to our one-dimensional (1-D) step-backwater calculations these three thresholds are 5200–5700 m3 s−1, 4900–5400 m3 s−1, and 3600–4000 m3 s−1 recording 6, 9, and 19 large Holocene floods, respectively. Dating the deposits enabled a correlation with the historical record. These paleoflood studies indicate that there are long gaps in flood occurrences on the Ardèche River; the floods are not randomly distributed in time but are clustered. They also indicate that the recent nineteenth century floods were the largest at the millennial timescale.
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Wollombi Brook in southeastern Australia is one of the most flood variable rivers in the world and was subjected to a catastrophic flood on 17–18 June 1949. At Paynes Crossing (1064 km 2), the flood peak discharge determined by the HEC-2 backwater model was ~22 times > mean annual flood. While this flood destroyed in-channel benches, it deposited up to 500 mm of slackwater deposits (SWDs) on a low flood plain inset into a high flood plain which was not inundated. Slackwater deposits preserved on the high flood plain record at least three late Holocene palaeofloods with peak discharges, estimated by the HEC-2 program calibrated against the 1949 flood data, up to ~32 times greater than the mean annual flood. Such events have been recorded in similar sized basins in New South Wales (NSW). However, at least one even larger flood occurred between 4280 and 3380 years BP which severely eroded most of the high flood plain. This cataclysmic late Holocene flood greatly exceeded the erosional effects of the catastrophic 1949 flood.
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The largest recorded flood on the Nepean River, with a peak discharge of 16600 m3 s-', occurred in 1867. Detailed field descriptions and particle-size analyses of sediments at six sites in the Fairlight Gorge below Warragamba Dam identified high-level flood deposits. Slackwater deposits (SWDs) are typically fine-grained sand and silt, which accumulate rapidly from suspension during large floods in areas where flow velocities are locally reduced. However, the higher-level SWDs were too thin and bioturbated to be clearly differentiated from locally derived colluvium. Heavy-mineral analysisof the very fine sand fraction of these high-level SWDs identified minerals (epidote and pyroxene) that were not present in the surrounding bedrock. Epidote and pyroxene were derived from distant sources and were emplaced by at least one palaeoflood. A radiocarbon date of 3756k72 years BP was obtained by tandem accelerator mass spectrometry on small fragments of charcoal contained in the high-level SWDs. Therefore, at least one palaeoflood larger than any historic flood occurred during the late Holocene. SWDs indicated that the largest palaeoflood had a peak height at least 8.0 m higher than the 1867 flood. If such a flood had occurred before the recent raising of the dam wall, the dam would have sustained substantial damage and might have failed. Additional modifications are being discussed; it is recommended that they should be completed as soon as possible to allow Warragamba Dam to discharge safely floods of similar magnitude to the largest palaeoflood indicated by SWDs.
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River basins in south-western USA are some of the most extensively studied arid land fluvial systems in the world. Since the early 1960s their hydro-climatic histories have been reconstructed from the analysis of alluvial cut-and-fill cycles, while from the late 1970s there have been investigations of slackwater deposits and palaeostage indicators for large floods in stable-boundary bedrock reaches. However, no studies have regionally integrated Holocene fluvial histories from these two different types of fluvial environments. The current study combines the alluvial archive with flood records from bedrock reaches to generate a probability-based 12,000 year record of flooding in south-western USA. Using more than 700 14C-dated fluvial units, the analysis produces a high resolution (centennial) flood record. Seven episodes of increased flooding occurred at 11,250–10,400, 8800–8350, 8230–7600, 6700–5700, 5600–4820, 4550–3320 and 2000–0 cal. BP. Bedrock reaches are found to record more frequent floods during the middle to late Holocene, while in alluvial rivers more flood units are dated to the early and middle Holocene. These differences are primarily the result of selective preservation with alluvial reaches tending to erode during periods characterised by very large floods. Episodes of major Holocene flooding recorded in slackwater deposits within bedrock systems correspond with periods of increased precipitation in the region and lower temperatures. In contrast, within alluvial rivers above-average flooding probabilities, as well as regionally extensive channel entrenchment episodes, match with reduced annual precipitation and lower temperatures. The results of this study clearly demonstrate the value of the Holocene fluvial archive for reconstructing regional, short-term hydro-climatic change in south-western USA. Copyright © 2010 John Wiley & Sons, Ltd.
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Along the eastern margin of Australia, hydrological variability reaches a peak in the subtropics of south-east Queensland and many rivers have entrenched characteristics. To address the nature of entrenchment and the relationship with adjacent alluvium, this paper presents the results of detailed chrono-stratigraphic analysis of alluvial units in the partly confined mid-reaches of Lockyer Creek, Australia. Four sites were investigated using topographic, sedimentological and chronological data. Radiocarbon and single grain optically stimulated luminescence dating indicate a large proportion of the valley fill reflects a major phase of aggradation of fine-grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period. Synchronous incision of Pleistocene alluvial fills between 11.5 and 9.3 ka suggests the current entrenched Lockyer Creek formed in response to changes in late Quaternary climate. Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages of ca. 7.5 ka, but whose proximal margins are still actively accreting. This Holocene fill has accreted over the mid- to late Holocene but overlaps with the contemporary hydrological regime. The sedimentary nature of the Holocene fill appears to be related to persistent antecedent controls in the form of bedrock and terrace constriction.
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This paper reconstructs past flooding from a range of settings in Lockyer Creek, a key tributary of the mid-Brisbane River, which experienced extreme flood events in AD 2011 and AD 2013. Optically stimulated luminescence samples (n = 110) were collected from alluvial material preserved in within-channel benches and floodplains. Age distributions from material in the bedrock reaches confirm an event ∼ 300 years ago which stripped the valley alluvium to bedrock. In the unconfined reaches floodplain deposits indicate lateral stability over the past 6000 years. Marked differences in the inundation patterns of the AD 2011 event highlighted changes in downstream channel geometry. The age distribution of alluvium in reaches not inundated during AD 2011 was older, ∼12 000 years, with no preserved evidence of deposition during the past 1000 years. A relatively continuous record of floodplain deposition in reaches which were inundated in AD 2011 identifies a major peak in flood activity also around 300 years ago (∼AD 1730) with five additional peaks occurring at approximately AD 1962, AD 1897, AD 1300, AD 550 and 5400 BC. The main climatic driver of changes in flooding over this timescale is oscillations in El Niño Southern Oscillation and although proxy records are scarce for this region, some correlations with high-resolution records of rainfall variability are apparent.
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Flood frequency analysis generally involves the use of simple parametric probability distributions to smooth and extrapolate the information provided by short flood records to estimate extreme flood flow quantiles. Parametric probability distributions can have difficulty simultaneously fitting both the largest and smallest floods. A danger is that the smallest observations in a record can distort the exceedance probabilities assigned to the large floods of interest. The identification and treatment of such Potentially Influential Low Floods (PILFs) frees a fitting algorithm to describe the distribution of the larger observations. This can allow parametric flood frequency analysis to be both efficient, and also robust to deviations from the proposed probability model's lower tail. Historically, PILF identification involved subjective judgement. We propose a new multiple Grubbs-Beck outlier test (MGBT) for objective PILF identification. MGBT PILF identification rates (akin to Type I errors) are reported for the log-Normal (LN) distribution, and the log-Pearson Type III (LP3) distribution with a variety of skew coefficients. MGBT PILF identification generally matched subjective identification from a recent California flood frequency study. Monte Carlo results show that censoring of PILFs identified by the MGBT algorithm improves the extreme quantile estimator efficiency of the expected moments algorithm (EMA) for negatively skewed LP3 distributions and has little effect for zero or positive skews; simultaneously it protects against deviations from the LP3 in the lower tail, as illustrated by distorted LN examples. Thus, MGBT generally makes flood frequency analysis based on the LP3 distribution with EMA both more accurate and more robust. This article is protected by copyright. All rights reserved.
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Summarises recent work in the Finke bedrock gorge, one of central Australia's larger and most spectacular river systems. The Finke has the best-explored slackwater deposit sequence on the continent. It therefore offers some insights on the magnitude and frequency of catastrophic floods and how they are expressed in the landscape. -from Authors
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This study aimed to determine the extent of geomorphic change resulting from the catastrophic flood of 2011 in the Lockyer Valley in southeast Queensland and to place these impacts within a history of geomorphic adjustment. Aerial photographs dated from 1933 to 2011, parish maps and historical on-ground photographs dating from 1865 to 1966 were examined for evidence of geomorphic adjustment since European settlement in the first half of the nineteenth century. Eleven forms of geomorphic adjustment were identified in three categories; erosional, depositional, and reorganisational. Only 26% of the Lockyer Creek channel length has been affected by some form of geomorphic adjustment since European settlement. Most of this adjustment was localised and dominated by reorganisation of geomorphic unit assemblages within the macrochannel and sediment deposition on floodplains. No wholesale river change in the form of lateral migration or avulsion has occurred, and the river’s morphology has remained relatively characteristic over time (i.e., morphology remains relatively uniform in a reach-averaged sense). Geomorphic responses to extreme flooding has been minor, and the geomorphic effectiveness of floods in this system (including the 2011 flood) has been limited over the last several hundred years. The system is likely still adjusting to past flooding events that ‘set’ the morphology of the current system (i.e., the macrochannel). A form of event resilience has resulted in this system such that it is less prone to geomorphic adjustment during events than would normally be considered geomorphically effective. As a result, antecedent controls on macrochannel presence and capacity are considered to be first-order controls on contemporary forms and processes in this system. Work is required to test whether the resilience of this system will hold in the future, with more extreme episodes of flooding predicted to occur in this region under future climate change.
Chapter
The estimation of rare, large magnitude floods is problematic due to short gauging station records and their limited spatial distribution. The instrumental record can be lengthened by hundreds or thousands of years by estimating discharges of past floods using geological and botanical evidence (palaeostage indicators) left by flood waters. In the former, stratigraphic sequences of sand and silt deposited in slackwater and eddy sedimentary environments are described and dated by geochronological methods (radiocarbon and luminescence techniques). In the later, flood impacts on trees producing scars and other damages (e.g., candelabrum trees) are identified and dated using tree-ring counting. These palaeostage indicators enable to calculate flood discharges using hydraulic modelling, and both flood ages and magnitudes are the input data necessary for improving flood frequency analysis. The scientific and technological interest of these studies is evident for design purposes of critical structures (dams, bridges), risk planning, and for understanding the response of flood patterns to climate change.
Article
Using well-established procedures for paleoflood hydrology and employing optically stimulated luminescence (OSL) geochronology, we analyzed a very well preserved natural record of 44 Upper Colorado River extreme floods with discharges ranging from 1800 to 9200 m3s-1. These are the largest floods occurring during the last 2140 ± 220 years, and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914. Most of these large floods occurred during the last 500 years, and the two largest floods in the record both exceeded the probable maximum flood (PMF) estimated at 8500 m3s-1 (300,000 cfs) for nearby Moab, Utah. Another 4 floods, with discharges greater than 7000 m3s-1, occurred during the last two millennia. Flood frequency analyses using the FLDFRQ3 model yields the following values, depending on the Manning n roughness coefficients: 100-yr flood – 4670-4990 m3s-1; 500-yr flood – 6675-7270 m3s-1; 1000-yr flood – 7680-8440 m3s-1. The presumed PMF discharge (8500 m3s-1) gets assigned a recurrence interval of about 1000 years, and the largest historical 1884 flood (3540 m3s-1) – a recurrence interval of <100 years. Flood frequency analysis for the Moab Valley based on the gaged record (1914-2012) yield 2730 m3s-1 for the 100-yr flood and 3185 m3s-1 for the 500-yr flood. This underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture.
Article
Fine-grained slackwater flood deposits provide a key to the paleoflood history of the Verde River in central Arizona. During large flows, the slackwater deposits accumulate in areas of sharply reduced flow velocity at channel expansions and contractions or in back-flooded tributaries. The upper surfaces of the highest deposits approximate the peak stages of the associated floods. Stratigraphic analysis, radiocarbon and archaeological dating, and correlation between slack-water sites along the study reach revealed multiple floods, including two paleo-floods that pre-date the 80-year observational record of flows on the Verde River. Discharges calculated with the HEC-2 computer flow model extended the record of rare flood events by 1000 years. The largest flow in that time period measured only 5000-5400 cms, which supports the contention of an upper limit on the maximum flood that can be reasonably expected. Revised recurrence intervals based on the extended paleoflood record were used as a basis to compare various methods of flood frequency analysis. A combination of the log-Pearson Type III distribution for recurrence intervals up to 100 years, and a regional envelope curve for less frequent flow events, produced a frequency curve that coincided with both the modern and paleoflood discharges. [Key words: flood frequency, flood deposits, paleohydrology.]
Article
Flooding is a persistent natural hazard, and even modest changes in future climate are believed to lead to large increases in flood magnitude. Previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change. This paper provides an assessment of the geomorphic effects of a rare, high magnitude event that occurred in the Lockyer valley, southeast Queensland in January 2011. The average return interval of the resulting flood was ~ 2000 years in the upper catchment and decreased to ~ 30 years downstream. A multitemporal LiDAR-derived DEM of Difference (DoD) is used to quantify morphological change in two study reaches with contrasting valley settings (confined and unconfined). Differences in geomorphic response between reaches are examined in the context of changes in flood power, channel competence and degree of valley confinement using a combination of one-dimensional (1-D) and two-dimensional (2-D) hydraulic modelling. Flood power peaked at 9800 W m- 2 along the confined reach and was 2-3 times lower along the unconfined reach. Results from the DoD confirm that the confined reach was net erosional, exporting ~ 287,000 m3 of sediment whilst the unconfined reach was net depositional gaining ~ 209,000 m3 of sediment, 70% of the amount exported from the upstream, confined reach. The major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width. In the unconfined reach, the benches and floodplains were the major loci for deposition, whilst the inner channel exhibited minor width increases. The presence of high stream power values, and resultant high erosion rates, within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development. Dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration. The confined reach experienced large-scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study.
Article
The expected moments algorithm (EMA) [, 1997] and the Bulletin 17B [, 1982] historical weighting procedure (B17H) for the log Pearson type III distribution are compared by Monte Carlo computer simulation for cases in which historical and/or paleoflood data are available. The relative performance of the estimators was explored for three cases: fixed-threshold exceedances, a fixed number of large floods, and floods generated from a different parent distribution. EMA can effectively incorporate four types of historical and paleoflood data: floods where the discharge is explicitly known, unknown discharges below a single threshold, floods with unknown discharge that exceed some level, and floods with discharges described in a range. The B17H estimator can utilize only the first two types of historical information. Including historical/paleoflood data in the simulation experiments significantly improved the quantile estimates in terms of mean square error and bias relative to using gage data alone. EMA performed significantly better than B17H in nearly all cases considered. B17H performed as well as EMA for estimating X100 in some limited fixed-threshold exceedance cases. EMA performed comparatively much better in other fixed-threshold situations, for the single large flood case, and in cases when estimating extreme floods equal to or greater than X500. B17H did not fully utilize historical information when the historical period exceeded 200 years. Robustness studies using GEV-simulated data confirmed that EMA performed better than B17H. Overall, EMA is preferred to B17H when historical and paleoflood data are available for flood frequency analysis.
Article
The hydrological literature on flood frequency analysis in the past has placed undue emphasis on solving the estimation problem. In this paper we argue that much better use should be made of the wealth of hydrological knowledge gained in the past century and that it is essential to expand the information beyond the flood sample at the site of interest. We suggest that the expansion of information can be grouped into three types: temporal, spatial, and causal. We present a number of examples from Austria to illustrate the rich diversity of flood processes that are often site specific and difficult to capture by formal methods. On the basis of these examples, and the expansion of information, we illustrate that hydrological reasoning can provide diagnostic findings that give guidance on how to adjust quantitative estimates from formal methods to more fully capture the subtleties of the flood characteristics at the site of interest. We believe that this approach gives a more complete representation of flood processes at a given site than the existing formal methods alone and propose the term "flood frequency hydrology," as opposed to flood frequency statistics, to reflect the focus on hydrological processes and hydrological reasoning.
Article
Understanding the frequency and causes of extreme events is crucial for environmental, social and economic protection and planning. In Australia this was never more apparent than January 2011 when widespread flooding across Queensland, New South Wales (NSW), and Victoria resulted in the loss of human lives and devastating impacts to infrastructure and local economies. However, understanding the interplay between the geomorphology of catchments and their hydrology remains poorly developed in floodplain planning guidelines. This paper seeks to explain spatial patterns of flood inundation in terms of downstream variations in channel morphometry; and to discuss the significance of these findings within the context of improving flood risk avoidance strategies and environmental outcomes for urban streams. A prominent characteristic of streams draining catchments in the Lockyer Valley south east Queensland and the Illawarra region of NSW, for example, are well developed macrochannels that have formed in mid-catchment zones. Detailed hydraulic modeling using HEC-RAS, HEC-GeoRAS and ArcGIS indicates that these macrochannels are scaled to accommodate high magnitude floods by operating as ‘bankfull’ channels during such events. In south east Queensland, locations where macrochannels debouch onto unconfined low gradient floodplains appear especially vulnerable to catastrophic flooding because of the efficient delivery and minimal attenuation of flood peaks generated in headwater catchments. Macrochannels and associated landforms can be clearly distinguished and mapped on fine-scale digital elevation models, offering the opportunity to integrate analyses of fluvial landforms and channel processes into hydraulic modeling studies, and ultimately, flood-risk avoidance strategies. Such an approach has the potential to improve on traditional flood risk avoidance methods that are focused primarily on design-flood heights by enabling the interpretation of hydraulic modeling outputs in the context of fluvial landforms that exert a significant control on flood behaviour.
Article
Palaeoflood studies enable estimates to be made of the age and magnitude of past flood events. The key points are the location, stratigraphy and age of slackwater deposits. These are often best preserved in gorge sections and typically occur in association with side gorges, caves and on terraces. The slackwater deposits are allocated to specific flood events and the associated discharges are estimated from step-backwater modelling methods. Absolute dating techniques are used to assess the age of the individual slackwater deposit sequences. Studies of this kind were undertaken for Windjana Gorge in the Kimberley region of northern West Australia. The fieldwork was a contribution to the Kimberley Research Project 1988, jointly organized by the Royal Geographical and Linnean Societies. A sequence of five slackwater deposits was established and palaeodischarges calculated. Both radiocarbon and thermoluminescence dating were used to establish a chronology for the deposits, which varied in age from a few years to over 2000 years. The palaeodischarges were compared with estimates of extreme discharges obtained using other hydrological methods. For remote areas, river discharge records are of short duration and variable quality. Therefore, the use of statistical techniques based on the extrapolation of discharge and frequency are of doubtful value. The palaeodischarges for Windjana Gorge were also compared with estimates of the unit area runoff for extreme floods in Australia and worldwide. There are indications that palaeoflood studies offer a useful and independent approach to the estimation of past extreme flows. The method is especially valuable for regions that have only sparse hydrological records.
Article
For historic settlement areas numerous flood level descriptions from times before the installation of river gauges are passed on, most of them are even dated. Typically, these written descriptions are qualitative such as "the water level peaked at 2 feet above the floor of the church" or "the water level topped the bridge before it failed". Furthermore, historic flood water levels are frequently marked at buildings and constructions. Such descriptions of flood water levels are used to determine periods of increased flood frequencies but are rarely transferred into palaeodischarge numbers due to methodological problems. One major problem is the estimation of the cross section area due to missing information on the topography and hydraulic roughness of the floodplain and the river channel in historic times. For the historic flood level records from the cities of Cologne (River Rhine) and Prague (River Vltava) an approach to estimate peak discharge is developed. Based on historic etchings, paintings and descriptions it is possible to reconstruct the characteristics of the river channel and floodplains to estimate cross-section areas during flood events. The reconstruction made use of all available data and estimations regarding channel incision as well as anthropogenic modification of the river and its floodplain. The mean flow velocity at the time of the historic flood events is estimated by the Manning-equation, based on the reconstructed river channel and floodplains. The slope of the water level is assumed to be comparable to recent values, while the estimation of the hydraulic roughness is a challenge as no studies on the hydraulic roughness of settled floodplains have been carried out so far. Sensitivity studies with different n-values within a reliable range of values are made to estimate the influence of this uncertainty. Finally, the reconstructed data are tested by estimating peak discharges of recent floods by the application of the described method and comparing the results with measured discharge data from the gauges located at Cologne and Prague. Herget, J. & H. Meurs (2009): Reconstructing peak discharges of historic flood levels in the citiy of Cologne, Germany. Global and Planetary Change (accepted)
Article
The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability. Slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1200 yrs. These floods range in magnitude from 11 000 to 30 000 m3s-1 and are characterized by large downstream variations in hydraulics. Downstream fluctuations help to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels. Boulder bars and flood levees form where shear stress and stream power decrease due to channel widening. Small-scale erosional features, such as potholes and troughs, are best developed at sites of channel constriction and associated increases in shear stress and stream power. The development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure, and by flow hydraulics. -from Author
Article
Paleoflood hydrologic analysis of the deposits, employing step-backwater flow modeling, allows quantitative estimates to be made of geomorphologically significant flows.-from Authors
Article
In flood frequency applications where the design flood is required to have a specified exceedance probability, expected probability should be used. Its computation, however, presents formidable difficulties. This study presents a Monte Carlo Bayesian method for computing the expected probability distribution as well as quantile confidence limits for any flood frequency distribution using data on gauged flows, possibly corrupted by rating curve error, and on censored flows. This is achieved by a three-step process: (1) Formulate the likelihood function for the given data; (2) approximate the likelihood function using a multinormal distribution; and (3) integrate the expected probability integral using importance sampling. The FLIKE software for performing this is described, and an example is given.
Article
Historic settlements which have developed in close proximity to rivers often contain numerous descriptions and/or epigraphic markings depicting flood levels relative to historic buildings, most of which are dated and have been inherited from long before the first river gauges were installed. Such evidence of historic flood water levels is regularly used to determine periods of increased flood frequency but, because of several methodological challenges, they are rarely used to calculate palaeodischarge. Due to missing information regarding the historic topography and hydraulic roughness of the floodplain and the river channel, the major problem tends to be the reconstruction of the river channel cross-section. The city of Cologne, located on the Lower Rhine in western Germany, has flood descriptions dating back to around 800AD. Since the extreme flood event of the year 1342, these descriptions are sufficiently detailed to permit the estimation of water levels. Based on historic etchings, paintings and descriptions it is possible to reconstruct the characteristics of the river channel and floodplain to estimate cross-section areas during flood events. The reconstructions make use of all available data, including estimates of channel incision as well as anthropogenic modification of the river and its floodplain. The mean flow velocity at the time of the historic flood events is estimated by the Manning equation, based on the reconstructed river channel and floodplain. The slope of the water level is assumed to be comparable to recent values. The estimation of hydraulic roughness is challenging since no studies of the hydraulic roughness of developed floodplains have been carried out. Sensitivity studies with different n values within a reliable range of values have been made to estimate the influence of this uncertainty. Finally, the reconstructed data are tested by estimating the peak discharges of recent floods by the application of the described method and comparing the results with measured discharge data from the gauging station at Cologne. This approach offers considerable potential to investigate the relationships between flood dynamics, climate change and land use history over extended timescales.
Article
The variability of flow in river channels influences the spatial and temporal variability of many biophysical processes including the transport of sediment and waterborne pollutants and the recruitment of aquatic animals and plants. In this study, inter- and intra-basin patterns of flood variability are examined for catchments east of Australia’s Great Dividing Range. Three measures of flood variability are explored with uncertainty quantified using bootstrap resampling. The two preferred measures of flood variability (namely a flood quantile ratio and a power law scaling coefficient) produced similar results. Catchments in the wet tropics of far north Queensland experience low flood variability. Flood variability increased southwards through Queensland, reaching a maximum in the vicinity of the Fitzroy and Burnett River basins. The small near-coast catchments of southern Queensland and northern New Wales experience low flood variability. Flood variability is also high in the southern Hunter River and Hawkesbury–Nepean basins. Using L-moment ratio diagrams with data from 424 streamflow stations, we also conclude that the Generalised Pareto distribution is preferable for modelling flood frequency curves for this region. These results provide a regional perspective that can be used to develop new hypotheses about the effects of hydrologic variability on the biophysical characteristics of these Australian rivers.
Article
Current statistical methods may be unable to accurately predict recurrence intervals of rare, large-magnitude floods, especially in semiarid regions having positively skewed annual flood distributions, great hydrologic variability, and widely spaced gaging stations. Current approaches rely on historical data, but catastrophic floods may have recurrence intervals far greater than the length of historical records. In the lower Pecos and Devils Rivers of southwestern Texas, paleoflood discharge and frequency estimates are extended over 10,000 years by the study of slack-water flood sediments. Slack-water deposits are typically fine-grained sand and silt that accumulate during floods in areas where current velocity is reduced, i.e., in back-flooded tributary mouths, channel expansions, downstream from bedrock spurs and/or slump blocks, and in shallow caves along bedrock walls. Radiocarbon dating of organic detritus in slack-water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope-area techniques. Paleoflood information extracted from slack-water sediments can greatly extend flood records. These floods may be weighted like historical data in log Pearson type 3 calculations of flood frequency. Our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency. This technique offers an inexpensive and rapid way to assess catastrophic flood risk.
Article
A database of published and unpublished radiocarbon dates from Late Holocene fluvial environments in Spain was critically analysed in order to obtain a fluvial chronology for this period. The principal types of depositional environment represented in the record were slackwater flood deposits (28 dates), alluvial overbank facies (12 dates) and alluvial channel facies (7 dates). The sedimentary context of each date was classified in order to identify those samples that specifically dated change in the fluvial environment. Analysis of the different depositional environment data-sets using summed probability plots identified phases of alluviation at 2800–2350 and 800–500 cal. BP. Increased frequency of large magnitude floods occurred at 2850–2500, 1000–800 and 520–250 cal. BP. The results indicate that in Mediterranean regions slackwater flood deposits are valuable archives of climatic variability even during periods of increased human impact.
Article
An investigation is made of flood quantile estimators which can employ “historical” and paleoflood information in flood frequency analyses. Two categories of historical information are considered: “censored” data, where the magnitudes of historical flood peaks are known; and “binomial” data, where only threshold exceedance information is available. A Monte Carlo study employing the two-parameter lognormal distribution shows that maximum likelihood estimators (MLEs) can extract the equivalent of an additional 10–30 years of gage record from a 50-year period of historical observation. The MLE routines are shown to be substantially better than an adjusted-moment estimator similar to the one recommended in Bulletin 17B of the United States Water Resources Council Hydrology Committee (1982). The MLE methods performed well even when floods were drawn from other than the assumed lognormal distribution.
Article
Slackwater sediments and palaeostage indicators record floods occurring prior to discharge gauging along the Fitzroy and Margaret Rivers. In upper Geikie Gorge along the Fitzroy River, preserved sequences record six floods ranging from 5000 to 30 000 ms during the last 2000 years. Approximately 13 floods between 2000 and 20 000 ms are recorded in an unnamed gorge along the upper Margaret River during the last 4000 years. These floods are at the upper limit of world‐wide discharge‐drainage area curves.The length and resolution of the flood record preserved in slackwater deposits depend on the exposure of flood sediments to post‐depositional weathering, the nature of flood sedimentation, channel geometry and stability, and the frequency of floods. Conditions along the Fitzroy and Margaret Rivers are well‐suited to preserving long, fairly accessible sedimentary flood records, in comparison to sedimentary records described from similar bedrock channels in the tropical regions of Queensland and the Northern Territory. The Fitzroy and Margaret River flood deposits indicate that, similar to other rivers with highly variable hydrologic regimes, the relatively short‐term systematic gauge records from these basins may not adequately represent extreme discharges.