Changes In Flood Characteristics Research Articles

Future Changes in Floods, Droughts, and Their Extents in the Alps: A Sensitivity Analysis With a Non‐Stationary Stochastic Streamflow Generator

AbstractSpatially compounding droughts and floods challenge water management and may become more severe in a warming climate. However, the influence of climate change on widespread hydrologic extremes remains largely unknown because they are neither well represented in observations nor in models. Here, we present a non‐stationary stochastic streamflow generator that captures streamflow dependencies between different catchments and represents seasonal covariations of streamflow with temperature. We run this model for 925 nearly natural catchments in the Alps to generate daily streamflow time series for a reference period and three warming levels of 1, 2, and 3°C. Then, we identify drought and flood events, determine their spatial extents, and assess changes in all of these characteristics. This sensitivity analysis for different warming levels suggests that changes in flood characteristics and spatial extent are substantially weaker than those in drought characteristics and spatial extent. While floods show changes in their timing toward earlier in the year, simulated changes in magnitude, volume, duration, and spatial extent are negligible. In contrast, droughts show changes not just in timing but also intensity, deficit, duration, and extent. Specifically, droughts are projected to intensify, last longer, and slightly increase in spatial extent, with the magnitude of change increasing with the warming level. These projected changes highlight the need to develop adaptation strategies in particular for droughts. Such adaptation strategies should go beyond local adaptation and consider that extreme events become more widespread in a warming world.

Identifying changes in flood characteristics and their causes from an event-based perspective in the Central Taihu Basin

Increasing rainstorms induced by climate change and modification in the land surface due to urbanization have greatly altered floods at different spatio-temporal scales. However, investigating flood events in urbanized plains is challenging as anthropogenic behaviors can change river flow without rainfall. In addition, while the frequency and magnitude of floods have been well examined, knowledge about variations in the rate of flood change is still limited. To fill these gaps, we proposed a scheme that focused on flood responses to rainfall to detect changes in flood characteristics in the Central Taihu Basin, a highly urbanized region in the Yangtze River Delta of China. Four characteristic metrics were adopted to summarize the flood hydrograph, including the peak, increment, rising rate, and falling rate. We then examined trends of these metrics based on the selected rainfall-flood events from ten hydrological stations during 1970–2020. Subsequently, the reduction method was used to separate the impacts of regional climate change and human activities on flood characteristics alterations. Furthermore, the importance of fifteen factors was quantified by the random forest model. We found that there is a significant upward trend in the evolution of flood characteristics, except for the increment of floods. Flood characteristics exhibit higher values when rainfall accumulates, indicating stronger responses of floods to a large amount of rainfall. The results also show that human activities dominate and impact the peak, rising rate, and falling rate of floods more than climate change. Meanwhile, although cumulative precipitation is the most important factor, flood characteristics are also susceptible to anthropogenic factors, such as land use change and hydraulic engineering construction. Our findings, which provide insights into flood event identification and enhance the understanding of regional flood changes, will serve as a reference for water resource management and flood mitigation in urbanized areas.

Changes in Mediterranean flood processes and seasonality

Abstract. Floods are a major natural hazard in the Mediterranean region, causing deaths and extensive damages. Recent studies have shown that intense rainfall events are becoming more extreme in this region but, paradoxically, without leading to an increase in the severity of floods. Consequently, it is important to understand how flood events are changing to explain this absence of trends in flood magnitude despite increased rainfall extremes. A database of 98 stations in southern France with an average record of 50 years of daily river discharge data between 1959 and 2021 was considered, together with a high-resolution reanalysis product providing precipitation and simulated soil moisture and a classification of weather patterns associated with rainfall events over France. Flood events, corresponding to an average occurrence of 1 event per year (5317 events in total), were extracted and classified into excess-rainfall, short-rainfall, and long-rainfall event types. Several flood event characteristics have been also analyzed: flood event durations, base flow contribution to floods, runoff coefficient, total and maximum event rainfall, and antecedent soil moisture. The evolution through time of these flood event characteristics and seasonality was analyzed. Results indicated that, in most basins, floods tend to occur earlier during the year, the mean flood date being, on average, advanced by 1 month between 1959–1990 and 1991–2021. This seasonal shift could be attributed to the increased frequency of southern-circulation weather types during spring and summer. An increase in total and extreme-event precipitation has been observed, associated with a decrease of antecedent soil moisture before rainfall events. The majority of flood events are associated with excess rainfall on saturated soils, but their relative proportion is decreasing over time, notably in spring, with a concurrent increased frequency of short rain floods. For most basins there is a positive correlation between antecedent soil moisture and flood event runoff coefficients that is remaining stable over time, with dryer soils producing less runoff and a lower contribution of base flow to floods. In a context of increasing aridity, this relationship is the likely cause of the absence of trends in flood magnitudes observed in this region and the change of event types. These changes in flood characteristics are quite homogeneous over the domain studied, suggesting that they are rather linked to the evolution of the regional climate than to catchment characteristics. Consequently, this study shows that even in the absence of trends, flood properties may change over time, and these changes need to be accounted for when analyzing the long-term evolution of flood hazards.

Changes in flooding in the alpine catchments of the Tarim River Basin, Central Asia

AbstractFloods are one of the most affective climate‐related disasters, and climate change has altered their intensity and frequency worldwide. This study examined long‐term changes in flood characteristics (including magnitude, frequency, and timing) in 30 alpine headwaters of the large endorheic Tarim River Basin, Central Asia. The contributions of climatic factors to flood (magnitude and timing) changes were investigated using numerical experiments in combination with the random forest approach. The following results were obtained: (1) Annual maximum flood peaks increased at most stations (89% stations) during 1961–2015 with increased flood frequency. Earlier flood peaks were observed in spring with a rate of 1.38 day per decade; for other seasons, changes in the occurrence time of flood peaks showed strong spatial variability. (2) Precipitation was the dominant factor for the increased flood magnitude in most catchments of the southern slope of the Tianshan Mountains, and temperature played a greater role in the northern Kunlun Mountains. (3) For flood timing changes, melt level height and precipitation were the most influential factors in the alpine catchments in the Tarim River Basin. The results provide information on the spatiotemporal variations of floods and their driving factors in this alpine basin under climate change.

Projected changes in Rhine River flood seasonality under global warming

Abstract. Climatic change alters the frequency and intensity of natural hazards. In order to assess potential future changes in flood seasonality in the Rhine River basin, we analyse changes in streamflow, snowmelt, precipitation and evapotranspiration at 1.5, 2.0 and 3.0 ∘C global warming levels. The mesoscale hydrological model (mHM) forced with an ensemble of climate projection scenarios (five general circulation models under three representative concentration pathways) is used to simulate the present and future climate conditions of both pluvial and nival hydrological regimes. Our results indicate that future changes in flood characteristics in the Rhine River basin are controlled by increases in antecedent precipitation and diminishing snowpacks. In the pluvial-type sub-basin of the Moselle River, an increasing flood potential due to increased antecedent precipitation encounters declining snowpacks during winter. The decrease in snowmelt seems to counterbalance increasing precipitation, resulting in only small and transient changes in streamflow maxima. For the Rhine Basin at Basel, rising temperatures cause changes from solid to liquid precipitation, which enhance the overall increase in precipitation sums, particularly in the cold season. At the gauge at Basel, the strongest increases in streamflow maxima show up during winter, when strong increases in liquid precipitation encounter almost unchanged snowmelt-driven runoff. The analysis of snowmelt events for the gauge at Basel suggests that at no point in time during the snowmelt season does a warming climate result in an increase in the risk of snowmelt-driven flooding. Snowpacks are increasingly depleted with the course of the snowmelt season. We do not find indications of a transient merging of pluvial and nival floods due to climate warming. To refine attained results, next steps need to be the representation of glaciers and lakes in the model set-up, the coupling of simulations to a streamflow component model and an independent validation of the snow routine using satellite-based snow cover maps.

Changes in flood characteristics and the flood driving mechanism in the mountainous Haihe River Basin, China

ABSTRACTRecently, the land surface in the Haihe River basin has changed, influencing the flood processes in the basin. To quantify this impact, seven typical sub-catchments were selected from different hydrological regions of the Haihe River basin for study. The non-parametric Mann-Kendall test was used to analyse for trends, and the non-parametric Pettitt test was adopted to detect any change point in the flood time series. Then, a hydrological model was established to simulate the effects of each potential driving factor on flood peak and volume. It was shown that flood peak and volume time series had decreased significantly, and the change point was around the year 1980. Groundwater depletion was not the main contribution to flood peak (FP) and volume (FV) decrease. In the Shifokou, Mubi and Lengkou sub-catchments, small hydraulic structures are the main driving factors for FP and FV decreasing. In the Xitaiyu, Daomaguan and Fuping sub-catchments, both land-use change and hydraulic structures are the main driving factors. The decreasing percentage decreases with the increase of the flood magnitude. The results provide valuable information for flood simulation and control in the Haihe River basin.

Multimodel assessment of flood characteristics in four large river basins at global warming of 1.5, 2.0 and 3.0 K above the pre-industrial level

This study assesses the flood characteristics (timing, magnitude and frequency) in the pre-industrial and historical periods, and analyzes climate change impacts on floods at the warming levels of 1.5, 2.0 and 3.0 K above the pre-industrial level in four large river basins as required by the Paris agreement. Three well-established hydrological models (HMs) were forced with bias-corrected outputs from four global climate models (GCMs) for the pre-industrial, historical and future periods until 2100. The long pre-industrial and historical periods were subdivided into multiple 31-year subperiods to investigate the natural variability. The mean flood characteristics in the pre-industrial period were derived from the large ensemble based on all GCMs, HMs and 31-year subperiods, and compared to the ensemble means in the historical and future periods. In general, the variance of simulated flood characteristics is quite large in the pre-industrial and historical periods. Mostly GCMs and HMs contribute to the variance, especially for flood timing and magnitude, while the selection of 31-year subperiods is an important source of variance for flood frequency. The comparison between the ensemble means shows that there are already some changes in flood characteristics between the pre-industrial and historical periods. There is a clear shift towards earlier flooding for the Rhine (1.5 K scenario) and Upper Mississippi (3.0 K scenario). The flood magnitudes show a substantial increase in the Rhine and Upper Yellow only under the 3.0 K scenario. The floods are projected to occur more frequently in the Rhine under the 1.5 and 2.0 K scenarios, and less frequently in the Upper Mississippi under all scenarios.

Questions in the quantitative analysis of sediment load - example of three major rivers in Hungary

The importance of the monitoring of sediment processes is unquestionable: sediment balance of regulated rivers suffered substantial alterations in the past century, affecting navigation, energy production, fish habitats and ecosystems alike. The changes in flood characteristics and rating curves of our rivers are being researched and described, involving state-of-the-art measurement methods, modeling tools and traditional statistics. Sediment processes however, are much less known. Sediment-related research is scarce, which is partly due to the outdated methodology and poor database background in the specific field. Regular sediment sampling was developed in the first half of the 20th century, with different station density and monitoring frequencies in different countries. Sampling frequency of suspended load is 3 to 7 per year in Hungary, and even lower for the bed-load, not only on the Danube river but also on large tributaries like the Drava and the Tisza rivers. Data related to sediment quantity are unreliable and often contradictory. It is difficult to produce high quality long-term databases that could enable the calibration of sediment transport models. It is a challenge to compare measurements on international rivers. The authors give an overview of sediment sampling methods, an inventory of the available datasets and data management in Hungary on the rivers Danube, Drava and Tisza, based on field data.

Has dyke development in the Vietnamese Mekong Delta shifted flood hazard downstream?

Abstract. In the Vietnamese part of the Mekong Delta (VMD) the areas with three rice crops per year have been expanded rapidly during the last 15 years. Paddy-rice cultivation during the flood season has been made possible by implementing high-dyke flood defenses and flood control structures. However, there are widespread claims that the high-dyke system has increased water levels in downstream areas. Our study aims at resolving this issue by attributing observed changes in flood characteristics to high-dyke construction and other possible causes. Maximum water levels and duration above the flood alarm level are analysed for gradual trends and step changes at different discharge gauges. Strong and robust increasing trends of peak water levels and duration downstream of the high-dyke areas are found with a step change in 2000/2001, i.e. immediately after the disastrous flood which initiated the high-dyke development. These changes are in contrast to the negative trends detected at stations upstream of the high-dyke areas. This spatially different behaviour of changes in flood characteristics seems to support the public claims. To separate the impact of the high-dyke development from the impact of the other drivers – i.e. changes in the flood hydrograph entering the Mekong Delta, and changes in the tidal dynamics – hydraulic model simulations of the two recent large flood events in 2000 and 2011 are performed. The hydraulic model is run for a set of scenarios whereas the different drivers are interchanged. The simulations reveal that for the central VMD an increase of 9–13 cm in flood peak and 15 days in duration can be attributed to high-dyke development. However, for this area the tidal dynamics have an even larger effect in the range of 19–32 cm. However, the relative contributions of the three drivers of change vary in space across the delta. In summary, our study confirms the claims that the high-dyke development has raised the flood hazard downstream. However, it is not the only and not the most important driver of the observed changes. It has to be noted that changes in tidal levels caused by sea level rise in combination with the widely observed land subsidence and the temporal coincidence of high water levels and spring tides have even larger impacts. It is recommended to develop flood risk management strategies using the high-dyke areas as retention zones to mitigate the flood hazard downstream.

Copula-based bivariate flood frequency analysis in a changing climate—A case study in the Huai River Basin, China

Copula-based bivariate frequency analysis can be used to investigate the changes in flood characteristics in the Huai River Basin that could be caused by climate change. The univariate distributions of historical flood peak, maximum 3-day and 7-day volumes in 1961–2000 and future values in 2061–2100 projected from two GCMs (CSIRO-MK3.5 and CCCma-CGCM3.1) under A2, A1B and B1 emission scenarios are analyzed and compared. Then, bivariate distributions of peaks and volumes are constructed based on the copula method and possible changes in joint return periods are characterized. Results indicate that the Clayton copula is more appropriate for historical and CCCma-CGCM3.1 simulating flood variables, while that of Frank and Gumbel are better fitted to CSIRO-MK3.5 simulations. The variations of univariate and bivariate return periods reveal that flood characteristics may be more sensitive to different GCMs than different emission scenarios. Between the two GCMs, CSIRO-MK3.5 evidently predicts much more severe flood conditions in future, especially under B1 scenario, whereas CCCma-CGCM3.1 generally suggests contrary changing signals. This study corroborates that copulas can serve as a viable and flexible tool to connect univariate marginal distributions of flood variables and quantify the associated risks, which may provide useful information for risk-based flood control.

Tool to address green roof widespread implementation effect in flood characteristics for water management planning

Abstract. In the last decades, new approaches were adopted to manage stormwater as close to its source as possible through technologies and devices that preserve and recreate natural landscape features. Green Roofs (GR) are examples of these devices that are also incentivized by city's stormwater management plans. Several studies show that GR decreases on-site runoff from impervious surfaces, however, the analysis of the effect of widespread implementation of GR in the flood characteristics at the urban basin scale in subtropical areas are little discussed, mainly because of the absence of data. Thereby, this paper shows results related to the monitoring of an extensive modular GR under subtropical weather conditions, the development of a rainfall–runoff model based on the modified Curve Number (CN) and SCS Triangular Unit Hydrograph (TUH) methods and the analysis of large-scale impact of GR by modelling different basins. The model was calibrated against observed data and showed that GR absorbed almost all the smaller storms and reduced runoff even during the most intense rainfall. The overall CN was estimated in 83 (consistent with available literature) with the shape of hydrographs well reproduced. Large-scale modelling (in basins ranging from 0.03 ha to several square kilometers) showed that the widespread use of GRs reduced peak flows (volumes) around 57% (48%) at source and 38% (32%) at the basin scale. Thus, this research validated a tool for the assessment of structural management measures (specifically GR) to address changes in flood characteristics in the city's water management planning. From the application of this model it was concluded that even if the efficiency of GR decreases as the basin scale increase they still provide a good option to cope with urbanization impact.

Upstream structural management measures for an urban area flooding in Turkey

Abstract. In recent years, flooding has become an increasing concern across many parts of the world of both the general public and their governments. The climate change inducing more intense rainfall events occurring in short period of time lead flooding in rural and urban areas. In this study the flood modelling in an urbanized area, namely Samsun-Terme in Blacksea region of Turkey is performed. MIKE21 with flexible grid is used in 2-dimensional shallow water flow modelling. 1 × 1000−1 scaled maps with the buildings for the urbanized area and 1 × 5000−1 scaled maps for the rural parts are used to obtain DTM needed in the flood modelling. The bathymetry of the river is obtained from additional surveys. The main river passing through the urbanized area has a capacity of 500 m3 s−1 according to the design discharge obtained by simple ungauged discharge estimation depending on catchment area only. The upstream structural base precautions against flooding are modelled. The effect of four main upstream catchments on the flooding in the downstream urban area are modelled as different scenarios. It is observed that if the flow from the upstream catchments can be retarded through a detention pond constructed in one of the upstream catchments, estimated Q100 flood can be conveyed by the river without overtopping from the river channel. The operation of the upstream detention ponds and the scenarios to convey Q500 without causing flooding are also presented. Structural management measures to address changes in flood characteristics in water management planning are discussed.

A European Flood Database: facilitating comprehensive flood research beyond administrative boundaries

Abstract. The current work addresses one of the key building blocks towards an improved understanding of flood processes and associated changes in flood characteristics and regimes in Europe: the development of a comprehensive, extensive European flood database. The presented work results from ongoing cross-border research collaborations initiated with data collection and joint interpretation in mind. A detailed account of the current state, characteristics and spatial and temporal coverage of the European Flood Database, is presented. At this stage, the hydrological data collection is still growing and consists at this time of annual maximum and daily mean discharge series, from over 7000 hydrometric stations of various data series lengths. Moreover, the database currently comprises data from over 50 different data sources. The time series have been obtained from different national and regional data sources in a collaborative effort of a joint European flood research agreement based on the exchange of data, models and expertise, and from existing international data collections and open source websites. These ongoing efforts are contributing to advancing the understanding of regional flood processes beyond individual country boundaries and to a more coherent flood research in Europe.

Effects of alpine swamp wetland change on rainfall season runoff and flood characteristics in the headwater area of the Yangtze River

The effects of changes in alpine swamp wetland (ASW) on rainfall season runoff and flood characteristics have not been quantified adequately in the headwater area of the Yangtze River (HYR).To address this, we focus on detecting changes in ASW based on remote sensing data. In particular, we analyzed the effects of ASW on runoff (both runoff and runoff coefficient) and flood characteristics (i.e., peak flow, rising time, duration and flood runoff coefficient) during the rainfall season based on observed runoff and climate data. Our results indicate that the area of ASW decreased by 28.22% and 30.54% at Zhimenda (ZMD) and Tuotuohe (TTH), respectively, from 1975 to 2004. Moreover, 76% and 71% of the decreases at ZMD and TTH, respectively, occurred during the period from 1990 to 2004. Both runoff and the runoff coefficients decreased at ZMD and increased at TTH with decreasing ASW in the rainfall season; we attribute this to the variability in runoff mechanisms. The effects of ASW on runoff and runoff coefficients were more pronounced at ZMD than at TTH. A slightly lower maximum peak flow and a greater frequency of moderate and small peak flows were observed at ZMD. Conversely, maximum peak flow increased and the frequency of maximum and moderate peak flows increased at TTH. Flood events increased at both ZMD and TTH, whereas both the rising time and duration decreased at both sites. Overall, the volume of snowmelt water increased and the ability to conserve water decreased in response to decreased ASW and permafrost were responsible for the observed changes in flood characteristics.

ASSESSMENT OF RIPARIAN VEGETATION SENSITIVITY TO RIVER HYDROLOGY DOWNSTREAM OF A MAJOR TEXAS DAM

Dams may impact the health of downstream riparian vegetation communities through flow modifications such as decreased flood frequency and duration. Without historical vegetation data, however, it is difficult to relate changes in vegetation composition to hydrology patterns downstream of dams. We studied bottomland hardwood forests downstream of Toledo Bend Dam on the Sabine River in Texas and Louisiana to determine their sensitivity to minor changes in river hydrology with a particular focus on floods. Current riparian vegetation was characterized within three topographic zones at three selected sites below the dam. Using 80 years of hydrologic records from two gauging stations downstream of the dam, we evaluated trends in flood frequency, flood duration, peak discharge and total flood discharge in those periods before (1926−1965) and after (1971−2005) dam construction, as well as related flood stage to floodplain elevations to link topography to flood frequency. Plant species diversity in this system is highly dependent on minor changes in elevation, and the proportion of wetland-dependent species changes rapidly with only a few centimeters difference in elevation. Although 50% of trees, shrubs and herbs in the sloughs were wetland adapted, their numbers were only 21% in the levees (74–284 cm higher in elevation) and 14% in the mid-floodplains. Since dam construction, total flood discharge and duration at the most upstream gauge on the Sabine River decreased by 49%. At both gauges, mean discharge was also altered with higher summer flows. Patterns of tree regeneration point to less recruitment by wetland-dependent species in the years following dam construction. These results suggest that minor changes in flood magnitude might limit occurrence of wetland species to the lowest topographic zones and illustrate the need to analyse sensitivity of plants to minor changes in flood characteristics when historical data for the vegetation community are lacking. Copyright © 2012 John Wiley & Sons, Ltd.

Impact of Climate and Land Use Changes on the Flood Hazard of the Middle Brahmaputra Reach, India

Being the highest specific discharge river in the world, the Brahmaputra has a large floodplain area of 700 km in length in its middle reaches falling in the high flood vulnerability category. Floods generated in upland Himalayan catchments are mainly controlled by land use and land cover, storm characteristics, and vegetation dynamics. Floods propagate through a floodplain region consisting of wetlands, paddy agriculture, and wide braided river reaches with natural constraint points (nodals) that make the reaches more vulnerable to flood hazards. In this study, a macroscale distributed hydrological model was used to obtain the flood characteristics of the reaches. A hydrological model with spatially distributed input parameters and meteorological data was simulated at (1 km × 1 km) spatial grids to estimate flood hydrographs at the main river and itsmajor tributaries. Aftermodel validation, “best guess” land use change scenarios were used to estimate potential changes in flood characteristics. Results show that at the middle reaches of the Brahmaputra, peak discharge increases by a maximum of 9% for land use change scenarios. The same model with bias-corrected climatological data from a regional climate model (RCM) simulation (PRECIS) was used to obtain future changes in flood generation and its propagation through the basin in the projected climatological scenario. Changes in flood characteristics with reference to the baseline period show that the average duration of flood waves will increase from 15.2 days in the baseline period (1961-1990) to 19.3 days in the future (2071-2100). Peak discharge will increase by an average of 21% in the future in the projected climate change scenario. After statistics on changes of flood characteristics in the projected climate change scenario (2071-2100) were obtained, a 2-dimensional hydrodynamic model was used to obtain flood inundation and velocity distribution on the floodplain. Distribution of velocity and inundation depth was spatially analyzed to obtain flood hazard zones in the projected climate change scenario. Results show that spatial variation in flood hazard zones will be significantly altered in the projected climate change scenario compared to land use/land cover changes.

Analysis of flood vulnerability and assessment of the impacts in coastal zones of Bangladesh due to potential sea-level rise

The 4th IPCC report highlights the increased vulnerability of the coastal areas from floods due to sea-level rise (SLR). The existing coastal flood control structures in Bangladesh are not adequate to adapt these changes and new measures are urgently necessary. It is important to determine the impacts of SLR on flooding to analyse the performance of the existing structures and corresponding impact to plan for suitable adaptation and mitigation measures to reduce the impacts of floods on coastal zone. The study aims to develop a comprehensive understanding of the possible effects of SLR on floods in the coastal zone of Bangladesh. A hydrodynamic model, which is a combination of surface and river parts, was utilized for flood simulation. The tool was applied under a range of future scenarios, and results indicate both spatial variability of risk and changes in flood characteristics between now and under SLR. Estimated impact on population, infrastructure and transportation is also exposed. These types of impact estimation would be of value to flood plain management authorities to minimize the socio-economic impact.