Floods Of Different Magnitude Research Articles

Critical linkages among floodplain hydrology, geomorphology and ecology along a lowland meandering river, Illinois, USA

AbstractThe ecology of forested floodplains is intricately linked to river hydrology through the frequency, magnitude, timing and duration of floodplain inundation. Spatial variability in inundation characteristics is influenced by the geomorphic template of a floodplain, both in terms of the topography of floodplain features and connectivity of these features to the main river channel. Spatial variability in inundation, in turn, has the potential to produce spatial variability in forest ecological characteristics. This study examines the influence of floodplain geomorphic features on spatial variability in inundation frequency as well as the relationship between these geomorphic features and the ecological characteristics of a floodplain forest. The frequencies of floods of different magnitudes are determined from flow‐duration analysis of over 100 years of discharge data for a lowland meandering river in Illinois, USA. Data on discharge, stage, and topography are then used to calibrate a two‐dimensional hydraulic model of flow across the floodplain at different levels of inundation. Integrating the frequency and inundation data yields mapping of average annual inundation frequency for different parts of the floodplain. Significant differences in inundation frequency correspond to three distinct floodplain geomorphic features: secondary channels (frequency = 12%), closed depressions (frequency = 4%) and the floodplain surface (frequency = 3%). Tree density is similar among the three types of geomorphic features, but tree species composition and canopy density differ significantly between secondary channels and the floodplain surface. The results provide insight into linkages among hydrology, geomorphology and tree characteristics of forested floodplains of lowland meandering rivers.

Response of Floods to the Underlying Surface Changes in the Taojiang River Basin Using the Hydrologic Engineering Center’s Hydrologic Modeling System

Due to underlying surface changes (USCs), the changes in the Taojiang River Basin’s flood generation conditions could impact the flooding process in the basin. However, most studies have typically focused on either land-use changes (LUCs) or soil and water conservation measures (SWCMs) to assess the impact of the USCs on floods, which may not provide a more comprehensive understanding of the response of floods to the USCs. To investigate how the USCs have altered the floods in the Taojiang River Basin, located upstream of Poyang Lake, China, the HEC-HMS model, which incorporates the influence of the USCs into the parameter calibration, is established in this study to investigate the flood processes on an hourly scale. The flood peak and the maximum 72 h flood volume are selected as two indexes and are applied to analyze the changes in floods caused by the USCs. The 1981–2020 period is divided into three sub-periods (i.e., 1981–1992, 1993–2007, and 2008–2020) based on the conditions of the USCs. It is found that the two indexes have exhibited decreasing trends, mainly due to the USCs during 1981–2020. Benchmarked against the baseline period of 1981–1992, the two indexes decreased by 3.06% (the flood peak) and 4.00% (the maximum 72 h flood volume) during 1993–2007 and by 5.92% and 7.58% during 2008–2020. Moreover, the impacts of the LUCs and SWCMs are separated through parameter adjustments in the model, revealing that the SWCMs played a dominant role in the USCs in the Taojiang River Basin. The quantification and assessment of the impact of the USCs on floods of different magnitudes revealed that the influence decreases with increasing flood magnitude. The results of this study improve our understanding of how USCs affect the flooding process and therefore provide support for flood control management under changing environments.

Exploring an intelligent adaptation method of hydrological model parameters for flood simulations based on the light gradient-boosting machine

Traditional hydrological modeling methods use a set of parameters to simulate flood processes with complex causes and variable intensity, which can easily lead to parameter instability. To address the problem of parameter instability, this study proposes an approach integrating the hydrological model with Intelligent Adaptation Parameters (IAP), whose intelligent adaptation relationship is established by the light gradient-boosting machine (LightGBM) based on individual calibration parameters by each flood event and flood characteristics including flood-caused rainstorm information and initial soil moisture. A widely used hydrological model, Xin 'anjiang (XAJ) model, is chosen to be integrated with IAP (XAJ-IAP) in this study, which has a relatively complex structure and a total of 15 model parameters. The obtained findings demonstrate that: (1) recalibrating the sensitive runoff concentration and separation parameters with a single flood leads to a notable enhancement in simulation accuracy, while simultaneously considering the model's physical significance; (2) the XAJ overestimates large floods and underestimates small floods. Compared with the XAJ, the XAJ-IAP has a better rain-flood response relationship and simulation accuracy for floods of different magnitudes, solving the problem of parameter instability that exists in XAJ; and (3) evaluated in terms of information gain, sensitive parameters contribute the most to the establishment of the intelligent adaptation relationship in the LightGBM compared to flood-caused rainstorm information and initial soil moisture, indicating that sensitive parameters are the most important input features of the LightGBM. It can be concluded that the intelligent adaptation system can not only solve the problem of parameter instability that exists when traditional hydrological models simulate complex and changeable floods, but also further reveal the relationship between the model and floods.

Prediction of flash flood susceptibility using integrating analytic hierarchy process (AHP) and frequency ratio (FR) algorithms

The landscape of Pakistan is vulnerable to flood and periodically affected by floods of different magnitudes. The aim of this study was aimed to assess the flash flood susceptibility of district Jhelum, Punjab, Pakistan using geospatial model and Frequency Ratio and Analytical Hierarchy Process. Also, the study considered eight most influential flood-causing parameters are Digital Elevation Model, slop, distance from the river, drainage density, Land use/Land cover, geology, soil resistivity (soil consisting of different rocks and soil formation) and rainfall deviation. The rainfall data was collected from weather stations in the vicinity of the study area. Estimated weight was allotted to each flood-inducing factors with the help of AHP and FR. Through the use of the overlay analysis, each of the factors were brought together, and the value of drainage density was awarded the maximum possible score. According to the study several areas of the region based on the parameters have been classified in flood zones viz, very high risk, high risk, moderate risk, low risk, and very low risk. In the light of the results obtained, 4% of the study area that accounts for 86.25 km2 is at high risk of flood. The areas like Bagham, Sohawa, Domeli, Turkai, Jogi Tillas, Chang Wala, Dandot Khewra were located at the very high elevation. Whereas Potha, Samothi, Chaklana, Bagrian, Tilla Jogian, Nandna, Rawal high-risk zones and have been damaged badly in the flood history of the area. This study is the first of its kind conducted on the Jhelum District and provides guidelines for disaster management authorities and response agencies, infrastructure planners, watershed management, and climatologists.

Flood spatial location in a Mediterranean coastal city: Ibiza (Balearic Islands) from 2000 to 2021

<abstract> <p>Floods are a common occurrence in the Western Mediterranean basin, causing daily life disturbances, economic impacts and fatalities as the population living near the Mediterranean shores face a great risk. The city of Ibiza has been historically affected by floods of different magnitudes. After almost three decades without large events, with the 1977 flood being the latest, 20 floods have been identified within the city boundaries since the beginning of the 21<sup>st</sup> century, causing a great social impact. The aim of the research herein presented is to identify the locations of floods and their possible causes. The methodology is based on a comprehensive survey of newspapers articles and technical reports, thus allowing mapping of the location of the events over a city map. Among the causes, there are the lack of a runoff network and deficient infrastructure planning. Regarding the spatial distribution of floods, the most affected areas are the circumvallation freeways around the city and the neighborhoods urbanized after the 1960's. The results highlight the importance of the urban sprawl and malpractices related to flood risk areas, which increase the occurrence of floods. Moreover, the identification of the most affected areas can help city planners to create prevention measures and systems to reduce flood hazards and vulnerability.</p> </abstract>

The role of flood wave superposition in the severity of large floods

Abstract. The severity of floods is shaped not only by event- and catchment-specific characteristics but also depends on the river network configuration. At the confluence of relevant tributaries with the main river, flood event characteristics may change depending on the magnitude and temporal match of flood waves. This superposition of flood waves may potentially increase the flood severity downstream in the main river. However, this aspect has not been analysed for a large set of river confluences to date. To fill this gap, the role of flood wave superposition in the flood severity at downstream gauges is investigated in four large river basins in Germany and Austria (the Elbe, the Danube, the Rhine and the Weser). A novel methodological approach to analyse flood wave superposition is presented and applied to mean daily discharge data from 37 triple points. A triple point consists of three gauges: one in the tributary as well as one upstream and downstream of the confluence with the main river respectively. At the triple points, differences and similarities in flood wave characteristics between the main river and the tributary are analysed in terms of the temporal match and the magnitudes of flood peaks. At many of the confluences analysed, the tributary peaks consistently arrive earlier than the main river peaks, although high variability in the time lag is generally detected. No large differences in temporal matching are detected for floods of different magnitudes. In the majority of cases, the largest floods at the downstream gauge do not occur due to perfect temporal match between the tributary and the main river. In terms of spatial variability, the impact of flood wave superposition is site-specific. Characteristic patterns of flood wave superposition are detected for flood peaks in the Danube River, where peak discharges largely increase due to inflow from alpine tributaries. Overall, we conclude that the superposition of flood waves is not the driving factor behind flood peak severity at the major confluences in Germany; however, a few confluences show the potential for strong flood magnifications if a temporal shift in flood waves was to occur.

Preliminary study on the erosion impact of breach floods based on numerical simulation and physical model test

This paper established the combination model of numerical simulation and physical model test to fully demonstrate the influence of dike-breach floods on the pipeline. Numerical models were used to analyze the effects of different flooding schemes with different breach locations and different breach widths on the engineering line. The results show that the large flow velocity is mainly distributed in the vicinity of the breach for each scheme, and the influence is most serious when the breach occurs near the pipeline. Comparing the floods of different magnitudes comprehensively, the greater is the flood magnitude, the more serious is the influence on the pipeline. As the breach width increases, most of results show some characteristics, such as the decrease of the maximum flow velocity at the breach, the increase of the maximum flow velocity in other areas of the pipeline, the increase of the maximum flow velocity contour range, and the decrease of the large flow velocity duration. The plane and range of the scouring pit were obtained by physical model test. Considering results of numerical and physical model comprehensively, the protection ranges of different grades along the project were determined according to different ranges of impact velocities and scouring pits. The research results can provide reference for the protection of the pipeline in the Jiahetao area.

Regularity of sediment transport and sedimentation during floods in the lower Yellow River, China

The flood season is the main period of flow, sediment transport, and sedimentation in the lower Yellow River (LYR). Within the flood season, most of the flow, sediment transport, and sedimentation occurs during flood events. Because of the importance of floods in forming riverbeds in the LYR, the regularity of sediment transport and sedimentation during floods in the LYR was studied. Measured daily discharge and sediment transport rate data for the LYR from 1960 to 2006 were used. A total of 299 floods were selected; these floods had a complete evolution of the flood process from the Xiaolangdi to the Lijin hydrological stations. For five hydrological stations (Xiaolangdi, Huayuankou, Gaocun, Aishan, and Lijin), a correlation was first established for floods of different magnitudes between the average sediment transport rate at a given station and the average sediment concentration at the closest upstream station. The results showed that the sediment transport rate at the downstream station was strongly correlated with the inflow (upstream station) sediment concentration during a flood event. A relation then was established between sedimentation in the LYR and the average sediment concentration at the Xiaolangdi station during a flood event. From this relation, the critical sediment concentrations were obtained for absolute erosion, sedimentation equilibrium, and absolute deposition during floods of different magnitudes in the LYR. The results of the current study contribute to a better understanding of the mechanisms of sediment transport and the regularity of sedimentation in the LYR during floods, and provide technical support to guide the joint operation of reservoirs and the regulation of the LYR.

Urban neighbourhood flood vulnerability and risk assessments at different diurnal levels

Diurnal changes within communities can significantly alter the level of impacts during a flood, yet these essential daily variations are not currently catered for within flood risk assessments. This paper develops a flood vulnerability and risk model that captures crucial features of flood vulnerability; integrating physical and socio‐economic vulnerability data, combined with a flood hazard analysis, to give overall flood risk at neighbourhood scale, at two different times of day, for floods of different magnitudes. The flood vulnerability and risk model, the resulting diurnal coastal flood vulnerability and risk indexes, and corresponding maps for the ward of Hilsea (Portsmouth, United Kingdom), presented within this paper, highlight three previously unidentified neighbourhoods in particular in the northwest of the Hilsea ward, which have the highest levels of risk during both time zones and for flood events of different magnitude. Critically, these neighbourhoods lie further inland and not directly on the Hilsea coastline, yet by analysing at this resolution (including diurnal impacts), substantial levels of underlying vulnerability were identified within these areas.

Recovery and resilience of urban stream metabolism following Superstorm Sandy and other floods

AbstractUrban streams are exposed to multiple different stressors on a regular basis, with increased hydrological flashiness representing a common urban stream stressor. Stream metabolism, the coupled ecosystem functions of gross primary production (GPP) and ecosystem respiration (ER), controls numerous other ecosystem functions and integrates multiple processes occurring within streams. We examined the effect of one large (Superstorm Sandy) and multiple small and moderately sized flood events in Baltimore, Maryland, to quantify the response and recovery of urban stream GPP and ER before and after floods of different magnitudes. We also compared GPP and ER before and after Superstorm Sandy to literature values. We found that both GPP and ER decreased dramatically immediately following floods of varying magnitudes, but on average GPP was more reduced than ER (80% and 66% average reduction in GPP and ER, respectively). Both GPP and ER recovered rapidly following floods within 4–18 d, and recovery intervals did not differ significantly between GPP and ER. During the two‐week recovery following Superstorm Sandy, two urban streams exhibited a range of metabolic activity equivalent to ~15% of the entire range of GPP and ER reported in a recent meta‐analysis of stream metabolism. Urban streams exhibit a substantial proportion of the natural variation in metabolism found across stream ecosystems over relatively short time scales. Not only does urbanization cause increased hydrological flashiness, it appears that metabolic activity in urban streams may be less resistant, but also more resilient to floods than in other streams draining undeveloped watersheds, which have been more studied. Our results show that antecedent conditions must be accounted for when drawing conclusions about stream metabolism measurements, and the rapid recovery and resilience of urban streams should be considered in watershed management and stream restoration strategies targeting ecosystem functions and services.

The role of flood hydrograph in the remobilization of large wood in a wide mountain river

Floods can mobilize large amounts of unconsolidated material, which also includes large wood in forested river basins. Yet, the influence of the shape and volume of flood hydrographs on wood dynamics in rivers remains poorly understood. Quantitative data on this relation are, however, critically needed to properly address management strategies and to improve the relevant understanding of wood dynamics in rivers. In this work we used a deterministic model, run in a multi-scenario mode, to simulate the transport of wood pieces fully coupled to hydrodynamics. The goal was to analyze how the transport of large wood occurs under different unsteady flood scenarios. We applied the model to two contrasting geomorphic configurations (channelized, single-thread and multi-thread reaches) in the Czarny Dunajec River in Poland, where extensive field observations of wood transport and deposition after floods of different magnitude were available to validate, interpret, and discuss model results. We show that the peak of wood transport is generally reached before the flood peak, and that the wood remobilization ratio is not always positively correlated to peak discharge. We found a positive correlation between the number of mobilized wood pieces and the duration of the rising limb. In addition, hysteresis was observed in the relationship between wood remobilization and discharge. We conclude that numerical modelling allows analysis of wood dynamics in a detail which cannot typically be achieved in field observations. Therefore, modelling improves our understanding of the process and helps disentangling the complex linkages between flood hydrographs and large wood transport dynamics in rivers.

Exploring large wood retention and deposition in contrasting river morphologies linking numerical modelling and field observations

AbstractLarge wood tends to be deposited in specific geomorphic units within rivers. Nevertheless, predicting the spatial distribution of wood deposits once wood enters a river is still difficult because of the inherent complexity of its dynamics. In addition, the lack of long‐term observations or monitored sites has usually resulted in a rather incomplete understanding of the main factors controlling wood deposition under natural conditions. In this study, the deposition of large wood was investigated in the Czarny Dunajec River, Polish Carpathians, by linking numerical modelling and field observations so as to identify the main factors influencing wood retention in rivers. Results show that wood retention capacity is higher in unmanaged multi‐thread channels than in channelized, single‐thread reaches. We also identify preferential sites for wood deposition based on the probability of deposition under different flood scenarios, and observe different deposition patterns depending on the geomorphic configuration of the study reach. In addition, results indicate that wood is not always deposited in the geomorphic units with the highest roughness, except for low‐magnitude floods. We conclude that wood deposition is controlled by flood magnitude and the elevation of flooded surfaces in relation to the low‐flow water surface. In that sense, the elevation at which wood is deposited in rivers will differ between floods of different magnitude. Therefore, together with the morphology, flood magnitude represents the most significant control on wood deposition in mountain rivers wider than the height of riparian trees. Copyright © 2015 John Wiley & Sons, Ltd.

Hydrological and sedimentological processes of flood layer formation in Lake Mondsee

AbstractDetrital layers in lake sediments are recorders of extreme flood events. However, their use for establishing time series of past floods is limited by lack in understanding processes of detrital layer formation. Therefore, we monitored hydro‐sedimentary dynamics in Lake Mondsee (Upper Austria) and its main tributary, Griesler Ache, over a 3‐year period from January 2011 to December 2013. Precipitation, discharge and turbidity were recorded continuously at the river outlet to the lake and compared to sediment fluxes trapped with 3 to 12 days resolution at two locations in the lake basin, in a distance of 0·9 (proximal) and 2·8 km (distal) to the Griesler Ache inflow. Within the 3‐year observation period, 26 river floods of different magnitude (10 to 110 m3 s−1) have been recorded resulting in variable sediment fluxes to the lake (4 to 760 g m−2 d−1) including the ‘century‐scale’ flood event in June 2013. The comparison of hydrological and sedimentological data revealed (i) a rapid sedimentation within 3 days after the peak runoff in the proximal and within 6 to 10 days in the distal lake basin; (ii) empirical flood thresholds for triggering sediment flux at the lake floor increasing from the proximal (20 m3 s−1) to the distal lake basin (30 m3 s−1) and (iii) various factors that control the detrital sediment transport in the lake. The amount of sediment transported to the lake is controlled by runoff and catchment sediment availability. The distribution of detrital sediment within the lake basin is mainly driven by mesopycnal interflows and closely linked to flood duration and the season in which a flood occurred. The combined hydro‐sedimentary monitoring revealed detailed insights into processes of flood layer formation in a meso‐scale peri‐Alpine lake and, thereby, improves the interpretation of the depositional record of flood layers.

Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude

AbstractThis work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley & Sons, Ltd.

Fragile Landscape: Impact and Consequences of May 2014 Flash-flood Disaster in the Aqaba Area, Southern Jordan

Flash-flood disasters are common in the Aqaba area. It always has a significant impact on the city and causes immense damages to lives and the infrastructure (highways, roads and dwelling areas). Before the 1990s several flash-floods of medium and high magnitudes were recorded (i.e., the 1963 and 1966 floods). However, low and medium magnitude floods (6-7 and 20 year return period) have been a recurrent phenomena over the last three decades. The city and the port of Aqaba are considered crucial to the Jordanian economy and cosequently the protection of the city/port from repetitive flooding is important to maintain future urban and economic development. Since the 1960s all formulated development plans for the city and the southern coast have suffered from a remarkable lack of appreciation and awareness regarding environmental hazards, where 90% of the Aqaba area is exposed to flood and sediment hazards. An intensive post-event geomorphic survey was carried out following a flash-flood that struck the Aqaba area on 6-7 th May 2014 to identify the damages that occurred and to recognize the main hydrological characteristics of the flash-flood, the terrain favoring flooding, fluvial processes which initiate large gullies and finally to gain experience regarding natural hazards. Hydrological studies related to previous flooding events indicate that the Aqaba area is exposed to recurrent floods of different magnitudes, associated with high peak discharge, gully erosion, slumping and high sediment load. These caused serious damages to the properties and infrastructure, loss of life and hardship for the people of Aqaba. The resultant large gullies are considered an additional cause of instability and a sediment source which threatened the Aqaba area.

Problems with binary pattern measures for flood model evaluation

As the calibration and evaluation of flood inundation models are a prerequisite for their successful application, there is a clear need to ensure that the performance measures that quantify how well models match the available observations are fit for purpose. This paper evaluates the binary pattern performance measures that are frequently used to compare flood inundation models with observations of flood extent. This evaluation considers whether these measures are able to calibrate and evaluate model predictions in a credible and consistent way, i.e. identifying the underlying model behaviour for a number of different purposes such as comparing models of floods of different magnitudes or on different catchments. Through theoretical examples, it is shown that the binary pattern measures are not consistent for floods of different sizes, such that for the same vertical error in water level, a model of a flood of large magnitude appears to perform better than a model of a smaller magnitude flood. Further, the commonly used Critical Success Index (usually referred to as F<2 >) is biased in favour of overprediction of the flood extent, and is also biased towards correctly predicting areas of the domain with smaller topographic gradients. Consequently, it is recommended that future studies consider carefully the implications of reporting conclusions using these performance measures. Additionally, future research should consider whether a more robust and consistent analysis could be achieved by using elevation comparison methods instead. Copyright © 2013 John Wiley & Sons, Ltd.

Flood generation during the SW monsoon season in northern Thailand

Abstract This paper analyses the annual floods for the city of Chiang Mai in northern Thailand using a relatively long dataset (1921–2009) to study the mechanisms behind flood generation in this region. Four floods of different magnitudes were chosen for the empirical analysis. Daily rainfall, flow and baseflow data are analysed together with information about tropical storm occurrence–frequency and El Ninõ Southern Oscillation (ENSO) events to identify the occurrence and causes of floods. We found that floods are caused by a variety of factors but the most extreme floods are often caused by a combination of wet catchment conditions and heavy rainfall due to monsoonal effects or tropical storms. Tropical storms play a variable role in that their occurrence in the earlier part of the monsoon season may only wet up the catchment and have no part to play in the peak-flow generation. However, heavy rainfall due to tropical storms during the later part of the monsoon season may result in extreme floods. Typhoon Damrey (2005) is such an example as a 100 year flood event occurred after the passage of this typhoon across northern Thailand. Flood forecasting and management therefore need more high-resolution information about rainfall patterns and tropical storm activity.

Analysis of the effect of regional lateral inflow on the flood peak of the Three Gorges Reservoir

The Three Gorges Region (TGR), located at the lower reach of the Upper Yangtze River Basin (UYRB) in China, suffers from heavy rainstorm frequently. The runoff generated from TGR composes an important part of the total flood at the famous Three Gorges Reservoir (TGRe). During the severe flood period in 1954, for example, the water from TGR accounted for up to 13.2% of the 30-days maximum flood volume of the UYRB. Considering the short and steep tributaries with rapid concentration, the regional lateral inflow (RLI) may induce more serious effect on the flood peak of the TGRe than the volume. However, hydrological data of the sparse gauge stations is too insufficient to evaluate the effect of RLI. This paper studied the impact by analyzing 880 flood events during 1956–2000. By comparing the observed hydrograph and simulated hydrograph with HEC-RAS software regarding no RLI, the effect of RLI on flood peak value and timing properties was identified and quantified. The variability of this effect among floods of different magnitudes was also analyzed. To evaluate the analysis uncertainty associated with the parameter of roughness coefficient, four sets of roughness coefficients from different research groups were employed in this study. The results showed that RLI contributes discharge of 3524 m 3 /s to flood peaks of the TGRe on average, with the contribution ratio of 15.9%. RLI contributes 12000 m 3 /s to the flood peaks larger than 50000 m 3 /s on average, with 25000 m 3 /s as its upper bound, while the contribution ratio can reach up to 50%, with an average of 20%. The variability of this effect is great among different events. Statistical analysis showed that to larger flood peak of the TGRe, RLI contributes more discharge with higher variability, and the contribution ratio and its variability are slightly larger, and events with higher contribution ratio occur more frequently. RLI can reshape the hydrograph, leading to earlier appearance of flood peak. This effect and its variability increase with the contribution ratio. This study has revealed that RLI plays an important role in large flood peak of the TGRe, which calls for more reliable flood forecasting methods to prolong the forecast lead time and improve the accuracy for the safety of the Three Gorges Dam and the protection of its lower reaches during severe flood disaster period.

Inclusion of potential vorticity uncertainties into a hydrometeorological forecasting chain: application to a medium size basin of Mediterranean Spain

Abstract. The improvement of the short- and mid-range numerical runoff forecasts over the flood-prone Spanish Mediterranean area is a challenging issue. This work analyses four intense precipitation events which produced floods of different magnitude over the Llobregat river basin, a medium size catchment located in Catalonia, north-eastern Spain. One of them was a devasting flash flood – known as the "Montserrat" event – which produced 5 fatalities and material losses estimated at about 65 million euros. The characterization of the Llobregat basin's hydrological response to these floods is first assessed by using rain-gauge data and the Hydrologic Engineering Center's Hydrological Modeling System (HEC-HMS) runoff model. In second place, the non-hydrostatic fifth-generation Pennsylvania State University/NCAR mesoscale model (MM5) is nested within the ECMWF large-scale forecast fields in a set of 54 h period simulations to provide quantitative precipitation forecasts (QPFs) for each hydrometeorological episode. The hydrological model is forced with these QPFs to evaluate the reliability of the resulting discharge forecasts, while an ensemble prediction system (EPS) based on perturbed atmospheric initial and boundary conditions has been designed to test the value of a probabilistic strategy versus the previous deterministic approach. Specifically, a Potential Vorticity (PV) Inversion technique has been used to perturb the MM5 model initial and boundary states (i.e. ECMWF forecast fields). For that purpose, a PV error climatology has been previously derived in order to introduce realistic PV perturbations in the EPS. Results show the benefits of using a probabilistic approach in those cases where the deterministic QPF presents significant deficiencies over the Llobregat river basin in terms of the rainfall amounts, timing and localization. These deficiences in precipitation fields have a major impact on flood forecasts. Our ensemble strategy has been found useful to reduce the biases at different hydrometric sections along the watershed. Therefore, in an operational context, the devised methodology could be useful to expand the lead times associated with the prediction of similar future floods, helping to alleviate their possible hazardous consequences.

The effect of anthropogenic disturbance on the hydrochemical characteristics of riparian wetlands at the Middle Ebro River (NE Spain)

In natural systems, the chemistry of floodplain waters is a function of the source of the water, which is influenced by geomorphic features of riparian wetlands. However, anthropogenic disturbances may alter both geomorphic features and the natural balance of water mixing in the floodplain. The aim of this study was to classify riparian wetlands and characterize their water characteristics in one reach of the Middle Ebro River to assess the hydrochemical functioning of the system. In order to accomplish that goal, water samples were collected at 40 sampling sites during low-water conditions and two floods of different magnitude. Moreover, geomorphic characteristics of riparian wetlands were also analyzed to interpret the results at broader spatio-temporal scales. Three group of wetlands were identified using multivariate ordination: (1) major and secondary channels highly connected to the river by surface water, containing weakly ionized water with high nitrate levels during floods; (2) secondary channels and artificial ponds located in riparian forests near the river, most of which were affected by river seepage during the examined events. This type of sites had high major ions concentrations and elevated spatial variability with respect to nutrient concentrations during floods; (3) Siltated oxbow lakes, whose hydrogeochemical features seemed to be unaffected by factors related to river fluctuations. Total dissolved solids, major ion (sulfate, chloride, sodium, calcium, magnesium, and potassium) and nutrient (nitrate, ammonium and organic nitrogen, and phosphate) depended upon the relationships between surface and subsurface water flows. Seasonal changes and geomorphic characterization indicated that a strong functional dependence of floodplain wetlands close to the main river channel is established, whereas most of the floodplain area remains disconnected from river dynamics. Moreover, the effect of nitrate-enriched agricultural runoff seems to affect water quality and hydrochemical gradients of the system. Based on our results, we propose different types of actions for the management of the Ebro River flow to ensure a more natural ecological functioning of its floodplains.