Return Period Flood Research Articles

Restoration of the shifting mosaic of floodplain forests under a flow regime altered by a dam

A braided gravel-bed river provides essential habitats for various plants and animals. However, human regulation of rivers, such as dams, channelization, and other engineering works, alter flow and sediment regimes, which generally cause the degradation of river and riparian ecosystems. One of the prominent changes prevailing in Japanese rivers is forest expansion over gravel bars, and many native plants and animals that depend on gravel-bar habitat are now endangered. The Satsunai River is typical of rivers experiencing forest expansion, so the Japanese government launched a restoration project in 2012 to partially restore its riparian ecosystems. This is a large-scale experiment developed jointly by an interdisciplinary science team and river managers, who conduct monitoring and evaluation under an adaptive management scheme. The main measure to restore gravel bed habitat was an artificial flood regime, releasing a maximum water volume of 120 m3/s, which was a 2-year return period flood before dam construction. A unique feature of the project is that we considered the role of high-magnitude floods with recurrence intervals greater than 20 years after dam construction. We hypothesize that the artificial floods can be timed seasonally to create sites for regeneration and nesting of riparian species, and the high-magnitude floods contribute to maintaining a shifting mosaic structure of floodplain forest and unvegetated gravel-bar patches. We also used critical non-dimensional shear stress analysis to define “flood-disturbance areas” that can be disturbed under the artificial flow regime created by a dam. Artificial floods have been initiated once a year since 2012 at the end of June, synchronized with the seed dispersal period of Salix arbutifolia, which is endangered and a high conservation priority in the project. Thus far, the idea of setting floodplain-disturbance areas and the strategy of using both artificial and high-magnitude floods to restore a shifting mosaic of floodplain habitat patches is appropriate, as we found successful regeneration of S. arbutifolia and an exponential decay curve of the age distribution of floodplain forest patches. However, the sediment regime regulated by the dam was not addressed in this research, so future monitoring should track changes in river morphology associated with reduced sediment supply caused by the dam.

Ultra-high resolution regional climate projections for assessing changes in hydrological extremes and underlying uncertainties

The frequency and intensity of extreme hydrological events (droughts and floods) have been increasing over the past few decades, which has been posing a threat to water security and agriculture production. Thus, projecting the future evolution of hydrological extremes plays a crucial role in sustainable water management and agriculture development in a changing climate. In this study, we develop the high-resolution projections of multidimensional drought characteristics and flood risks using the convection-permitting Weather Research and Forecasting (WRF) model with the horizontal grid spacing of 4 km for the Blanco and Mission River basins over South Texas. Uncertainties in model parameters are addressed explicitly, thereby leading to probabilistic assessments of hydrological extremes. Our findings reveal that the probabilistic multivariate assessments of drought and flood risks can reduce the underestimation and the biased conclusions generated from the univariate assessment. Furthermore, our findings disclose that future droughts are expected to become more severe over South Texas even though the frequency of the occurrence of droughts is projected to decrease, especially for the long-term drought episodes. In addition, South Texas region is expected to experience more floods with an increasing river discharge. Moreover, the Blanco and Mission river basins will suffer from higher flood risks as flood return periods are expected to become longer under climate change.

Pemetaan Tingkat Rawan Bencana Banjir di Daerah Aliran Sungai Maros Provinsi Sulawesi Selatan

Indonesia is an archipelago with a tropical climate with very high rainfall. In the rainy season floods occur which cause losses, namely loss of life and property. This condition is a routine disaster that always threatens people's lives. Therefore, research needs to be done to identify flood-prone areas and flood-causing factors in the Maros River Basin. The method used to determine flood-prone areas is used by a combination of remote sensing, terrestrial, secondary data and interviews using the criteria of Sutikno et al (1995) with development. The variables used topography (flat and sloping), soil texture, drainage, inundation time and flood return period. The analysis shows that in the Maros watershed there are 3 classes that are prone to flooding namely not prone, prone and very prone. Most are vulnerable because more than 50 percent of the area is at a vulnerable and very vulnerable level. The class is very prone to spread from downstream to the middle of the watershed, covering the southern sub-districts of Maros Baru, Marusu, Turikale, southern Bantimurung, western Simbang, northern Mandai and northern Tanralili. The contributing factors are high rainfall, flat and sloping topography, fine soil texture in the downstream river, poor drainage due to poor waste management, land use dominated by ponds and paddy fields and high flood return periods.

Catchment-scale flood hazard mapping and flood vulnerability analysis of residential buildings: The case of Khando River in eastern Nepal

Study regionThis study considers the Khando River (a tributary of Koshi River) in eastern Nepal. Study focusTo quantify the hazard and vulnerabilities across one of the frequently flooding catchments, i.e. Khando River, we conducted flood hazard assessment for 20, 50, 100, and 200 years return periods. We coupled flood hazard analysis with vulnerability analysis of the most dominant construction system along the river channel, i.e. wattle and daub houses. Based on the measured inundation depths, we created vulnerability and fragility functions. The flood hazard maps, damage mechanisms due to the 2017 flood, and vulnerability, as well as fragility, curves are reported in this paper. New Hydrological Insights for the RegionThe flood hazard analysis highlighted that the 2017 flood was equivalent to 20 years return period flood. Flood hazard analysis shows that the variation in the maximum inundation depth is not so wide between 20 and 200 years return periods for the Khando River catchment. Flood vulnerability analysis of residential houses along the riverbank highlighted that the wattle and daub construction system is highly vulnerable even for 20 years return period flood. Thus, the floods equivalent to 50, 100, and 200 years may have detrimental consequences in the future.

Modeling of magnitude and frequency of floods on the Narmada River: India

The concept of magnitude–frequency was introduced by Fuller, and since then, it has been widely used, especially in Europe and Asia. Over the past, 9–11 decades, several probability distribution models have been developed and applied. The principal objective of the present study is to identify the best-fit magnitude–frequency model at various sites on the Narmada River amongst the Gumbel Extreme Value type I (GEVI) and Log Pearson type III (LP-III). Therefore, Kolmogorov–Smirnov (KS) and Anderson–Darling (AD) tests of goodness-of-fit were applied to find out best-fit model at each site on the river under review. The result shows that LP-III model is the best fit for Dindori, Manot, Barman, Sandia, Hoshangabad, Handia, and Mandleshwar, and GEVI model is the best fit for the Garudeshwar site. Accordingly, flood magnitudes for 2, 5, 10, 25, 50, 100, and 200 year return period were predicted. The analysis shows that the return period of largest peak flood on record (69400 m3/s) on the Narmada River at Garudeshwar is 96 years. These models demonstrate satisfactory results for predicting discharges and return periods. In addition to this, magnitude–frequency curves reveal that fitted lines are fairly close to the most of stream flow data points. Therefore, GEVI and LP-III are the best-fit distributions for modeling of magnitude and frequency of floods on the Narmada River.

Flood Hazard Spatialization Applied to The City of Batna: A Methodological Approach

Flood flows can cause destruction to properties and infrastructure or even cost human lives. Batna is an Algerian city that is highly exposed to the risk of flooding, with an average of one flood every three to four years. The current methods utilized to analyze flood hazards are limited to the hydrology of the watershed. Limiting the analysis of flood hazards could mislead the decision-makers from proper management of such risks. The objective of the current study is to propose a simplified flood hazard model called HEC RAS-DTM (Hydrologic Engineering Centers River Analysis System (HEC RAS)-Digital Terrain Model (DTM)) and to evaluate it utilizing data gathered from the hydrological context and the hydraulic modeling of Batna city. The model entails two distinct phases. Initially, it attempts to use descriptive statistical methods based mainly on frequency analysis, which consists of studying flood flows in order to determine the probability of future flood occurrence. The analysis of the hydrological context of the city of Batna has revealed that peak flows from stream floods have been predicted at various return periods. Subsequently, HEC RAS was deployed to produce hydraulic modeling in order to extract the water heights and speeds corresponding to these expected flows. These data, along with DTM, are crucial for the spatialization of flood hazards. The hydraulic modeling and simulation using HEC-RAS and Geographic Information System (ArcGIS) of water flow at the two main valleys, Oued Batna and Oued El Gourzi, allowed predicting the extent of flooding that could occupy a large part of the city. The mapping of the flood hazard revealed the sectors that would be most exposed. The results obtained from the suggested model confirm that a significant portion of the city of Batna remains vulnerable to floods in relevance with the predicted flood return periods. The suggested model has indicated significant growth in flood locality. Additionally, the model was proved to be efficient for the analysis of flood flows, and it could easily substitute conventional analysis methods. Further studies or investigations are advised in order to replicate the study in different contexts. The article entails suggestions for properly managing flood risks. Future studies on flood risk alleviation in Batna city could be likewise considered.

RAINWATER HARVESTING FOR FLOOD PEAK REDUCTION IN WAY AWI CATCHMENT, INDONESIA

This study is motivated by floodings which occur frequently in Jagabaya village, Way Awi catchment, Bandar Lampung city, Indonesia. In addition to the intensity of rainfall, this area is considered as an urban area with intense housing and insufficient drainage capacity during heavy rain. This study aims to analyze flood peak reduction by simulating the application of rain barrels for each house. The volume of rain barrels used in the simulation is 0.5 to 5 m3. An Intensity-Duration-Frequency (IDF) curve is generated using short-duration rainfall data (i.e. 5, 10, 15, 30, 60, 120, 180, 360 and 720 minutes) recorded from Panjang Meteorological Bureau, Lampung. The rational method, a simple rainfall-runoff model, is used to calculate runoff generated both from catchment and rooftop. The result shows that the application of rain barrels may reduce flood peak up to 28% for 2 year return period of flood, while for 100 year return period of flood, the reduction may reach 13.64%. From this study, it can be concluded that rainwater harvesting utilizing rain barrel is significant in reducing flood peak. Furthermore, this result can be used to persuade both the society as well as the local government to do such flood mitigation in this densely populated urban area

Validation of flood risk maps using open source optical and radar satellite imagery

AbstractFlood risk maps delineate areas potentially at risk of flooding and are thus a crucial tool in flood risk management. In Spain, such maps are provided as open geospatial data. This article compares the flood‐prone areas according to those maps for floods of different return periods with the spatial extent of two floods that severely affected southwestern Spain using open source optical and radar satellite imagery. Results using the recall metric were found to be very good and are the most relevant to emergency preparedness services as this metric focuses on the correctly predicted flooded areas. It was also found that the 500‐year flood risk map was the one with the best precision and accuracy for both flood events. The results confirm the accuracy of the flood risk maps based on open source remote sensing imagery and hence demonstrate the potential of publicly available and freely distributed remote sensing data in their development.

Wetropolis extreme rainfall and flood demonstrator: from mathematical design to outreach

Abstract. Government and consulting experts on flood mitigation generally face difficulties when trying to explain the science of extreme flooding to the general public, in particular the concept of a return period. Too often, for example, people perceive they are safe for the next 100 years after a 1:100-year return-period flood has hit their town. UK flood practitioners therefore gave us the challenge to design an outreach tool that conceptualises the science of flooding in a way that is accessible to and directly engages the public, and in particular demonstrates what a return period is. Furthermore, we were tasked with designing a live 3-D physical model rather than a graphical or animated 2-D game on a screen. We show here how we tackled that challenge by designing, constructing, and showcasing the Wetropolis Flood Demonstrator. Wetropolis is a transportable and conceptual physical model with random rainfall, river flow, a flood plain, an upland reservoir, a porous moor, representing the upper catchment and visualising groundwater flow, and a city which can flood following extreme and random rainfall. A key novelty is the supply of rainfall every Wetropolis day. Several aspects of Wetropolis are considered. i. We present the modular mathematical and numerical design on which Wetropolis is based. It guided the choice of parameter values of Wetropolis, which was loosely inspired by the Leeds Boxing Day floods of the River Aire in 2015. The design model further serves as the building block and inspiration for adaptations suited to particular local demands. Moreover, the model is purposely lean and therefore quick to compute, serving flexibility in the outreach-tool design, but is less suitable for any detailed scientific validation.ii. The constructed Wetropolis is described here in broad terms, but we include a link to a GitHub site with details to inspire other bespoke designs. The goal, again, is to facilitate new adaptations of Wetropolis for particular catchments different to the Leeds River Aire case.iii. Our experience in showcasing Wetropolis is summarised and discussed, with the purpose of giving an overview as well as inspiring improved and bespoke adaptations. While Wetropolis should be experienced live, with videos found on the GitHub site, here we provide a photographic overview. To date, Wetropolis has been showcased to 500 to 1000 people at public workshops and exhibitions on recent UK floods, as well as to flood practitioners and scientists at various research and stakeholder workshops.iv. We conclude with some ongoing design changes, including how people can experience natural flood management in a revised Wetropolis design. Finally, we also discuss how Wetropolis, although originally focussed solely on outreach, led to a new cost-effectiveness analysis and protocol for assessing flood-mitigation plans and inspired other physical models for use in education and water management.

Capturing transformation of flood hazard over a large River Basin under changing climate using a top-down approach

Existing flood modeling studies over coastal catchments involving different combinations of model chain setup imparting complex information fails to entail the needs of policy or decision-makers. Thus, a comprehensive framework that pertains to the requirements of practitioners and provides more perspicuous flood hazard information is required. In this paper, a novel approach translating complex flood hazard information in the form of decision priority maps derived using a rational combination of models (physical and statistical) is elucidated at the finest administrative scale. The proposed methodology is illustrated over a highly flood-prone deltaic region in Mahanadi River Basin, India, to characterize impacts of climate change for a 1:100 years return period flood event under future conditions (2026–2055). The modeled flood events are further analyzed to capture the transformation dynamics of flood hazard classes (FHCs) in near-future, for prioritizing areas with greater hazard potential. Interestingly, the results capture a high transformation characteristic from low to high FHCs in agriculture-dominated areas, which are significantly greater than the areas experiencing flood hazard reduction. The results show a significant increase of 12.5% and 27.35% in areas with high FHCs under RCP4.5 and RCP8.5 scenarios, respectively. Moreover, a notable climate change response is indicated under both climate change scenarios, with approximately 22% (RCP4.5) and 25% (RCP8.5) in villages showing a drastic increment in flood hazard magnitude. The results thus highlight the importance of identifying and prioritizing the areas for flood adaptation where a relative change in flood hazard potential is higher due to climate change. Therefore, we conclude that this study can provide an insight into the implication of new approaches for effective communication of flood information by bridging the gaps between scientific communities and decision-makers in appraisal for better flood adaptation measures.

Combined effects of biological control of an invasive shrub and fluvial processes on riparian vegetation dynamics

Plant community responses to biocontrol of invasive plants are understudied, despite the strong influence of the composition of replacement vegetation on ecosystem functions and services. We studied the vegetation response to a folivore beetle (Diorhabda genus, Coleoptera) that has been introduced along southwestern US river valleys to control the invasion of non-native shrubs in the genus Tamarix (Tamaricaceae). We collected detailed plant compositional and environmental data during four different surveys over 7 years (2010–2017), including two surveys prior to when substantial beetle-induced dieback occurred in summer 2012, along the lower Virgin River, Nevada. The study river was of special interest because it is one of only a few largely unregulated rivers in the region, and a large flood of 40-year return period occurred between the first and second surveys, allowing us to study the combined effects of fluvial processes, which typically drive riparian plant community assembly, and biocontrol. Vegetation trajectories differed as a function of the dominant geomorphological process. Tamarix cover declined an average of 75% and was replaced by the native shrub Pluchea sericea as the new dominant species in the floodplain, especially where sediment deposition predominated. Following deposition, and especially erosion, opportunistic native herbs, Tamarix seedlings, and noxious weeds colonized the understory layer but did not increase in cover over time. Stands of the native shrub Salix exigua, a desirable replacement species following Tamarix control, only increased slightly and remained subordinate in the floodplain. Overall, our results showed that, by successfully controlling the target non-native plant, a biocontrol agent can substantially modify the replacement plant communities in a riparian system, but that fluvial processes also strongly influence the resulting communities.

Predicting River Embankment Failure Caused by Toe Scour Considering 1D and 2D Hydraulic Models: A Case Study of Da-An River, Taiwan

Physically based numerical models can predict scour depth at embankments located in bend reaches. However, methodologies for utilizing these numerical models to assess the risk of reinforced concrete embankment failure are rarely investigated. Therefore, a new assessment methodology is proposed to predict the riverbank failure caused by bend scour. The methodology is primarily based on a bend scour simulation model that integrates a one-dimensional (1D) hydraulic model, a two-dimensional (2D) hydrodynamic finite-volume model, and an empirical equation of bend scour prediction. The model was first applied to the Shuiwei Embankment located in a river bend reach of Da-An River in Taiwan and verified against results from the 1D hydraulic model and field data. The model was then used to simulate 2D flow field and the temporal evolution of bend scour depth under different return period flood events to examine the relationships between river discharge, water level, shear stress, and bend scour depth. The influence of shear stress on the stability of toe protections was also investigated. The field data (from two events) and numerical solutions (four scenarios) were assessed to conceive two empirical equations for predicting shear stress and bend scour depth. A new assessment methodology was proposed using these two equations to predict the risk of river embankment failure during flood periods. The proposed methodology can be easily applied in other disaster-prone regions to mitigate the effects of disasters caused by bend scouring.

Geomorphic response of a mountain gravel-bed river to an extreme flood in Aberdeenshire, Scotland

This study uses the 2015 ‘Storm Frank’ flood on the River Dee, Aberdeenshire, to assess the impact of extreme events on river dynamics. The Storm Frank flood (>200 year recurrence interval) caused significant local morphological change that was concentrated in the middle portion of the 140 km long river and overall net degradation that primarily occurred through lateral adjustment processes. Although the flood did not cause widespread change in channel planform, morphological change at the reach scale (<1 km) was significant. Bank scour resulted in channel expansion and lateral migration as well as widespread aggradation on existing gravel beds. The HEC-RAS and CAESAR–Lisflood models were used to determine the impact of morphological changes from the Storm Frank flood on a series of future hypothetical floods. The results show that inundation is highly influenced by the degree of morphological change for moderate floods, but not for high magnitude ones. In-channel scour and bank erosion can lead to an increase in channel capacity, thereby decreasing inundation. Conversely, where conveyance capacity is decreased by aggradation, flood risk inherently increases. The impact of these changes was great for a five-year return period flood, but minimal for a magnitude flood comparable to that of Storm Frank. Our modelling results also reveal that the inundation model is sensitive to the grain size and channel bed roughness input parameters, as these parameters impact flow discharge and flood hydraulics. Accurate determination of sediment parameters and degree of morphological change is therefore critical in flooding modelling and flood hazard management. Supplementary material : Peak discharge and rainfall during the 2015 Storm Frank storm, parameters used in the hydrological model CAESAR–Lisflood and sediment budget statistics of each DEM of difference threshold are available at: https://doi.org/10.6084/m9.figshare.c.4847946 Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Homogeneity in Patterns of Climate Extremes Between Two Cities—A Potential for Flood Planning in Relation to Climate Change

Information about potential scenarios and causes of floods is important for future planning. Historical weather data of Fredericton (New Brunswick) and Charlottetown (Prince Edward Island), the two coastal cities of Atlantic Canada, were analyzed using RClimDex, Mann–Kendall test, and Sen’s slope estimates for potential scenarios and causes of floods. Flood hazard analyses were conducted using GIS (Geographical Information System) and ArcSWAT software. The watersheds of Fredericton and Charlottetown were delineated from 25 × 25 m resolution DEMs (Digital Elevation Models) of the two cities followed by percent watershed area calculations for different elevation classes for flood generation. Over the past 100 years, there was a significant decreasing trend in the high intensity precipitation in Charlottetown supported by a significant decrease in the number of heavy precipitation days. However, maximum one-day precipitation and maximum five-day precipitation significantly increased in Charlottetown and Fredericton, respectively. Charlottetown received more annual precipitation than Fredericton. In the last 30 years, there was an event exceeding 50 mm precipitation (considered as a threshold for the return period of urban floods) in Charlottetown; Fredericton experienced such events for more than 1.5 times. For twelve times, these events occurred more than once in a year in Charlottetown as compared to fourteen times in Fredericton. Despite statistically proven similarities in the occurrence of extreme events in the two cities, the visualized flood hazards, and the mapping of watershed characteristics, no devastating floods were reported for Charlottetown. This does not necessarily mean that there had never been risks of flooding in Charlottetown. These findings may help policymakers for future developments.

Climate Change Impacts on Extreme Flows Under IPCC RCP Scenarios in the Mountainous Kaidu Watershed, Tarim River Basin

In the 21st century, heavier rainfall events and warmer temperatures in mountainous regions have significant impacts on hydrological processes and the occurrence of flood/drought extremes. Long-term modeling and peak flow detection of streamflow series are crucial in understanding the behavior of flood and drought. This study was conducted to analyze the impacts of future climate change on extreme flows in the Kaidu River Basin, northwestern China. The soil water assessment tool (SWAT) was used for hydrological modeling. The projected future precipitation and temperature under Intergovernmental Panel on Climate Change (IPCC) representative concentration pathway (RCP) scenarios were downscaled and used to drive the validated SWAT model. A generalized extreme value (GEV) distribution was employed to assess the probability distribution of flood events. The modeling results showed that the simulated discharge well matched the observed ones both in the calibration and validation periods. Comparing with the historical period, the ensemble with 15 general circulation models (GCMs) showed that the annual precipitation will increase by 7.9–16.1% in the future, and extreme precipitation events will increase in winter months. Future temperature will increase from 0.42 °C/10 a to 0.70 °C/10 a. However, with respect to the hydrological response to climate change, annual mean runoff will decrease by 21.5–40.0% under the mean conditions of the four RCP scenarios. A reduction in streamflow will occur in winter, while significantly increased discharge will occur from April to May. In addition, designed floods for return periods of five, 10 and 20 years in the future, as predicted by the GEV distribution, will decrease by 3–20% over the entire Kaidu watershed compared to those in the historical period. The results will be used to help local water resource management with hazard warning and flood control.

Perkiraan Tinggi Standar Lantai Jembatan Terhadap Pengaruh Muka Air Banjir

One of the bridge crossing of the Ciliwung River at the upstream side entered DKI-Jakarta area is MT. Haryono Bridge, where the bridge has the heavy and jam traffics. Every rainy season, water level of Ciliwung River flood always increase, where the peak point of the flood was occured on February in 2007, the water level exceeded the bridge deck level. Therefore, it would be necessary to determine a height standard of the bridge deck of MT.Haryono bridge against to the water level of Ciliwung River when the peak flood occured, the existing of the elevation at the below of the bridge deck is + 18.50 meter and the elevation above the bridge deck is + 19.50 meter. Hydrology analysis using Log Pearson type III performed to determine the data of average daily and yearly rainfall around of catchment area of Ciliwung River with using the water level control point near Kalibata Bridge and the flood hydrograf was calculated using synthetic hydrograph method from Nakayasu. Hydraulic analysis utilized the of HEC-RAS software (4.1) using the maximum value of hydrograph unit and consists of two simulations, where the first is simulation of calibration model of Ciliwung River at the upstream side based on the flood occured on February 04th 2007 (called as Q2007) and the second simulation using some scenarios for some return period of 2, 5, 10, 20,50, 100 years. Every result of both simulations generates the height of bridge deck from the water level of flood according to the exsisting requirements. From these results, it could be determined the elevation of bridge deck for return period of flood for the peak flood 2007 and for 2, 5, 10, 20, 50, 100 yearly, and, such as +19.50, +20.50, +21.00, +21.10, +21.80, + 22.10 dan 22.00 meter.

An Approach of Travel Time of Flood Peaks and Runoff Model towards Low Impact Development

The most crucial point for the planning of the city should be considered flooding conditions. In manipulating the length of the channel, of course also manages the direction of the stream, so that the size of the catchment area in the area will be different. The characteristics of surface runoff of urban drainage systems are essential to determine the effects of runoff reduction towards Low Impact Development (LID). In this study, to make modelling of stormwater runoff characteristics in a city can be analysed by using the Stormwater Management Model (SWMM). This research is to explain the method of approaching travel time of flood peaks and runoff in a drainage network system that needs a rainfall-runoff model, EPA SWMM model that shown flood peak in the return period 5-yr and 10-yr, and need representatives to the graph of travel time and runoff. The result shows that the channel only accommodates return period of flood 5-yr, not the 10-yr. The drainage network system consists of minor drainage and major drainage (river) that can be simulated to reduce the runoff. The approach influenced by the direction of flow and the roughness. These parameters are the vital point to manage the travel time of peak floods. The, by redesign and update the capacity of the channel can reduce the overflow over the nodes (junctions).

A Model-Based Flood Hazard Mapping on the Southern Slope of Himalaya

Originating from the southern slope of Himalaya, the Karnali River poses a high flood risk at downstream regions during the monsoon season (June to September). This paper presents comprehensive hazard mapping and risk assessments in the downstream region of the Karnali River basin for different return-period floods, with the aid of the HEC-RAS (Hydrologic Engineering Center’s River Analysis System). The assessment was conducted on a ~38 km segment of the Karnali River from Chisapani to the Nepal–India border. To perform hydrodynamic simulations, a long-term time series of instantaneous peak discharge records from the Chisapani gauging station was collected. Flooding conditions representing 2-, 5-, 10-, 50-, 100-, 200-, and 1000-year return periods (YRPs) were determined using Gumbel’s distribution. With an estimated peak discharge of up to 29,910 m3/s and the flood depths up to 23 m in the 1000-YRP, the area vulnerable to flooding in the study domain extends into regions on both the east and west banks of the Karnali River. Such flooding in agricultural land poses a high risk to food security, which directly impacts on residents’ livelihoods. Furthermore, the simulated flood in 2014 (equivalent to a 100-YRP) showed a high level of impact on physical infrastructure, affecting 51 schools, 14 health facilities, 2 bus-stops, and an airport. A total of 132 km of rural–urban roads and 22 km of highways were inundated during the flood. In summary, this study can support in future planning and decision-making for improved water resources management and development of flood control plans on the southern slope of Himalaya.

A Framework Proposal for Regional-Scale Flood-Risk Assessment of Cultural Heritage Sites and Application to the Castile and León Region (Central Spain)

Floods, at present, may constitute the natural phenomenon with the greatest impact on the deterioration of cultural heritage, which is the reason why the study of flood risk becomes essential in any attempt to manage cultural heritage (archaeological sites, historic buildings, artworks, etc.) This management of cultural heritage is complicated when it is distributed over a wide territory. This is precisely the situation in the region of Castile and León (Spain), in which 2155 cultural heritage elements are registered in the Catalog of Cultural Heritage Sites of Castile and León, and these are distributed along the 94,226 km2 of this region. Given this scenario, the present study proposes a methodological framework of flood risk analysis for these cultural heritage sites and elements. This assessment is based on two main processing tools to be developed in addition: on the one hand, the creation of a GIS database in which to establish the spatial relationship between the cultural heritage elements and the flow-prone areas for different flood return periods and, on the other hand, the creation of a risk matrix in which different variables are regarded as associated both to flood hazard (return period, flow depth, and river flooding typology) and to flood vulnerability (construction typology, and construction structural relationship with the hydraulic environment). The combination of both tools has allowed us to establish each cultural heritage flood risk level, making its categorization of risk possible. Of all the cultural heritage sites considered, 18 of them are categorized under an Extreme flood risk level; and another 24 show a High potential flood risk level. Therefore, these are about 25% to 30% of all cultural heritage sites in Castile and León. This flood risk categorization, with a scientific basis of the cultural heritage sites at risk, makes it possible to define territories of high flood risk clustering; where local scale analyses for mitigation measures against flood risk are necessary.

Biomorphological scaling laws from convectively accelerated streams

AbstractWorldwide convectively accelerated streams flowing in downstream‐narrowing river sections show that riverbed vegetation growing on alluvial sediment bars gradually disappears, forming a front beyond which vegetation is absent. We revise a recently proposed analytical model able to predict the expected longitudinal position of the vegetation front. The model was developed considering the steady state approximation of 1‐D ecomorphodynamics equations. While the model was tested against flume experiments, its extension and application to the field is not trivial as it requires the definition of proper scaling laws governing the observed phenomenon. In this work, we present a procedure to calculate vegetation parameters and flow magnitude governing the equilibrium at the reach scale between hydromorphological and biological components in rivers with converging boundaries. We collected from worldwide rivers data of section topography, hydrogeomorphological and riparian vegetation characteristics to perform a statistical analysis aimed to validate the proposed procedure. Results are presented in the form of scaling laws correlating biological parameters of growth and decay from different vegetation species to flood return period and duration, respectively. Such relationships demonstrate the existence of underlying selective processes determining the riparian vegetation both in terms of species and cover. We interpret the selection of vegetation species from ecomorphodynamic processes occurring in convectively accelerated streams as the orchestrated dynamic action of flow, sediment and vegetation characteristics. © 2019 John Wiley & Sons, Ltd.