Hydrological and flood hazard assessment using a coupled modelling approach for a mountainous catchment in Portugal

Floods may lead to destruction of property, to damage to the environment and ultimately to loss of lives. Although it is not possible to avoid them, they are enhanced by human activities that increase the probability of occurrence of these natural events. Preliminary flood risk assessment and determination of areas of potential significant flood risk is mandatory according to the European Floods Directive (2007). In order to meet the established legislation, a methodology was developed that couples two modelling approaches: the Hydrological Simulation Program—FORTRAN (HSPF) and IBER. A target watershed, with complex orography and known to be vulnerable to flood hazards, is selected: the Vez River (northern Portugal). The performance of the HSPF model, driven by a climate gridded dataset, was assessed, followed by the reconstruction of the flow rate in the catchment for the period from 1950 to 2015. The results hint at an agreement between simulated and observed daily flow rates, with high coefficient of determination value and of the Nash–Sutcliffe coefficient of efficiency (> 0.7 daily timescale). A satisfactory performance was also found in reproducing flood peak events. An average deviation of 10% was found between observed and simulated flood peaks. The output of HSPF was subsequently used to drive IBER, thus determining flood hazard areas for a 10, 50 and 100-year return periods. The methodology presented here provides basic tools for decision-makers to evaluate hydrologic responses to climate data, namely the determination of flood hazard maps, but also risk assessment, water management, environmental protection and sustainability.     

Reconstruction of a flash flood with large wood transport and its influence on hazard patterns in an ungauged mountain basin

The reconstruction of past flash floods in ungauged basins leads to a high level of uncertainty, which increases if other processes are involved such as the transport of large wood material. An important flash flood occurred in 1997 in Venero Claro (Central Spain), causing significant economic losses. The wood material clogged bridge sections, raising the water level upstream. The aim of this study was to reconstruct this event, analysing the influence of woody debris transport on the flood hazard pattern. Because the reach in question was affected by backwater effects due to bridge clogging, using only high water mark or palaeostage indicators may overestimate discharges, and so other methods are required to estimate peak flows. Therefore, the peak discharge was estimated (123 ± 18 m³ s^–1) using indirect methods, but one‐dimensional hydraulic simulation was also used to validate these indirect estimates through an iterative process (127 ± 33 m³ s^–1) and reconstruct the bridge obstruction to obtain the blockage ratio during the 1997 event (~48%) and the bridge clogging curves. Rainfall–Runoff modelling with stochastic simulation of different rainfall field configurations also helped to confirm that a peak discharge greater than 150 m³ s^–1 is very unlikely to occur and that the estimated discharge range is consistent with the estimated rainfall amount (233 ± 27 mm). It was observed that the backwater effect due to the obstruction (water level ~7 m) made the 1997 flood (~35‐year return period) equivalent to the 50‐year flood. This allowed the equivalent return period to be defined as the recurrence interval of an event of specified magnitude, which, where large woody debris is present, is equivalent in water depth and extent of flooded area to a more extreme event of greater magnitude. These results highlight the need to include obstruction phenomena in flood hazard analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

Uncertainty in floodplain delineation: expression of flood hazard and risk in a Gulf Coast watershed

This paper investigates the development of flood hazard and flood risk delineations that account for uncertainty as improvements to standard floodplain maps for coastal watersheds. Current regulatory floodplain maps for the Gulf Coastal United States present 1% flood hazards as polygon features developed using deterministic, steady‐state models that do not consider data uncertainty or natural variability of input parameters. Using the techniques presented here, a standard binary deterministic floodplain delineation is replaced with a flood inundation map showing the underlying flood hazard structure. Additionally, the hazard uncertainty is further transformed to show flood risk as a spatially distributed probable flood depth using concepts familiar to practicing engineers and software tools accepted and understood by regulators. A case study of the proposed hazard and risk assessment methodology is presented for a Gulf Coast watershed, which suggests that storm duration and stage boundary conditions are important variable parameters, whereas rainfall distribution, storm movement, and roughness coefficients contribute less variability. The floodplain with uncertainty for this coastal watershed showed the highest variability in the tidally influenced reaches and showed little variability in the inland riverine reaches. Additionally, comparison of flood hazard maps to flood risk maps shows that they are not directly correlated, as areas of high hazard do not always represent high risk. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of an automated LSPIV system in a mountainous stream for continuous flood flow measurements

An imaging‐based automated large‐scale particle image velocimetry (LSPIV) system for flash flood monitoring is developed and deployed in a mountainous stream in the Longchi Catchment, Chengdu, China. This system is built from a low‐cost Raspberry Pi board‐level computer with a camera module, which can acquire continuous images/videos automatically at programmed intervals. The minimum quadratic difference algorithm tracks surface patterns as flow tracers to estimate the distribution of surface velocities. Meanwhile, a stereo imaging‐based ‘virtual pole’ method has been developed to reconstruct the three‐dimensional topography with a stereo digital camera, and a cross‐sectional bathymetry has been generated without manual surveying. The varying water stage and water surface gradient, which are critical parameters that affect image rectification and surface velocity measurements, can also be directly resolved by applying the two imaging modules together. Discharge can then be estimated with the velocity–area method through selected cross sections. A flash flood that occurred between 24 July 2014 and 25 July 2014 is selected for analysis. The water surface level reconstructed from image processing was validated with marked water levels, and a good agreement was found with a root mean square error of 3.7 cm. The discharge recorded during the flood recession process ranged from approximately 3.5 to 27 m³/s. The rating curve obtained can be well described by a power function, and the linear regression suggested a Manning's n roughness coefficient of 0.18 of one specific cross section. Some limitations of the presented large‐scale particle image velocimetry system are also put forward, and possible solutions are provided for future improvements. With these proposed upgrades, the system can provide valuable datasets of flash floods in steep mountainous streams, which are critically needed for improving our understanding and modelling of many hydrological processes associated with flood generation, propagation and erosion, as well as for real‐time forecasting. Copyright © 2016 John Wiley & Sons, Ltd.     

Geomorphological and historical data in assessing landslide hazard

Traditionally, earth scientists assess landslide occurrence on the basis of geomorphological investigations carried out through aerial photograph interpretation and ?eldwork. Conversely, local administrators primarily evaluate the impact of natural catastrophes, such as landsliding, on the basis of historical records and data. Owing to the substantial difference in the structure and spatial density of these two types of information, it is dif?cult to compare them directly and few investigators have attempted this. We compared landslide information derived from geomorphological mapping and historical data in a pilot area (the Staffora river basin, northern Italy). To do this we generated two multivariate statistical models where the dependent variable was either the mapped landslide deposits (geomorphological model), or the historical sites affected by landslide‐induced damage (historical model). By quantitatively comparing these two model maps, we demonstrate that the geomorphological model performs better in terms of percentage of terrain units correctly predicted as stable or unstable. The historical model underestimates landslide hazard mainly where human structures are lacking. However, it highlights slopes where landslide movements take place with a high frequency at the temporal scale of human life. Hence, the joint use of these two models may facilitate the knowledge of the overall instability conditions of a given region and the identi?cation of the landslides that are most frequently reactivated. Copyright © 2003 John Wiley & Sons, Ltd.     

Temporal and spatial variation of infilling processes in a landslide scar in a steep mountainous region,Japan

The duration of the soil‐depth recovery needed for reoccurrence of shallow colluvial landslides at a given site in humid regions is much longer than the return period of rainfall needed to generate sufficient pore water pressure to initiate a landslide. Knowledge of the rate of change in soil depth in landslide scars is therefore necessary to evaluate return intervals of landslides. Spatial variation in sediment transport at the Kumanodaira landslide scar in central Japan was investigated by field observations. Spatial distribution of the rate of change in soil depth was estimated using sediment transport data and geographic information system (GIS) analysis. Observations revealed that the timing of sediment transport differed for shallow and deep soil layers. Near‐surface sediment transport (mostly dry ravel and some shallow soil creep at depths ≤0·05 m) measured in sediment traps was active in winter and early spring and was affected by freezing–thawing; soil creep of subsoil (i.e. >0·05 m), monitored by strain probes, was active in summer and autumn when precipitation was abundant. Near‐surface sediment flux was estimated by a power law function of slope gradient. Deeper soil creep was more affected by relative location to the landslide scar, which influences soil depth, than by slope gradient. Our study indicated that the rate of soil‐depth recovery is high just below the head scarp of the landslide. Abrupt changes in the longitudinal slope topography immediately above, within and just below the head scarp became smoother with time due to degradation proximate to the landslide head scarp and flanks, as well as aggradation just below the head scarp. Copyright © 2014 John Wiley & Sons, Ltd.     

1993年太湖流域的洪涝灾害及水利工程的作用

1993年汛期太湖最高水位高居建国以来的第3位,仅次于1991年和1954年,达到4.51m(平均水位,下同),局部地区发生了洪涝灾害。本文对1993年太湖流域汛期的雨情和水情做了论述,并对1993、1991、1954年三个典型大水年的降雨和洪水特征作了比较。同时,还对洪涝灾害和水利工程的作用进行分析。太湖流域的雨季一般为5—7月,但是1993年汛期的降雨在时间上的分布有些异常。降雨集中在8月,而河道最高水位则出现在8月下旬。降雨的空间分布有以下3个特征:(1)上游地区的降雨集中在浙西山区;(2)太湖湖区的降雨量很大;(3)下游地区的降雨集中在淀泖和杭嘉湖地区。淀泖和杭嘉湖地区一些水位站的实测河道水位,比发生大洪水的1991年还要高。发生洪涝灾害的原因可归纳为,上游地区洪水来量大,当地的降雨强度高,以及下游河道排水不畅通。为了改进防汛调度和完善治理规划,需要对不同典型洪水年份的降雨和洪水模式做进一步研究。     

淮河具有行蓄洪区河系洪水预报水力学模型研究

针对淮河流域河系特点,建立淮河具有行蓄洪区河系洪水预报模型.干流河道洪水演进采用一维水动力学模型,钐岗分流量利用分流曲线法推求,利用虚拟线性水库法解决大洪水时支流洪水受干流顶托作用,临淮岗闸作为水力学模型的内边界条件进行处理,利用分流比法概化行洪过程,行洪区内只有蓄满时,才会有出流,行洪区内的洪水利用Muskingum...     

Mapping landscape evolution in 3D: Climate change,natural hazard and human settlements across the 1618 Piuro landslide in the Italian Central Alps

Natural and anthropogenic mountain landscapes coevolve responding on different temporal scales to climate changes and geodynamics by a series of increments that cause the dynamic association of morphological stabilization surfaces, stratigraphic units and landforms. Understanding the incremental history of palimpsest landscapes helps to recognize and forecast the effects of climate change on the sensitive mountain environments, contributes to archaeological and historical reconstruction and supports management strategies for natural risks prevention and mitigation. The Italian Bregaglia Valley provides an excellent site to unravel the recent/historical increments of evolution of landforms and human settlement, permitting to map the paleo-digital terrain models (DTMs) corresponding to the relevant landscape turning points. After the last de-glaciation, two large-scale landslides reshaped the valley floor, both predisposed by deep-seated gravitational slope deformations and one surely triggered by intense rainfalls. The most recent and impacting event buried in 1618 the rich border town of Piuro, the ancient one occurred in the same area at least 1.5 ka before. Combining stratigraphic, geomorphological, topographic, archaeological and historical data, we drew the paleo-DTMs of the pre- and post-1618 settings of Piuro, sketching the landscape evolution. Since two millennia, human settlements took advantage of the decadal to secular most stable surfaces, represented by the inactive lobes of debris-flow fans, the highest trunk river terraces and the top of humps formed by the ancient landslide body in the valley centre. Stratigraphic relationships, archaeological findings and age determinations show that both landslides diverted the trunk river and covered the existing fan lobes. On a secular timescale, fan progradation and trunk river terracing buried and reworked both the landslide bodies. The paleo-DTMs show their original areal extent and permit to compute their volume and to sketch the setting of the buried Piuro settlements, drawing the changes of the Mera trunk river course and the chronology of activity of the lateral debris-flow fan lobes.     

Development of a global flood risk index based on natural and socio-economic factors

Abstract

A global flood risk index (FRI) is established, based on both natural and social factors. The advanced flood risk index (AFRI) is the expectation of damage in the case of a single flood occurrence, estimated by a linear regression-based approach as a function of hazard and vulnerability metrics. The resulting equations are used to predict potential flood damage given gridded global data for independent variables. It is new in the aspect that it targets floods by units of events, instead of a long-term trend. Moreover, the value of the AFRI is that it can express relative potential flood risk with the process of flood damage occurrence considered. The significance of this study is that not only the hazard parameters which contribute directly to flood occurrence, but vulnerability parameters which reflect the conditions of the region where flood occurred, including its residential and social characteristics, were shown quantitatively to affect flood damage.

Citation Okazawa, Y., Yeh, P., Kanae, S. & Oki, T. (2011) Development of a global flood risk index based on natural and socioeconomic factors. Hydrol. Sci. J. 56(5), 789–804.     

Nonstationarity-based evaluation of flood risk in the Pearl River basin: changing patterns,causes and implications

Nonstationary GEV-CDN models considering time as a covariate are built for evaluating the flood risk and failure risk of the major flood-control infrastructure in the Pearl River basin, China. The results indicate: (1) increasing peak flood flow is observed in the mainstream of the West River and North River basins and decreasing peak flood flow is observed in the East River basin; in particular, increasing peak flood flow is detected in the mainstream of the lower Pearl River basin and also in the Pearl River Delta region, the most densely populated region of the Pearl River basin; (2) differences in return periods analysed under stationarity and nonstationarity assumptions are found mainly for floods with return periods longer than 50 years; and (3) the failure risks of flood-control infrastructure based on failure risk analysis are higher under the nonstationarity assumption than under the stationarity assumption. The flood-control infrastructure is at higher risk of flood and failure under the influence of climate change and human activities in the middle and lower parts of Pearl River basin.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR G. Thirel     

地震滑坡灾害评估中地震影响因素的联合应用

地震滑坡灾害是一种致灾性极高的地震次生灾害,其中的地震触发因素是导致滑坡发生的重要影响因素．可以表征地震影响因素的参数有多种,但它们代表地震动水平的能力有差异,对灾害评估预测结果有很大影响．因此,本文提出利用层次分析法将多种地震动参数联合应用以弥补不同参数间存在的不足和差异．计算实例表明, 参数联合应用所得评估结果与实际破坏情况最为接近,优于参数单独使用所得评估结果,说明该方法是可行的．文中给出的参数联合应用的方法对于其它地震灾害的评估也有很重要的借鉴意义．      

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

ABSTRACT

Recently, 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.     

Uncertainty in levee heights and its effect on the spatial pattern of flood hazard in a floodplain

Levees are not usually built to a uniform height due to the varying priority of protecting urban and agricultural lands and they are often maintained in segments. Ad hoc alteration of the heights of these segments may aggravate flood conditions. Alterations lead to complex feedback loops in velocity and depth of water that are difficult to predict. A large number of possible configurations of the levee segments renders a deterministic modelling approach ineffective. The current analysis, based on a two-dimensional hydrodynamic model involving 1000 Monte Carlo realizations of randomly varying levee heights in segments, presents a methodology of dealing with the effect of uncertainty in levee heights on the inundation pattern in a probabilistic framework. Spatially distributed model outcomes include the likelihood of inundation, range and standard deviation of flood depths and maximum speed of water. The results indicate the necessity of adopting a probabilistic approach for robust flood hazard assessment when dealing with levee segments with uncertain heights.

EDITOR M.C. Acreman; ASSOCIATE EDITOR H. Kreibich     

Probabilistic evaluation of flood hazard in urban areas using Monte Carlo simulation

The goal of the presented research was the derivation of flood hazard maps, using Monte Carlo simulation of flood propagation at an urban site in the UK, specifically an urban area of the city of Glasgow. A hydrodynamic model describing the propagation of flood waves, based on the De Saint Venant equations in two‐dimensional form capable of accounting for the topographic complexity of the area (preferential outflow paths, buildings, manholes, etc.) and for the characteristics of prevailing imperviousness typical of the urban areas, has been used to derive the hydrodynamic characteristics of flood events (i.e. water depths and flow velocities). The knowledge of the water depth distribution and of the current velocities derived from the propagation model along with the knowledge of the topographic characteristics of the urban area from digital map data allowed for the production of hazard maps based on properly defined hazard indexes. These indexes are evaluated in a probabilistic framework to overcome the classical problem of single deterministic prediction of flood extent for the design event and to introduce the concept of the likelihood of flooding at a given point as the sum of data uncertainty, model structural error and parameterization uncertainty. Copyright © 2011 John Wiley & Sons, Ltd.     

Impact of antecedent conditions on simulations of a flood in a mountain headwater basin

A devastating flood struck Southern Alberta in late June 2013, with much of its streamflow generation in the Front Ranges of the Rocky Mountains, west of Calgary. To better understand streamflow generation processes and their sensitivity to initial conditions, a physically based hydrological model was developed using the Cold Regions Hydrological Modelling platform (CRHM) to simulate the flood for the Marmot Creek Research Basin (~9.4 km²). The modular model includes major cold and warm season hydrological processes including snow redistribution, sublimation, melt, runoff over frozen and unfrozen soils, evapotranspiration, subsurface runoff on hillslopes, groundwater recharge and discharge and streamflow routing. Uncalibrated simulations were conducted for eight hydrological years and generally matched streamflow observations well, with a NRMSD of 52%, small model bias (?3%) and a Nash–Sutcliffe efficiency (NSE) of 0.71. The model was then used to diagnose the responses of hydrological processes in 2013 flood from different ecozones in Marmot Creek: alpine, treeline, montane forest and large and small forest clearings to better understand spatial variations in the flood runoff generation mechanisms. To examine the sensitivity to antecedent conditions, ‘virtual’ flood simulations were conducted using a week (17 to 24 June 2013) of flood meteorology imposed on the meteorology of the same period in other years (2005 to 2012), or switched with the meteorology of one week in different months (May to July) of 2013. Sensitivity to changing precipitation and land cover was assessed by varying the precipitation amount during the flood and forest cover and soil storage capacity in forest ecozone. The results show that runoff efficiency increases rapidly with antecedent snowpack and soil moisture storage with the highest runoff response to rainfall from locations in the basin where there are recently melted or actively melting snowpacks and resulting high soil moisture or frozen soils. The impact of forest canopy on flooding is negligible, but flood peak doubles if forest canopy removal is accompanied by 50% reduction in water storage capacity in the basin. Copyright © 2016 John Wiley & Sons, Ltd.     

1995年太湖流域东南地区的洪涝灾害

太湖流域继１９９１、１９９３年大水后,１９９５年东南地区再次遭受大水。太湖最高水位达４．３２ｍ,为建国以来的第６位。流域东南地区的浙西、杭嘉湖、淀泖及上海浦东、浦西等地普降大到暴雨,部分地区水位超过大水的１９９１、１９９３年,杭嘉湖地区水位超过有记载以来的最高水位。德清水位高达６．４３ｍ,超过记载最高水位６．４０ｍ;嘉兴水位达４．４０ｍ,超记载最高水位的４．３８ｍ;王江泾水位高达４．３８ｍ,超记载     

Significance of sediment transport processes during piedmont floods: the 2005 flood events in Switzerland

A long‐lasting rainstorm event from 20 to 22 August 2005 affected a large part of the northern Alps and Prealps in Switzerland. It resulted in elevated discharges and flooding in many headwater catchments and mountain rivers. The associated geomorphic processes included shallow landslides, deep‐seated slope instabilities, debris flows, and fluvial sediment transport. In many parts of the affected areas human activities are important, including many buildings, traffic lines and other infrastructure. In the steeper parts, geomorphic processes were mainly responsible for flow overtopping and sediment deposition both in and outside of the channel network. In the lower parts, lateral erosion and exceedance of the channel discharge capacity were the main reasons for morphologic channel modification and flooding. Sediment‐related processes contributed a lot to the overall damage. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessing the significance of wet-canopy evaporation from forests during extreme rainfall events for flood mitigation in mountainous regions of the United Kingdom

There is increased interest in the potential of tree planting to help mitigate flooding using nature-based solutions or natural flood management. However, many publications based upon catchment studies conclude that, as flood magnitude increases, benefit from forest cover declines and is insignificant for extreme flood events. These conclusions conflict with estimates of evaporation loss from forest plot observations of gross rainfall, through fall and stem flow. This study explores data from existing studies to assess the magnitudes of evaporation and attempts to identify the meteorological conditions under which they would be supported. This is achieved using rainfall event data collated from publications and data archives from studies undertaken in temperate environments around the world. The meteorological conditions required to drive the observed evaporation losses are explored theoretically using the Penman–Monteith equation. The results of this theoretical analysis are compared with the prevailing meteorological conditions during large and extreme rainfall events in mountainous regions of the United Kingdom to assess the likely significance of wet canopy evaporation loss. The collated dataset showed that event Ewc losses between approximately 2 and 38% of gross rainfall (1.5 to 39.4 mm day⁻¹) have been observed during large rainfall events (up to 118 mm day⁻¹) and that there are few data for extreme events (>150 mm day⁻¹). Event data greater than 150 mm (reported separately) included similarly high percentage evaporation losses. Theoretical estimates of wet-canopy evaporation indicated that, to reproduce the losses towards the high end of these observations, relative humidity and the aerodynamic resistance for vapour transport needed to be lower than approximately 97.5% and 0.5 to 2 s m⁻¹ respectively. Surface meteorological data during large and extreme rainfall events in the United Kingdom suggest that conditions favourable for high wet-canopy evaporation are not uncommon and indicate that significant evaporation losses during large and extreme events are possible but not for all events and not at all locations. Thus the disparity with the results from catchment studies remains.