B. Merz

Flood Risk Mapping At The Local Scale: Concepts and Challenges

Maps give a more direct and stronger impression of the spatial distribution of the flood risk than other forms of presentation (verbal description, diagrams). Thus, maps are valuable for presenting and assessing the local flood situation, and they provide information for many applications in flood defence and disaster management. In Europe, there are no standardised nomenclature or agreed practices for flood mapping. The paper reviews the concepts of flood risk mapping at the local scale, discusses the challenges and proposes a systematic presentation of flood hazards, vulnerabilities and flood risks, spanning from flood danger maps to damage risk maps

Flood trends and variability in the Mekong river

Annual maximum discharge is analyzed in the Mekong river in Southeast Asia with regard to trends in av- erage flood and trends in variability during the 20th century. Data from four gauging stations downstream of Vientiane, Laos, were used, covering two distinct hydrological regions within the Mekong basin. These time series span through over 70 years and are the longest daily discharge time se- ries available in the region. The methods used, Mann Kendal test (MK), ordinary least squares with resampling (OLS) and non-stationary generalized extreme value function (NS- GEV), are first tested in a Monte Carlo experiment, in or- der to evaluate their detection power in presence of changing variance in the time series. The time series are generated us- ing the generalized extreme value function with varying scale and location parameter. NSGEV outperforms MK and OLS, both because it resulted in less type II errors, but also be- cause it allows for a more complete description of the trends, allowing to separate trends in average and in variability. Results from MK, OLS and NSGEV agreed on trends in average flood behaviour. However, the introduction of a time-varying scale parameter in the NSGEV allowed to iso- late flood variability from the trend in average flood and to have a more complete view of the changes. Overall, results showed an increasing likelihood of extreme floods during the last half of the century, although the probability of an aver- age flood decreased during the same period. A period of en- hanced variance in the last quarter of the 20th century, esti- mated with the wavelet power spectrum as a function of time, was identified, which confirmed the results of the NSGEV. We conclude that the absence of detected positive trends in the hydrological time series was a methodological mis- conception due to over-simplistic models.

Development and evaluation of FLEMOps - a new Flood Loss Estimation MOdel for the private sector

The estimation of flood losses is an essential component for risk-oriented flood design, risk mapping or financial appraisals in the reinsurance sector. However, only simple models, e.g. stage-damage curves, have been used frequently. Further, the reliability of flood loss and risk estimates is fairly unknown, since flood loss models are scarcely validated. In the aftermath of flooding in August 2002 large data sets of flood losses were collected at affected properties in Germany. These data were used to derive multi-factorial loss models. This paper presents FLEMOps – the Flood Loss Estimation Model for the private sector, which estimates direct monetary flood losses of residential buildings and household contents considering water level, building type and building quality. In an additional model stage (FLEMOps+), the effects of private precautionary measures as well as of the contamination of the floodwater can be quantified. Together with census data and land use information the model is adapted for applications on the meso-scale. Further, different data sets of repair costs for single buildings and in whole municipalities were used to validate loss estimates on the micro- as well as on the meso-scale. First results show that the model FLEMOps+ outperforms simple stage-damage-functions.

Flood risk assessments at different spatial scales

Managing flood risk, i.e. both the hazard and the potential consequences, is an important aspect of adapting to global change and has gained much traction in recent decades. As a result, a priori flood risk assessments have become an important part of flood management practices. Many methodologies have been set up, ranging from global risk assessments for the world as a whole, to local assessments for a particular stretch of a river/coast or small town. Most assessment frameworks generally follow a similar approach, but there are also notable differences between assessments at different spatial scales. This review article examines these differences, for instance those related to the methodology, use of assessments and uncertainties. From this review, future research needs are identified in order to improve flood risk assessments at different scales. At global/continental scale, there is a clear need for harmonised information on flood defences to improve assessments. Furthermore, inclusions of indirect economic effects at the macro-/meso-scale would give a better indication of the total effects of catastrophic flooding. At the meso-/micro-scale, there is an urgent need to improve our understanding of the effects of flooding on critical infrastructures, given their importance to society, the economy, emergency management and reconstruction. An overarching theme at all scales is the validation of flood risk assessments, which is often limited. More detailed post-disaster information would allow for improved calibration, validation and thus performance of flood risk models. Lastly, the link between spatial scales also deserves attention, for instance up- or downscaling methodologies.

Influence of dike breaches on flood frequency estimation

Many river floodplains and their assets are protected by dikes. In case of extreme flood events, dikes may breach and floodwater may spill over into the dike hinterland. Depending on the specific situation, e.g. time and location of breach, and the capacity of the hinterland to contain the floodwater, dike breaches may lead to significant reductions of flood peaks downstream of breach locations. However, the influence of dike breaches on flood frequency distributions along rivers has not been systematically analysed. In order to quantify this influence, a dynamic-probabilistic model is developed. This model combines simplified flood process modules in a Monte Carlo framework. The simplifications allow for the simulation of a large number of different scenarios, taking into account the main physical processes. By using a Monte Carlo approach, frequency distributions can be derived from the simulations. In this way, process understanding and the characteristics of the river-dike-floodplain system are included in the derivation of flood frequency statements. The dynamic-probabilistic model is applied to the Lower Rhine in Germany and compared to the usually used flood frequency analysis. For extreme floods, the model simulates significant retention effects due to dike breaches, which lead to significant modifications of the flood frequency curve downstream of breach locations. The resulting probabilistic statements are much more realistic than those of the flood frequency approach, since the dynamic-probabilistic model incorporates an important flood process, i.e. dike breaching, that only occurs when a certain threshold is reached. Beyond this point, the behaviour of the flood frequency curve is dominated by this process.

Analysis of changes in climate and river discharge with focus on seasonal runoff predictability in the Aksu River Basin

The River Aksu is the principal tributary to the River Tarim, providing about three quarters of its discharge. It originates in Kyrgyzstan and flows into the arid areas of the Xinjiang Uyghur Autonomous Region in China, where an extensive irrigated agriculture has been developed in the river oases. The aim of the present contribution is to review the current trends in temperature, precipitation, and river discharge and links between these variables. The temperature in the region and the river discharge have been rising. Changes were studied using multiple trend analyses with different start and end years. Correlations between daily temperature and discharge are high and statistically significant for two headwater subcatchments of the Aksu for most of the time. However, there are episodes in late summer or beginning of autumn when correlations between temperature and discharge for the Xiehela station are absent. This can only be explained by Glacial Lake Outburst Floods from the Lake Merzbacher that are not routinely monitored. On an annual time scale, changes in summer discharge in the highly glacierized Xiehela subcatchment are dominated by changes in temperature. In contrast, in the subcatchment Shaliguilanke, variations in summer streamflow are more strongly influenced by variations in precipitation. A comparison of links between climatic variables and streamflow at different temporal scales is offered. Perspectives for seasonal forecasting are examined.

Hydrological and sedimentological processes of flood layer formation in Lake Mondsee

Detrital 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.

Flood precaution and coping with floods of companies in Germany.

During August 2002 and again in April 2006 the Elbe catchment was hit by floods. The flood in 2002 was an extreme event, e.g. with a return period of 150 years at the Dresden gauge. In comparison, the 2006 flood had only a return period of about 15 years at Dresden. This special situation enables a comparison of the preparedness of companies in the flood endangered federal state of Saxony after a long period of relatively low flood discharges and in 2006 just a few years after a severe flood event. Before August 2002, the flood risk awareness and flood preparedness of companies in Saxony was low. Only 25% of the companies had flood experience before August 2002 and for instance, only 12% of the companies had used their flood prone stories and areas in a flood adapted way. After 2002, many companies were motivated to undertake precautionary measures. On average, the percentage of companies which had undertaken different building precautionary measures was doubled in 2006. Additionally, the percentage of companies which had undertaken emergency measures before and during the flood events has increased from 67% in 2002 to 91% in 2006. It is an important challenge for the future to keep preparedness at a high level also without recurrent flood experiences.

Das Forschungsnetzwerk „Water in Central Asia“ (CAWa) – Von Klimawandel bis Wassermanagement

Das Forschungsnetzwerk „Water in Central Asia“ (CAWa) hat es sich zur Aufgabe gemacht, wissenschaftliche und technische Grundlagen fur ein grenzuberschreitendes Wasserressourcenmanagement in den funf zentralasiatischen Staaten Tadschikistan, Kirgistan, Usbekistan, Turkmenistan und Kasachstan zu erarbeiten. Dazu sollen in einem Netzwerk zentralasiatischer und deutscher Forschungseinrichtungen Fragestellungen aus den Bereichen Monitoring, Klimamodellierung, Hydrologie, Hydrogeologie und Geoinformatik mit Hilfe modernster wissenschaftlicher Methoden bearbeitet werden.