Heiko Apel

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.

Quantification of uncertainties in flood risk assessments

Abstract By a common definition, flood risk assessments are comprised of two parts: a hazard and vulnerability assessment. The hazard assessment investigates the extent and magnitude of usually large flood events, which are associated to a certain exceedance probability, whereas the vulnerability part assesses the impact of the flooding on specified targets, e.g., building, people or infrastructure. Being inherently speculative flood risk assessments should always be accompanied by an uncertainty assessment in order to assist consequent decision properly. In this paper a dynamic‐probabilistic method is proposed, which enables a cumulated flood risk assessment of a complete river reach considering dike failures at all dike locations. The model uses simple but computational efficient modules to simulate the complete process chain of flooding. These modules are embedded into a Monte Carlo framework thus enabling a risk assessment which is physically based thus mapping the real flooding process, and which is also probabilistic and not based on scenarios. The model also provides uncertainty estimates by quantifying various epistemic uncertainty sources of the hazard as well as the vulnerability part in a second layer of Monte Carlo simulations. These uncertainty estimates are associated to defined return intervals of the model outputs, i.e., the derived flood frequencies at the end of the reach and the risk curves for the complete reach, thus providing valuable information for the interpretation of the results. By separating single uncertainty sources a comparison of the contribution of different uncertainty sources to the overall predictive uncertainty in terms of derived flood frequencies and monetary risks could be performed. This revealed that the major uncertainties are extreme value statistics, resp. the length of the data series used and the discharge‐stage relation used for the transformation of discharge into water levels in the river.

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.

Adaptation to flood risk - results of international paired flood event studies

As flood impacts are increasing in large parts of the world, understanding the primary drivers of changes in risk is essential for effective adaptation. To gain more knowledge on the basis of empirical case studies, we analyze eight paired floods, that is, consecutive flood events that occurred in the same region, with the second flood causing significantly lower damage. These success stories of risk reduction were selected across different socioeconomic and hydro-climatic contexts. The potential of societies to adapt is uncovered by describing triggered societal changes, as well as formal measures and spontaneous processes that reduced flood risk. This novel approach has the potential to build the basis for an international data collection and analysis effort to better understand and attribute changes in risk due to hydrological extremes in the framework of the IAHSs Panta Rhei initiative. Across all case studies, we find that lower damage caused by the second event was mainly due to significant reductions in vulnerability, for example, via raised risk awareness, preparedness, and improvements of organizational emergency management. Thus, vulnerability reduction plays an essential role for successful adaptation. Our work shows that there is a high potential to adapt, but there remains the challenge to stimulate measures that reduce vulnerability and risk in periods in which extreme events do not occur.

Monsoon Variability and the Mekong Flood Regime

The Mekong River in Southeast Asia owes its tidal annual flood regime to the monsoon. But the monsoon is a spatially and temporally variable circulation, with different annual to millennial variability for different regions. We focus on the separation between the Indian and the Western North-Pacific monsoon component to shed light on the interannual flood variability of the Mekong River.

Flood Hydraulics and Suspended Sediment Transport in the Plain of Reeds, Mekong Delta

The Mekong Delta is one of the largest and most intensively used estuaries in the world. It witnesses widespread flooding annually, which is both the basis of the livelihood for more than 17 million people, but also constitutes the major natural hazard in the region. Therefore, a thorough understanding of the hydrologic and hydraulic features as well as the associated sediment dynamics is urgently required for various planning purposes. In order to gain quantitative support for the existing knowledge of the floodplain inundation processes, a comprehensive monitoring scheme of the inundation hydraulics and sediment dynamics in channels and floodplains was established in a study area in the Plain of Reeds in the north-eastern part of the Vietnamese delta. With the collected data the inundation and sediment transport in the floodplains and the interaction between channels and floodplains could be observed and quantified for the first time. Differences between the two major dike systems and management strategies in terms of inundation and sediment transport in the floodplains became obvious. Combining this finding with the observed reduction of sediment loads in the secondary channels with distance from the main river, a recommendation for the operation of the sluice gates in the area is given in order to optimize sediment input to the floodplains.