Supplementary Materials (SM) SM1: a) Input parameter features ind SwanOne, b) The bathymetry profile and respective location of the wave sensors.Example from transect 2.

Das Mekong-Delta ist Lebensraum für ca. 21,5 Mio. Menschen und sieht sich seit einigen Jahren mit zunehmend existenzbedrohenden Landsenkungsprozessen (bis zu 3 cm/a) konfrontiert. Als einer der Hauptgründe dafür gilt die starke Grundwasserentnahme. Um die Prozessdynamik besser untersuchen zu können, wurde ein im Mekong-Delta bislang einmaliges Messsystem zur tiefendifferenzierten Landsenkungsmessung an einem Pilot-Standort aufgebaut, dessen vorläufige Messdaten eine Korrelation zwischen Landsenkung und dem fallenden Grundwasserspiegel zeigen.

Abstract. Against the background of the rising sea level and land subsidence, protecting the progressively eroding coast along the Vietnam Mekong Delta becomes of tremendous importance. Within the presented work, design conditions for breakwaters were derived from offshore climate reanalysis data (ERA5), which were transferred to the nearshore by two numerical approaches, i.e. SwanOne and Delft3D, for different average and extreme wave and weather conditions. Within this process, design wave heights and periods at the nearshore could be determined for 10- to 100-year recurrence intervals. Both models thereby showed sufficient accuracy according to measurements in the field. Limitations must be made regarding the available spatio-temporal resolution, where reanalysis data showed a lack of short but high peak values compared to the observed measurements. Both numerical approaches showed their capabilities, where SwanOne offers a simple and fast calculation method, while it lacks of continuous effects like wind-generated swell or bottom friction. The Delft3D software on the other hand provides a more complete representation, not only of wave but also current dynamics, while it requires a much broader amount of input parameters and more complex boundary conditions. Within this study, the advantages and disadvantages of both models could be demonstrated, whereas for the final calculation of nearshore wave characteristics, only SwanOne was applicable based on the input parameters extracted from statistical analysis of long term ERA5 data.

The Mekong Delta is home for approx. 21.5 million people and is confronted with existence-threatening land subsidence processes up to 3 cm/a since several years. Groundwater overexploitation is considered to be the main reason for this. In order to investigate the process dynamics of land subsidence, a depth-differentiated land subsidence monitoring system was set up at a pilot site, which is so far unique in the Mekong Delta. The preliminary monitoring data shows a correlation between land subsidence and the lowering groundwater level.

Coastal erosion has become a pressing problem all over the world, especially in areas where the hinterland is only slightly elevated over the sea level. The ongoing progression reveals the urging need for engineered coastal protection measures like breakwaters. Amongst others, curtain wall breakwater types (CWB) have proven their potential to dissipate the wave energy in multiple studies. Their application is often considered in situations where only a partial protection of the coast is needed (e.g. to maintain a hydraulic connectivity or enable sediment transport for land reclamation). Due to their slender design, they are assumed to offer an economical alternative in comparison to massive breakwater constructions, while their pillar-based foundation shows advantages for applications under soft soil conditions. Within the development process of a detached breakwater to face coastal erosion in the Mekong Delta, different types of CWB configurations have been investigated under regular wave conditions. Several characteristics of CWB structures such as the inclination and thickness of the wall, the height of the structures, the rate of submergence and emergence were examined for different water depths and wave parameters. The wave-structure interaction was analyzed using FLOW3D software, which is capable of simulating wave transformation. It showed a high agreement in comparison with own experimental investigations and the wave theory. The results showed a continuous reduction of the wave transmission coefficient with increasing inclination from 90° to 135°, whilst the orientation of the inclination (e.g. 60° vs 120°) only showed a minor effect regarding the wave reduction. All CWB arrangements showed increasing performance with decreasing wave periods. Besides, the wave transmission was mainly impacted by the level of submergence together with the amount of supporting piers and the thickness of the structures. Water depth changes due to tidal influence revealed an increase in wave transmission coefficient once the wave started to overtop the structure.

This study aims at evaluating the geographical influences of rice-based protection dykes on floodwater regimes along the main rivers, namely the Mekong and the Bassac, in the Vietnamese Mekong Delta (VMD). Specifically, numerous low dykes and high dykes have been constructed particularly in the upper delta’s floodplains to protect the double and triple rice cropping against the annual flooding. For the whole deltaic domain, a 1D-quasi-2D hydrodynamic model setup was used to simulate seventy-two (72) scenarios of dyke construction development in the context of low, medium, and high floods that occurred in the VMD to examine the effects of different flood magnitudes on a certain dyke construction area. Based on the model simulation results, we established an evaluation indicator, the so-called Geographical Impact Factor (GIF), to evaluate the impacts of zone-based dyke compartments on the floodwater regimes along the main rivers for different kinds of floods. Our findings revealed different rates of influences on the floodwater levels along the Mekong and Bassac Rivers under different scenarios of zone-based high-dyke developments. GIF is a useful index for scientists and decision-makers in land use planning, especially in rice intensification, in conjunction with flood management for the VMD and for similar deltas worldwide.

Climate change and its consequences can only be stochastically assessed on the basis of available data over a long time series. Extreme weather events, floods, storms and rising sea water level already show strong changes and effects on the water systems and the water balance in short periods. Holistic and interdisciplinary approaches are therefore needed, which are not available at comparable levels around the world

This paper presents the first attempt to capture a comprehensive spatial view of land use change in the Vietnamese Mekong Delta (VMD) for a long period, i.e., from 2000 to 2020. It is aimed at monitoring holistically the land use change and flooding situation in the region, addressing the reasons for land use change, and assessing the impacts of land use change on hydraulic aspects and farmer livelihoods during the last 21 years. MODIS products, in particular, are used to study the dynamics of land use and floods after demonstrating high validation with statistical data and radar satellites, with R2 = 0.96 and R2 ≥ 0.97 for land use and flood maps, respectively. The results show that rice cultivation is the most dominant land use type, accounting for 40% to 46% of the delta area, while aquaculture accounts for 10% to 22%, respectively. The total rice cultivation area increased from 3764 thousand hectares (thous. ha) in 2001 to 4343 thous. ha in 2015 based on the intensive development of triple rice cropping in the upper zone, then decreased to 3963 thous. ha in 2020. In contrast, aquaculture areas are farmed mainly in the coastal area and remained relatively steady, increasing slightly from 619 thous. ha in 2001 to 856 thous. ha in 2020. The massive construction of dikes for triple rice cropping in the upper zone appears to cause a significant impact on the annual flooding regime. Land use policies have influenced the changes in land use patterns, flooding situations, and the livelihoods of local farmers.

Eine stochastisch basierte Bewertung von Klimaveränderungen und den Folgen ist nur auf der Grundlage von verfügbaren Daten über lange Zeitreihen möglich. Extreme Wetterereignisse, Hochwasser, Stürme und ansteigender Meereswasserspiegel zeigen bereits jetzt spürbar starke Veränderungen und Auswirkungen auf die Gewässersysteme und den Wasserhaushalt. Es bedarf daher ganzheitliche und interdisziplinäre Ansätze, die nicht überall auf der Welt auf vergleichbarem Niveau vorhanden sind.

For the simulation of urban flooding methods from river modeling are used. However, transferring the empirical formulas and parameters to the widely differing boundary conditions (a.o. including flow depth, slope) is only possible to a limited extent. The article discusses the evaluation of a large number of published field and laboratory tests as well as fundamental considerations on the practical suitability of common approaches and parameters taking into account the flow resistance in extreme precipitation events. In this context it is shown that the application of the Darcy-Weisbach formula using logarithmic equations (e.g. Colebrook-White) for the calculation of the resistance co-efficient. can lead to unintended retention effects. Also small-scale runoff concentration on natural surfaces result in a significant spread of corresponding resistance parameters.

Bei der hydraulischen Starkregensimulation kommen Verfahren aus der Fließgewässermodellierung zum Einsatz. Eine Übertragung der empirischen Formeln und Parameter auf die stark unterschiedlichen Randbedingungen (u. a. Fließtiefe, Neigung) ist jedoch nur eingeschränkt möglich. Der Beitrag stellt die Auswertung einer Vielzahl publizierter Feldund Laborversuche sowie grundsätzliche Überlegungen zur Praxistauglichkeit gängiger Ansätze und Parameterangaben zur Berücksichtigung des Fließwiderstandes bei Starkregen zur Diskussion.

Within the joint project Integrated Water Governance Support System (iWaGSS) funded by the German Federal Ministry for Education and Research (BMBF, reference numer: 02WGR1424C) the Institute of Water and River Basin Management (IWG) of the Karlsruhe Institute of Technology (KIT) developed a benthic flume. The benthic flume HIPPO (Hydro-morphological Investigation of riverbed Particle Performance On-site) is an adjustable in situ device to reliably determine the start of erosion of fine sediments. In advance 3D-CFD simulations have been carried out to optimize the components and the setup of the measurement system. The final product is primarily a benthic flume, which has a downwardly opened sampling area at the bottom and is placed on the river or reservoir bed. This underwater flow channel can be adapted to the local conditions with further components and is connected via a tube system to a measurement boat or raft. On the boat a pump creates a steady flow velocity in the system. The velocity in the benthic flume is gradually increased at fixed time intervals and is monitored using a built-in flow velocity meter (Acoustic Doppler Velocimeter). In addition the entire erosion process is recorded visually with video cameras. Also the turbidity of the water flowing through the system is continuously measured by a turbidity probe installed behind the pump. The amount of flow induced by the pump is controlled by a valve close to the end of the system. With the pump currently installed flow velocities of up to v = 0.8 m/s at the sampling area can be achieved, which is sufficient for the determination of the critical flow rate for erosion of most types of clay, silty and fine sandy sediments. During the process of erosion also the remobilization of fluid mud can be monitored. The critical flow velocity for the start of sediment transport is determined on the basis of the turbidity of the pumped water and data from the flow velocity probe and is verified using the camera system. In addition to the critical threshold flow velocities, the critical bed shear stress is often required as input or evaluation variables for morhpodynamic numerical models. The conversion can be made, for example, using the quadratic velocity approach originally used in pipe hydraulics. The determination of the required resistance coefficient λ is based on the Moody Chart. However, it should be considered that this procedure entails some uncertainties with regard to the measurement system presented here. Still for cohesive sediments, the natural values measured in this way represent a significant added value compared to common estimates based on only partially known bed parameters, since factors such as vegetative cover, consolidation or even a developed biofilm can influence the timing of erosion. Especially against this background, possible effects of the change of hydraulics by the measuring system (geometry, velocity profile) seem to be small compared to the uncertainties of contemporary morphodynamic analyses.

In many structurally disadvantaged regions of the world, limited or highly fluctuating water resources lead to a deficient water distribution. Since conventional water distribution systems are designed for a demand-covering operation, a non-demand-covering operation results in a hydraulically induced unfair distribution of the available water. Therefore, the subject of this report is the development of an adapted water distribution system based on simple approaches that allows a fair and adequate water supply also during non-demand-covering operation scenarios. In order to test the function of the adapted water distribution system under real conditions, a demonstration project was implemented in a karst region in northern Vietnam as part of the joint research project KaWa-Tech sponsored by the German Federal Ministry of Education and Research (BMBF). Based on the developed concept, an adapted water distribution system was designed, built and put into operation to supply several thousand people in the project region. In course of the system evaluation, a proper operation was proven, thus ensuring the sustainable and fair supply of the local people.

Extensive research of the variability of flows under the impact of climate change has been conducted for the Upper Indus Basin (UIB). However, limited literature is available on the spatial distribution and trends of suspended sediment concentrations (SSC) in the sub-basins of UIB. This study covers the comparative assessment of flows and SSC trends measured at 13 stations in the UIB along with the variability of precipitation and temperatures possibly due to climate change for the past three decades. In the course of this period, the country’s largest reservoir, Tarbela, on the Indus River was depleted rapidly due to heavy sediment influx from the UIB. Sediment management of existing storage and future planned hydraulic structures (to tap 30,000 MW in the region) depends on the correct assessment of SSC, their variation patterns, and trends. In this study, the SSC trends are determined along with trends of discharges, precipitation, and temperatures using the non-parametric Mann–Kendall test and Sen’s slope estimator. The results reveal that the annual flows and SSC are in a balanced state for the Indus River at Besham Qila, whereas the SSC are significantly reduced ranging from 18.56%–28.20% per decade in the rivers of Gilgit at Alam Bridge, Indus at Kachura, and Brandu at Daggar. The SSC significantly increase ranging from 20.08%–40.72% per decade in the winter together with a significant increase of average air temperature. During summers, the SSC are decreased significantly ranging from 18.63%–27.79% per decade along with flows in the Hindukush and Western–Karakorum regions, which is partly due to the Karakorum climate anomaly, and in rainfall-dominated basins due to rainfall reduction. In Himalayan regions, the SSC are generally increased slightly during summers. These findings will be helpful for understanding the sediment trends associated with flow, precipitation, and temperature variations, and may be used for the operational management of current reservoirs and the design of several hydroelectric power plants that are planned for construction in the UIB.

Im Norden Vietnams haben Fachleute des Karlsruher Instituts für Technologie (KIT) ein wasserkraftbetriebenes Pumpsystem zur Trinkwasserversorgung installiert. Die Wasserverteilung basiert auf innovativen Verteilerbauwerken und ermöglicht auch in trockenen Zeiten eine gerechte Verteilung des wenigen verfügbaren Wassers.

The accurate estimate of sediment load is important for management of the river ecosystem, designing of water infrastructures, and planning of reservoir operations. The direct measurement of sediment is the most credible method to estimate the sediments. However, this requires a lot of time and resources. Because of these two constraints, most often, it is not possible to continuously measure the daily sediments for most of the gauging sites. Nowadays, data-based sediment prediction models are famous for bridging the data gaps in the estimation of sediment loads. In data-driven sediment predictions models, the selection of input vectors is critical in determining the best structure of models for the accurate estimation of sediment yields. In this study, time series inputs of snow cover area, basin effective rainfall, mean basin average temperature, and mean basin evapotranspiration in addition to the flows were assessed for the prediction of sediment loads. The input vectors were assessed with artificial neural network (ANN), adaptive neuro-fuzzy logic inference system with grid partition (ANFIS-GP), adaptive neuro-fuzzy logic inference system with subtractive clustering (ANFIS-SC), adaptive neuro-fuzzy logic inference system with fuzzy c-means clustering (ANFIS-FCM), multiple adaptive regression splines (MARS), and sediment rating curve (SRC) models for the Gilgit River, the tributary of the Indus River in Pakistan. The comparison of different input vectors showed improvements in the prediction of sediments by using the snow cover area in addition to flows, effective rainfall, temperature, and evapotranspiration. Overall, the ANN model performed better than all other models. However, as regards sediment load peak time series, the sediment loads predicted using the ANN, ANFIS-FCM, and MARS models were found to be closer to the measured sediment loads. The ANFIS-FCM performed better in the estimation of peak sediment yields with a relative accuracy of 81.31% in comparison to the ANN and MARS models with 80.17% and 80.16% of relative accuracies, respectively. The developed multiple linear regression equation of all models show an R2 value of 0.85 and 0.74 during the training and testing period, respectively.