Live Pole Drains (LPDs) are a plant-based drainage system used to drain natural slopes and prevent shallow gully erosion. LPDs are a Nature-based Solution built by placing a live fascine in a shallow ditch or gully along the slope direction, allowing moderate fluxes of surface runoff or seepage to infiltrate and high water fluxes to be conveyed along the fascine without further eroding the slope. Despite their practical implementation, the transient and long-term eco-hydrological behavior of LPDs is not well understood. We aim to better understand the LPD’s water balance, the seasonal and life-span changes in hydrological behavior, as well as the impact of an LPD on surface runoff water quality. To this end, we built and instrumented an artificial slope with full-scale LPDs in an open-air lab (OAL) at TU Delft. The design of the setup and the monitoring plan of the LPDs were developed in collaboration with Glasgow Caledonian University with insights from the construction and monitoring of three LPDs at different growth stages in their OAL on the east coast of Scotland. Herein, the design and possible research experiments that can be performed over the next 5 years are presented, generating a data set to further develop and validate hydrological modeling of LPDs. We expect this long-term demonstrative setup to generate interest and facilitate a more comprehensive understanding of LPD functions, ultimately leading to the incorporation of LPD design and maintenance standards in engineering toolboxes for slope and gully stabilization.

Rivers play a crucial role in shaping landscapes and supporting ecosystems. This is demonstrated by the tiger habitats in and around Bardia National Park in West-Nepal, which rely on the Karnali River. This study contributes to a larger effort aimed at sustainably managing these tiger habitats. Monitoring the rivers in this remote area is challenging, suggesting a role for remote sensing. An exploration is presented regarding the potential of satellite synthetic aperture radar altimetry (sat-SARA) for monitoring rivers situated in diverse topographic landscapes. Focusing on the Bheri, Karnali, and Geruwa Rivers, the applicability of sat-SARA techniques for water level monitoring, multiple channel identification, and channel activation detection was evaluated. For deriving water surface heights from sat-SARA data, an empirical Gaussian retracker was used. The findings are promising. While resulting water level variations align with field observations, complementary in-situ measurements are imperative for a comprehensive evaluation. Additionally, the study reveals the potential for identifying multiple channels from sat-SARA return signals, extending to channel classification and detecting channel activation. Leveraging the labour-intensive nature of sat-SARA data processing, the technique holds great promise for monitoring rivers in remote and difficult-to-access landscapes. Therewith, this study contributes to advancing the understanding of the hydrodynamics of the Lower Karnali River and opens doors for sat-SARA applications for river monitoring in challenging terrains.

The study of water is without question an important one. We can learn a lot by studying the theory but in the end, it is essential to actually go out into the field and see the hydrological processes in practice for ourselves. The educational value of fieldwork lies not only in seeing theory become reality but also in experiencing the difficulties and limitations of gathering data first-hand.

The water resources department of the Hanoi University of Natural Resources and Environment (HUNRE) wanted to expand its curriculum with a fieldwork excursion that will be part of three courses related to surface water, groundwater, and water quality. During our ten weeks in Vietnam, we assisted the teaching staff of the Water Management faculty in the development of these courses. We selected multiple experiments and found suitable locations for their execution, about a three-hour drive from the university. Several times we visited the fieldwork site together with students and teachers to explore the catchment, perform experiments, and gather data. For every experiment, a comprehensive manual was written with accompanying assignments and sheets tailor-made for the fieldwork excursion. All manuals are combined into a practical document that can be brought into the field.

The experiments require a wide variety of specific pieces of equipment. Most but not all of these were present at the university. With financial support from the Orange Knowledge Project (OKP), we were able to repair existing equipment and acquire new equipment that was lacking in order to facilitate the experiments that were deemed essential. We also re-organized part of the water lab where all equipment is stored to improve the equipment’s maintenance and organization.
With the completion of this project, we believe to have made a contribution to the improvement of the quality of education at HUNRE. We hope that a lot of students can benefit from our efforts as they go on the fieldwork excursion in the coming years.

Groundwater extraction has increased significantly in Nepal. In combination with climate change, this might lead to accelerated groundwater resource depletion. No recent research has been done in the assessment of these resources in the Banke district development area in the Terai. This study aims to assess whether the intensification of extractions has led to a depletion of groundwater in the upper aquifer. We investigated six local case studies during the dry season with a sole focus on the upper aquifer. The hydrological fluxes were quantified at every location. Evaporation and recharge were estimated using remote sensing data and locally obtained meteorological data. Domestic extractions, irrigation return flow and irrigation extractions were approximated by fieldwork. The groundwater storage change over the period was estimated using a recorded groundwater table time series and the Water Table Fluctuation method. The net subsurface flow- and percolation were estimated by closing the water balance for every case study. Subsequently, the fluxes at local case studies were extrapolated using Groundwater Response Units. The groundwater table replenishment was estimated using the relation between effective precipitation and groundwater levels over two years. The net extraction from the groundwater was insignificant compared to the contribution of evaporation. The approximate minimum precipitation needed for the monsoon season to recover the shallow aquifers after the dry season was on average comfortably exceeded over the last ten years. Thus, the groundwater resources are currently not depleting due to the intensification of extractions in the upper aquifer. However, the current extractions from deeper aquifers might decrease the pressure in deeper layers increasing percolation. Eventually, this endangers the groundwater resources in the upper aquifer. The limited number of measurements in deeper wells already indicated potential depletion in deeper layers. Further research is therefore recommended in deeper aquifers.

The Vietnamese Mekong Delta, a vital region in the country’s economy, faces the dual challenges of coastal erosion and mangrove degradation, which threaten its long-term sustainability and flood protection capabilities. This research focuses on the coastal area of the Bac Lieu province, characterized by severe erosion and degrading mangrove forests. The study investigates the applicability and potential impacts of hydraulic measures to decrease the net rate of coastal erosion, utilizing numerical modeling with Delft3D and a comprehensive socio-economic analysis. The research hypothesizes that the coastal erosion is partly driven by the placement of a sea-dike to protect aquaculture farms, initiating a positive feedback loop. This loop explains the relation between coastal erosion and mangrove degradation. The proposed hydraulic measures to interfere with this feedback loop are a porous detached breakwater, a shoreface nourishment and the removal of the existing sea-dike. The socio-economic analysis involves questionnaires for local residents, field investigations, and insights from experts in Ho Chi Minh City. While the questionnaires provide inconclusive results, the overall socio-economic impact of the nourishment and breakwater is deemed positive and worth further exploration, particularly in light of the critical role of mangroves in future flood protection. On the other hand it is concluded that the measure of removing the sea-dike will have a negative impact on the coastal area of Bac Lieu due to the intensive land-use and the lack of individual protection of the farms and villages. Therefore, this measure is not modelled. Numerical modeling with Delft3D assesses the hydraulic impact of the breakwater and nourishment on the heavily eroded and partially eroded coasts of Bac Lieu. Results indicate that the nourishment method exhibits a positive effect in reducing net erosion, especially in low energy conditions. Conversely, the porous breakwater shows minimal impact on cumulative erosion and sedimentation. Since this is against all expectations, the validity of the schematization of the porous breakwater is questioned. It is observed that the schematization does not grasp the complex behaviour of the breakwater and therefore it is concluded that Deft3D is not a suitable modelling tool for modelling a porous breakwater. The findings suggest that the nourishment method is a promising approach for reducing erosion in Bac Lieu, benefiting both the heavily and partially eroded coasts. To determine the best course of action for Bac Lieu, further research into the long-term effects and configurations of nourishment is recommended. Additionally, informing local inhabitants on the threats of relative sea-level rise and flood protection, and fostering consensus between the government and engineering agencies on the importance of protecting the Mekong Delta and its mangrove ecosystems are essential steps toward a more resilient future.

The Mekong Delta is facing some complicated challenges in the near future. Its geographical location and the fact that it is a delta result in low elevation levels which makes it vulnerable to inundation. This problem will only become bigger in the future due to the effects of climate change. To get funding from organisations like the world bank it is important to propose multiple solutions that are beneficial to multiple parts of society. It is favourable to split
a complex system like the Mekong Delta into subsystems to make it more feasible to build realistic models. The subsystem defined in this report is the Hau River estuary. This area mainly suffers from riverine inundation caused by tidal variation in the South Chinese Sea. The biggest city in the region is Can Tho with 1.3 million inhabitants.

The research question is: Which integrated solutions reduce riverine inundation problems in the Hau River estuary while also considering socio-economical aspects? To answer this research question the following four solutions are proposed and designed.

• Discharge sluice in the mouth of the Hau River to reduce the tidal influence

• Wetland with a double levee system and buffer zones to reduce peak discharge

• Bypass channel to the Gulf of Thailand to reduce discharge during the wet season

• Protection of valuable assets and adaptation of local citizens to the new natural balance

Based on desired discharges and water levels preliminary design parameters of the proposed hydraulic structures were determined. The effectiveness of these solutions was assessed based on their ability to reduce the water level in Can Tho. The reduction that the discharge sluice achieved was determined with a zero-dimensional model, whereas the water level reduction that the wetlands and bypass option achieved were determined by Delft3D models. The discharge sluice performed best in reducing the water level in Can Tho, as it opposes the tidal influence in the Hau River.

To assess the quality of the solutions relative to each other a best-worst multi-criteria analysis is done. In this assessment other factors such as financial aspects, socio-economics and transportation are taken into account. The most important criteria are flood reduction and funding opportunities. According to the assessed criteria, the discharge sluice and the wetland are the best-scoring solutions. These solutions have the most potential in reducing the river inundation problems in the Hau River estuary. This does not mean that the bypass and adaptation solutions should be neglected or are not useful. For a complex problem in a complex system like the Hau River estuary, one solution is not going to solve all the problems. A good balance between different aspects has to be determined by also considering other problems like sand mining, subsidence and salt intrusion.

For the development of regional landslide early warning systems, empirical-statistical thresholds are of crucial importance. The thresholds indicate the meteorological and hydrological conditions initiating landslides and are an affordable approach towards reducing people’s vulnerability to landslide hazards. This thesis defined different landslide hydro-meteorological thresholds in Rwanda and evaluated their predictive capabilities. Chapter 1 identifies the landslide problem to society, opportunities for possible solutions, overview of the previous research and knowledge gap. It defines the research concepts, research objectives and outlines...

Climate change causes an increase of precipitation intensity and frequency, which leads to an increased amount of landslide occurrences. Therefore, there is an urgent need for prevention measures. The risk of landslide occurrence can be mitigated by decreasing the pore water pressure in a hillslope, e.g. by drainage from the subsurface. The implementation of Nature Based Solutions (NBS) is advantageous, as these are not disruptive for the hillslope ecosystem. A Live Pole Drain (LPD) is a type of NBS which is inexpensive and can be installed at remote locations. However, its effect on hillslope hydrology has not been studied before.

In this study, fieldwork at an Open Air Lab, a laboratory experiment with miniature LPDs and the development of a conceptual model were used to characterise the hydrological behaviour of a slope in which a LPD is installed. After comparison of this hydrological behaviour to that of a slope with bare soil, it was found that a LPD facilitates rapid infiltration and drainage of precipitation and subsurface water. Furthermore, runoff volumes decrease after installation of a LPD and this effect is enhanced through plant development. Vegetation also contributes to the decrease of the stored volume of water in the subsurface through evapotranspiration. An increase of the design aspects diameter and macropore fraction of the LPD increases its drainage capacity. This study resulted in a first version of a numerical model to quantify the hydrological processes in a hillslope with LPD and the confirmation of the expected effect of drainage through preferential flow by a LPD.

Due to rising temperatures, rainfall patterns around the world are being affected, causing extreme precipitation events to become more frequent and intense, resulting in an increased probability of severe flash floods. Thailand is no exception to the increased risk of these hazards, which is why Early Warning Systems are being set up. Since flash floods occur within a few hours after the triggering precipitation event, timely and accurate precipitation observations are critical, to enable timely warning and evacuation of inhabitants to mitigate the risk. One solution for this is the use of rain radar, which provides rain data with high spatial and temporal resolutions.

An analysis of this technique was chosen to form the basis of this research in Northeastern Thailand's Lam Takhlong basin. The objective was to investigate the importance of using distributed precipitation data. In this research, the hydrological response of the catchment of interest was studied. The ability of the physically-based, conceptual and distributed CALEROS model to capture this response was assessed. Different modelling strategies and sources of precipitation input were analysed, and the additional value of using distributed precipitation data was determined. The examination of multiple hydrological characteristics in the study area shows great heterogeneity of the catchments response to precipitation. A clear differentiation can be made between the hydrological response of the upstream and downstream part of the catchment. Additionally, the results from the catchment characterisation indicate great spatial variability of the precipitation patterns in the study area. This is confirmed when using the CALEROS model to recreate the catchments hydrology.

Four modelling strategies were used, varying by spatial and temporal constancy. Calibration performed using uniform precipitation showed to be incapable of capturing the discharge trends in the catchment, failing to properly catch the observed peaks as well as the base flow. Runs performed using distributed precipitation maps obtained by Inverse Distance interpolation of rain gauge data showed significantly better results, adequately capturing the trend of the discharge observed in the catchment. Differences in parameterisation only had limited effect on the outcome, making the precipitation input the most important parameter. Runs using synthetic precipitation data demonstrated the importance of properly capturing the precipitation pattern and movement across the catchment, as it greatly influences the timing of occurring discharge peaks.

A comparison between rain gauge data and rain radar data shows that, although data quality of the rain gauges seems acceptable, rain gauges are not capable of properly capturing rainfall patterns, while rain radar does. Precipitation patterns were found to be the most crucial parameter for modelling of flash floods in the area of interest, signifying the importance of using rain radar data for accurate flash flood forecasting.

Leaching of phosphorus (P) from agricultural land is an important source of eutrophication in the Bollenstreek in the Netherlands. This is because floriculture has led to high P concentrations in the sandy soils with low organic carbon content, which are characterised by a low retention capacity for P. In this study, P concentrations in groundwater from three plots in the Bollensteek were found to exceed the water framework directive (WFD) by a factor of 40. The case study on a single plot showed a phosphate load of 5.5 kg/ha/year leached through the tile drains, which is an order of magnitude higher than fields outside the Bollensteek. To understand where phosphate comes from and how hydrological processes influence leaching, hydraulic and chemical measurements of soil, groundwater and tile drain outflow were taken in a field with regulated tile drainage, and two hydrological models were created to simulate wet winter and dry summer conditions. The results showed the flow paths from this plot under wet and dry conditions and illustrated how hydrological processes can influence phosphate leaching through the soil and tile drains.

Recirculation and infiltration of leachate in landfills are performed to accelerate the process of stabilizing organic matter in waste. At landfill De Kragge (Bergen op Zoom, the Netherlands), leachate recirculation and infiltration measures started in March 2018. This research aims to provide insight related to leachate flow throughout the landfill. Knowledge about the leachate flow is essential for evaluating the success of the stabilization measures. In this research, uniform and point borehole dilution tests were conducted to investigate the horizontal and vertical flow velocities. In addition, measured leachate levels throughout the landfill were analyzed.

The leachate levels indicated perched leachate zones: above the basal drainage system and below the injection drains at the top. Leachate injected through the infiltration drains cannot efficiently infiltrate the waste body, and the effects of the infiltration events were not picked up in the wells and piezometers throughout the landfill, implying little hydraulic connectivity.

The results of the dilution tests indicated horizontal and vertical flow within the landfill. Vertical velocities measured in the wells were estimated to be considerably higher (77 - 225 m/d) than the average horizontal velocities (0.02 - 1.0 m/d). The wells provide a path for vertical flow. Apart from the highest horizontal velocities measured in deeper sections of the landfill (15-18 m below ground level), velocities varied without a clear relation to landfill depth, indicating preferential flow paths. Uniform dilution tests performed with the infiltration drains turned off suggested that leachate infiltration does not increase the horizontal velocities. This research suggests that waste stabilization through recirculation is not optimal at De Kragge.

\noindent Due to an overall lack of understanding about the well and filter pack installation and the high spatial heterogeneity of the waste, the calculated velocities are uncertain. Further research into possible error sources (e.g., the borehole correction factor) is recommended. Additional tracer tests, including tests on the neighboring compartment without stabilization measures, are recommended to further assess the effectiveness of the recirculation system.

Under the current climate change projections, droughts and extreme rainfall events are becoming more common and intense. During sustained droughts, soil cracks can develop in the topsoil of a dike. These soil cracks can enhance the infiltration and drainage of the dike, affecting the dike stability. This study’s main objective is to better understand the potential influence of cracks on the hydrology and dike stability of canal dikes. A finite element model (FEM) was built to study the hydrological response and stability of a canal dike due to rainfall infiltration through a uniformly distributed cracked top layer. This model included the fast hydrological response of the fractures and their exchange with the soil. A surrogate model (a simplified data-driven machine learning model) was built using training data generated by the FEM to reduce the computational burden and allow for a sensitivity and stochastic reliability assessment. An evaluation of important model features, such as the connectivity among cracks and soil swelling, revealed that the cracks have a very localized influence on the infiltration and drainage of water through the dike. Water drainage in the toe of the dike is restricted by soil swelling resulting in a larger decline in dike stability. The sensitivity analysis found that the cracks facilitate more infiltration than drainage before the crack parameters, such as the crack aperture and the amount of cracking, reached a threshold. Above this threshold, drainage provided by the cracks is greater than the infiltration. The failure probability was calculated for a variety of different rainfall patterns. A pattern where the rainfall intensity incrementally increases over time results in the highest failure probability. The main conclusion from this study is that soil cracks influence the stability of the dike by enhancing infiltration and drainage under rainfall. Under low-intensity rainfall, the cracks in the toe facilitate more water to drain out of the dike than rainfall to infiltrate, stabilizing the dike. However, in the absence of cracks in the toe, the cracks provide more infiltration than drainage resulting in a lower dike stability. Under high-intensity rainfall, cracks in the crest and the inner slope of a dike advance the wetting front resulting in a larger decline in dike stability than a dike without soil cracks. Rainfall infiltration and drainage in a cracked dike depend on both rainfall characteristics, such as the average rainfall intensity, duration, and pattern, and the crack features, such as the crack width, amount of cracking, and the connectivity of the cracks. Since this study was not validated with observations, it is recommended that further studies aim to validate the observed influence of the cracks by conducting field experiments.

Empirical-statistical rainfall landslide initiation thresholds are popularly used for early warning systems to discriminate between the occurrence and non-occurrence of rainfall-induced landslides. However, the few studies that have derived landslide initiation thresholds for landslide-prone and data-scarce Rwanda rely solely on the limited in situ data. Therefore, our objective is to explore the feasibility of using satellite data and hydrological model derived data to derive both trigger and trigger-cause thresholds for landslides in Rwanda. We firstly evaluated seven precipitation products (TRMM 3B42v7, CHIRPS, PERSIANN-CDR, GLDAS 2.1, CFSv2, IMERG, and ERA5) using the rain gauge data as a reference and found that IMERG was the most suitable product for obtaining rainfall triggering conditions. We then studied the added value of incorporating the antecedent soil moisture from both a high spatial satellite data and from a distributed hydrological model following the trigger-cause framework. The results showed that the event precipitation volume E, the event duration D and the bilinear threshold E-D are the landslide initiation thresholds that accurately predict the highest number of landslide events while keeping the false and the failed alarms low. Including the antecedent soil moisture products as the causal variables -expected to account for the hillslope hydrologic processes predisposing the slopes to near failure- did not lead to any improvement with respect to the trigger only thresholds for predicting landslides in Rwanda.