Plastic pollution and accumulation in the riverine environment is of increasing concern. While most research focuses on microplastic contamination, the dynamics of macrolitter remain largely unknown. Large scale riverbank monitoring initiatives in the Netherlands reveal that macrolitter hotspots occur at several locations. Unfortunately, current knowledge on how these hotspots emerge and how this is influenced by hydrology and meteorology remains limited. As most studies are based on data from seasonal monitoring activities, short term variability remains unknown. This study is the first attempt to monitor and analyse the variability of riverbank macrolitter within a single location for over a period of three months. Behaviour of individual items is tracked and macrolitter exchange between water and riverbank is studied with regards to hydrology and wind. Finally, a conceptual model on riverbank macrolitter dynamics is presented in favour of supporting future research design.
A remote groyne field in the Waal has been monitored 21 times within the period of November 2021 until January 2022. The location of macrolitter items was recorded using Real Time Kinematic positioning. This allowed for analysing spatial patterns throughout time. Additionally, photographs of items were made in order to categorise the items without removing them from the riverbank. The river OSPAR protocol was used for item categorisation.The data shows that macrolitter primarily accumulates in the floodmarks. Rising water pushed items higher on the riverbank. Wind had a limited effect on item mobilisation as most items are wet and sandy. Analysis of item exchange between riverbank and water revealed that macrolitter deposition was observed at a relatively constant rate with minor deviations. Item uptake was heavily dependant on changes in water level. Uptake was initiated when the water level rises (dH > 0). The rate of uptake was higher with a larger water level increase. However, correlation was not statistically significant as riverbank morphology, substrate and vegetation may also influence uptake. After three months and two moderate discharge peaks, almost all items found on day 1 (estimated 99.6%) had been taken up. This indicates that under normal hydrologic conditions, the retention time of items within groyne fields is defined by the timing and magnitude of moderate water level fluctuations (assuming no accumulation under water within the groyne fields).
A conceptual model of riverbank macrolitter dynamics under natural processes is presented. Macrolitter can be stored in three domains: water, sediment and riverbank surface. Exchange within these domains occurs in four directions: in/out of sediment (storage/mobilisation) and in/out of water (uptake/depositions). Exchange is promoted by an interplay between item attributes, environmental processes and riverbank morphology. Future research on the interaction between these variables is needed in order to fully understand macrolitter dynamics. ... Read more

Flash floods are a damaging and recurring problem in Cebu city, Philippines. Very little data is known about the intensities and precipitation amounts and the resulting river discharges. This research project firstly aims to gather as much data as possible on precipitation and river discharges that could cause the floods, it focuses on a small catchment in the city called the Mahiga catchment. Data is gathered by installing three tipping buckets and two trail cameras. The cameras were able to calculate the river discharges using an innovative open-source program called OpenRiverCam. Thanks to this program a hydrograph can be
made of the river for each precipitation event. The used cameras were trail cameras of the Brand Bushnell. During this project it was concluded that, due to their unreliability, using trail cameras with OpenRiverCam is really not recommended. Security cameras with a Raspberry Pi are more suited. Due to bad luck with the weather and faulty material only three different hydrographs could be made during our time abroad (10 weeks). These hydrographs however remained useful for the second part of this research project. The second part consists of modelling the discharge of the Mahiga catchment to different
precipitation amounts using HEC-RAS. HEC-RAS is a computer program meaning Hydrologic Engineering Center’s River Analysis System. The model has been calibrated using the gathered precipitation data from the tipping buckets and the discharge results from OpenRiverCam. Graphs have been made about discharges and accumulated volumes and rating curves. The accuracy of the model is reasonable but should be improved using more discharge events. What stood out was the high infiltration rate and the fast response time of the Mahiga catchment. In section three, the results from the HEC-RAS model are used to understand the impact gabion dams make on reducing the peak flow in the Mahiga creek.
The third part summarises the effectiveness of the gabion dams in preventing flash floods. Unfortunately there is no ’real’ flash flood event captured by the tipping buckets, so three precipitation events are used based on analog measurements of a tipping bucket nearby the catchment. The gabion dams are tested on a maximum precipitation intensity of 35 mm/h, 30 mm/h and 25 mm/h with a total amount of 40 mm. Higher amounts of total precipitation
are realistic, but have a larger time duration and are not considered flash floods anymore. The volume that gabion dams can retain is too little for these large amounts of precipitation and are therefore not in the scope of this report. The results show that with at least five gabion dams, the peak flow reduces for all above mentioned precipitation intensities, but for the 35 mm/h it is getting less effective. The model also showed that the effectiveness is very dependent on the volume that can be retained by the dams. Maintenance of the gabion dams is therefore of crucial importance especially with the large amount of sediments and
debris in the creek. ... Read more

Plastic pollution in aquatic ecosystems is a global problem. Rivers transport large quantities of litter from land to the oceans. Plastics of different sizes and properties are widely present at various locations in river systems. Concerning macroplastics, previous studies mainly focussed on floating plastics, however, substantial parts of the litter can be transported underneath the water surface. Currently, this suspended litter load remains understudied. Submerged litter is presently monitored with the use of nets. The use of nets has several disadvantages. In large rivers, nets can usually, not be deployed over the full depth and width of the river. Moreover, using nets is labour intensive and requires fixed structures to be deployed. To overcome this problem, this thesis aimed to develop and test a new method for monitoring submerged riverine litter. In accordance with the fish detection abilities of sonar, it seems to have a high potential for identifying suspended underwater objects.

In this study, the litter detection ability of a low-cost single beam echosounder (Deeper CHIRP+) were investigated. Three different experiments were executed, in specific, controlled tests in an artificial environment, semi-controlled tests in a natural environment and litter monitoring in a naturally flowing river. The controlled tests, to get an insight into the scanning technique and detection abilities of the echosounder, were performed in the Kerkpolderbad in Delft. During these tests, the influence of actual object size, object depth and flow velocity on the sonar signal was investigated. The semi-controlled tests were carried out in the Rio de San Pedro, in Andalusia, Southern Spain. During these tests, several plastic targets were used and repeatedly released in the river, passing the sensor. For this, objects of different material properties and sizes were used. Lastly, plastic was monitored in the Guadalquivir and Guadalete rivers in Andalusia. In the Guadalquivir river, the sensor was operated together with nets for validation purposes. In the Guadalete river, monitoring took place for 18 hours from a pedestrian bridge, at different locations over the cross-section of the river and under varying tidal conditions.

The performed tests showed a significant relationship between the dimensions of the reflection signal, derived from the sonar observations and the actual object size. However, object orientation and deformation play a role and lead to deviations in the signal dimension results. A second relation, regarding flow velocity and signal dimensions, was observed. The larger the flow velocity, the smaller the sonar signal. Additionally, signal intensities can, for four out of the eight objects tested, be related to material properties but differences in signal intensities are relatively small. Regarding the river monitoring activities, suspended litter items can be counted, river tide is influencing litter transport and litter is present over the full river depth.

The following main conclusions are drawn based on this research:
- Echo sounding can be used for detecting suspended riverine macroplastics. Litter items can be counted, and fish can be discarded from the sonar readings by their specific displayed shapes.
- Litter size can be estimated when looking at the sonar readings, however, several factors, such as flow velocity, object orientation and deformation have to be taken into account when estimating litter size.
- In the Guadalete river, significantly more suspended litter is transported when river water flows into the sea compared to river water flowing inland. The counted litter items were approximately uniformly distributed over the river depth.

In general, using echo sounding for suspended litter monitoring is potentially useful to gain a better understanding of the suspended litter transport, from which prevention and mitigation strategies could be optimised. For further research, it is recommended to use an echosounder for which the raw sonar data can be exported as a standard digital file. Moreover, the set of test objects should be extended, including more variation in object size. To separate signal size and signal intensity, objects of different size but same material properties and objects of the same size but different material properties should be used for testing. Finally, other types of sonar such as side scan or multibeam sonars may potentially lead to more accurate sonar readings regarding litter size and material estimations.
... Read more

Along river banks, on the transition zone between the river and see, tidal marshes can develop. The tidal marshes accommodate multiple plant and fish species. As a result of the stressful conditions in tidal marshes, unique intertidal ecology develops. However, due to the densifying of the river banks, tidal marshes are disappearing.
Restoration of tidal marshes can reintroduce the unique intertidal ecology on several locations in the estuary. To obtain restoration of tidal marshes, constructed tidal marshes come into play, which can be built at designated places along the river.
However, constructed tidal marshes are not necessarily built to restore the unique tidal nature, but can also have other functions as recreation and contribution in green city area.

Nevertheless, is it still unknown how constructed tidal marshes behave and what the optimal design is. Namely, a mismatch exists between the policymakers and designers on the one side and academic knowledge on the other side. To fill up this gap, accessible knowledge from the experts should be available for designers.
Therefore, this research provides guidelines for designers and gains more insight into the behaviour of constructed tidal marshes. This thesis focusses on extracting general knowledge from the results of a numerical model, applied on a case study.

Simulations of a numerical 1D Sobek model applied on the case study achieves the influence of model configurations on accretion. Therefore, tidal forcing, marsh design and system adjustments are divided into multiple components. The model simulations of the separate components give the influence on bed shear stress and potential sedimentation.
Furthermore, executed fieldwork calibrates the model on measured flow velocities. Besides, the Manning coefficient is estimated, and cross-sections are obtained by gps measurements. The parameters are subsequently used as model input.

From the simulations, it can be concluded that tidal asymmetry determines the duration of stagnant water and thus the settling of fine particles. A considerable increase in sedimentation is reached when flats are participating. However, an increasing flat area encourages ebb-dominance and can even lead to erosion.
Next, when the width of the cross-section is large compared to the depth, more sedimentation is predicted. With flood-dominant bed shear stresses, a broad cross-section leads to accretion.
In the case study, the presence of sand particles is not expected, as sand is deposited close to the inlet, where the bed shear stress is ebb-dominant. In contrast, silt settles throughout the system. Especially at low energetic conditions, such as bends, silt settles. As the occurring bed shear stress is mostly flood-directed, it is likely that sediment entering the system, does not leave the system anymore, and accretion of the bends is presumed.
The placement of gate culverts in flood direction leads to higher accretion rates due to the longer slack duration. However, at the exact locations of structures, higher bed shear stresses can develop, and erosion is expected. ... Read more

This study investigates the many dynamics of the Manyame Catchment and comes up with recommendations on how to adapt or become more resilient to the risks at hand. The bad politics and forthcoming economic situation of Zimbabwe has led to instability and dramatic inflation over the last decade. This has resulted in a terrible investment climate, limited aid received from foreign countries and an outflow of educated population. Exploitation of water resources by industries, mines and urban centres located close to the rivers resulted in a decline in the quality and quantity of the environment. Especially the Manyame River Catchment, that supplies the capital Harare and flows north into the Zambezi, has been greatly affected. The catchment suffers from several pollution sources such as agriculture, mining, industrial dumps and wastewater inflow. Poor management and the deterioration of the drinking water supply and sanitation infrastructure has led to the recent outbreak of cholera. Although the Manyame Catchment has enough water, another main issue is the spatial distribution of its water resources. In some areas there are dams, but no farmers to utilize the water. In other areas farmers are desperate for water, but don’t receive any. The increasing dry spells due to climatic changes, has had disastrous effects for the non-irrigating smallholder farmers dependent on their maize production for survival, while floods created by the backwater curve from the Cahora Bassa Dam in the Lower Manyame sub-catchment have washed away the livestock, crops and infrastructure on the fertile areas in the flood plains. The situation stresses for better awareness, monitoring and guardance of the water resources. This will provide information for improvements in the planning, policy and management of water resources. Although the history of Zimbabwean governance would suggest it is purely acting on crisis management, proactive planning would be preferable to the alternative; to wait for the aquifers to dry up only for people to realize the real value of water. ... Read more

Conventional practices of rating curve computation fall short in many aspects. They are data-intensive and are notoriously inaccurate in high-flow regimes as a result of extrapolation of low-flow curve fitting. By making use of a simple commercial UAV, a physically-based rating curve is computed that is substantiated by a detailed representation of the section's geometry. Discharge data is still required for calibration of the roughness coefficient and determination of the stage at zero flow. The physically-based rating curve improves the accuracy of the rating curve in the high flows and allows for an easy separation between flow regimes. ... Read more

Climate models predict increased Arctic precipitation and subsequent Arctic freshening as a response to increased green house gas concentrations. Eulerian studies have shown that with increased Arctic precipitation AMOC (Atlantic Meridional Overturning Circulation) strength decreases. Decrease in AMOC strength comes with a decreased redistribution of heat from lower to higher latitudes which can have severe effects on our climate. Therefore, understanding the effects and mechanisms of Arctic precipitation change is a crucial building block for predicting and possibly preventing climate change. This study used a Lagrangian approach. The pathways of water at Fram Strait were investigated for present-day climate (control run) and two scenario runs with increased Arctic precipitation (+50% and +300% respectively). Importantly, it was found that Arctic water reaches the Labrador Sea through Denmark Strait for all three runs. Thus, the extra fresh water in the Arctic can possibly impact sinking and convection zones in the Labrador Sea. The total amount of Arctic water, passing Denmark Strait from Fram Strait, increases for the weak scenario and decreases for the strong scenario of this study. On the other side of Iceland, for the strong scenario of this study, Arctic water stops passing the Iceland-Faroe-Ridge through the Faroe Bank Channel. The amount of Arctic water going into and staying in the Nordic Seas remained almost unchanged with increased Arctic precipitation. The two routes passing from Fram Strait into the North Atlantic were analysed further with respect to depth changes and properties. On both routes particles were fresher compared to the control run when increasing Arctic precipitation. For the weak scenario particles were usually colder than the control run on both routes. For the strong scenario, particles were only colder at Fram Strait, but got warmer than the control run along the pathway. ... Read more

Direct measurement of river discharge is difficult, time consuming and costly. Therefore a rating curve is often used to estimate the river discharge. Limited measurements under extreme conditions result in extrapolation of the rating curve for high flow conditions. This induces uncertainties and errors in the stage-discharge relation. Recently there has been a gradual shift to more physically based rating curves, where the geometry of a river is included and no extrapolation is needed. This seems to be a promising shift to improve traditional river rating. However, the challenge now is to accurately determine the parameters bed roughness and hydraulic slope. The aim of this research is to develop and evaluate a method to better estimate the hydraulic parameters bed roughness and hydraulic slope. To do so a case study has been carried out in the Luangwa river catchment in Zambia. ... Read more

The main focus of this project is to gain insight into feasible strategies for sustainable water supply in a community inhabited by indigenous people. This was done by an intervention study in a small village called El Progreso in the province of Colon, on the Caribbean coast of Panama. The project aims to provide safe drinking water to the 80 members of the Embera tribe, living in El Progreso. This is done by constructing rainwater harvesting tanks where rainwater will be collected from roofs of houses and school buildings and stored within the tanks. We want to investigate the impact of this system, which supplies safe drinking water on a daily basis, on the community. The construction of the storage tanks (ferrocement) will stimulate and create local entrepreneurship. In these ways we will take our first step towards a sustainable local enterprise and sustainable employment. This can ultimately result in a long-term development in the El Progreso area. The project consists of 3 study topics: the physical construction of the rainwater harvesting tanks and the project control and quality that comes with this, evaluating the quality and quantity of available water sources in the area of rainwater and rainwater and the possible changes in the community due to this project (both the influence of the newly constructed tanks on daily life as well as the presence of aid in the community and social impacts that comes along with this). The project resulted in the construction of 11 tanks, improved job opportunities for several community members, improved trust in aid organizations as well as connecting the community to a national aid network for future projects. ... Read more

The direct measurement of discharges in rivers can be time-consuming and costly. The discharge is commonly estimated indirectly by means of a curve, relating water level to discharge. This so-called rating curve is traditionally determined by fitting a curve to a number of observations, which induces several uncertainties and difficulties: 1) the curve is approximated by a function type, 2) in general there are no observations for high flow conditions and 3) it needs to updated regularly due to cross sectional changes.

In this research a physics-based rating curve is developed and evaluated that is more reliable and easy to update. By the use of a photogrammetry techtnique 3D surface maps of river banks are generated by pictures obtained from an Unmanned Aerial Vehicle (UAV). The topography of the main channel is determined by using an Acoustic Doppler Current Profiler (ADCP) or it is approximated by the making use of expert judgement and the method developed by Lane (relates the water depth of the channel to the width and natural angle of repose). By knowing the geometric profile of a river reach, only the roughness coefficient and water surface slope are the unknown parameters to derive the rating curve, where the Manning’s formula acts as the basis of the rating curve. In this way, the rating curves can be made physically substantiated and the calibration parameter is the combination of roughness and water surface slope.

Instead of using the power law function as a approximate function of the rating curve, it is used as approximation of the conveyance-water level relationship. Therefore, the exponents in the power law function are physical substantiated and reduce uncertainties in the extrapolation zone of the traditional method, because the profile during high flow conditions is also known. Further more, the local invariabilities that are specific to single cross-section analyses are minimized by analysing river reaches. It is demonstrated that local invariabilities arises easily in natural rivers that are causing non-uniform flows, which make in general the the open-channel flow analysis much more complex. However, the average conveyance of a reach may led to the a valid assumption of a uniform flow (which is part of the assumption in the Manning’s formula), depending on how the local variabilities behave. The calibration of rating curves are much easier compared with the traditional way. Instead of collecting new discharge
measurements during dispersed flows, you just have to visit the area ones with a drone and surveying equipment. During this visit, the new new geometric profile can be captured and the rating curve can be updated, assuming that the roughness and water surface slope remains constant. ... Read more

De psychrometrische constante wordt gebruikt om met behulp van een psychrometer de relatieve luchtvochtigheid vast te stellen. Onderzoek geeft aan dat deze psychrometrische constante niet constant is en dat voor lage windsnelheden de algemeen geaccepteerde formules niet zonder meer toepasbaar zijn. In dit onderzoek wordt vastgesteld wat het verband is tussen de waarde van de psychrometrische constante en de snelheid van langsstromende lucht aan de psychrometer. Hierdoor kunnen de bekende formuleringen om de relatieve luchtvochtigheid vast te stellen ook bij lage windsnelheden worden toegepast.

The psychrometric constant is used to determine the relative humidity of air with help of a psychrometer. Previous research indicated that the psychrometric constant is in fact not constant and widely accepted equations cannot be applied in low wind speed conditions. In this research the relation between the psychrometric constant's value and ambient wind speed is determined, so existing equations can be adapted to low wind speed conditions. ... Read more