Due to climate change, temperatures are rising worldwide resulting in sea level rise, higher evaporation and possible changes in precipitation patterns. These effects of climate change influence salt intrusion, particularly by affecting the freshwater supply. In The Gambia also a planned hydropower dam and possibly future irrigation development will affect the freshwater supply. The combined effects of climate change, the hydropower dam and irrigation development on salt intrusion are unknown. A river south of The Gambia, the Casamance, turned strongly hypersaline during the Sahelian droughts, which strongly affected the rivers discharge in the early 80s. The freshwater supply in the Casamance after these droughts was insufficient to flush away the salt and the estuary never restored. This led to the decay of the entire ecosystem that existed in and around the Casamance. The Gambia river is of major importance to the country, the agricultural sector relies on the freshwater it supplies for growing crops, the fishing industry relies on sufficient numbers of fish to catch and the forestry industry makes use of the mangroves growing around the river. All these industries combined account for ±75% of the working force in The Gambia. If the Gambia estuary turns strongly hypersaline such as the Casamance, this would have disastrous results. This thesis aims to clarify the effect that climate change, the hydropower dam and irrigation development have on salt intrusion in the Gambia river. A 1D model, SALNST, is used to model salt concentrations over the river length up to the year 2100. The effects of climate change, the hydropower dam and irrigation development are represented by four model parameters: climate projections, sea level rise, dam operations and irrigation development. Multiple parameter values, ranging from mild to strong conditions or impact are implemented, creating a set of 27 scenarios. These scenarios treat the parameters independently to distinguish the individual parameters effect on salt intrusion. It can be concluded that The Gambia is prone to climate change when considering salt intrusion. Due to the importance of the river, monitoring sea level rise and the development of climate change will be essential for planning and taking counter measures. Understanding the effects that climate change can have regarding salt intrusion can also be used in negotiations to reserve some freedom in the hydropower dam operations. In addition, solutions that limit the increase in evaporative surface for strong sea level rise (e.g., dikes along a part of the Gambia river) prevent extreme salt intrusion lengths and hyper salinity. The hydropower dam causes a reduced range of salt intrusion lengths, this creates easier to predict salt concentrations which is beneficial for aquaculture. Furthermore, the reduced maximum salt intrusion is also beneficial for agriculture that makes use of irrigation.

Near-surface wind speed is typically only measured by point observations. The so-called Actively Heated Fiber-Optic (AHFO) technique, however, has the potential to provide high-resolution distributed observations, allowing for better understanding of different processes. However, before it can be widely used, its performance needs to be tested in a range of settings. Therefore, in this work, experimental results on this novel observational wind-probing technique are presented. We utilized a controlled wind-tunnel setup to assess both the accuracy and the precision of AHFO as well as its potential for outdoor atmospheric operation. The technique allows for wind speed characterization with a spatial resolution of 0.3 m on a 1 s time scale. The flow in the wind tunnel is varied in a controlled manner, such that the mean wind, ranges between 1 and 17 m/s. Comparison of the AHFO measurements with observations from a sonic anemometer shows a high overall correlation, ranging from 0.94-0.99. Also, both precision and accuracy are greater than 95 %. As such, it is concluded that the AHFO has potential to be employed as an outdoor observational technique in addition to existing techniques. In particular, it allows for characterization of spatial varying fields of mean wind in complex terrain, such as in canopy flows or in sloping terrain. In the future the technique could be combined with regular Distributed Temperature Sensing (DTS) for turbulent heat flux estimation in micrometeorological/hydrological applications.

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.

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.

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.

This report introduces the use of Genetic Programming (GP) into hydrology by describing the results of GP using conceptual hydrological models as physical representation. First the possibilities of GP are tested on synthetic data, which results in a shortlist of good working objective functions and insight in the most important GP settings. The test on real data in the Belgium Ardennes showed that GP using the objective functions KG10, MM and Shafii performed better. Nevertheless all three models performed not well on simulating the low flows and high peaks. Furthermore GP using KG10 and MM both results in simple serial models which perform well overall, but bad on quick response runoff. Shafii resulted in parallel models which show quick response flow, however GP it is not able to capture the fast responses correctly (yet). GP has the potential to improve the understanding in the behaviour of catchments, however it still needs the human mind to observe, compare and analyse the modelling results. The main consideration with GP is to look for a balance between: model search space, objective function, randomness and (computational) time. The challenge is how to lead GP in an efficient way without removing the possibility of finding unknown patterns.

Valdivia is a Chilean city located near an estuary system, 800 km south of Santiago. The navigation capacity of the Valdivia river mainly determines the present state and future possibilities for the welfare of the city. Since sedimentation problems arose in the river, the capacity for navigation became limited. The ministry of Valdivia therefore desires a solution for this problem in order to create possibilities for future growth. However, it is not known to what extent solutions for the sedimentation problem will actually contribute to an increase of welfare. By identifying the problem for the Ministry of Public Works of Chile, the boundaries and goal of the project could be set. In this research an attempt is made to answer the following research question: How can the surplus of sediment in the Valdivia river be remedied and to what extent will this contribute to the economic and social values of the city of Valdivia? In order to know how to remedy the surplus of sediment, a qualitative analysis of probable causes of the sedimentation was necessary. The main subjects which were researched by means of data and literature are tidal influence, river discharges, sediment composition and salt intrusion. With this knowledge it was concluded that the directions and magnitudes of flow lines in the river system can give good indications on what locations sedimentation can occur. The interaction between tidal currents and river discharges are the main drivers behind these flow characteristics. To verify the theoretical analysis, a basic Delft3D model was set up containing only tidal movement and river discharges. The pattern of the flow lines which was obtained from the Delft3D model supports the possibility that sedimentation occurs on the current identified sedimentation locations. Although other hydrological and morphological processes were found to possibly influence sedimentation rates in the river, qualitative data for studying these processes were missing. The current Delft3D model is therefore a good result regarding the available data and can be seen as a part of preliminary research in order to support further studies. Because the exact causes of the sedimentation were not identified in this research, it was not possible to come up with suitable solutions and to research how these solutions could affect the system. However, in order to gain insight in the effects of a river system without sedimentation problems, a fictitious scenario was studied by means of a social cost benefit analysis. The goal of the social costs and benefits analysis was to give insight in what factors should be taken into account when considering a project plan which solves the sedimentation problem. A dredging design was made in order to make the Valdivia river navigable for larger cargo vessels to increase the trading capacity of Valdivia. This dredging design comprises the deepening of the Valdivia river to a minimum water depth of 10 meters. Considering investment and maintenance costs of the dredging activities of this magnitude, it is concluded that the costs are not profitable compared to the social and economic effects of a higher traffic intensity on the river. With the social costs and benefits analysis all effects of solving the sedimentation problem are inventoried. When the Delft3D model can identify what the remedy of the sedimentation could be, the impact on the economic and social values of Valdivia can be determined. This research was not sufficient to answer the research question. Both the Delft3D model and the SCBA model were lacking accurate and reliable data. To expand this research, it is necessary to collect the data of which an overview has been made. Therefore, this research can serve as a preliminary study for further research. It is recommended that after data collection, solutions will be identified which can solve the sedimentation problem. These solutions need to be compared with a scenario analysis in order to decide on which is the most suitable solution for the sedimentation problem. Besides this, it is advised to research whether it is actually necessary to expand the river for transport when looking at all actors involved.

Standard conceptual hydrological models can rarely accommodate stream tracer dynamics at the catchmentscale. They rely on the generation of runoff through the propagation of a pressure wave and do not accountfor the actual advective movement of particles. Over the last years different model frameworks have beendeveloped to account for this shortcoming. The difference between the frameworks lies in whether they arebased on mixing coefficients or storage age selection functions. Both methods have shown their ability tocapture the stream chemistry response. It is however not clear how these distinct approaches compare toeach other and to reality.To provide more clarification in this matter, this research applied both frameworks separately to model thehydrological and stream water chemistry response for a specific catchment. The results are analysed usingthe concept of transit times, where information on the fluxes and states in all model components is used togenerate distributions that describe the age structure of water. By comparing the distributions generated byboth methods and by evaluating the overall model performances, more insight is gained on the two frame-works and on their ability to capture subsurface mixing dynamics.From the results of this research it is concluded that both frameworks show similar performance. They arehowever conceptually different, which might have implications for their ability to capture specific dynamics.These implications depend on the characteristics of the catchment of interest. It is still concluded howeverthat performance for each frameworks is likely to be similar. Because the model description of the MixingCoefficient framework is less complex, it is recommended to continue research with this framework.