With rapid urbanization, Low Impact Development (LID) is promoted as an alternative to Conventional Drainage (CD), seeking a natural solution for current urban water problems. The positive effects of LID were the main theme of recent LID researches, but this project aims to deeply explore the hidden troubles about the extreme peak runoffs influenced by LID city on catchment scale. In this research, the SUPERFLEX conceptual model was adapted to a rural-urban semi-distributed model to simulate the current rainfall-runoff relationship of the catchment where San Antonio city is located in. Besides, the model expressions of 4 representative LID practices (bioretention cells, vegetated swales, green roofs, and permeable pavements) were devised under SUPERFLEX framework. To deal with the prediction uncertainty, three urban development scenarios in 2040 and five LID implementation scenarios were designed for San Antonio city. And their influences on the basin peak runoffs will be quantitively studied. Research result shows that, the urban runoff tended to swing between extreme flood and extreme drought in the reference situation; And next, the infill urban development strategy, which means developing the vacant or undeveloped land within an existing community, was more helpful on peak runoff and total runoff volume control than sprawl urban development strategy with the same population growth; Thirdly, the bioretention cells, vegetated swales, and permeable pavements had similar good performance on peak runoff reduction, which can be mainly ascribe to the stormwater infiltration process. As for the retention of total runoff volume, the bioretention cells, permeable pavements, and green roofs perform better than vegetated swales since the rapid water transportation character of vegetated swales decrease the water residence time for infiltration; The runoff reduction function of LID practices performs effective on the large peaks in dry and normal seasons, but it will be restrained significantly in flood season. According to model result, the rural peak runoffs happened 6.5 to 15.5 hours after the urban peaks. And for 4 LID implementation scenarios in which 15% of urban grey areas is covered by LID practices, the urban peaks are delayed between 0.5 and 2.5 hours. And for the scenario with the LID cover areas as 50% of the urban grey areas, the time lag of urban peaks varies from 0.5 to 6.5 hours. For this scenario, since the obvious time delay of urban peaks, more stack of urban and rural peaks is caused by the time approaching of urban and rural peaks, which causes the increases of two total basin peaks in flood season from 3.57 to 3.65 mm/d and from 6.35 to 6.47 mm/d respectively. In conclusion we may say that the stacking effect of LID implementation on total basin runoff is limited in the case of San Antonio basin, partly due to the fact that only a small part of this basin is urbanized.

The social dimension in water management and the interaction of stakeholders is a challenge when modeling hydrological systems. Case studies are essential to uncovering relations and unexpected results. In this research, a case study of the Guayllabamba basin in Ecuador is proposed. Mainly, the interactions, perspectives, and relations between key stakeholders are analyzed. NGOs, water management government institutions and water users’ associations are the main focus. The political changes in Ecuador, the creation of new institutions and the new water law have shaken up the relations between these stakeholders and have provided a great opportunity to see what factors have affected the interactions of these stakeholders and have exacerbated some issues. Moreover, the capital of the country, Quito, is located in the Guayllabamba basin which allows the governmental institutions to be more hands on, thus generating interesting exchanges among all stakeholders. An assessment of the current legal framework is carried out to understand how the relations (mainly with the governmental stakeholders) should work, then interviews and surveys were applied to get a grasp on how the different stakeholders perceive each other and the main issues in the basin. Finally, direct observation was crucial to see the different stakeholders interacting and draw conclusions as to whether the responses to the interviews and survey are coherent with the observations made by the researcher. The methods used were mainly qualitative and the research followed an emergent design, meaning that modifications to the research were done along with the gathering of the data.

The undertaken thesis work conducts a research study based on the study area — Laakhaven, The Hague, to develop an implementation example of the Adaptation Pathway approach, in order to support long-term adaptive stormwater management planning on urban adaptation measures to mitigate pluvial flooding under the climatic and socio-economic uncertainties. The methodology is presented in the stepwise procedure to develop adaptation pathways. The core part of this method is expressed as the risk-based approach, which considers the flood risk from the aspects of the probability and the consequence. Different climate and socio-economic scenarios are developed to represent the uncertain environment for policymaking resulting from long-term changes. An urban water balance model is applied to produce the novel empirical performance indicator for the effectiveness of adaptation measures as the critical input to this assessment. Sell-by dates of adaptation actions are computed based on the assumption that, once a policy action reaches the perspective-based socially acceptable risk, it is said to encounter an adaptation tipping point thus requiring additional interventions. With the computed sell-by dates, the adaptation pathways maps are assembled under certain rules that exclude illogical sequences. Robust adaptation pathways that can succeed over various future scenarios are outlined from the pool of pathways. The developed adaptation pathways map provides the policymakers with a range of possible options. The results indicate the significance of investing in the modular rainwater harvesting devices on private space since it is effective and flexible action that supports the development of dynamic robust strategies for the long-term adaptive stormwater management planning. The implementation methodology of this case study is theoretically viable and its potential to make a more comprehensive study has been proven. Therefore, it is recommended to take the undertaken study as a starting point and further improve it to find the ultimate answer through sub-selecting preferred pathways.

One sixth of the world's coastline consist of coral reefs and provide natural flood defence for the people who live in the coastal region behind the reef. However, a rising sea level, changing wave conditions and degradation of corals threaten the coastal safety of these reefs.Numerical models can be applied to study the reef-hydrodynamics and the effects of coral degradation on the reef-hydrodynamics. When non-linear processes are important or the individual waves need to be determined, a phase resolving model is preferred. Within this thesis two issues regarding the application of non-hydrostatic models to coral reefs were studied. Due to the large bottom gradient in front of a reef, the offshore boundary has to be located in deep water, which means that frequency dispersion becomes important. The accuracy of frequency dispersion within non-hydrostatic models depends on the number of vertical layers. However, the addition of a vertical layer increases the computational time extremely. Therefore, a reduced two layer non-hydrostatic model (XBeach-nh+) was developed with the assumption of a constant non-hydrostatic pressure in the lower layer. In theory, XBeach-nh+ is capable of modelling the wave transformation from deep to shallow water, but the applied boundary conditions cannot force deep water waves. On top of that XBeach-nh+ has never been properly validated for reef environments. Furthermore, the corals (growing on the reef flat) have a large effect on the reef-hydrodynamics by dissipating a large part of the wave energy. There exist different formulations to include vegetation into a non-hydrostatic wave model, but these formulations are mainly applicable for cylinder shaped geometries, whereas corals are more complex in shape. Apart from the shape, the in-canopy velocity can be significantly different from the free stream velocity. Therefore, a porous in-canopy model was implemented to model the in-canopy velocity, which was used to determine the canopy-induced force on the depth-averaged flow computation. Firstly, the inclusion of the second reduced layer improves the dispersion relation up to a relative depth ($kh$) of 5 for linear waves. A simulation of biochromatic waves over a plane beach showed that XBeach-nh+ is capable of modelling the energy transfer between the major wave components. Both steeping and reflection of the sub-harmonic were modelled according to the measurements. Furthermore, the validation of random waves over a fringing reef showed the capability of XBeach-nh+ to model the reef-hydrodynamics for different wave conditions (rel. bias of -0.003 for total wave height, -0.081 for LF-waves and -0.103 for the setup). Moreover, the addition of the second reduced layer gives a more robust prediction for all modelled wave conditions, whereas the one-layer model contains more scatter. Secondly, the in-canopy model captures the canopy-induced force when the canopy parameters were known. Both the in-canopy flow of unidirectional and oscillating flow fields was accurately modelled when the results were compared to the measured velocity though cylinders and corals. Although, the canopy parameters were not always known, it was shown that an un-calibrated in-canopy model, based on porosity and canopy height, gives a competitive result compared to a fully calibrated shear stress formulation. The applicability of XBeach-nh+ in 2-dimensional domain with a coral covered reef flat was shown by modelling a 5 day Swell event at Ningaloo Reef. Reasonably accurate results were achieved when using the in-canopy model, based on the canopy properties.