The risk of flood risk is increasing with global trends such as rising sea levels, land subsidence, increasing populations in coastal areas and economic growth. Traditional solutions to reduce flood risk are a levee or a dam; however, the research on hydraulic engineering is increasingly promoting nature-based solutions for flood protection. Salt marshes and mangroves can attenuate waves and could thus help to reduce failure probability. This research focuses on the attractiveness of mangroves in flood risk reduction. The essence of the reasoning followed in this research is that mangroves in front of a levee enable a lower levee and therefore reduce costs while maintaining the same failure probability. There are three aspects related to this assessment: the biological behavior of mangroves; the physical behavior of waves and storm surge in a mangrove forest; and the economics of flood risk, levees, and mangroves. Mangroves grow under specific conditions, such as high temperatures, saline water with fresh water input, between mean sea level and high tide, and with low wave impact. Mangroves attenuate waves, with the attenuation rates depending on mangrove height, stem diameter and density, in addition to the water depth, wave height and wave period. Lastly, the costs of restoring mangroves and building levees have been considered. In this thesis a method has been developed to define the optimal configuration of a levee-mangrove system. The goal is to reach a desired safety level while keeping costs as low as possible. Costs consist of building a levee, restoring mangroves, and the expected annual damage and maintenance. This depends on the ratio of mangrove and levee costs, the ability of mangroves to attenuate waves, and levee characteristics. Although all variables have equal significance in defining the mangrove forest width, the variety in levee and mangrove costs and significant wave height are the variables most likely to affect the optimal mangrove forest width. Mangroves are only useful to decrease the height of waves, thereby reducing wave run-up. This wave-attenuating effect decreases exponentially with the mangrove forest width. all Based on the literature, when considering common values for levee costs and mangrove restoration costs, levee characteristics and wave attenuation, wave attenuation is strong enough such that the optimal mangrove forest width is larger than zero. The costs of restoring mangroves depend on the required measures to enable mangroves to grow back. Hydrological restoration involves returning the mangrove area to the natural condition where mangroves are able to grow. This could, for example, be restoring a fresh water source that has been blocked by human intervention. If hydrological restoration is required and can be done cheaply, then this is the most effective option. Planting mangroves can also be a cost-effective option. Sheltering the mangrove area by using a permeable structure requires more financial resources. If large-scale filling or excavation is required, costs may increase significantly, and it is likely to be too expensive to use mangroves in flood-risk reduction. However, this cost assessment may change if the ecosystem services of mangroves are taken into account. As a rule of thumb, for mangroves to be economically effective, restoration costs [USD/m/m] in case of a 1 m wave height should remain below 0.003 times the variable levee costs [USD/m/m]. This value increases linearly with the wave height. For levee costs of 1 million USD/km/m, of which 0.6 million USD/km/m variable costs, the mangroves restoration costs should not exceed 0.18 USD/m/m, or 18,000 USD/ha. For Kaback, Guinea, the optimal mangrove forest width and height of the levee are assessed. These are 900 meter and 1.1 meter respectively, with the levee located just behind the mangroves. 900 meter is the maximum width possible, considering the physical conditions where mangroves grow. Since mangroves are growing back naturally already, costs for mangroves restoration is limited to fixed costs. The model developed in this study can be used to create a ``mangrove opportunity map”. This map can indicate—based on physical attributes—the costs, hydraulic conditions such as storm surge and wave height, and whether mangrove restoration for flood risk reduction is effective. Governments or organizations such as the World Bank can use this method to explore whether mangrove-based strategies are an option. Further research can focus on improving the cost estimates on levees and mangroves. In this study, costs are assumed to be linear with the levee height or mangrove area. In practice, levee costs might especially increase exponentially with the levee height, which would make mangroves more effective.

This report contains the conceptual lay-out for two possible expansions of the port of Bahía Blanca. To determine the best conceptual lay-outs, emphasis is drawn to understand the physical system to determine the effect of the expansion of the port on the natural system. The port of Bahía Blanca is situated at the end of a ria, or tidal basin. For the designs, different conceptual lay-outs are developed and simulated in a hydrodynamic model called MOHID. This is a 2D depth-averaged model (2DH), which uses a rough bathymetry grid of the ria to determine the effect of the port development. There are three mutations of the different port expansions on the environment, which are investigated using the MOHID-model: (1) the East expansion, containing reclamation of tidal flats and closure of a side channel; (2) the South expansion, containing a widening and elongation of the channel and reclamation of tidal flats; and (3) the deepening of the entire navigation channel to various minimum depths. From the results of the MOHID-model on the East expansion conclusions on the mutations of the different port expansions are drawn. For the East expansion, only small changes are predicted; only local erosion in the navigation channel near the expansion may occur. For the South expansion, the flow velocities reduce in the entire stretch and there seems to be sedimentation at the eastern part of the expansion.
As a conclusion the best and most feasible designs are chosen. The best design is the lay-out that obtained the highest score in the MultiCriteria- Analysis (MCA). The most feasible design is the design having the highest cost/benefit ratio determined by a Cost-Benefit Analysis (CBA). The east bank is located close to the current port, Ingeniero White, on tidal flats which are inundated at high-water and dry at low-water. For the East expansion, different port lay-outs are developed mainly differing in amount of reclaimed land, length of viaducts and the presence of a mooring basin. The best design on the east is characterised as being very compact and having small viaducts between the dry bulk and agribulk terminals and jetties. The main advantage of this design is the small expected increase of siltation, good safety and sufficient future expansion possibilities. The most feasible design, however, is characterised by long viaducts reducing the costs of the design. The other appointed location for the port expansion is the south bank, opposite of the current port development. This location, however, is characterised by one main disadvantage; It is far from any form of connection with the hinterland. Nevertheless, in 2013, the port authority (CGPBB) initiated the start of small reclamation works. The best and most feasible design fully utilises this reclaimed portion of land. Moreover, the best design has a small expected increase of siltation in the port area. For a final designs, all previous designs are combined to create a design in which all the advantages of each of the designs are fully incorporated. Therefore, this design has little reclamation as well as viaducts with only intermediate lengths.