The enormous damage caused by hurricane Ike in the year 2008 has been the main incentive for several structural flood risk reduction plans in the Houston-Galveston Bay area. One of them is the ’Ike Dike’, a large-scale coastal barrier which closes Galveston Bay to prevent penetration of hurricane surges. A momentum gain for flood risk reduction plans enables opportunities for nature-based or Building-with-Nature (BwN) solutions in the area. However, no structured approach with respect to BwN for flood risk reduction has been followed for Galveston Bay yet. BwN focuses on working with processes of nature, promoting and using habitats and increasing the contribution to ecosystem services and other potential benefits. In order to apply this principle to flood risk reduction, more understanding of the hydrodynamic effect of BwN measures under hurricane forcing is required. The main topic of this research is to explore the opportunities for BwN solutions for flood risk reduction in the Galveston Bay area. This study focuses on two aspects: flood risk and natural value. Thereby, the goal is to create solutions which add value to both aspects. Concerning flood risk, surge and waves are considered key processes because they contribute greatly to damage during hurricane conditions. In this study a conceptual design is made and evaluated by means of a qualitative evaluation and a quantitative evaluation. The qualitative evaluation involves two toolboxes. The quantitative evaluation involves a 2DH hydrodynamic model. The Galveston Bay system is a large estuary, which is prone to hurricanes and vulnerable to flooding. Important variables that affect surge and waves inside Galveston Bay are fetch, depth, landfall location and hurricane surge at the open coast. Fetch and depth can be influenced by BwN measures. Galveston Bay is a productive ecological system and its most important habitats are wetlands (e.g. salt marshes) and oyster reefs. However, significant erosion of shorelines and wetlands in Galveston Bay as a result of relative sea level rise and insufficient sediment supply has been observed. A BwN solution for flood risk reduction in Galveston Bay can consist of several elements or ’building blocks’: nourishments, wetlands, oyster reefs and an eco-island. Oyster reefs are considered as three-dimensional structures. Moreover, an eco-island is defined as a large island with ecological development and e.g. recreational functions. These building blocks have been identified because they have the potential to reduce flood risk as well as provide natural benefits to the system. In order to qualitatively evaluate these building blocks for Galveston Bay, a framework has been developed. This framework involves two toolboxes and the formulation of a global, conceptual BwN design. The toolboxes emphasize on flood risk and natural value of the building blocks. Simplified one-dimensional calculations, where possible, and qualitative literature are used to formulate the toolboxes. The potential effect of a building block is illustrated by a color classification and a description. The effect on flood risk is assessed for surge and waves. The effect on natural value is assessed for five relevant criteria. The framework has been proved to work well for evaluation of building blocks for Galveston Bay. On the one hand, with respect to surge, the toolbox shows potential for a reduction of several decimeters to a meter in Galveston Bay for emerged nourishments which compartmentalize the Bay by limiting fetch and therefore wind set-up. The eco-island, however, is significantly less effective in surge reduction as water can easily flow around it. Wetlands and oyster reefs are considered not effective for surge reduction in Galveston Bay at all. On the other hand, with respect to waves, wetlands are promising for attenuating waves nearshore. However, wide stretches are required and the quantification of their effect is difficult due to various parameters like vegetation type, stem density and stemstiffness. At the shore, oyster reefs are effective in attenuating waves and reducing erosion as well, although the effect in hurricane conditions requires more investigation. Concluding, the evaluation of the toolboxes shows that most promising flood risk reduction measures are least beneficial to the natural value in Galveston Bay and vice versa. The most effective design according to this qualitative evaluation method is a continuous emerged island that reduces peak water levels close to the western and northwestern shore of the Bay. This can be combined with oyster reefs along the western shore until Texas City and wetlands at the foreshore of the Texas City Levee for wave attenuation and erosion reduction. The qualitative evaluation showed promising solutions for surge reduction. However, quantification has been difficult. Therefore a hydrodynamic 2DH model has been created that incorporates the complex dynamics of a hurricane, two-dimensional flow mechanisms and the bathymetry of the Bay. This model is used to evaluate the system behavior and surge reduction measures for different hurricanes, with or without Ike Dike. Three hurricanes have been applied: a ’regular’ Ike as reference hurricane, and two shifted Ike tracks in which landfall occurs to the southwest of Galveston. These two shifted hurricanes cause higher storm surge levels in Galveston Bay due to higher surge levels at the open coast and stronger onshore winds in Galveston Bay. The model has been calibrated with measurements of peak water levels during Ike. Although this model is a coarse resolution model with limited simulation time, its performance is acceptable. Simulations show that peak surge levels of 3.5 - 7 m are expected in the Bay for different storms in the open Bay situation. However, in the case of a closed Bay (Ike Dike) peak surge levels are limited to 1.5 - 2.5m. Surge measures are evaluated for three different areas of interest: West, Northwest or Houston Shipping Channel (HSC) and Northeast Bay. Outcomes of the model show that peak surge level can be reduced by emerged islands which are as continuous as possible. For an island to be emerged even in the worst simulated hurricane conditions an elevation of 6 m above mean sea level (MSL) is required. In that case, peak surge level reductions with a minimumof 0.5mand amaximumof 1mare expected with emerged islands in theWest and the Northwest (HSC) for different storm tracks in the open Bay situation. The measures in the case with an Ike Dike are generally less effective in reducing peak surge levels. A final, optimized design for surge reduction is defined as the combination of a continuous island forWest and Northwest (HSC), the two most economically valuable areas. This design is presented in Figure 1, along with results for peak water level reduction without Ike Dike. The conclusion can be drawn that various limitations are found with respect to BwN solutions for flood risk reduction in Galveston Bay. Bwn solutions can significantly improve natural conditions in the Bay. However, they cannot eliminate flood risk inGalveston Bay, because a large emerged structurewith a few open passages would reduce peak surge levels at thewestern and northwestern shore by a maximum of 1.5 m. The presented BwN design is a significant intervention in the system and might have limited benefits for natural value. Furthermore, flood risk reduction of BwN measures highly depends on whether an Ike Dike will be constructed and which storm is considered because the landfall location is highly influential to peak surge levels in the Bay. With respect to waves, oyster reefs and wetlands along the shore are considered to be promising measures as they clearly add value to the natural system, protect the shore and attenuate waves. However, the effect of these measures requires quantification. To reduce flood risk significantly, BwN solutions in Galveston Bay should be constructed alongside of hard flood protection structures.