Urban areas of coastal cities are increasingly susceptible to the consequences of climate change. In particular, the threat of increasing floods due to sea level rise and heavier rainfall is growing. Cartagena de Indias in Colombia is such a coastal city, where flooding is a significant issue, with floods becoming more frequent in recent decades. Arcadis, together with the ConAgua consortium, is currently investigating and designing mitigation strategies for the city’s water-related issues. The thesis focuses on defining and quantifying the sources of urban flood problems and assessing the impact of potential mitigation measures for reducing these flooding problems.

The study consists of four parts:
1. Identifying the main causes of floods using a data analysis;
2. Quantifying these findings with a numerical D-Hydro model;
3. Investigating suitable kind of mitigation measures and quantifying the impact of the best feasible mitigation options;
4. Evaluating the investigated mitigation options based on several criteria with the help of a multicriteria analysis.
Parts 1 and 2 focus on understanding the system. Parts 3 and 4 of the study focus on finding the best option to reduce floods.

The data analysis allows the development of a system description identifying the main causes of flooding. The analysis reveals that the limited capacity of the drainage system, high topographic variations, and extreme rainfall are the main causes of flooding in Cartagena. Extreme rainfall flows quickly from steep upstream areas to milder slope lower-lying areas, particularly those at the edge of the Ciénaga de la Virgen. It is hypothesised that downstream water level plays a role in the accumulation of runoff water, thereby influencing the frequency and intensity of floods in the neighbourhoods at the southern edge of the Ciénaga de la Virgen, especially with a relative sea level increase of approximately half a meter factored in. This hypothesis is checked by quantifying the impact of different scenarios with a numerical model.

A numerical D-Hydro model is used to quantify the impact of different meteorological conditions and downstream water levels on floods. The model’s simulation results show that rainfall intensity is a more dominant factor in flood generation than downstream water levels. Even with a return period of two years, rainfall causes significant flooding of almost 25% in the study area. The model also indicates an increasing influence of tidal conditions due to relative sea level rise. Still, the increased influence is mainly visible in the borders of the Ciénaga de la Virgen with minimal residential impact. These conclusions highlight the need for mitigation efforts to focus primarily on improving the areas influenced by the rainfall, especially for short return period rainfall events.

Various flood mitigation measures are considered for the southern border of the Ciénaga de la Virgen to reduce floods caused by the dominant intense rainfall. Wetland creation and evaporation measures are ruled out due to the dominant influence of rainfall-induced floods and the inability to prevent floods caused by extreme rainfall events respectively. After investigating the scale required, considering feasibility, the three types of measures that are converted into mitigation options are:
• Mitigation Option 1: Increasing channel dimensions,
• Mitigation Option 2: Increasing infiltration,
• Mitigation Option 3: Constructing retention areas.
These mitigation options are assessed for their impact on reducing flooded areas and flood levels. The three mitigation options are evaluated to recommend the best-preferred mitigation design. This evaluation employs a Multi-Criteria Analysis (MCA)... ... Read more

Sint Eustatius is a Caribbean island located in the north of the Lesser Antilles. The coastline west of the capital, Oranjestad Bay, is of vital importance to the island as it is here where tourism, recreation, the harbour and historical ruins are located. On the other hand, Oranjestad Bay has a heavily fluctuating beach, which is undesirable. The Public Entity of St. Eustatius is investigating the possibility of a coastal solution that could preserve and extend this beach. However, there is insufficient knowledge of the hydrodynamic processes and morphology available to design a solution. By analysing the morphological system, this study aims to advise the Public Entity of St. Eustatius on the decision for a coastal solution to create a beach in Oranjestad Bay.

A literature study was performed to obtain basic knowledge of the coastal system of Oranjestad Bay. A two-dimensional numerical Delft3D model was then developed to gain more in-depth knowledge of the wave, current and sediment patterns. The result of both findings is combined in a conceptual model, where the cause of the beach fluctuations along Oranjestad Bay was analysed. Based on the system knowledge and stakeholders’ demands and regulations, three alternatives to create a beach at Oranjestad Bay are proposed. The alternatives are qualitatively verified and evaluated with a multi-criteria analysis and cost-value ratio.

The literature review shows that Oranjestad Bay has the characteristics of a wave-dominated coast. The seasonal wave climate has a strong influence on the beach fluctuations that occur in Oranjestad Bay. The influence of tides and wind on sediment dynamics is expected to be minimal. The modelling study concluded that the wave climate can be described by three main wave conditions, namely a sea condition, a storm condition and a swell condition. The different hydrodynamic and morphodynamic characteristics of these conditions provide a reasonable simulation of the coastal processes in Oranjestad Bay. The model results give an impression of the seasonal wave climate and a net northward sediment transport capacity is found. These results are consistent with the expected coastal morphodynamics found in the literature study. The effect of a storm is less well represented in the modelling study. This limitation is caused by the modelling approach. In addition, there are some uncertainties in the model due to the lack of detailed coastal data in the nearshore. The conceptual model indicates that the coast of Oranjestad Bay may experience seasonal gradients
in alongshore sediment transport leading to beach fluctuations. From April to September, beaches are likely to erode due to net northward sediment transport. In addition, beaches erode due to periodic storm conditions that occur from June to November. It has been found that the recovery of beaches after a storm varies along Oranjestad Bay. Scubaqua beach recovers much faster than the beach at Smoke Alley. This is due to the larger sediment transport capacity in the alongshore direction at Smoke Alley beach, the amount of larger cobbles on the shoreline of Smoke Alley and the lack of sediment north of Smoke Alley... ... Read more

The awareness of climate change has grown worldwide in the past years. Possible consequences of climate change are sea level rise, heavier storms and high river discharges, but also prolonged drought. For a country such as the Netherlands, these water level variations, high discharges and droughts could have a great impact. Therefore, solutions of flood protection and water management are required to cope with these possible consequences.

The Delta21 project proposes a solution for flood protection of the Southwest delta in the Netherlands, while contributing to energy storage and nature preservation. The Delta21 project is a dam structure located in the Haringvliet mouth and, when constructed, results in a large change of the environment in the Haringvliet mouth. The remaining area of the Haringvliet mouth becomes the tidal lake. The important functions of the tidal lake are to be navigable and to have the capacity for extreme discharges to flow through the lake, without posing threats to safety. The functions are fulfilled by a channel through the tidal lake. The current channel (Slijkgat) requires highly frequent maintenance to allow for navigability and contains bottlenecks to meet the discharge capacity function. Hence, the objective of this research is to determine a suitable channel configuration in the tidal lake of Delta21, considering navigability and discharge capacity.

A suitable channel configuration is determined by designing a channel, accompanied by a morphological modelling study. The functional requirements and criteria for the design are defined based on the stakeholders interests. The requirements and the expected hydrodynamic and morphodynamic conditions in the tidal lake have resulted in three channel concepts that are considered for the design process. One of the concepts is the current bathymetry of the Slijkgat. The other concepts are Channel Concept 1 and Channel Concept 2, which are the same in cross-sectional area but different in orientation and location. The orientation of Channel Concept 1 is a small adjustment from the current Slijkgat, where the bend is slightly straightened. Channel Concept 2 is an almost straight channel and has the shortest length through the tidal lake.
The numerical modelling study is applied on the design in order to determine the morphological response of the system to the concepts and indicate the influence on the navigability, discharge capacity and maintainability. A Delft3D-FLOW model is applied for two simulation periods; normal conditions of one year of morphological modelling and extreme conditions for one week. The normative hydrodynamic drivers for normal conditions are the tide and the river discharge and for the extreme conditions only (extreme) river discharge.

The hydrodynamic and morphological results from the modelling study are processed in order to evaluate the three concepts in a qualitative and quantitative manner. The evaluation criteria are based on navigability, maintenance and nature preservation. Based on the results, Channel Concept 2 is most suitable in terms of navigability and maintenance with respect to the other concepts. Therefore, evaluation has led to the selection of Channel Concept 2 as the most suitable channel configuration. Additionally, an optimisation is proposed for the selected channel concept in order to improve the fulfilment of the maintainability criteria. A dam around the north western end of the channel is proposed in order to decrease the sedimentation volume in the channel. Modelling computations including the dam resulted in a significant decrease of sedimentation in the channel. Hence, the dam is an improvement to the selected channel concept.

Concluding, Channel Concept 2 is considered the most suitable channel configuration in the tidal lake with respect to the other concepts. Implementing the dam in the tidal lake further improves the channel concept in terms of maintainability and navigability.

... Read more

San Andrés is a Colombian-Caribbean Island located 800 km from the Colombian coast. On its Eastern side there is a barrier reef formation protecting the island from offshore incident waves. Due to the protected environment created by the coral reef, sandy beaches can be formed on the East side of the island.
During normal conditions, waves come predominantly form the East (90% of the wave climate), have significant wave height of 2 m and period of 8 s. Besides, San Andrés is situated on the Caribbean hurricane route, which can cause an enormous damage to the island. The storm season at San Andrés is between October and December, which is also when major erosion events take place.
The economy of San Andrés is mostly built upon tourism, specially related to its biodiverse ecosystems and Caribbean beaches. The island’s ecological environment is composed by mangroves, seagrasses, and coral reefs, attracting a wide spectrum of fauna and flora to its ecosystems.
During the Masterplan for Coastal Erosion (PMEC), San Andrés was pointed out as a location in which coastal erosion is problematic. In a follow-up of this Masterplan, the island was elected to be part of a program in which solutions against coastal erosion would be presented. This research is part of this project, as a parallel trajectory to get a more profound understanding of the system and the possible mitigation measures that could be applied on the island.
With increasing urbanization and frequency of extreme weather events, erosion is becoming a problem with which San Andrés and its residents are repeatedly having to deal. Erosion is specially problematic for the Northern part of the island, called Spratt Bight. This region is not only the most densely populated area of the island, but also economically and touristically very important. Its beach presents periodically eroding patterns during storm seasons, when wave action drives the sediment towards the East, decreasing its beach width almost to none. A decreasing beach width has a direct negative impact on tourism, making coastal erosion in Spratt Bight not only a coastal safety problem, but also an economic issue.
This study aims to look into the main hydro- and morphological processes driving coastal erosion in Spratt Bight and, using the Building with Nature philosophy, propose a set of solutions to mitigate this problem. To reach this objective data analysis and literature research has been carried out, after which different environmental conditions were modelled using the numerical model Delft3D.
During these activities it was found that independently of its direction, waves approaching San Andrés break upon the coral reef and induce a water level set up inside the coral lagoon. The difference in water level in- and outside the lagoon generates a current and sediment transport, which is directed towards the western opening in the coral reef.
When the Northern waves approach the island (1.5% of the wave climate), the same water level set-up phenomenon is observed. However, as waves are approaching form the North, they not only break upon the reef, but are also able to enter the sheltered lagoon through the western opening in the coral reef. These waves are able to bend around the reef reaching the shore and the headland on the Northern part of the island, inducing a longshore current and a sediment transport that is southeastward directed. The result is that Northern waves are mostly responsible for a strong westward and erosive sediment transport pattern. These waves are mostly observed between October and March, which coincides with the storm season in San Andrés.
Besides, it was found that the Eastern waves are responsible for restoring the (dynamic) equilibrium profile of Spratt Bight Beach. However, this restoring force has a less strong intensity, taking more time to restore the beach than to disrupt its equilibrium.
The solutions proposed include seagrass restoration to enhance ecology, restrain sediment transport and attenuate wave heights; the beneficial reuse of dredged material, to nourish Spratt Bight Beach; and finally, the implementation of artificial coral reefs as breakwaters to prevent the newly nourished sediment to be lost from the system. Besides, artificial coral reefs enhance the ecosystem by attracting fauna and flora increasing biodiversity. All proposed solutions have a positive impact on the beaches and therefore on tourism and the economy of the island. This makes them multifunctional solutions, serving the main goal of protecting the beach while at the same time creating benefits for other functions and values in the area. Following in this way the prescriptions of the Building with Nature design approach by van Eekelen and Bouw (2020).
... Read more

The Oosterschelde has been an area of morphological change for centuries. Both floodings and human influences have caused the Oosterschelde to have its current shape. During the last century before implementation of the Delta plan (1953) the estuary was still expanding as the channels deepened and the tidal flats increased. With the construction of the Delta works the tidal range decreased with 12 % whereas the tidal prism reduced with 31 %. As a consequence of this reduced tidal motion, the tide is not able anymore to counteract the erosion of the tidal flats which is caused by waves/wind. Therefore, the surface area and the height of the tidal flats reduces (‘Zandhonger’). This decrease of tidal area is an undesirable situation as the unique tidal nature of the Oosterschelde provides a lot of functions for both economy (oysters, mussels), ecology and recreation. Furthermore, the Oosterschelde is used as a transport route by cargo vessels.

This research studies the morphological development of a model of the Oosterschelde with two hypothetical interventions: removal of the storm surge barrier (SSB) and applying 2 metre sea level rise (SLR) in 50 years. This is done with four model scenarios: a run with SSB in place, without SLR (1), a run without SSB, without SLR (2), a run without SSB, with SLR (3) and a run with SSB, with SLR (4). This last run was used as sensitivity run in order to see which hypothetical interventions has more impact.

The model results led to conclusions which parts of the model are represented well and which parts of the model need to be improved. The results made clear that current model is promising as the tidal range was modelled correctly for the majority of the Oosterschelde. Also the model represented the tidal prism well compared to the calculated tidal prism (tidal prism = tidal range * wet surface area of the basin – the sediment volume of the tidal flats). It was found too that SLR can be modelled in a correct way by forcing a water level at the boundaries of the model.

It was found that wave activity is an important process with respect to the development of the tidal flats. Therefore, the frequency of the wave computations should be chosen in such way that reliable wave heights are present for each water level during the tidal cycle at all locations in the Oosterschelde. Another key factor which should be improved is the availability of sediment in the model as this determines the (desired) growth of the tidal flats with SLR. Processes/indicators which give information about this availability (ebb/flood dominance, sediment characteristics) can give insight in the development of the tidal flats. With improvement of modelling these two processes it may be possible to improve model capabilities. ... Read more

Ports are located in deltas and dynamic coastal regions with large loads of fine sediment. Ports also enhance the import of marine sediment by channel and basin deepening. This results in the need for extensive maintenance and sediment management. Finding an effective and efficient sediment man­agement strategy is difficult. This research dives into the current sediment management approaches and innovations to develop a decision-­making approach to find the most suitable and sustainable sed­iment management strategy for a port.

There are several ways to optimize the sediment management in a port. With the principle to con­ sider sediment as a value resource to the port and the coastal system, rather than a waste, the sediment management hierarchy is obtained. This hierarchy consists of Prevent and Reduce, Recycle and Reuse and Disposal to find a sustainable sediment management strategy. Various Prevent and Reduce and Recycle and Reuse examples and methods are analysed in this research to identify their potential for sustainable sediment management. Furthermore, based on literature study, expert interviews and a workshop at Arcadis input is generated to propose a decision­-making approach. This research devel­ops an approach and a tool considering all the aspects of sediment management and for assessing potential strategies for sediment management in a port.

The decision­-making approach considers four steps to find a sediment management strategy. The first step, sediment management objectives, defines the objectives of the stakeholders for the strategy. In the second step, fit to case, all the sediment management methods are considered and the potential strategies are defined. The third step, assessment of potential strategies, assesses the performance of each strategy with performance indicators in the decision­-making tool. The performance indicators are selected based on the assessment categories and objectives. In the final step, meeting the objectives, the most suitable strategy is proposed and described for the long-­term sediment management in the port. This approach supports the process to optimize sediment management in a port.

The proposed decision-­making approach is applied and further developed with a case study on Porto Amboim. Porto Amboim is a port on the west coast of Angola. The port operates for the offshore industry and is currently experiencing sedimentation problems. The northward directed natural long­ shore sediment transport is interrupted by the port’s breakwater, resulting in sedimentation around the breakwater and at the lee side of the breakwater. This interruption disturbs the sediment balance in the region causing erosion problems at the down drift side of the port. The decision­making approach and tool are used to find a potential sediment management strategy for Porto Amboim. This potential strat­egy for Porto Amboim uses a sediment trap to manage the sediment in the region, ensuring sufficient port operations and restoring the sediment balance along the coast.

In order to use the approach in a generic way the results are reflected upon and a validation on the approach is performed. The reflection showed some uncertainties and limitations to the approach and tool because of the available information on the case. However, still two suitable strategies have been obtained for Porto Amboim. In the validation the approach has been applied on a second case study. This validation showed the potential for generic use of the decision-making approach and tool for sediment management. From the reflection and validation can be concluded that the available information determines the level of detail of the assessment of the potential strategies.

The main objective of this study was to explore the potential dredging and sediment management strategies to optimize sediment management in the port towards more sustainable methods, develop a decision-­making tool and apply the tool in a specific case study. From this research the following can be concluded, the decision­-making approach is considered a suitable approach to find a sediment management strategy for a port. All aspects of sediment management are considered and assessed. Therefore, further development of the tool and innovations in more aspects of sediment management are recommend. Yet, this research, the approach, and the innovative methods for sediment manage­ ment show a great potential for more sustainable maintenance strategies in the future.
... Read more