The Weg naar Zee (WnZ) region of Suriname’s capital Paramaribo is dealing with coastal erosion. Images from the past 30 years have shown coastal retreats of up to 1.3 km. Mangrove forest used to be present in large numbers at the location. These forests enhance accretion, but due to different reasons, the number of mangroves has decreased significantly. Several attempts have been made to rehabilitate the mangrove at WnZ. Sediment Trapping Units (STUs) have been constructed, with the purpose to enhance accretion and thereby create ideal conditions for mangrove development. The units consist of walaba poles with bamboo as filling material. Walaba is a suitable wood type for this and also grows in Suriname. From bed level change results, field observations and interviews it could be concluded that there is room for improvement. Therefore, the following project goal was formulated: ‘To present methods for mangrove rehabilitation at the WnZ coast, which can be supported by the key stakeholders’.


To present mangrove rehabilitation methods, it was necessary to gain insight regarding the WnZ stakeholders’ involvement when it comes to coastal management. Fourteen key stakeholders were identified and analyzed. To gain even more insight into the views, perceptions and wishes of the WnZ region’s inhabitants, a survey was also conducted. Knowing the success factors and design requirements resulted in the analysis of which mangrove rehabilitation designs are applicable and supported by the region’s stakeholders. The first step was conducting a Multi-Criteria Analysis (MCA) in which five alternatives are weighed. From this analysis the top three variants were chosen as preliminary designs: 1) Adapted STU, 2) STU with Nourishment and 3) STU with chenier. Finally, support by stakeholders for the mangrove rehabilitation designs can only be gained if they are engaged. This was tackled by composing a stakeholder engagement plan. In this plan, the following three strategies were formulated: 1) Setting up a bamboo farm, 2) Setting up a Mangrove Vacation Resort and 3) Growing salt-tolerant plants. These strategies can contribute to the economic development of the region if implemented, which could then also benefit the various stakeholders.
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Due to the removal of mangrove forests, coastal zones can suffer from severe erosion. One of the proposed solutions is the construction of permeable structures. This study aims to optimise the design of permeable (brushwood) structures in order to restore the sediment balance and encourage mangrove re-establishment on tropical mud coasts. Preferably wave transmission should be low in order to create a calmer climate behind the structure. In that way sediment is able to settle down, which could lead to a recovery of the mud profile. It is also preferred that reflection by the structure is low. High reflection rates cause scour holes that lead to instability of the structure. Furthermore, scour holes could hinder future mangrove re-establishment. Aiming to achieve low reflection and transmission rates, the dissipation inside the structure has to be as high as possible. Experiments were conducted in the 40 meter wave flume at the Environmental Fluid Mechanics Laboratory at TU Delft. The permeable structure was schematized as an array of cylinders. With the physical scale model various effects could be tested, including the porosity, structure width, arrangement, orientation, etc. The tests were done for 5 different wave cases, from which the wave energy distribution over reflection, dissipation and transmission was determined. The existing brushwood structures require intensive maintenance. This is partly due to the sinking of the material into the soft mud. Also, the brushwood material washes away often as it is lighter than water and difficult to constrain in vertical direction. An alternative design that requires less maintenance would be preferred. Therefore, it was interesting to see whether a comparable amount of wave dissipation could be achieved by using vertical elements only. One important finding is that in more shallow water regions, vertical and horizontal orientations have similar dissipation rates. In water regions that go more towards deep water, the horizontal structures have higher dissipation rates. This can be explained by the relative importance of the horizontal and vertical velocities due to the wave motion. In deep water vertical velocities are relatively high. As the horizontal elements have more exposure to this component in comparison to the vertical elements, they provide more dissipation. In shallow water the relative importance of the vertical velocities is lower, which explains the similar dissipation rates of the two orientations. The analytical model of Dalrymple (1984) was used to describe the energy dissipation through the structures. Drag coefficients were derived by using the calibration method. For KC<15 the drag coefficients start increasing. This is possibly due to the relative importance of the inertia force. Comparing the drag coefficients to the ones derived from direct force and velocity measurements in previous studies showed relative high values. This could be due to an underestimation of the horizontal velocity due to the wave motion. The velocity that is used is the undisturbed velocity in front of the structure. However, the velocity inside the structure might be higher as the flow accelerates in between the gaps of the elements. Furthermore, the wave cases in this research are in the Stokes 2nd and 3rd order region, indicating that the waves cannot be fully described by linear wave theory. The inertia, permeability and non-linear effects among other possible effects are not included in the analytical model of Dalrymple. Therefore, the drag coefficients do not only represent drag forces, but also other processes. To gain more insight on the physical mechanisms that affect the wave energy dissipation, it is recommended to test the same scale model with direct force and velocity measurements. ... Read more