For the assessment of the morphological development a modelling study for four cases is executed: three construction alternatives (Variants 1-3) and the reference scenario (Variant 0). Until this phase the modelling calibration analysis was executed over a period of 20 days. In this modelling study the stages in the scenarios are simulated for each week. This means that the new nourished sand is added once per week to the model. In practice the sand nourishment will be executed continuously 24 hours
Important model parameters
Setting
D50 600
D90 1000
Bed friction Manning
Morfac 10 CFL 0.8 Wetslp 0.24 Form Soulsby_vanRijn Alpha 0.95 Gamma 0.59 Gammax 1.64 facuA 0.09 dtheta 5 Cyclic 1 roller 0
Table C.13: Overview of important model parameters as a result of the calibration analysis
114 per day, 7 days per week. However, the sediment losses for each stage are compared relatively to each other therefore this modelling of the construction phases of the different Sandbar breakwater strategies enables us to draw conclusions on what strategy would be an optimal form regarding reducing sediment losses. The different executed steps for this modelling study are briefly summed in the next section. For the simulations of the different construction stages, the model settings of the best runs obtained from the calibration analysis is used (section C.7.). The boundary conditions which have been applied are further explained in the sections C.8.3.
C.8.3. Steps in construction phase modelling
In the methodology there was already stated that in order to assess the morphological behavior of the construction scenarios each construction stage is been simulated. The different steps were summarized in a scheme in Chapter 6 (Figure 6.1) which is further explained in this section.
i. Start:
Each scenario starts with the bathymetry before the construction stage of the Sandbar breakwater. Depending on the specifications such as production rate, location of the nourishment, sequence of construction the bathymetry before construction phase is updated for the situation after a week of sand suppletion.
ii. Bottom updating:
The raising of specific areas with the correct volumes in the Sandbar breakwater design is done by determining polygons and volume calculations using MatLab. The slope during the construction phase was measured to be 1/8. This is implemented along the edges of the raised area in order to have a realistic sand nourishment in the model bathymetry.
iii. Simulation of updated bottom for week:
The next step is the simulation of the new situation after one construction week for 7 days with the corresponding (hydrodynamic) boundary conditions which occur at the start of the project.
iv. Processing bathymetric output:
After simulation the XBeach model output can be obtained; the bathymetric output file will be processed into MatLab.
v. Quantification of losses & captured sediment:
After the simulation of the first week, the sand which is moved outside the design profile by natural processes to locations where it is not contributing to the stability of the Sandbar i.e. the sediment losses as well as the captured sand into the design profile are quantified for the first stage. This is done based on volume balances between the start and the end of the simulation. The concept of the volume balances is already explained in section C.4.1. Volume balance However, in this analysis it became clear that there was a need to split the natural sediment losses from a nourishment to the ‘loss’ areas (Inner Lake and West side) and the natural movement of sand at from other locations in these ‘loss’ areas.
The sediment losses quantification can be explained by the following steps:
- Volume flux from nourished bund: around the updated bottom a polygon is drawn and the volume flux between the start and the end of the simulation is determined. Usually the flux has a negative value indicating a net erosion from the nourished bund. By knowing this flux, there is known how much sediment is moved by natural processes to other locations. However not all this sand is moved to areas where it is considered as a loss. - Volume flux at designated ‘losses’ polygon areas: now there is known how much sand it
moved, the volume of sand which is considered as a loss needs to be quantified. The change in volume flux between the start and the end of the simulation in the ‘losses’ polygons is considered as the volume of nourished sand which is lost. However, for the
115 reclamation of the Sandbar Road one extra remark needs to be made. As the Sandbar Road from the coast is reclaimed sand will starts accumulated at the east side in the Inner Lake polygon which cannot be counted as a sediment loss. The processes of sediment capturing from LST and movement of sand from the nourished sand bund to the Inner Lake will be split up. This is done by calculating the difference between the volume flux from the Sandbar Road bund (erosion) and the volumes flux at the east and westside of the Sandbar Road. The amount of sand which has accumulated more at the east side than was eroded from the bund is considered as sediment trapping from LST.
The capturing of sediment quantification can be determined as follows:
- The volume flux of bathymetry within the design profile: For the construction variants where temporary groynes are implemented into the design which have the objective to capture free sediment into the Sandbar breakwater design profile, the volumes at the westside of the groyne are calculated. This ‘captured’ volume depends on when the construction of the groyne is finished, the location and dimensions of the groyne (see construction variants description, Variant 1& Variant 3The quantity of trapped sediment for the stages when the capturing takes place is determined by drawing a design profile polygon at the east side of the groyne and applying a volume flux between the start and the end of a construction week. This volume difference indicates the volume of accumulated sand behind the temporary groynes.
vi. Repetition of steps for all stages:
The steps for the first stage will be repeated for all construction weeks apart from the construction stages where only further widening and heightening of the Sandbar is executed. This is done since in this stage no significant morpho dynamic development will occur anymore and since it is time consuming to schematize and develop bathymetries for this stage.
vii. Determination of total captured sand/losses volume:
After the full simulation of the construction scenarios the total sediment losses will be determined and analysed (location, rate of losses) and compared to the actual construction method (scenario 0) and the other alternatives.
Input data/Boundary conditions
The wave boundary conditions for the assessment of the sediment losses is derived from the existing wave climate for the area of interest. This wave climate with nearshore representative wave conditions was developed based on a large scale SWAN model and calibrated against local measurements, resulting in a limited set of wave conditions for each month. Since the scenario simulations take place on weekly basis the durations of these conditions are scaled from monthly to weekly durations. For construction scenario simulations tide data from the existing tide input file is used after adaptation of the tide lengths.