Sensitivity analysis

30 MSc. Thesis Daan Poppema

3.3

SENSITIVITY ANALYSIS

A sensitivity analysis is used to determine the impact of modelling choices and parameter values. First, the sensitivity of the establishment to the parameters in the WoO framework is examined, followed by the sensitivity of the Delft3D model in section 3.3.2.

3.3.1

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W

O

Because the Windows of opportunity concept has been developed recently, there is a lack of data on the parameter values that should be used. To determine how much impact this uncertainty has, the results with different parameter values are examined. To present the results, the establishment patterns are aggregated into the number of cells within the area of interest where a window is possible or finished (i.e. cells where the conditions of a specific window are met, and cells where simultaneously also the previous window is finished.) All parameter values are varied independently. The values used are given in section 2.4.6, the result is plotted in Figure 3.12.

This analysis shows that the result is quite robust. As explained in the previous section, multiple criteria are simultaneously limiting establishment. Consequently, the end result (i.e. the prediction if establishment is successful, in the lower-right panel of Figure 3.11 and Figure 3.12) does not change, even if the new parameter set allows one of the conditions to be met. Nonetheless, the intermediate results do show some variation.

Whether window 1 would be possible (so the left panel of the fourth row of Figure 3.11) depends on the duration of this window (TWoO1). Figure 3.12 shows that durations longer than 4 days decrease the number of successful cells, up to the point that window 1 is no longer possible at the third row near the coast. At this point, the cells where window 1 and window 2 are finished coincide exactly (see the bottom row of Figure 3.11). Shorter durations do not increase the number of successful cells: as long as the duration remains more than a tidal cycle (12 hours), window 1 is unsuccessful outside the three rows at the coast.

Whether window 2 would be possible (so the mid panel of the fourth row of Figure 3.11) depends quite strongly on its duration (TWoO2). Figure 3.12 shows that lower durations increase the number of successful cells, longer durations decrease the success rate. Furthermore, the potential success rate of window 2 also depends on the value for CEDmature, with higher values increasing the success rate. However, this increase or reduction in establishment would not occur directly seaward of the already successful cells, but further away from the coast, where window 1 is not successful (not plotted). Consequently, the number of finished cells – which also depends on window 1 – does not change.

Whether the window 3 is possible (so the right panel of the fourth row of Figure 3.11), depends mostly on the CED of mature vegetation, the sensitivity to bed level change and the duration of the event (i.e. the period for which a maximum erosion is defined). However, like before, there is no effect on whether window 3 can be finished. Directly seaward of the cliff strong erosion occurs, so any additional cells where WoO3 would become possible are located further seaward. In those cells Window 1 is already limiting establishment, so the overall establishment pattern does not change. This means the pattern bottom right panel of Figure 3.11 remains the same, leading to the straight lines for WoO3 finished in Figure 3.12.

Chapter 3: Results 3.3 Sensitivity analysis

MSc. Thesis Daan Poppema 31

Figure 3.12: A sensitivity analysis of the Windows of opportunity parameters. The left column shows if a window is possible, so if all conditions of that window are met at the same time. The right column shows if a window can be finished so if at the same time the previous window is finished and all conditions of the new window are met. (WoO1 possible is not plotted: without the existence of previous windows, this is the same as WoO1 finished). Establishment is possible when window 3 is finished, so the bottom right panel shows the (in)sensitivity of establishment.

Chapter 3: Results 3.3 Sensitivity analysis

32 MSc. Thesis Daan Poppema

3.3.2

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3D

The establishment chances are determined by the combination of the WoO framework and the results from Delft3D. The previous section examined the impact of changes in the WoO framework, this section looks at the impact the model set-up in Delft3D. Three different factors are examined: the sediment type, the impact of vegetation and the wave forcing. The results are described in this section. For some of the figures, the reader is referred to the appendices.

3.3.2.1 SEDIMENT TYPE

Because of the direct dependency of establishment on sediment dynamics, the sediment is very important. This section describes how the results would change when a different sediment type is used. Instead of the single-graded sediment of the reference model, two types of clay with different settling velocities are used (for information on the exact settings, see section 2.4.7). With this sediment, the amount of erosion increases, leading to a slightly higher cliff. This is plotted in Figure B.1 and Figure B.2 in Appendix B.

The differences in the bed level dynamics also affect the establishment chances of the different windows (for details see Figure B.3). With the stronger erosion, window 1 becomes impossible at the third row near the coast, while further seawards window 2 and 3 become possible less often. However, because other conditions were already limiting establishment at these locations, the predicted establishment (i.e. the successful completion of window 3) does not change.

3.3.2.2 IMPACT OF VEGETATION

The impact of vegetation modelling is studied to determine how the results would change if the model includes the impact of vegetation on the hydrodynamics. This has very little impact on the bed level change and supports the choice to not include the impact of vegetation on hydrodynamics in the reference model. The erosion during the spin-up period is up to 1 cm smaller than in the reference model, so very small compared to the total erosion of up to 25 cm. After the entire model run, the maximum decrease in erosion is 3 cm, so also very small compared to the maximum erosion of 70 cm. Both results are plotted in Appendix C. The slight decrease in erosion has no effect on where the erosion is acceptable. Consequently, the predicted establishment remains the same.

3.3.2.3 WAVE FORCING

The impact of wave forcing is important because Marconi will use wave-dampening measures to make the area more suitable for salt marsh vegetation. Therefore, this section looks at the change in the results when the waves at the model boundary become up to 50% lower. As shown in Appendix D, the erosion would decrease significantly when the waves become lower (Figure D.1 and Figure D.2). The cliff, which is approximately 70 centimetres high at the end of the reference model run, decreases to 50 cm and 25 cm with when the waves are 25% and 50% lower respectively.

Chapter 3: Results 3.3 Sensitivity analysis

MSc. Thesis Daan Poppema 33

When the waves become 25 percent lower, the overall establishment does not change (see the green line in Figure 3.13). The number of cells where window 2 and 3 would be possible changes strongly, as shown by the red and purple lines. However, this change occurs further seawards, where window 1 is already limiting establishment. (For the detailed results with spatial patterns, see Figure D.3 in the appendix).

Figure 3.13: The dependency of establishment on wave height (wave heights are measured at the boundary, wave direction does not change)

When the waves become 50 percent lower, the overall establishment does change. At this point the establishment generally becomes possible at two more rows, as visible in the bottom right panel of Figure 3.14. Furthermore, the success per cells depends almost fully on window 1: the conditions for window 2 and 3 are met almost everywhere. This means that the water depth instead of erosion is limiting establishment. Another effect is that the sedimentation becomes so strong at the seaward end of the domain, that window 2 and 3 cannot occur anymore. These results show that the Marconi project needs wave-dampening measures to make the area suitable for a pioneer salt marsh.

Chapter 3: Results 3.3 Sensitivity analysis

34 MSc. Thesis Daan Poppema

Figure 3.14: The results of the Windows of opportunity model with 50% lower waves. Every grid shows a top view of the area of interest, with the coast located at y=0 and the deepest part at y=300. Row 1-3 show if the separate conditions are met, row 4 shows if a window is possible, so if these conditions are all met at the same time. Row 5 shows if a window can be finished, so if at the same time both the previous window is finished and the new window is possible. This means the bottom right panel shows in which cells establishment can be successful. Dark colours indicate results equal to the reference wave height, bright colours indicate a difference.

MSc. Thesis Daan Poppema 35

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