Ongoing sea level rise combined with significant levels of land subsidence cause the everlasting need for reinforcements of the current coastal protection measures to ensure a certain level of coastal safety. Developing countries in the Asian regions tend to suffer from even more severe levels of land subsidence and often lack the financial resources to fulfil these continuing needs for reinforcements, which put the coastal areas under the continuous tread for potential flood disasters. The Lease Polder is a new Building with Nature approach for the sustainable design of protective coastlines and can offer a durable solution to this current problem. The concept is built upon the principle of sharing a part of the land with the hydraulic system and thereby allowing the natural vertical growth of the area by siltation during in- and outgoing tide. The concept is, however, still in its infancy, causing numerous uncertainties regarding the feasibility. The critical design parameter that affects the feasibility is the Lease Polder period, denoting the duration for which the breach of the Lease Polder is kept open. This study aims to offer a quick assessment tool that can determine the optimal Lease Polder period and perform a feasibility analysis for arbitrary Lease Polder cases, based on a sensitivity analysis of influential parameters on the potential siltation process.

A simplified 2DH process-based Delft3D model was used to perform a sensitivity analysis on multiple sediment characteristics, hydraulic boundaries and dimensional parameters. The parameter sensitivity was determined by comparing the results from a set of parameter scenarios to a base-case model. This base-case model was a simplified version of the Tollesbury realignment site and was calibrated and validated on the available historical data sets. The base-case model was simplified to a square 430m by 430m basin area with a near-horizontal initial bed level 1m above average sea level and a 50m wide breach. The foreshore was assumed to be alongshore uniform with an offshore tidal boundary condition and an assigned level of suspended sediment concentration. A wave module was coupled to the flow module to include a constant input of wave forces. Processing the results consisted of exponential curve fitting through the average change in bed level of each scenario, followed by the determination of the initial sedimentation rate and the average equilibrium bed level. These two characteristics were then transformed into parameter specific functions describing the dimensionless
dependency for a given parameter value relative to the base-case scenario. A quick assessment tool called the WisLi-tool was created to predict the sedimentation curve for arbitrary Lease Polder cases, based on these parameter specific functions. It thereafter uses this curve to perform a cost-benefit analysis
on the complete life cycle of a Lease Polder, resulting in a quantitative net profit or loss. Finally, the tool will enter an optimization process by determining the optimal Lease Polder period which results in the maximum financial benefit.

The simplified model based on the Tollesbury was able to accurately reproduce the morphological changes which followed from the historical data analysis. A summary of the sensitivity of the most significant parameters is listed below.
Suspended sediment concentration The suspended sediment concentration (SSC) at the boundary showed a positive linear relationship with the initial rate of sedimentation, where the increase in SSC was directly proportional to the increase in initial sedimentation rate. The average equilibrium bed level was however independent of the SSC. Breach width The breach width showed to have a negative relationship for both the initial rate of sedimentation and the average equilibrium depth. Polder length and width The dimensional length and width of the polder showed to have an insignificant effect on the initial sedimentation rate, with differences remaining within the 10% range. The average equilibrium bed level decreased for increasing polder length and width. Initial polder bed level The sensitivity analysis on the initial polder bed level showed a dichotomy into two regimes, where the ’shallow regime’ was coherent to initial bed levels betweenODÅ1.0m andOD¡1.5m and the ’deep regime’ to the bed levels between OD ¡2.0m and OD ¡4.0m. The deep regime was characterized by the abrupt stop in sedimentation due to the high levels of wave energy entering the polder. Both regimes showed decreasing levels of initial sedimentation rate and average equilibrium bed level for increasing initial polder bed levels. Tidal amplitude The tidal amplitude showed to have a positive relationship with the initial sedimentation rate and the average equilibrium bed level for tidal amplitudes of 2.5m and lower. Higher tidal amplitudes caused high-velocity circular flow patterns within the polder, preventing further siltation. The above-mentioned relationships were translated into functions describing the dimensionless dependency for a given parameter value relative to the base-case scenario.

The analysis on the effect of sea level rise on the sedimentation process of a Lease Polder concluded that the siltation process was able to produce a local bed level change following the exact rate of sea level rise, for an average rate of sea level rise of 4.2mm/year . The Lease Polder can, therefore, be seen as a partly self-sustaining system of coastal defence. The quick assessment WisLi-tool was able to accurately predict the sedimentation curve based on case-specific input parameters using the dimensionless functions derived from the sensitivity analysis. By utilizing location-dependent financial input parameters, the tool was capable of performing an optimization process for the determination of the optimal Lease Polder period.

The final conclusion is that there is not a single uniform optimal duration for the Lease Polder period but that it may vary for each individual Lease Polder case. However, the WisLi-tool was able to assess arbitrary Lease Polder cases based on their parameter values and could determine the case-specific optimal duration of the Lease Polder period. Therefore, the WisLi-tool can act as a valuable resource during both the design and operational phases.