Usability of the climate-resilient nature-based sand motor pilot, The Netherlands

Abstract

Coastline maintenance in the Netherlands, formally in place since 1990, aims at a sustainable preservation of coastal flood protection. Over 25 years annual assessments, comparing the actual coastline positions with the 1990 reference position of the coastline, have governed the execution of sand nourishments following an adaptive management method. This method involves yearly assessment of the coastline based on measurements, design and adoption of nourishment strategies and measures and execution of the nourishments. This management approach has enabled learning and introduction of innovations in coastline maintenance. For instance, in comparison to the early nineties, nourishments are now placed more on the foreshore and the yearly nourishment budget has doubled. The most recent innovation in coastline maintenance is the ‘Sand Motor’, a nature-based nourishment approach, which concentrates the regular nourishments in space and time, given that natural processes should redistribute the sediment over the wider coastal system. In contrast to regular nourishments, the Sand Motor combines flood protection with nature and recreational objectives and is much larger in dimensions. Five years after the construction of the Sand Motor we investigate its usability in this article. We present the results of first Sand Motor evaluation and draw conclusion on the adoption and usefulness of it for coastline management from the perspective of the adaptive management method used in coastline maintenance. Recent evaluation of the monitoring data shows that the large amount of sand used for the Sand Motor has a positive impact on coastal protection. Bridging between the Sand Motor pilot and daily nourishement practice is however not yet achieved.

Appendix: morphodynamic modelling in support to design

A depth-average morphodynamic Delft3D model (Lesser et al. 2004) of the South Holland coast has been set up for the evaluation of the morphological effects of the 4 designs, selected in the exploratory phase, on a time scale of 20–50 years. The morphodynamic approach comprises the use of the SWAN wave module (Holthuijsen et al. 1993) and of TRANSPOR2004 (Van Rijn 2007a, b, c) as sediment transport model; see Mulder and Tonnon (2010) for more details.

The flow domain is about 26 km long and 15 km wide, while the wave domain stretches further to the south and north to minimize boundary effects (see Fig. 6a). The maximum grid resolution in both domains in the area of interest is 80 m alongshore by 20 m cross-shore. The flow model applies Neumann boundary conditions (Roelvink and Walstra 2004) at both lateral boundaries, water levels at the north-western (sea) boundary and a discharge at the entrance of the Nieuwe Waterweg entrance channel to the Port of Rotterdam in the South West. Boundary conditions were derived from the validated large-scale ZUNO-FIJN North Sea model (Roelvink et al. 2001) and the ZEEDELTA v8 model (Zijlema 2003) (Fig. 6b). Offshore wave data from wave buoys Euro Platform and Meetpost Noordwijk was classified into 10 wave height bins between 0.25 m and 5.25 m and in 12 directional bins between 195 °N and 15 °N.

Fig. 6

Left: model domains for flow (red) and wave model (black). Right: ZUNO-FIJN domain (black) and ZEEDELTA v8 domain (red). (From Mulder and Tonnon 2010)

Tidal and wave boundary data have been schematized to enable the long-term net total transports. A so-called representative, morphological tide was derived using the method of Latteux (1995). A harmonic analysis using 16 components was performed over this period to result in a cyclic tidal signal. A representative morphological wave climate of 10 conditions was derived using the so-called “Opti” method, based on a statistical description of the performance of wave conditions with respect to the net and gross transport fields. The parallel-online approach developed by Roelvink (2006), has been used to simultaneously compute the weighted bed changes resulting from these 10 representative wave conditions. The uncalibrated net transport rates thus computed by the model, were found to be in the order of the net transport rates reported by Van Rijn (1997).

Estimates of dune development were required given that both long term safety and nature development are main objectives of the pilot Sand Motor. A dedicated dune module has been developed and implemented following De Vriend and Roelvink (1989) to account for intertidal, beach and Aeolian processes. An empirical relation, considering that the migration rate of the dune foot is function of the difference between the actual beach width and its equilibrium width (i.e. approximately 125 m based on historic data of the Dutch coast), was implemented in the dune module to simulate horizontal migration of the dune foot.

A schematized nourishment program was implemented in the model for investigating the effect of the Sand Motor on the regular maintenance effort of the adjacent BKL and the coastal foundation. The schematization consists of a nourishment of the erosive stretches with a frequency of 5 years. The shoreface nourishments were placed at a depth of −5 m, with a volume equal to twice the calculated loss in the MCL-zone (i.e. Momentary Coast Line zone between +3 and −4.4 m; see e.g. Van Koningsveld and Mulder 2004). Sea level rise was assumed to be constant and equal to the present rate of 2 mm/year. As a consequence, the total nourishment volume to maintain the coastal foundation of the coastal cell of Delfland (between Hook of Holland and The Hague), in the reference case without a Sand Motor, is 1.1 million m³ per year. The implemented nourishment program was found to maintain the coastline at its position in the reference simulation with realistic nourishment volumes.

The different designs for the Sand Motor were evaluated based on computed wave fields, flow velocity fields, sediment transport fields, morphological evolution, sedimentation/erosion patterns, erosion of adjacent coastlines, necessary nourishment volumes and evolution of dune area. Results were compared to the reference simulation without a Sand Motor (Mulder and Tonnon 2010).

Sensitivity computations were carried out in which forcing conditions, model parameters and numerical settings were varied. For instance, an uncertainty of about 30% in magnitude and time was estimated for the erosion volume of the MCL zone.

Figure 7 shows the computed morphological development of the hook-shaped alternative after 5, 10 and 15 years. The top of the hook is elongated in northern direction and pushed onshore, creating a shallow, sheltered area that is filled and emptied through a small channel. In time the elongated sand body is breached and a shallow plain is formed, ultimately leading to wide beaches. Sand is spread in both northern and southern directions.

Fig. 7

Computed morphological development of hook-shaped design with the initial bathymetry in the upper left panel, the bathymetries after 5, 10 and 15 years in the upper right, lower left and lower right panel respectively. (From Mulder and Tonnon 2010)

The horizontal migration of the dune foot has been estimated, resulting in estimates of the change in total dune area for the Delfland coast. The results for three different designs compared to a reference situation are shown in Table 1. All alternatives implied 5 yearly nourishments with as purpose the maintenance of the coastline in case of the different Sand Motor designs, and the maintenance of both the coastline and the coastal foundation in case of the reference situation. The nourishments were schematised according to the approach mentioned above.

Table 1 Estimates of increase in dune area (ha)

The Sand Motor designs create approximately twice as much new dune area in comparison to the reference situation without a Sand Motor. The dune creation by the three designs hardly shows significant differences. As the dune growth contributes to a stronger dune, the model results suggest that a Sand Motor represents an effective method to enhance long term safety against flooding.

The effect of a Sand Motor on the nourishment effort is presented in Table 2. Firstly, for the reference situation, the schematized nourishment scheme implies a 5 yearly nourishment of an amount of 5.5 million m³ equal to the volume enabling the coastal foundation to grow with the sea level (with rise of 2 mm/year). When considering the placement of the Sand Motor, the 5 yearly nourishment volumes range from 1.3–2.0 million m³ after 5 years to 0.7–0.9 million m³ after 20 years. After 20 years, the total sand input by each of the Sand Motor alternatives is still but slightly larger than in the reference case: 23.3–25.6 million m³ compared to 22.6. In case of a Sand Motor design of 20 million m³ and for a seal level rise of 2 mm/year, additional nourishments will be required to maintain the coastal foundation after a period of approximately 22–25 years.

Table 2 Estimates of nourishment volumes to guarantee maintenance of the coastline and of the coastal foundation (millions m3)