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ABSTRACT

Shoreface nourishments, in principle, provide an attractive alternative for beach nourishments. In the NOURTEC project, it has been demonstrated that they can be applied with good success. In this contribution, some results of the Terschelling case are presented. Based on its success, several new shoreface nourishments are underway in The Netherlands.

INTRODUCTION

Beach nourishments, nowadays, are widely applied to mitigate shore erosion. They have generally replaced the traditional hard measures like seawalls and groynes, that not only are expensive to construct and maintain, but also show adverse side effects. Placing the sand under water, on the shoreface instead of on the beach, may offer further advantages. Costs may be reduced since some handling and the installation of, e.g. pipelines, are avoided, while any ongoing recreation is no longer hindered. Practical experience with this form of nourishing is limited, but has recently been increased significantly in the project NOURTEC (Innovative Nourishment Techniques Evaluation).

NOURTEC

The project NOURTEC (May 1993 - December 1996) of the MAST-II program of the EU involved three coastal sites in Europe with under water nourishments: Torsminde Tange (DK, 1993), Norderney (FRG, 1992) and Terschelling (NL, 1993). In NOURTEC, the effectiveness of the nourishments and the relevant physical processes have been studied, while the ecological impacts were the subject of the RIACON project (Essink & van Dalfsen, 1998).

The designs and objectives of the nourishments and the physical conditions at the sites differed (Niemeyer et al., 1995). Thus, a joint project like NOURTEC covers a relatively wide range of conditions, which is crucial at the present state of limited knowledge of the behavior of coastal systems. Better, and more cost-effective, results have been obtained than separate projects would have allowed. The NOURTEC results have been made public at conferences, in literature and in reports. Concerning the latter, a joint final report is available (Nourtec, 1997) that integrates the experience of the three cases, as are two reports per country. One national report covers the observed effectiveness of the nourishment in relation to its design objectives (Biegel & Spanhoff, 1996; Knaack et al., 1996a; Laustrup et al., 1997a), and the other gives a synthesis per site of morphologic observations, process measurements and model calculations (Hoekstra et al., 1996; Knaack et al., 1996b; Laustrup et al., 1997b). Furthermore, a series of underlying reports has been produced.

An important NOURTEC lesson, which turned out to be much less trivial than it might seem to an outsider, is the necessity to define beforehand clear objectives and design parameters of a shoreface nourishment that can be evaluated quantitatively later. The objective of the Terschelling nourishment was to prevent, for a period of 8 years, the natural retreat of the coast line in a 4 km long stretch of coast in the center of the island, by placing 2 million m3 sand there (taken from the deep sea bottom some 10 km away). In The Netherlands, the coast line is derived from the observed volume of sand in a horizontal control slice between fixed levels (NAP+3m and NAP-5m at Terschelling) bounded by the profile and a fixed inland vertical reference line.

R. Spanhoff

R. Spanhoff

R. Spanhoff

Most sand was dumped below this slice, in the trough of the outer two of the three bars that are characteristic for this part of the Terschelling coast (Fig. 1). The idea of the design was that by natural processes half of the dumped sand would gradually feed the control slice while moving on-shore. The dumped sand had similar grain sizes as the native sand. At Torsminde a weak spot in the most seaward bar has been reinforced by placing coarser than native sand there, with the idea that the coast behind it would be less subject to erosion than before. The dumped (coarse) sand should stay in place sufficiently long. At Norderney parts of the coast are regularly nourished to protect an existing seawall against wave attack. Placing part of the needed sand on the shoreface adjacent to the beach, instead of all the sand on the beach, was supposed to result in a more efficient use of the sand. All three NOURTEC cases were successful. For the conclusions, reference is made to the above mentioned reports and publications. Some results came as a surprise, as illustrated by the Terschelling case.

TERSCHELLING

Figure 2 shows the original behavior of the coastline position of the coastal section to be nourished at Terschelling, and the response to the nourishment. Coastal profiles have been measured each year since 1965, with echo sounding and beach levelling. The mean coast line position has been derived from the measured sand volume in the control slice mentioned above. The position has been seen to gradually retreat a few meters per year, with inter-annual fluctuations that are partly natural and partly due to sounding errors.

The nourishment was supposed to feed the coast by a (net) cross-shore transport of a significant part of the dumped sand, thus compensating the natural losses in 8 years (anticipated net coastline gain of 20 m). However, the figure shows the gain to be much greater (already 30 m at the end of 1996, and still increasing), so obviously, huge extra amounts of sand have been gained behind the nourishment. The explanation is that the nourishment acts also as a submerged breakwater that traps (shoreward of it) part of the sand that is normally longshore transported by waves and currents, in the surf zone (here to the East). Now being starved partly from its natural sand supply, the Terschelling coast might show some erosion east of the nourishment site. Luckily enough, this hardly plays a role in this case, since this part of the coast has a natural trend to accrete.

Based on the success of the Terschelling nourishment, more shoreface nourishments are now being planned and carried out in The Netherlands. For example, this year a nourishment is foreseen at Ameland (Fig. 1). Now, the sand will be placed outside the most seaward bar, which definitely has practical advantages. It is anticipated that its effect, though smaller than that of the Terschelling nourishment, will still meet its design objectives.

ACKNOWLEDGEMENT

The NOURTEC project has been co-sponsored by the Commission of the European Communities, Directorate General for Science, Research and Development (DG XII) under the Marine Science & Technology (MAST-II) Program, under contract MAS2-CT93-0049.

R. Spannhoff

REFERENCES

Biegel, E.J. and R. Spanhoff, 1996. Effectiveness of a shoreface nourishment Terschelling, The Netherlands. Rijkswaterstaat, National Institute of Coastal and Marine Management, 42 pp.

Essink, K. and J.A. van Dalfsen, 1998. Ecological risks of shoreface nourishment. Wadden Sea Newsletter, this volume.

Hoekstra, P., Houwman, K.T., Kroon, A. and Ruessink, B.G., 1996. Morphodynamic Behavior of the Terschelling Shoreface Nourishment - Morphological changes in response to hydrodynamical and sediment transport processes and sediment characteristics. Utrecht University, Institute for Marine and Atmospheric Research, IMAU report R96.14, part 1, 64 pp. and part 2.

Knaack, H., Kaiser, R. and Niemeyer, H.D., 1996a. Behavior and Effectiveness of the NOURTEC experimental shoreface nourishments -Norderney case, CRS Norderney, 14 pp.

Knaack, H., Kaiser, R. and Niemeyer, H.D., 1996b. Dominant processes and synthesis - Norderney case. CRS Norderney, 19 pp.

Laustrup, C., H. Toxvig Madsen and P. Sorensen, 1997a. NOURTEC-Torsminde Tange; Effectiveness report. Danish Coastal Authority.

Laustrup, C., H. Toxvig Madsen and P. Sorensen, 1997b. NOURTEC-Torsminde Tange; Synthesis report. Danish Coastal Authority.

Niemeyer, H.D., E. Biegel, R. Kaiser, H. Knaack, C. Laustrup, R. Spanhoff, and H. Toxvig Madsen, 1995. General Aims of the NOURTEC project - effectiveness and execution of beach and shoreface nourishments. Proc. 4th Conf.on Coast. & Port Eng. in Develop. Countr., Rio de Janeiro, Brazil.

NOURTEC, 1997. Innovative nourishment techniques evaluation, Final report. Coord. Rijkswaterstaat, National Institute for Coastal and Marine Management/RIKZ, The Hague, The Netherlands, 105 pp.

Authors address:

R. Spannhoff

Rijkswaterstaat - RIKZ,

Postbus 20907,

NL - 2500 EX Den Haag

e-mail: R.SPANHOFF@rikz.rws.minvenw.nl