Coastal Guide on Dune Management

Sandy beaches and intertidal flats are among the most dynamic environments. They occur on all sedimentary shorelines exposed to waves, the so-called ‘coastal-plain’ or shoreface respectively (see: Coastal Landscapes). The shoreface, as defined morphodynamically, extends from the limit of wave runup on the beach face, seaward to the limit of effective influence by gravity waves in shaping seabed morphology. The geomorphology and nature of sandy shorelines and dunes is totally dependent on waves and sediment, but independent of most of other surface processes. Consequently they occur in all latitudes, in all climates, in all tidal ranges and on all types of coasts (for Europe see: Coastal Systems of Europe). However, they are influenced and modified by other processes, particularly wind and tidal energy, parameters such as size and type of sediment, biota, air and water temperature, and water chemistry (7, 9). In very few cases sandy beaches and intertidal flats can be found in a natural condition (see: Natural processes), mostly they are influenced and managed by man (see: The management of tidal inlets, Coastal defence management, Sand replenishment for coastal defence).

Beach morphodynamics can be defined as the ‘mutual adjustment of topography and fluid dynamics involving sediment transport’. On beaches this implies that the surface topography of the beach (swash, surf and shoaling wave zones) will adjust to accomodate the fluid motions produced by waves, tides and other currents, which in turn will influence the wave and tide processes (9). Alternations of advance and retreat even in the same part of the coastline can constantly be noticed (1). Connected to the shorface are the backshore and the dune area, both being in a constant exchange of material with the beach because of aeolian and hydrodynamic sediment transport processes (see: Sand replenishment for coastal defence, Natural foredunes; case studies: Ameland, Terschelling, The Netherlands).

Despite nowadays those dynamic processes are well-known and accepted in a wide range, people still tend to accept the dynamic character of the sea as long as the dynamical processes lead to an increase of the landscape (e.g. the island Memmert, Germany), but they tend to negate a natural development and want to interfere as soon as erosion starts to prevail. In such situations nature conservation and public interests can quickly come into a conflict (see: the islands Rottumeroog and Rottumerplaat, The Netherlands).

A natural dune coastline is characterised by the occurence of tidal inlets. They may have developed in (former) shallows as a combination of various factors:

(i) as remnants of old rivulets or creeks (e.g. The Zwin, Belgium, many French inlets),
(ii) storm impacts (the island Schiermonnikoog, The Netherlands),
(iii) after reclamation (the ‘slufter’ on the island Texel, The Netherlands) or
(iv) on purpose by man (see: The management of tidal inlets).

The management of tidal inlets may range from (i) closing of inlets, e.g. as part of an operation that opens a new one, to (ii) stabilizing inlets, e.g. to maintain predictable navigation channels of shoreline positions, (iii) increasing inlet mobility, e.g. for the creation profound alterations to the adjacent shoreline and (iv) creating inlets, e.g. to improve navigation or water quality (6) (see: Ringkøbing Fjord, Denmark). With respect to nature conservation management, there is room for natural processes in places where dune ridges are several kilometres wide and the sand dune body high enough to prevent the hinterland from flooding by the sea. In these places coastal defence is not critical and tidal inlets can be allowed. They may develop as a result of natural processes (see: Schiermonnikoog, The Netherlands) or by human influence. In many French dune systems tidal inlets have developed as natural dune streams carrying water from the dunes to the beach. At spring tides these inlets are flooded with sea water. This phenomenon is comparable with the recently restored rivulet of the Wadden Sea Island Terschelling, The Netherlands. In the Schoorl dune area (also The Netherlands) an inlet was made in the foredunes to let the sea in from time to time. This way, the dunes are enabled to more natural conditions.

Historically, coastal landscapes and dunes respectively have been exploited in a variety of ways. Man interposed hard structures like houses into a relatively soft and dynamic environment (and still do so). The wish of dunes being permanent led in the past to a coastal defence management, which oppressed dynamics and created a so-called hard coastline (3). Much of the protection work carried out in the past involved the construction of relatively simple hard structures built in response to periodic emergencies without investigating even the short term implications of such structures on the coastline as a whole. Inappropriate structures can accelerate erosion of soft coasts or erosion even takes place further upshore or downshore (see: Ringkøbing Fjord, Denmark). If this would be continued the overall coastal environment would be at risk (8). Some countries react on such bad experiences in the past and the increased knowledge of dynamic processes of the coastline with new projects or even new policies. The Government of the Netherlands decided in 1990 on dynamic preservation of the coastline, allowing dynamic processes where coastal defence is not critical while maintaining the coastline in its 1990s position with sand replenishment (2, 4) (see: Meijendel dunes, The Netherlands). Ireland launched in 1995 the project ECOPRO – ‘Environmentally Friendly Coastal Protection’ -.

Over the last ten years sand replenishment has become widely practiced in Europe. It is used as a way to counteract coastal erosion and to restore or maintain beaches and their facilities (see: Sylt, Germany). The method is a soft engineering alternative, furthermore it is a cheap option (contrary to the "solid" and expensive ones like piers and groins; see: Coastal defence management) and fits more into the natural dynamic character of sandy coasts. Beach and dune nourishment adds a ‘wearing layer’ to absorb coastal energy so that the beach and dunes remain intact. In general, nourishment sand is borrowed in the nearshore zone of the sea, usually by dredging and transported to a discharge location in the vicinity of the beach to be nourished. The material is brought ashore and spread out over the beach. Bulldozers and cranes are used to remodel the fill. The material may be placed under the water, on the beach or in the dune area (5, 6). The management technique has many positive features and benefits, but sand replenishment may have ecological effects, too. This applies both to sand extraction sites and to the nourishment site as well as several indirect effects on the geomorphology and vegetation of the outer dunes (5).

During the extraction the amount of suspended sediment in the water increases, which has adverse effects on many biological processes. Furthermore, benthic organisms locally disappear except for less vulnerable species. The area may be recolonised which often results in a population with less species diversity and less vitality (see: Ringkøbing Fjord, Denmark). In the Mediterranean many projects have damaged underwater ecosystems (see: Posidonia beds), so in Spain the sand is now taken from inland sources (5).

On the nourishment site the original geomorphology, the flora and the fauna may be affected. Furthermore, nourishment sand has characteristics unlike beach sand and can lead to a disturbance of the natural equilibrium between the natural sand and the hydrodynamic or aeolian forces (see: Norderney, Germany and Ameland, The Netherlands) In the case of underwater shore nourishment, the nourished sand may move to the beach or in the foredunes by natural processes without affecting them (see: Natural processes). Because of that this type of nourishment is now preferred (5) (see: The NOURTEC and the RIACON projects on Terschelling, The Netherlands).