HOLWERD AND THE DUNE
Design project about adding ecological and recreational value to the Frisian Wadden Sea coast, by a coastal defense sample for the year of 2117.
Project type · Ecological research and landscape architecture design
Date · 2017
Location · Holwerd, Friesland, The Netherlands
Mentors · Ir.PhD. Nico Tillie & Berrie van Elderen
Keywords · Coastal defense, sea level rise, wetland introduction, dune landscape, salt marshes, flood protection, landscape ecology, social reconnection, design with nature, nature based design.
In collaboration with:
A design nature based design on the border of land and sea
Holwerd needs attention. All passersby on their way to Wadden Island Ameland rush past this Frisian village, without even taking a look. The Dune will make a difference. This enormous mount of sand frames a new world between Holwerd and the Wadden Sea. It is the opposite of the residents’ initiative, the break through, which is not climate proof. With a height up to 25 meters, this coastal defense element is ready for the expected sea level rise of the year 2117.
The concept is illustrated above. In yellow, the dune as the main element, which lies both inside and outside the sea wall. It is connected to the existing salt marsh marked dark green and the new wetlands north of the village. The existing canal, the Holwerter Feart, is now connected to this new swampy ecology. As a counterpart to the improvements inside the sea wall, on the other side of the seawall the ferry pier has been transformed to a port.
In the harbor, there is room for pleasure boats (d) and a hotel with restaurant. In addition, this is the place where tidal energy is generated (e) and a business park (c) with a quay can be found.
The dune offers space to 80 holiday homes (b) and 30 new houses (a). The adjacent wetland has trails which are connected to the salt marsh and is together with dune home to a new ecology.
The new dune landscape and the unique inter tidal area host an extensive flora and fauna which is shown below. Sea and wind constantly distort the new dune and during storms lots of sand can be replaced. Here, only tough grass species can grow, like European Beach grass (Ammophilia arenaria). This essential plant grows in linear clusters that keep the sand in place and prevent the dune from eroding. Behind the dunes, there is a zone with larger shrubs and pine trees. Sand which is blown over the dunes is captured here by the vegetation. In the dune, fresh water is hidden which surfaces in the inland valley as ponds between the pines. Specific plant- and animal life can be found here, like orchids (Platanthera bifolia) and roe deer (Capreolus capreolus).
The sea side of the dune can be rough, the mudflats between come dry twice a day and supply food for many coastal birds. The Arctic Tern (Sterna paradisaea) breeds on the higher parts of the salt marsh, but as close to the sea as possible. Here they can find enough food for their chicks, which can stay safe and dry in their nests. During low tide the birds can find shellfish, worms and small crustaceans on the mudflats and during high tide they can feed on insects that are found on the salt marshes. In contrast to the dune, the fragile marsh is not accessible for people, but can be observed from the dune, the port and the sea.
Sand and silt can be found locally between Ameland and Holwerd. The Wadden Sea has many sandbanks, but not every bank can be excavated. In certain places, mussel beds attach with high ecological value. They filter the turbid water of the Wadden Sea, improving the habitat of different fish species. Silt comes from dredging the fairways. It is a residual product that can be reused for the dune. For example, 365 days a year the fairway between Holwerd and Ameland is dredged for the ferry. The silt is being dropped further, but within moments two-thirds of the silt is deposited back in the fairway by the current. Using the silt on land could solve this problem.
Finally, the entire construction of clay basins is covered with sand. Vegetation as European Beach grass largely prevents the sand from blowing away inland. Nevertheless, the dune will be subject to erosion and a sand supply is required. This can be done in two ways, shown in figure 6. Firstly by beach nourishment, where sand is deposited on land. The wind blows the sand towards the dune. Secondly, foreshore nourishment, in which sand is applied in the shallow bank just in front of the dune. The current settles the sand on land and there it is blown up by the wind, like the beach nourishment method.