Investigating Changes in Aeolian Sediment Transport at Coastal Dunes and Sand Trapping Fences: A Field Study on the German Coast

Abstract

For the restoration and maintenance of beach and dune systems along the coast, knowledge of aeolian sediment transport and its interaction with coastal protection measures is required. As a nature-based solution, sand trapping fences can be an integral part of coastal protection measures initiating foredune development. There are few detailed studies on aeolian sediment transport rates on coastal dunes and sand trapping fences available to date. Thus, in this work, we present the results of field experiments conducted at the beach, coastal dune, and sand trapping fence on the East Frisian island Langeoog. The vertical sediment flux profile was measured by vertical mesh sand traps, and saltiphones measured the instantaneous sediment transport. A meteorological station was set up to obtain wind data. On the beach, dune toe, and dune crest, the stationary wind profile can be described well by the law of the wall. Saturated aeolian sediment transport rates on the beach and dune toe were predicted by widely used empirical models. Between the sand trapping fence, these empirical transport models could not be applied, as no logarithmic wind profile existed. The upwind sediment supply reduced after each brushwood line of the sand trapping fence, thereby, leading to increased deviation from the saturated conditions.

Influence of Sand Trapping Fences on Dune Toe Growth and Its Relation with Potential Aeolian Sediment Transport

“…The East Frisian island Langeoog (see Figure 2b) covers ~20 km², and its morphology generally consists of a natural sandy beach followed by foredunes and older dune landscapes. The fully established foredune is ~ 20 m high and partially covered with the European Marram grass (Ammophila arenaria) [2,36]. The island has a coastal dike line of 5.5 km (red line) in the southwest and a coastal dike line (red line) of 0.3 km in the south, while coastal dunes (orange areas) over ~20.3 km protect the coastline from the southwest side of the island northward towards the east.…”

“…The study site Langeoog is located north of the island at the dune toe, see Figure 2b. The beach comprises quartz sand with a median grain size of d50 = 218 µm [36]. The beach has a relatively steep slope with m ~ 1:50.…”

“…On the other hand, in coastal regions, sand trapping fences are placed with the following purposes: rehabilitating eroded areas from storm surges or blowouts in the coastal dunes, for reinforcement of the coastal dune toe, for protecting transport infrastructure or monetary assets from drifting sand, to control human access to nature reserve, and to initiate the formation of coastal dunes by supporting the selective deposition of sand [30,31]. While wind velocity reduction has already been extensively studied for wind fences [27,30,32,33], only a few detailed studies, e.g., [23][24][25]31,[34][35][36][37], about sand trapping fences for initiating and facilitating the establishment of the dune toe are available. There is currently a research demand on sand-trapping fences in coastal areas for the position relative to the beach profile, the porosity and height of the fence, and the arrangement of the fence [25,36].…”

“…While wind velocity reduction has already been extensively studied for wind fences [27,30,32,33], only a few detailed studies, e.g., [23][24][25]31,[34][35][36][37], about sand trapping fences for initiating and facilitating the establishment of the dune toe are available. There is currently a research demand on sand-trapping fences in coastal areas for the position relative to the beach profile, the porosity and height of the fence, and the arrangement of the fence [25,36].…”

“…Currently, the design of these sand trapping fences on the East Frisian islands, i.e., the arrangement of the brushwood lines parallel and orthogonal to the coastal dunes, the position of the brushwood lines relative to the dune profile as well as the porosity and the height of the sand trapping fences is based on empirical knowledge, creating further uncertainties for implementing these coastal protection measures [36].…”