A Nature-Based Solution for Coastal Protection: Wind Tunnel Investigations on the Influence of Sand-Trapping Fences on Sediment Accretion

Abstract

Sand-trapping fences are a frequently used nature-based solution in coastal protection for initiating and facilitating coastal dune toe growth. However, only a few researchers have evaluated the trap efficiency of sand-trapping fences based on their porosity and height. Subsequently, the design of their properties has only been based on empirical knowledge, to date. However, for restoring and maintaining coastal beach–dune systems, exact knowledge of sand-trapping fence’s optimal properties is essential. Thus, we conducted physical model tests focusing on the most crucial parameters: fence height (h = 40, 80, 120 mm) and fence porosity (ε = 22.6, 41.6, and 56.5%). These tests were conducted in an indoor subsonic, blowing-sand wind tunnel equipped with a moveable sediment bed (d50 ∼ 212 µm). The experimental mean wind velocities were u1 = 6.1 m/s, u2 = 7.4 m/s, and u3 = 9.3 m/s. We used a hot-wire anemometer to measure the flow fields, a vertical mesh sand trap to determine the sediment fluxes, and a 2D laser scanner to record the sediment accretion around the sand-trapping fences over time. The study results provide substantial theoretical and practical support for the installation and configuration of trapping fences and improving their design. The fence porosity, for example, should be chosen depending on the installation purpose. While denser fence porosities (ε1 = 22.6% and ε2 = 41.6%) can be used for initiating and facilitating the dune toe growth, fences with higher porosity (ε3 = 56.5%) are more suitable to favor the sediment accretion between foredunes and white dunes as they allow further dune growth downwind.

Review of the Quantification of Aeolian Sediment Transport in Coastal Areas

“…Continued evaluation included assessing the reliability Lastly, when looking into the relationship between wind velocities and fence effectiveness, the need for further research presents itself. Recent results from field experiments conducted at coastal dunes and on sand-trapping fences underline this need for knowledge on aeolian sediment transport rates and the interaction with coastal protection measures [155]. But their study showed that empirical transport models could not be applied between the sand-trapping fence due to the absence of a logarithmic wind profile, indicating a more complex interaction between the wind flow and the fence structure that impacts sediment transport rates.…”

“…With denser fence porosity, the deposited sediment on the windward side of the sand-trapping fence increased. The sediment accretion on the leeward side of the fence developed simultaneously but faster than the sediment accretion on the windward side of the fence [155].…”

“…Experiments in a wind tunnel at Aachen University, Germany, were conducted to test the wind regime and sand-trapping efficiency of sand-trapping fences, with different fence porosities and fence heights (Figure 5) [155]. In these experiments, a total of 27 cases of different heights and porosities of sand fences were tested.…”

“…With denser fence porosity, the deposited sediment on the windward side of the sand-trapping fence increased. The sediment accretion on the leeward side of the fence developed simultaneously but faster than the sediment accretion on the windward side of the fence [155]. To better understand the optimal fence configuration of height and porosity, eight different configurations of fences were tested in field experiments by Ning et al [150] (Figure 6).…”