Reducing aeolian sand transport and beach erosion by using armour layer of coarse materials

摘要

A man-made dune-beach-spit system at the south-east side of the island of Texel (Prins Hendrik site) has been built in 2018?2019 to strengthen the traditional dike. The core of the dune-beach-spit system consists of medium fine sand with a d50 of 0.25?0.3 mm. The beach is covered with an armour (protection) layer of coarse materials with relatively large gravel and shell fractions to reduce wind erosion and thus maintenance costs. In the design phase of the project the aeolian sand transport model of Bagnold was used to estimate the long-term erosion losses of sand at the new dune-beach system. This transport model was validated in the design phase by using detailed sand transport and bed roughness measurements at a nearby site called The Hors. This site is a wide natural beach plain of sand (d50 = 0.23 mm), where 147 high-quality datasets have been collected using a wind mast equipped with 5 cup anemometers and various sand traps. It is shown that the measured sand transport rates at the Hors can be reasonably well represented by the modified Bagnold-equation for dry sand. After completion of the new dune-beach system, a field experiment was performed at the Prins Hendrik site to verify the sediment transport predictions and erosion loss of sand. Data from two permanent wind masts and one short, mobile wind mast were used to derive the effective roughness of (stationary) bed forms. Sand transport rates were measured at various locations using a new trap sampler. The measured sediment transport in the armoured beach zone can be reasonably well represented by the Bagnold-equation using a multi-fraction approach with hiding-exposure coefficient. The predicted transport rates have been used to estimate the annual loss of sand from the Prins-Hendrik site.