Wind flow dynamics and sand deposition behind excavated foredune notches on developed coasts

Abstract

AbstractThis study examines wind flow dynamics and downwind sand deposition behind engineered notches in a foredune system on a developed coast. Recent studies have examined the morphodynamics downwind of such notches, however, wind flow dynamics and sand deposition in the lee of such notches is not well understood, particularly in relation to artificial foredunes on developed coasts. Wind flow dynamics behind an excavated notch at St Kilda beach, Dunedin, New Zealand is measured using vertical arrays of ultrasonic anemometers. Field data is used to validate computational fluid dynamic flow simulations to examine flow dynamics behind a foredune section with multiple notches. Sand deposition behind the notches is measured using sand pot traps.Wind direction across the depositional lobe aligns with notch orientation and the normalized wind speed increases when the incident wind direction is >10° to the shoreline in a notch that orientates 67° to the shoreline. Wind speed in the swale downwind a notch decreases when the incident wind direction shifts from alongshore to onshore normal. Flow reversal occurs when the incident wind direction approaches onshore normal (>72° to the shoreline). Helicoidal flow appears to occur in the lee of the higher foredune section when the incident wind direction is 74° to the shoreline, while alongshore flow dominates in the lower section. Most sand deposition occurs within 5 m of the depositional lobe of the notch. The orientation of excavated notches should be less than 74° relative to the shoreline to prevent the occurrence of flow reversal in the swale, which might reduce the distance that sand can be transported inland from the notches when the incident wind direction is parallel to the notch axis.