Video monitoring of meso-scale aeolian activity on a narrow beach.

Abstract

Dune erosion takes place during high storm surges, which is a process that takes place on a short time scale (hours to days). They can recover from these storm events, but this process takes much longer (months to years). Dunes need sediment to grow back and recover which comes from the beach and is transported towards the dunes by the wind. It is therefore important to understand the process of aeolian transport. High aeolian transport rates and strong dune growth are expected to happen with high wind velocities with a wind direction that is orthogonal to the dune row, but this is not always the case for narrow beaches (with a width of a few tens of meters). Weather conditions that seem quite favourable for aeolian transport do not always result in actual transport on those kinds of beaches, so there is something that limits the aeolian transport rate. This MSc research focuses on aeolian transport on a narrow beach at Egmond aan Zee, the Netherlands, to figure out what limits aeolian transport here and what kind of conditions are needed to create aeolian transport towards the dunes. Data from the KNMI was used to calculate potential transport rates for this beach. This data was later compared to Argus images from the Coast3D tower, which monitors the coastline with 5 cameras since 1995. Two of these cameras have a good view on the beach, which are used to search for traces of aeolian transport in the form of sand strips or streamers. These images are visually classified based on these traces to indicate how strong the actual aeolian transport rate is. An event has a limited transport if it has a high potential transport rate, but a much lower transport rate in reality. It turned out that the other way was also possible: events that have a low potential transport rate can still cause a strong aeolian transport. The width of the beach and the wind direction play a very important role in this; every wind that is strong enough to transport sediment can cause a high aeolian transport rate on a narrow beach, as long as the wind direction comes from an alongshore direction. All events that produce strong aeolian transport on the Argus images have a wind direction like this. However, this is not favourable for dune growth, because only a small part of this transported sediment ends up in the dunes. A lower wind velocity with a southwesterly wind direction will probably contribute the most to aeolian transport towards the dunes. High wind velocities are rare and occur often during storms. It is not uncommon that the dunes erode, due to the storm surge. This happens often here, which is quite a good wind direction for dune growth because quite a large part of the transported sediment still ends up in the dunes. Other limiting factors are moisture, the location of the bar and snow/ice, but they seem to be more important for events with a low wind velocity. The location of the bar, with a moist area between the bar and the beach, seems to be a larger problem for winds that have a seaward direction. These wind directions cause aeolian transport close to the shoreline, while the other wind directions cause transport close to the dunes. This area of transport can spread out along the entire beach width if the wind is strong enough for it.