Sand in motion – What does wind do to wet sand?
This UT FieldLab experiment falls under the theme Water cycle and climate. View all experiments related to this theme.
Introduction
On the beach, there's a lot of sand and it's almost always windy. Yet, all this sand doesn't simply blow away. How does that work? A key explanation lies in the adhesive force of moisture on the beach. In this wind tunnel, we simulate beach conditions to understand how sand grains can detach from a wet beach and how evaporation of soil moisture by wind influences this. This everyday natural phenomenon may seem simple, but the details are complex. Besides research into the behavior of sand grains on beaches, this wind tunnel is also used to investigate evaporation from surfaces other than sand. Examples include non-natural surfaces such as those found in urban environments, as well as short vegetation like grass. The focus in those studies will be on the influence of wind on the rapid change in surface temperature of these surfaces. Moreover, this is not only a research facility but also a place for education.
Currently, the test section of the wind tunnel is being modified to enable simulation of a beach surface and regulation of the moisture content. Therefore, below are some examples from the natural situation we aim to simulate, and a computer simulation of sand grains set in motion by the wind.
Wind-driven sand transport over a wet beach. (link to video)
Computer simulation of wind-driven movement of sand grains (link to animation)
Partly wet beach (darker) due to the daily ebb and flood cylces of the sea in combination with the groundwater level.
Due to a rain shower, the top layer of the (usually dry) higher part of the beach has been wetted (see the darker area in the upper right of the photo). Because it isn't evenly wet everywhere and doesn't dry evenly through evaporation, the sand is blown away unevenly, creating intricate shapes (foreground, approximately 1-2 cm high), where the adhesive force of moisture holds the sand turrets together.
What are we researching?
We want to know how factors like soil moisture and wind speed influence the transport of sand. Exactly how much moisture must evaporate from the surface before a grain of sand can be set in motion by the wind on a damp beach? How easily can a loose grain of sand set other grains in motion? Once a grain is loose, do others follow quickly? Is evaporation uniform? And many more questions remain.
How does it work?
This building houses a wind tunnel (see photo below) in which we can simulate beaches (or other moist surfaces, if desired). By varying the wind speed and moisture content of the sand bed, we simulate in our experiments the diverse conditions that occur in the field. Thermal cameras and sensors track heat, allowing us to monitor how soil moisture patterns change with wind. We also measure wind speed, of course, as well as air temperature and relative humidity. Using powerful LED lighting and a high-speed camera, we track the movement of sand grains as they fly through the air, allowing us to analyze sand transport in detail.
The wind tunnel. On the right side is a fan, which sets the air in motion. In the wider section, the air movement is conditioned so that the fan's vortices are not reflected in the measurement section. Experiments are conducted in the test section (in the middle with the windows). On the left side, the air is blown out. The test section is currently being modified to simulate wet beaches.
Why is this important?
In a low-lying country like the Netherlands, dunes are crucial for protection against coastal flooding, as they are the only line of defense against the sea along large stretches of coast. Dunes grow and recover from storm erosion due to beach sand blown into the dunes. By better understanding how sand can be transported by wind under different weather conditions, we can more accurately calculate how much sand is blown away from the beach. By incorporating this knowledge into computer models that can simulate an entire beach and its dunes, we can better predict how quickly these protective dunes can grow in the coming decades, taking into account that climate change will alter future weather conditions in terms of wind and evaporation. Beach management can also be improved. For example, if extra sand is added to the beach for coastal maintenance (sand nourishment), how can this best be done if we also want to stimulate dune growth? In this way, we can make coastal management smarter and more sustainable, helping to anticipate the consequences of climate change and contributing to safe, resilient coasts.