Presentation Information
[O12-P91]Sand Ripple Diversity on Beaches: Exploring the Secrets Behind Mysterious Patterns
*Tomoya Miyano1, Ritsuki Kuroda1 (1. Hamamatsu Gakugei Junior High School / High School Science clab)
Keywords:
coast,terrain model,sandy beach
1.Purpose
The purpose of this study is to identify the factors that cause the distinctive differences in morphology between wind ripples and subaqueous ripples formed on the Enshu-nada coast. Wind ripples, developed by the movement of sand grains (suspension, creep, and saltation) driven by winter monsoon winds, exhibit uniform, wavy patterns that are continuous and perpendicular to the wind direction (Nishimori, 1995). In contrast, subaqueous ripples, developed by water waves, exhibit irregular, diverse shapes with frequent discontinuities. This study focuses on these differences to examine how the properties of fluids—gases versus liquids—affect ripple formation and to elucidate the mechanisms by which sand ripples form diverse shapes.
2.Methods and Data
To achieve the objectives of this study, we conducted four surveys and experiments. The first was a field survey to investigate the formation environments, structural differences, and commonalities between wind ripples and subaqueous ripples. The second was a wind tunnel experiment to demonstrate the flow characteristics of sand. The third was a simulation experiment aimed at elucidating the mechanism of subaqueous ripple formation under flowing water conditions. The fourth was an experiment to generate subaqueous ripples by rotating a water tank.
2-1. Field Survey
We investigated the field conditions of the Enshu-nada coast and measured the height of ripple crests, the wavelength over five cycles, and the slope angles of the stoss (upwind) and lee (downwind) sides. Based on these data, we analyzed structural similarities and differences between the two types.
2-2. Wind Tunnel Experiment
In the front section of a custom-made cardboard wind tunnel (60cm x 45cm x 45cm), we placed a 3.0 cm mound of sand from the Enshu-nada coast. We set the wind velocity at approximately 4.0 m/s and observed changes in sand movement and depositional features under different moisture levels of the deposit (dry vs. wet sand). The experiment was divided into two cases: with and without saltating sand (sand supply).
2-3. Sand Ripple Formation Experiment
Sand and water were placed in a transparent container (75cm x 40cm x 5.0cm). Wind was applied to the water surface to generate waves. We recorded the time-lapse of ripple formation and sand movement. Similar to the wind tunnel experiment, we compared cases with and without sand supply under a fixed wind velocity of 4.0 m/s.
2-4. Rotating Water Tank Experiment
Using a chiffon cake mold (230mm diameter), we induced a continuous water flow by rotating a paddle attached to the central axis of the setup at speeds ranging from 0 r/s to 3.5 r/s. We observed how the flow velocity affected the morphology of the resulting ripples.
3. Results and Discussion
3-1 Field Survey
Subaqueous ripples tended to be taller and steeper than wind ripples. Notably, while wind ripples showed no significant difference between upwind and downwind angles, subaqueous ripples exhibited steeper angles on the downstream side. The wavelength of wind ripples was concentrated between 26 and 92 cm, whereas that of subaqueous ripples exhibited a broader range, from 30 to 142 cm, indicating that subaqueous ripples are less regular and larger in scale.
3-2 Wind Tunnel Experiments
Without sand supply, no significant changes occurred in either dry or wet sand mounds. However, with sand supply, the dry sand mound became lower, and the angle of repose decreased. In contrast, the wet sand mound showed less change, likely due to surface tension and the mass of the moisture inhibiting sand movement.
3-3 Ripple Formation Experiments
Ripples formed regardless of sand supply, but the scale was significantly smaller than those observed in the field. The wavelength matched the water surface waves, suggesting that ripple morphology is highly dependent on the scale of waves determined by container size and flow velocity.
3-4 Rotating Tank Experiment
A strong positive correlation was found between the rotation speed and the ripple dimensions (length, height, and spacing). Increased kinetic energy from higher flow velocities promotes ripple development. Conversely, a negative correlation was found between flow speed and the number of ripples; as ripples grow larger within the constrained space, their total count naturally decreases.
4. References
Nishimori, Taku (1995). Suna no hyomen no patan dorikigaku [Pattern dynamics of sand surface]. Hyomen Kagaku (Surface Science), 16(4), 267-272. (in Japanese).
The purpose of this study is to identify the factors that cause the distinctive differences in morphology between wind ripples and subaqueous ripples formed on the Enshu-nada coast. Wind ripples, developed by the movement of sand grains (suspension, creep, and saltation) driven by winter monsoon winds, exhibit uniform, wavy patterns that are continuous and perpendicular to the wind direction (Nishimori, 1995). In contrast, subaqueous ripples, developed by water waves, exhibit irregular, diverse shapes with frequent discontinuities. This study focuses on these differences to examine how the properties of fluids—gases versus liquids—affect ripple formation and to elucidate the mechanisms by which sand ripples form diverse shapes.
2.Methods and Data
To achieve the objectives of this study, we conducted four surveys and experiments. The first was a field survey to investigate the formation environments, structural differences, and commonalities between wind ripples and subaqueous ripples. The second was a wind tunnel experiment to demonstrate the flow characteristics of sand. The third was a simulation experiment aimed at elucidating the mechanism of subaqueous ripple formation under flowing water conditions. The fourth was an experiment to generate subaqueous ripples by rotating a water tank.
2-1. Field Survey
We investigated the field conditions of the Enshu-nada coast and measured the height of ripple crests, the wavelength over five cycles, and the slope angles of the stoss (upwind) and lee (downwind) sides. Based on these data, we analyzed structural similarities and differences between the two types.
2-2. Wind Tunnel Experiment
In the front section of a custom-made cardboard wind tunnel (60cm x 45cm x 45cm), we placed a 3.0 cm mound of sand from the Enshu-nada coast. We set the wind velocity at approximately 4.0 m/s and observed changes in sand movement and depositional features under different moisture levels of the deposit (dry vs. wet sand). The experiment was divided into two cases: with and without saltating sand (sand supply).
2-3. Sand Ripple Formation Experiment
Sand and water were placed in a transparent container (75cm x 40cm x 5.0cm). Wind was applied to the water surface to generate waves. We recorded the time-lapse of ripple formation and sand movement. Similar to the wind tunnel experiment, we compared cases with and without sand supply under a fixed wind velocity of 4.0 m/s.
2-4. Rotating Water Tank Experiment
Using a chiffon cake mold (230mm diameter), we induced a continuous water flow by rotating a paddle attached to the central axis of the setup at speeds ranging from 0 r/s to 3.5 r/s. We observed how the flow velocity affected the morphology of the resulting ripples.
3. Results and Discussion
3-1 Field Survey
Subaqueous ripples tended to be taller and steeper than wind ripples. Notably, while wind ripples showed no significant difference between upwind and downwind angles, subaqueous ripples exhibited steeper angles on the downstream side. The wavelength of wind ripples was concentrated between 26 and 92 cm, whereas that of subaqueous ripples exhibited a broader range, from 30 to 142 cm, indicating that subaqueous ripples are less regular and larger in scale.
3-2 Wind Tunnel Experiments
Without sand supply, no significant changes occurred in either dry or wet sand mounds. However, with sand supply, the dry sand mound became lower, and the angle of repose decreased. In contrast, the wet sand mound showed less change, likely due to surface tension and the mass of the moisture inhibiting sand movement.
3-3 Ripple Formation Experiments
Ripples formed regardless of sand supply, but the scale was significantly smaller than those observed in the field. The wavelength matched the water surface waves, suggesting that ripple morphology is highly dependent on the scale of waves determined by container size and flow velocity.
3-4 Rotating Tank Experiment
A strong positive correlation was found between the rotation speed and the ripple dimensions (length, height, and spacing). Increased kinetic energy from higher flow velocities promotes ripple development. Conversely, a negative correlation was found between flow speed and the number of ripples; as ripples grow larger within the constrained space, their total count naturally decreases.
4. References
Nishimori, Taku (1995). Suna no hyomen no patan dorikigaku [Pattern dynamics of sand surface]. Hyomen Kagaku (Surface Science), 16(4), 267-272. (in Japanese).