Sediment Transport:
Sediment can travel with the water flow of a stream. Smaller particles such as clay and silt can become suspended in the water column and make the water appear opaque or translucent. However, for larger particles such as sand, these types of sediment tend to settle and travel near the bottom of the stream bed in structures known as ripple marks.
What are ripple marks?
Ripple marks or ripples are structures produced by a current (water or wind) on the surface of sandy sediment. This structure is known as a bedform, because it forms at the bottom of a basin, such as a stream bed, at the contact between the sediment and air/water.
Image 1: An example of a ripple mark
Two types of Ripples: Asymmetrical and Symmetrical
Asymmetrical: When ripples (or other bedforms) are produced by a water current in a single direction, they show an asymmetric shape with a gentler and a steeper side. On the other hand, oscillatory currents like waves often produce symmetric ripples. The gentler stoss side leans against the flow, and a steeper lee side leans towards the direction of the flow as seen in the image below. Sediment is pushed forward by eroding the stoss side of the ripple and depositing it on its lee side. The lee side accretes new sediment that falls from the crest of the ripple. In bodies of water with a series of ripples, this process can transport sand downstream.
Image 2: Diagram of an asymmetrical ripple
Symmetrical: In places with oscillatory or bidirectional currents, water can move in two opposite directions. This oscillatory movement produces symmetric ripples or wave ripples, as the sediment is dragged in a direction and then in the opposite one. The crests of wave ripples can be pointed or rounded. These types of ripples are extremely prevalent near beaches where uprush and backwash of seawater act as bidirectional currents on the shoreline.
Image 3: Diagram of a symmetrical ripple
Logging Tasks:
- Stand on the bridge and face the upstream portion of Walnut Creek. Describe the color and appearance of the sediment at the streambed.
- Do you observe any small ripple marks at the bottom of the streambed? If so, estimate the size of the ripples. Depending on the day, you might experience difficulty in observing ripples. If you don't see any during your visit, explain the ripple formation process at Walnut Creek.
- Based on what you've learned in the description and your observations at the GZ, which of the two types of ripples is most likely present at Walnut Creek? Explain your rationale in a couple of sentences.
- As of June 2019, earthcaches now contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://geologyistheway.com/sedimentary/bedforms-ripple-marks-and-dunes/#:~:text=Asymmetric%20ripples%20form%20at%20the,the%20flow%20of%20the%20current.
https://en.wikipedia.org/wiki/Sediment#Fluvial_bedforms
https://blogs.egu.eu/divisions/ts/2019/10/11/features-from-the-field-ripple-marks/#:~:text=Symmetric%20ripples%20are%20formed%20by,water%20moves%20back%20and%20forth
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