What is a tombolo?
A tombolo is a deposition landform in which an island is attached to the mainland by a narrow piece of land (that usually consists of sand). Once attached, the island is then known as a tied island. A tombolo is a sandy isthmus. Several islands tied together by bars that rise above the water level are called a tombolo cluster. Two or more tombolos may form an enclosure (called a lagoon) that can eventually fill with sediment.
How are tombolos formed?
Tombolos are formed by wave refraction and diffraction. As waves near an island, they are slowed by the shallow water surrounding it. These waves then bend around the island to the opposite side as they approach. The wave pattern created by this water movement causes a convergence of longshore drift on the opposite side of the island. The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there, conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up, the beach shoreline, known as a spit, will connect with an island and form a tombolo.
Longshore drift occurs in both oceans and lakes; however, there are major differences. Oceanic shorelines generally have larger sand accumulation and face larger wave action due to more pronounced tides. As a result, longshore drift occurs more frequently. In lacustrine environments such as in Raccoon River Park, the lakeside beaches do not experience much wave action. Oceanic waves average >2-4 meters in height on most continental beaches compared to the extremely small ripple-like waves in Iowa’s lakes. Large storms lead to the generation of larger waves in Blue Heron Lake, allowing the process of longshore drift (and thus tombolo formation) to occur.
What is Longshore Drift?
Sand moves to and from the beach in an onshore-offshore direction, but it also moves along the shoreline (“longshore”). This longshore sand transport is referred to as longshore drift, and because of it, beaches have been called “rivers of sand.” There have been special instances where the longshore currents have changed direction during storms, hurricanes, or an abrupt change of wind for a brief period of time.
The longshore current, the movement of water along the shoreline, is caused by:
- Waves approaching at an angle to the shoreline.
- Tidal currents.
- Wind pushing water along the beach.
Sand:
- Sand is divided into five categories based on its size:
- Very fine sand (1/16 – 1/8mm)
- Fine sand (1/8 – 1/4mm)
- Medium sand (1/4mm – 1/2mm)
- Coarse sand (1/2mm – 1mm)
- Very coarse sand (1 mm – 2mm)
These are accurate measurements according to the Udden-Wentworth Scale
Logging Requirements:
- At the GZ, observe the water around the tombolo. Is it shallow or deep? Are there any small waves or ripples in the water present? What are the weather conditions during your visit?
- Based on your observations at Blue Heron Lake and the information in the description, do you think that over time the tombolo will build up or erode away?
- Examine the sand at the tombolo. What category on the Udden-Wentworth scale does it fall in? How wide is the stretch of sand that makes up the tombolo?
- 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://en.wikipedia.org/wiki/Tombolo
https://www.bbc.co.uk/bitesize/guides/zgnhhyc/revision/4
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