MoʻIkeha Canal
After walking the Coconut Trail or the Ke Ala Hele Makalae (the path that travels by the coast) multiple times, I wondered why the MoʻIkeha Canal existed. This Earthcache will help explain its purpose to you. The trail is used quite a bit for walking, jogging, biking and traveling around Kapaa and is family friendly. The makai (toward the ocean) bridge next to you and the mauka (toward the mountain) bridge are State Historical sites that were part of the Ahukini Terminal Railroad that served the pineapple and sugar industries.
Kauai gets a significant amount of rainfall each year and receives approximately 460 inches of rain a year in the mountains. As you look westward, you can see mountains and the map shows streams and rivers draining towards the ocean and locally through Kapaa. Flooding in 1940 resulted in the dredging and construction of the Waikaea and MoʻIkeha canals in the 1940s. A total of three natural drainages were constructed to direct rainwater into the ocean. Rock jetties were also installed to help ensure that rainwater would drain and minimize the risk of future flooding in Kapaa.
At the posted coordinates note the construction of the canal in relation to Kapaa and the ocean. Observe the canal, the jetties, and the beach sand condition in this area to answer the questions below.
Longshore Drift
Part of Kapaa’s flooding issues involved the beaches in 1940. 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. The interaction of the waves and swells changes throughout the year in Hawaii.
The longshore current moves the sand along the beach. At a particular time, the longshore drift of the sand by the longshore current may be in either direction along the beach. Over the course of a year and at most locations, the volume of sand moved in one direction will exceed the amount moved in the opposite direction. The difference in the amounts of longshore drift between the two directions over a period of one year is called the annual net longshore drift.
The ultimate sources of sand for beaches are either new sand introduced to the coastal zone by rivers, old sand reworked by the waves, and currents from deposits in shallow offshore areas.
Jetties and Groins
In Hawaii and other coastal areas, it is not uncommon to see jetties and groins installed to help mitigate coastal erosion. Groins are structures perpendicular to the shore and are used to restrict longshore sediment transport. Groins will capture sand transported by longshore current on one side of the groin and prevent it from moving down the shore. Jetties are also perpendicular to the shoreline that have been placed adjacent to tidal inlets and harbors to control inlet geometry and minimize sediment deposition in the inlet. In fact, the similarly constructed Waikaea canal approximately 0.8 miles south along this trail also has a public boat launch that provides water access to the ocean. Jetties and Groins may be constructed with a variety of materials such as concrete, granite, or in this instance, volcanic rock.
Logging Requirements
- How does the jetties here help prevent flooding in Kapaa?
- Walking along the trail, which side of the jetty (north or south) appears to currently hold more sand and provide a larger beach? In your own words, what is happening to cause this?
- Please include in your online log a photo of you (or your personal geocaching related item such as gps, a trackable or a piece of paper with your geocaching name. I appreciate your photos in the logs.
🥇🏆 Congratulations to Skyhawk Star for the email to claim the FTF! 🏆🥇
References:
1. Atlas of Natural Hazards in the Hawaiin Coastal Zone. Atlas of Natural Hazards in the Hawaiian Coastal Zone | USGS I Map 2761
2. Exhibit M A Cultural Impact Assessment for the Proposed Kapaʻa Highlands Phase II Kapaʻa Ahupuaʻa, Kawaihau District, Kauaʻi. HoKua-Place-Vol-II-B-Exhibit-M-P.pdf (hawaii.gov)
3. Lydgate Park-Kapa‘a Bike/Pedestrian Path. fea-ch4a-affected-environ.pdf (kauai.gov)