The Okinawa archipelago, located in a subtropical area, has well-developed coral reefs along its coastline, and the excellent natural environment of the coasts had previously well maintained. This natural environment of the islands, however, is currently in a critical state because of the rapid development and the deterioration of natural coral reefs, particularly on the main island of Okinawa. Although there used to be many sandy beaches composed of coral sand, the number of such natural sandy beaches has been rapidly decreasing. The rise in the sea level due to global warming will also affect the stability of sandy beaches in Okinawa. To mitigate damage to the coasts and sandy beaches, it is necessary to determine present state of Okinawa’s coasts.
　For this purpose, the basic concepts on coastal engineering, by which anyone can understand the various phenomena on coasts, were first introduced in Chapter 1. Then, Okinawa’s coasts were geographically classified in Capter 2. Furthermore, in Chapters 3 and 4, the results of the field observations of the east and west coasts on Okinawa’s main island were summarized, respectively. Then, the present situation of the coasts on Miyako and Irabu Islands were summarized in Chapter 5. It is also important to understand the difference between the island’s coasts with coral reefs and the coasts located in Honshu, where large rivers flow into the ocean with abundant sediment discharge. In Chapter 6, therefore, the results of the field observation on the Abe River watershed and the field observation of the Chigasaki coast immediately after a large typhoon which hit this coast in 2019 and caused severe damage were described.

1.1

Definition of technical terms on beach topography

1.2

Shoreward sand transport on coral reef

1.3

Beach changes owing to construction of artificial structures

2.1

Geographical classification of Okinawa’s coasts

2.2

Classification of Okinawa’s coasts by category

3.1

Oku coast – Okinawa’s original natural coast

3.2

Higashi fishing port – Longshore sand movement on coral beaches

3.3

Fukuchi River mouth – Beach nourishment and river mouth closure

4.1

Uga in Kunigami Village – Wave run-up

4.2

Tsuha Beach in Ogimi Village – Failure of seawall and river mouth closure

4.3

Yagaji coast on Yagaji Island – Beach changes trriggerd by construction of offshore fishing port

5.1

Higashi-hennazaki Point – Criff erosion and tsunami stones

5.2

Bora coast located on Higashi-hennazaki Point – Natural sandy beach

5.3

Separation of beach by seawall at Nagakita coast

6.1

Abe River Watershed

6.2

Chigasaki coast – Damage due to storm waves associated with Typhoon No. 19 in 2019