Abstract
The chart “Hoko Suido (1/150000) ” was published by Naval Hydrographic Office of Japan for the first time in 1904 and showed the development of three great sand spits along the coast of Formosa. In 1932, a descriptive remark was added on the chart that these sand spits appear to be migrating southward. After the World War Second, Hydrographic Office of Taiwan surveyed around the same area and published the new chart “Hoko Kodo”. From the comparison of two charts mentioned above, it was able to observe clearly that these sand spits had migrated greatly to southwestward or southward direction during the past 60 years.
North spit is a cuspate spit. Middle spit is about 20km in length and extends southwe-stward from the coast. It's termination has a recurve (hook) trending southward. Two older recurves on the landward side of the spit mark former terminations of a spit stretched inter-mittently. South spit is a barrier spit which extends about 30km in the SSW direction. Each barrier are about 3 or 6km long. Salt marshes develop on the sheltered landward sides of the middle and south spits. Seaward side slopes of each spits are steep, and flat terraces develop at the depth of -7.5 to 15m on. the seaward slopes of spits.
Sandy shoals exist off recurves of spits beyond the depression.
All sand spits were eroded remarkably on the seaward (northwestward) side and were deposited on landward (southward or southeastward) side.
The seaward side slope of the north spit retreated landward about 2km over a distance of about 7km during the past 60 years. It retreated about 4.5km over a distance of about 15km on the middle spit and 1 km over a distance of 20km on the south spit during the same period. The rate of recession is about 75m/year on the middle spit. Each spit extended its length southwestward or southward about 11km at north spit, about 6km at middle spit and about 5km at south spit. And the terminations of these new spits reached to the position of shoals which situated off the distal ends of former spits. That is to say, a spit may grow by the process that a shoal off the distal end of spit grows by material supply fed by abrasion of seaward side slope of spit, then it connects with the termination of the former spit. We observed a embryo of new shoal happened off the distal end of north new spit in 1969.
As a result of abrasion of seaward side slopes, flat planes developed at the foots of new spits. These flat planes are at the depth of -7.5 to 15m. The formation of new abrasion planes demonstrated that the base of vigorous abrasion related to the present sea level exists at a depth of -7.5 to -15m. North cuspate spit stretched southwest ward and a depression was formed between the coast and the spit.
Orientaion of middle spit was changed from N 39°E to N 49°E. New middle spit entered to Hoko Channel deeper than the former stage by southward migration, so that the direction of prevailing wave with relatively long fetches which determines the shape of spit altered from northwest to north. It seems that middle spit increased the angle against the general direction of the coast in order to adjust its orientation to make right angle with the direction of prevailing wave from north. South spit did not change its orientation because the angle was similar to right angle against the prevailing wave from Hatto Channel.
