INGENIERÍA DEL PROYECTO

raised shore platform (Plate 3.3-E). This type o f gravel beach has been commonly observed along the coast from the Fengpin River to the Shuimuting River and also the southern side o f the Hualien River.

The occurrence o f sandy beaches is very limited. They may only exist where the wave energy is gentler or where the sand supply is abundant, either from cliff erosion (e.g. Chilin and south o f Chintsun), or from rivers north o f Lungchang (Plate 3.3-D). In most cases, gravels occur either at the bay ends or in the up-drift half o f the bays. The former usually occur in the pocket bays (e.g. Tuli) and the latter in the sinuous shape bays (e.g. Hsiaoma).

Coastal sand dunes usually appear where the hinterlands are wide enough for them to develop and also where the sand supply is abundant (Plate 3.3-F). Most of them occur south o f the Hsiukuluan River, where low marine terraces are much better developed than north o f the river. Sandunes used to be more extensive than they are now. Two segments, Fengtien to the Mawuku River and the Chili River to Tulanpi, had formerly extensive sand dune systems. Most o f these have been cultivated, with the exception o f the dunes at Hsiaoma and Tulanpi. Changkuang settlement, on the Changpin uplifted alluvial fan, is a site where palaeo sand dunes still exist. They may be much older than those presently developing near the shore.

The segments where beaches are backed by lowlands mostly occur in the south o f the Hsiukuluan River. In the Tuluanshan Formation outcrop, they may be formed when the volume o f gravel is large enough to spread over the raised shore platform so that the beach adjoins the lowland directly. Where the outcrops are o f sedimentary rock, they occur where beach sediments are abundant enough to protect the lowland against the effects o f wave erosion. Moreover, in the segments o f sandy beaches, ‘LS’ type only occurs where the environment is proper for the development o f sandunes (e.g. Chungan). Some segments have no beaches developed at all. Among them, Fanshuliao, Shihmen (Plate 3.3-B), Shihtiping, Wushihpi, Chihui and Ésiaoyehliu segments are the longest five. The rest are small protrusions.

The extent o f engineered coastline is increasing rapidly along the Huatung coast, especially since mid-1980s (W ater Conservancy Bureau 1990a; see also section 6.5). This is mainly the result o f coastal protection and the construction and extension o f harbours. Two types o f constructions are especially common (W ater Conservancy Bureau 1990a). Concrete tetrapods have been extensively placed to protect the

Plate3.3: M ajor coastal types along the Huatung coast. ( A ) n a r r o w b e a c h e s b a c k e d b y m a r in e te r r a c e s ; ( B ) s h o r e p la tfo r m ( a b o v e s e a le v e l l - 2 m ) ; ( C ) g r a v e l b e a c h

b a c k e d b y th e s l o p i n g h ill; ( D ) s a n d b e a c h b a c k e d b y th e lo w la n d ; ( E ) th in g r a v e l b e a c h e s

settlements, important buildings or coastal highways from erosion. Seawalls and breakwaters have also been extensively constructed, mainly in relation to harbour works. Most o f the harbours are located to the south o f headlands to minimise exposure to strong northeasterly winds in the winter. The majority are o f small size and have been built without any major change in existing coastal configuration. Some more recent constructions have been on a larger scale and have resulted in changes o f coastal configuration. Among them, the most serious is the very protrusive Yenliao breakwater. This has interrupted the southerly littoral drift since it was built (refer to section 4.2).

In brief, gravel beaches are dominant all along the coast; they may be backed by steep hillslopes, marine terraces, lowlands or sandunes. No sandy beaches develop in front o f the steep slopes. In certain segments where plunging cliffs occur or lack of sediment supply, beaches may not be able to develop at all. As a result o f the combination o f the shore and backshore features, gravel beaches backed by marine terraces (type TG) is the most common coastal type along the Huatung coast. They are particularly dominant along the coast north o f the Hsiukuluan River, where the relief is high along the coastal zone. The second dominant type, gravel beaches backed by lowlands (type LG), on the other hand, occurs more frequent in the south of the Hsiukuluan River. Sandy beaches appear exclusively in the outcrop o f sedimentary rock and so are coastal sandunes, thus type TS, DS, LS and DG, are mainly present in the south o f the Hsiukuluan River, with the only exception o f Chichi in the north. It is also notable that sea-cliffs with bare faces are common, either appearing as steep slopes or cliffs o f marine terraces. The distribution o f the retreating sites o f the sea-cliffs is discussed in the Chapter 5.

3.8 Summary

In summary, the Coastal Range o f Taiwan is located at the collision boundary o f the Eurasian and Philippine Sea Plates. The large-scale topography o f the Coastal Range is largely determined by neotectonic history and major geological structures. The fringing Huatung coast has experienced rapid uplift and is also characterised by differential uplift movement. Contrasting rock types outcrop along the coast, which mainly consists o f volcanic and sedimentary rocks. The local occurrence o f resistant rock outcrops along the shore largely determines the location o f the major headlands. The arrangement o f the major ridges makes the drainage basins very small and with high relief. The continental shelf is narrow and steep, providing less favorable conditions for

the retention o f sediments and for the development o f beaches.

In profile, coastal morphology is characterised by Holocene marine terraces, o f which at least three flights can be identified. South o f the Hsiukuluan River is where the marine terraces are the most well developed, which results from the favorite condition o f the exposure o f weak rocks and higher uplift rate in the area. In terms o f the shoreline configuration, two orders o f headlands and protrusions have made the rather straight coastline more indented, especially in the southern half o f the coast.

The climate o f the area is a subtropical monsoon type, with rainfall concentrating in the hot season. Maximum rainfall amount and intensity is usually induced by typhoon events. Together with the rainfall type, short and steep river channels and high- relief river basins, the river discharge and sediment transport is closely related to heavy rainfall events. The sediment discharge is particularly characterised by coarse particles in comparison with other rivers in Taiwan. In terms o f coastal oceanography, the Huatung coast might be classified as a micro-tidal and high wave energy environment. The largest maximum wave heights exceed 13 m. These events should have great importance on the timing and rate o f coastal change.

Based on the shore and backshore features, a new classificatory scheme is proposed which divides the Huatung coast into 9 categories. Among them, gravel beaches backed by marine terraces (type TG) is the most common type in the study area. They are particularly dominant along the coast north o f the Hsiukuluan River. The second is the type o f gravel beaches backed by lowlands (type LG): this type is more common in the south o f the Hsiukuluan River. Sandy beaches appear exclusively in the outcrop of sedimentary rock and so are coastal sand dunes, thus type TS, DS, LS and DG, are mainly present in the south o f the Hsiukuluan River.

Chapter Four

DELINEATION OF LITTORAL CELLS ALONG THE HUATUNG COAST

4.1 Introduction

Galvin (1990) stated that “ ...all examples o f shore erosion on non subsiding sandy coasts are traceable to manmade or natural interruptions o f longshore sediment transport.” Although this is perhaps an over-statement, experiences o f side-effects at the larger regional scale due to the tackling coastal erosion problems merely from a site- and/or problem-specific viewpoint, has led to the idea o f managing the coast based on physical-system units instead o f administrative units (Bray et al. 1995). This leads naturally to the concept o f coastal or littoral ‘cells’.

The identification o f the littoral cell is a task that has not been attempted for the Huatung coast before, partly because the necessary contextual information (Chapter 3) has previously never been collated in any coherent fashion. This chapter is denoted to conduct the preliminary analysis of the structure of littoral cells along the whole Huatung coast. Although the quantifiable assessment of the sediment budget is yet achieved, the delineation of littoral cells, mainly based on the morphological and sedimentological evidences, is accomplished for the first time.

4.2 Definition and Methods

The concept of the ‘sediment unit’ was probably first applied to the coastal research by Stapor (1971), May and Tanner (1973) and Tanner (1974). Similar terms like littoral cell, sediment unit, drift-cell, and drift-sector have been used by other authors (refer to Jacobsen and Schwartz 1981 or Taggart and Schwartz 1988). The definition o f the ‘cell’ and ‘boundary’ used in this research mainly follows Motyka and B ram pton’s (1993) work. In their work, identification o f the cell is based on the idea that interruptions to the movement o f sands or shingles along the beaches, or the near shore seabed, within one cell should not significantly affect beaches in an adjacent cell. In another words, based on the continuity of sediment transport, a ‘cell’ is a relatively self-contained unit within which sediment circulates (Bray et a l 1995). A littoral cell itself is a dynamic system, which consists of major components, including sediment sources (input), transport pathways (throughput), stores and sinks (output) (Figure 4.1). Sediment transport pathways are

o ff shore