ESTUDIO Y EVALUACIÓN FINANCIERA

O f the many geomorphic features which may be used as indicators o f longshore drift direction (e.g. Jacobsen and Schwartz 1981), those considered applicable to the Huatung coast are described in Table 4.1. The nature and interpretation o f each o f these is explored more fully below.

Table 4.1: G eom orphic in d icato rs su itab le for the H u atung C oast to determ ine net long sho re d rift d irectio n (deriv ed fro m Jaco b sen and S chw artz 1981).

Geomorphic indicators

Descriptions

object interrupting Accumulation of sediment will occur on the updrift side, while the shore drift downdrift side will experience erosion due to sediment starvation.

beach w idth Beaches tend to widen, develop one or more berms, and develop a larger backshore in their downdrift direction.

sedim ent size Sediment size generally decreases in the direction of net shore-drift

gradation due to decreasing wave energy, (many exceptions)

b lu ff m orphology Through the drift sector, in the direction of net shore-drift, the bluffs gradually become more vegetated and their slopes less steep.

Headlands and submarine canyons

In the context o f the environmental context developed in Chapter 3, major headlands and submarine canyons are the most readily recognised boundaries o f littoral cells (refer to section 3.7.1), especially those very protrusive headlands (refer to Figure 3.5 for location) and submarine canyons extending very close to the coast (Table 4.2). From an examination o f large-scale coastal landforms, five absolute boundaries are recognised (Table 4.3). These include the Fanshuliao, Shihtiping, Wushihpi, Hsiaokang, Sanhsientai and Hsiaoyehliu blocks. Each comprises either an extensive and protrusive headland or a combination o f a headland and a submarine canyon. Since gravels, the predominant beach materials on the Huatung coast, are mainly transported in the swash zone {of Carter 1988), thus presumably have little chance to pass these barriers.

Some headlands which are less protrusive or submarine canyons which are not so close to the coast are likely to be less effective as boundaries than the absolute ones defined above. These are tentatively recognised (more or less intuitively) as partial boundaries. These boundaries include the Changpin protrusion, Saopieh submarine canyon and the Tulanpi protrusion (Table 4.3).

Rivers

Rivers are usually the main suppliers in the context o f the coastal sediment budget (e.g., Norris 1964) and, as such, form boundaries o f the longshore drift. Inspection of the aerial photographs o f major river mouths, it is found that under the wave-dominated coastal environment, rivers blocked by river mouth bars are a very common feature along the Huatung coast. Even the major rivers (Hualien, Hsiukuluan and Peinan River) can only maintain a narrow exit most o f the year, o f which the location changes very often. Shih et al. (1995) concluded that the seasonal variation o f geomorphic characteristics is significant due to the relative importance o f fluvial vs. coastal processes along with the seasonal change o f the river discharge, dominant wind direction and wave conditions. In general, the river-mouth bar is w ider and gentler in the summer (the wet season), and narrower and steeper in the winter when the river discharge is lower and the wave energy is higher. High waves induced by typhoons, however, usually make the bar even steeper and narrower. This phenomenon is especially true for those smaller rivers.

It seems most o f the rivers o f the Huatung coast are too small to function as effective boundaries. Information learnt from aerial photographs (five sets o f the whole

Table 4.2: The distance between 20 m and 200 m contour in depth to the land along the Huatung coast S i t e s D i s t a n c e t o 2 0 m & 2 0 0 m in d e p t h ( k m ) S i t e s D i s t a n c e t o 2 0 m & 2 0 0 m in d e p t h ( k m ) H u a lie n R iv e r 0 .4 5 / 2 .8 5 * W u s h ih p i 0 .0 1 / 0 . 4 5 s o u th tip o f S h u ilie n e m b a y m e n t 0 . 5 8 / 1 .8 3 P a is h a u lie n 0 . 2 8 / 1 .0 8 F e n g p in R iv e r 0 . 5 8 / 1 .5 8 T u li 1 . 1 0 / 2 . 0 5 S h ih m e n 0 .0 1 / 1 .8 0 H s in g c h a n g 1 .0 5 / 2 .7 0 * H s iu k u lu a n R iv e r to S h u im u t in g R iv e r 0 .6 5 / .0 9 8 H sin la n 0 .4 5 / 1 .2 8 S a o p ie h R iv e r 0 . 4 3 / 0 . 8 8 H s ia o y e h liu 0 . 3 5 / 1 . 9 0

Note: * measured from 1:150,000 sea chart (C hinese Naval Hydrographic & O ceanographic O ffice, 1977), otherw ise from 1:25,000 topographical maps, published in 1967 (bathym etric data based on sea chart no. 3 4 8 , published in 1953). Refer to Figure 3 .7 for location.

Table 4.3: Some boundaries o f littoral cells identified from morphological evidence along the Huatung coast

L o c a t i o n o f b o u n d a r ie s I n d i c a t o r s T y p e * H u a lie n R iv e r m a jo r r iv e r & s u b m a r in e c a n y o n P Y e n lia o B r e a k w a te r m a n -m a d e str u c tu r e (b r e a k w a te r ) P F a n s h u lia o b lo c k p lu n g in g c l i f f A F e n g p in s u b m a r in e c a n y o n & m a jo r riv er P S h ih t ip in g h e a d la n d A C h a n g p in p r o tr u sio n & m a n -m a d e str u c tu r e (b r e a k w a te r ) P S a o p ie h s u b m a r in e c a n y o n P W u sh ih p i h e a d la n d & su b m a r in e c a n y o n A H s ia o k a n g h e a d la n d & S h ih y u s a n h e a d la n d A S a n h s ie n ta i h e a d la n d & su b m a r in e c a n y o n A M a w u k u R iv e r m a jo r r iv e r P T u la n p i p r o tr u s io n P H s ia o y e h liu b lo c k h e a d la n d A P e in a n R iv e r m a jo r r iv e r P

Huatung coast and more than ten sets for three major rivers) shows that some rivers may, however, constitute boundaries in conjunction with other geomorphic features. Learning from the aerial photographs, which range from 1951 ~ 1990, it is found that the exit o f river channels does widen temporarily during the periods o f high water and sediment discharges, which are usually following intensive rainfall events. A very obvious sedim ent plume, which could be vividly seen on photographs, usually forms and flows directly offshore, with a resulting net loss o f sediment from the fluvial- coastal system (Plate 4.1). Also learnt is that the extension o f the sediment plume from the H siukuluan River and the Peinan River is no farther beyond the neighboring headland in the north, i.e., Shihtiping (e.g., Plate 4.1-B) and Hsiaoyehliu headland. The distribution o f sediments from these two rivers is thus confined by the headlands to the north. It is justifiable, therefore, to consider these two headlands as absolute boundaries since even the finer particles can hardly pass them.

The M awuku River, the largest developed on the eastern flank o f Coastal Range, usually possesses a slightly protrusive river mouth. No photographs on hand cover a time o f flood discharge for this river. One photograph, which was taken ju st a day before Typhoon Offila (magnitude 13, track II), with turbid river water does not produce a plume-like sediment ejection (Plate 4.1-C). It may imply that the river discharge o f this river is too small to produce such a strong sediment ejection due to its small catchment or that the water level is not high enough to produce one. It is not entirely certain how effective a river mouth bar o f this kind can prevent the longshore sediment transport, thus is regarded as a partial boundary for the moment. The Fengpin River, the second largest, seems to be the same.

Certain small protrusions in the Huatung coast are formed by boulder beaches which are attached to the river mouths, as discussed in section 3.7. The long axis of their constituent boulders is usually, at least, around 1 m. These boulder beaches are believed to be formed by debris flow events associated with typhoon conditions. A good example has been documented at the Tunghsing River (Plate 3.2-G;). Among them, some rivers have experienced uplift since the rivers developed, for example, the Papien River mouth (Plate 3.2-H). Such protrusive river mouths very often form the boundaries o f sub-cells, and they are the most common in the southern part o f the coast.

The diversion o f river mouths is one o f the geomorphic indicators which is most often applied to infer the direction o f net shore drift (Jacobsen and Schwartz 1981). As mentioned above, most o f the rivers along Huatung coast are small and their mouths are

C. Peinan R iver and D. M aw uku R iver (refer to Figure 3.11 for location).

very often completely blocked. It may be misleading to adopt the direction o f diversion as the net drift direction given the extreme variability in river mouth geometry, not just seasonally but also in response to particular events.

Beach width and b luff/cliff morphology

Generally speaking, beach width increases through a drift cell in a down-drift direction (Taggart and Schwartz 1988). If the widening tendency shows a seasonal variation, it may also imply a seasonal change of the longshore current direction. Accordingly, the backed bluffs/cliffs usually gradually become more vegetated and gentler through a drift sector. The reasoning is that the wider the beach, the less frequently the bluff/cliff behind will be subject to direct attack by waves (e.g., Cambers 1976).

Aerial photographs which taken in five different years, ranging from 1951 ~ 1990, were examined carefully and beach width were measured from large-scale maps (1:1,000 and 1:5,000) for the whole Huatung coast (refer to section 2.2.1). It is found the planforms o f beach morphology do not show a significant widening trend although several stretches do show a southward widening trend. More commonly, within the stretches that confined by protrusions the beach width is usually larger in the central portion and narrows down towards two ends.

Man-made structures

With increasing coastal development, a wide range o f protective structures have become significant in interrupting longshore transport and in separating larger littoral cells into sub-cells. Case studies documented elsewhere include that by Chrzastowski et al. (1994) o f the southern Lake Michigan coast. Here, two primary (i.e. first-order) littoral cells have been artificially divided into ten primary cells and 25 sub-cells by engineering structures. O f all the coastal structures along this coast, those that are perpendicular to the shoreline have the most profound effect on the longshore transport. It is the length o f the structure which mainly determines whether it functions as an absolute or a partial boundary. In this case, the structure must extend to a water depth o f at least 6 m (the accepted depth o f closure for this lake coast), for it to function as a primary cell boundary. Chrzastowski et al. also note that cells o f this kind are time- dependent features, the persistence o f which is related to bathymetric changes and coastal management practices. For example, a structure separating primary cells may become a sub-cell boundary if updrift accretion progresses and natural bypass eventually occurs, or if dredging and artificial bypass are conducted.

Along the Huatung coast, ‘seaw alls’ composed o f interlocking concrete tetrapods are the most common kind o f coastal protection structures. These may retard the transport o f sediments to the nearshore system from the backing cliffs but may have less effect on longshore drift direction. Breakwaters may have a much greater effect on lognhsore transport. O f them, Yenliao breakwater (225 m long, built in 1991), which extends to 3 m in depth (Council o f Agriculture 1996), has had a particularly obvious effect. Comparison o f aerial photographs dating from before and after the construction o f the breakwater shows a clear increase in beach width on the north side and a narrowing due to deficit o f sediments to its south, where severe cliff erosion has ensued (Plate 4.2-A). This indicates that the structure has formed a fixed boundary and the net littoral drift direction is towards south, which is in accordance with the predominant wind direction (NE wind). It is not certain, however, whether the breakwater entirely impedes the longshore drift from the north. Considering the depth the structure extends, it is classified as a partial boundary for the moment.

Changpin breakwater (148 m long, built firstly in 1982 and then 1988 ~ 1996), extending southwards, is located on the south side o f the head o f the Changpin uplifted fan. The accumulation of sediments on its eastern side indicates the longshore current is south-westward (Plate 4.2-B). It has also learnt from the aerial photographs (69P05- 131, 24 January, 1980) that the higher permeability o f the Changpin fan head. Neither Changping fan nor Changping breakwater can entirely prevent the sediment passing.

The other harbours are either excavated from the bedrock (e.g. Shihti Harbour and Fukang Harbour), built with minor change on the coastal configuration (e.g. Chintsun Harbour) or located at the upstream position within the littoral cells (e.g. Chengkung Harbour). Thus their breakwaters reveal little about the longshore drift direction, and can be presumed to have relatively little impact. Along the Huatung coast, the fact that breakwaters on the northeastern side o f harbours are always longer in order to provide protection against the predominant northeasterly waves may also imply a southward net longshore direction under average conditions.