Human Impact on Coastal Erosion in Taiwan

Transcription

1 Journal of Coastal Research West Palm Beach, Florida July 2007 Human Impact on Coastal Erosion in Taiwan Tai-Wen Hsu, Tsung-Yi Lin, and I-Fan Tseng * Department of Hydraulic and Ocean Engineering National Cheng Kung University Tainan 701 Taiwan twhsu@mail.ncku.edu.tw Department of Geography National Taiwan Normal University Taipei 106 Taiwan ABSTRACT HSU, T.-W., LIN, T.-Y., and TSENG, I.-F., Human impact on coastal erosion in Taiwan. Journal of Coastal Research, 23(4), West Palm Beach (Florida), ISSN Coastal erosion is an islandwide problem in Taiwan. On the basis of the result of the most recent survey, more than 80% of the island s sandy coasts have undergone erosion over the past 3 decades. Naturally and historically with the specific tectonic environment and uplifting rate in Taiwan, sufficient sediments had been yielded and transported to the coastal area, resulting in an advancing shoreline along the trailing edge coast during the last several centuries. However, human interventions have contributed significantly to the erosion in more recent times. In this paper, documented examples are chosen to address how human-induced changes have made profound impacts on the coastal erosion in Taiwan. The information presented will provide a better understanding of the coastal processes, and solutions to the erosion problem for the benefit of others in different parts of the world. ADDITIONAL INDEX WORDS: Shoreline change, sediment transport, groin effect, overmining. INTRODUCTION Taiwan is an island generated by the tectonic activities in the circum-pacific belt. The mountain-building process might still be active in the eastern part of the country. Vertical displacement velocity was estimated at 1 3 cm/yr (LIU and YU, 1990). Because of this process, sufficient amounts of sediment have been yielded and transported to coastal areas, which resulted in an advancing shoreline on the west coast. A comparison of maps from the 17th century and those most recently published shows a rapid shoreline progression in part of the western coast, the trailing edge coast of Taiwan. However, on the basis of the most recent survey, more than 80% of the island s sandy beaches have undergone erosion over the past 3 decades. Figure 1 presents a spatial distribution of beach changes from 1980 to Coastal erosion has occurred along most of the sandy shores at an alarming rate and become an islandwide problem in Taiwan. Many local coastal scientists have shown a keen interest in finding the underlying causes to the coastal changes. Factors affecting shoreline retreat can be divided into two main categories: natural and human-induced (GILLIE, 1997). Natural causes include short-term beach dynamics, longterm natural shoreline evolution, sea-level rise, and catastrophic geohazards in the coastal zone. The episodic coastal erosion occurring during a storm is a specific example. Stormy waves breaking near shore have been known to gen- DOI:10:2112/ R.1 received 9 September 2004; accepted in revision 22 August *Present address: Center for Marine Technology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan. erate a return flow that causes offshore transport of beach/ berm sediment, resulting in shoreline retreat. Consequently, a bar beach profile is produced from the material eroded from a beach. At the wane of a storm, subsequent swell waves act upon the bar profile and move the sediment back to the beach to form an accreted berm profile. This cycle of beach erosion and accretion caused by a storm and subsequent swell is a natural process of beach evolution. Human-induced causes include sand mining from riverbeds and beaches, construction of coastal structures (such as jetties, groins, seawalls, and breakwaters), reclamation of shorefront lands, and human-induced land subsidence. A typical example of human impact on coastal erosion can be found at the Nile Delta (KOMAR, 2000). The Aswan High Dam was constructed in 1964 on the upper reach of the Nile River, which cut off supply of sediment reaching the coast. With a decrease in sediment from the river and continuing transport of sand alongshore by waves, beach erosion has occurred at several hundred kilometers downstream of the dam construction. Taiwan has a land area of 36,000 km 2, and a total length of shoreline about 1500 km, including offshore islands. The shoreline/land area ratio is 44.4 m/km 2, which is about 2.4 times as large as that of the United States and is half that of Japan (KUO, 1990). Since mountain areas account for about three-fifths of Taiwan, most of the population tends to concentrate along the coastal regions. Each person on the island state has a share of 7 cm of shoreline. During the process of recent economic development in Taiwan, population increases and available land became scarce. Coastal land reclamations (e.g., harbors, recreational parks, and industrial

2 962 Hsu, Lin, and Tseng Figure ). Schematic view of sediment budget to a coast (Horikawa, Figure 1. Spatial diversity of coastal erosion in Taiwan. For color version of this figure, see page 940. processes, due to waves, tides, currents, and winds, continually change the state of a coast and attempt to achieve a dynamic balance on it. Human activities cause coastal changes by modifying the coastal environments and disturbing the natural processes. Since most physical changes are associated with the movement of sediments, the sediment budget analysis is used to evaluate the possible erosion or accretion condition at a beach. The use of a sediment budget concept usually provides a proper explanation for erosion problems and may offer an avenue for remedial action. The application of a sediment budget concept also helps determine what human effects have been made on the near-shore sediment system. A sediment budget model presented by HORIKAWA (1988) is used to elucidate coastal change within a specified compartment, such that parks) have increasingly occurred. Such human interventions are considered to be the major factors in causing recent beach erosion. As the shoreline recesses landward, a natural buffer zone could be destroyed by storm waves, leading to the destruction of homes and properties. Some typical examples of coastal erosion in Taiwan chosen to address the impacts caused by human activities, such as construction of harbors or fishing ports, land reclamation for industrial parks, sand mining from riverbeds for structure materials, construction of reservoirs or dams, and overextraction of groundwater for aquaculture activities, may have direct or indirect effects on the changing coast. This study considers not only the full range of spatial scales important to coastal variation, but also temporal scales of long-term shifts of the shoreline. In terms of sustainable development for an island state like Taiwan, the examples presented will provide a better understanding of the coastal processes, and contribute to a better response to the coastal erosion problem. APPLICATION OF SEDIMENT BUDGET MODEL A coast is a dynamic system; shorelines and beaches are subject to continual change. Diverse and complex natural V R E W L O (W L O ) (1) in in in out out out t where V is the volume of sediment, t is the time, R is the rate of sediment discharge from rivers, E is the rate of sediment supply from eroding coasts, W is the rate of windblown sand, L is the longshore sediment transport rate, O is the cross-shore sediment transport rate, and the subscripts in/ out indicate an inflow/outflow to the control volume. If the sum of sediment inflow rates is smaller than that of outflows, i.e., V/ t 0, the coast tends to erode and vice versa. A control system for sediment budget shown in Figure 2 illustrates the concept of sediment balance that can be applied to examine changes in various sediment sources to subsequent coastal process. An empty arrow indicates inflow of sediment to the control volume, whereas a solid arrow shows an outflow. The processes and factors that bring about coastal change are inherent in the spatial and temporal elements in the present study. As a result, we are able to predict in a quantitative fashion human-induced coastal erosion based on the balance in sediment budget. Human activities that interrupt the natural sediment movements have either direct or indirect effects on coastal processes. They may affect the supplement of sediment to a coast, and the movement of sediment within a coastal envi-

3 Human Impact on Coastal Erosion in Taiwan 963 Figure 3. Distribution of major rivers and their average annual runoff in Taiwan ( ). For color version of this figure, see page 941.

4 964 Hsu, Lin, and Tseng Figure 4. Shoreline changes at Linco and Bali coast due to improvement of the flood channel at Tanshui River, Taipei, Taiwan. Construction of Taipei Harbor has been in progress since January ronment, or promote a land subsidence or relative sea-level rise locally. Human-induced causes of coastal erosion in Taiwan may include reduction of sediment supply from rivers, construction of improper engineering structures, and a land subsidence due to overextraction of underground water. In this paper, some typical examples of human interruption are chosen to address these significant impacts on coastal erosion. REDUCTION OF SEDIMENT SUPPLY FROM RIVERS Referring to Equation (1), we notice that fluvial sediment is one of the most important contributors of sediment sources to a coast. Most rivers in Taiwan are short and have a steep slope with distinctive runoff difference in wet and dry seasons (Figure 3). The annual rainfall in Taiwan reaches 2500 mm, which is 2.5 times the world average. However, rainfall concentrates in the wet season from May through October, which amounts for 3/4 of the annual rainfall. Because rivers have a steep slope, flow discharges respond rapidly to rainfall intensity and flood flows, and usually carry a large amount of sediments from the watershed. The sediment discharges to the river mouths mainly take place during flood season, especially on typhoon or storm events. During the dry season, the sediments are redistributed by coastal processes, such as waves, tides, currents, and winds to form wide beaches updrift of a littoral barrier and dune fields at certain locations. Reduction in sediment supply from rivers due to damming and losses of sediment due to interception by coastal structures have led to significant beach erosion at several popular sites. There are 129 rivers in Taiwan; most of them are short, steep, and with small drainage basins. Downstream reaches of most rivers are heavily deposited because of poor geologic Figure 5. Shrinkage and landward migration of a series of barriers north of the Tsengwen River mouth, resulting from loss of sediment supply of the Tsengwen River. conditions in the watersheds and concentrated rainfall; therefore, they re difficult to manage and flood easily. River improvements have been the major concern by the Water Resources Agency under the Ministry of Economic Affairs since the 1970s for the protection of houses and properties along the river banks. Constructions of flood channels and building of dams or reservoirs could have a negative impact on the budget, decreasing the amount of sediment reaching the shore, while waves and currents continue to transport sand alongshore. As a result, the extreme reduction of longshore sediment sources from rivers reflects a rapid retreat of the shoreline at the coast. In addition, because of the shortage of sand/gravel resources on the land, large amounts of these materials have been mined either legally or illegally from the riverbed to be utilized as the aggregate for concrete structures. All these activities have reduced sediment supply from rivers to the coast and consequently resulted in beach erosion. Tanshui River is located in the north of Taiwan. The river has a total length of 159 km and a watershed of 2726 km 2. The flood channel project was initiated in 1970 by the government to prevent flood damage on the Taipei alluvial plain. Jetties were gradually extended to the sea around the river

5 Human Impact on Coastal Erosion in Taiwan 965 Figure 6. Dune erosion and shoreline retreat at Wangziliao barrier, Tainan County. For color version of this figure, see page 942. mouth to trap littoral drift, and were quite successful in achieving the disaster prevention purpose. But the completion of this project also reduces sediment supply to the river mouth and results in the rapid erosion to the Linco and Bali coast southwest of the river mouth. The chronic events of shoreline recession at Linco and Bali coasts are shown in Figure 4. It is noted that shoreline retreated over 250 m from 1958 to 1993, and the same trend remains south of the Tanshui River. A dam or reservoir constructed on a river may cut off sediment supply to the coast and produce a significant imbalance in the budget of littoral drift, leading to beach erosion (e.g., FRIHY et al., 1992; LY, 1980). There are several instances of dam construction on rivers that have been identified by their impacts on the coast. Construction of a reservoir in the middle reach of Tsengwen River is one example that results in the coastal erosion at the river mouth in Taiwan. The Tsengwen River, located in the southwest of Taiwan (Figure 5), has a total length of 138 km and a drainage area of 1177 km 2. The Tsengwen reservoir, containing a drainage area of 481 km 2 and capacity of 7.08 billion m 3, was built in 1973 to supply water for multiple uses such as agriculture, power generation, and domestic purposes. Shoreline retreated around the mouth of the Tseng- Figure 7. Satellite photos showing Waisanding barrier in 1986, 1994, and For color version of this figure, see page 942.

6 966 Hsu, Lin, and Tseng Table 1. Variations in total area of the Waisanding barrier. Date (Year) Area (km 2 ) Variation (km 2 ) Figure 8. Historical evolution of Waisanding barrier. wen River, and the depositional system has been transformed from a deltaic system to an estuarine system (LIU,YUAN, and HUNG, 1998). The Tsengwen reservoir blocked approximately 99% of the river s sediment supply to the coast, inevitably resulting in large-scale shoreline erosion north of the Tsengwen River mouth. As shown in Figure 5, a series of barriers north of the Tsengwen River have gradually shrunk and moved landward. Among them, the maximum rate of shoreline retreat of the Dingtoue barrier exceeds 50 m/y. Figure 6 presents a detailed barrier erosion and shoreline retreat at Wangziliao barrier. In addition to dam construction on rivers, other human activities have altered the sediment transport processes along the beaches and consequently affected local sediment budget. A more direct human intervention is the sand mining from riverbeds that reduces supply to the beaches. The purpose of sand mining from rivers is to provide sand and gravel for various constructions such as roads, bridges, harbors, and buildings. Examples of overmining riverbeds are from the Choushui and Lanyang rivers. As shown in Figure 7, the Waisanding barrier is an offshore island located south of the Choushui River mouth, and is thought to be the sink of the discharging sediments from the Choushui River and others to its north. On the basis of the reports of the Water Resources Agency, the peak discharge per unit area of the Choushui River was 7.7 cms/km 2, which is approximately 450 times that of the Yangtze River in China and 25 times that of the Sinno River in Japan. The sediment transport rate was estimated to be ton/y. With the huge amount of sediment mining from the riverbed at the lower reach of the Choushui River, the Waisanding barrier has been shrinking and shorelines on both the ocean and lagoon sides have been retreating. Comparing the satellite photos of three different years, it seems that the total area of the barrier island has obviously been reduced as shown in Figure 7. Figure 8 demonstrates the progressive retreat of the shoreline on the northeast (ocean) site of the barrier up to a value of 1100 m during and , respectively. The variations in total area of the Waisanding barrier are given in Table 1 for comparison. It is noted that the area shrunk was about 10.7 km 2 during the past 40 y ( ). Dune erosion along the sandy coast of Ilan is another instance of sand overmining on the river bed. The Lanyang River is situated in the northeast of Taiwan (Figure 3) and is the main sediment source supplied to the Ilan coast. This Figure 9. Beach erosion and shoreline retreat at Ilan coast. (a) Aozujia; (b) Daken. For color version of this figure, see page 943.

7 Human Impact on Coastal Erosion in Taiwan 967 Figure 10. Extension of the north breakwater at Taichung Harbor with sand accretion updrift and erosion downdrift: (a) initial stage (1972); (b) original layout (1986); (c) after breakwater extension (1995); (d) comparison of shoreline changes. river is the largest in northeast Taiwan, and flows northeastward along the fault valley in the upper and middle reaches, then turns eastward on the Lanyang plain to the Pacific Ocean. On the basis of a report of Water Resources Agency, sediment transport rate of the Lanyang River was estimated to be about 8 million m 3 /y (SHIH, CHANG, and LIN, 1996). Because of the seriousness of sand overmining in many major rivers in the western part of Taiwan, the government established a new policy to transport sand and gravel from the east to the west coast in the 1980s. According to KUO, CHEN, and YU (1995), one-eighth to one-fourth of the fluvial sediments may become the source for longshore sediment transport on Ilan coast. Without a proper regulation to restrict sediment mining from the riverbed, annual inputs of river sediment to the littoral system become much less than it used to be. Figure 9 depicts two locations with severe beach and dune erosion at Aozujia and Daken, respectively, in Ilan county. KUO, CHEN, and YU (1995) used historical maps and aerial photos to study shoreline changes along the Ilan coast and found that the maximum retreat rate at the river mouth of the Lanyang River was 32 m/y from 1977 to CONSTRUCTION OF IMPROPER ENGINEERING STRUCTURES Coastal erosion could also be induced by improper construction of structures along a coast. These structures (e.g., break-

8 968 Hsu, Lin, and Tseng Figure 11. Beach volume changes in each sector adjacent to Taichung Harbor. The sector between north breakwater and north groin (sector B) was in accretion from 1976 to 1993; the sectors south of the north breakwater (sectors C E) were in erosion from 1976 to waters, jetties, and groins) tend to interrupt the longshore sediment transport or to alter the patterns of nearshore current system. Shore-parallel structures (like seawalls) tend to intensify wave energy because of strong wave reflection from them. An example to be discussed is the construction of Taichung Harbor at the Wuchi, facing the Taiwan Strait, on the middle western coast of Taiwan (Figure 1). It is an artificial harbor comprising north and south breakwaters. In October 1976, Taichung Harbor was officially completed and commissioned as the second international commercial harbor in Taiwan. It was anticipated to serve as a transport center of Asia. After

9 Human Impact on Coastal Erosion in Taiwan 969 Figure 12. Sand deposition in the harbor basin due to north breakwater extension at Hommei fishing harbor: (a) top view; (b) wave-induced nearshore currents before breakwater extension; (c) wave-induced nearshore currents after the extension. For color version of this figure, see page 943.

10 970 Hsu, Lin, and Tseng Figure 13. The Gold Coast in Tainan city. A 150-m-wide beach was destroyed and retreated during Typhoon Herb in 1996:(a) vertical seawall at Gold Coast; (b) beach erosion in the front of the seawall; (c) shoreline recovery at the Gold Coast after Typhoon Herb.

11 Human Impact on Coastal Erosion in Taiwan 971 Figure 14. Beach erosion at Chijin coastal park in Kaohsiung city (a) before construction of coastal park building; (b) after construction of the park building; (c) after protection (2003). For color version of this figure, see page 944. the 1980s, further extension of its north breakwater has blocked the longshore sediment transport that moves southward. As seen in Figure 10, interruption of longshore sediment has caused accumulation updrift of the north breakwater and severe coastal erosion downdrift to a distance about 1500 m south. The beach volume changes in each sector adjacent to Taichung Harbor are presented in Figure 11. The sector between the north breakwater and north groin (sector B) experienced sand accretion during , whereas the beaches downdrift of the south breakwater had severe erosion from 1976 to Suffice it to say that this is a typical example of downdrift beach erosion due to the construction of a harbor on a sedimentary coast. Although breakwater extension could provide better sheltering and reduce wave heights in the harbor, it also results in changes to the wave field, nearshore current system, and sediment transport pattern. Hommei fishery port in Taipei County (Figure 12a), for example, had its north breakwater extended about 200 m in 1984 to protect it from wave attack. But the extension caused wave sheltering in its lee, with rapid beach erosion at the neighboring Jinshawan (literally stands for golden sand bay ) coast, which was a beautiful recreational beach to the south (Figure 12a). CHANG (1993) and HSU, CHIEN, and CHANG (1997) pointed out that sheltering waves behind the extended breakwater caused reversed currents, which transport sediment from Jinshawan toward the entrance of the harbor. As shown in Figures 12b and 12c, the results of numerical simulations (HSU, CHIEN, and CHANG,1997) indicate the changes in the wave-induced nearshore current pattern before and after the construction of breakwater. The clockwise northern current pattern generated off the Jinshawan coast in the lee of the breakwater might have removed sand from Jinshawan to fill the port basin. A beach in sufficient width is a natural buffer against waves and storm surges. Construction of seawalls, especially the steep to vertical types, has proven detrimental to the fronting beaches and resulted in downdrift erosion. One such example of vertical seawall construction is on the Gold Coast in Tainan City (Figure 1). Figure 13a demonstrates that onehalf of the 150-m-wide fronting beach was destroyed during Typhoon Herb in The vertical seawall reflected the approaching waves back to the sea during a storm and the reflected waves could double the energy applied to the seabed that eventually results in the disappearance of beach in front of the seawall. When a wave approached obliquely and reflected from the wall, they set up a short-crested wave system with a strong potential to scour the bed and move sediment downdrift (SILVESTER and HSU, 1993). Figure 13b shows a temporal erosion pattern in front of the vertical seawall during Typhoon Herb, and Figure 13c indicates beach recovery after the event. Figure 14 shows another example of coastal erosion at Chijin coastal park in Kaohsiung city, where improper construction of a vertical seawall had accelerated local beach erosion in A remedy incorporating layers of stones and breakwaters was taken in 2003 with fair outcome. LAND SUBSIDENCE HOM-MA and HORIKAWA (1961) presented an example of beach erosion due to land subsidence resulting from the removal of subsoil natural gas in Japan. A great amount of groundwater was pumped and drained and the gas separated from the dissolved water. The excessive pumping of groundwater had accelerated ground settlement because of subsoil consolidation. The area of subsidence extended to the sea bottom and hence caused beach erosion. The total number of aquaculture farms along the Taiwan coast has steadily increased since the 1970s. A large amount of groundwater has been extracted to mix with seawater for land-based aquaculture activities. The overpumping of groundwater lowers the water table and permits soil compaction of the land subsurface layer that eventually results in land subsidence followed

12 972 Hsu, Lin, and Tseng Figure 15. Distribution of land subsidence around Taiwan. (SA subsidence area recently; AS accumulated subsidence depth; SR subsidence rate in last 4 y.) by seawater advance and shoreline retreat. At present, many places with severe ground subsidence remain in this country, and the distribution is shown in Figure 15. As shown in Figure 16, Wenfong coast in Pingtung County, southwest of Taiwan, is an example of shoreline retreat due to land subsidence resulting from over pumping of groundwater. The amount of subsidence had reached a maximum of 4.3 cm/y ( ). The accumulated subsidence was 3.2 m at some places. Such a large-scale subsidence extends to the sea bottom and hence accelerates beach erosion. As shown in Figure 16, shoreline receded about 80 m within 11 y ( ). The coast was, at that time, exposed to coastal hazards in many typhoon events. Since the simple seawall built in 1960s to prevent storm wave attacking was inadequate, eight units of detached breakwater were constructed 100 m offshore in 1984 to provide protection of the seawall and the fishing village. At present, the subsidence has been managed, but the breakwater had collapsed in many typhoon events. CONCLUSIONS The basic question addressed in this paper is the human impact that has caused coastal erosion in Taiwan. The main objective is to highlight our awareness of the important environmental factors affecting coastal changes. Human activities, such as damming a river and mining from a riverbed could alter the sediment budget to be delivered to a coast. Building breakwaters for harbors and groins or detached breakwaters along an eroding coast may block longshore sed-

13 Human Impact on Coastal Erosion in Taiwan 973 the problem is necessary before any solution is considered. With limited living space for the 23 million people of Taiwan, coastal development is an irreversible trend. The often undesirable experiences provided in this paper may serve as a lesson for all coastal engineers and managers to provide better assessment, planning, and management for sustainable development. Since coastal processes and erosion problems are very site specific, cooperative investigation into the coastal system around Taiwan is not only necessary but also a pressing need to provide the crucial information for minimizing the unintended effects of human disturbances on the coasts. LITERATURE CITED Figure 16. Land subsidence and shoreline retreat at Wenfong coast: (a) shoreline retreat; (b) land subsidence. iment or remove it from a littoral system. In these ways, human intervention could interrupt the natural hydrodynamic system of waves, currents, and the littoral movement. Coastal erosion has been a serious problem in Taiwan, and it has become difficult to use public expenses for any new requirements for coastal protection other than repairing the damaged units. From the examples discussed in this paper, it is obvious that even the engineering structures intended to stabilize the shoreline can inadvertently cause rapid erosion. Therefore, a better understanding of the physical nature of CHANG, S.J., Natural environment and estimation of territory resources of Taiwan coast. Engineering Environment, 5, (in Chinese). FRIHY, O.E.; Nasr, S.M.; Ahmed, M.H., and RAEY, M.E.I., Temporal shoreline and bottom changes of the inner continental shelf of the Nile delta, Egypt. Journal of Coastal Research, 7, GILLIE, R.D., Causes of coastal erosion in Pacific island nations. Journal of Coastal Research, Special Issue 24, HOM-MA, M. and HORIKAWA, K., A study on submerged breakwaters. Coastal Engineering in Japan, 4, HORIKAWA, K., Nearshore Dynamics and Coastal Processes. Tokyo: University of Tokyo Press, 522p. HSU, T.W.; Chien, C.H., and CHANG, R.H., Application of the planar sediment transport model to Homei fishing port. Journal of the Chinese Institute of Civil and Hydraulic Engineering, 9(1), (in Chinese). KOMAR, P.D., Coastal erosion underlying factors and human impacts. Shore & Beach, 68(1), KUO, C.T., Coastal topographic changes, development and utilization along Taiwan coast. Department of Hydraulic and Ocean Engineering, National Cheng Kung University, Technical Report CKHOPJ , 219p (in Chinese). KUO, C.T.; Chen, W. J., and YU, C.F., Evolution of Ilan shoreline. In: Proceedings of the 17th Conference on Ocean Engineering, Taiwan, pp (in Chinese). LIU, C.C. and YU, S.B., Vertical crustal deformations in eastern Taiwan and its tectonic implications. Tectonophysics, 183, LIU, J.T.; YUAN, P.B., and HUNG, J.J., The coastal transition at the mouth of a small mountainous river in Taiwan. Sedimentology, 45, LY, C.K., The role of Akossombo dam on the Volta river in causing erosion in central and eastern Ghana (West Africa). Marine Geology, 35, SHIH, T.T.; CHANG, J.C., and LIN, H.M., A geomorphological study on the estuaries in northern Taiwan. Geographical Research, 26, SIVESTER, R. andhsu, J.R.C., Coastal Stabilization: Innovative Concepts. New Jersey: Prentice-Hall, 578p.