Marine-related physical natural hazards affecting coastal megacities of the Asia–Pacific region – awareness and mitigation

Ocean & Coastal Management 40 (1998) 65—85

Marine-related physical natural hazards affecting coastal megacities of the Asia—Pacific region — awareness and mitigation Russell S. Arthurton* British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Abstract The fast-growing, coastal megacities of the Asia—Pacific region are expanding into areas that are vulnerable to marine-related physical natural hazards, or, because of physical environmental changes, will become increasingly vulnerable within the timescale of city planning. The hazards comprise those that are due to extreme events such as storm surge and tsunami which may be catastrophic in their impacts; and those that relate to continuing changes over the long-term, notably global sea-level rise, sedimentary consolidation and coastal erosion. The latter may be exacerbated by human activities such as the increasing production of ‘greenhouse’ gases and over-abstraction of groundwater, and, while not threatening catastrophic loss of life or destruction of property, do have important economic and social implications for the future. There are two complementary approaches to hazard mitigation — constraining the hazard, and reducing vulnerability to the hazard. The contributions that science can make in the planning and implementation of sustainable adaptive measures are to improve the quantification of the incidence and severity of the various hazards, establishing realistic timescales of incidence, estimating return periods; and to establish the geographical limits of vulnerability to the hazards in a range of likely scenarios over timescales appropriate to the planning cycle. Contemporary, high risk, hazard scenarios for existing city developments demand an approach which focuses on effective warning networks and emergency planning; long-term, incremental hazards that are forecast to affect both developed and periurban areas can be addressed with a strategic planning approach, involving relocation and capital protective works. The selection of strategic measures demands the best possible predictive information on hazards and on vulnerability, including its full socio-economic evaluation so that the costs and benefits of the possible mitigation options can be realistically assessed. A predictive capacity, developed through modelling, requires the collection of reliable baseline and monitoring data relating

* Tel.: 0115 936 3486; Fax: 0115 936 3460. 0964-5691/98/$19.00  1998 Natural Environment Research Council. Published by Elsevier Science Ltd. PII: S 0 9 6 4 - 5 6 9 1 ( 9 8 ) 0 0 0 7 7 - 5

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to the hazards over a range of timescales in local, regional and global perspectives.  1998 Natural Environment Research Council. Elsevier Science Ltd.

1. Introduction Coastal megacities have grown from historic port development. The attributes which favoured the use and development of such sites as ports were essentially physiographic — places on otherwise exposed coasts which afforded sheltered anchorage and wharfage for hinterland trade. While the port function generally remains a focus of economic activity, most coastal megacities have grown far beyond their original, protected port location. Cities have expanded rapidly, subject to physiographic constraints, not only along the waterfront and into the hinterland [1], but in many cases also seaward on land reclaimed from the sea. This trend of coastal urban growth is set to continue [2]. The geographic settings of individual coastal megacities have provided specific opportunities for urban development but they impose constraints to sustainable growth. Important among such constraints are those which are due partly or wholly to natural hazards. The understanding of these natural hazards, and a recognition of a city’s vulnerability to them, are key elements in the planning and management of effective adaptive measures.

1.1. Natural hazards of the Asia—Pacific region Coastal megacities of the Asia—Pacific region (Fig. 1) are subject to different suites and intensities of natural hazard. Depending on a range of physiographic and developmental factors, they differ greatly in their vulnerability. Some (Hong Kong, Manila) suffer, on an extensive scale, the extreme wind and rainfall effects of seasonal tropical cyclones (typhoons). Some (Karachi, Jakarta, Osaka) are located in regions prone to potentially damaging earthquakes, while Manila, e.g., lies within range of significant ash fall from nearby active volcanic centres [3]. In addition to the natural hazards described above, there are those which are specific to coastal megacities because of their maritime location. A major concern is the possibility of accelerated sea-level rise as a consequence of global climate change [4]. Most coastal countries in the Asia—Pacific region have long shorelines and major centres of population located in low-lying, coastal areas. Even under today’s sea-level conditions, these coasts are prone to wave erosion or marine inundation, in particular that caused by storm surges [5]. The prospect of significantly higher global sea levels during the next century and the consequent exacerbation of these marinerelated hazards have captured the attention of coastal scientists and managers alike (see, e.g., Ref. [6]). Marine-related hazards occur over a wide range of timescales. They include catastrophic events, some of which may last perhaps only a few minutes, and incremental

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Fig. 1. The Asia—Pacific area showing principal locations referred to in the text.

physical changes, notably relative sea-level rise, that take place over much longer periods — from years to millennia and longer. These hazards differ greatly in their predictability. The ability of city managers and planners to set in place effective adaptive measures to cope with specific hazards depends on reliable information on the likely incidence and severity of hazard events. It is a role of science to improve knowledge and understanding of these natural hazards, and thus contribute to sound management and planning decisions. The priorities for action at local as well as regional and global levels need to be based on a sound and scientific assessment of the vulnerability of coastal areas to global change [7]. The research agenda is to set the spatial and temporal contexts of the hazards and the limits within which realistic assessments of vulnerability and risk can be made [8]. This paper reviews the marine-related, physical, natural hazards that pose risks [9] for the vulnerable elements of developing coastal megacities in the Asia—Pacific region. It considers the timescales relating to specific hazards and the geographical extents of vulnerability to those hazards. It identifies ways in which science can contribute to the appraisal of those risks by quantifying the hazards and assessing the context of vulnerability to them. It discusses how city management and the strategic planning of city development might respond to these risks by mitigation and adaptive measures implemented both through emergency procedures and by civil and social engineering over the medium and long terms.

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2. Recognising vulnerability to marine-related hazards Proximity to the sea is an obvious, necessary condition for the existence and growth of port facilities. However, this condition may be coincidental and irrelevant to the wider functions of the coastal megacity. While the megacity’s waterfront may provide a recreational resource and sites for prestige property development and urban infrastructure, most of the city’s inhabitants are likely to be indifferent to their maritime location. As a consequence, that population, whether as individuals or municipally, may be poorly aware of their vulnerability to hazards posed by the sea, and thus the risks that they might face in those respects. Vulnerability to hazards tends to occur where people lack the resources, awareness, knowledge, power or choices to mobilise defences against them [10]. In areas prone to such hazards people often appear ignorant of the potential for serious consequences, or, if aware of them, seem prepared to take unnecessary risks [11]. For some longer-term hazards and infrequent catastrophic events, they may simply be unaware of their vulnerability [12]. Important issues here are the city’s expectations of the incidence and magnitude of hazard events. These are factors that science can address. They are described and discussed below in the context of specific hazards. Vulnerability to hazards differs greatly both between and within cities. It depends upon a range of physical, environmental, economic, social and cultural factors [13]. In particular, vulnerability depends on the spatial distributions of these factors (notably the extent of potentially floodable areas and their closely interrelated socio-economic systems) and their changes over time within the city’s planning perspective. These are factors to be assessed by the application of the natural and social sciences, and also economics, because of the need in vulnerability assessment to quantify the economic activity (and infrastructure) at risk in hazardous coastal zones [14]. The pressures for urban growth are such that much of the development of coastal megacities takes place without the benefit of reliable information on natural hazards and vulnerability. Because of this, the need for mitigation measures, or the scale of such measures, may not be well understood by city managers. Even where plans and protocols are in place, they may be inappropriate to the real risk. The public’s perception of risk today may be ill-informed and the scope of mitigation out of date, a consequence perhaps of unplanned development, or of changes in vulnerability due to changing physical and social conditions. Mitigation measures are unlikely to eliminate risk. Rather they should aim to reduce risks to levels that are acceptable within the limits of available resources. The marine-related hazards which affect the coastal megacities of the region are of two main types, those caused by extreme physical events and those due to continuing changes over the long term. Extreme event hazards, which may be catastrophic in their impacts, are listed in Box 1. Hazards of the second type, listed in Box 2, impact incrementally over much longer periods. While they may not constitute a direct threat of catastrophic loss of life or property damage, they do have important economic and social implications over the long term.

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Box 1. Marine-related, physical extreme event hazards affecting coastal megacities z Severe waves (high amplitude, storm-generated waves, may be cyclone-induced). These may cause local flooding of unprotected coastal lowlands and recession of erodible shores. They may also cause disruption to operations at exposed port terminals. Impacts tend to be most severe during high tidal states. z Storm surges (cyclone-induced, abnormally high sea levels, commonly associated with high amplitude waves). These may cause the extensive inundation of unprotected coastal lowlands over periods ranging from a few hours to several days (events which may be exacerbated by landwater floods). z ¹sunami (high amplitude, long-period waves and run-up generated by near- or far-field submarine earthquakes and submarine ‘land’ slides). These may cause severe damage to waterfront land areas and associated coastal defences, with potential consequent loss of life and destruction of property and infrastructure over periods as short as only a few minutes. z Coastal earthquakes. These may result in ground surface displacements in the coastal zone over seconds to days. The vertical component of displacement may induce flooding of coastal land or, depending on the sense of movement, emergence of the intertidal to shallow subtidal sea bed. These earthquakes may be accompanied by near-field tsunami.

Box 2. Marine-related, physical long-term hazards affecting coastal megacities z Relative sea-level change. This is the increase (or decrease) at any given coastal location between mean sea level and the level of a reference point on the adjacent land surface or sea bed. Contributing factors include the possible human contribution to the forecast accelerated global sea-level rise as a predicted response to global warming. Continuing relative sea-level rise leads to an increasing frequency and severity of the marine inundation of low-lying coastal land, and, in the absence of engineering intervention, to long-term inundation. z Coastal erosion and accretion (including siltation of navigation channels). These are physical manifestations of coastal change caused by a wide range of possible forcing factors, including some which are induced or exacerbated by human interventions, both local and regional. Consequences include progressive loss or gain of land and a possible enhanced need for maintenance dredging. z Saline intrusion of coastal aquifers. This causes the progressive reduction or degradation of the coastal groundwater resource and is a likely consequence of relative sea-level rise. However, saline intrusion today being exacerbated by the over-abstraction of groundwater by coastal communities.

2.1. Extreme event hazards — knowing the risk The extreme event, physical hazards are difficult to plan for. They may provide no warning of their incidence, or so little warning that, even if emergency procedures are in place, there may be insufficient time to implement them. They range from events that are unpredictable in their timing and location, notably earthquakes, to those of a seasonal nature that have relatively predictable return periods and severities, such as storm surges (Table 1). Scientific uncertainties continue to shroud the scale and significance of potential combined sea-level rise and storm event risks at the regional level [13]. 2.1.1. Severe waves Any unprotected coast exposed to an extensive, uninterrupted, marine fetch is prone to damage by local flooding and shoreline erosion by waves, notably during high tidal states and storm surge conditions. While port facilities have generally been developed in sheltered inlets or estuaries where wave energy is subdued, there are

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Table 1 Conditions and likely coastal impacts of marine-related physical extreme event hazards Extreme event

Main vulnerable locations

Likely return period

Likely impact

Critical tidal state

Severe waves

Coasts exposed to long oversea wind fetch

Storm-related: months to years

Inundation and erosion of unprotected water front

High

Storm surge

Areas prone to tropical cyclones

Storm-related: months to years

Extensive inundation of unprotected lowland

High

Tsunami

Coasts and critical inlets exposed to far-field events

Years to centuries

Inundation and severe damage to waterfront zone

High

Coastal earthquake

Any seismically active area

Unknown

Unknown

—

many instances worldwide of urban expansion having taken place along less protected shores, resulting in vulnerable waterfronts. Wave damage is probably the commonest of the marine-related extreme event hazards. Because of this, its incidence is well documented in most urbanised coastal areas and the risks related to wave damage in existing, normal tidal conditions are usually well understood in respect of today’s sea level and climatic conditions. 2.1.2. Storm surges Storm surge (and related wave) inundation of coastal lowlands threatens human safety, and damages property and infrastructure. Surge levels are at their highest when these cyclone-related events coincide with high tidal states. Any unprotected land area at or below the surge sea level is vulnerable, as are its inhabitants and its susceptible service infrastructure, notably power and water supply, and sewerage. Landlocked harbours such as Tokyo and Osaka afford some protection from surges, though funnel-shaped coastal configurations and islands can accentuate the effect towards their closures [15]. Surges are normally associated with tropical cyclones, thus the incidence of this hazard varies greatly within the Asia—Pacific region. Tropical cyclones in the Bay of Bengal typically make landfall about three times a year [16]. Since 1882 Bangladesh has suffered significant marine flooding of coastal lowlands on average once in five years, although since 1950 less than once in two years [17]; such an event in 1991 killed 140 000 people. 2.1.3. Tsunami Many coasts within the region are prone to tsunami impact. Elsewhere low-lying coasts exposed to the ocean and, notably, bays and estuaries are vulnerable, such embayments tending to enhance wave amplitude in run-up or bore effects. ‘Tsunami’

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is the Japanese word for ‘harbour waves’ [18]. As with storm surge hazards, the existence of coastal defences may constrain some events, but the destructive power of tsunami waves in the shore zone may breach such defences and lead to significant loss of life and waterfront property before their energy is dissipated. The extent of the vulnerable zone depends on the wave magnitude and on local factors such as the nearshore bathymetry and shoreface profile. The impact of the 1960 Chilean far-field tsunami on the city of Hilo in Hawaii caused 61 deaths and carried blocks weighing more than 20 tonnes as much as 200 m inland [19]. The same tsunami claimed 119 lives and caused extensive damage to property and aquaculture in Japan [20]. The incidence of tsunami hazards, or tsunamigenic earthquakes, can be forecast and return periods estimated only in the most general way. Historical records provide some guidance in the prediction of impacts and the estimation of return periods [21], while the recent geological record at coastal sites may also provide evidence of past incidence [18]. In the Asia—Pacific region major tsunami affected many Pacific coasts in 1960 and 1964. The 1970s were generally uneventful but in 1983 a tsunami in the Sea of Japan killed 100 people [22], and in 1992 an event off Flores, Indonesia, also claimed many lives. Compared with the incidence of storm surges within the region, destructive tsunami are infrequent at any one site and, on many shores, e.g. western Kamchatka [23], they are regarded as the lesser of the two hazards. 2.1.4. Coastal earthquakes Coasts in much of the Asia—Pacific region are earthquake-prone; these include the Indonesian archipelago and the islands of Japan. The 1995 earthquake off Kobe, Japan, provides a recent example of such devastation in a coastal urban area. Coastal lowlands are vulnerable to the marine-related impacts of coastal earthquakes in three ways. They may be affected directly by vertical ground displacement causing possible relative sea level rise or fall (e.g. Wellington, New Zealand, in the 1860s); or indirectly by marine inundation as a result of sediment consolidation triggered by the earthquake shock, or the impact of a near-field tsunami. Prediction of the incidence of catastrophic coastal earthquake events, as earthquakes in general, is imprecise. Information from the analysis and interpretation of the recent geological and historical records may provide the most realistic indication of the severity and return period of events which might affect coastal megacities. While such information may inform structural engineering and building regulations, its lack of precision offers little practical guidance to city planners in respect of the timing of significant events. 2.2. Understanding the risk from long-term, incremental hazards Unlike extreme event hazards, the long-term, incremental hazards (Box 2) generally provide ample warning of their incidence. The processes causing the hazards can be analysed and the resulting changes monitored in space and time. The rates of change may be sufficiently slow and predictable to provide the opportunities for the planning and implementation of sustainable adaptive measures (Table 2).

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Table 2 Components of relative sea-level change in coastal areas, with indications of operative timescales Global and regional processes

Local effects— main processes

Contributing factors

Relative sea-level rise #

Global sea-level change Neotectonics

Consolidation

Shrink-swell in clay Floodland sedimentation Isostatic adjustments

fall !

# # # #

Long term Long term (some extreme events) Decades/millennia Months/decades Months/decades Years/decades

#

Months/decades

!

Months/decades

#/!

Seasonal

!

Extreme events to long term Centuries to millennia

#/! #/!

(Neotectonics) Natural loading Artificial loading Land drainage Weathering/ soil formation Groundwater abstraction Groundwater recharge

Fines from suspension/peats (Sea) water loading (or unloading) Sediment loading (or unloading)

Timescale

#/!

#/!

Centuries to millennia

2.2.1. Sea-level change The assessment of relative sea-level change poses considerable problems in city planning. It is difficult to accurately assess the rate of this change at the local level with a resolution that is relevant to planning and management. The difficulty stems from the large number of separate contributory processes. Some components are due to global and regional processes, some to local. Relative sea-level change depends on a number of component factors of vertical displacement, some marine and some ‘terrestrial’ (though affecting the sea bed as well as the land), acting at different rates (Table 2) which themselves may be expected to change with time. While some of these factors, e.g. neotectonic crustal displacements in coastal areas, have strictly natural causes, most are either the result of human interventions of natural systems or of natural changes exacerbated by human activities. The net displacement of the land/sea-bed surface and the mean sea level at a given coastal location is the relative sea-level change at that site. In this review, relative sea-level change is described as a natural hazard, even though some of its contributing factors may be partly or wholly induced by human activities.

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While the vulnerability (to sea-level change) of coastal megacities, their inhabitants and their economic infrastructure, may usually be geographically defined and quantified with some precision, the uncertain, multigenic, nature of this hazard makes its related risk difficult to assess. The likely incidence and severity of the hazard, in terms of the rates of net relative sea-level change and the changes of those rates with time, may be only poorly known. This presents a major challenge to science. Until the likely contributions from the various component factors are better understood and quantified at their respective scales, a precautionary approach should prevail in mitigation planning [24]. The difficulties implicit in relative sea-level change prediction at coastal locations arise from uncertainties at global, regional and local scales. At the global scale, forecasts by Warrick and Oerlemans [25] of accelerated sea-level rise, as a predicted — though yet unproven [26] — response to global climate warming, were 200 mm by the year 2030 and 660 mm by 2100. The best estimate given by Raper et al. [27] was that between 1990 and 2100 sea level would rise by 490 mm; taking uncertainties into account, estimates of the rise during this period were in the range 200—860 mm. Regional and local circumstances may temper or exacerbate the effects of such a global change at the coastal site. For example, many major river outflows are sited in areas that have been subsiding tectonically over millions of years [28, 29]. While many coastal megacities in the Asia—Pacific region may be involved in similar regional tectonic subsidence, such neotectonic effects may be difficult to distinguish from other contributions. Notwithstanding this difficulty, subsidence of 1—2 mm/yr in Tianjin, China’s third largest urban area, has been ascribed to neotectonics [30], a rate that is of concern in the timescale of coastal planning and management. Also acting at the regional scale are the processes of isostatic adjustment of the earth’s crust. Perhaps of greatest significance in the context of coastal megacities in the region is the crustal loading effect of sediment isostasy, in which the crust responds to increasing sediment load by regional downwarping. This may be especially important in the vicinity of major deltas, where there is a substantial added crustal load due to the long-term accumulation of sediment, but distinguishing such subsidence from that due to neotectonics may not be feasible. The increase in crustal loading in coastal regions due to relative sea-level rise may also be a significant factor, this crustal deformation process referred to as hydro-isostasy (see, e.g., Ref.[26]). The post-Glacial global sea-level rise of more than 100 m [31] has differentially loaded what is now the inshore sea bed, the added load depending upon the pre-transgression relief. As with glacio-isostatic crustal deformation, both of these isostatic adjustments lag the loading in time, and are reversible. At the local scale, natural physical processes and human interventions may lead to ground-level displacement over the short term, at rates considerably in excess of those predicted for global sea-level rise. The potential for local displacement effects depends mainly on the geology of the coastal area. While some cities, e.g. Hong Kong, are founded largely on bedrock (or weathered bedrock) or, like Karachi, on well consolidated sediments, many, including Bangkok, Shanghai, Greater Tianjin, Jakarta and Calcutta, are sited partly or wholly on poorly consolidated sedimentary formations, including marine muds and sands. Some of these sedimentary formations have

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been formed in (geologically) very recent times, or, as in the Yangtze River delta adjoining Shanghai [32, 33], are actively accreting. The land that these recent sedimentary deposits forms lies within only a few metres of mean sea level and may become increasingly vulnerable to relative sea-level rise within the planning timescale. Largely irreversible land-surface subsidence due to the consolidation of these coastal and deltaic sediments can locally make an important contribution to relative sea-level rise. The potential for consolidation depends upon the types and thicknesses of sediments involved, some clay-rich muds reducing to about half of their deposited volume and peats to as little as little as one ninth [28, 29]. The rate of consolidation depends on natural and anthropogenic factors. Natural loading by superincumbent deposits and urban development enhances consolidation and consequent subsidence over the long term [34]. Lowering of the groundwater table, as a result, for example, of land drainage schemes, promotes consolidation of the superficial sediments and thus subsidence, while the weathering and oxidation of emergent associated peat deposits further enhances this effect [28]. The natural consolidation of coastal and deltaic muddy sediments may be catastrophically triggered by earthquakes. In part of Greater Tianjin, for example, abrupt land-surface subsidence of 500—600 mm occurred during the 1976 earthquake, in which some quarter of a million people are reported to have perished in Tangshan alone [30]. The over-abstraction of groundwater from aquifers within coastal and deltaic sediments is an especially important contributor to consolidation-related ground subsidence. There are several well documented instances of this problem within the region. In Bangkok between 1960 and 1988, up to 1.6 m of subsidence have been reported [35, 36], and in parts of Shanghai subsidence rates have exceeded 10 mm/yr [37, 38]. In the coastal plain of Tianjin, subsidence rates due to natural consolidation (1—3 mm/yr) are dwarfed by those induced by the extensive pumping of freshwater aquifers. Between 1960 and 1982 cumulative land subsidence of 1.5 m occurred [30]. When aquifers that have been over-pumped are recharged with water, there may be some expansion of the host sediments and thus some rebound of the land surface, though generally only a small proportion the original ground subsidence is recovered. The monitoring of land-surface changes on clay land needs to take account of possible seasonal variations due to the shrinking and swelling effects of clay minerals within the soil and sub-soil. The magnitude of such changes — perhaps up to 100 mm within a seasonal cycle — may mask subtler changes due to consolidation or neotectonic effects. The natural processes of sedimentation in coastal and deltaic areas are themselves significant contributors to relative sea-level change at the local scale as well as regionally. The natural flooding of undefended coastal lowlands by sediment-charged waters, whether of riverine or marine origin, leads to the progressive addition of sediment to the lowland surface. Many such low-lying coastal floodlands have been reclaimed for agricultural development during the last two millenia through the construction of protective dikes. Shanghai itself has grown on land that has been successively reclaimed as seaward sediment accretion has occurred (Tan, Q.X., 1987 [33]). The construction of coastal flood defences has protected the reclaimed land; but it has also effectively prevented the reclaimed land from being recharged with sediment

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during flood events. The natural sedimention history of reclaimed land has thus been interrupted by the reclamation, and floodland sedimentation has ceased, temporally, to be a factor contributing to relative sea-level change in such circumstances. 2.2.2. Coastal erosion Coastal erosion may be a significant hazard for some coastal megacities in the region, though its incidence is seldom catastrophic. Of all the coastal hazards, erosion is perhaps the easiest to forecast. Indeed it is perhaps questionable whether coastal erosion should be classified as a hazard, but rather a predictable consequence of the impact of waves on physically vulnerable coasts. Coastal erosion is the natural response of geological materials forming a shoreface to wave impact, or in some cases the impact of strong tidal currents. While there is widespread concern that the expected sea-level rise related to global warming will aggravate the effect [39], erosion can, and does, occur on coastlines where the sea level is stable or even falling [40]. Wherever and whenever sufficient wave energy reaches vulnerable coasts, erosion can take place. Vulnerability depends on (a) the degree of exposure to the waves, (b) the level of protection afforded by beach deposits and (c) the geological composition of the shoreface and its adjoining hinterland. Erosion tends to be most severe when storm conditions coincide with high tidal states, particularly during surge events. In such conditions protective beach materials are drawn downshore, seawards from the backshore, exposing hinterland sediments or rocks to wave attack. Well lithified rocks forming the shoreface and hinterland resist erosion, while poorly lithified rocks and unlithified sediments may be readily eroded, with a consequent recession of the shoreline. The protection afforded to erodible coasts by beach deposits depends on the maintenance of those deposits. Various factors in addition to the drawdown process may result in beaches becoming starved of sediment and thus ineffective in their protective role. In deltaic environments, the discharge of sediment which feeds beaches may change with time; climate variability may change the direction of net alongshore drift of beach sediment; or sediment may be removed from beaches, or its littoral transport impeded, by human interventions. The process of coastal land loss by erosion has a counterpart process of coastal change — coastal land growth by sedimentary accretion. Such coastal accretion is commonly a feature of deltaic areas, where sediment-bearing rivers discharge to the sea. Coastal megacities sited in deltaic areas, e.g. Shanghai [33], may be wholly or partly founded upon sediments which have accreted to form new land within the last few millenia. The long-term stability of this type of land may be threatened by sea-level rise, but also by any significant reduction in the rates at which sediments are discharged by rivers to the sea [33]. It is therefore particularly important to understand the impacts at river mouths of dam construction schemes within catchments which might greatly reduce discharge. 2.2.3. Saline intrusion of coastal aquifers For completeness, the process of saline intrusion is referred to in this review of marine-related natural hazards. In reality the effect is not a hazard in itself but rather

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a consequence of one or more of the following factors — relative sea level rise, the excessive pumped abstraction of groundwater from coastal aquifers and impeded flow in rivers due to human activities. The problem is now common in deltas and coastal urban areas [39].

3. Approaches to mitigation This section considers the information requirements of city managers and planners to guide policies of mitigation to cope with marine-related natural hazards acting over time scales ranging from the immediate to the long term — say 50—100 years. Mitigation measures are those taken in advance of a hazard event aimed at decreasing or eliminating its impact on society and the environment [9]. It is a long-term investment in the welfare of all, and there is a growing need in the formulation of city development plans to emphasize mitigation rather than response [41]. The perspectives of vulnerability and hazard need careful definition. A hazard relates to vulnerability. Thus a storm surge is a real hazard in respect of unprotected coastal lowlands, but not a hazard in respect of (non-vulnerable) rock-cliffed coasts or the open ocean. Where a seawall provides protection, a surge becomes a hazard in respect of the lowlands only when the wall is in danger of being over-topped. From the perspective of vulnerable lowland inhabitants, the construction of the seawall constrains the hazard. The vulnerability of the coastal lowland relates to expected surge sea levels rather than the standard of protection afforded by seawalls. For example, much of the population and economic activity of The Netherlands lives, or takes place, below mean sea level and is therefore inherently vulnerable to flooding. However, because the standard of protection from storm surge (provided by seawalls and other tidal defences, and naturally by sand dunes) is high, the risk in respect of that hazard is low. The problem in many coastal megacities in the Asia—Pacific region is that uncontrolled or poorly planned growth has created, or is creating, vulnerability (of people, services, economic activity) without the concurrent provision of an adequate standard of protection. The provision of adequate protection, e.g. by engineering intervention, may not be a realistic option, particularly if it is difficult to resource. Another possible adaptive option is the reduction of vulnerability, an approach which aims to safeguard people, economic activity and infrastructure by encouraging city development in areas removed from potential hazard impacts. Thus there are two mutually complementary approaches to mitigation. One is to constrain the hazard, the other to reduce vulnerability to the hazard. Both contribute to a reduction in the level of risk. Planning and management strategies that aim to achieve a sustainable reduction in vulnerability should be designed to cope with the failure of protective systems [42]. Human intervention activities which are unsustainable contribute to vulnerability. In any event the approaches to mitigation must reflect vulnerability assessments and the different levels of immediacy attached to the various hazards. National and city governments are under an increasing obligation to assess risk and to ensure that mitigation strategies feature prominently in their development agendas [41].

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Contemporary high risk scenarios for existing city developments demand an approach which focuses on immediate emergency planning. Long-term scenarios of increasing marine-related hazard which may be forecast to affect both the developed and yet-to-be developed, periurban areas can be addressed with a considered, strategic planning approach, perhaps one involving capital protective works. Between these extreme scenarios, there is scope for tactical adaptive measures leading to a reduction in vulnerability over the shorter term. In practice, mitigation measures are likely to be pursued in all three modes — emergency, tactical and strategic. In establishing a strategy for mitigation [41], a priority activity is the preliminary assessment of hazard, vulnerability and risk, using existing sources of information wherever possible (Box 3). Vulnerability is defined as being an estimate of the degree of loss resulting from a potentially damaging phenomenon [9]. The integration of the resulting assessments provides the basis for an initial quantification of risk and identifies the need for follow-up studies. With policy objectives in mind, a plan of action can then be developed by identifying the options for mitigation and the costs and benefits of these actions. The plan may identify the contributions that science can make to this quantification (Table 3). 3.1. Emergency measures Recommendations for emergency procedures in the face of impending or actual extreme event hazard impacts have been well documented elsewhere [41], and are referred to only briefly in this review. The recommendations focus on short-term to immediate goals of reducing vulnerability. They include the development of public awareness and emergency warning systems, and the establishment of co-ordinated procedures for evacuation, rescue and rehabilitation. There may also be provision for the protection of vulnerable key service installations. Warning networks for extreme marine-related events, other than coastal earthquakes, are generally well established in the Asia—Pacific region. The global forecasting of tropical cyclones through real-time observational and predictive modelling techniques [43] enables the issue of storm and related surge warnings, which may trigger the implementation of emergency procedures and provide an opportunity for the evacuation of vulnerable areas. The regional forecasting of tsunami impacts [21] involves a network of warning centres around the Pacific and uses data from specific recorded earthquake events or wave monitoring. Depending on the distance of the predicted impact from the source, the network can ideally provide vulnerable communities with up to a few hours’ warning, and thus the opportunity of taking vital precautionary action. The effectiveness of the emergency procedures, however, may be hampered by a reluctance of the threatened community to take evasive action [11]. Box 3. Determinands for the preliminary assessment of city hazard vulnerability and risk [41] z z z z

The The The The

nature of potential hazards: their predicted frequency, intensity and duration. geographical areas of the city which are most vulnerable. communities, business sectors and infrastructure components which are most vulnerable. estimated losses which would result from hazard events of different magnitudes.

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Table 3 Science activities at local, regional and global scales in respect of the quantification of marine-related physical hazards Hazards

Analytical and monitoring activities Local/city

Regional

Storm surge

Wave recording, monitoring Monitoring, modelling

¹sunami

Monitoring, modelling

Coastal earthquake

Monitoring

Weather/wave monitoring, forecasting Weather monitoring/forecasting Earthquake/tsunami warning network Monitoring network

Extreme events Severe waves

Long term Relative sea-level change Global sea-level change Neotectonic crustal displacement Isostatic crustal adjustment Sediment consolidation Floodland siltation

Coastal erosion

Coastal progradation, channel siltation Saline intrusion of coastal aquifers

Global

Earthquake/tsunami warning network

Tide-guage monitoring

Land surface altimetry monitoring

Tide-guage monitoring, sea surface altimetry Land surface altimetry monitoring Land surface monitoring/modelling

Tide-guage monitoring, sea surface altimetry

Land surface and sub-surface monitoring Land surface monitoring Catchment/coastal sediment transport monitoring Shoreline/estuary Catchment/coastal monitoring sediment transport monitoring Shoreline/estuary/ bathymetric monitoring Groundwater monitoring/modelling

3.2. Tactical measures Implementation of the dual approach of marine-related hazard constraint and vulnerability reduction, in the light of cost-benefit analysis, is appropriate to the management of established urban development (Table 4). The various regulatory and financial instruments (including insurance incentives) designed to divert or relocate people and economic activity from hazard-prone areas have been dealt with elsewhere [41]. Some possible mitigation options aimed at hazard constraint which may help to safeguard developed low-lying or waterfront urban areas are considered here. They

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Table 4 Mitigation and adaptive measures (excluding emergency measures) at local, regional and global scales in respect of the constraint of marine-related physical hazards Hazards

Mitigation and adaptive options Local/city

Extreme events Severe waves

Storm surge

¹sunami

Coastal earthquake

Long term Relative sea-level change Global sea-level change Neotectonic crustal displacement Isostatic crustal adjustment Sediment consolidation Floodland siltation Coastal erosion

Coastal progradation and channel siltation Saline intrusion of coastal aquifers

Regional

Construct/enhance/ maintain offshore breakwaters, seawalls. Promote/conserve saltmarsh Construct, enhance/ maintain seawalls. Promote/conserve sand dunes Construct/enhance/ maintain offshore breakwaters, seawalls. Conserve/promote saltmarsh. Restrict development on waterfront (Take precautionary measures in construction and planning)

Develop and relocate in low risk areas in response to rise, defend immovable assets Reduce emissions of ‘‘greenhouse’’ gases

Manage groundwater abstraction, regulate wetland drainage Consider planned siltation in periurban areas Restrict development in waterfront areas, regulate coastal defence interventions and beach/nearshore sand extraction Dredge to maintain/ enhance navigation/port function Manage groundwater abstraction

Assess impacts of climate and catchment land use change, river/coastal engineering affecting sediment discharge/ transport regimes

Global

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include regulatory measures but not major capital works. They mostly aim to control the human activities which exacerbate the natural hazards of relative sea-level rise in vulnerable low-lying areas and coastal erosion by wave action. Regulatory management of groundwater abstraction from aquifers underlying coastal cities can significantly reduce the rate of land surface subsidence. At Hangu, in Greater Tianjin, the aim has been to reduce pumping and to spread the abstraction over a wider area, reducing land-surface subsidence to about 20 mm/yr [30]. In the central part of Bangkok, where groundwater pumping is now prohibited, the rate of subsidence has decreased and in some places the ground level has shown a slight rebound [35]. Elsewhere, e.g. in Shanghai, programmes of pumped recharge of aquifers have led to some recovery of the land surface [29]. Land drainage schemes, carried out in advance of the development of periurban areas and which contribute to land-surface subsidence and the risk of inundation, need to be planned with a view to the provision of strategic protection works against relative sea-level rise. Regulation of human interventions which impact on the urban and periurban shoreline may help to inhibit wave erosion of soft shorefaces and the consequent threat of recession of the city waterfront. The protective role of beach sediments can be enhanced by controlling or prohibiting the extraction of sand from the shoreface and adjoining sea bed. It may also be helped by avoidance of sea defence works that interrupt the natural transport of sediment in the littoral zone and result in beach sediment starvation, while such regulatory measures can be complemented by the periodic artificial replenishment of beach materials (see, e.g., Ref. [44]). 3.3. Strategic measures Strategic mitigation measures concern long-term city planning and, if appropriate and affordable, major capital works arising. They address extreme event and longterm, incremental marine-related hazards. They have the dual aim of hazard constraint and vulnerability reduction, in part through hazard avoidance in urban development. They are concerned with the future development of periurban areas as well as with the management of existing urban areas. They should be considered, not only in a long (50—100 yr) time context, but in a wide geographical (e.g. catchment) context as well. Strategic mitigation measures above all demand the best possible information on hazards and vulnerability, as well as careful cost-benefit analysis. In view of the many uncertainties over hazard prediction over the long term, they should be implemented with due caution [24]. Long-term coastal city planning in respect of these hazards has three main strategic aims. These are (a) to encourage development outside areas vulnerable to marinerelated hazards; (b) to relocate people, economic activities and key infrastructure to reduce vulnerability; and (c) to enhance the standard of protection from marinerelated hazards where there is existing vulnerability but where relocation is not a feasible option (Table 4, Box 4). Recognising the geographical extent, and assessing the scale, of vulnerability in the coastal urban and periurban areas under different hazard and developmental scenarios forms a basis for strategic planning (Box 3). Computer mapping techniques

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Box 4. Strategic aims for coastal city planning to take account of marine-related, physical hazards z Promote new urban development away from areas vulnerable to marine-related hazards using financial incentives and regulatory constraints as appropriate. z Relocate vulnerable urban population, economic activities and key infrastructure to areas of low hazard susceptibility. z Enhance the standard of protection where there is existing vulnerability but where relocation is not a viable option.

provide the means of identifying areas that are unsuitable for development. These may involve hazard maps [41, 45], or maps depicting vulnerability, based on social and economic factors; or composite risk maps, combining hazard information with vulnerability assessment. The analysis of possible development scenarios against the predictions of hazard impacts within the timescale of the planning cycle can inform the strategic planning process, including decisions on the implementation of capital works for hazard constraint. Strategic, socio-economic adaptive measures, intended to encourage development or relocation in areas of low hazard risk within the planning guidelines, may include a range of regulatory and financial constraints and incentives [41]. Strategic coast and flood defence capital works, designed to constrain hazards in respect of existing, key development and infrastructure, include structures that are expected to withstand or dissipate extreme events and progressive long-term relative sea-level rise. These engineering interventions may include seawalls or other tidal defences; also offshore breakwaters, designed for extreme event wave, and especially tsunami, control. There may also be scope for the introduction of soft-engineered schemes, such as managed coastal ‘setback’ in periurban areas and the consequent promotion of saltmarsh wetland as a means of dissipating wave energy. Assessment of environmental impact over the long term is an essential part of any such engineering intervention.

4. Research priorities Local research agendas relating to marine-related hazards as they affect coastal megacities in the region may be viewed as having three foci. One is the spatiotemporal prediction of the hazard impacts through the application of geological, oceanographic and climatological disciplines; this is considered briefly below. The second focus concerns the socio-economic issues relating to vulnerability in the context of predicted hazard scenarios, and aims to establish the geographical limits in vulnerability assessments and their predicted changes with time. Digital terrain mapping techniques, coupled with ground-level change data, provide a cost-effective geographical definition of vulnerability under likely hazard scenarios. The third focus is the translation of the scientific and socio-economic information to a format appropriate to the planning and integrated management of coastal megacities. In addition to this agenda, there are areas of generic research and development to be addressed by the international scientific community concerning, e.g., the development

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of marine-related hazard warning systems, or the optimization of groundwater abstraction in coastal urban environments. Research needs for hazard impact prediction at the local or city-specific scale involve global, regional and local data (Table 3). Global and regional information, including global sea-level change predictions, tropical cyclone monitoring and tsunami modelling, must be taken into account when assessing potential local impacts. Complementary knowledge of the local physical conditions, in particular, the various effects that may be contributing to ground level change, is of paramount importance in the formulation of effective hazard mitigation strategies. Overall, there is a need to put in place the means to collect these data, establishing baseline measurements against which future events and changes can be monitored and measured. The aim should be to produce predictive models which are soundly based on adequate observational information at the local to the global scale, and covering the range of timescales relevant to the quantification of extreme event and long-term incremental hazards.

5. Concluding summary z Marine-related physical hazards affect, and will increasingly impact on, coastal megacities in the Asia—Pacific region. z The hazards comprise those due to extreme events such as storm surges and tsunamis which may be catastrophic in their impacts; and those due to continuing changes over the long term, notably eustatic sea-level rise, sedimentary consolidation and coastal erosion. z The long-term hazards may be exacerbated by human activities such as the increasing production of ‘greenhouse’ gases and over-abstraction of groundwater, and, while not threatening catastrophic loss of life or destruction of property, do have important economic and social implications for the future. z There are two complementary approaches to mitigation: to constrain the hazard; and to reduce the vulnerability. z The contributions that science can make to the planning of sustainable adaptive measures are to improve the prediction of the incidence and severity of the various hazards, and their impacts in space and time. A priority task is to establish the means to collect the essential baseline data for predictive modelling. z Contemporary high risk hazard scenarios for existing city developments demand an approach that focuses on effective warning networks and emergency planning. z Long-term, incremental hazards that are forecast to affect both developed and peri-urban areas can be addressed with a strategic planning approach, involving relocation and capital protective works. z The selection of strategic measures demands the best possible predictive information on hazards and vulnerability, including its full socio-economic evaluation so that the costs and benefits of the mitigation options can be realistically assessed.

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Acknowledgements The author is grateful to Chris Evans, Robin Wingfield, Martin Culshaw, Kerry Turner and Edmund Penning-Rowsell for their critical review of the manuscript. The paper is published with the approval of the Director, British Geological Survey (N.E.R.C.).

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