Geographic Information System Assessment of Tsunami ... - BioOne

Journal of Coastal Research

25

1

131–141

West Palm Beach, Florida

January 2009

Geographic Information System Assessment of Tsunami Vulnerability on a Dune Coast Deirdre E. Hart† and Gemma A. Knight†‡ † Department of Geography University of Canterbury Te Whare Wananga o Waitaha Private Bag 4800 Christchurch, New Zealand [email protected]

‡ Otago Regional Council Private Bag 1954 Dunedin, New Zealand

ABSTRACT HART, D.E. and KNIGHT, G.A., 2009. Geographic Information System assessment of tsunami vulnerability on a dune coast. Journal of Coastal Research, 25(1), 131–141. West Palm Beach (Florida), ISSN 0749-0208. This paper documents a novel procedure for assessing the vulnerability of an open-coast dune system to tsunami hazard. Geographic Information System (GIS)–based analyses of Light Detection and Ranging (LIDAR) data are employed to classify a range of dune topographies in terms of four tsunami inundation scenarios, run-up to 3, 6, 8, and 10 m above mean sea level, along the coast of Christchurch, New Zealand. Analysis reveals two key characteristics which together define the tsunami vulnerability of a narrow vegetated dune system: (i) the elevation or average height of the dune ridge and (ii) the continuity or standard deviation of height of its longshore profile. We find that the Christchurch dune coast currently offers a high degree of protection against inundation from small to medium tsunami (run-up ⱕ6 m above mean sea level), with the degree of vulnerability under more extreme scenarios varying longshore in relation to the height and continuity of the dune system and its vegetation. A tsunami inundation vulnerability index is developed to assist coastal managers in quickly assessing the relative vulnerability of sections of dune, while simultaneously identifying the nature and location of weaknesses. At the local level this index may be used to effectively allocate management resources, while at a regional level it can be used in coastal development and hazard planning. Relative to current field survey–based methods of determining tsunami inundation risk, the GIS-based procedures and vulnerability index developed here offer significant improvements in accuracy and efficiency at local to regional scales. ADDITIONAL INDEX WORDS: Tsunami hazard, run-up, inundation, Light Detection and Ranging.

INTRODUCTION Tsunami waves have catastrophic potential: they can cause complete destruction of settlements, severe erosion of beaches, and injury or loss of life to those in affected areas. The 26 December 2004 Indian Ocean tsunami brought to the fore the devastation which is possible during such events (KATHIRESAN and RAJENDRAN, 2005). The global reach of this tsunami (TITOV et al., 2005), combined with predictions that 600 million people will occupy coastal floodplains by 2100 (N ICHOLLS and MIMURA, 1998), highlights the need for coastal communities worldwide to identify vulnerabilities to, and prepare for, tsunami hazards. Geographic Information Systems (GISs) provide effective opportunities for detailed regional investigations of the likely effects of such hazards (WOOD and GOOD, 2004). In this paper we employ GIS techniques in a novel assessment of the vulnerability of an open-coast dune system to tsunami hazards, classifying a range of topographies in terms of overtopping and inundation risk. Tsunami waves are distinctly different from wind-generated waves, not only in terms of their mode of generation, but also DOI: 10.2112/07-0960.1 received 30 October 2007; accepted in revision 8 January 2008.

in terms of their long-period, shallow-water nature and shoreline behaviour. Tsunami-induced damage at the coast is a function of wave run-up, the maximum level that waves swash up to at the shore. According to Plafker’s Rule, earthquake-generated tsunami wave run-up may reach elevations twice the height of the seafloor displacement that initiated them, significantly beyond the reach of large storm waves (LI and RAICHLEN, 2001; OKAL and SYNOLAKIS, 2004; YEH et al., 1994). The 2004 Indian Ocean disaster graphically demonstrated the value of natural coastal barriers to tsunami run-up (KATHIRESAN and RAJENDRAN, 2005; LIU et al., 2005; MARRIS, 2005; MORTON, GOFF, and NICHOL, 2006). Field and satellite observations from Indonesia, Thailand, and Sri Lanka indicate that intact sand dunes, rock platforms, mangrove forests, coral reefs, and barrier islands all offered enhanced protection against wave impact, flooding, and scour. These barriers reduced incoming wave energies, run-up, and inundation levels and, thus, the return flows that devastated adjacent unprotected areas. Significantly, areas in close proximity to each other experienced very different levels of property damage and casualties because of differences in the development of these natural coastal defences. The focus of this research is the protective function of open-coast dunes. The uneven contours and jagged geometry of the coastal zone

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often renders the prediction of tsunami waves a matter of complex engineering modelling (BHATTACHARJEE, 2005). Most models typically propagate tsunami to the 10-m depth contour, producing wave heights which differ from observed values by factors of 4–10 (SYNOLAKIS, 1995). As such, they have limited applicability to coastal community vulnerability assessment and, thus, in informing coastal managers and planners. GIS tools offer an effective alternative for predicting the behaviour of tsunami waves at the shore. GIS is used extensively in terrestrial hazard assessment, but its application in the coastal zone has been slower, not least because of the challengingly dynamic nature of this environment (DANIEL and ABKOWITZ, 2005; WOODROFFE and FURNESS, 2003). Our investigation involves creating a snapshot of coastal vulnerability to tsunami run-up and inundation from GIS analysis of high-resolution Light Detection and Ranging (LIDAR) topographic data. The method is analogous to a riverflood inundation analysis, with the dunes functioning as stop banks. LIDAR data are obtained from an aircraft-mounted laser (WEHR and LOHR, 1999), allowing the collection of highly accurate, subaerial coastal elevation data across large areas, including three-dimensional dune morphology surveys (ROBERTSON et al., 2004; ZHANG et al., 2005). The high vertical accuracy of LIDAR is ideal for use in predicting coastal community vulnerability to a range of run-up scenarios, while the potential to efficiently cover extensive coastal areas makes LIDAR-based methods ideal for assessing risks from tsunami events.

STUDY AREA Situated on the east coast of the South Island, Christchurch is the second largest city in New Zealand, with a population of approximately 400,000 (Figure 1). We examine the potential for tsunami waves to run up, overtop, and inundate the 9.5-km-long stretch of dunes fronting the city’s open-coast suburbs, Southshore, New Brighton, and North Beach, but exclude the effects of reflected waves on the areas immediately adjacent to the Avon-Heathcote Estuary. The open coast of Christchurch is characterised by intermediate-type sandy beaches, which are stable or accreting in the long-term, according to the classification scheme of SHORT (1999); and a broad, shallow, gently-sloping continental shelf (KIRK, 2001). The local wave climate is dominated by southerly waves refracted around Banks Peninsula and locally-generated wind waves from easterly and northeasterly synoptic systems (6–7 second mean period, 2 m significant wave height; GORMAN, BRYAN, and LAING, 2003). The tidal regime is semidiurnal and microtidal, with a spring range of 1.87 m. The dunes form a semicontinuous line ranging in height up to 10 m above mean sea level (AMSL). They are typically hummocky with vegetation dominated by introduced marram grass, Ammophila breviligulata, except where artificial contouring has produced a smooth, ice pant–covered, Carpobrotus edulis incipient foredune. Collectively this vegetation provides an almost-continuous and stabilising cover, with the exception of a few large, predominantly unvegetated topographic gaps. Along New Brighton Spit and the adjacent

coast the dunes compose a single line of incipient foredunes, with houses typically set back behind a road along their landward edge, while towards the northern end of the field area the dunes spread inland several rows deep. This coastline is Holocene in age, the broad northern section having prograded to its present position since sea levels stabilised 6000 years ago, while the spit has formed, been breached, and reformed over the last 2500 years (MCFADGEN and GOFF, 2005). In historical times the Christchurch coast dunes provided important resources for local Ma¯ori, including weaving materials from the native sand binder pingao or golden sand sedge, Desmoschoenus spiralis. Much of their native vegetation was removed in the 19th century by early European settlers via grazing and burning. The value of the dunes as a sand trap and coastal protection resource was soon realised, and they were rebuilt and stabilised with marram grass and other introduced plants. Today native vegetation is slowly being reintroduced while the line of the dunes is broken by sea walls and other built features at several locations. Adjacent to central New Brighton, for example, a retaining wall that is 150 m long and a height of 3 to 5 m AMSL protects the public library and car park from the sea. Similarly, there is a 200-m-long, 3-m-AMSL seawall at North Beach with no landward dunes. Our independent investigation coincided with one by the Christchurch City Council (CCC) into the feasibility of lowering the New Brighton dunes to 6 m AMSL. The current legal minimum elevation for dune lowering along this coast is 8 m AMSL (CELG, 1997), a level that corresponds to a tsunami event with about a 1000-year return period (BERRYMAN, 2005). Geological, geomorphological, and archaeological records of palaeotsunami in New Zealand indicate that, over the last millennium, several large tsunami (wave heights 5 to 15 m) have occurred with nationwide effects (GOFF, CHAGUE-GOFF, and NICHOL, 2001). Between 1840 and 1982, it is estimated that New Zealand received 32 tsunami, 23 of which were near-field. The Christchurch coast experienced 9 tsunami over this period. Two were major events, 1868 and 1960, both originating from Chile (DE LANGE and HEALY, 1986). New Zealand’s high level of exposure to tsunami is due to its geographic location. The country’s plate margin setting means that almost the entire coast is vulnerable to near-field tsunami generated by earthquakes, landslides, and volcanic eruptions (DE LANGE and HEALY, 1986; GOFF, CHAGUEGOFF, and NICHOL, 2001). In addition, the east coast is exposed to far-field tsunami propagating across the Pacific Ocean. South America tends to produce the highest such waves (DE LANGE and HEALY, 1986), although the coast is also vulnerable to those from Japan, Alaska, Indonesia, and the Tonga Trench. The city of Christchurch is at greatest risk from far-field tsunami, rather than the more frequent nearfield tsunami, since the former are locally amplified by shelf resonance (BERRYMAN, 2005; DE LANGE and HEALY, 1986).

METHODOLOGY LIDAR beach topography data were collected for the Christchurch region from a fixed-wing aircraft on 6–9 July

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GIS Assessment of Tsunami Vulnerability

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Figure 1. Location of the Christchurch city open-coast dune study area, from Rothesay Road to the southernmost end of Rocking Horse Road, within the regional setting (inset) north of Banks Peninsula, New Zealand.

2003 (see ZHANG et al., 2005 for details of LIDAR procedures). A triangulated surface model was used to derive a 2-m gridded data set based on every classified ground laser strike (AAMHATCH, 2003). This data was provided by the CCC as eight ASCII (American Standard Code for Information Interchange) text files, each comprising a 2 ⫻ 2 km tile (AAMHATCH, 2003). Using ArcGIS 9.1, the ASCII tiles were converted to 2 ⫻ 2 m raster grids, mosaicked together, and clipped to the size and shape of the study area. The accuracy of this data was investigated by comparison with cross-sectional profile sur-

veys conducted by the Canterbury Regional Council, Environment Canterbury (ECAN). Twenty-five profiles were measured in the study area in July 2003 with a survey-grade electronic total station. At the exact benchmarked location of each profile, an equivalent line was interpolated from the LIDAR raster data, graphed, and compared to the field data. The two data sets were found to be acceptably similar for the purposes of this study, with maximum variations of 0.15 m vertically and 0.55 m horizontally. Using the raster data, a series of elevation points were interpolated along the topmost dune or seawall surface in a line


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Table 1. Description of the four tsunami run-up scenarios. Run-Up (m AMSL)

3 6 8 10

Inundated by Tsunami Event

Reason Chosen

current height of dune toe and seawall top proposed dune lowering target current legal minimum for dune lowering current maximum dune height

small moderate large extreme

parallel to the shore and graphed. This longshore profile represents the threshold elevations at each point along the coast that wave run-up would need to reach in order to overtop the beach and inundate the land behind. The beach topography raster data was also reclassified into binary raster layers denoting levels above and below four run-up scenarios: 3, 6, 8, and 10 m AMSL (hereafter referred to as the 3-, 6-, 8-, and 10-m scenarios). Each scenario corresponds to a key physical or planning-related elevation in the local coastal environment (Table 1). Collectively, these scenarios represent the run-up of waves from tsunami events with estimated return intervals of approximately 100–2500 years (BERRYMAN, 2005). For each run-up scenario, ‘‘gaps’’ in the dune system were identified as areas lower than the scenario elevation. All run-up elevations used in this study are assumed to include the tsunami wave run-up as well as windgenerated wave run-up and tidal water level variations. Tsunami inundation of the dune system was predicted for the initial wave of each scenario. These volumes represent minimum estimates for the four events since the gaps would be eroded and enlarged by the inundation and return flow of the first wave, allowing subsequent waves to pass through the dune system more readily. For gaps identified in each scenario, the volume of inundation, V (m3), was calculated according to: V ⫽ rAt

(1)

where r is the rate of inundation (m/s), A is the cross-sectional area of the gap (m2), and t is half the wave period (s). A 300-second wave period was used, based on the 1960 tsunami wave record from the Port of Oamaru, which is situated 360 km south of Christchurch and has a similar aspect and continental-shelf setting. The rate of inundation was estimated for each gap according to BRYANT (2001, p. 56): 0.5 ⫺1 n r ⫽ H0.7 s [tan(␤w)]

(2)

where Hs is wave height (m), which was considered equivalent to the water depth through the gap; tan(␤w) is the inclination of the water surface and is assumed to be 0.001; and n is a Manning’s roughness coefficient 0.023. Inundation rates were calculated using both (i) average gap height and (ii) actual elevation at every (2 m horizontal) grid point across the gap.

RESULTS In places where the longshore profile of the dune system dips below the elevation specified in each scenario, gaps were identified through which tsunami run-up could pass to in-

undate areas behind. Figure 2 shows a plan view of the dune gaps at 3, 6, 8, and 10 m AMSL. This two-dimensional depiction also illustrates the width and length of dunes at each scenario elevation, showing that the dune volume below 3 m AMSL is considerable. As such, the dunes would likely continue to operate as an effective barrier to 3-m tsunami waves after erosion by an initial wave. By contrast, the dunes are much narrower at 6, 8, and 10 m AMSL. Thus, erosion by waves with run-up exceeding 6 m AMSL would be much more likely to lower the dunes, reducing their effectiveness as a barrier to inundation by subsequent waves of similar height. Another characteristic of the morphology of the Christchurch dune system is that areas which are particularly discontinuous also tend to be narrow (Figure 2). Gaps through narrow dunes are highly vulnerable to erosion and, thus, inundation. Because of the small volume of sand and generally thinner vegetation in such areas, tsunami waves would rapidly erode and widen these areas on the inundating and return flows. A wider dune creates a greater distance for a wave to cross and volume of sediment to erode, thereby offering a more effective buffer against inundation. Figure 3 illustrates the longshore profile of the dune system. The longshore and vertical dimensions of each gap are shown, as well as longshore changes in the pattern of gap width and spacing. In order to examine this pattern more closely, the longshore profile was divided into 10 distinct sections. The characteristics of each section and their predicted volumes of inundation under each scenario are summarised in Table 2. Under the 3-m scenario, the dune system is again shown to operate as an effective barrier to tsunami run-up, with the total expected inundation ⬍80 m3 (Table 2). At first glance, most of the open coast does not appear particularly vulnerable to inundation under the 6-m scenario either, since the average height of seven of the ten dune sections is between 7 and 8.6 m AMSL (Table 2). However, predicted inundation under this tsunami scenario totals ⬎500,000 m3. Approximately 75% of this volume is expected to pass through a few large gaps in Sections 6 and 8, with most of the remainder inundating a series of small gaps and low dunes at the end of the spit (Section 1). Notably, the former two sections are also the only areas vulnerable to overtopping under the 3-m scenario. Their low profile is due to the presence of seawalls and absence of dune development. And unlike the low dunes and small gaps at the end of the spit, which are predominantly well vegetated, Sections 6 and 8 are characterised by a sparsely vegetated dune toe. The reduced vegetative cover here would offer little protection from erosion to any sand accumulations present at the time of a tsunami because of the reduced roughness and cohesion offered by this cover relative to that of adjacent dune systems. All ten sections of the open-dune coast would experience overtopping under the 8-m scenario, with inundation volumes ranging from 16,450 to 1,057,130 m3 (Table 2). Sections 5, 7, and 10 are predominantly elevated above 8 m AMSL, but they are still vulnerable under this scenario as they are characterised by tall dunes interspersed by small gaps below 8 m AMSL. Only a few isolated dune peaks along the Christchurch coast exceed 10 m AMSL (Figures 2 and 3). It therefore could



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Figure 2. Planform cross-sections of the Christchurch dune coast at 3, 6, 8, and 10 m AMSL.

be expected that the 10-m scenario would result in overtopping along the entire coast, with extensive inundation of areas behind. Under this scenario volumes ⬎500,000 m3 are predicted to pass through nine of the ten dune sections, with total inundation exceeding 13,000,000 m3 (Table 2). Figure 4 illustrates the rate of increase in predicted vol-

umes of inundation under the four scenarios calculated using point heights and using average gap heights. It is important to note that each inundation level would be followed by a proportionate return flow. Inundation estimates calculated using the two methods do not vary significantly, indicating that average gap-height data are sufficient for estimating lev-


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Figure 3. Longshore profile of the Christchurch city dune system, showing 10 distinct dune sections (and their vulnerability class).

els of inundation in this open-coast setting. As illustrated, the dune system as a whole is predicted to experience an exponential increase in inundation with increasing run-up height, with large increases occurring at levels above 6 m AMSL.

DISCUSSION The GIS LIDAR data–based approach developed in this investigation yielded indications of vulnerability to a range of tsunami scenarios for Christchurch’s open-coast dune system as a whole, for individual gaps within that system, and for the different types of dune examined. Of interest are how our results compare with previous assessments of tsunami vulnerability; whether our approach provides significant methodological improvements in terms of efficiency and resolution; and what implications this has for tsunami vulnerability assessment and coastal planning.

Methodological Advances Tsunami inundation volumes were predicted for the Christchurch dune coast in 1997 using cross-sectional surveys of major gaps identified in the field, overtopping Equations 7–10 from the Coastal Engineering Research Center (CERC,

1984), and an 8.2-m wave run-up and 10,800-second wave period scenario (CELG, 1997). Since 1997, the pattern of gaps in the dune system has not changed markedly. The locations of the major gaps identified in our study mirror those identified in the non-GIS-based CELG (1997) investigation, indicating a level of stability in areas of major tsunami vulnerability between 1997 and 2003. Housing development in the areas behind several of the large gaps, however, has expanded considerably over the last decade. The 10,800-second (3-h) wave period was estimated using Lyttelton Harbour water level records from the 1960 Chilean earthquake tsunami. This gauge site is located within a sheltered-harbour environment, prone to much-stronger attenuation and amplification of long-period waves than the open coast of Christchurch (HEATH, 1976). Given the open-coast records from eastern New Zealand of the 1960 event and the 2004 Indian Ocean event, it is now believed that the 1997 wave period was about an order of magnitude too long. In the CELG (1997) study, the total volume of inundation calculated was approximately 3,000,000 m3, a similar order of magnitude to the total volume of inundation found under our 8-m scenario (3,624,870 m3), despite the unrealistically long wave period used in the former study.

Table 2. Estimated average height and volume of inundation for ten sections of the Christchurch dune coast under 3, 6, 8, and 10 m AMSL tsunami runup scenarios. Inundation Volume (m3) Section

Locality

Average Height (m)

3m

6m

8m

10 m

1 2 3 4 5 6 7 8 9 10 Total

distal end of spit Southshore South Brighton (south) South Brighton (centre) South Brighton (north) Pier North Beach (south) North Beach (centre) North Beach (north) Waimairi Christchurch coast

5.2 7.0 7.9 7.8 8.8 4.5 8.6 3.6 7.5 8.5 6.9

0 0 0 0 0 70 0 5 0 0 75

148,050 0 860 0 0 192,320 0 218,700 2010 4400 566,350

1,057,130 494,640 82,300 135,720 32,460 945,160 16,450 561,220 244,850 54,930 3,624,870

2,582,560 2,254,590 1,057,250 851,250 717,300 2,111,910 455,580 1,028,310 1,241,670 721,010 13,021,430



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Table 3. Predicted coastal vulnerability to tsunami run-up and inundation based on dune characteristics. Coastal Area Vulnerability

Figure 4. Changes in the predicted volume of inundation with increasing tsunami run-up elevation AMSL. Black bars indicate predicted volumes calculated using average gap height (mean), while grey bars indicate those calculated using gap height at 2 m horizontal resolution (2 m).

Advances in understanding tsunami wave periods aside, the major difference between the two Christchurch studies is that our estimates include inundation through all the gaps in the dune system below the run-up scenario, whereas the 1997 study summed inundation estimates for the large gaps alone. The 1997 estimate of large gap length below 8.2 m was 2170 m, compared to our estimate of the total gap length below 8 m of 5616 m. The influence of this methodological difference on the accuracy of total inundation estimate for this dune system can be illustrated by recalculating total inundation based on the total area of gaps identified in 1997 and the more accurate 300-second wave period. The result is a total inundation estimate of 170,000 m3, around 20 times less than our present estimate of 3,624,870 m3 using the total gap area at 8 m AMSL. This comparison has significance for dune systems beyond New Zealand. We conclude that methods focussed on large gaps alone are likely to lead to significant underestimation of tsunami inundation through single-line hummocky dune systems, since inundation will not only occur through large gaps but also through smaller gaps and discontinuous areas.

Class

Low

I

Moderate

II

Moderate

III

High

IV

Dune Elevation and Continuity

high, continuous dunes; average height ⬎8 m AMSL medium, continuous dunes; average height 6–8 m AMSL high, discontinuous dunes; average height ⬎6 m AMSL absent or low, discontinuous dunes; average height ⬎6 m AMSL

Christchurch Coast Sections

5, 7 2, 3 4, 9, 10 1, 6, 8

a section of dune, energy will be focussed into the gaps, causing scour, erosion, and gap enlargement. In the case of the Christchurch dune, this effect will be enhanced in large gaps because of the sparse vegetative cover in these areas. COOPER and MCLAUGHLIN (1998) consider that a good classification index is based on the minimum amount of necessary information. We suggest that dune ridge height and continuity characteristics are sufficient for a rapid assessment of tsunami vulnerability on open coasts with a narrow dune field. Narrow dune coasts backed by urban infrastructure are becoming increasingly common worldwide as development pressures combine with a global ‘‘rush’’ to the coast (NICHOLLS and MIMURA, 1998). However, in situations where the inland extent of the dune field is greater, a third variable, derived from the sheltering effect of multiple lines of dunes that run parallel to the coast, should be added. Multiple lines of dunes would significantly increase the capacity of the coast to protect the hinterland from tsunami inundation and return flows because of the increased width, sediment volume, and roughness of the more-complex barrier.

Tsunami Inundation Vulnerability Classification Our analysis of the Christchurch coast reveals two different characteristics which together define the vulnerability of a vegetated dune system to tsunami inundation: (i) the elevation of the dune crest and (ii) the continuity of its longshore profile. Based on these two characteristics we developed a relative tsunami vulnerability index (Table 3, Figures 3 and 5). Our classification assumes that vegetated dunes offer the best form of natural shore protection from tsunami inundation on temperate sandy open coasts. It also assumes that, when tsunami waves run up, breach, and return flow through

Figure 5. Relative vulnerability classification (roman numerals in grey) of the 10 dune sections (italic numbers) based on a plot of their crest height versus standard deviation (SD). Risk of tsunami inundation increases towards the top right of the plot.


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According to our classification, the least vulnerable sections of dune coast are characterised by a high, continuous, well-vegetated ridge (Table 3, Class I). Such continuous dune systems offer the most protection from tsunami inundation since, in addition to functioning as an effective barrier to wave run-up and overtopping in extreme events, this type of system is the least susceptible to breaches through gaps. Medium, continuous dunes are classified as moderately protective since they would operate as an effective barrier during tsunami scenarios ⬍6 m AMSL (Table 3, Class II). Further, these dunes would likely dissipate a significant amount of wave energy during large events (6- and 8-m scenarios), despite the significant overtopping that would occur. Of the four tsunami vulnerability classes, dune sections in Classes I and II would experience the least flow channelling and focussed erosion due to the absence of gaps. As such, they would be less likely to be compromised during a large tsunami than if their profile was of a similar or greater height but discontinuous. In contrast, sections of coast with a low, discontinuous dune ridge or without dunes are classified as most vulnerable (Table 3, Class IV). Waves from a moderate tsunami could pass over and through these dunes with little obstruction. High but discontinuous dunes are classified as moderately vulnerable since under the 8- and 6-m tsunami scenarios their large deep gaps would be breached regardless of whether or not run-up reached the level of the average dune height (Table 3, Class III). Further, under both the 6- and 8-m scenarios, the initial wave run-up and return flow would likely scour and erode gaps, further compromising the effectiveness of the dune barrier against subsequent waves. Perhaps not surprisingly, we found that dunes characterised by a discontinuous ridge (Table 3, Classes III and IV) also tended to be narrow. If a tsunami were to travel through such dunes, the relatively small volumes of sediment contained within would provide a small barrier to erosion and limited surface for energy dissipation. This finding is relevant to other dune systems where the dominant vegetation type produces narrow, hummocky dunes and/or where the inland extent of the dune field is limited by infrastructure or housing development. In the case of the Christchurch coast, marram grass dominance produces hummocky dunes while backshore road, residential, and commercial infrastructure severely limits the inland extent of the dune field. Overall, it appears that along the Christchurch coast the present dune system offers a moderate degree of protection from tsunami inundation. However, the numerous deep gaps along the dune system mean that large amounts of overtopping and inundation will result from tsunami in which runup exceeds 6 m AMSL. If the deeps gaps were fewer and/or less pronounced than at present and well-vegetated, the area inundated could be restricted significantly. It is important to note that the classification scheme developed here refers to the susceptibility of a section of coast to tsunami inundation, and not to the vulnerability of the dunes to perturbation during a tsunami (although, in the event of a tsunami, these two effects are likely to be highly correlated). As such, our index differs from the dune vulnerability indices developed by WILLIAMS et al. (2001) and GAR-

CıÁ-MORA et al. (2001), which focus on dune system responses to change. However, as with the latter indices, our classification scheme can assist coastal managers to quickly assess the relative vulnerability of dune sections while simultaneously identifying the nature and location of the main weaknesses. At the local level this can lead to efficient allocation of dune management resources, while at a regional level it can be used as a tool in coastal development and hazard planning. Our dune classification concerns one major element of the vulnerability of the Christchurch coast to tsunami inundation. An overall assessment of the physical vulnerability of Christchurch city to tsunami inundation would also need to consider the likely response of the Avon-Heathcote Estuary, landward of the spit (Figure 1), a factor which is outside the scope of the present study.

Tsunami Hazard Risk BURTON, KATES, and WHITE’s (1993) hazard risk model asserts that natural hazards are negative or unwanted interactions between human-use and natural-event systems. He argues that hazard risk increases not only with the increasing likelihood and magnitude of natural events, but also with the increasing extent and development of the human-use system. In the context of tsunami hazard we may infer that, relative to undeveloped areas exposed to the same potential for inundation, highly developed areas are characterised by greater hazard risk (WOOD and GOOD, 2004). With the exception of a small area at the end of the New Brighton Spit, the area landward of Christchurch’s dune coast is highly developed and, therefore, at some risk of tsunami hazard. In general, north of the pier where the dunes have been artificially contoured (Sections 7–10), the majority of the gaps identified are related to human-use system features, including surf clubs, beach access tracks, and their associated sea walls. These gaps are largely unvegetated, being covered instead with artificial surfaces such as concrete and tarmac, which provide surface cohesion but little roughness. South of the pier, where the coast is less-anthropogenically altered (Sections 1–5), the hummocky nature of the marram-covered dunes determines the locations and dimensions of many of the smaller gaps. Hazard ‘‘hotspots,’’ where highly developed areas overlap those with the greatest potential for tsunami inundation (Classes III and IV), include new subdivisions landward of Section 10, the long-established North Beach suburbs landward of Section 8, and the central New Brighton business and retail district landward of Section 6. Two of these hazard ‘‘hotspots,’’ Sections 6 and 8, have undergone historical dune removal and replacement with seawalls. The public library building and associated car park, playground, and ramp structures are situated behind the New Brighton Pier sea wall (Figure 6). Although the library structure was designed with the intention of providing a degree of protection from tsunami inundation for areas immediately landward, it is debatable how much protection it can offer since run-up will be able to flow under the ramp, across the adjacent car park, and around the building itself under tsunami scenarios greater than 3 m AMSL, with the building potentially operating to focus energy and erosion onto adja-



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Figure 6. The New Brighton Library structure situated landward of the pier. (a) Looking north along the seaward side of the spit; (b) aerial photograph of the structure.

cent dunes (Figure 6). We conclude that the preservation and development of ‘‘natural’’ dune barriers provides a greater level of protection along the Christchurch city coast than the construction of seawalls and buildings.

The Value of Natural Barriers to Tsunami Inundation Worldwide the value of natural barriers as a form of coastal protection has been clearly demonstrated by local variability in the effects of major tsunami events (BHATTACHARJEE, 2005; KATHIRESAN and RAJENDRAN, 2005; KERR, 2005; LANDER, WHITESIDE, and LOCKRIDGE, 2003). As indicated earlier, sand dunes, mangrove forests, and reefs have been shown to reduce tsunami wave energy, lessening the impacts of run-up on the coastal zone and adjacent communities. KATHIRESAN and RAJENDRAN (2005) found that settlements on the southeast coast of India located behind sand dunes and mangroves suffered considerably less damage during the 2004 Indian Ocean Tsunami than those that were not sheltered by such features. Similarly, FERNANDO et al. (2005) found that the presence of coral reefs in Sri Lanka operated to deflect tsunami waves along the reef, visibly reducing their height. An important finding of this study concerned the effect of gaps in the reef system caused by coral mining. Where tsunami waves blocked by the reef found major gaps in its structure, their energy was focussed and amplified through the gap, causing increased inundation and damage in areas inland of the gaps. In Sri Lanka, areas behind sand dunes received considerably less damage than those located where the dunes had been removed, which were almost completely destroyed. Where gaps in the dunes occurred, wave focussing caused increased erosion and scour (LIU et al., 2005; MORTON, GOFF, and NICHOL, 2006).

As demonstrated by our results, the same principals of the protective function of natural barriers to tsunami inundation apply to the open dune coast of Christchurch. Dune barriers may be used to effectively reduce vulnerability to tsunami inundation. However, if the dune system contains any large gaps or numerous small gaps, these areas may operate to focus tsunami energy and erosion, increasing the risk of inundation for areas behind. Further, since flow rate is related to water depth (BRYANT, 2001), volumes of inundation occurring through deep gaps may exceed those overtopping areas of the dune system with a continuous longshore profile. Thus, infrastructure and assets located directly behind dune gaps compose the areas of greatest hazard risk. A major implication of our study is that it is possible to effectively mitigate the hazard risk posed by extreme tsunami events in urban areas, by analysing the strengths and weaknesses of coastal dune systems and enhancing their barrier function through targeted management.

CONCLUSIONS AND MANAGEMENT IMPLICATIONS This paper documents two advancements in the field of coastal hazard assessment: (i) a GIS-based methodology for characterising the longshore continuity and height of narrow dune fields, and (ii) a tsunami inundation vulnerability index for open-coast dune systems. The simplest but potentially most significant finding of our study is that dune profiles that are high, continuous, and vegetated offer the best protection from tsunami hazard in open-coast settings characterised by narrow dune fields and heavy urban development. Dune sections where the profile is low and/or discontinuous, with patchy vegetation, are vul-


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nerable to tsunami inundation and require the most immediate planning and management attention. The findings of this study demonstrate that the Christchurch dune coast currently offers a high degree of protection against inundation by small to medium tsunami (0–6 m AMSL run-up), with the degree of vulnerability varying locally along the shore in relation to the height and continuity of the dune system. Most areas are, however, vulnerable to inundation under more-extreme tsunami scenarios. Major areas of vulnerability include deep gaps characterised by low seawalls and other built features, and sections of dune with highly discontinuous longshore profiles and average heights ⬍8 m. The simple GIS-based methodology for dune characterisation developed here offers significant improvements in assessing volumes of inundation from tsunami waves with differing run-up elevations, both in terms of accuracy and in terms of efficiency. A tsunami inundation vulnerability index is developed to assist coastal managers in quickly assessing the relative vulnerability of sections of dune while simultaneously identifying the nature and location of weaknesses. At the local level this classification scheme can be used to efficiently allocate dune management resources, while at a regional level it can be used as a rapid assessment tool in coastal development and hazard planning.

ACKNOWLEDGMENTS The authors would like to thank Derek Todd for useful discussions throughout this investigation; Graham Furnis, Paul Bealing, and Dr. Clive Sabel for technical assistance; and Dr. James Goff and an anonymous reviewer for useful comments.

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