gis modelling of sea-level rise induced

8/2/2019 GIS Modelling of Sea-Level Rise Induced

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Natural Hazards 31: 253276, 2004.

2004 Kluwer Academic Publishers. Printed in the Netherlands.253

GIS Modelling of Sea-Level Rise InducedShoreline Changes Inside Coastal Re-Rntrants

Two Examples from Southeastern Australia

WERNER G. HENNECKESchool of Geography and Environmental Studies, University of Tasmania, Private Box 252-76, TAS

7001, Australia, E-mail: [email protected]

(Received: 28 May 2001; accepted 7 February 2003)

Abstract. Shoreline recession as a result of rising sea level has been recognised as a potential

near-future hazard by a number of countries. However, the collection of high spatial resolution data,in particular elevation data, is often too costly and time consuming to be applied routinely for a

detailed assessment of the potential physical and economic impacts of this hazard. Based on work

undertaken for the Dutch Wadden Sea, a GIS-based coastal-behaviour model has been developed to

formulate simple algorithms for simulating the potential physical impacts of rising sea level on the

coastal environment, focussing here on coastal re-entrants. The GIS model developed is suitable for

providing first estimates of potential shoreline change, based on readily available information. To

enhance the suitability of such initial assessment, the GIS model output, that is the rate of shoreline

change, has been analysed in greater detail using a spreadsheet-based hazard probability model.

The advantage of using a combination of both models is a rapid assessment of the probability of

shoreline changes, instead of a single impact zone, as modelled with the GIS. The hazard probability

rates received from the spreadsheet model are returned to the GIS to be displayed as a grading of

risk instead of a single impact zone. The model introduced in this paper has been applied to two field

sites in southeastern Australia to model regional variations in shoreline response to rising sea level.

Key words: Sea-level rise, flood-tide delta aggradation, GIS-based coastal-behaviour modelling,

shoreline change, probability assessment.

1. Introduction

The subject of the present paper was to develop a method that can be utilised

to provide gross estimates of the potential effects of sea-level rise on coastal re-

entrants, in particular flood-tide deltas and their adjacent erodible shorelines. A

tidal inlet is defined as the link between the ocean and a protected embayment to

exchange water, sediments, nutrients, planktonic organisms and pollutants betweenthem (Bruun, 1978; Boothroyd, 1985; Aubrey and Weishar, 1988). The term

coastal re-entrant used in this paper also includes barrier-island and bay-barrier

re-entrants, and bays opening to the sea that contain flood-tide deltas (Figure 1).

The topic of the study has arisen from the limited existing knowledge about

the range of near-future (50 to 100 years) physical impacts on coastal re-entrants,


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254 WERNER G. HENNECKE

Erodible shorelines(eg. barriers)

Erosion-resistant shorelines(eg. rocky headlands)

Erodible or erodion-resistantshorelines enclosing theflood-tide delta, their proportiondepending on local settings.

Sediment deposition(flood-tide delta)

c) Baysb) Bay-Barrier Re-entranta) Barrier-Beach Re-entrant

Restricted or nosediment transportaround headland

Restricted or nosediment transportaround headland

Littoral sedimenttransport

Bay

Flood-tide Delta

Channel

Flood-tide Delta Flood-tideDelta

Channel

Figure 1. Schematic sketch of re-entrant conditions considered in the Flood-Tide Delta

Aggradation Model.

their flood-tide deltas and the adjacent erodible shorelines due to a climate-change

induced sea-level rise (Cowell et al., 1996). The majority of studies that have ad-

dressed the effects of sea-level rise on coastal environments to date, have focused

on shoreline responses per se to sea-level rise (e.g., Bruun, 1962, 1988; Bruun

and Schwartz, 1985; Gornitz and Kanciruk, 1989; Gornitz, 1991; Healy, 1991;

Leatherman and Nicholls, 1995). This paper focuses on the effects of sea-level

rise on the morphology of flood-tide deltas and their adjacent erodible shorelines

in coastal re-entrants and erodible shorelines adjacent to coastal re-entrants on the

open coast.

Despite application of GIS for environmental hazard modelling (Brinkley,

1997) (e.g., modelling of fire spreading) little attention has been paid to GIS-based

coastal-behaviour modelling (Bartlett, 1999). The GIS-based Flood-Tide Delta Ag-

gradation Model (FTDAM) has been developed to formulate simple algorithms

for simulating the potential impacts of rising sea level on coastal re-entrants. Themodel is based on work undertaken for the Dutch Wadden Sea and has been applied

to two field sites in southeastern Australia (Narrabeen Lagoon and Batemans Bay).

2. Background

Coastal re-entrants have been accorded special consideration within coastal and

estuarine science and engineering, because of their commercial, recreational and

ecological importance (Mehta, 1996). Among the broad range of applications of

research on coastal re-entrants, the maintenance of navigation channels has histor-

ically been the most important, driving the research into physical processes that

determine re-entrant flow characteristics and morphodynamics (Mehta, 1996).Sediments required for the aggradation of the flood-tide delta are received from

ocean beaches exposed to wave action and littoral sediment transport as well as

ebb-tide deltas. The material is transported through the tidal channel and deposited

in lee of the channel on the flood-tide delta (Bruun, 1978). Coastal re-entrants,

therefore, often act as sinks for sand (Curray, 1964; Swift, 1976; Eysink, 1991;


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GIS MODELLING OF SEA-LEVEL RISE INDUCED SHORELINE CHANGES 255

Nicholls, 1993). Indirect effects of sea-level rise such as sediment transport from

open-coast beaches or barriers alongshore into coastal re-entrants are often more

important with regard to shoreline erosion than offshore sediment transport and

deposition described by the two-dimensional Bruun Model (Stive et al., 1990;

Nicholls, 1993; Cowell and Thom, 1994). Those indirect effects are modelled in

the FTDAM.

The FTDAM (Hennecke, 2000; Hennecke and Cowell, 2000) is based on re-

search concerned with potential impacts of rising sea level on the morphology

of the Dutch Wadden Sea (Van Straaten, 1954; Eysink, 1991; Peerbolte et al.,

1991; Louters and Gerritsen, 1994; Buijsman, 1997; Stive and Wang, 1998). Eysink

(1991) and Louters and Gerritsen (1994) suggested that the sediment supply from

ebb-tide deltas and barrier islands is sufficient to allow the floor of the Wadden

Sea to aggrade approximately at the rate of sea-level rise, though, with some lag in

time.

The underlying principle here is the assumption that one of the main processes

of sedimentation in the Wadden Sea is the vertical deposition and upward growth ofhorizontal parts of tidal flats approximately at the rate of relative sea-level rise. This

assumption is based on work by Van Straaten (1954) and suggests that the water

depth of the Wadden Sea remains approximately constant over time. Initially, the

tidal volume increases, resulting from an increase in sea level, causing higher water

flow velocities in the channels (Peerbolte et al., 1991). The sediment transport

capacity increases, leading to higher sediment transport towards the shoals during

flood tide. The water level above the shoals increases also, as a result of rising sea

level. This increase is relatively large due to the limiting water depth above the

shoals, possibly leading to a reduction in sediment transport towards the channel

during ebb-tide, and consequently improving conditions for sedimentation in the

Wadden Sea (Peerbolte et al., 1991). The tidal channel increases in depth first, rel-ative due to the increase in the tidal volume (Figure 2). The tidal volume gradually

decreases with the rising shoals over time, causing an adjustment of the channel

bed. The channel then follows the rising shoals with some lag in time (Figure 3)

(Peerbolte et al., 1991). In summary, the sediment volume supplied from outside

the Wadden Sea (Vext.) is sufficient for the aggradation of the floor of the Wadden

Sea; that is Vext. = Vdem., where Vdem. is the sediment demand volume for the

aggradation.

In southern southeastern Australia, however, the rate of littoral sediment trans-

port is negligible (Chapman et al., 1982). Therefore, the assumption for the

FTDAM here is that the external sediment supply is less than the sediment ac-

commodation space; i.e., Vext. < Vdem.. The FTDAM then assumes that

erodible shorelines along the flood-tide delta supply the remaining sediment de-mand volume (Vi ) to allow for the aggradation of the flood-tide delta, causing

shoreline recession inside the re-entrant.

In addition to the assumptions made above, the FTDAM considers the pos-

sibility of an oversupply of sediment for locations where Vext. > Vdem.. The


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Beach

SL 1 SL 2

Profile 1(before sea-level rise)

Profile 2(after sea-level rise)

Beach

Flood-Tide Delta

Re-entrant Channel

Ds

Dagg.

Ds

Figure 2. Aggradation of the flood-tide delta following rising sea level. The floor of the

flood-tide delta follows the rising sea level while the tidal channel deepens relatively. s

= rate of sea-level rise; agg. = rate of flood-tide delta aggradation.

Dagg. Ds

Beach

SL 1 SL 2

Profile 1(before sea-level rise)

Profile 2(after sea-level rise)

Beach

Flood-Tide Delta

Re-entrant Channel

Ds

Figure 3. Aggradation of the flood-tide delta following rising sea level. The tidal channel

follows the rising sea level and the aggrading tidal flats with some lag in time.

FTDAM in its current configuration then assumes an even distributed of the

sediment surplus along the unconsolidated shorelines inside the re-entrant.

The magnitudes of six model parameters (Figure 4) are critical for the rate of

shoreline change (R) inside a re-entrant. These are:

(1) the size of the flood-tide delta (A);

(2) the rate of sea-level rise (s);(3) the volume of marine sediments available for the aggradation (Vm);

(4) the volume of fluvial sediments available for the aggradation (Vfl.);

(5) the length of erodible shorelines along the flood-tide delta (Les); and,

(6) the dune elevation of erodible shorelines along the flood-tide delta (Dx ).

The demand volume (Vdem.) required for the aggradation of the flood-tide

delta in any application of the model is a function of the area of the flood-tide delta

and the (local or regional) rate of sea-level rise; i.e.,

Vdem. = A s. (1)

The model assumes initially that the area of the flood-tide delta remains constantover time. However, a decrease ofA, for example as a result of land reclamation,

can also be considered (Figure 5). Three main sources of sediment supply for the

aggradation of the floor of the flood-tide delta are identified as:

(1) marine sediments (Vm), being transported into the embayment by flood-tide

currents and waves;


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R

DVm + DVfl.

DA

Ds DLes

DDx

Figure 4. Parameters defining the rate of shoreline change in the FTDAM, where A = area

of the flood-tide delta, s = rate of sea-level rise, Vm + Vfl. = external sediment supply,

Dx = dune elevation, and Les = length of erodible shorelines.

Flood-tide Delta

Barrier BarrierRe-entrant channel

Lower (seaward) limit of the FTD

Upper (landward) limit of the FTDFluvial delta / Central basin

Land reclamation

Figure 5. Generalised re-entrant conditions defining the FTDAM.

(2) fluvial sediments (Vfl.), being deposited in the re-entrant; and,

(3) a remaining sediment volume (Vi ) derived as a result of shorelines recession

along the flood-tide delta.

More specifically, Vm is defined as a combination of three sources of net marine

sediment input:

(1) littoral sediment transport (Vlit.);

(2) offshore sediment supply (Voff.); and,(3) overwash processes (Vov.) (Figure 6).

The total sediment demand (Vdem.) for the aggradation of the flood-tide delta can

be defined therefore as:

Vdem. = Vm +Vfl. +Vi


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Offshore supply, DVoff.

Fluvial sediments, DVfl.

Littoral transport, DVlit.

Overwash processes, DVov.

DVi from erodible shoreline, DLes

Flood-tide Delta

BarrierRe-entrant channel

Barrier

Figure 6. Sediment sources for the aggradation of a flood-tide delta in the FTDAM.

= Vlit. +Voff. +Vov. +Vfl. +Vi . (2)

The model assumes that the shorelines along the flood-tide delta erode only to sup-

ply sediment if the external sediment supply is less than the total demand volume;

i.e., (Vm +Vfl.) < Vdem.. Hence, the larger (Vm +Vfl.) the smaller is Viand therefore the rate of shoreline recession inside a re-entrant. The magnitude

of every component of Vdem. can vary between 0% to >100% of the required

sediment volume to raise the floor of the flood-tide delta. As stated above, in a

scenario where the sediment supply is greater than 100% of the demand volume,

a sediment surplus in the re-entrant occurs according to the model. This surplus

results in sediment deposition along the erodible shorelines in the re-entrant; i.e.,

shoreline progradation.

The rate of shoreline change (R) for erodible shorelines inside the embaymentis expressed in the FTDAM as a function of the sediment demand (Vi ) along the

erodible shorelines (Les) and their dune elevations (Dx ); i.e.,

R = Vi (Les)1 (Dx)

1. (3)

As such, three general trends of shoreline change are modelled with the FTDAM:

(1) shoreline recession in coastal re-entrants where the external sediment supply is

less than the total demand volume (Vm + Vfl.) < Vdem.; i.e., Vi > 0 and

thus R > 0;

(2) the shoreline remains in its position where the external sediment supply

matches the demand volume ((Vm + Vfl.) = Vdem.); i.e., Vi = 0 and

thus R = 0; and,

(3) shoreline progradation in coastal re-entrants where the external sediment sup-

ply exceeds the demand volume (Vm + Vfl.) > Vdem.; i.e., Vi < 0 and

thus R < 0.


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3. GIS Procedures

The FTDAM is embedded into a Geographic Information System (Arc/Info TM)

(Environmental Systems Research Institute, 1995) for modelling the spatial extent

of shoreline change. The FTDAM has been designed to use publicly availablemaps such as nautical charts and 1 : 25,000 topographic maps. Also, the model

can run with spatial data of higher resolution if these are available. However, the

field collection of high spatial resolution data, in particular elevation data, is often

too costly and time consuming to be applied routinely (Nicholls, 1993).

To run the FTDAM, the user of the model must define boundary conditions,

such as the seaward and landward boundaries of the flood-tide delta, and local

parameter values for A, s, Vlit., Voff., Vov., Vfl., Les, and Dx . Mor-

phological variations alongshore, such as changes in dune elevation can be taken

into account in the GIS if such information is available. In locations where suf-

ficiently detailed data are not available, one can utilise best estimates for those

parameter values for an initial assessment of the potential extent of shorelinechange. The FTDAM can be simply adjusted to local conditions by re-defining

or updating parameter values in the GIS database.

After definition of the parameter values, the GIS is used to calculate the total

demand volume for the aggradation of the flood-tide delta (Vdem.) as a function

of the area and the rate of sea-level rise (Equation 1). The external sediment supply

(Vov. + Vlit. + Voff. + Vfl.) is subtracted from the total demand volume to

determine the sediment volume Vi required from the erodible shorelines adjacent

to the flood-tide delta; i.e.,

Vi = Vdem. (Vov. +Vrmlit. +Voff. +Vfl.). (4)

Five generic scenarios of shoreline change for a modelled site are shown in Table I

and Figure 7 to illustrate the concept of the FTDAM. Parameter values are set for

the area (A) of the flood delta to aggrade, the rate of sea-level rise (s), and

following Equation (1), the demand volume (Vdem.). Also, the length of erodible

shorelines (Les) and dune elevations (Dx ) are fixed. Only the amount of the

external sediment supply (Vov. + Vlit. +Voff. + Vfl.) varies in the scenarios

outlined below, and subsequently the extent of shoreline change between the five

scenarios. Scenarios 13 describe situations where the sediment supply from out-

side the re-entrant is less than the sediment required for the aggradation of the

flood-tide delta, causing shoreline recession along the flood-tide delta. In Scenario4, the sediment supply from outside the re-entrant matches the demand volume,

the rate of recession equals zero and the shoreline remains unchanged. Scenario

5 finally illustrates a scenario where the external sediment supply is greater than

the demand volume. Here, the rate of shoreline recession is negative, resulting in

shoreline progradation according to the FTDAM.


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Table I. Sample FTDAM scenarios for a generic site A showing the range of potential

impacts simulated with the GIS-based model.

Model scenario 1 2 3 4 5

A (m2) 100,000 100,000 100,000 100,000 100,000

S (m) 0.2 0.2 0.2 0.2 0.2

Vdem. (m3) 20,000 20,000 20,000 20,000 20,000

Vlit. (m3) 2,000 1,000 1,000 15,000 18,000

Vov. (m3) 5,000 3,000 1,500 3,000 3,000

Voff. (m3) 3,000 2,000 1,000 1,750 4,500

Vfl. (m3) 250 250 250 250 250

Les (m) 500 500 500 500 500

Dx (m) 2 2 2 2 2

R (approx) (m) 0.7 13.8 16.3 0 5.8

Offshore supply, DVoff.

Fluvial sediments, DVfl.

Littoral transport, DVlit.

Overwash processes, DVov.

Erodible shoreline, DLes

Flood-tide Delta

BarrierInlet channel

Recession Scenario 3



Accretion ScenarioNo change Scenario

Barrier

Figure 7. Range of shoreline changes anticipated with the FTDAM, depending on local

conditions.

4. Application of the FTDAM and Field Experimentation

Although the FTDAM is based on work for the tide-dominated Dutch Wadden Sea,

it is, in principle, applicable also to wave-dominated environments. This is because

the model considers the restriction of external sediment supplies for the aggrad-

ation of a flood-tide delta in a coastal re-entrant, as outlined above. To examine

its applicability in environments other than the Dutch Wadden Sea, the model hasbeen applied to two locations in New South Wales, southeastern Australia.

The coast of New South Wales is a drowned embayed coast which is oriented

SSW to NNE and exposed to moderately high-energy ocean waves and a small

(


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Projection: UTM, Zone 56, AGD66

Batemans Bay

NEW SOUTH WALES

SYDNEY

Australia

W.A. S.A.

N.T.Qld.

N.S.W.

Tas.

Vic.Sydney

Narrabeen

Sydney CBD

0 200 400 Kilometres

Figure 8. Location of the field experimentation sites. (Source: Australian Bureau of Statistics,

CData 1996).

net direction of sediment movement is normally from the inner continental shelf

towards the re-entrant (Thom, 1974; Chapman et al., 1982). Characteristic flood-

tide deltas formed in re-entrants where shelf sands continued to accumulate after

Holocene sea levels stabilised about 6,500 years B.P. (Roy, 1984b). Longshoresediment transport is restricted by deeply embayed re-entrants and prominent head-

lands extending into deep water. Fluvial sand is trapped at the upstream estuary

margins of drowned river valleys (Roy and Crawford, 1977; Boyd and Penland,

1984).

4.1. NARRABEEN LAGOON

Narrabeen Lagoon is located within the local government area of Warringah Coun-

cil, approximately 16 km north of Sydneys Central Business District (Figure 8).

The re-entrant is separated from the open sea by a 3.4 km long barrier beach

(Collaroy/Narrabeen Beach) raising to about 10 m above sea level at its northernend. The beach is bound by Narrabeen Headland in the north and the prominent

headland of Long Reef in the south (Figure 9). Sediment transport from the south

around Long Reef is negligible (P.S. Roy, personal communication).

The FTDAM was applied to investigate the potential response of the flood-tide

delta of Narrabeen Lagoon and the adjacent erodible shorelines to a rise in sea level.


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Narrabeen Lagoon

Collaroy/Narrabeen Beach

Long Reef Headland

North Narrabeen Headland

0 1 2 Kilometres

-30m

Collaroy Plateau


- 2 4

m - 2 0

m - 1

0 m

Figure 9. Location of Collaroy/Narrabeen Beach and Narrabeen Lagoon. Source: Central

Mapping Authority of New South Wales, 1978.

Table II. Sediment demand volume required for the

aggradation of the flood-tide delta in Narrabeen La-

goon for a mid-range 50-year (0.2 m) and 100-year

(0.49 m) sea-level rise scenario.

Scenario s (m) A (m2) Vdem. (m3)

50 y 0.20 458.295 91,659

100 y 0.49 458,295 224,564

A 1 : 25,000 orthophoto map was utilised to digitise relevant sections of the area

and to determine the surface area of the flood-tide delta (458,295 m2, or 22.2% of

the total area of the lagoon) (Figure 10). The erodible shoreline along the flood-tide

delta was calculated as approximately 5,800 m with an average elevation of about1.5 m. Estimates for near-future sea-level rise published by the Intergovernmental

Panel on Climate Change (IPCC, 1996, 2001) were utilised for a mid-range 50-

year (0.2 m) and 100-year (0.49 m) scenario. The demand volumes (Vdem.) for

the aggradation of the flood-tide delta for both scenarios were calculated following

Equation (1) and are shown in Table II.


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0 10.5 Kilometre

Ocean Street

Narrabeen Lagoon

North Narrabeen Headland

Flood-tide Delta

Approximate areadredged in 1995

Coll

aroy/N

arrab

eenB

each

Figure 10. Area of the flood-tide delta in Narrabeen Lagoon. Figure based on Figure 9.

The entrance of Narrabeen Lagoon is located at the northern end of Col-

laroy/Narrabeen Beach and frequently infills with marine sediments, causing its

closure approximately every three to five years (Public Works Department, 1990).

Flood waters can re-open the outlet naturally, but at the same time threaten to

flood properties adjacent to the Lagoon if the entrance remains closed. As part

of their coastal management, Warringah Council dredges the lagoon in a cycle ofapproximately three to five years to ensure good water quality in the lagoon and

reduce the risk of flash flooding (Public Works Department, 1990). The entrance of

the lagoon and part of the flood-tide delta was dredged early in 1995 (Figure 10) but

littoral sediment transport along Collaroy/Narrabeen Beach into the lagoon again

caused its closure in November 1997. The entrance to the lagoon was re-opened


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Table III. Estimate rate of shoreline erosion along Col-

laroy/Narrabeen Beach as a result of flood-tide delta aggrad-

ation in Narrabeen Lagoon.

Scenario Vdem.

(m3) Les

(m) B (m) R (m)

50 y 91,659 3,400 2.5 11

100 y 224,564 3,400 2.5 26

during the Easter Weekend 1998, when heavy rainfall threatened to flood properties

adjacent to the lagoon.

Based on current sediment regimes, it is anticipated for this modelling exper-

iment that the present sediment transport conditions will continue over the next

50 to 100 years. Therefore, the external sediment supply (Vext.) is expected to

match the demand volume (Vex. = Vdem.) required for the aggradation of theflood-tide delta in the next 50 to 100 years even under near-future sea-level rise

conditions. The erodible shoreline adjacent to the flood-tide delta (i.e., inside the

re-entrant) is not expected to recede under these conditions (Vi = 0). There-

fore, properties located along Narrabeen Lagoon are considered to be safe from

shoreline recession. It is anticipated that the littoral sediment transport along Col-

laroy/Narrabeen Beach causes shoreline recession along the barrier-beach since

Long Reef Headland obstructs northward sediment transport and therefore the sed-

iment supply for the beach. The average rate of recession along Collaroy/Narrabeen

Beach was determined here as a function of the sediment volume required for the

aggradation of the flood-tide delta (Vdem.), the length of erodible shoreline along

the beach (Les) and the average beach elevation (B). The average recession rateswere calculated as approximately 11 m for the 50-year scenario and 26 m for the

100-year scenario for this model configuration (Table III).

4.2. BATEMANS BAY

Batemans Bay is located on the south coast of New South Wales about 300 km

south of Sydney (Figure 8). The south-east facing, funnel-shaped drowned river-

valley estuary is approximately 8 km long, 500 m wide near the Princess Highway

Bridge, and 5.7 km at its mouth (Figure 11). The Clyde River, one of the larger

rivers on the New South Wales south coast with a catchment of approximately1,800 km2 discharges into Batemans Bay (May et al., 1996). The bay has a total

shoreline length of approximately 25 km from Moscito Bay in the south to Three

Islet Point in the north, excluding Cullendulla Creek. The total length of erodible,

sandy shoreline in the embayment is 12 km. Erosion-resistant shorelines as well

as training walls are approximately 13 km long in total. The fluvial and littoral


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0 1.5 3.0 Kilometres

Erodible (sandy) shorelines

Non-erodible (rocky) shorelines


PrincessHighwayBridge

Three Islet Point

Moscito Bay

Cullendulla Creek

CBD

Caseys Beach

-5 m

-15 m

-10 m

Seawall

ClydeRiver

Figure 11. Erodible and non-erodible shorelines in Batemans Bay. Map source: Hydrographic

Service of the Royal Australian Navy, 1985.

sediment supply into the re-entrant is negligible within Batemans Bay (Chapman

et al., 1982).

A bathymetric chart (1 : 50,000) was utilised for the modelling due to the lack

of more detailed bathymetric data. Since precise information was not available for

the area of the flood-tide delta, two modelling scenarios were conducted, using

different seaward boundaries for the flood-tide delta (10 m, and 15 m depth

contours). The landward limit of the flood-tide delta was defined as the shorelineof the embayment and the Princess Highway Bridge for both modelling experi-

ments. The delineated areas for the flood-tide delta for both scenarios are shown in

Figure 12.

Again, mid-range near-future sea-level rise estimates by the IPCC (1996, 2001)

were utilised for a 50-year (0.2 m) and a 100-year (0.49 m) scenario. Based on


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0 1.5 3 Kilometres

Cullendulla Creek

PrincessHighwayBridge Three Islet Point

Moscito Bay

Sb = 10 m

Sb = 15 m

Area of the flood-tide delta

Figure 12. Areas of the flood-tide delta for different modelling scenarios in Batemans Bay.Figure based on Figure 11.

Equation (1), the demand volumes for the aggradation of the flood-tide delta for

both scenarios were calculated (Table IV). An average dune elevation of 3 m above

Mean Sea Level (MSL) was employed for the entire re-entrant, based on work

undertaken by the New South Wales Department of Public Works (Public Works

Department, 1989). Assuming a negligible marine and fluvial sediment supply, the

assumption following the FTDAM was that the sediment demand for the aggrad-

ation of the flood-tide delta under sea-level rise conditions will be supplied fromthe erodible shorelines along the flood-tide delta. The average rate of shoreline re-

cession inside the re-entrant for both scenarios was determined following Equation

(4). Parameter values employed in this scenario as well as the modelled rates of

recession for the 50-year and 100-year sea-level rise and the 10 m and 15 m

seaward limit of the flood-tide delta are shown in Table V.


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Table IV. Estimated demand volume for the aggradation of the

flood-tide delta for the 0.2 m and 0.49 m sea-level rise scenarios.

Seaward Area (m2) Demand volume Demand volume

Boundary 0.2 m slr (m3

) 0.49 m slr (m3

)

10 m 19,628,498 3,925,700 9,617,964

15 m 26,612,168 5,322,434 13,039,962

Table V. Parameter values and simulated rates of shoreline change in the Batemans

Bay experiment.

Scenario 1 2 3 4

s (m) 0.2 0.2 0.49 0.49

h (m) 10 15 10 15A (m2) 19,628,498 26,612,168 19,628,498 26,612,168

Vdem. (m3) 3,925,700 5,322,434 9,617,964 13,039,962

Vm (m2) 0 0 0 0

Vfl. (m2) 0 0 0 0

Vi (m3) 3,925,700 5,322,434 9,617,964 13,039,962

Les (m) 12,000 12,000 12,000 12,000

D (m) 3 3 3 3

R (m) 109 147 267 362

Limitations of the GIS-Model

The GIS-based FTDAM is designed to model morphological impacts of rising sea

level on erodible shorelines in coastal re-entrants, and the model is capable of

accounting for morphological variability alongshore. However, the limitation of

the GIS model is that the outcome of a model run provides only a single impact

zone at a fixed distance from the shoreline (Figure 13). This value, at the same time,

is surrounded by an area of uncertainty (Cartwright, 1993), but the GIS model in

its current configuration is not yet capable of considering probabilities of shoreline

change within a single GIS model scenario. The use of sharply defined boundaries

of recession, however, is regarded as misleading especially for coastal manage-ment and planning purposes. This is because the probability of shoreline recession

does not diminish immediately landward of the calculated impact zone (Cowell

et al., 1996). A decision not taking the gradual decrease of impact probability

into account may be inappropriate and would not have been made if probability

distributions were known.


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Cullendulla Creek

CBD

Shoreline Recession



Figure 13. Single impact zone of shoreline recession modelled with the GIS for Batemans

Bay. Figure based on Figure 11.

Determining the probability of the hazard, here shoreline recession, provides

decision makers with a better understanding of the potential range of hazard im-

pacts. To assess the probability of the hazard, the recession rate modelled with

the GIS is utilised as the statistically most likely value for the hazard probability

modelling. The probability assessment procedure is shown in Figure 14 and an

example outlined below for Batemans Bay.

Due to the absence of specific information for the probability assessment pro-cedure in Batemans Bay, a normal frequency distribution was applied for all

analyses. Standard deviations (S) for all parameters were based on estimates, due to

the lack of detailed data. However, every parameter value can be updated if more

detailed information becomes available. Input values and standard deviations for

all parameters of all four modelling scenarios are listed in Table V.


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GISModel

Scenario

Probabilityvalues

returnedtoGIS

GISModelscenario

Hazardprobabilityassessment

GISdisplay

Parameter values(Ds, DA, DV

ext.)

Shoreline Recession(crisp line)

Probability assessmentof shoreline recession

statistical probability

of shoreline change

=>

Spatial displayin GIS

Figure 14. Generic probability assessment procedure based on a GIS model result.

The model was run with 15,000 iterations in this experiment to ensure reliable

statistics being generated for the modelling output. Running a sufficiently high

number of iterations means that output distributions become more stable because

the statistics describing each distribution change less and less with additional itera-

tions. Choosing a high number of iterations therefore ensures the quality, accuracy

and stability of the results (Cartwright, 1993). Statistical probabilities of shoreline

change for all scenarios employed in this experiment are presented in Table VI.

These probability values were then returned to the GIS model to be displayed as

a hazard map, showing hazard in a grading of risk probability classes instead of a

single impact zone (Figure 15).

5. Results and Discussion

The potential impacts of rising sea level on coastal re-entrants for the next 50

to 100 years have been discussed widely in the last few decades for example


18/24


Table VI. Hazard probability model parameters and standard deviation for the simula-

tion experiments in Batemans Bay.

0.2 m sea-level rise FTD10 S FTD15 S

A 19,628,498 5,000 26,612,168 5,000

Vfl. 50 y 0.2 0.05 0.2 0.05

Les 0 500 0 500

Vm 0 50 0 50

s 12,000 50 12,000 50

D 3 0.5 3 0.5

Recession 50 y 109 147

0.49 m sea-level rise

A 19,628,498 5,000 26,612,168 5,000

s 100 y 0.49 0.05 0.49 0.05

Vm 0 500 0 500Vfl. 0 50 0 50

Les 12,000 50 12,000 50

D 3 0.5 3 0.5

Recession 100 y 267 362

Where FTD10 = area of the flood-tide delta with a seaward limit of10 m; FTD15 =

area of the flood-tide delta with a seaward limit of15 m; S = standard deviations for

individual model parameters, based on estimates.

for the Dutch Wadden Sea. The main concern of coastal scientists and engineers

investigating the Wadden Sea has been the recession and protection of shorelines

outside coastal re-entrants along barrier islands. The prospect of shoreline reces-sion inside coastal re-entrants as a result of rising sea level, however, has not been

previously addressed in detail. De Ronde (1993, 1996) showed that the sediment

supply from barrier islands and shallow zones of the North Sea is sufficient for

the aggradation of the tidal flats in the Wadden Sea, even under sea-level rise con-

ditions anticipated for the next 50 to 100 years. Similar suggestions can be made

for Narrabeen Lagoon according to the FTDAM. The littoral sediment transport

along Collaroy/Narrabeen Beach at present is sufficient to cause the closure of

the entrance to Narrabeen Lagoon approximately every three to five years. The

outcome of the modelling experiment undertaken here suggests that the rate of

littoral sediment transport along Collaroy/Narrabeen Beach is sufficient to raise

the floor of the flood-tide delta approximately at the rate of sea-level rise over thenext 50 to 100 years, causing shoreline recession along the barrier beach.

The application of the FTDAM to a location like Batemans Bay, where the

sediment supply from outside the re-entrant is negligible, suggests that erodible

shorelines inside this re-entrant are likely to recede severely under conditions

defined in this experiment. The probability assessment for Batemans Bay suggests


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Cullendulla Creek

CBD

< 554 m

< 368 m

< 238 m

< 225 m

< 208 m

< 174 m

< 82 m

< 61 m

Impact Scenarios


Figure 15. Hazard probability modelling, based on results shown in Figure 13. Hazard

probabilities are displayed in grey shades. Figure based on Figure 11.

a statistical chance of 5% that the rate of recession is 61 m for the 50 year sea-

level rise scenario and a 10 m offshore limit of the flood-tide delta. The statistical

worst case scenario suggests a 95% probability that the rate of shoreline recession

for a 100 year scenario and a 15 m seaward boundary of the flood-tide delta is

554 m (Table VII).

Overall, results derived with the FTDAM, in particular for locations like Bate-

mans Bay, must be regarded as estimates of potential shoreline recession only.Climate change modelling is inherently difficult (Henderson-Sellers, 1993) and

GIS model inaccuracies, such as data capturing and processing, add to modelling

uncertainties. Further, the simulated rate of recession is potentially higher for the

Batemans Bay site than it may experience in reality. This is because the actual

sediment volume available from beaches along the flood-tide delta is potentially


20/24


Table VII. Probability range of shoreline recession for Batemans Bay according to

the Aggradation-Risk Model.

Scenario Seaward limit s 10 m 15 m

5% 95% 5% 95%

50 y 0.2 m 61 m 174 m 82 m 238 m

100 y 0.49 m 208 m 368 m 225 m 554 m

less than the calculated sediment demand. Bedrock close to the surface may limit

the sediment volume available to feed the aggrading flood-tide delta. If the required

sediment demand is not available; i.e., the floor of the flood-tide delta cannot follow

rising sea level, then an increase in the height of the water column is likely to

occur. Larger waves inside the re-entrant can be expected as a consequence of

the rising water column (resulting from reduced wave breaking and bed friction).

The implication is that more frequent overtopping and destruction of protective

structures or other assets could occur as a result of a progressive rise in sea level.

6. Conclusions

The potential impacts of rising sea level over the next 50 to 100 years have been

recognised by a number of countries as a potential future hazard. Detailed in-

formation, in particular elevation data, however, is often not available for such

hazard assessments. The use of existing information, in combination with coastal-

behaviour models, appears to be an appropriate way to simulate shoreline recessionunder such circumstances.

The primary aim of this study was to develop a simple but at the same time

widely and easily applicable GIS-based model for simulating shoreline change

inside coastal re-entrants. The model had to be applicable to readily and publicly

available information, such as topographic maps and boating charts at medium

scales. The FTDAM accommodates a range of re-entrant types (barrier-beach re-

entrants, bay-barrier re-entrants and bays). In combination with data, GIS was

utilised to run the model based on parameters values in accordance with local

conditions. Nevertheless, the simulation results derived from the modelling ex-

periments served only as an illustration for the potential range of feasible impacts

between different locations.GIS-based models may be used as modelling tools that allow users, such as

local or regional government authorities, to combine available data from different

sources (topography, geology, bathymetry, cadastral data) into a single software.

However, issues of uncertainty remain and must be addressed in any model

application. Three aspects of uncertainty need to be addressed here:


21/24


(1) the definition of parameter values, such as the rate of sea-level rise or the know-

ledge of local/regional sediment transport regimes. A best-practice protocol

here is simply to adopt information readily available, such as the range of IPCC

recommendations for sea-level projections;

(2) data-resolution and accuracy. This includes map resolution and GIS-related

factors such as inaccuracies in digitising, data conversion and interpolation, as

they contribute to the uncertainty and generalisation of the modelling results;

and,

(3) the validity of models (Oreskes et al., 1994), as they are highly generalised

approximations and representations of the real world that often cannot be

proved.

Overall, the main messages emerging from this study can be summarised as fol-

lows: (1) readily available data often provide sufficient information for the initial

assessment of sea-level rise impacts on coastal re-entrants. The FTDAM can serve

as a tool to simulate and map a first overview of regional variation in re-entrant sus-

ceptibility to sea-level rise; and, (2) the capabilities of GIS allow for the simulation

of a range of scenarios based on changes in parameter values.

The application of a probability assessment, however, remains crucial in dealing

with the unavoidable uncertainty inherent in the results of the modelling. Once the

uncertainty has been evaluated, hazard results can be used not only to map impact

probabilities, but also to assess economic or ecological vulnerability. Modelling

output can be used by (local and regional government) authorities for decisions

regarding re-entrant management and planning. Based on these results, coastal

managers can decide if further, more detailed investigations with higher spatial

resolution data are necessary.Credibility of analyses such as that undertaken in this study is required to make

the FTDAM suitable for application. The credibility of this analysis would be en-

hanced through further study of areas potentially at risk according to the modelling

results achieved here. This would be accomplished through the systematic applic-

ation of the model referred to above. However, this was beyond the scope of this

paper and remains future work.

Utilising GIS-based models for the evaluation and mapping of hazard impacts

can assist local and state authorities in defining the appropriate zoning for an area

and describing the conditions for future development and building standards for

different coastal settings. This information can be incorporated in the preparation

of local and regional plans (such as Local and Regional Environmental Plans (LEPand REP) in Australia) or in the establishment of general setback limits for build-

ings from the foreshore to reduce the risk of damage or loss in the future. The

examples presented here show that GIS-based coastal-behaviour modelling based

on readily available data for the gross estimation of sea-level rise impacts is a vital

part of coastal re-entrant management in the near future.


22/24


Acknowledgements

The author wishes to acknowledge the useful comments provided on this manu-

script by Prof. Michael Roberts, A/Prof. Richard Whitlow and Dr Ray Merton.

References

Aubrey, D. G. and Weishar, L.: 1988, Hydrodynamics and Sediment Dynamics of Tidal Inlets (Lecture

Notes on Coastal and Estuarine Studies), Springer Verlag, New York.

Australian Bureau of Statistics: 1997, CDATA 1996. Release 1, October 1997, Australian Bureau of

Statistics.

Bartlett, D. J.: 1999, Working on the frontiers of science: Applying GIS to the coastal zone, In: D.

Wright and D. Bartlett (eds), Marine and Coastal Geographical Information Systems, Taylor and

Francis, London.

Boothroyd, D. F.: 1985, Tidal inlets and tidal deltas, In: R. A. Davies (ed.), Coastal Sedimentary

Environments, Springer Verlag, New York, pp. 445532.

Boyd, R. and Penland, S.: 1984, Shoreface translation and the Holocene stratigraphic record: Ex-amples from Nova Scotia, the Mississippi Delta and Eastern Australia, Marine Geology 60,

391412.

Brinkley, A.: 1997, GIS and environmental modelling, GIS User21, 2223.

Bruun, P.: 1962, Sea-level rise as a cause of shore erosion, J. Waterways and Harbours Division

88(13), 117130.

Bruun, P.: 1978, Stability of Tidal Inlets. Elsevier, New York.

Bruun, P.: 1988, The Bruun rule of erosion by sea-level rise: A discussion on large scale two- and

three-dimensional usages, J. Coast. Res. 4(4), 627648.

Bruun, P. and Schwartz, M. L.: 1985, Analytical prediction of beach profile change in response to a

sea-level rise, Zeitschrift Geomorphology N.F. 57, 3350.

Buijsman, M. C.: 1997, The impact of gas extraction and sea level rise on the morphology of the

Wadden Sea, M.Sc. thesis, public version, Faculty of Civil Engineering Delft, University of

Technology, Delft, The Netherlands.

Cartwright, T. J.: 1993, Modeling the World in a Spreadsheet, Environmental Simulation on aMicrocomputer, John Hopkins University Press, Baltimore.

Central Mapping Authority of New South Wales: 1978, Orthophoto Map 1 : 25,000, Mona Vale,

Sheet 9130-I-S, Bathurst.

Chapman, D. M., Geary, M., Roy, P. S., and Thom, B. G.: 1982, Coastal Evolution and Coastal

Erosion in New South Wales, A Report Prepared for the Coastal Council of New South Wales,

West-Government Printer, New South Wales, Sydney.

Cowell, P. J. and Thom, B. G.: 1994, Coastal Impacts of climate change modelling procedures for

use in local government, Proceedings 1st National Coastal Management Conference, Coast to

Coast94, Hobart, Australia, pp. 4350.

Cowell, P. J., Zeng, T. Q., Hennecke, W., and Thom, B. G.: 1996, Regional predictions of climate

change impacts, In: Australian Coastal Management Conference, Coast to Coast 96, Adelaide,

S.A., pp. 185193.

Curray, J. R.: 1964, Transgressions and regressions, In: R. Miller (ed.), Papers in Marine Geology,

McMillan, New York, pp. 175203.

De Ronde, J. G.: 1993, What will happen to The Netherlands if sea-level rise accelerates? In: E. M.

Warrick, E. M. Barrow and T. M. L. Wigley (eds), Climate and Sea Level Change, Observations,

Projections and Implications, Cambridge University Press, Cambridge, pp. 322335.

De Ronde, J. G.: 1996, Man-made projects and sea-level rise, In: J. D. Milliman and B.U. Haq (eds),

Sea-Level Rise and Coastal Subsidence, Kluwer Academic Publishers, Dordrecht, pp. 327342.


23/24


Environmental Systems Research Institute (ESRI): 1995, Understanding GIS, The Arc/Info Method,

John Wiley & Sons, New York.

Eysink, W. D.: 1991, Morphological response of tidal basins to changes, In: Proceedings 22nd

International Conference on Coastal Engineering, New York, 19481961.

Gornitz, V.: 1991, Global coastal hazards from future sea level rise, Paleogeography, Palaeoclimato-logy, Palaeoecology 379398.

Gornitz, V. and Kanciruk, P.: 1989, Assessment of global coastal hazards from sea level rise, Coastal

Zone 89, Charleston, SC, pp. 13451359.

Healy, T.: 1991, Coastal erosion and sea-level rise, Z. Geomorph. N.F., Suppl. Bd. 81. Borntrager,

Berlin, pp. 1529.

Henderson-Sellers, A.: 1993, An antipodean climate of uncertainty. Climatic Change 25, 203224.

Hennecke, W. G.: 2000, GIS-Based Modelling of Potential Sea-Level Rise Impacts on Coastal Re-

Entrants, Unpublished PhD thesis, School of Geosciences, The University of Sydney, Sydney,

Australia.

Hennecke, W. G. and Cowell, P. J.: 2000, GIS Modelling of impacts of an accelerated rate of sea-

level rise on coastal inlets and deeply embayed shorelines, American Association of Petroleum

Geology, Special Issue on Coastal Hazards 7(3), 137148.

Hydrographic Service of the Royal Australian Navy: 1985, Batemans Bay, Chart No. AUS 191,

1 : 50,000. Published by the Hydrographic Service of the Royal Australian Navy, Wollongong

New South Wales.

IPCC (Intergovernmental Panel on Climate Change): 1996, Climate Change 1995, The Science of

Climate Change, Vol. 1, Cambridge University Press, Cambridge.

IPCC (Intergovernmental Panel on Climate Change): 2001, Climate Change 2001: The Scientific

Basis, Cambridge University Press, Cambridge.

Leatherman, S. P. and Nicholls, R. J.: 1995, Accelerated sea-level rise and developing countries: An

overview, J. Coast. Res. Special Issue No. 14, 114.

Louters, T. and Gerritsen, F.: 1994, The riddle of the sands, a tidal systems answer to a rising sea

level, Ministry of Transport, Public Works and Water Management, National Institute for Coastal

and Marine Management/RIKZ: The Hague, Netherlands.

May, P., Spurway, P., Waterman, P., Gray, L., Clark, T., Baxter, T., Good, R., Watt, M., Mills, R.,

Elias, G., Hibbert, K., Stone, P. B., Harding, G., McPhee, D., Clarke, A., and Neville, J.: 1996,

Batemans Bay Vulnerability Study, A Report to Eurobodalla Shire Council and the Common-wealth Department of the Environment, Sport and Territories. Eurobodalla Shire Council, New

South Wales, Australia.

Mehta, A. J.: 1996, A perspective on process related research needs for sandy inlets, J. Coast. Res.

Special Issue No. 23, 321.

Nicholls, R. J.: 1993, Coastal evolution and accelerated sea-level rise. In: Large Scale Coastal

Behaviour Conference, St. Petersburg, FL, pp. 137140.

Oreskes, N., Shrader-Frechette, K., and Belitz, K.: 1994, Verification, validation, and confirmation

of numerical models in the earth sciences, Science 263, 641646.

Peerbolte, E. B., Eysink, W. D., and Ruardij, P.: 1991, Morphological and ecological effects of sea-

level rise: An evaluation for the western Wadden Sea, In: R. F. C. Mantoura, J. M. Martin, and

R. Wollast (eds), Ocean Margin Processes in Global Change, J. Wiley & Sons, Chichester, pp.

329348.

Public Works Department (PWD): 1989, Batemans Bay Oceanic Inundation Study, Public Works

Department Coast and River Branch New South Wales, Sydney, Australia.Public Works Department (PWD): 1990, Narrabeen Lagoon Flood Study, Public Works Department

Coast and River Branch New South Wales, Sydney, Australia.

Roy, P. S. and Crawford, E. A.: 1977, Significance of sediment distributions in major coastal rivers,

northern New South Wales, In: Third Australian Conference on Coastal and Ocean Engineering,

Melbourne, Australia, pp. 177184.


24/24


Roy, P. S. and Thom, B. G.: 1981, Late Quaternary marine deposition in New South Wales and

southern Queensland: An evolutionary model, J. Geologic. Soc. Australia 28, 471489.

Roy, P. S.: 1984a, New South Wales estuaries: Their origin and evolution, In: B. G. Thom (ed),

Coastal Geomorphology in Australia, Academic Press, Sydney, pp. 99121.

Roy, P. S.: 1984b, Holocene sedimentation histories of estuaries in southeastern Australia. In:53rd Congress of the Australian and New Zealand Association for the Advanced Science in

Conjunction with the Australian Marine Science Association, Perth, Australia, pp. 2359.

Stive, M. J. F. and Wang, Z. B.: 1998, Morphodynamics of a tidal lagoon and the adjacent coast, In:

8th International Biennial Conference on Physics of Estuaries and Coastal Seas, 912 September

1996, The Hague, Netherlands, pp. 397407.

Stive, M. J. F., Dano, A., Roelvink, D. J. A., and de Vriend, H. J.: 1990, Large-scale coastal evolution

concept, Proceedings 22nd International Conference Coastal Engineering, American Society of

Civil Engineering, New York, 19621974.

Swift, D. J. P.: 1976, Coastal sedimentation, In: D. J. Stanley and D. J. P. Swift (eds), Marine

Sediment Transport and Environmental Management, Wiley, New York, pp. 255310.

Thom, B. G.: 1974, Coastal erosion in eastern Australia, Search 5(5), 198209.

Van Straaten, L. M. J. U.: 1954, Composition and structure of recent marine sediments in the

Netherlands, Leidse Geological Medelin 19, 1110.

gis modelling of sea-level rise induced

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