NZ1-2406103-South Tarawa Coastal Condition Assessment FINALclimate.gov.ki/wp-content/...Coastal...Assessment.pdf · Coastal exposure reflects whether the coastline is exposed to coastal

Report

KAP - II - FS6 - Improving the Protection of Public Assets

South Tarawa Coastal Condition Assessment Prepared for Office of Te Beretitenti, Government of Kiribati

By Beca Infrastructure Ltd (Beca)

16 February 2010

© Beca 2010 (unless Beca has expressly agreed otherwise with the Client in writing).

This report has been prepared by Beca on the specific instructions of our Client. It is solely for our Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which Beca has not given its prior written consent, is at that person's own risk.

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Revision History

Revision Nº Prepared By Description Date

A Paul Kench Alex Fullerton Lisa Hardwick

Draft for client comment 22 Dec 2009

B Lisa Hardwick Final, incorporating client comments. 16 Feb 2010

Document Acceptance

Action Name Signed Date

Prepared by Lisa Hardwick 16 Feb 2010

Reviewed by Richard Frankland 16 Feb 2010

Approved by Richard Frankland 16 Feb 2010

on behalf of Beca International Consultants Ltd

KAPII FS6 – Improving the Protection of Public Assets – South Tarawa Coastal Condition Assessment

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Table of Contents List of Acronyms and Abbreviations .........................................................................3 1 Introduction ..........................................................................................................4 2 Coastal Condition Assessment ...........................................................................5

2.1 Design of the South Tarawa Coastal Condition Assessment.....................................5 2.2 The South Tarawa Coastal Condition Assessment ...................................................6

3 Integration of Wave Data....................................................................................28 3.1 Coastal Calculator Input Data.................................................................................28 3.2 Results of Coastal Calculator .................................................................................29

4 Analysis and Interpretation of the STCCA........................................................31 4.1 Data Storage..........................................................................................................31 4.2 GIS Data Analysis and Mapping.............................................................................31 4.3 Synthesis and Interpretation of Threatened Assets.................................................34 4.4 Decision Support Tool............................................................................................35

References.................................................................................................................36

Appendices Appendix 1 – South Tarawa Coastal Condition Assessment Form Appendix 2 – South Tarawa Coastal Processes Assessment Appendix 3 – MapInfo Procedures Document

Appendix 4 – MapInfo ProViewer User Guide Appendix 5 – List of High Risk Sites



List of Acronyms and Abbreviations

AEP Annual Exceedence Probability

CCA Coastal Condition Assessment

FMC Foreshore Management Committee

GIS Geographic Information Systems

GOK Government of Kiribati

GPS Global Positioning System

IPCC Intergovernmental Panel for Climate Change

KAPII Kiribati Adaptation Project (Phase II)

MELAD Ministry of Environment, Lands & Agriculture Development

MFMRD Ministry of Fisheries & Marine Resources Development

MPWU Ministry of Public Works and Utilities

NIWA National Institute of Water and Atmospheric Research

STCCA South Tarawa Coastal Condition Assessment

UoH University of Hawaii datum



1 Introduction The Government of Kiribati commissioned Beca to carry out one of the components of the Kiribati Adaptation Project – Phase II (KAPII Component 2.2.1 FS6). The purpose of this component is to improve protection of public assets along the coast of South Tarawa. Effective management of public assets can be divided into two parts. First, it is necessary to identify those assets currently at risk from erosion. Second, once ‘at risk’ assets have been identified it is necessary to evaluate the most effective solution to protect assets. An important element in the assessment of risk at the coast and of the design of an appropriate solution is to understand the coastal processes (wave and current conditions) that impact shorelines promoting erosion and flooding.

This report addresses the first of these steps. It outlines the design and implementation of a methodology called the ‘South Tarawa Coastal Condition Assessment’ (STCCA) to identify public assets at risk from erosion in South Tarawa.

The specific objectives of the report are to:

(i) To develop the South Tarawa Coastal Condition Assessment. The purpose of the Coastal Condition Assessment is to map and assess the condition of South Tarawa coastline and existing coastal structures with respect to erosion and risk to climatic change impacts. In particular, this component of the work programme will:

a) Establish a methodology to evaluate the condition of the coast and existing coastal structures.

b) Develop a user friendly GIS interface to map the results of the coastal condition assessment.

c) Transfer the methodology to MPWU, MELAD, MFMRD and FMC for ongoing surveillance.

(ii) To synthesise the coastal process regime of South Tarawa to support planning and design of coastal solutions to erosion and flooding.



2 Coastal Condition Assessment

The rationale for developing a coastal condition assessment is threefold.

1. To evaluate the relative physical condition of the coast with respect to erosion and the protection of public infrastructure and assets. In particular, identify sections of coast in very poor physical condition and that will likely lead to short-term (0-2 years) coastal impacts on infrastructure and assets.

2. To document the existing state of coastal protection structures so that maintenance programmes can be prioritised.

3. To identify spatial differences in shoreline condition and process regime that will guide the selection of appropriate management responses.

The following sections provide an overview of the South Tarawa Coastal Condition Assessment methodology. The document also provides explanatory information on the reasons behind the method selected.

2.1 Design of the South Tarawa Coastal Condition Assessment

The STCCA method includes two separate assessment procedures:

1. Coastal Structures Condition Assessment. This assessment procedure is designed to be used on sections of coast in which coastal engineering structures exist, or attempts to protect assets already exist.

2. Coastal Condition Assessment. This assessment procedure is for sections of coast that have no existing coastal structure but are adjacent to public assets. This procedure should also be used where structures have failed so that the physical characteristics of the coast provide the only protection to assets.

Development of the assessment procedures were guided by the following principles.

n User-Friendly Field-Based Procedures To maximise the possibility for the condition assessment to be adopted and maintained by the

responsible agencies in Kiribati (MPWU, MELAD, MFMRD, FMC) the procedures must be user friendly and be able to be implemented during short site visits. Therefore, the procedures require simple observations to be recorded and tools adopt straight forward rating scales. The rationale behind these rating scales is explained in detail in this report.

n Point in Time Assessment

It is important to stress that the development of the STCCA is a point in time assessment of coastal condition (as of September – November 2009). The assessment will provide a baseline assessment that can be updated in the future in order to document changes in coastal condition through time.

n Focus on Public Assets at the Coast

The condition assessment is concerned with the shoreline and assets that are under the management of the Government of Kiribati. This includes major assets such as the Hospital, roads, runway and causeways but also includes sections of coast that currently have no coastal protection but adjoin public assets.



2.2 The South Tarawa Coastal Condition Assessment

This section outlines the field-based procedure to assess the condition of coastal structures and physical coastline that adjoins public assets. The procedure is split into four parts, which is presented in a table format that can be taken on site visits (Figure 1). A template of the South Tarawa Coastal Condition Assessment form is contained in Appendix 1.

The STCCA form should be completed for each section of coast or structure which adjoins public assets. In developing this assessment only public assets located within 20 m of the shoreline were included in the analysis.

Part 1: Description of Coastal Location

The first part of the STCCA form requires information on the location and broad setting of the coastline under consideration. The broad section of coast being examined and date of the site visit should be entered in the sections in the top of the form.

1a. Identifying Code

A unique code given to the stretch of coast under consideration. In the pilot study a 3-letter code was given to each site which was consistent with the island name or causeway linking two islands. The three letter code was followed by a number to identify each site visited on an island or causeway.

For example, on the island of Bairiki 38 sections of coast were examined. The first site was given the following identifier:

BAI_1 = the island Bairiki and site 1

The last site was given the identifier

BAI_38 = the island Bairiki and site 38

These identifying codes can be viewed on the final MapInfo product so that the data from each assessment site can be located on maps of South Tarawa.

1b. Co-ordinates Lat and Long

The latitude and longitude co-ordinates of each site should be recorded. This will require a hand held global positioning system (GPS) unit. This data can be used to identify an accurate location of each inspection site that can be located on MapInfo and relocated in the field. The co-ordinates need to be transformed to the Tarawa Local Grid system for inclusion in the MapInfo dataset using the MapInfo Procedures document generated by the Land Information Division, Ministry of Home Affairs & Rural Development. Republic of Kiribati, Central Pacific. 2000 (see Appendix 3).

1c. Island Name

Insert the name of the island or coastal site.

1d. Coastal Exposure

Coastal exposure reflects whether the coastline is exposed to coastal processes of the ‘ocean’ or ‘lagoon’. The orientation of the coastline to the reef edge and lagoon is important in controlling the magnitude of wave energy and processes that affect the shoreline. In particular, shorelines oriented toward the ocean reef are generally higher in energy than shorelines oriented toward the lagoon. This difference is attributed to the different types of wave energy that impact ocean and lagoon shorelines.



South Tarawa Coastal Condition AssessmentNotes:

Coastal Section: Ambo Taborio Causeway Location description

Date: 14-Sep-09 Record date surveyed

1 Description of Coastal Location1a Identifying Code ATC_4 Three letter code for island/location and each site numbered

1b Co-ordinates lat long To be obtained by GPS

1c Island Name Ambo-Taborio Causeway Describe physical location

1d Coastal exposure Lagoon (ocean or lagoon)

1e Coastal orientation N (N, NE, E, SE, S, SW, W, NW)You may need a compass

1f Energy setting Medium (High, Medium, Low)

1g Coastal type Causeway shoreline (linear beach and reef flat, bay, passage, intertidal lagoon, headland.)See glossary for an explanation of terms

1hApproximate length of coast where asset is at risk 200

Scale from aerial photograph or measure in field

2 Condition of Coastal Structures

2a Type of structure Road and wall (vertical seawall, sloping seawall, revetment, groyne, culvert)

2b Make of structure inderterminate (coral block, wood, sand bags, geotextile matting)

2c Length of structure (m) 300 Estimate / measure

2d Height of structure above reef (m) Estimate / measure

2e Distance structure to asset <0.5 Estimate / measure

2f Condition of structure 3 Grade from 1 to 5 as below

Grading - Condition of Structure1 = good condition and doing job

2 = performing with some signs of wear and tear

3 = significant signs of damage4 = extensive damage structure not performing

5 = derelict structure not performing

3 Physical Characteristics of Coast3a Beach present Y (Y / N)

3b Beach width (m) 4 (Width from low water mark to top of beach)

3c Composition of beach Gravel (sand, gravel, boulders)

3d Width vegetated backshore (m) < 2 m (Edge of vegetation to asset; <2m; 2-5 m; 5 - 10 m; >10 m)

3e Is there a conglomerate outcrop? N (Y / N)

3f Is beachrock present on beach? N (Y / N)

3g Is erosion scarp present Y (Y / N)

3h Height of erosion scarp (m) 0.4 Estimate / measure

3i Are engineered structures present N (Y / N)

3j Coastal Condition 5 Grade from 1 to 5 as below

Grading - Coastal Condition1 = more than 20m between edge of vegetation and asset2 = 10 to 20 m between edge of vegetation and asset

3 = 5 to 10 m between edge of vegetation and asset

4 = 2 to 5m between edge of vegetation and asset OR where there is not beach, 5 to 10m between edge of vegetation and asset5 = less than 2m between the edge of vegetation or scarp and the asset

4 Criticality of Asset4a Importance of Asset 1 main road Grade from 1 to 3 as below

Grading - Importance of Asset1 = critical e.g. hospital, airport, causeways2 = moderate criticality - school buildings offices etc3 = not critical - domestic housing

5 Notes

6 Link to photographs ..\Ambo-Taborio Causeway 14 Sept 09

Figure 1 – South Tarawa Coastal Condition Assessment Form. Highlighted area in box is section dealing with site description. The form contains a completed example.



1e. Coastal Orientation

Coastal orientation refers to the angle of the shoreline with respect to north. It is expressed as the compass direction at right angles to the shoreline. This measure provides an indication of the orientation of the coastline with respect to the prevailing wind and wave regime (east – southeast for Tarawa). For example:

n Shorelines that face the prevailing wind and wave direction are likely to have higher energy shorelines.

n Shorelines at oblique angles to the wind are expected to have lower energy coastal conditions and may be dominated by alongshore current processes.

n Shorelines facing away from prevailing winds are expected to have the lowest energy conditions.

1f. Energy Setting

The physical condition of each section of coast results from the interaction of waves and currents with the existing coastline and the available supply of sand and gravel. The term ‘Energy Setting’ represents the combination of wind, wave and current processes that are able to influence the coastline transport of sand and gravel, cause coastal erosion or coastal accretion (land building). A review of these processes and their relevance in South Tarawa is presented in Appendix 2. A summary of this review is presented below.

On coral reef systems the exposure of coastlines to energy (waves and currents) is controlled by:

n The direction and magnitude of ocean swell that is delivered to the coral reef. In general prevailing swell propagates from the east around Tarawa.

n The elevation and width of the reef which act to reduce incident ocean swell energy reaching shorelines. Coral reef platforms cause ocean swell to transform and break, releasing wave energy at the reef edge. In general, higher reef platforms are more efficient at promoting wave breaking. The reef platform at Tarawa is considered to be elevated and is exposed at lower tidal stages and is efficient at breaking incident wave energy.

n Wave energy that is not dissipated at the reef edge travels onto the reef platform where it can form secondary waves. In this instance wave breaking can occur at island shorelines. However, as waves travel across a reef platform interaction with the reef surface causes waves to lose their energy. Therefore, wider reef flats are more efficient at extracting energy from waves.

n In summary wave energy is greatest on prevailing windward locations and where the distance from island shoreline to reef edge (the reef flat) is narrow. In contrast, wave energy is generally lowest at leeward locations and where the reef width is large.

Based on these principles the coastline of South Tarawa has been divided into six broad energy settings. The different zones are presented in Figure 2 and summarised in Table 1. This classification reflects broad differences in the amount of wave energy reaching island shorelines and as a result the magnitude of wave setup and storm surge expected at island shorelines. On the ocean shoreline there are three divisions which reflect differences in gross wave energy conditions from the exposed high energy eastern end of South Tarawa to the lower energy western South Tarawa ocean shoreline (Table 1). The lagoon shoreline is also divided into three sections which also reflect differences in incident energy. However, on the lagoon shoreline the energy gradient is reversed. Lowest energy is the eastern lagoon shoreline. The incident energy conditions increase toward the west as wind generated waves increase in height and as ocean swell wraps around the western end of Betio.


Figure 2 – Summary of coastal process regions in South Tarawa.


The process assessment also provides a coarse-scale indication of the direction of alongshore sediment transport. On the ocean shoreline it is inferred that alongshore sediment transport occurs toward the west. However, it is likely that there are small pocket beaches and sections of coast in which there is little alongshore sediment flux. On the lagoon shoreline it is also inferred that alongshore sediment transport is toward the west. However, it is also likely that there is easterly drift along the lagoon shoreline of Betio in response to swell refraction around the western tip of the atoll. It should also be noted that at the local scale reversals in littoral drift can be expected due to changes in prevailing winds and presence of human structures (e.g. walls and reclamations).

Table 1 summarises the six energy settings. Depending on the location of a site being assessed the appropriate energy setting should be inserted on the STCCA form.

Table 1 – South Tarawa coastal exposure categories as shown in Figure 2.

Orientation Energy Key factors Location on Tarawa

Ocean High Windward setting Narrow reef width <200 m

East to southeast ocean shoreline

Medium Non-windward setting Intermediate reef width 200 – 500 m

Central southern ocean shoreline

Low Leeward setting Wide reef width >500 m

Western southern shoreline

Lagoon Low Sheltered lagoon setting (upwind) Eastern lagoon shoreline

Medium Moderate fetch lagoon setting Central lagoon shoreline

High Long fetch lagoon setting (downwind) Western lagoon shoreline

1g. Shoreline Type

The plan form configuration of the shoreline varies considerably in South Tarawa. Figure 3 presents a range of different shoreline types that include: linear (straight) beaches; indented embayments; intertidal lagoon beaches; headlands; passages. Differences in shoreline type may have implications for the coastal system and processes of coastal change. The shoreline type should be noted on the STCCA form.

1h. Approximate Length of Coast

The length of shoreline under consideration is estimated based on: measurement in the field or scaling from a map or aerial photograph. Field measurements may include GPS, pacing the length of the coastal section or use of a tape measure.

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Indented embayment

Linear Beach

Intertidal Lagoon Beach

Headland

Figure 3 – Shoreline types on South Tarawa.


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Part 2: Condition of Coastal Structures

Part two of the South Tarawa Coastal Condition Assessment Form (Figure 4) examines the characteristics and condition of structures that exist at the coast.







































5 Notes


Figure 4 – The South Tarawa Coastal Condition Assessment Form. Highlighted area in box is the assessment of existing coastal structures. The form contains a completed example.


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2a. Type of Structure

Three typical types of coastal structure can be observed in South Tarawa.

i) Vertical or sloping seawall

These are self supporting structures built along the coast to prevent erosion by wave action or currents (e.g. reinforced concrete wall, rock armour). These structures are typically able to withstand greater wave forces than revetments (Figure 5A).

ii) Revetment

A facing of stone / concrete, supported by the land behind it, to protect an embankment or shore structure from erosion (e.g. sandbag walls, coral rock walls, Figure 5B).

iii) Groyne

A narrow structure built perpendicular to the shoreline to impede longshore currents and/or trap sediments. In Kiribati structures acting as groynes are constructed as part of a reclamation.

C. Groyne

Figure 5 – Seawall and revetment structures.

Land

Built perpendicular to the shoreline to trap sediment

Water


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2b. Make of Structure

Note the structure could be comprised of materials which may include:

n Sand bags

n Stacked coral rock (ungrouted)

n Grouted coral rock

n Gabions (wire baskets filled with rock)

n Concrete

n Fabriform matting (e.g. Nippon causeway

material)


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n Tetrapods

n Drums

n Other ad hoc materials

2c. Length of Structure (m)

Estimate the length of the structure. This can be measured by a survey or tape measure.

2d. Height of Structure (m)

Estimate the height of the top of the structure from the reef flat level or top of the beach.

2e. Distance Between Structure and Asset (m)

This is the distance between the top landward edge of the structure and the asset potentially at risk.

2f. Condition of Structure

The final step of this stage of the assessment is to evaluate the physical condition of the coastal structure. The criteria developed to assess the physical condition of existing structures is presented in Table 2. The categories are differentiated based on the degree of wear of the structure and its ability to perform its function. A range of examples are shown in Figure 6 and Figure 7.


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Table 2 – Scale for assessing the physical condition of coastal structures.

Rating Grade Indicators

1 Very Good Sound physical condition, no work required

2 Good Acceptable physical condition, minimal short-term failure risk but potential for deterioration. Only minor work required.

3 Average Significant deterioration evident, failure unlikely in the near future but further deterioration likely

4 Poor Failure likely in the short term, substantial work required in the short term, asset barely serviceable

5 Very Poor Failed or failure imminent. Major work or replacement required urgently.


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Figure 6 – Examples of structures in different physical condition 1 - 4.


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Figure 7 – Examples of structures in physical condition 5.


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Part 3: Physical Condition of the Coast

The third part of the coastal condition assessment gathers information on the physical properties of the coast (Figure 9). In many locations Public assets are situated in close proximity to the coast where there are no existing coastal structures. In order to develop appropriate management strategies for the protection of these assets it is also necessary to evaluate the physical condition of the coast.

This part of the assessment should be used to assess the characteristics and condition of the coast where no structures exist or where a structure has been rated as 5 (failed).

The physical condition of the coast is assessed through capture of simple descriptive information on the shoreline features. The questions and responses required are summarised below. Figure 8 also shows key coastal features and defines dimensions to be estimated. The vertical scale is exaggerated to allow all features to be clearly seen.

Figure 8 – Schematic cross-shore profiles from reef island to reef edge on South Tarawa. Figures show topographic and morphological zones at the coast.


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5 Notes


Figure 9 – South Tarawa Coastal Condition Assessment Form. Highlighted area in box is the assessment of the physical condition of the coast. The form contains a completed example.



3a. Beach Present (Y/N)

A beach is an unconsolidated deposit of sand or gravel located between the edge of vegetation and reef flat surface or subtidal lagoon (Figure 8 and Figure 10). This question is to establish whether a beach exists at each field location. In general, the sand volume stored in beaches provides a protective buffer to the island shoreline as it absorbs wave energy.

Figure 10 – Beach on the lagoon shoreline of South Tarawa.

3b. Beach Width (m)

The distance from the edge of vegetation to the toe of beach that intersects the reef flat surface. On lagoon shorelines beach width is the distance from the edge of vegetation to the lowest tide mark. The beach width measurement provides an easily quantifiable indicator of the extent of the beach sand volume (conducting surveys at low tide is recommended).

3c. Composition of Beach

Is the beach composed of sand size (particles less than 2 mm in size) or gravel size (particles greater than 2 mm in size) material. In general, gravel beaches are more stable than sand beaches.

3d. Width Vegetated Backshore (m)

This is the distance between the edge of vegetation and asset potentially at risk (Figure 11). This is a horizontal distance in metres and forms a critical component of the overall physical condition assessment. In general, sand and gravel stored at the coast in the form of beaches, or island margins that have been vegetated provide a buffer to erosion of assets. Consequently, the wider the width of vegetated backshore provides a greater buffer to erosion and greater protection to assets.


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Figure 11 – Examples of differences in width of the vegetated backshore at South Tarawa. Also note rating scale values that identify differences in condition of the coast (discussed in section

3j).



3e. Is Conglomerate Present (Y/N)

Along the shoreline of South Tarawa cemented rock can occur at the shoreline and provide a natural measure of protection. There are two different types of cemented rock. Conglomerate platform is one type of such rock and has the key distinguishing characteristics:

n It typically protrudes from the shoreline and extends across the reef and underneath islands. n It has a horizontal surface (no slope). n It is commonly elevated above the surrounding reef flat (by approximately 0.5 – 1.0 m); n It is comprised of cemented together coral blocks and gravel. n Has a heavily eroded surface and black/dark grey appearance.

Figure 12 shows images of conglomerate outcrops along the South Tarawa shoreline.

Figure 12 – Examples of conglomerate platform outcropping at island shorelines in South Tarawa.



3f. Is Beachrock Present? (Y/N)

Beachrock is a second type of cemented rock. It forms on the face of beaches (in the intertidal zone) and has the following distinguishing characteristics:

n It exists as shore parallel bands of cemented rock. n It has a sloping surface that has a similar gradient to the surrounding beach. n It is generally composed of cemented sand (and less commonly gravel).

Figure 13 presents examples of beachrock present along the South Tarawa shoreline.

Figure 13 – Examples of beachrock on island shorelines in South Tarawa.

3g. Is an Erosion Scarp Present? (Y/N)

A key indicator of recent erosion of a beach is the presence of a vertical scarp present at the edge of the island (usually at the edge of vegetation, Figure 8). This indicates scour and undermining of the vegetated shoreline landward of the beach and is diagnostic of erosion. Examples of erosion scarps are shown in Figure 14.

3h. Height of Erosion Scarp (m)

The height of the erosion scarp from the sand surface to top edge of vegetation.

3i. Are Engineered Structures Present (Y/N)

Establishes whether there are engineered structures present or in the near vicinity of the coastal section.



Figure 14 – Examples of erosion scarps in edge of vegetation on South Tarawa.

3j. Coastal Condition

Based on the information gathered a summary estimate of the coastal condition is determined. A rating scale has been devised to assess the overall physical condition of the coast. The rating scale is presented in Table 3. The rating scale is based on the principle that the natural coast can protect assets. In particular, beaches and the width of the backshore between an asset and edge of vegetation provide a large volume of sediment that affords protection to assets. The width of the vegetated shoreline therefore, provides a coarse indicator of the extent and volume of this sediment reservoir protecting an asset and provides the basis for the rating scale (Table 3). In general, the wider the vegetated backshore the greater the level of protection for an asset.

For example, a coastal site in which the width of the vegetated backshore is less than 2 m has very little sand reservoir protecting assets and therefore, it is considered to be in very poor condition (Rating 5, Table 3) and there is a high short-term risk to assets.

In contrast, a coastal site in which the width of vegetation is greater than 20 m suggests there is a large sediment volume protecting assets. Consequently, such a site would be considered to have a very good rating (Rating 1, Table 3) and there is a minimum risk to assets.

A range of examples are shown in Figure 11 and Figure 15.



Table 3 – Rating Scale for Assessing the Physical Condition of the Coast.

Rating Grade Indicators

1 Very Good > 20 m between edge of vegetation and asset

2 Good 10-20 m between edge of vegetation and asset

3 Average 5-10 m of backshore between beach and asset and with beach fronting vegetated shoreline

4 Poor 2-5 m between edge of vegetation and asset

5 Very Poor < 2 m between edge of vegetation/scarp and asset

Figure 15 – Examples of coastal condition 5 in South Tarawa. Note the width of vegetated backshore (distance between the edge of vegetation and asset) is less than 2 metres in each

case.



Part 4. Criticality of Asset

The last part of the South Tarawa Coastal Condition Assessment Form seeks an estimate of the level of importance of the asset at the coast. A three scale rating scheme has been developed that reflects different levels of importance of assets.

Criticality 1:

Lifeline assets that are critical to the operation of the government and country. Such assets include: hospitals, ports, causeways and airports.

Criticality 2:

Assets that have a moderate level of importance and include: school buildings, government offices etc.

Criticality 3:

Assets that are not considered critical to government functioning including: dwellings, secondary store spaces etc.

It is important to note that this scale can be adjusted and/or refined at any stage following discussions with the FMC to assign assets different levels of criticality.

SUMMARY

• This section has outlined the different parts of the South Tarawa Coastal Condition Assessment Form.

• It is intended that the STCCA Form is completed at each site that an assessment of the condition of the coast in front of assets is required.

• At each site photographs should also be taken of the structure and/or the physical coastline being assessed.

• The position of each site should also be marked on a map of the island.


3 Integration of Wave Data

NIWA were commissioned by the Government of Kiribati, under a separate contract, to develop climate risk information to be adopted as national standards for options analysis and technical design work (“climate proofing parameters”). The output from NIWA’s work is a spreadsheet calculator (Coastal Calculator) which can be used to derive site specific wave run-up, overtopping and sea level rise information. This water level and wave data has been incorporated into the coastal condition assessment to assist with planning and design of shoreline protection works.

3.1 Coastal Calculator Input Data

The Coastal Calculator was used to predict future storm tide levels, significant wave heights and minimum crest levels for sloping revetments and vertical seawalls. To derive this information a number of parameters need to be selected. For the purposes of this project the following inputs in Table 4 were used to determine wave height and water level information at specific sites.

Table 4 - Input Parameters for NIWA's Coastal Calculator

Input Parameter Value Source of Data

Baseline year 1980 – 1999 average IPCC

Tarawa datum University of Hawaii GoK MapInfo Procedures

Future timeframe comparison

2050s (2050-2059) 2070s (2070-2079)

Foreshore Management Committee

Emission scenario A1F1 (upper range of model output – 95%, including 0.2m ice sheet discharge by 2090s)

Foreshore Management Committee

Change in extreme wave heights

0% Foreshore Management Committee

Annual exceedence probability

1% (based on existing data)

Foreshore Management Committee advised a 0.5% or 0.2% AEP should be used. However existing data only provides for 1% AEP.

Reef width (ocean or lagoon)

Varies Aerial photography provided by GoK.

Landward level of reef (lagoon side only)

Varies GoK GIS layer Tar_topo_contours plus additional survey data provided by NIWA

Average reef flat level (ocean side only)

Varies GoK GIS layer Tar_topo_contours plus additional survey data provided by NIWA

Wave breaking location Reef edge & rim Default value recommended in Coastal Calculator

Angle of reef face slope 1 in 3 Default value recommended in Coastal Calculator

Ocean side reef flat characteristics

75%-100% smooth rock or coral pavement

Site visit to Kiribati and review of photographs


Input Parameter Value Source of Data

Shoreline type Sloping (revetment) seawall or Vertical seawall

Used to determine the minimum crest levels for structures

Seawall (revetment) slope 1 in 1 Consistent with existing sandbag seawall construction

Seawall crest width 0.6m Width of sandbag currently used by MPWU

Seawall (revetment) armouring

Smooth concrete blocks Similar to existing sandbag seawall construction

Revetment crest wall Yes MPWU standard seawall design drawings

3.2 Results of Coastal Calculator

Outputs from the Coastal Calculator include predictions for future sea level rise, storm tide levels, significant wave heights and minimum crest elevations for sloping revetments and vertical seawalls. As the final version of the NIWA Coastal Calculator was not available at the time this work was carried out. A draft version 4.3 was used to accommodate the timeframes for this project.

Sea level rise predictions as derived from the Coastal Calculator are shown in Table 5.

Table 5 – Coastal Calculator Sea Level Rise Predictions

Timeframe Sea Level Rise (m) Water Level (m UoH)

Present day 0 1.20

2050 – 2059 0.35 1.55

2070 – 2079 0.55 1.75

Storm tide, significant wave height and minimum crest elevations for the ocean and lagoon shorelines are shown in Table 6 and Table 7. This data has been included in the South Tarawa Coastal Condition Assessment as part of the Coastal Processes Map in the GIS Viewer (refer to section 4.2.3).

Due to the limited survey data which was available as inputs into the Coastal Calculator it is recommended that this data is used for planning purposes only. Profile surveys should be carried out at individual sites prior to design and the Coastal Calculator run to provide more accurate data for the design of shoreline protection works.

Table 6 – Output Data from Coastal Calculator (Lagoon Shoreline)

Lagoon Shoreline

2050s 2070s 2050s 2070s 2050s 2070s 2050s 2070sStorm Tide Level (m UoH) 2.98 3.17 2.98 3.17 2.98 3.17 2.98 3.17

Significant Wave Height (m) 0.61 0.80 0.48 0.57 0.59 0.80 1.05 1.23

Sloping Revetment Minimum Crest Elevation (m UoH) 2.80 3.40 3.37 3.80 3.10 3.70 3.60 4.10

Vertical Seawall Minimum Crest Elevation (m UoH) 2.90 3.25 3.17 3.43 2.90 3.30 3.20 3.80

Low Energy Medium Energy Dai Nippon CausewayHigh Energy1% Annual Exceedence Probability


Table 7 – Output Data from Coastal Calculator (Ocean Shoreline)

Ocean Shoreline

2050s 2070s 2050s 2070s 2050s 2070s 2050s 2070sStorm Tide Level (m UoH) + wave set-up 3.23 3.39 3.23 3.39 3.23 3.39 3.23 3.39

Significant Wave Height (m) 0.41 0.46 1.06 1.12 0.82 0.88 0.53 0.57

Sloping Revetment Minimum Crest Elevation (m UoH) 3.70 4.10 6.20 6.50 5.60 5.85 4.30 4.70

Vertical Seawall Minimum Crest Elevation (m UoH) 3.40 3.50 3.40 3.60 3.40 3.50 3.40 3.50

Dai Nippon CausewayHigh Energy1% Annual Exceedence Probability

Low Energy Medium Energy


4 Analysis and Interpretation of the STCCA

This section outlines the steps taken to store and analyse the data collected using the STCCA to support planning and effective management of public assets.

4.1 Data Storage

After the completion of each field visit the information gathered must be transferred to digital form and stored on a computer system. In particular the following steps should be undertaken:

1. Transfer information on each STCCA Form to an individual excel spreadsheet. The file should be saved using the 3 letter code for the site entered at box 1a on the STCCA Form.

2. Photographs of each individual site should be transferred to computer and stored in an individual folder. The folder should be named using the same 3 letter code for the site as entered at box 1a on the STCCA Form.

3. Both the excel spreadsheet and photographs should be stored in an individual folder.

4. Hard copies of the STCCA Form should be stored in a filing system.

4.2 GIS Data Analysis and Mapping

Data collected using the STCCA has been integrated into a Geographic Information System (GIS). The result is an interactive map viewer using MapInfo ProViewer version 10. The software allows the user to rapidly identify the information for each site and view the photographs. All data is provided in MapInfo .tab file format and is in terms of Tarawa Local Grid (WGS84).

4.2.1 Methodology for Data Collection and Analysis

1. A set of hard-copy plans were generated for the field team to use during the Inception Site visit in September 09. Areas were identified on-site and their extent’s recorded and marked-up on these hard copy plans. A unique identifying code was used to reference each site on the STCCA form (see section 2.2, item 1a above). Due to the absence of any geo-referenced orthophotography, at the time of the assessment, imagery from Google Earth was exported and geo-registered against a provided building footprint GIS layer “Tar_Topo_Bld.TAB”.

(Note: Due to the currency and accuracy of the provided GIS data used to geo-register the Google Earth imagery, the spatial accuracy of assessment areas recorded against them may be considered variable).

2. The attribute information collected on each STCCA form was appended to the generated assessment area polygons. Photographs relating to each area were also linked to the pologons.


3. A second site visit occurred in November and additional public assets identified through discussions with MPWU. GPS coordinates for the additional sites were recorded and further assessment area polygons added to the dataset.

4.2.2 GIS Viewer (MapInfo ProViewer, version10)

The following GIS datasets are packaged into the GIS viewer:

• Existing GIS datasets provided by the Government of Kiribati:

- Tar_topo_misc_lin.tab

- Tar_topo_Spot_hts.tab

- Tar_topo_roads.tab

- Tar_topo_top_bch.tab

- Tar_Topo_MHWS.tab

- Tar_topo_reef.tab

- Tar_topo_contours.tab

- Tarawa_villages.tab

- Kiribati_Images_Tawarwa_Local_Grid.tab (orthophotography)

• Coastal Assessment Areas – Extents identified utilising the process described in this report and attributed with the information collected through the STCAA form.

• Critical Assets – Public assets classified as grade 1 level of importance (see section 2.2, part 4 above).

Re-projection to Tarawa Local Grid (WGS84) was performed using the process outlined in the MapInfo Procedures document generated by the Land Information Division, Ministry of Home Affairs & Rural Development. Republic of Kiribati, Central Pacific. 2000. The document contains all metadata relating to the above GIS datasets and is attached as Appendix 3.

(Note: This data is derived in part or wholly from sources other than Beca, and therefore, no representations or warranties are made by Beca as to the accuracy or completeness of this information.)

4.2.3 User Guide for MapInfo ProViewer

A MapInfo ProViewer User Guide is contained in Appendix 4. This document provides detailed information on how to install the viewer, minimum software requirements and how to operate the viewer. A summary of the key tools required to access the relevant information in the viewer is outlined in Table 8 below.

To view the coastal assessment map change the viewing window by selecting “Window” from the top menu toolbar and change to “Coastal Assessment Areas – Map”. The legend for this map can be viewed by selecting the “Coastal Assessment Area – Legend” as above.

Similarly to view the coastal processes map and legend change the viewing window as outlined above to “Coastal Processes – Map” or “Coastal Processes – Legend” as required.


Table 8 - Key Tools for Accessing Data in MapInfo ProViewer

Icon Name Description

Zoom In Allows the user to zoom in to view an area in greater detail.

Zoom Out Allows the user to zoom out to view a larger area in less detail.

Grabber Allows the user to move the image across the screen to view adjacent areas.

Info Tool Displays the information collected through the South Tarawa Coastal

Condition Assessment for the selected site by clicking on a coloured area.

Hot Link Displays the photographs taken as part of the South Tarawa Coastal

Condition Assessment for a selected site when viewing the Coastal Assessment Areas Map by clicking on a coloured area. Displays the outputs from the Coastal Calculator when viewing the Coastal Processes Map by clicking on the coloured arrows for each energy setting.

Ruler Allows the user to measure the distance between points.

Show/Hide Legend

Displays or hides the map legend indicating the colour theme of the classification scheme.


4.3 Synthesis and Interpretation of Threatened Assets

In order to assist the identification of high risk and low risk assets data has been manipulated into a 5 point rating scale in the GIS (Table 9, Figure 16). High risk areas are denoted as Red on the GIS system. The scale is consistent with both rating scales of the structural condition assessment and physical condition assessment.

If there is a structure present at a site then the Structural Condition Assessment value is assigned to the area and it is highlighted in the GIS according to the 5 point rating scale. If there is no structure present then the Physical Condition Assessment rating is assigned to the area and highlighted accordingly. For example if a site has a structure present which is in sound physical condition (i.e. yields a Structural Condition Assessment of 1 (the lowest risk rating)) but has less than 2m between the edge of vegetation and the asset (i.e. yields a Physical Condition Assessment of 5 (the worst rating)) it would be highlighted as green on the GIS as the structure is performing its function and the asset is not at risk.

Finally for sites that have assets that are considered most critical (criticality level 1) a star is placed in the GIS system at that location (Figure 16).

Table 9 – Risk scale developed for presentation of results of the structural condition and physical coastal condition assessments.

GIS Risk Scale Structural Condition Assessment Physical Coastal Condition Assessment

Rating Risk level Rating Grade Indicators Rating Grade Indicators

1 Low 1 Very Good

Sound physical condition

1 Very Good

> 20 m between edge of vegetation and asset

2 2 Good Acceptable physical condition

2 Good 10-20 m between edge of vegetation and asset

3 Moderate 3 Average Significant deterioration

3 Average 5-10 m of backshore between beach and asset

4 4 Poor Failure likely in the short term

4 Poor 2-5 m between edge of vegetation and asset

5 High 5 Very Poor

Failed 5 Very Poor

< 2 m between edge of vegetation/scarp and asset

Note: Risk ratings are in colour as they appear in the GIS tool. The structural condition assessment and physical coastal condition assessment are also shown to illustrate how the rating values are in alignment.


Figure 16 – Example of the MapInfo product. Note coloured areas reflecting different risk levels and stars that denote assets with critical rating of 1. Users are able to click on each

labelled site to investigate the STCCA form data and photographs taken at each site.

4.4 Decision Support Tool

The GIS tool can be examined to assess the relative risk of coastal sites. For instance, it is possible to scan the entire coastline of South Tarawa to identify all high risk sites. These sites could include all areas which are highlighted in red and have a star (denoting critical asset rating 1). This exercise was undertaken in the Workshop held in MPWU on November 19th, 2009. The high risk sites identified by the coastal condition assessment are contained in Appendix 5.


References

Abelson, A., Denny, M., 1997. Settlement of marine organisms in flow. Ann. Rev. Ecol. Syst., 28, 317–339.

Atkinson M., Smith, S.V., Stroup, E.D., 1981. Circulation in Enewetak Atoll lagoon. Limnol Ocean 26: 1074-1083.

Brander, R.W., Kench, P.S., Hart, D.E., 2004. Spatial and temporal variations in wave characteristics across a reef platform, Warraber Island, Torres Strait, Australia. Mar. Geo., 207, 169-184.

Damlamian, H. 2008. Hydrodynamic Model of Tarawa: Water Circulation and Applications. Kiribati Technical Report. EU EDF – SOPAC Project Report 134, Reducing Vulnerability in Atoll States.

Gallagher, B.S., Shimada, K.M., Gonzalez, Jr. FI., Stroup, E.D., 1971. Tides and currents in Fanning Atoll lagoon. Pac. Sci. 25: 191-205.

Gerritsen, F., 1981. Wave attenuation and wave set-up on a coastal reef. Proc. 17th Int. Conf. Coast. Eng., ASCE, pp. 444−461.

Gourlay, M.R., 1988. ‘Coral Cays’ products of wave action and geological processes in a biogenic environment. Proc. 6th Int. Coral Reef Symp., Australia, pp. 491−496.

Gourlay, M.R., 1994. Wave transformation on a coral reef. Coast. Eng., 23, 17−42.

Hamner, W.H., Wolanski, E., 1988. Hydrodynamic forcing functions and biological processes on coral reefs: a status review. Proc. 6th Int. Coral Reef Symp., 1, 8-12.

Hardy, T.A., Young, I.R., Nelson, R.C., Gourlay, M.R., 1990. Wave attenuation on an offshore coral reef. Proc. 22nd Int. Coast. Eng. Conf., Vol. 1, pp. 330–344.

Hearn, C.J., 1999. Wave-breaking hydrodynamics within coral reef systems and the effect of changing relative sea level. J. Geophys. Res., 104 (C12), 30007−30019.

Hearn, C.J., Atkinson, M.J. and Falter, J.L., 2001. A physical derivation of nutrient-uptake rates in coral reefs: effects of roughness and waves. Coral Reefs, 20, 347−356.

Henderson, R.S., Jokiel, P.L., Smith, S.V., Grovhoug, J.G., 1978. Canton Atoll lagoon physiography and general oceanographic observations. Atoll Res. Bull. 221.

Stroup, E.D., Meyers, G.A., 1974. The food-tide jet in Fanning Island lagoon. Pac. Sci. 28: 211Ð223

Kench, P.S., 1994a. Hydrodynamic observations of the Cocos (Keeling) Islands lagoon. Atoll Res. Bull. 408, 21 pp.

Kench, P.S., 1998. Physical processes in a semi-enclosed Indian Ocean atoll. Coral Reefs, 17, 155−168.

Kench, P.S., Brander, R.W., 2006. Wave processes on coral reef flats: implications for reef geomorphology using Australian case studies. J. Coast. Res., 22, 209-223.

Kench, P.S., Brander, R.W., Parnell, K.E., McLean, R.F., 2006. Wave energy gradients across a Maldivian atoll: Implications for island geomorphology. Geomorphology, 81, 1-17.


Kench, P.S., Brander, R.W., Parnell, K.E., O’Callaghan, J.M., 2009a. Seasonal variations in wave characteristics around a coral reef island, South Maalhosmadulu atoll, Maldives. Mar. Geo., 262, 116-129.

Kench, P.S., Parnell, K.E., Brander, R.W. 2009b. Monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef islands. Marine Geology, 266, 91–108.

Kraines, S.B., Yanagi, T., Isobe, M. and Komiyama, H., 1998. Wind-wave driven circulation on the coral reef at Bora Bay, Miyako Island. Coral Reefs, 17, 133−143.

Lee, T.T. and Black, K.P., 1978. The energy spectra of surf waves on a coral reef. Proc. 16th Int. Conf. Coast. Eng., ASCE, pp. 588−608.

Lugo-Fernández, A., Roberts, H.H. and Wiseman, W.J. Jr., 1998a. Tide effects on wave attenuation and wave set-up on a Caribbean Coral Reef. Est. Coast. Shelf Sci., 47, 385−393.

Massel, S.R., Brinkman, R.M., 1999. Measurement and modelling of wave propagation and breaking at steep reefs. In: Saxena, N. (Ed.), Recent Advances in Marine Science and Technology, 1998. Pacon Int.

Munk, W.H., Sargent, M.C., 1948. Adjustment of Bikini atoll to ocean waves. Trans Am Geophys Union 29: 855-860.

Nakamori, T., Suzuki, A., Iryu, Y., 1992. Water circulation and carbon flux on Shiraho coral reef of the Rykuyu Island, Japan. Cont. Shelf Res., 12, 951–970.

Pugh, D.T., Rayner, R.F., 1981. The tidal regimes of three Indian Ocean atolls and some ecological implications. Estu, Coast Shelf Sci, 13: 389-407.

Roberts, H.H. and Suhayda, J.N., 1983. Wave current interactions on a shallow reef (Nicaragua). Coral Reefs, 1, 209−260.

Roberts, H.H., Wilson, P.A. and Lugo-Fernandez, A., 1992. Biologic and geologic responses to physical processes: examples from modern reef systems of the Caribbean-Atlantic region. Cont. Shelf Res., 12 (7/8), 809−834.

Symonds, G., Black, K.P. and Young, I.R., 1995. Wave-driven flow over shallow reefs. J. Geophys. Res., 100 (C2), 2639−2648.

von Arx, W., 1948. The circulation systems of Bikini and Rongelap lagoons. Trans. Am. Geophys Union 29: 861-870.


Appendix 1 – South Tarawa Coastal Condition Assessment Form




Coastal Section: Location description

Date: Record date surveyed

1 Description of Coastal Location1a Identifying Code Three letter code for island/location and each site numbered


1c Island Name Describe physical location

1d Coastal exposure (ocean or lagoon)

1e Coastal orientation (N, NE, E, SE, S, SW, W, NW)You may need a compass

1f Energy setting (High, Medium, Low)

1g Coastal type(linear beach and reef flat, bay, passage, intertidal lagoon, headland.)See glossary for an explanation of terms

1hApproximate length of coast where asset is at risk



2a Type of structure (vertical seawall, sloping seawall, revetment, groyne, culvert)

2b Make of structure (coral block, wood, sand bags, geotextile matting)

2c Length of structure (m) Estimate / measure


2e Distance structure to asset Estimate / measure

2f Condition of structure Grade from 1 to 5 as below

Grading - Condition of Structure1 = good condition and doing job2 = performing with some signs of wear and tear





3 Physical Characteristics of Coast3a Beach present (Y / N)

3b Beach width (m) (Width from low water mark to top of beach)

3c Composition of beach (sand, gravel, boulders)

3d Width vegetated backshore (m) (Edge of vegetation to asset; <2m; 2-5 m; 5 - 10 m; >10 m)

3e Is there a conglomerate outcrop? (Y / N)

3f Is beachrock present on beach? (Y / N)

3g Is erosion scarp present (Y / N)

3h Height of erosion scarp (m) Estimate / measure

3i Are engineered structures present (Y / N)

3j Coastal Condition Grade from 1 to 5 as below

Grading - Coastal Condition1 = more than 20m between edge of vegetation and asset




4 Criticality of Asset4a Importance of Asset Grade from 1 to 3 as below


5 Notes

6 Link to photographs



Appendix 2 – South Tarawa Coastal Processes Assessment

The coastal process regime (waves, tides, currents, sediment transport) of South Tarawa varies considerably between different island shorelines and between the ocean and lagoon. It is important to recognise such differences in the assessment of coastal change (erosion and accretion) and flooding. Furthermore, consideration of differences in processes between sites is critical in the design of asset protection measures at the shoreline.

Circulation in coral reef systems is typically driven by a combination of tidal flow, wind-driven flow, and flow induced by the breaking of wind-waves at the reef edge. In large part the relative role of each of these mechanisms is controlled by reef morphology. Existing studies of coral reef oceanography can be divided into those that have examined broad scale circulation (von Arx 1948; Gallagher et al. 1971; Stroup and Meyers 1974; Henderson et al. 1978; Atkinson et al. 1981; Pugh and Rayner, 1981; Kench, 1994; 1998; Kraines et al., 1998) and those that have examined the interaction of waves with coral reefs (e.g. Roberts et al., 1992; Symonds et al, 1995; Brander et al., 2004; Kench and Brander, 2006; Kench et al., 2009a, b).

A.1 Tides

Tidal flow is an important mechanism of water exchange between the open ocean and lagoon where there are large (wide and deep) passages in the reef rim (Kench, 1994, 1998) or where the lagoon is relatively small in area. Of relevance to this study the direct influence of tides is limited where there are no passages connecting the lagoon and ocean and/or where the tides have low amplitude.

The tidal characteristics of Tarawa lagoon are semi-diurnal (2 high tides every 24 hour period) with the tidal range (difference in tidal elevation between low and high tide) ranging from 2.4 m under spring tide conditions to 0.5 m under neap tide conditions (Figure A.1). Notably there is considerable difference in the height of successive high tides.

Figure A.1 Water level record from Betio wharf Seaframe tide gauge. From Damlamian, 2008.



Damlamian (2008) undertook a modelling study of circulation in Tarawa atoll lagoon. Results from this modelling work indicated that the lagoon shoreline of South Tarawa is subject to reversing tidal flows, which are controlled by the rising (incoming) and falling (outgoing) tides. Current meters deployed close to the south Tarawa Lagoon shoreline, used to validate the model, measured peak currents on the order of 0.15 to 0.2 m/s. In general such currents are insufficient to entrain sediments but are capable of transporting sediment once it has been mobilised by wave energy.

Changes in tidal elevation also play a major role in modulating the magnitude of wave energy that propagates onto reef surfaces and can access the reef island shorelines. This will be discussed in greater detail below.

A.2 Wind

The wind regime is important in forcing surface currents in atolls and can force surface and return currents in atoll lagoons (Munk and Sargent, 1948). Wind also plays a major role in the generation of waves that can affect shorelines (Kench et al., 2006). Wind-generated wave energy can be divided into two types. First, incident ocean swell at the ocean reef edge is generated by wind systems in the Pacific Ocean. These waves are the largest (1-3 m) to impact the atoll on the ocean reef and have a long period (10-14 seconds). Second the interaction with wind with the lagoon surface generates local wave energy. Wave height is related to the wind speed and distance of lagoon across which wind blows. These waves are typically lower than ocean waves, have shorter period and propagate in the direction of wind flow. In Tarawa the wind regime is dominated by winds from the easterly quarter. Consequently, wind-generated wave energy increases westward across the lagoon. However, reversals in this energy gradient can be expected under northwesterly events.

A.3 Waves

The interaction of waves with coral reefs is known to control reef flat wave and current processes, ecological and geological processes in coral reef systems (Hamner and Wolanski, 1988; Nakamori et al., 1992; Roberts et al., 1992; Abelson and Denny, 1997; Hearn et al., 2001). Incident waves and their interaction with coral reef platforms are also acknowledged as the main mechanism controlling the formation, erosion and change of coral reef islands (Gourlay, 1988).

Coral reefs act to filter incident waves and control the energy that leaks on to reef platform surfaces. The physical processes of wave breaking at the reef edge have been well documented. Numerous studies have documented large reductions in wave energy of up to 97% as incident swell is transformed and breaks at the reef edge (Lee and Black, 1978; Gerritsen, 1981; Roberts and Suhayda, 1983; Hardy et al., 1990; Lugo-Fernández et al., 1998; Hearn, 1999; Massel and Brinkman, 1999). However, there has been comparatively little emphasis placed on examining the character and importance of wave processes on reef flats for the formation, change and stability of reef sedimentary landforms such as beaches, sand aprons and reef islands.

Despite dramatic energy losses at the reef edge, residual wave energy still leaks onto reef flat surfaces. Transmission of this energy onto reef surfaces and toward shorelines is strongly influenced by tidal stage. Transfer of wave energy across reef flats to shorelines is greatest at higher tidal stages (Brander et al., 2004) and in situations where the wave height is small compared with water depth (Kench and Brander, 2006).



Once on reef flats energy travels toward island shorelines. Studies have shown that the maximum size of waves on reef flats is controlled by water depth, with the maximum wave height approximately 0.6 times the water depth (Gourlay, 1994; Brander et al., 2004; Kench and Brander, 2006; Kench et al., 2009). Consequently, as water depth decreases toward shore wave height also diminishes. In addition waves can lose their energy through interaction with the reef surface. Consequently, this frictional loss in energy is dependent on the width of the reef flat surface. In general, reduction in energy increases with reef flat width (Kench and Brander, 2006).

A.4 Summary Coastal Processes Regime: South Tarawa

The physical condition of each section of coast results from the interaction of waves and currents with the existing coastline and the available supply of sand and gravel. Figure A.2 presents a summary of the coastal process regime in South Tarawa and represents a synthesis of the wind, wave and current processes that are able to influence the coastline transport sand and gravel, cause coastal erosion or coastal accretion (land building).

On coral reef systems the exposure of coastlines to energy (waves and currents) is controlled by:

n The direction and magnitude of ocean swell that is delivered to the coral reef. In general prevailing swell propagates from the east around Tarawa.

n The elevation and width of the reef which act to reduce incident ocean swell energy reaching

shorelines. Coral reef platforms cause ocean swell to transform and break, releasing wave energy at the reef edge. In general, higher reef platforms are more efficient at promoting wave breaking. The reef platform at Tarawa is considered to be elevated and is exposed at lower tidal stages and is efficient at breaking incident wave energy.

Wave energy that is not released at the reef edge travels onto the reef platform where it can form secondary waves. In this instance wave breaking can occur at island shorelines. However, as waves travel across a reef platform interaction with the reef surface causes waves to lose their energy. Therefore, wider reef flats are more efficient at extracting energy from waves.

In summary wave energy is greatest on prevailing windward locations and where the distance from island shoreline to reef edge (the reef flat) is narrow. In contrast, wave energy is generally lowest at leeward locations and where the reef width is large.

Based on these principles the coastline of South Tarawa has been divided into a number of broad exposure settings. The different categories are presented in Figure A.2 and summarised in Table A1. This classification reflects broad differences in the amount of wave energy reaching island shorelines, and the magnitude of wave setup and storm surge at island shorelines.


Figure A.2. Summary of coastal process regions in South Tarawa.

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The assessment divides the shoreline into six categories. On the ocean shoreline there are three divisions which reflect differences in gross wave energy conditions from the exposed high energy eastern end of South Tarawa to the lower energy western South Tarawa ocean shoreline (Table A1). The lagoon shoreline is also divided into three sections which also reflect differences in incident energy. However, on the lagoon shoreline the energy gradient is reversed. Lowest energy is the eastern lagoon shoreline. The incident energy conditions increase toward the west as wind generated waves increase in height and as ocean swell wraps around the western end of Betio.

The process assessment also provides a coarse-scale indication of the direction of alongshore sediment transport. On the ocean shoreline it is inferred that alongshore sediment transport occurs toward the west. However, it is likely that there are small pocket beach and sections of coast in which there is little alongshore sediment flux. On the lagoon shoreline it is also inferred that alongshore sediment transport is toward the west. However, it is also likely that there is easterly drift along the lagoon shoreline of Betio in response to swell refraction around the western tip of the atoll.

Table A1 summarises these categories. These coarse divisions will be used in the Coastal Condition Assessment.

Table A.1. South Tarawa energy setting categories.

Orientation Energy Key factors Location on Tarawa

Ocean High Windward setting Narrow reef width <200 m

East to southeast ocean shoreline

Medium Non-windward setting Intermediate reef width 200 – 500 m

Central southern ocean shoreline

Low Leeward setting Wide reef width >500 m

Western southern shoreline

Lagoon Low Sheltered lagoon setting (upwind) Eastern lagoon shoreline

Medium Moderate fetch lagoon setting Central lagoon shoreline

High Long fetch lagoon setting (downwind) Western lagoon shoreline


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Appendix 3 – MapInfo Procedures Document


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Appendix 4 – MapInfo ProViewer User Guide


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Appendix 5 – High Risk Sites Identified by the South Tarawa Coastal Condition Assessment

(FSS0942 - FS – 06/2009)

The following sites have been identified as high risks sites as part of the South Tarawa Coastal Condition Assessment. Six sites need to be selected for outline design of shoreline protection works as part of the FS6 project.

Beca has recommended six sites from the list in table 1 below where works could be undertaken. These sites have been selected to demonstrate a range of options for undertaking shoreline protection works i.e. ocean and lagoon shorelines, causeways, airports, protecting the main road where existing structures are present and where there are no existing structures. This is to build capacity within the MPWU to design a range of options for shoreline protection works.

It is suggested that the Government of Kiribati, through the Foreshore Management Committee, consider the recommendations provided by Beca and advise the six sites which are to be taken through outline design as part of the FS6 project.


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Figure 1 – Approximate Location of High Risk Sites Identified from the South Tarawa Coastal Condition Assessment

6 8 12

1 2 3

4

5

9 10 11

13

14 15

7


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Table 1 – List of High Risk Sites Identified from South Tarawa Coastal Condition Assessment

Site Code Description Location Asset at risk Length (m) Existing Structure Energy

1 BNC_12 Bairiki – Nanikaai Causeway Lagoon Main road 100 N High

2 TNC_2 Teaoraereke – Nanikaai Causeway Ocean Main road 20 Y Medium

3 RD_2 Main road in Teaoraereke Lagoon Main road 30 N Medium

4 RD_10 Main road in Antenon Lagoon Main road 75 N Medium

5 ATC_4 Ambo – Taborio Causeway Lagoon Main road 200 Y Medium

6 RD_12 Main road in Antenon Lagoon Main road 200 N Medium

7 RD_14 Main road in Taborio Lagoon Main road 20 N Medium

8 RD_20 Main road in Taborio Lagoon Main road 170 N Medium

9 RD_24 Main road in Tangintebu Lagoon Main road 20 N Medium

10 RD_51 Main road in Nawerewere/Temaiku Ocean Main road 30 Y High

11 RD_54 Main road in Nawerewere/Temaiku Ocean Main road 100 N High

12 RD_57 Main road in Bikenibeu/Nawerewere Ocean Main road 80 N High

13 RD_60 Main road in Bikenibeu/Nawerewere Ocean Main road 70 Y High

14 AC Main road to Bonriki Airport Lagoon Main road 200* N Low

15 RUN_1 Bonriki Airport runway eastern end Ocean Airport runway 300 Y High

* The actual length of coast at risk is 2000m however it is proposed to undertake works on approximately 200m as part of the FS6 pilot project.

Note: The shaded sites are those recommended for outline design under the FS6 project. The South Tarawa Coastal Condition Assessment does not conclude Tungaru Hospital to be a high risk site and therefore has not been included in the above list, however it can be considered as part of FS6 if requested by the GoK.



Site 1 Bairiki – Nanikaai Causeway (BNC_12) Lagoon shoreline Length 100m

Site 2 Teaoraereke – Nanikaai Causeway (TNC_2) Ocean shoreline Length 20m

Site 3 Teaoraereke (RD_2) Lagoon shoreline Length 30m



Site 4 Antenon (RD_10) Lagoon shoreline Length 75m

Site 5 Ambo – Taborio Causeway (ATC_4)

Lagoon shoreline Length 200m

Site 6 Antenon (RD_12) Lagoon shoreline Length 200m



Site 7 Taborio (RD_14) Lagoon shoreline Length 20m

Site 8 Taborio (RD_20) Lagoon shoreline Length 170m

Site 9 Tangintebu (RD_24) Lagoon shoreline Length 20m



Site 10 Nawerewere/Temaiku (RD_51, previously NAW_7D) Ocean shoreline Length 30m

Site 11 Nawerewere/Temaiku (RD_54, previously NAW_4) Ocean shoreline Length 100m

Site 12 Bikenibeu/Nawerewere (RD_57, previously NAW_1) Ocean shoreline Length 80m



Site 13 Bikenibeu/Nawerewere (RD_60) Ocean shoreline Length 70m

Site 14 Bonriki Airport Causeway (AC) Lagoon shoreline Length 2000m

Site 15 Bonriki Airport Runway eastern end (RUN_1) Ocean shoreline Length 300m

Documents

NZ1-2406103-South Tarawa Coastal Condition Assessment FINALclimate.gov.ki/wp-content/...Coastal...Assessment.pdf · Coastal exposure reflects whether the coastline is exposed to coastal