SimpleCoast: Simple assessments of Coastal Problems and Solutions Alessio Giardino(1), Leo van Rijn(2), Ellen Quataert(1), Andrew Warren(1), Ad Jeuken(1), Kees Nederhoff(1), Nicolas Desramaut(3) (1)
Deltares, Unit Marine and Coastal Systems Rotterdamseweg 185, P.O. Box 177, 2600 MH Delft, The Netherlands Tel: + (31) 88 335 8132 Fax: +31 (0)88 335 8582 E-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]; (2)
WWW.LEOVANRIJN-SEDIMENT.COM (LVRS-Consultancy) E-mail:
[email protected] (3)
GFDRR, The World Bank, 1818 H Street, NW Washington DC 20433, United States
[email protected]
Abstract Small Island States are very susceptible to the impact of natural disasters. Flooding events due to cyclones, tsunamis, storm surges, extreme rainfalls, flash floods from rivers, and coastal erosion can have devastating consequences on the populations and the economies of those islands. Moreover, these effects are likely to be exacerbated by the effects of climate change and sea level rise. As their population, agricultural land and infrastructure tend to be concentrated in the coastal zone, any rise in sea level will have significant effects on their economies and living conditions. Hence, a very efficient way to support resilient socio-economic development of these islands is by placing them in the conditions of predicting the effects of natural disasters, and providing them with suitable solutions to mitigate the relative effects. The aim of SimpleCoast is to develop simple and freely-available tools, tutorials and knowledge notes downloadable from a website (http://www.simplecoast.com/), adequate for a data-limited environment, and to combine them with targeted capacity building actions. This paper provides a general description of the available material, with a practical application example to a Small Island State (Ebeye Island on Kwajalein Atoll in the Republic of the Marshall Islands).
Introduction In most islands around the world, people, agricultural land, tourist resorts and infrastructures are concentrated in the coastal zones, and are thus especially vulnerable to any change in climate and rise in sea level. Two thirds of the countries with the highest disaster losses relative to GDP are Small Island States – with average annual losses between 1 and 9 percent of GDP. These averages hide extremes – sometimes a single disaster can overwhelm an island’s entire economy (GFDRR, 2016; https://www.gfdrr.org/small-island-states-resilience-initiative). On the other hand, Small Island States in developing countries often lack the data and technical capacity in hydraulic and coastal engineering as well as the socioeconomic skills to properly assess the most cost-effective interventions appropriate to each vulnerable coastal area. With this in mind, the SimpleCoast project aimed at developing simple and freely-available tools, tutorials and knowledge notes downloadable from a website (http://www.simplecoast.com/), adequate for a datalimited environment, and combined them with targeted capacity building actions. This paper provides a general description of the available material, i.e. the SimpleCoast toolkit. In addition, the toolkit is applied on the island of Ebeye in the Kwajalein Atoll. The SimpleCoast tools are used to quantify the impacts of coastal hazards on Ebeye and possible interventions to mitigate these hazards are designed.
SimpleCoast toolkit The website All the available material can be freely downloaded via the website: http://www.simplecoast.com/ (Figure 1). The website is arranged under the following main topics, as described in the following sections: § § § §
Knowledge Notes Tools & Tutorials Events & Trainings Contact
Fig. 1: SimpleCoast website (http://www.simplecoast.com/). Knowledge notes The knowledge notes provide basic insight and definitions, generally used in coastal engineering applications and coastal zone management. The notes have been arranged under these four main topics: §
Adaptive approaches to coastal zone management: provides an overview of approaches, options and practices to Integrated Coastal Zone Management. This includes the formulation of general objectives and a framework of analysis for the set up of an ICZM plan, which guides the user to make robust and flexible choices to cope with future undertainties.
§
Coastal processes and problems: focuses on the description of the main physical processes in coastal areas, and with a particular emphasis to Small Island States. A collection of basic formulas and graphs widely used in coastal engineering is also provided. Those formulas, among others, are also implemented in the freely available tool-kit provided alongside the knowledge notes.
§
Data collection and monitoring: describe simple methods for data collection and monitoring, crucial for the understanding of the coastal systems and to evaluate possible changes to the coastal indicators. Coastal indicators in particular are very useful to evaluate the efficiency of different adaption options.
§
Coastal adaptation solutions: a number of different adaptation technologies are described and grouped under the following main approaches: soft protection, hard protection, accommodation and retreat. For each of the approaches, basic technologies, advantages and drawbacks are described alongside a rough costing. Simple tools for a preliminary design of these options are also presented as part of the simple tools.
Tools and tutorials Simple tools have been developed for a preliminary assessment of coastal problems and solutions in coastal areas. All tools are straightforward to use (.xls based) and can be freely downloaded from the website. Each tool is illustrated by short tutorials, describing relevant formulas and practical application examples. The full list of tools is shown in Table 1, while a screenshot of one of these tools (Wavemodels.xls) is shown in Figure 2. Table 1: Full list of tools accompanied by a brief description for each of them. Tool name Pathway Generator Flooding.xls Waveparameters.xls Sedimentparameters.xls Beachnourishment.xls Armour.xls Littoral.xls Dunebeacherosion.xls Riverflowandsandtransport.xls Wavemodels.xls Vegetation.xls
Description Tool to design and present adaptation pathways Calculate flood levels for extreme conditions Calculate typical wave characteristics Calculate typical sediment characteristics Calculate the lifetime of a beach nourishment Calculate armour size for seadikes, revetments, breakwaters and toe protections Calculate the longshore transport and dune erosion Calculate dune and beach erosion during storms Compute the flow discharge and sand transport in a river Compute wave propagation using two simple wave models Compute the impact of vegetation on wave propagation using two simple wave models
Fig. 2: General layout of the Wavemodels.xls tool. Events and trainings Events and hands-on trainings are organized as part of SimpleCoast. During the training real problems at pilot locations are analyzed and assessed with the support of the tools and materials developed as part of the project. Trainings include: interactive theoretical classes supported by the help of a serious game, field work and data collection, data analysis and modeling using the toolkit developed as part of this project. Pictures from one of the trainings carried out at the island of Sao Tome are shown in Figure 3.
Fig. 3: Pictures from a SimpleCoast training session on the island of Sao Tome. Application example Case study Ebeye is built on a small islet on the south eastern side of Kwajalein Atoll (8.78°N 167.74°E), see Figure 4. The part above sea level stretches about 2 km from north to south and is approximately 250 m wide bordering a large lagoon to the east and
the open ocean to the west. The lagoon is shallow with an average depth of approximately 40 m. On the eastern ocean side, the islet is fronted by a reef flat. This reef flat varies slightly in width between 100-130 m. From there on, the depth quickly increases, reaching depths of ~6000 m just a few kilometres out from the coast. The islet is covered entirely with buildings and infrastructure and hence densely populated. About 12,000 inhabitants live in an area of merely 0.36 km2, i.e. 1 person per 30 m2.
Fig. 4: Aerial view of Kwajalein Atoll (left) and Ebeye Island (right). The island, already in the current situation, is highly affected by natural hazards such as flooding due to swell waves, typhoons, tsunamis, wind waves from the lagoon and coastal erosion. The frequency and impact of these hazards is likely to increase in the future as a result of sea level rise and climate change effects (Quataert et al., 2015). In this application example, we apply the SimpleCoast tools to Ebeye to quantify the impacts of typhoons and extreme swell conditions, and design possible interventions to mitigate these hazards. Wave propagation Giardino et al. (2016) quantified offshore and nearshore coastal hazards and risks at Ebeye. In Table 2, the maximum offshore wave height and storm surge at Ebeye, as derived from this study, are provided. In particular, values are available for four return periods (RP = 5, 10, 30 and 50 years). This information is used as input in the Wavemodels.xls tool in order to calculate the wave heights at the beach. The resulting wave height transformation from offshore towards the beach for a 5 year return period is shown in Figure 5. The resulting maximum wave height and water level at the beach for all different return periods are shown in Figure 6. Coral reefs affect the nearshore hydrodynamics by dissipating short wave energy, generating long waves and increasing water levels due to wave-induced setup. However, long wave effect is not included in the simple formulations for wave transformation available through the SimpleCoast toolkit. To include the impact of long waves, the storm surge level is increased with half of the long wave height, which can be computed for example by using a process-based model such as XBeach (Roelvink et al., 2009).
This is a reasonable assumption, since the long wave period (>120 seconds) will have an effect on the water levels. Table 2: Maximum offshore significant wave height (Hs) and storm surges (SSL) for Ebeye as a result of typhoons and swell events, from Giardino et al. (2016). Typhoons Return period in years 5 10 30 50
Maximum Hs (m) 3,16 4.61 7,21 8,57
Maximum SSL (m) 0,08 0,10 0,15 0,16
Swell Maximum Hs (m) 3.56 3.74 4.04 4.17
Maximum SSL (m) 1.37 1.40 1.43 1.45
Fig. 5: Results from Wavemodels.xls on wave propagation across a cross-section of Ebeye for a 5 year return period typhoon wave conditions and SSL.
Fig. 6: Maximum short wave height and storm surge levels at the beach, for different return periods, both for swell and typhoon wave conditions.
Flooding and overtopping With the Flooding.xls tool, the overtopping discharges at Ebeye are calculated using the formulations for smooth and rough impermeable slopes (EUROTOP Manual, 2007). The estimated wave heights and water levels at the toe of the beach in Figure 6 will be the input for the calculations. Similar results can also be obtained by using the Overtopping Neural Network calculator : https://www.deltares.nl/en/software/overtopping-neural-network/.
Figure 7 shows overtopping rates at a 4 meter high revetment. Already for a return period of 1 year overtopping of about 93 l/m/s occurs. The overtopping increases when the return period increases. Considering sea level rise projections, the amount of wave overtopping will increase as shown for the RCP 4.5 scenario. Note that for large overtopping rates (e.g. q > 500 l/m/s) the revetment will behave rather as a weir than as a revetment. Allsop et al. (2009) suggests that damages occur to paved or armoured promenade behind a seawall when a mean discharge exceeds 50 l/s/m, indicated as dashed red line in Figure 7.
Fig. 7: Overtopping discharges calculated with Flooding.xls for different time horizons in the RCP4.5 scenario (coloured bars) and return periods (horizontal axis). Beach erosion During large storm events the fine sediment fraction (i.e. sand) is eroded and transported to the reef within a short time period. With the Dunebeacherosion.xls tool storm-induced beach erosion is calculated using the Van Rijn (2009) formula. Figure 8 shows maximum dune erosion volumes for the swell events and typhoons for different
time horizons (coloured bars) and return periods (horizontal axis). Dune erosion increases as the return period increases, which is the result of the increasing wave height and water level. Additionally, erosion volumes become higher as the time horizon increases, due to the rise in sea level.
Fig. 8: Beach erosion volumes calculated with Dunebeacherosion.xls for different time horizons in the RCP4.5 scenario (coloured bars) and return periods (horizontal axis). Armouring The large overtopping discharges (Figure 7) and beach erosion volumes (Figure 8) suggest that some sort of coastal protection is required in order to improve the current and future living conditions at the island. Coastal structures such as seawalls, groins, and breakwaters and, where possible, more sustainable nature-based flood defences, can be constructed to mitigate flooding and erosion. Hard structures are commonly made of rock or concrete elements and therefore stability is required to grant structural integrity of the proposed structure. The required stone size for a stable structure under breaking waves at a sloping surface can be calculated with the Armouring.xls tool. Figure 9 shows the resulting stability-required nominal stone diameter using Van Gent (2014). For a return period of 1 year a nominal stone diameter of 0.41 meter is required. The wave height at the toe of structures increases when the return period increases, resulting in a large required nominal stone diameter (0.52 meter for 1/50). Considering future sea level rise scenarios, the wave attack at the structure will increase. For example a 1/50 year storm under RCP 4.5 in 2100 requires a nominal diameter of 0.59 meter.
Fig. 9: Stability-required nominal stone diameter calculated with Armouring.xls for different time horizons in the RCP4.5 scenario (coloured bars) and return periods (horizontal axis). Dynamic adaptive coastal zone management In order to evaluate alternatives and set-up a long-term coastal management plan, the Dynamic Adaptive Policy Pathways (DAPP) approach can be used (Haasnoot et al., 2013). The Dynamic Adaptive Policy Pathways (DAPP) approach has been recently developed to better incorporate such uncertainties into adaptive planning methodologies and to enable the creation of longer-term adaptation strategies. User-defined objectives are used to connect short-term actions to potential future actions, and adaptation pathways are constructed to illustrate how short-term decisions can enable or constrain future options. An example of a fictitious adaptation pathway map for Ebeye is shown in Figure 10, generated using the ‘Pathways Generator’ software program (pathways.deltares.nl). In this example, three possible solutions suitable for different time windows are shown: construction of a rock revetment, construction of elevated houses, and finally relocation of the inhabitants combined with new land reclamation works. The example and the values in the example are only provided for illustrative purpose and have not been used in real-world planning for the island. Two SLR scenarios are also shown (RCP 4.5 and RCP 8.5). A possible pathway, represented by the orange dotted line, shows how the current situation will become unsustainable at year 2025 for RCP 4.5. Therefore, a rock revetment may be constructed and which can be effective until year 2045 for RCP 4.5 and year 2040 for RCP 8.5. At this time, the increase SLR will cause very frequent flooding events which are likely to affect most of the island. The only option available will be to start elevating some of the houses on stilts. Finally, in year 2090 (RCP 4.5), the only feasible option will be the
relocation of some of the inhabitants to other islands, which will need to be artificially heightened by the dumping of aggregate resources in combination with land reclamation works.
Fig. 10: Fictitious adaptation pathways map for Ebeye. The possible pathway described in the text is indicated by the dotted orange line. The pathways shown in Fig. 10, illustrate that there are still some options available before relocation becomes necessary. However a cost benefit analysis should always be part of the evaluation. For example from Fig. 10 the question could arise whether investment in the rock revetment would be worthwhile if it only postpones additional investments by 10-20 years? If this period is insufficient to provide an acceptable return on the investment, it may be preferred to move directly towards investing in subsidies to elevate vulnerable houses. Discussion and conclusion The application example shows that a first assessment of the impacts of natural hazards can be derived with the SimpleCoast tools, also at areas poorly covered by data. Subsequently, possible interventions to mitigate these hazards can be assessed using the knowledge notes. Quantification of the size of these interventions and long-term management approaches can be proposed using other SimpleCoast tools. The SimpleCoast tools and knowledge notes are useful for a first screening of problems and solutions in coastal zones. As the tools and knowledge notes are freelyavailable (http://www.simplecoast.com/) and easy to use, they are very suitable for local practitioners and policy makers, in particular for Small Island States. After this first screening and for a more in depth and accurate analysis, the use of process-based model, accompanied by the use of more accurate data, is recommended at most cases.
Acknowledgements The project has been kindly supported by the Water Partnership Program and frames within the Small Island State Resilient Initiative (SISRI) implemented by the GFDRR (The World Bank). We are also thankful to the Deltares research programme
“Climate Adaptation” which has co-financed the study. We are also thankful to our Deltares colleagues: Hans de Vroeg, Alex Capel, Almar Joling, Rens van den Bergh, Marjolijn Haasnoot for their support during the development of the SimpleCoast toolkit. Finally, we would like to thank Denis Jordy (World Bank) and Richard Croad (Gillrich Consulting Limited) for their help during the development of the Ebeye case study.
References Allsop, N. W. H., Bruce, T., Pullen, T., & van der Meer, J. (2008). Direct Hazards From Wave Overtopping – the Forgotten Aspect of Coastal Flood Risk Assessment ? 43rd DEFRA Flood and Coastal Management Conference, (July), 1–11. EUROTOP, 2007. Wave overtopping of sea defenses and related structures; Manual. Die Küste, Heft 73 Hallermeier, R.J. and Rhodes, P.E.(1988). Generic treatment of dune erosion for 100-year event. 21st ICCE, Malaga, Spain, 1197-1211 GFDRR, (2016). “The Small Island States Resilience Initiative (SISRI)”, SISRI Knowledge Note No. 1. Small Island States Resilience Initiative, The World Bank and Global Facility for Disaster Reduction and Recovery (GFDRR), Washington DC. Giardino, A., Nederhoff, K., Gawehn, M., Quataert, E. and Capel, A. (2016). Coastal risk assessment for Ebeye, Technical Report Deltares, 1230829-001-ZKS-0001. Haasnoot, M., Kwakkel, J. H., Walker, W. E. and ter Maat, J. (2013). Dynamic Adaptive Policy Pathways: A New Method for Crafting Robust Decisions for a Deeply Uncertain World. Global Environmental Change, 23(2), 485-498. Quataert, E., C. Storlazzi, A. van Rooijen, O. Cheriton, and A. van Dongeren (2015). The influence of coral reefs and climate change on wave-driven inundation of tropical coastlines, Geophys. Res. Lett., 42,6407–6415, doi:10.1002/2015GL064861. Roelvink, D., Reniers, A., Van Dongeren, A., Van Thiel de Vries, J., McCall, R., and Lescinksi, J. (2009). Modelling storm impacts on beaches, dunes and barrier islands. Journal of Coastal Engineering, 56, 1133-1152. Van Gent, M. (2014). Oblique wave attack on rubble mound breakwaters. Coastal Engineering. Van Rijn, L.C. (2009). Prediction of dune erosion due to storms. Jourmal of Coastal Engineering, 56, 441-457. Keywords: Simple Tools, Hazard Quantification, Design of Solutions, Disaster Risks and Reduction, Small Islands
