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Barriers, canals and fake islands: how we can save cities from rising sea levels

The Conversation

This article was originally published on The Conversation. Read the original article.

Sally Brown, University of Southampton; Ivan Haigh, University of Southampton, and Robert Nicholls, University of Southampton

Extreme storms and rising sea levels will threaten the existence of coastal cities worldwide, unless preventative action is undertaken. With population growth and sea-level rise set to continue, research has estimated that by 2050, we can expect more than US$1 trillion worth of damages per year to be incurred by 136 of the world’s largest cities, if there is no attempt to adapt.

The game changer came in 2005, when we saw one of the most active hurricane seasons in US history. Hurricane Katrina, the fifth hurricane of that season, resulted in nearly 1,600 deaths. Almost half of these fatalities occurred in New Orleans: 80% of the city was flooded, at a cost of US$40 billion. When the water subsided, so did the population: ten years on, the city that used to house 500,000 is now home to only 300,000 people.

There are a number of ways to go about changing cities to account for rising sea levels: we can raise coastal defences, build houses on stilts, or simply move cities and their populations away from the coast. Which of these strategies works best was one of many questions set out in Climate Change: A Risk Assessment – a new report led by Sir David King and the Foreign and Commonwealth Office.

Constant threat

Globally, sea levels have been remarkably stable since civilisation started to develop several thousand years ago. During the 20th century, sea levels rose about 17cm, at an average rate of 1.8mm per year. Over the past few decades, that rate has doubled to more than 3mm per year. This trend is expected to continue and accelerate. According to the latest Intergovernmental Panel on Climate Change report, the sea level is projected to rise up to 1m by 2100. If the large ice sheets of Greenland and Antarctica melted, even higher rises are considered possible, albeit highly uncertain.

Importantly, if carbon emissions are stabilised, or even decrease, the sea level will continue to rise for many centuries, as the deep ocean slowly warms and the large ice sheets reach a new equilibrium. Simply put, sea-level rise is here to stay. It is likely to lead to greater flooding, salinisation (the build up of salt in surface and groundwater) and erosion in coastal areas, affecting millions of people worldwide and costing billions of dollars of damage.

The devastation of Typhoon Haiyan.
EU Humanitarian Aid and Civil Protection/Flickr, CC BY-ND

The high costs of economic damage and loss of life are becoming less acceptable in a world where extreme weather events can be accurately forecast and coastal protection is possible. In many parts of the world, damages and loss of life remain high, as seen during Typhoon Haiyan, which hit the Philippines in 2013. Preparing coastal cities for extreme events and adapting them to cope with sea-level rise remains challenging: King’s report highlights the engineering, financial and socio-political limits of the adaptation challenge.

But cities are starting to embrace these challenges. For example, last year, Boston put forward the bold, novel idea of becoming an American Venice – a city full of canals to hold water as sea levels rise. New York has considered building a barrier to keep water out, in light of the fact that, with a 1m rise in the sea level, a 1-in-100 year event (that is, a severe storm one would expect to occur once every 100 years) could become 200 times more likely to occur.

London has also developed a range of flexible options that would protect the Thames Estuary against up to 5m of sea-level rise. These include raising defences, implementing flood storage and constructing a new and bigger Thames Barrier further downstream.

Developing better cities

In developing countries, few cities are preparing for sea-level rise, despite the awareness that this is a long-term hazard. Developing cities also frequently have rapid population growth. In Shanghai and Kolkata more than 400,000 people live less than 2m above the present-day sea level. A rise of 1m will increase the frequency of a current 1-in-100 year event by 40 times in Shanghai, and about 1,000 times in Kolkata.

Local ground subsidence is another factor to worry about. This involves the sinking of the land relative to the sea due to natural and sometimes human processes (such as groundwater withdrawal). Local ground subsidence will worsen conditions in about a quarter of coastal cities – namely, those built on susceptible deltaic soils (those at the mouth of a river).

Malé’s protective tetrapods.
Sally Brown, Author provided

Small islands and their cities are also under serious threat from sea-level rise as they are low-lying, remote and dispersed in their territories, and often have limited financial resources. Far from being a green, spacious island, Malé – the capital of the Maldives – is one of the world’s most densely populated cities. Building protective structures is one way of reducing the impacts of extreme events: Malé is surrounded by a sea wall and giant tetrapods (a four-pronged concentrate structure about 2m high). But a lack of space limits future coastal protection.

To overcome this, a new island has been constructed, Hulhumalé, with sea-level rise also in mind. The solution to sea-level rise is simply to build upwards: The island was raised to 2m above present day sea level to protect against storms. This buys time, but moving into the late 21st or early 22nd century this may not be enough. Other Maldivian islands are following suit, with the Safer Islands programme selectively raising parts of islands. This may help the parts of the country, but clearly much more work is required to ensure the long-term prospects of this fragile island nation.

Ultimately, these case studies show us that there’s no one-size-fits-all approach to adapting cities to rising sea levels. Rather, the best bet for cities to adapt against rising sea levels is to dare to be different. Both engineering design, government authorities and social attitudes must acknowledge that change needs to occur, if we’re to avoid disaster.

Sally Brown, Research Fellow, University of Southampton; Ivan Haigh, Lecturer in Coastal Oceanography, University of Southampton, and Robert Nicholls, Professor of Coastal Engineering, University of Southampton

Blog, personal

I am back with a new web-site design

My last post was on the 4th January 2014 – almost 9 months ago! The year has been a very busy one for me so far, but that I know is not a good excuse for my lack of posts. Anyway I am back, feeling refreshed have a months holiday. One of the highlights of my holiday was building sea creatures on the beach at New Quay, Wales.

I hope to post more regularly in the coming weeks and months. For a fresh start I have also changed the design of the web site – hope you like it.

Blog, coast, coastal flooding, science, sea level, storm surge, tides

Fieldwork at the Steart managed realignment scheme, Somerset, September 2014

My new PhD student Clementine Chirol has recently been undertaking field work at the Steart peninsula Managed Realignment Scheme.

Steart

The Steart peninsula is the largest managed realignment scheme undertaken in the UK, with 400 hectares of new habitats created to compensate for the losses related to coastal squeeze. To that end, flood defences are moved further inland and previously reclaimed farmlands are opened up to tidal inundation. As saltmarshes naturally attenuate wave and tide energy, the new flood defences will be more durable.

The project was designed and modelled by CH2M Hill on behalf of the Environment Agency; the construction phase was carried out by Team Van Oord. The completed site is now managed by the Wildfowl and Wetlands Trust (Tim McGrath).

The flood defences were breached on 1st September 2014 by Team Van Oord. With the increasing spring tide, the site’s channel was first flooded to full bank on the 7th. The highest spring tide was reached on 10th September (Hinkley Point: 7.05 m).

steart peninsula breach

The aim of Clementine’s PhD is to monitor the morphological evolution of the entry channel and creek network over several years as the site transitions to a more natural shape. Results from this project should help improve the design of future realignment schemes.

This early fieldwork campaign undertaken around the time of the first inundation focused on the creek system: in fact, due to the rapid erosion and turbulent flow, no deployment could be made in the breach area. We had two objectives: firstly, to perform a baseline survey of one of the creeks’ morphology and sedimentology that would help quantify all future changes. Secondly, to observe the early effects of the tide on the morphology and sediment strength at the Steart managed realignment site.

TGPS survey o this end a GPS survey of one of the creeks’ outline was realised, as well as several cross-sections along the length of the creek. Stakes in the ground were used to evaluate the accumulation or erosion of sediment at the banks. Sediment samples and syringe cores were taken to assess the bulk density and the organic matter concentration. The cohesive strength of the sediment was measured every day along the creek with a CSM (Cohesive Strength Meter) during the time of the fieldwork. The successive inundations of the site were monitored by two Gopro cameras covering the studied creek and the entry channel.

This fieldwork was a great opportunity to witness the realisation of an ambitious realignment project. It will also provide a valuable baseline to monitor the evolution of this area.

Blog, coast, coastal flooding, extreme events, flooding, science, sea level, storm surge

New paper just published – Variability in coastal flood risk

We have just had a new paper (Assessing the variability in extreme high water levels for coastal flood risk assessment) published in the Journal of Geophysical Research-Oceans – see here.

The probability of extreme storm-tide events has been extensively studied, however the variability within the duration of such events, and implications to flood risk, is less well understood. This research quantifies such variability during extreme storm-tide events (the combined elevation of the tide, surge, and their interactions) at 44 national tide gauges around the UK. Extreme storm-tide events were sampled from water level measurements taken every 15 minutes between 1993 and 2012. At each site, the variability in elevation at each time step, relative to a given event peak, was quantified. The magnitude of this time-series variability was influenced both by gauge location (and hence the tidal, and non-tidal residual characteristics) and the time relative to high water. The potential influence of this variability on coastal inundation was assessed across all UK gauge sites, followed by a detailed case study of Portsmouth. A two-dimensional hydrodynamic model of the Portsmouth region was used to demonstrate that given a current 1 in 200 year storm-tide event, the predicted number of buildings inundated differed by more than 30% when contrasting simulations forced with the upper and lower bounds of the observed time-series variability. The results indicate that variability in the time-series of the storm-tide event can have considerable influence upon overflow volumes, hence with implications for coastal flood risk assessments. Therefore, further evaluating and representing this uncertainty in future flood risk assessments is vital, while the envelopes of variability defined in this research provides a valuable tool for coastal flood modellers.

 

MATLAB Handle Graphics

 

Blog, coast, coastal flooding, energy, flooding, Journal paper, science, sea level, waves

New paper just published in Coral Reefs – Influence of the Great Barrier Reef on wave attenuation

We have just had a new paper (The large-scale influence of the Great Barrier Reef matrix on wave attenuation) published in the journal Coral Reefs. Click here to see a copy. This was our press release:

New research has found that the Great Barrier Reef, as a whole, is a remarkably effective wave absorber, despite large gaps between the reefs. This means that landward of the reefs, waves are mostly related to local winds rather than offshore wave conditions. 

As waves break and reduce in height over reefs, this drives currents that are very important for the transport of nutrients and larvae. This reduction in wave height also has implications for shoreline stability. Transition 

The Great Barrier Reef in Australia is the largest coral reef system in the world, extending 2,300 km alongshore. The reef matrix is a porous structure consisting of thousands of individual reefs, with gaps in between. The porosity varies in that is it much lower in the north where the continental shelf is narrow and there is extensive reef flats; and is greater in the south where the shelf reaches up to 300 km wide and there are extensive lagoons. 

Previously, there have been several studies investigating how individual reefs in the Great Barrier Reef influence ocean waves. However, this was the first, comprehensive, large-scale study of the influence of an entire offshore reef system on ocean wave transmission. The researchers used a 16-year record of satellite altimeter measurements of wave heights. 

The team was led by Dr Shari Gallop, Research Fellow in Geology and Geophysics at the University of Southampton, and included Dr Ivan Haigh, also from the University of Southampton; Professor Ian Young, Vice-Chancellor of the Australian National University (ANU); Professor Roshanka Ranasinghe, Professor of Climate Change Impacts and Coastal Risk (UNESCO-IHE, Deltares, ANU), and Dr Tom Durrant (Bureau of Meteorology, Australia). 

The aim was to see how wave height reduction is influenced by the porosity of the reef matrix, sea level and wind speed. Dr Gallop says: “There was no evidence that in less porous areas wave heights are lessened. This is because individual reefs, like islands, cast a ‘wave shadow’ over a large area, so that a matrix of individual reefs is remarkably efficient at reducing waves.” 

Dr Haigh adds: “As sea level varies, due to tides and storm surges, the submergence of the reef in water also varies. Wave heights are not strongly affected by water level over the reef matrix.” 

Professor Young says: “A number of previous studies have investigated the attenuation (height reduction) of ocean waves as they spread across individual coral reefs. This research is unique as it looks at the impact of a large scale reef matrix, such as the Great Barrier Reef, on wave height. Such studies are important in providing wave climate information for physical, biological and planning processes in such areas.” 

This new research, published in Coral Reefs, has important implications for wave modelling near reef systems. This is because models that consider individual reefs only may underestimate the wave reduction potential of a full reef matrix. 

Professor Ranasinghe comments: “Plans are under-way to investigate the wave attenuation characteristics over the reef in more detail, using sophisticated numerical modelling. It is of critical importance to know the potential impacts of climate change effects, such as sea level rise and variations in wave conditions, on wave attenuation and current circulation on the Great Barrier Reef. This will aid in the sustainable management of this natural wonder and the surrounding marine national park.”

 

 

 

 

Blog, coast, Journal paper, science, sea level

New paper in Nature Climate Change – Shifting perspectives on coastal impacts and adaptation

Here is our new paper in Nature Climate Change. The below is from our press release:

Coastal regions under threat from climate change and sea-level rise need to tackle the more immediate threats of human-led and other non-climatic changes, according to a team of international scientists.

The team of 27 scientists from five continents, led by Dr Sally Brown at the University of Southampton, reviewed 24 years of Intergovernmental Panel on Climate Change (IPCC) assessments (the fifth and latest set being published in 2013 and 2014). They focused on climate change and sea-level rise impacts in the coastal zone, and examined ways of how to better manage and cope with climate change. 

They found that to better understand climate change and its impacts, scientists need to adopt an integrated approach into how coasts are changing. This involves recognising other causes of change, such as population growth, economic development and changes in biodiversity. Dr Brown emphasised that: “Over the last two and half decades, our scientific understanding of climate change and sea-level rise, and how it will affect coastal zones has greatly increased. We now recognise that we need to analyse all parts of our human and natural environments to understand how climate change will affect the world.”

The scientists also acknowledged that long-term adaptation to climate change can greatly reduce impacts, but further research and evaluation is required to realise the potential of adaptation. “Many parts of the coast can, with forward planning, adapt to sea-level rise, but we need to better understand environments that will struggle to adapt, such as developing countries with large low-lying river deltas sensitive to salinisation, or coral reefs and particularly small, remote islands or poorer communities,” said Dr Brown.

For example, in the Maldives, many small, remote low-lying islands are at risk from climate change and will struggle to adapt. But around the densely populated capital city and airport, adaptation has already occurred as land claim is a common practice in order to relive population pressure. Sea-level rise has already been considered into newly claimed land. Thus in decades to come, potential climate change impacts, such as flooding, will be reduced for this island, benefiting both the local population and economy.

Dr Jochen Hinkel from Global Climate Forum in Germany, who is a co-author of this paper and a Lead Author of the coastal chapter for the 2014 IPCC Assessment Report added: “The IPCC has done a great job in bringing together knowledge on climate change, sea-level rise and is potential impacts but now needs to complement this work with a solution-oriented perspective focusing on overcoming barriers to adaptation, mobilising resources, empowering people and discovering opportunities for strengthening coastal resilience in the context of both climate change as well as existing coastal challenges and other issues.”

This new research, published as a commentary in Nature Climate Change, will help in the understanding of the impacts of climate change and how to reduce impacts via adaptation. Its multi-disciplinary approach could be useful if future IPCC assessment reports are commissioned.

Australia, Blog, coastal flooding, Journal paper, science, sea level, storm surge

New paper just published – land sinks around parts of Australia with passage of tropical cyclones.

We have just had a new paper (‘Non-linear motions of Australian geodetic stations induced by non-tidal ocean loading and the passage of tropical cyclones’) published in the journal of Journal of Geodesy – see here.

In this paper we examined movements in land around Australia using time-series from stations fitted with continuous GPS. Most people think the land is completely stable, but actually the land moves each day by a few millimetres for a variety of reasons. Here we show that land around parts of Australia sunk when cyclone Yasi crossed the Australian coast in January/February 2011. This cyclone generated a large storm surge and the extra weight of this large volume of water near the coast actually resulting in the land dropping slightly.

A earlier study (see here) showed the same thing happens around the North Sea coastlines when big storm surges happen there.