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What will happen with Åhus? - Will it again become a bay as it was once upon a time?

Two profound, life changing events are converging to create the most significant crises in modern time – the warming of the earth’s atmosphere by burning fossil fuels, and the rapid depletion of global petroleum and natural gas reserves. Huge crises will occur if we fail to take decisive action to reduce greenhouse gas emissions, with intensified weather events and rising sea levels as some of the examples. This is a case study concerning Åhus, a city on the eastern coast of Scania (southern Sweden) that shows a fearsome scenario. The method behind these pictures is developed with the support of the modern climate research that creates conditions to identify affected areas within housing and infrastructure, using Geographical Information Systems (GIS). We have made three-dimensional snapshots and 3D models of a simulated sea level rise to give you a realistic picture and understanding of the physical impacts of sea level rise of a familiar area.

Climate changes/ Case study Åhus

Still, despite the alarming forecast more and more people want to settle in these coastal locations. In Åhus, and along the south coast, lives about 10 000 people in addition around 5-7000 time

The question of future sea levels has become increasingly relevant over the years since the

residents (About 11 600 buildings).

IPCC presented its fourth Assessment Report (AR4) in January 2007 (IPCC, 2007) The assessment

Result and recommendations

included the effect of the ocean’s volume expansion caused by warming and the contribution from melting land-based ice sheets. AR4 stated 18-59 cm as a range of sea level rise, with regional variations. Later on in year 2011 a climate study conducted by the Copenhagen climate adaptation plan

Based on the projected inundation levels, lower bounds of IPCC projections (IPPC, 2007) as

indicated an average water elevation of just below 1 meter as probable until 2100 (Municipality of

well as high risk scenario of four meter flooding, using an increment of a one or two meter sea level

Copenhagen, 2011). The same year another knowledge compilation performed by AMAP (Arctic

rise, we have a series of pictures showing relatively small, but also huge catastrophes. In a case of

Monitoring and Assessment Program) 2011 (AMAP, 2011) dealt with the sea level trend. Their hy-

4-meter waves, in a heavy storm in year 2100, more than half of the community would be affected!

pothesis was that sea levels could rise between 0.9 m and 1.6 m from 1990 to 2100 and that a large

Figure 1.1: Scania.

part of the increase is due to the melting of the Arctic ice.

SMHI have produced a report for Kristianstad municipality with an estimated sea level rise of 0.8 meters and a maximum water level of about 2 meter in 2100 (Dahlman, 2008). The natural variability of sea level due to extreme climate may probably lead to further increased sea levels Figure 1.2: Map showing sea level rise inundation on the east coast of Scania (for location see frame in Figure 1.1).

with about two meters additionally. (Persson.G Et al., 2012) The GIS based calculations publis-

Sea Level Rise 1 Meter 2 Meters 3 Meters 4 Meters

Figure 1.2

hed in 2008 brought sharp reactions, like the author writes himself;

2.1 Sea level rise inundation (+1m). 6 houses (red colored) affected.

”We were surprised by how low-lying the large surfaces were compared to the feeling on site, as seen at Äspet in southern Åhus. Sometimes there is a beach dune that gives a sense of protection.”

(Dahlman, 2008)

The Report presented in 2008, also created a concern among the residents. A non-governmental organization consisting of holiday- and homeowners in Äspet was formed in order to protect Äspet beach (the eastern part). To protect the dune, that today protects the houses along the coastline, was their most important question. ”We, as we during summers and winters regularly visit Äspet beach, have for many years

Existing road network in Åhus

been able to observe a strong beach erosion between Snickarhaken and Revhaken. In addition, the beach has become much narrower in many places because of the ocean’s constant movement,

2.2 Sea level rise inundation (+2m). 786 houses (red and orange colored) affected.

storms and currents. ” (Letter to Michael Dahlman, Kristianstad municipality from the NGO ”Save the Äspet beach”, 2011-05-30) Higher groundwater, flood and erosion are all examples of threats caused by a raised sea

Road network affected by sea-level rise (+4 m)

level. The protective dune, along the coast, is extremely important; a higher sea level will increase

Pictures from the erosion in year 2007

erosion on this dune. Pictures form 2007 shows examples of when the sea severely reduced the important dune in this area. The pictures below (2.4) shows worst-case scenario of a 4-meter rise, a scenario possible if not the dune is strengthened and better protected in the future.

2.3 Sea level rise inundation (+3m) .3246 houses (red, orange and pink colored) affected .

IMPLEMENTATION OF OPERATIONS

(land use and buildings) Vector and raster format Selection of road network and bulidings

Raster format

Set sea level rise

Select layers of interest

Reclassify

Raster

convert to Vector

information about acres under sea level Not presented in the report

Vector

Intersect polygons

Tabulate Area calculation

Figure 2.4 The worst-case scenario, a 4-meter sea level rise, 6000 of the totally 11 600 houses affected

FINISH

Fastighetskartan,

DEM, Höjddata 2m ,

Raster

VISUALIZATION

Figure 1.2 Map showing sea level rise inundation

Maps showing houses and road network, inundation (1- 4 meter)

Orthophoto

DEM, Höjddata 2m Raster format

Visualisation using ArcScene

Method results showed the water surfaces tightly coherent in chunks, we decided to stay

on each level the inundation layers is clipped or extracted to qualitatively define

with the simplified “bathtube” method. A source of error is discovered, the sand

spatial analysis and to bring together multiple dimensions and analyzing rela-

each regions on this severity data. The flowchart (right) describes briefly the th-

dune, which today protects large parts of the settlement, is not counted in an

tionship. In this case, we looked at land use, hydrology and infrastructure of

ree parts of this project. The first part ”implementation of operations” contains

exposed position because of their topographical location today. On the contrary,

the study area, in order to model different sea level scenarios and thus identify

the four steps aforementioned.

I think erosion can quickly break down the barrier, when the sea level raises.

ways and containing multiple parameters such as storm, wave action and costal

Presented GIS analyses of rising sea levels

Figure 3.1 Flowchart

This could result in catastrophic consequences for large sections of society.

Discussion and conclusion

AMAP 2011. Snow, water, ice and permafrost in the Arctic. SWIPA 2011 Executive Summary.15 pp.

Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp. Københavns Kommune, 2011. Københavns Klimatilpasningsplan.

For a more accurate model of surface flooding around geological and man

Bergelo, M(2011) Havsvattenståndets påverkan längs Sveriges kust, SMHI OCANOGRAFI. Nr106. page12. Dahlman, M. Att planera för högre havsnivåExempel Kristianstad och Åhuskusten. Kristianstad Kommun.

erosion. This analysis is made by the “bathtube” approach. A cell is considered inundated if it’s elevation value is less than or equal to the projected sea lever

Graphical corrections Adobe Photoshop

the land use data is made to identify range of vulnerabilities. And last, step four,

vulnerable structures. Calculations of these scenarios can be done in many of

(land use and buildings) Vector format

visualisation

Export

Geographical Information System (GIS) is a powerful tool for doing

Fastighetskartan

(Oswald M.R and Treat. C, 2013). This process can simplified be described in

made features Oswald M.R and Treat. C proposes an alternative approach to

four steps. First, the DEM (höjddata 2m) is reclassified into output of land and

modelling sea level rise by accounting for hydrological connectivity to the major

sea level rise increments including possible sea level rise, the land value includes

water body. With this approach a cell is only considered inundated if at least

all other elevations. Second, a conversion into vector layers is made in order to

one neighbouring cell is adjacent to another major water body or inundated cell.

use the layers in processes with layers from Fastighetskartan (land use including

During the project a similar approach were considered, but as the cast study

houses). Third, an overlay of the inundation layers onto the selected layers from

area consists almost entirely of sand pits, (high risk of erosion) and also that the

Dahlman, M FloodWatch Kristianstad Prognossystem för översvämningar längs nedre Helge å, C4 Teknik, Kristianstad Kommun. Dahlman, M (2008) Konsekvenser för Åhuskustens bebyggelse vid en framtida, Bilaga 1 till Program för kustens utveckling från Åhus till Juleboda höjd havsnivå. Kristianstad Kommun. Persson.G, Sjökvist.E,Åström.S, Eklund, D. Andréasson, J. Johnell, Asp,M. Olsson.J och Nerheim.s, (2012) Klimatanalys för Skåne län, Rapport Nr 2011-52, Norrkäping:SMHI. IPCC 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.).

Oswald M.R and Treat.C (2013) Identifying sea level rise vulnerability using GIS: Development of a transit inundattion on modeling method. Department of civil and environmental Engineering, Bucknell University.

Johanna HOLGERSSON Advanced Digital Landscape Analysis with GIS, Swedish University of Agricultural Sciences


Etude d'impact de la montée des eaux  
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