Erosion Risk Assessment at the Bay of Vigsø

EROSION RISK ASSESSMENT AT THE BAY OF VIGSØ

Spring Semester 2016 – Geography Project

1. JUNI 2016 STUDENT: ROBIN MIKAELA KOTSIA Supervisor: Stig Roar Svenningsen Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 Table of Contents 1. Introduction ...... 3 1.1. Foreword ...... 3 1.2. Intro ...... 4 1.3. Problem formulation ...... 5 Research questions: ...... 5 1.4. Hypothesis ...... 5 2. Theory ...... 5 2.1. Geological history of the area of interest ...... 5 2.2. Evolution of the bay ...... 7 2.3. Coastal processes ...... 8 Waves ...... 8 Nearshore currents and sediment transport ...... 9 Sediment budget and drift cells ...... 10 Tides ...... 11 2.4. Coastal protection ...... 11 Beach Nourishment ...... 11 Groins ...... 12 Seawalls and Revetments ...... 12 Offshore breakwaters ...... 12 Sand bypassing ...... 12 2.5. Coastal classification ...... 13 2.6. Hanstholm harbor ...... 14 2.7. Hanstholm harbor after the extension ...... 15 3. Method ...... 16 3.1. Description of the Vigsø Bay ...... 16 3.2. Selection of sample sites ...... 18 3.3. Collecting the soil samples ...... 19 3.4. Analyzing the soil samples ...... 19 3.5. Limitations and uncertainties ...... 20 4. Results ...... 21 5. Analysis ...... 22 5.1. How will the extension of the harbor affect the Vigsø Bay ...... 22

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

Future sediment transport around the harbor ...... 22 Shoreline evolution after the harbor extension ...... 23 5.2. Assumptions based on field observations ...... 24 5.3. Future coastal protection ...... 25 Artificial bypass ...... 25 Protection by coastal structures ...... 25 5.4. Maintenance and efficiency of protection measures ...... 26 6. Discussion ...... 27 7. Conclusion ...... 27 8. Acknowledgements ...... 28 9. Bibliography ...... 28

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 1. Introduction

1.1. Foreword Initially, the starting point of the project was an interest in the erosion of the Vigsø coast in relation to climate change. However, as we went on, we realized that in order to get an understanding of the coastal erosion we would need local data on the wind and wave directions in the future. Unfortunately, such specific data was not available through IPCC (Intergovernmental Panel on Climate Change) and DMI (Danish Meteorological Institute), which were our main sources, and at that point we realized that we would either have to model these conditions ourselves or drop the climate change part. Another factor, when looking into the erosion of the coast was the sea-level rise, which also turned out to be a challenge. After the last glaciation, the Weichsel glaciation, the north part of Jutland is rising due to the isostatic rebound, and it is therefore difficult to estimate the effect the sea-level rise will have on the coasts. Due to limited time and our limited experience this far in our studies, we could not create a model and go on with this project as it started. We were therefore forced to change our focus away from climate change. However, we had already spent a lot of time investigating the specific area, including a field trip at the bay, and decided to turn the focus on the Hanstholm harbor, and the impacts of a possible extension of the harbor, on the Vigsø Bay. In this way, we kept our study area and interest in erosion, but instead of looking into how the erosion will be affected by climate change, we look into the human activity on a more local scale.1

Further in our working process, I decided to continue working on my own, due to collaborative difficulties within the group. This happened quite late in the writing process, so I only had a week to finish the work. This was also because we started working on the final subject (the effects of a potential extension of the Hanstholm harbor) relatively late, a fact that gave me less time to realize what the rest of the group wanted. At this point, I decided to focus more on coastal protection and how the preventative measures need to be maintained. To distinguish the parts of the paper that were written before I decided to leave the group, I have made footnotes stating “Group work”.

To begin with, the new coastal equilibrium created by the extension of the harbor, and the impacts on the Bay of Vigsø are studied. Later, the preventative measures to avoid negative consequences (mainly erosion), are presented. Last, I focus on whether these measures will be realistic and what maintenance they will require.

1 Group work

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

1.2. Intro2 The past centuries, the entire West Coast of Jutland has suffered from erosion. It is stated, that up until 1980 the rate of natural erosion (in cases of no coastal protection), was approximately 1-2 meters on an annual basis. Due to stormier weather, this rate increased to about 3-5 m/yr during the 1980's and the beginning of the 1990's3 and the present rate at which the coastline retreats is estimated to about 6 m/yr - a case in which Vigsø Bay is no exception.4 Thanks to coastal protection, this number is significantly reduced. But aside from alterations in meteorological conditions or even eustatic changes in the sea level (both phenomena affected by present changes in the global climate), coastal erosion can also on a local scale be more directly affected by human activity. This is the case in the coastal area around the "shoulder" of Jutland where an extension of the existing harbor at Hanstholm will disturb the process of sediments being transported in and out of Vigsø Bay and thus directly influence the erosion rate.5

With Thisted being one of many outlying municipalities experiencing depopulation, it is only natural that the city council is welcoming a project that will increase the number of local jobs related to the harbor industry. As it is, the harbor prides itself on being the largest harbor in Denmark in relation to commercial fishing. In order to maintain this status and to fulfil a vision of the harbor being the largest of its kind in Europe, the harbor is planning to expand about 150 hectares, which is 3 times its existing size.6 In accordance, EIA investigations (Environmental Impacts Assessment) have been carried out resulting in subsequent reports concluding that the extension will have a negative impact on the erosion of Vigsø Bay. In order to mitigate, or perhaps even prevent, this expected impact on the coast, some preventative measurements are taken into consideration, such as dredging accumulating sand along the future North-west pier and instead depositing it along the shore of Vigsø Bay.7

2 Group work 3 Jensen (1994): p. 266 4 Klimatilpasning.dk 5 Aaen and Gammeltoft-Pedersen (2012): p. 7 6 DHI: p.1 7 Municipality of Thisted (2012): p. 22

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

Image 1 Bunkers just out of Vigsø Bay consist a perfect example to illustrate the erosion extent since the World War 2. Photo taken by Svog Blens (23-10-2011) 1.3. Problem formulation8 How will the extension of the Hanstholm harbor affect the erosion of Vigsø bay? Research questions: How has the Vigsø bay evolved until now?

Which effect do the processes of sediment transport and thus the sediment budget have on the erosion of the bay?

How does the harbor affect these coastal processes now and in the future?

What kind of coastal protection needs to be applied in the area after the extension?

To what extent will the preventative measures applied prevent the expected impacts on erosion? 1.4. Hypothesis The coastal protection measures suggested in the EIA will be sufficient to mitigate the negative impacts of the Hanstholm harbor extension.

2. Theory 2.1. Geological history of the area of interest9 The Danish landscape that consists primarily of large areas of moraine, is a product of glacial processes. Still today, old moraine from the second last ice age, the Saale, can be found in the southwest corner of Jutland. Even though this area was not covered by ice during the last ice age (the Weichsel ice age, which ended at around 11.500 years ago) the melt water from the ice sheet turned large portions of it into out wash plain.10 The cliff of Hanstholm, a formation of which the creation is much older and dates back to an entirely different geological period, is covered by these fairly young traces of the last ice age.

8 Group work 9 Group work 10 Vejbæk (2006): pp. 132-136; Antonsen and Schou (1960): p. 101

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

In the Atlantic period (9.000 – 6.000 years BP), the last great ice sheets of North America and Scandinavia melted off and retreated pole wards resulting in a rapid eustatic rise of the sea level. As a result, vast areas of Denmark became submerged by the transgression of the Litorina Sea (or the Stone Age Sea) and the northern part of Jutland was submerged with the high ground forming islands. Here, the two distinct hilltops, the cliffs of Hanstholm and Bulbjerg, located on the shoulder of today’s Jutlandic landscape could be found.11

Map 1: The Stone Age Sea in Northern Jutland. Source: Houmark Nielsen et al. (2012), p. 325

These isolated formations consisting of limestone (chalk) and flint were created some 65 million years ago (also called the Danien). In this period a warmer climate provided nourishing conditions for Bryozoans, microscopic organisms, which after reaching the sea bottom eventually formed banks and reefs of chalk: hence the name bryozoan chalk. (bog kap 10 plus museum). However, the topographic distinctiveness of these two formations can only be explained by looking even further back. During the Zechstein (around 250 million years BP), the last epoch of the Permian period, the North Bassin was part of a narrow and shallow sea covering the area where Denmark is today. Here, a dry climate caused large volumes of this sea to evaporate thus creating large variety of salt pans. Later on these layers of salt where covered by younger sediments but because of their fluent like properties, they turned into salt

11 Houmark-Nielsen et al. (2012): pp. 322-325

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 domes (diapirs) which eventually pushed the upper layers resulting in the exposure of the cliffs of Hanstholm and Bulbjerg.

From the Atlantic period and on, eventually the elevation of the surface due to the post glacial isostatic rebound caused the gap of open waters between Limfjorden and Skagerak to close off. However, the visual landscape of sand dunes behind the Bay of Vigsø is created by later periods of sand drift, which explains an area consisting of sand dunes.12

2.2. Evolution of the bay The following map shows the changes in the Vigsø Bay coastline for the years 2004, 1978, 1960, 1918, 1883 and 1780, provided by the Geographical Institute of Copenhagen University. The information has been taken from topographic maps and except for the earlier survey (1780) which cannot be considered due to high uncertainties, the rest of the coastlines should be quite reliable. By observing the evolution of the coastline it becomes obvious that the most erosive part of the Bay is located 7-12 km east of the harbor. This is because of the hard bottom in the stretch between the harbor and the erosive point of the bay.13

Map 2: Coastlines extracted from geological maps, courtesy of Institution of Geography and Geology, University of Copenhagen. Source: Christensen (2012): p. 8

The map also shows us that the building of the harbor has already accelerated the erosion of the Bay, compared to how it was before the harbor existed. The Hanstholm harbor was built in 1967 and in the

12 Granat and Secher (2006): p. 13 13 Christensen (2012): pp. 7-8

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 map we can clearly see the erosion being significantly larger after that. This is mainly due to the imbalances in the sand transportation caused by the harbor.14

2.3. Coastal processes15 Waves One of the main agents that contribute to shaping the coasts is ocean waves. They play a large role, not only for shaping the coast, but also because waves transport nearshore sediments. In most places around the world, they are the major force to take into consideration, even dominating over wind and tides, which also have strong shaping capabilities. Waves are created by wind, and the stronger the wind is, the larger the waves it gives. A basic distinction among waves is the one between regular and irregular (random) waves. Regular waves are characterized by a repetitive motion over space, thus, being periodic. They are normally represented by a wave height (H), a wavelength (L) and a wave period (T). However, waves in nature are usually highly irregular with unstable wave heights and periods. These waves are difficult to be described and explained in detail, and require therefore advanced techniques.

To understand the behavior of ocean waves, the linear wave theory can be used. The theory is based on the ratio of water depth (h) to deep-water wave length (Lo), and according to this ratio we distinguish three different wave regions that differ when it comes to the water particle motions under the waves. These regions are:

1. Deep water (h/Lo > 0.5): The water particles under the sea surface, where the wave is travelling, are moving in an almost closed circular path. There is a forward motion of the water particles under the wave crest and a seaward motion of the particles under the wave trough. The circular path the water particles are following under the waves are called orbits. The orbits have a diameter that decreases as the sea depth increases. The waves are unable to stir be sediment at a sea depth called the wave base (and bellow), at which the wave motion also ceases.

2. Intermediate water (0.5 < (h/Lo) < 0.05): When the water depth is decreasing, then the wave motion interacts with the sea bed, which results into the water particles moving elliptically instead of in a circular path. As the wave moves towards shallower water, the ellipses of the water particles become smaller and flatter. The water particles that are in contact with the sea bed are moving horizontally after a to-and-fro movement.

14 Christensen (2012): pp. 7-8 15 Group work

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

3. Shallow water (h/Lo < 0.05): When a wave is moving very close to the shore, all water particles are moving horizontally, following the to-and-fro movement which depends on the depth.16

Figure 1: Motion of water particles under waves according to linear wave theory. Source: Holden (2012): p. 433

Nearshore currents and sediment transport When the waves insert the surf zone of a bay, they eventually break and their energy is released on the point of wave breaking. A large amount of this energy is then creating nearshore currents and causing sediment transport. The larger the energy that is released from the wave breaking – and therefore the larger the wave energy itself – the larger the nearshore currents will be. Since waves are wind generated, the strongest currents are generated during storms. There are three types of wave generated currents:

1. Longshore currents that flow parallel to the shore within the surf zone. These currents depend on the wave energy as well as on the angle of the wave approach, and reach velocities higher than 1 m/s. 2. Bed return flow or undertow is a current with an offshore flow that is created near the bottom of a water column that is moving towards the shore. These currents have low velocities ranging 0,1-0,3 m/s.

16 Holden (2012): pp. 433-434

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

3. Rip currents are strong and narrow currents with an offshore flow, flowing through the surf zone in channels. Their velocities depend on the whether there is a low or high tide and range between 0,5-1 m/s, but may reach up to 2 m/s under extreme storm conditions.

Sediment transport is possible because of nearshore currents. The transport usually starts at the place of wave breaking where part of the energy released turns into stirring motion and entrains the sediments. Then, currents with significant flow velocities are capable of transporting these sediments.17

Sediment budget and drift cells By measuring the sediment transport in a given area (cell), it is possible to determine the sediment budget for a section of coastline. The littoral sediment budget, often along with the concept of a littoral drift cell, can constitute vital information for building a coastal management framework in order to assess the potential impact of human actions on shorelines.18 It is therefore very important to understand these concepts in relation to this project, since we are investigating the effects on the Vigsø coast by the man made Hanstholm harbor and its future extension.

A littoral drift cell is a section of coast that should ideally contain 3 elements to be recognized: a source or several sources of sediment, a clearly continuous zone of alongshore sediment transport, and a downdrift sink, which is a zone where sediments can either deposit or retreat offshore. Such cells are broadly used, when possible, to determine the sediment transport, and further the sediment budget.19

A sediment budget is the sum of all sediment inputs (gains or sources) and outputs (losses or sinks), within a certain section of the shoreline or control volume (cell), over a given time. Commonly, a series of connecting calculation cells is being used to calculate the sediment budget. In order for the sediment budget to have a value for the possible engineering activities in the area of interest, it is necessary that the algebraic difference between the sediment inputs and outputs of each cell, and for the entire sediment budget, reflects the rate of change in sediment volume occurring within that region. The sediment budget can therefore be expressed as a volumetric rate of change and can be calculated as such by the following equation:

∑ Qsource − ∑ 푄푠푖푛푘 − 훥푉 + 푃 − 푅 = 푅푒푠푖푑푢푎푙 (1)

17 Holden (2012): pp. 437-438 18 Davidson-Arnott (2010): p. 169 19 Davidson-Arnott (2010): p. 169

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 where Qsource and Qsink are the sources and sinks to the control volume, respectively, ΔV is the net change in volume within the cell, P and R are the amounts of the material placed in and removed from the cell, respectively, and Residual represents the degree to which the cell is balanced.20

Even though calculating the sediment budget based on drift cells seems to be the best way to understand the actual processes that contribute to the sediment transport, in the case of Vigsø Bay, it was done by using a model called LITDRIFT. The model calculates the wave parameters, the wave driven current and the longshore sediment transport along the coast, through four representative nearshore points. Data from 32 years by a regional survey give 4 wave roses that are used when simulating the wave parameters and sediment transport. When comparing the sediment transport that was estimated by the LITDRIFT model, the results agreed with the data from the survey, except for the one directly east of Hanstholm harbor where there is a hard bottom and therefore the transport is reduced.21

Tides Even though tides can be of great importance as a forming agent for coasts, it is almost not present in the area of interest for this paper.

2.4. Coastal protection Coasts that are suffering from erosion, or are in danger to potentially be exposed to erosion, can be protected in three ways: by soft intervention (beach nourishment), hard intervention (such as breakwaters or groins) or by relocating the existing structures from eroding shores. Some of these preventative methods will be explained here, since they are relevant to the specific study.22

Beach Nourishment Beach nourishment is done by removing large quantities of sand from a site and placing them on a beach that is retreating, aiming in this way for the shoreline to advance seaward. In order to get the most out of this measure, the sand is usually deposited on the beach at a slope steeper than the one naturally created by the beach. In this way, it is possible to gain more time, perhaps several years, during which profile equilibration takes place. By adding the external material to the beach, additional components of the longshore sediment transport are prevented from moving away from their original location.23

20 Rosati (2005): p. 308 21 Christensen (2012): pp. 8-9 22 Committee on Coastal Erosion Zone Management (1990): p.56 23 Committee on Coastal Erosion Zone Management (1990): pp. 56-57

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 Groins Groins are perpendicular structures to the shore, that are usually made of timber, concrete, metal sheet piling, or rock. They might consist of a single groin or a series of them along a coast. Their main function is to reduce the longshore sediment transport, and therefore, when they are placed on an open coast they will end up widening the beach on the updrift side. Groins should preferably be built with a height according to the beach profile since in that way they have less potential of causing downdrift beach erosion compared to a high profile and/or long structured groin that could lead water and sediments offshore.24

Seawalls and Revetments Seawalls and revetments are an expensive option for coastal protection due to the constant need for maintenance. It is crucial that they are properly engineered when constructed since they have often been accused for causing additional erosion to the adjacent coasts. However, seawalls are meant to protect the land behind them without affecting the fronting beaches.25

Offshore breakwaters Breakwaters are long constructions usually made from rock or concrete armor units that protect the shoreline by reducing the wave energy reaching it. The advantage of these structures is that they don’t disturb the longshore sediment transport on a large scale, and at the same time they promote sediment deposition at the downwind side of the structures. Segmented and detached breakwaters also protect the natural currents, unlike breakwaters that project from the land which could potentially relocate them.26

Sand bypassing Inlets, navigation channels and harbor entrances, are structures that disturb the natural longshore sediment transport. When it comes to harbors, the interrupted flow of sand is usually diverted into the harbor entrance as well as offshore. This sediment transport disturbance results in the erosion of the downdrift coastline. Sand bypassing can be achieved with the help of a floating or fixed pumping system which enhances the natural flow of sand to the downdrift shoreline and reduces the need for dredging.27

24 Committee on Coastal Erosion Zone Management (1990): p. 59 25 Committee on Coastal Erosion Zone Management (1990): p. 59 26 Committee on Coastal Erosion Zone Management (1990): p. 60 27 Committee on Coastal Erosion Zone Management (1990): p. 61

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

Figure 2: Figure showing the bypass concept. Source: Brøker (2003): p. 7 2.5. Coastal classification28 There are several criteria that can be the focus when classifying a coast, as well as several classification schemes that have been developed and further modified by numerous scientists throughout time. According to Sheppards comprehensive classification (1963), the coast of Vigsø is a secondary coast, dominated by the coastal processes taking place and giving the coast its form as we know it. More specifically, we are dealing with a wave erosion coast, and according to Valentins classification (1952) the shoreline is retreating.29 However, the coast consists of a large area around the coastline being covered by sand dunes. When looking at the coast from the macro-scale, we would categorize it as a flat coast, which seems to have been the case for centuries, since it expands over several square kilometers without any sudden changes in the elevation, despite of the sand dunes height. Due to various reasons (e.g. climate change), the conditions in the area seem to be changing. The erosion caused by the dominating waves are creating an image (see image 2) of a steep coast, to a certain level. We could therefore say that when looking at the coast from the micro-scale, it could also be categorized as a steep coast.30 It should also be mentioned that part of the erosive picture we get in the micro-scale of the Bay, could be a seasonal phenomenon and not a general thing. Due to limited time for this study we do not have a holistic idea of how the coast looks during the whole year round. If we had been making

28 Group work 29 Davidson-Arnott (2010): pp. 13-14 30 Holden (2012): pp. 442-443

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 observations throughout the four seasons of a year, we would be able to specifically determine what is due to the seasonal weather and what is due to the general erosion of the coast.

Image 2: Signs of erosion on the coast of Vigsø. Photo taken by Simon Overgaard (7/5/2016)

2.6. Hanstholm harbor Hanstholm harbor is located at the headland of Hanstholm that consists of a limestone cliff, on the northern west part of the Jutland coast. It was built in the 1960s and is a fishery and ferry harbor. The harbor is built with a bypass layout in order to avoid sedimentation. Specifically, a symmetrical and streamlined layout leads the sand past the harbor entrance by creating a smooth convergence. At the same time the front of the harbor consists of vertical breakwaters, which in combination with the smooth shape creates very successful bypass conditions. The entrance depth is approximately 9 m and requires a dredging of about 100.000 m3 annually. The factors affecting the sediment flow around the

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 harbor are the wind and pressure as well as the wave breaking to a smaller extent. The tide is almost of no importance in this area.31

Image 3: The Hanstholm harbor as it looks today. Source: Nordjyske archive photo, 27/4/2016 2.7. Hanstholm harbor after the extension The future harbor, after the planned expansion, will look quite different. It will be several times larger than the existing harbor, the entrance will be located northeast of where it is to be found today and the active entrance will be closed. In addition, the future entrance will aim for a depth of 12,5 m which will of course require certain dredging maintenance but will be approximately 3-4 m deeper than the existing. The new conditions are expected to affect the sediment transport around the harbor as well as to the coast located on the east of the harbor, the coast of Vigsø. This topic is further analyzed under the analysis.32

31 Mangor (2010): p.17 32 Christensen (2012): p. 2

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

Figure 3: Planned extension of the Hanstholm harbor. Source: Christensen (2012), p. 2 3. Method33 While working on this project we have used two different approaches on how to solve the problem. The paper is primarily literature based and is supplemented by samples and observations taken on a field trip. In order to answer our research questions, we combine literature based theory with the results from our field trip findings and subsequent lab work. In addition, and particularly in relation to the effect the Hanstholm harbor will have on the bay, we used reports made by the municipality of Thisted in cooperation with the Danish Coastal Authority, and a study conducted by DHI (the company building and extending the harbor), estimating the possible future consequences by the future extension of the harbor.

3.1. Description of the Vigsø Bay The Vigsø Bay is located in the west coast of northern Jutland, on the eastern part of the Northern Sea, meaning that it is exposed to waves coming from westerly and northwesterly directions34, created by relatively strong winds. During winter, the area suffers a few storms, with severe cyclones being rare. A relatively high annual sea temperature, ranging approximately between 1° in the winter and 20° in the summer, combined with the dominating westerly winter storms, keeps the along shore sea free of ice35 - a condition which in return sustains an active sea surface.

Supported by Geodatastyrelsen, it is stated by Den danske havnelods, which gives information relevant to the maritime traffic, that the difference between the mean high tide and the mean low tide amounts to no more than 0,3 meters within the harbor basin. However, the direction and flow velocity of the

33 Group work 34 Christensen (2012): p. 1 35 Anthony and Leth (2001): p. 248

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 wind is highly influential on the sea surface level. Thus the maximum high tide during severe westerly winds is measured to 1,5 m above mean sea level and on the other hand the ebb during severe easterly winds can be as low as 1,7 m below mean sea level.36

The North Sea circulation creates the West Jutland Coastal Current which flows northward.37 This causes large amounts of eroded sediments from the coastline south of Hanstholm to be transported and deposited along the coastline east and north of Hantholm. On an annual basis an amount of 500.000 m3 of sand reaches the Hanstholm harbor where the amount of 100.000 m3 gets trapped in the harbor entry. However, with the harbor of Hanstholm causing the longshore sediment transport to bypass, the remaining 400.000 m3 travels further on towards Vigsø Bay. In direction of the bottom of the Vigsø Bay, along the Hanstholm Cliff, the sea bottom is hard, consisting mainly of limestone38. This prevents deposition of the sand sediments and along with new eroded material from the bay, the sediment transport continues accumulating as it flows forward north along the beach of Vigsø Bay towards the Lild Strand and the Cliff of Bulbjerg.39

Map 3: Sediment budget and sediment transport along the coast of Vigsø Bay, Source: Christensen et al. (2012)

36 Den danske hanvelods - Hanstholm havn (last updated: 19/2/2014) 37 Anthony and Leth (2001): p. 248 38 Municipality of Thisted (2012): p. 21 39 Christensen et al. (2012): p. 8

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242

Map 4: Sediment budget in the West Coast 2001, numbers in m3/year, Source: The Danish Coastal Authority 3.2. Selection of sample sites The sample selection was done by dividing the area of focus into smaller areas, and taking a sample that represented each sub-area. The division was based on a geomorphological map of the area. Our goal was to collect a sample from all different soil types in the area and to analyze them in order to figure whether these soil types consist of highly or poorly erosive materials – meaning high or low risk to erode. Based on the geomorphological map, it was possible to estimate the soil types expected in each area and we decided to take a sample of each geomorphological area, given in the map below. The division of the area into sub-areas was a solution to the problem of the rather large scale of the case area we are analyzing in this project. The bay we are working with exceeds over more than 205 km2 and is therefore difficult to cover during the limited available time for fieldwork. In total we collected 7 soil

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 samples and ended up discarding one of them because we decided it was not within the area of interest after all.

Map 5: Geomorphological map used when choosing the sampling sites. Each soil type in our area of interest constitutes a sub- area that is represented by one sample. Our area of focus is indicated with a red square around it. Source: Smed, P. (1981). Landskabskort over Danmark. Geografforlaget , Brenderup 3.3. Collecting the soil samples At the actual locations chosen to take the soil samples, we used a hand-drill of 1 m length and all our samples were taken approximately at that depth. For every soil sample we used a GPS (Garmin e-trex 30), which gave us the exact coordinates of the location as well as the elevation. For every sample we collected information about: soil color, texture and whether it was rocky.

3.4. Analyzing the soil samples When returning to the lab with the collected samples, we dried the soil in an oven at the temperature of 15°. When looking and feeling the samples after being dried, it was possible to identify that four of them were sand samples, based on observation and with the help of our teacher and soils scientist Niels H. Jensen. This saved us the time we would have used if he had to analyse all the samples and ended up only analysing the samples taken from the two cliffs slopes, in order to indicate the presence of

Geography Project Erosion Risk Assessment at the Bay og Vigsø Robin Mikaela Kotsia Student number: 54242 limestone. This was done by dripping a few drops of HCl 10% on the white stones that could potentially be chalk.

3.5. Limitations and uncertainties We recognize that the amount of 7 samples is a limited repetitiveness to represent the entire bay. We are also aware of the possibility of error when collecting these samples. Some of the samples we took from the two cliffs, Hanstholm and Bulbjerg, on the two ends of the Vigsø Bay were from the deeper layers and were taken from the sides of the eroded cliff, where the original geological material was exposed. The sampling sites were perhaps not always chosen correctly as we experienced a sample containing material that we didn’t expect. We also had some uncertainties with the elevation numbers given by the GPS machine we used. At certain locations where we could obviously see that we were at the same level as the sea surface, the elevation given by the GPS was surprisingly 3 m, which cannot have been correct. We are therefore assuming that our elevation numbers given in the table below can be imprecise.

4. Results

Sample Drilling Lab Location Coordinates Elevation Observations Soil type No. depth analysis Sand dunes Oven landscape, we Middle sized 1 South of Hanstholm 3 m 110 cm drying of expect to find sand sample 57° 06.108 (N), 008° 34.453 (E) sand. Dark soil with By the bunker museum on 2 57° 07.280 (N), 008° 37.244 (E) 20 m 70 cm traces of iron and HCl 10% Moraine Hanstholm cliff rocks. Moraine Chalk like, light with 3 14 m 50 cm HCl 10% grey clayey soil weathered Slope of the Hanstholm Cliff 57° 07.219 (N), 008° 38.366 (E) flint stones 80 cm Oven (from We expected to Fine grained 4 Bottom of Vigsø Bay beach 0 m drying of erosive find sand sand sample 57° 06.019 (N), 008° 43.994 (E) cliff) Red-brown wet Oven soil. We expect to Middle sized 5 10 m 100 cm drying of Near road 26, south of find marine sand sample Hanstholm 57° 04.525 (N), 008° 40.906 (E) foreland soil. Bulbjerg cliff (from the side of 6 0 m 0 cm Hard whit cliff HCl 10% Limestone the cliff) 57° 09.487 (N), 009° 01.416 (E)

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242

5. Analysis 5.1. How will the extension of the harbor affect the Vigsø Bay DHI – the company building and expanding the Hanstholm harbor – as well as the Environmental Impact Assessment (EIA) have conducted studies that include several simulation models, to get an understanding of the harbor extension impacts on the harbors surroundings. The estimation of the future conditions include the sediment transport and the shoreline evolution. The estimates are based on data related to waves, currents, sediment transport and morphological evolution of the Bay taken from previous year’s conditions (specifically the years 1960-2004) and simulated as future conditions including the harbor extension.40 For the calculation of the shoreline evolution and sediment budget, data were also taken from the several beach profiles along the Vigsø Bay, from a survey made for the Danish Coastal Authorities. From the profiles available, some representative ones were chosen according to their sensitivity.41 I will briefly present the impacts given based on these simulations in this section.

Future sediment transport around the harbor The future sediment transport has been investigated by DHI with the help of the numerical model system MIKE21. By simulating the wave, current and sediment transport that will be possible to pass by the future harbor entrance during an extreme storm, it was found that the sediment transport capacity will be way lower compared to the one at the current harbor entrance. This is due to two reasons: the future harbor will cause less contraction of the passing current, and the larger harbor entrance will not be able to support the sediment transport as well as the current entrance does. The latter problem will cause increased sedimentation by the harbor mouth, which will consequently require a significant maintenance dredging. Some alternative layouts for the harbor entrance were therefore tested and simulated in the search of a more efficient construction. However, it turns out that the increase in the construction costs will be larger than the dredging costs. So, the extension plan will stay as it is and dredging will have to be the solution to this problem. The sediment deposition at and around the harbor mouth has also been modeled. The sediments will be accumulating along the harbors main northern breakwater. For this reason, a reservoir will be installed west of the harbor entrance (updrift) that will be able to absorb the sand sediments deposited under normal conditions as well as during severe

40 Christensen (2012): p. 3 41 DHI (2012): pp. 31,34

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242 storms. The reservoir will be dredging along the main northern breakwater, which is where the core of the sedimentation problem takes place, and will aim to maintain the water depth west of the harbor entrance. This maintaining measure is expected to be efficient since the reservoir has a capacity of 1.000.000 m3, an amount that has been estimated to give sufficient results. Specifically, it is expected of the reservoir to take in 80% of the total sediment transport going by the harbor, which corresponds to approximately 400.000 m3 annually, depending of course on the possible variation from year to year in the number and severity of storms.42

Shoreline evolution after the harbor extension The change in the sediment transport past the harbor will have impacts on the coastline that would normally receive these sediments - the Bay of Vigsø. In order to predict the future development of the coastline in Vigsø Bay, a simulation was made by DHI, using the same weather data and the historical evolution of the bay, for the years 1960-2004, and in this way an estimation for the next 44 years was made. The model for the simulation is called LITLINE and is using the model LITDRIFT, which has been analyzed earlier under the theory section (see 2.3. Coastal processes – Sediment budget and drift cells). The model is taking into account the presence of hard bottom (limestone bed) at certain parts of the bay, which will prevent the erosion even though there is a sediment deficit compared to the transport capacity. With the help of these data and model, DHI conducted a few simulations of the future coastline according to different scenarios. One for the same conditions as they were before the extension of the harbor (full bypass of sediments) and four with a bypass of sediment of 75%, 50%, 25% and 0%. The simulated shorelines for these different scenarios are shown in map 6 below.43 In all cases, the shoreline will retreat, even with a fully efficient bypass system that will ensure conditions as the ones before the extension. However, the less sufficient the bypassing might be, the larger the erosion of the Bay will be. These simulations show how vital these sediments are for the prevention of the erosion of the Bay, and warn us for the importance of an excellent bypassing system that is necessary to be established along with the harbor expansion.

42 Christensen (2012): pp. 3-6 43 Christensen (2012): pp. 9-12

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242

Map 6: The simulated shoreline evolution from 2004 to 2048 for different bypass conditions with background from Google Earth. Source: Christensen (2012): p. 12 5.2. Assumptions based on field observations By observing the Bay of Vigsø during the field trip, first thing to be noticed were the stones placed on the beach to prevent the erosion. Obviously, coastal protection is already needed and it is therefore that the extension of the harbor is an environmental threat. An area that is already suffering from erosion will have to take an intervention that is known will have negative impacts on an already existing problem. First, the sand drift coming from the coast west of the harbor will find obstacles on its way and might therefore be reduced when reaching the coast of Vigsø. The limestone that can be seen along the coast in certain spots and depending on the weather, indicates that there are several wave resistant and slowly erosive parts of the coast. These parts, are not well known, and we can therefore not estimate the exact erosion that will be taking place in the future according to the new conditions. By observing the stones placed on the beach to prevent erosion, it becomes clear that the most erosive part of the coast lies approximately 3-4 km from the harbor. At the inner point of the beach (the most eroded part), the stones placed are almost gone. When walking from the most erosive point and towards the harbor, the stones are more and more concentrated and at the beginning of the beach they look almost untouched, as if they were put there the day before. It is easy to understand from these observations that the coast will need very smooth operations and a concrete plan to prevent further erosion when extending the harbor.

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242 5.3. Future coastal protection Artificial bypass As mentioned earlier, the reservoir placed for dredging and capturing the sediments before the harbor mouth will be enclosing 80% of the sediment transport passing by the harbor. It is therefore natural to think that these sediments will need to be further bypassed after the harbor in order to secure the natural flow and avoid the erosion of Vigsø Bay. To achieve the full bypass it is necessary to artificially bypass all the sediments that tend to deposit along the main north breakwater. In practice, this corresponds to approximately 400.000 m3 of sediments annually, that would normally be flowing by as part of their natural route from the coast southwest of the harbor and towards Vigsø Bay. After the extension, only 100.000 m3 of sediments are expected to manage to flow by the harbor naturally. The sediments captured by the reservoir (400.000 m3) should therefore be deposited on the coast at a short distance east of the harbor, which is where the present bypass reaches the coast and continues travelling further along the coast as littoral transport. The place of deposition of the reservoir sediments has been estimated to be approximately 1-3 km east of Roshage, where there is a jetty, and would be wise to also be bypassed in order to minimize extra losses. Usually, artificially bypassed material is used as nourishment on the dry beach and is placed there by pumping or rain bowing. Unfortunately, this is an expensive choice and might cause limitations as to the type of material that can be used. DHI is therefore proposing a direct sediment deposition from the actual reservoir on the zone of active transport and probably at a water depth of 7 m or less. However, the actual location for placing the dredged sand has not been determined yet, and will be part of a more detailed investigation and planning of the bypass operation.44

Protection by coastal structures Suggested by DHI, is not only the artificial bypass of the sediments, but also the possibility of coastal protection by structures. It is with some doubt though, since the artificial bypass is clearly more effective and a safer protection. Structures like groins, detached breakwaters and revetments or seawalls, are all suggested in their report as possible coastal protection for the Bay of Vigsø. In all of the named protections, the goal is to keep the transport capacity along the coast at a low level, matching the amount of sediments passing by from the west. In this way, erosion is prevented in the protected area, but at the same time, the erosive pressure will be lead downdrift to the next unprotected coast. To avoid this consequence, coastal structures will have to be built all the way to a coastline that can be

44 DHI (2012): pp. 55-56

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242 allowed to erode, or to the location where the littoral drift terminates. It is therefore, a way larger project than just protecting the specific coast and can cause larger problems if it is not applied correctly.45

5.4. Maintenance and efficiency of protection measures It is at this point understood that the protection measures suggested are not going to fully prevent the erosion of the Vigsø Bay. Even with the full bypass, the Bay is expected to lose 250-300.000 m3 of sand due to erosion annually.46 Since the Bay is already eroding, it is vital that the preventative measures are as sufficient as possible in order to mitigate extra erosion, caused by the harbor extension. For this reason, maintenance of the chosen method will be required. More specifically, in the case of the artificial bypass, the reservoir in front of the main breakwater will need certain maintenance and regular measurements of the sediment transport around the harbor should be conducted, in order to ensure that the bypass is sufficient. If the sediment transport was to change, the reservoir capacity would perhaps not be enough to support such amounts, or the placing of the sand at the chosen location after Roshage would have to be more frequent. In the case of protection by structures, the hazards have already been mentioned. If groins, breakwaters, revetments or seawalls were to be built, the risk of erosion would simply be moved further to the next coast, without solving the real problem. It is therefore understood that such measures will require constant expanding in order to fully secure the west coast of Jutland and are not a realistic option for the Bay of Vigsø, unless a greater protection plan is established for the entire coast. This is of course not investigated within this paper.

In the DHI report, it is also mentioned that there are some uncertainties regarding the hard bottom of limestone along the coast. The exact locations and extend of the hard bottom have not been determined, and since it constitutes a major protection for the coast from erosion, the extend of it should be known.

There is also some potential uncertainty in the sediment transport calculations47, which are the numbers that the protection planning is based on. The efficiency of the protection measures is therefore also uncertain, as it is not ensured that they will be able to live up to the possible future erosion.

45 DHI (2012): p. 55 46 DHI (2012): p. 15 47 DHI (2012): p.53

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242 6. Discussion It should be mentioned that for this study, certain factors have not been taken into account. Future climate change as well as changes in the local weather conditions might affect the erosive factor of the Bay, and the entire west coast of Jutland, differently and in an unexpected degree. Studies conducted by IPCC and DMI show that there will be an increase in the storms frequency. The exact wave and wind conditions have not been explained, but it can be expected that the erosion of the Bay will increase due to the future weather. Another factor that is related to climate change is the sea level rise. Even though there is a debate as to whether the sea level rise in Denmark will actually cause severe erosion and flooding, it is not taken into consideration within the study carried out by DHI. This leads to the assumption that the suggested coastal protection might not be as efficient as expected. It could be assumed that due to these omissions, there is a lack of theoretical assumptions covered in the EIA, since the simulations are based on few variables and not representing a realistic case scenario.

The simulations carried out for the study by DHI, are using data from previous years to model the future conditions after the harbor expansion. However, as just mentioned, the future weather conditions are very likely to change and therefore, the simulations based on storm data from past years could be inaccurate. The study has consequently an empirical issue that does not allow us to fully trust the results. Since these simulations are the base for the coastal protection planning, it is crucial to get a better assumption of what is expected in the future.

7. Conclusion The Bay of Vigsø will suffer from erosion regardless the extension of Hanstholm harbor. The expansion will only increase the erosion hazard if the artificial bypass fails to move the entire 80% of the transported sediments that it is expected to dredge/capture. The plan made by DHI is still to be completed, especially regarding the coastal protection and the bypassing method, which has not been fully studied yet. However, the artificial bypass is clearly the choice number one and obviously vital for the coast of Vigsø. The EIA seems to be missing certain aspects when it comes to the simulations done to predict the future conditions around the harbor. Possible future climate change is not taken into account when conducting the modellings. That means wave, wind and sea level data could be completely different than expected. In addition, data from past years are being used for simulating the coast development, which leads to the question: how accurate are the results given from the simulations? To conclude and to answer the hypothesis, the coastal protection measures suggested in the EIA may not be sufficient to mitigate the erosion impacts caused by the harbor extension. Further

Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242 research is needed in order to evaluate the true rate of coastal erosion and preferably measures on how to prevent that. Preferably, future models need to implement information on future wave, wind and sea level rise data.

8. Acknowledgements I would like to thank my supervisor Stig Roar Svenningsen for his help and support throughout the course of this project, both during the teamwork but also after I decided to leave the initial group and finish the paper on my own. I am also grateful for all the advice, inspiration and help with the soil samples I got from my teacher Niels H. Jensen.

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Geography Project Erosion Risk Assessment at the Bay of Vigsø Robin Mikaela Kotsia Student number: 54242

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