Coastal Dynamics

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Delft University of Technology Coastal Dynamics Bosboom, J.; Stive, M.J.F. DOI 10.5074/T.2021.001 Publication date 2021 Document Version Final published version Citation (APA)

Bosboom, J., & Stive, M. J. F. (2021). Coastal Dynamics. TU Delft Open. https://doi.org/10.5074/T.2021.001

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Coastal Dynamics

Judith Bosboom and Marcel J.F

. Stive

Judith

Bosboom

and

Marcel

J.F

. Stive

| Coastal Dynamics

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2021 TU Delft Open ISBN 978-94-6366-370-0 DOI 10.5074/T.2021.001 textbooks.open.tudelft.nl Cover image:

Atlantic coast, Angola (Courtesy Stefanie Ross) Coastal Dynamics

Judith Bosboom and Marcel J.F. Stive

This textbook on Coastal Dynamics focuses on the interrelation between physical wave, flow and sediment transport phenomena and the resulting morphodynamics of a wide variety of coastal systems. The textbook is unique in that it explicitly connects the dynamics of open coasts and tidal basins; not only is the interaction between open coasts and tidal basins of basic importance for the evolution of most coastal systems, but describing the similarities between their physical processes is highly instructive as well. This textbook emphasizes these similarities to the benefit of understanding shared processes such as nonlinearities in flow and sediment transport. Some prior knowledge with respect to the dynamics of flow, waves and sediment transport is recommended.

Judith Bosboom

TU Delft | Faculty of Civil Engineering and Geosciences

Dr. Judith Bosboom is a senior lecturer Coastal Engineering. She developed innovative teaching methods for the topic of Coastal Dynamics. Judith has been elected best lecturer for the MSc Hydraulic Engineering multiple times. In 2016, she received both the best lecturer award for the Faculty of Civil Engineering and Geosciences and the best lecturer award for Delft University of Technology. She has successfully taught Nanjing University graduates (China) for several years.

Marcel J.F. Stive

TU Delft | Faculty of Civil Engineering and Geosciences

Prof. Marcel Stive is an emeritus professor Coastal Engineering. He assisted Judith in developing and teaching the Coastal Dynamics lectures based on his earlier involvements in lecturing and teaching. He has successfully taught Indian, Chinese, Vietnamese, Brazilian and Iranian graduates and/or professionals for several years.

Judith

Bosboom

and

Marcel

J.F

. Stive

| Coastal Dynamics

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Delft University of Technology

Coastal Dynamics

Judith Bosboom & Marcel J.F. Stive

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iv

Bosboom, J. and Stive, M. J. F. (2021). Coastal Dynamics. Delft University of Technology,

Delft, The Netherlands. Revision no. 1269 logged at 2021-01-21 09:13.

Publisher TU Delft Open

Date 1st February 2021

Cover image Atlantic coast, Angola (courtesy Stefanie Ross)

ISBN (softback/paperback) 978-94-6366-370-0

ISBN (e-book) 978-94-6366-371-7

DOI 10.5074/T.2021.001

Corresponding author j.bosboom@tudelft.nl

The latest edition of this book is available for online use and for freedownloadfrom

the TU Delft Open Textbook repository attextbooks.open.tudelft.nl. Here, also aform

is available to subscribe for update notifications and provide feedback.

In line with TU Delft Open Science policies, this Open Textbook is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International Li-cense (CC BY-NC-SA 4.0), except where otherwise stated. This work can be re-distributed in unmodified form, or in modified form with proper attribution and under the same license as the original, for non-commercial uses only.

Every attempt has been made to ascertain the correct source of images and other potentially copyrighted material and ensure that all materials included in this book have been attributed and used according to their license. If you believe that a portion of the material infringes someone else’s copyright, please contact the

authorj.bosboom@tudelft.nl.

The softback is printed by Ridderprint (www.ridderprint.nl) on behalf of the Faculty

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of Coastal Engineering. It can be ordered via aformattextbooks.open.tudelft.nl.

Graphics produced with [Matlab, Inkscape, Adobe Illustrator, Adobe Photoshop]©

Typesetting with LA_TEX

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v

Preface

This Open Textbook is the Open Access version of lecture notes that have been de-veloped to support and supplement a Delft University of Technology first year Master course on Coastal Dynamics. Since its introduction in the academic year 2009-2010, this first year MSc course has been taught to MSc graduate students following the two-year Hydraulic Engineering MSc curriculum of the Faculty of Civil Engineering and Geosciences. It focuses on the interrelation between physical wave, flow and sediment transport phenomena and the resulting morphodynamics of a wide variety of coastal systems. The objective is to provide insights into the phenomenological and theor-etical as well as applied aspects for hydraulic and coastal engineering MSc students. It builds upon BSc and MSc courses treating the dynamics of flow, waves and sedi-ment transport, and may serve as a basis for a course focusing on coastal modelling, as in the Delft University of Technology curriculum. Although several valuable course monographs and books exist on the topic of coastal dynamics and coastal engineering

(see Sect. 1.7), we felt that no standard teaching books existed for the purposes that

we had in mind. It has been our explicit intention to connect coastal and estuarine dynamics. Not only is the interaction of coasts and estuaries of basic importance for the evolution of many coastal systems, but describing the similarities between their physical processes is highly instructive as well.

This textbook emphasizes these similarities to the benefit of understanding shared processes such as nonlinearities in flow and sediment transport. Furthermore, our text book differs from other coastal textbooks in that we connect engineering scales with larger Holocene scales, emphasizing among others the impact of leading and trailing edge coasts on coastal evolution. While primarily developed for the purpose of teaching, we hope that these notes are also useful as a reference book for MSc and PhD students.

The present form and format of these notes have been developed to follow the Open Access rules and is not final and far from perfect, both in language and contents. We feel that this is a matter of evolution, strongly steered by our continuous learning through and our experience with both teaching and examination. In the years to come we will provide regular updates of this Open Textbook, also in response to comments we expect to receive from our readers.

Our below list of acknowledgements is long but our excuse is that it reflects that the art of teaching is to accumulate and digest the efforts of many of our colleagues nationally and globally.

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xiv Preface First, we wish to acknowledge the efforts of all our national colleagues that developed earlier lecture notes that we used as inspiration for these notes: Professor Eco Bijker and Associate Professors Walt Massie and Jan van de Graaff, Professor Kees d’Angremond and MSc Liesbeth van der Velden, Professors Huib de Vriend and Zheng Bing Wang

and PhDs Anneke Hibma and Edwin Elias, and Professor Job Dronkers (see ‘Credits’

on page575).

Second, we wish to acknowledge some key international references that we used while

developing new material that we found missing. Chapter 2, especially Sect. 2.3, is

heavily inspired by Davis Jr. (1994), whose well-illustrated textbook offers a good

in-sight into larger-scale coastal behaviour and classification. The energetics approach

for cross-shore sediment transport in Sect. 6.1and Ch. 7 is largely based on Bowen

(1980), whose elegant work we still find very instructive. In a more general sense we

have been inspired by a number of books we reference in Sect.1.7, notably the books by

Fredsøe and Deigaard (1992), Kamphuis (2000) and Masselink and Hughes (2003). We

have tried to the most of our abilities to pay attention to cite where external material is used. If we have failed to do so, please inform us.

Further, we need to acknowledge many close Delft colleagues who have contributed to our knowledge by their work and comments and/or provided us with course and lecture materials. Regarding knowledge contributions, we mention Jurjen Battjes, Leo Holthuijsen, Jacobus van de Kreeke and Han Winterwerp. Stefan Aarninkhof, Jan van Overeem, Roshanka Ranasinghe and Zheng Bing Wang gave valuable contributions to the content of our lectures. Revisions of sections of early versions of our notes by Ad Reniers, Dano Roelvink, Ad van der Spek and Zheng Bing Wang are highly appre-ciated. Special thanks go to Howard Southgate, who has read through our complete lecture notes in detail making relevant comments and suggestions. PhDs Stuart Pear-son, Yorick Broekema, Alejandra Gijón Mancheño valuably contributed to improve the connection between the notes and the lecturing of the course. We thank our former Delft Hydraulics colleague Hans de Vroeg for providing cross-shore distributions of wave heights, longshore currents and sediment transports calculated with Unibest-CL+. Finally, we are grateful for the many suggestions we received from our students over the years.

We have invested heavily in making the illustrations and graphs “our own”. For this we were assisted by many of our talented students Liang Li, Dáire Stive, Marcio Boechat Albernaz, Janbert Aarnink, Ascha Simons, Viktorija Usevičiūtė and Tim van Dam, whose creative contributions we gladly acknowledge. It is thanks to Marcel Mol that we have a subject index.

Delft University of Technology (DUT) library convinced, stimulated and sponsored us to publish these notes Open Access. Their specialists Michiel de Jong, Jacqueline Michielen-van den Riet and Monique de Bont guided us professionally to understand and comply with copyright licenses. Besides their creative qualities, Ascha Simons and

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Preface xv Tim van Dam used their ample technical competences to successfully finish this ambi-tious project. The management qualities of Carolina Piccoli to streamline this process are highly appreciated. The broad support by Stefan Aarninkhof in this venture has been essential.

As our knowledge and experience is continuously developing, lecture notes are al-ways work-in-progress. We would be very grateful if our readers give us feedback on contents and didactics. Also, we would appreciate to be informed of any copyright infringements in the textbook. We will gladly adjust and/or correct.

Judith Bosboom and Marcel J.F. Stive Delft, The Netherlands

January 2021

Note to our readers: We would like to track the reach and use of this Open

Text-book. Therefore, we would be very pleased if you could provide information on

your intended use of this book using theform on the TU Delft Open Textbook

repository. The form also provides options to subscribe for update notifications and give feedback.

Note to our students: No one reads a textbook more thoroughly than a student

studying the material for a course. Therefore, if you find any smaller or larger mistakes in this book – from typographical errors and incorrect cross-references to unclarities and inconsistencies – we would greatly appreciate to be informed

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1

Overview

1.1. Introduction

Humans have extensively used the coastal zone for amongst others fishing, tourism, transport of goods, water treatment and housing. Agriculture has benefited from the very fertile grounds due to marine and riverine deposits. Approximately three billion people – half the world’s population – live and work within a couple of hundred kilo-metres of a coastline, notwithstanding the vulnerability of coastal areas to flooding. Due to the high population densities and extensive infrastructure and property devel-opment, disasters will have major consequences. Coastal engineers play an important role in both developing the coastal zone and protecting the coast and the hinterland.

In Sect. 1.2, the course contents and the position in the curriculum are explained.

Sect.1.3lists the study goals. Section1.4gives some examples of the problems coastal

engineers may be faced with. In doing so coastal engineers need a thorough know-ledge of the natural dynamics of the coastal system. An introduction on that topic

is given in Sect.1.5. Important players in the (Dutch) coastal engineering sector are

summarised in Sect. 1.6. The chapter concludes with a list of handbooks, journals,

conference proceedings and internet sources for further reading (Sect.1.7).

1.2. Coastal dynamics for coastal engineers

1.2.1. What is coastal engineering?

Coastal Engineering is the branch of civil engineering concerned with the planning,

design, construction and maintenance of works in the coastal zone. Coastal engineer-ing usually involves either 1) the transport and stabilisation of sand and other coastal sediments or 2) the construction of structures.

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2 1. Overview Measures in the first category are called ‘soft’ measures since they make use of natural (soft) coastal material. Examples are beach nourishments, maintenance dredging and land reclamation.

The second category of structures or ‘hard’ measures can be divided into various func-tional groups:

• Seawalls and revetments are built parallel or nearly parallel to the shoreline at the land-sea interface with the objective of preventing further shoreline reces-sion. Seawalls are usually massive and rigid, while a revetment is an armouring of the beach face with rock armour or artificial units. Although often used, the efficacy of seawalls and revetments is debatable as we will discuss later on; • Groynes are built perpendicular to the shore and usually extend out through

the surf zone under normal wave and water level conditions. They help widen and protect a beach by trapping sand from the alongshore transport system (see

Ch.8) or by retaining artificially placed sand;

• Jetties are structures built at the entrance to a river or tidal basin to stabilise the entrance as well as to protect vessels navigating the entrance channel;

• Breakwaters primarily protect a shoreline or harbour anchorage area from wave attack. Breakwaters may be located completely offshore and oriented parallel to the shore (detached breakwaters), or they may be oblique and connected to the shore. Traditionally, detached breakwaters have been designed as emerged structures, but submerged breakwaters have now also become a popular option. The latter are not easily noticeable because of their low crests;

• Other structures such as submerged pipelines. The purposes of these (‘soft’ and ‘hard’) works are diverse:

• Control of shoreline erosion;

• Defence against flooding caused by storms, tides;

• Development of coastal functions, for instance coastal recreation; • Development of navigation channels and harbours.

Coastal engineering works are carried out in a highly dynamic and energetic environ-ment. The various sources of coastal energy are:

• Marine forces (waves, tides, currents and other oceanographic phenomena); • Terrestrial forces (river outflow);

• Atmospheric forces (coastal winds and climate).

These forces not only directly impact the planned ‘soft’ or ‘hard’ measures, but also permanently change the physical shape and structure of a coastal system. This shape of the coast is called morphology. Coastal morphology for a sandy stretch of coast thus is the topography of the sandy dunes and beach and the underwater topography of the seabed. When changes occur in the external forcing, the coastal morphology Last change date: 2021-01-21

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1.2. Coastal dynamics for coastal engineers 3 will change accordingly. Changes in the forcing can have a natural cause or can be human-induced. Examples of changes in the forcing conditions are:

• High waves and piling up of water against the coast (surge) due to the occurrence of a storm;

• Long-term sea-level rise;

• Changes in the wave impact on the adjacent coast due to the construction of a harbour;

• The deprivation of a coastal system of sediment supply due to the construction of a dam in a river.

Although coastal changes take place on a variety of timescales, coastal engineers and managers are mostly interested in timescales ranging from 1 year to (a few) hundred years and in large impact events like storms causing dune erosion and flooding.

1.2.2. Position of Coastal Dynamics in the DUT curriculum

Coastal engineering is a very broad profession. As an illustration, the program of the two-yearly International Conference on Coastal Engineering (ICCE) includes discip-lines as hydrodynamics, coastal morphology, coastal protection, structures and ports and waterways. At DUT these disciplines are covered in a number of courses within the specialisations Coastal Engineering, Rivers, Ports and Waterways, Environmental Fluid Mechanics and Hydraulic Structures and Flood Risk.

Coastal Dynamics I and II

For an appropriate choice and design of measures the dynamics of the forcing and re-sponse of the coastal system should be taken into account. For that reason the courses Coastal Dynamics I and II focus on the dynamics of the coastal system (waves, currents, sediment transport and morphology). Coastal Dynamics I (CIE4305), which is served by this textbook, treats the most important coastal and estuarine phenomena. Besides, attention is paid to functional design and impacts of engineering works. Coastal Dy-namics II (CIE4309) goes into more detail regarding coastal processes and pays special attention to coastal modelling. Coastal Dynamics I is compulsory for Coastal Dynam-ics II.

In both Coastal Dynamics I and II the focus is on the dynamics of coastal systems built-up by loose and relatively fine material (we consider mainly sand and to a lesser extent mud). This material has been delivered to the coast in the recent geological history,

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4 1. Overview Relevant prior knowledge

Fluid mechanics and wave theories are indispensable topics in coastal dynamics. Coastal Dynamics I builds upon the Bachelor courses treating flow, wave and transport dynam-ics. Of the MSc courses, preferably Ocean Waves (CIE4325) should be followed prior to Coastal Dynamics I. A summary of the (offshore) aspects of wind waves and tides that

are relevant to Coastal Dynamics I are given in Ch.3and will be built on in subsequent

chapters.

1.3. Study goals

At DUT, this textbook accompanies the course Coastal Dynamics I. The study goals for Coastal Dynamics I are:

1. To be able to explain the characteristics of coastal systems on the basis of geotec-tonics, historic sea-level changes, nature and availability of material and forcing

by waves and tides (Chs.2and4);

2. To be able to assess the expected response of a coastal system in the case of nat-ural or human-induced changes and to explain – in words as well as in equations – and analyse the underlying physical processes, viz. the hydrodynamic forcing of a coastal system, the sediment transport processes and the resulting coastal

morphology (Chs.3and5to9);

3. To be able to discuss the nature and complexity of problems a coastal engin-eer will be faced with regarding coastal protection against flooding and erosion

(Chs.1and10, but also Chs.7to9);

4. To be able to discern various methods for protection against flooding and erosion

and to assess their merits and disadvantages for specific situations (Ch.10).

1.4. Examples of engineering applications

1.4.1. Overview of coastal area and problems

Figure 1.1 shows a schematic plan view of a coastal area with most of the natural

forcing conditions, natural features and some examples of man-made interventions in a natural coastal system. Most of the items are dealt with in these lecture notes.

The coastline in Fig.1.1receives sediment from rivers and is interrupted by openings

called tidal or coastal inlets (see Intermezzo9.2for the terminology). The name tidal

inlet refers to the fact that the tide is important in maintaining the inlet, viz. keeping Last change date: 2021-01-21

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1.4. Examples of engineering applications 5 militar y z one t A A beach dunes artificia l isla_nd ero sio_n

former channe l gro_yn es st_or m sur ge barrier ac cre tio n revetmen t

entrance channel

primary waves secondary waves ebb-tidal delta barrier island tidal basi n or lagoon

pipeline and cable

intertidal_area

wind farm harbour harbour river dis charge estua ry estuar y tidal inlet tidal in let jetties breakwaters delta sand engine High Water (HW) Low Water (LW)

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6 1. Overview the inlet from closing naturally. Tidal inlets are either found along barrier island coasts or along coasts interrupted by estuaries or lagoons. Tidal inlets and their associated basins are common features of lowland coasts all around the world.

Seen from the sea side, an estuary is an arm of the ocean that is thrust into the mouth and lower course of a river as far as the tide reaches. Estuaries receive fresh water from rivers, and salt water from the sea. Lagoons do not have a major point source of fresh water input, such as a river. These tidal systems play a crucial role in the sediment budget of the coastal zone and thus influence the long-term coastal evolution.

At the uninterrupted stretches of coast, waves are the dominant forcing agent. Wind

waves and tides are treated in detail in Chs. 3 and 5 for oceanic and coastal waters

respectively. The global variation in wave and tidal climate is discussed in Ch.4.

Some examples of practical cases are given in this section by briefly discussing the

following items from Fig.1.1:

• Cross-shore profile (section A – A in Fig.1.1);

• Morphological development in vicinity of a port; • Delta near a river mouth;

• Tidal inlet;

• Dune erosion during a severe storm surge; • Large artificial island in open sea.

1.4.2. Cross-shore profile

The upper panel of Fig.1.2shows the shape of a cross-section (called cross-shore

pro-file) as measured perpendicular to a sandy coast at two moments in time. The lower panel shows the variation in time around an average profile. Please notice that the vertical and horizontal scales of the plot are quite different. Dunes, beach and a part of the so-called shoreface can be discerned. The actual slope of the dune face is 1:3 to 1:4. The slope of the beach is decreasing from the upper part of the beach near the foot of the dunes (1:20) towards the sea; near the waterline the slope is approximately 1:50. At the shoreface some (breaker) bars are present. The average bottom slope becomes flatter with longer distance from the waterline. At the seaward end of the plot (water depth: MSL −10 m) the slope is approximately 1:125.

The water level as indicated in Fig.1.2reflects the Mean Sea Level (MSL). This is the sea

level averaged over a period of time such as a month or a year such that periodic sea-level changes e.g. due to waves and tides are averaged out. In the Netherlands, MSL is approximately equal to the Dutch reference level called NAP (Normaal Amsterdams

Peil in Dutch)1. The seabed consists of sandy material that generally fines when going

further offshore. A typical grain size for a sandy coast is 𝐷₅₀ = 200 µm; 150 million of

these particles fit into a volume of 1 litre!

1_{Recently, MSL has been around NAP + 0.06 m in the Netherlands.}

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1.4. Examples of engineering applications 7 -10 0 10 20 2004 2011 -200 0 200 400 600 800 1000 cross-shore distance [m] -10 0 10 20 Range 1970-2015 Time-averaged

Figure 1.2: Cross-shore profiles at Egmond aan Zee (RSP 7004125) from the long-yearly Jarkus dataset (JARKUS, n.d.), which contains profiles of the entire Dutch coast measured in sub-sequent summers. The upper panel shows two instantaneous profiles, whereas the lower panel displays the temporal variation from 1970–2015. The cross-shore coordinate is relative to a local beach pole or Rijksstrandpaal (RSP) in Dutch. The elevation is relative to the Dutch refer-ence level NAP.

Under the influence of waves the position of the coastline (say represented by the intersection of MSL and the profile) will continuously change. Variations will take place on the timescale of storms and seasons. During storms, high and long waves cause erosion of the beach. This sediment is deposited in the surf zone (the zone where the waves are breaking). A typical storm profile therefore has a narrow beach and a relatively flat slope. Seasonality is especially evident in the Northern Hemisphere

which experiences a large number of storms in winter (see Sect.4.3.1). Hence, a storm

profile is also often called a winter profile. During storms high water levels can also

cause dune erosion and flooding (see Sect.1.4.6). In summer the sand is moved back

towards the beach and dunes by lower and shorter waves. This cross-shore transport

of sediment causes oscillations of the coastline (Fig. 1.3), but in principle the mean

position of the coastline does not change. The mean position of the coastline will only change in the case of a structural loss or gain of sediment; structural erosion may occur

when sand disappears in offshore canyons or in the alongshore direction (Sect.1.4.3).

Seasonal variations are relevant to tourism (beach width) or the safety of property close to the brink (highest point) of the dune.

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8 1. Overview storm/winter MSL summer MSL storm/winter MSL summer MSL bar berm

Figure 1.3: Summer and winter profile showing the annual changes in beach profile.

Suppose that irregular waves (also called random waves) approach the coast

perpen-dicularly with a significant deep water wave height of 𝐻_𝑠,0 = 2 m and a peak period

of 𝑇_𝑝 = 10 s (for the definitions see Sects.3.4.2and 3.4.3). The wave height

distribu-tion along the profile (Fig.1.4a) can be calculated with numerical models based on a

spectral energy or action balance (Sects.3.5.3and5.2.1). Notice that the wave height

in Fig.1.4areduces gradually when the waves approach the waterline. This is due to

energy dissipation due to (partial) wave breaking and bottom friction.

In Fig. 1.4b the maximum horizontal components of the orbital velocities near the

bed are plotted as a function of the position in the cross-shore profile. Notice the rather large magnitudes (>1.0 m/s at many positions). Realizing that the critical

velo-city to initiate motion in uniform flow for particles with 𝐷₅₀ = 200 µm is approximately

0.2 m/s (see Sect.6.3), one may understand that the waves of Fig.1.4aare able to stir

up many particles in the cross-shore profile. The turbulence generated by breaking waves is very effective in keeping those particles away from the bed. The particles can subsequently be transported by for instance wave-generated and tidal currents

(see Ch.5). Also asymmetric waves can give a net sediment transport. Chapter6

dis-cusses sediment transport in general, whereas Ch.7focuses on cross-shore transport.

Both the magnitude and direction (onshore-offshore) of the cross-shore transport may change depending on the local hydrodynamic conditions.

Often the effect of cross-shore oscillations is assumed to average out on the longer term. In those cases structural trends in coastline position are due to sediment trans-port along the coast (or more correctly due to gradients in longshore transtrans-port, as we

will see in Sect.1.4.3). Nevertheless, some important practical problems are related to

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1.4. Examples of engineering applications 9 0 100 200 300 400 500 cross-shore distance [m] -6 -4 -2 0 2 el ev ation [m + MSL] 1 (a) 0 100 200 300 400 500 cross-shore distance [m] 0 0.2 0.4 0.6 0.8 1 orbital velocity [m/s] (b)

Figure 1.4: Wave height and orbital velocity along a cross-shore profile computed with XBeach (Roelvink et al., 2009) based on a JONSWAP spectrum (Sect.3.5) with peak period 𝑇𝑝 = 7 sec

and significant wave height 𝐻𝑠 = 1.9 m. (a) 𝐻𝑠 (solid line) and 1 % exceedance wave height 𝐻1%

(dashed line) calculated from a Rayleigh distribution (Sects.3.4.2and3.4.4); (b) Horizontal or-bital velocity near the bed (root-mean-square velocity 𝑢𝑟𝑚𝑠).

changes in the shape of the profile with time (e.g. dune erosion and the behaviour of beach and shoreface nourishments).

1.4.3. Morphological development in vicinity of a port

Figure1.5shows in plan view a part of a uniform sandy coast. Uniform means that the

depth contours are assumed to be straight and parallel. Waves obliquely approach the coast, viz. there is a non-zero angle 𝜑 between the wave crests and the depth contours (or equivalently between a wave ray and the shore normal). As the waves approach

the shore the angle becomes smaller due to refraction (Sect.5.2.3).

Inside (and a little bit outside) the surf zone, sediment is transported along the coast,

the so-called longshore transport (Ch.8). Waves continuously stir up material from

the bed. This sediment is then transported by the longshore current. This current is

generated by the breaking of obliquely incident waves in the surf zone, see Sect.5.5.5.

Due to the wave action in stirring up the material, a longshore current of 1 m/s is much more effective in transporting sand than a river flow with the same magnitude.

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10 1. Overview If the coast is uniform indeed, the sediment transport 𝑆 is constant along the coast and the coast remains stable. The coastal section under consideration will only change when the amount of sediment transported into the section is different than the sed-iment leaving the section; or in other words: when there is a gradient in longshore transport rates. Erosion will occur in the case of a positive gradient in the transport direction (the sediment transport is increasing along the shore, and hence more sedi-ment is leaving than entering the section). Accretion occurs in the case of a negative gradient in the drift direction. A uniform sediment transport along the coast (no gradi-ent) does not change the coast. This leads to one of the most important notions of these lecture notes:

Coastal changes occur in the case of transport gradients. A positive gradient (an increase in the sediment transport in the transport direction) leads to erosion. A negative gradient (a decrease in sediment transport in the transport direction) creates accretion. If the gradient is zero there are no changes in morphology. If along the uniform sandy coast a port is built with the help of two rather long

break-waters, much longer than the width of the surf zone (see Fig.1.6), the longshore

sedi-ment transport will be interrupted. Seaward of the breakwaters no sedisedi-ment transport is assumed to occur. On the updrift side of the port, accumulation of sand will occur (negative transport gradient); at the downdrift side (lee side) erosion will take place

(due to a positive transport gradient). In Fig. 1.6some coastline positions have been

sketched as a function of time.

After having studied these lecture notes, you should be able to understand and describe the shapes of the coastline at updrift and downdrift sides as a function of time. It is to be expected that sooner or later the accreting coastline on the updrift side reaches the end of the updrift breakwater; you should be able to say at what time after the construction of the port this will happen. For the time being it will be clear that as long

as no sediment will pass the breakwaters (𝑆 = 0 m3/yr seaward of the breakwaters) the

total accumulation of sand in 𝑡 years after completion of the port on the updrift side will

be 𝑡 × 𝑆 m3(with 𝑆 the undisturbed transport rate). That is, when we assume that 𝑆 in

waves

longshore current surf

zone S [m3_/yr]

Figure 1.5: Plan view of a uniform sandy coast. In the zone where waves are breaking (called

breaker or surf zone), a wave-driven sediment transport takes place along the coast.

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1.4. Examples of engineering applications 11

longshore current waves

S [m3_/yr]

S = 0

Figure 1.6: Plan view of a uniform sandy coast with port breakwaters. Note the updrift accretion and lee-side erosion. The latter is a (too) simplified sketch of the real shape of the lee-side erosion, see Ch.8.

m3/yr is expressed including pores between the grains, so that the volumes represent

deposited volumes. (In Sect.6.4other units of sediment transport are discussed). The

total erosion on the lee side will be 𝑡 × 𝑆 m3as well.

In most cases accumulation will not be considered problematic by the coastal zone manager involved; valuable new land has been gained (unless sediment is deposited in navigation channels). However, the erosion on the lee side of the port will sooner or later cause serious problems. How to resolve such types of erosion problems will

be discussed in Ch.10. An obvious solution will be to artificially transfer volumes of

sand from the one side of the port to the other (𝑆 m3/yr on average; a so-called sand

bypass system). Summarizing:

Engineering problems are often related to longshore transport gradients. Struc-tural coastal problems arise when the longshore transport is changing alongshore, for instance when the longshore current is interrupted by harbour breakwaters.

In the example of Fig.1.6, the longshore sediment transport was due to obliquely

ap-proaching waves only. If also tidal currents occur along the coast, the morphological behaviour becomes more complicated. The combination of a wave-induced and tidal

longshore current is discussed in Sect.5.7.2.

1.4.4. Delta near a river mouth

Figure1.7shows in plan view a part of a sandy coast with a river outfall. Waves are

assumed to approach perpendicular to the (initial straight) coastline. The river

dis-charges a volume of water 𝑄_𝑟 m3/s to the sea; at the same time sediment is transported

by the river; say 𝑆_𝑟 m3/yr. 𝑆_𝑟 is expressed in m3/yr since we are looking at large

mor-phological timescales.

Like many rivers all over the world, also the river of Fig. 1.7acts as source of

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12 1. Overview between river and sea and to be able to predict the morphological changes with time,

as shown in Fig.1.7. Such a clear delta coastline develops when the supply of riverine

sediments to the coast is faster than they can be dispersed along the coast by tidal and

wave-generated currents. (See Chs. 2and 8for a more detailed discussion of deltaic

coastlines.) The stability of a deltaic coastline is very dependent on the river sediment supply. If the sediment supply is cut-off or reduced by for instance the construction of dams, sand mining or irrigation schemes, the system is deprived of its regular supply of sediments. This leads to erosion of the coastline on either side of the river mouth and is quite common for present-day deltaic coastlines.

1.4.5. Tidal inlets and basins

Tidal inlets are openings in the shoreline, such as between two barrier islands that connect bays or lagoons to the open ocean. They are maintained (viz. kept from

closing naturally) by tidal currents. Figure1.8shows various tidal inlets in the Wadden

Sea, the Netherlands. Essential for a tidal inlet is the tidal variation in the open sea; the tide is the engine that determines most of the occurring features of the inlet and the basin it connects to.

The tidal range – the difference between High Water Level (HWL) and Low Water Level (LWL) – and the surface area of the tidal basin together determine in principle the volumes of water that have to flow in and out through the inlet during a tide.

This tidal prism is in some cases in the order of magnitude of one billion (1 × 109)

m3. For instance, during one tidal cycle about 109m3of water enters the Texel Inlet or

Marsdiep (between Texel and Den Helder, see Fig.1.8) and leaves the Texel Inlet again.

A typical tidal inlet system consists of several morphological units (see Ch.9for more

details):

• The actual entrance or tidal inlet, often dominated by a main channel;

• A shallow ebb-tidal delta seaward of the inlet that often folds around a deep channel. original coastline Q_r [m3_/yr] S_r [m3_/yr] (a) Q_r [m3_/yr] S_r [m3_/yr] (b)

Figure 1.7: Plan view of a sandy coast with a river outfall in initial situation (a) and with a devel-oping delta (b).

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1.4. Examples of engineering applications 13

Figure 1.8: Tidal inlets in the Wadden Sea (the Netherlands). The elevations are from the Vaklodingen dataset (https://publicwiki.deltares.nl/display/OET/Dataset+documentation+ Vaklodingen) and are w.r.t. to NAP.

• The flood basin with possibly a distinct flood-tidal delta just landward of the inlet and an inner tidal basin consisting of the channels that are followed by the tidal currents and of the lower and higher tidal flats that alternately inundated and exposed by the tides and possibly covered by salt marshes or mangroves. The position of the different elements of the tidal inlet system (e.g. ebb-tidal delta, flood-tidal delta, flood channels, ebb channels, shoals, tidal flats and gorge) is

chan-ging with time. Figure 1.9 shows a schematic plan view of a tidal inlet with some

characteristic notions.

water

shed tidal basin watershed

gorge ebb HW LW flood t η

Figure 1.9: Schematic plan view of a tidal inlet with the tidal water level variation, outflowing ebb and inflowing flood currents and watersheds. At watersheds tidal flow through different inlets meets and tidal currents are zero.

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14 1. Overview From a morphological point of view, tidal inlets form highly dynamical systems, which are interlinked with the adjacent coast and the tidal basin or backbarrier area to which they give access. Often, the natural morphodynamic behaviour interferes with unnat-ural constraints (e.g. coastal defence works) and with the effects of human utilisation (e.g. sand mining). The dynamic behaviour may complicate navigation. Sometimes measures are taken to restrict the dynamic behaviour of the morphological system.

1.4.6. Dune erosion and flooding during a severe storm surge

Figure1.10 shows an instantaneous cross-shore profile (cf. Fig. 1.2). Under normal

conditions the water level is changing due to vertical tidal variations. On top of that, shorter variations due to waves are present. Due to changing water levels and changing wave characteristics (wave height, wave period and wave direction), cross-shore sediment transport rates (and the cross-cross-shore transport gradients) continuously change in magnitude and direction. This normal variation was already discussed in Sect.1.4.2. 0 200 400 600 800 1000 cross-shore distance w.r.t. RSP [m] -10 0 10 20 el ev ation [m + N AP] MHW MSL MLW = ≈ = NAP + 0.82 m NAP + 0.06 m NAP – 0.79 m

Figure 1.10: Instantaneous (summer 2011) cross-shore profile at Egmond aan Zee (RSP 7004125). Tidal elevations are calculated for 2018 and assumed static for the entire JARKUS period of 1965-present (JARKUS,n.d.).

During a severe storm the waves generated in the open sea will be much higher than normal. Depending on the direction and strength of the wind during the storm and on the shape of the sea bordering the coast under consideration, water can pile up at the coastline raising the Still Water Level (SWL) (without the effect of waves). This piling up of water is called surge.

During the severe storm surge of January 31stand February 1st1953 in the Netherlands,

the water level along the Dutch coast was about 2.5 to 3.0 m higher than normal. The wind associated with this storm came from northwest directions, blew over the funnel shaped North Sea and forced the water to pile up against the coasts of the southern

part of the North Sea (see Fig.1.11). The rather small gap between United Kingdom

and France (The Channel) prevented the raised water from escaping from the North Sea basin. During the 1953 storm surge many dikes in the southwestern part of the Last change date: 2021-01-21

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1.4. Examples of engineering applications 15 Netherlands broke. Nearly 1850 lives were lost and there were large economical con-sequences as well. This disaster triggered the execution of the so-called Delta plan in the Netherlands.

NW

0 150 1000

depth [m]

Figure 1.11: Funnel shaped North Sea with the wind blowing from the northwest (NW). The darker the shade the deeper the water. Notice the distinction between the shallower shelf region and the deeper oceanic waters. Bathymetric data from GEBCO (https://www.gebco.net/data_ and_products/gridded_bathymetry_data/).

Figure1.12shows some characteristics of the measured and predicted water levels in

Flushing (Vlissingen in Dutch). Note that the storm effect (the surge) is the differ-ence between the actual measured and the predicted astronomical (tidal) water level variation. Note also that the surge lasted in fact a rather small period of time; within two days the storm effect rose from zero to approximately 2.8 m and fell down to zero again.

In Fig. 1.13the same cross-section as in Fig. 1.10 has been sketched, but now under

maximum storm surge conditions. Not only is the SWL much higher than in Fig.1.10,

but also much higher waves are present. The SWL even exceeds the level of the dune foot; all the beaches have disappeared under water and the waves hit the dunes. It can be argued that the shape of the cross-shore profile does not correspond to the storm surge conditions (the profile shape is far out of equilibrium for these conditions). Large offshore directed sediment transports ensure that the profile shape is transformed to-wards the equilibrium shape associated with the storm conditions. The dunes erode and the eroded dune material settles in deeper water, where the cross-shore profile

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16 1. Overview -2 -1 0 1 2 3 4 el ev ation [m + N AP] 09/0 2/20 10/0 2/20 11/0 2/20 12/0 2/20 13/0 2/20 14/0 2/20 -1 0 1 2 meteorological effect [m] recorded level predicted level -2 -1 0 1 2 3 4 5 el ev ation [m + N AP] 31/0 1/53 01/0 2/53 02/0 2/53 03/0 2/53 0 1 2 3 storm surge [m] recorded level predicted level Vlissingen

Figure 1.12: Measured and predicted water levels at Vlissingen (the Netherlands). The upper panel displays the predicted astronomical tide (grey line) and the actual measured water levels (black line) during a storm in March 2020 (https://waterinfo.rws.nl/). The difference between these two is the meteorological effect (second panel) and consists of both the storm surge and the interaction between the tide and the storm surge (see Bijlsma et al. (1989)). The third panel shows the astronomical tide and recorded water levels during the disastrous storm in 1953. The fourth panel show the corresponding storm surge only, which is not exactly equal to the difference between predicted and measured water levels, because of the effect of interaction between the tide and the storm surge.

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1.4. Examples of engineering applications 17 MSL ≈ NAP + 0.06 m SSL = MSL + 5 m -100 0 100 200 300 400 500 600 cross-shore distance w.r.t. RSP [m] -5 0 5 10 15 20 ele vation [m +N AP] pre-storm post-storm erosion deposition

Figure 1.13: Illustration of a storm impact on the cross-shore profile. The pre-storm profile (dashed line) is a characteristic summer profile (summer 2015) for the Dutch coast, averaged over a 3 km long stretch near Zandvoort (data from JARKUS,n.d.). The post-storm profile (solid black line) as a consequence of a storm surge with Storm Surge Level (SSL) and storm wave conditions is estimated after (Vellinga,1986), see Fig.7.18for details.

In Ch.7a more detailed description is given of the associated processes. For the time

being it is sufficient to understand that a coastal zone manager likes to know what the loss of dune area will be under a given set of storm surge conditions e.g. to assess the safety of properties built close to the brink of the dunes. In the Netherlands also the safety of a large part of the population, living well below MSL behind the dunes, is at stake if a break-through of the dunes occurs.

1.4.7. Large artificial island in open sea

In densely populated areas (e.g. Japan, Taiwan or the Netherlands) it is becoming increasingly complicated to find large open areas at the mainland to start new devel-opments (e.g. for a new airport, hosting new industries or even housing the growing population).

The open sea in front of the existing coast may be used to build an artificial island. In the Netherlands, for instance, there has been ongoing discussion on whether it is useful to build an artificial island off the coast near IJmuiden for hosting an extension of Schiphol Airport. A specific airport-island requires rather large dimensions (amongst others due to the length of runways). A typical size for such an island is 5 km. With respect to the distance of an island from the existing coast, an optimum has to be found taking into account transportation requirements, noise limitations and morphological implications.

An artificial island in open sea has large morphological implications for the existing coasts and affects the stability of coasts in a wide area. In the shadow area behind the island the wave characteristics will fundamentally change. Another aspect of an airport-island is that the (tidal) current patterns will be affected in the vicinity of the island. The adaptation time of the existing coastal area is generally relatively long. The water depth off the coast of IJmuiden, where such an island would probably be

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18 1. Overview built, is approximately 15 m. Because of this depth, the required dimensions of an island and the required level above MSL (approximately 5 m) huge volumes of sand

are required to construct such an island (order of magnitude 600 million m3 of sand.)

These volumes are easily available from the bed material of the North Sea, but call for large borrow pits which also impact the morphology.

During dredging operations also fine-grained silt (see Sect. 2.6.2) will be mobilised,

since within the sand deposits at the seabed often small volumes of silt occur (say 2 %).

With a required volume of 600 million m3and 2 % silt, this results in 12 million m3of

silt that is mobilised in the North Sea environment. The associated turbidity can have large ecological effects.

It is clear that in the final decision whether to build an artificial island or not, coastal morphology topics have to play a role.

1.4.8. Other examples

A few examples relevant to coastal engineering practice have been discussed in this section. Other examples are:

• Siltation of (dredged) navigation channels;

• Erosion near toe of breakwaters, sea-walls or offshore structures; • Structural / gradual erosion of coasts;

• Impact of coastal protection tools, like groynes, offshore breakwaters and sea-walls and revetments;

• Behaviour of artificial beach and shoreface nourishments; • Blockage of sediment supply by a river.

In all cases sediment transports due to waves and currents are important. Compared to sediment transports due to currents alone (like in rivers), the waves enhance the sediment transport and make the calculation of the transport rates more complicated.

In Ch.6, sediment transport due to waves and currents is discussed in detail.

1.5. Coastal (morpho)dynamics

1.5.1. Definition of the coast

So far we have assumed that it was clear what we meant by ‘coast’. The definition of coast however depends on the objective and the timescale under consideration. Coasts are the transition zones between oceans and continents. The coastal zone is made up of:

• the part of the land that is affected by being near to the oceans (coastlands); and • the part of the oceans that is affected by being near to the land (coastal waters).

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1.5. Coastal (morpho)dynamics 19 The coastlands encompass all terrain features that are influenced by coastal processes like dunes, cliffs and low-lying areas (coastal plains). In a practical situation, the in-land extent depends on the timescale under consideration. A coastal engineer, who is mostly concerned about timescales of years, would say that the coast extends inland as far as the influence of the tides and storm waves reaches. Although for a large estuary the limit of inland tidal propagation can be up to hundreds of kilometres, this defini-tion of the landward extent is relatively narrow in the eyes of a geologist. A geologist would be aware of the fact that in former times the sea has reached higher levels than the present levels and might find evidence of that in the form of coastal deposits far from the present influence of the sea. Therefore, a geologist would include all these areas in a definition of the coast. Half of the Netherlands consisting of polders below sea level would then belong to the coast.

Similarly, the seaward limit of the coast is dependent on the timescale under consider-ation. Engineers have introduced the so-called depth of closure as the most seaward point of interest. The depth of the beach profile closure is the depth beyond which repeated field observations of the bed height over a certain period of time show no significant changes. It can be empirically determined by examining a series of profile measurements over a period of months to years so that both calm periods and storm

conditions are included (Fig.1.14).

seasonal range of sand level mean sand level

offshore zone shoal zone littoral zone

shelf lower shoreface upper shoreface

SSL MHWS MLWS MSL

Figure 1.14: Envelope of beach profiles measured at different times, over a period of for instance a year. The two depth limits 𝑑𝑖 and 𝑑𝑙correspond to the closure depth definition of Hallermeier

(1978,1981), see also Sect.7.2.3. The profile is dynamic landward of the outer depth limit 𝑑𝑖.

The majority of the bed dynamics take place at depths smaller than the inner depth limit or annual closure depth 𝑑𝑙, where 𝑑𝑙is the maximum water depth for nearshore erosion by extreme

conditions exceeded twelve hours per year. The tidal levels Mean High Water Spring (MHWS) and Mean Low Water Spring (MLWS) are explained in App.C.

The beach profile envelope in Fig. 1.14 includes typical accretional profiles built up

during months of moderate wave conditions and typical erosional profiles under the influence of large storm waves. It shows that the profile is divided into an inactive offshore and a more active nearshore zone called shoreface. The shoreface is the part

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20 1. Overview of the sandy profile affected by wave action and typically extends to water depths of 10 m to 20 m. The shoreface can further be separated into the shoal zone (or shoaling zone or lower shoreface) where waves gain amplitude up to the point of breaking and the most active zone where waves are breaking and the majority of the changes take place (breaker or surf zone, littoral zone or upper shoreface).

When considering larger timescales we have to take into account that coastal processes

have reached up to the continental shelf (Fig.1.15) when sea levels were lower than

present. The continental shelf is the edge of a continent and is covered with relatively shallow seas up to 100 m to 200 m water depth (up to the shelf break).

estuary barrier

Quaternary coastal morphology

coastal plain continental

slope continental shelf shoreface

shelf break

waves start ‘feeling’ bottom

Quaternary sediments

Figure 1.15: Spatial boundaries of the coastal zone. The Quaternary coastal morphology reaches up to the shelf break. The transition between the shelf and the shoreface is located where waves propagating towards the shore start feeling the bottom.

In the broadest sense of the word, we can now characterise coastal systems as relative shallow areas (i.e. depths less than order 100 m) bordered by or partly enclosed by land that are influenced by the sea and connected to the oceans, and in which ocean disturbances propagate. Due to the intensive interaction between land and water at the interfaces of the two, a large variety of coastal ocean systems has developed. The

landward side includes the partially enclosed basins (such as the estuaries of Fig.1.16b).

1.5.2. Coastal morphodynamics

In Sect. 1.4.3 we had already concluded from continuity considerations that coastal

changes occur in the case of gradients in sediment transport rates. This of course not only holds for the coastal stretch considered there, but for coastal systems in general. Consider for instance the sediment budgets along the estuarine and deltaic coasts of

Fig. 1.16. Changes in the morphology of these systems depend on the spatial and

temporal fluctuations in the sediment transport rates. In terms of a continuity equation or mass balance: 𝜕𝑧_𝑏 𝜕𝑡 + 𝜕𝑆_𝑥 𝜕𝑥 + 𝜕𝑆_𝑦 𝜕𝑦 = 𝑉 (1.1)

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1.5. Coastal (morpho)dynamics 21 where:

𝑧_𝑏(𝑥, 𝑦, 𝑡) bed level above a certain horizontal datum m

𝑆_𝑥(𝑥, 𝑦, 𝑡), 𝑆_𝑦(𝑥, 𝑦, 𝑡) sediment transport rates per m width of flow

in the horizontal 𝑥- and 𝑦- direction, including the effect of porosity

m3/m/s

𝑉 (𝑥, 𝑦, 𝑡) sink or source term per unit area

representing local sediment gains and losses, often taken as zero

m3/m2/s cliff erosion progressi ve beach ridge plain overwash inputs outputs output input output input inpu t (a) output input rocky headland inputs outputs transgressiv e beach ridge plain barrier overwash storm cut inpu t output (b)

Figure 1.16: Sediment budgets along (a) deltaic and (b) estuarine coasts. In the example, the deltaic coast gains sediment; the sum of the inputs is larger than the sum of the outputs. This is reversed for the estuarine coast that loses sediment.

If the net sediment flux into a certain area is negative, meaning that the outgoing sediment flux is larger than the incoming one, the bottom will supply the sediment

Coastal Dynamics

Coastal Dynamics

Judith Bosboom and Marcel J.F

. Stive

Judith

Bosboom

and

Marcel

J.F

. Stive

| Coastal Dynamics

Judith

Bosboom

and

Marcel

J.F

. Stive

| Coastal Dynamics

Delft University of Technology

Coastal Dynamics

Judith Bosboom & Marcel J.F. Stive

Contents

Preface

1

Overview

1.1. Introduction

1.2. Coastal dynamics for coastal engineers

1.2.1. What is coastal engineering?

1.2.2. Position of Coastal Dynamics in the DUT curriculum

1.3. Study goals

1.4. Examples of engineering applications

1.4.1. Overview of coastal area and problems

1.4.2. Cross-shore profile

1.4.3. Morphological development in vicinity of a port

1.4.4. Delta near a river mouth

1.4.5. Tidal inlets and basins

1.4.6. Dune erosion and flooding during a severe storm surge

1.4.7. Large artificial island in open sea

1.4.8. Other examples

1.5. Coastal (morpho)dynamics

1.5.1. Definition of the coast

1.5.2. Coastal morphodynamics