687

1 INTRODUCTION

Tombolo is a term used to describe a

geomorphological feature [6, 46], a narrow strip (a

sandy spit) connecting the mainland to the coastal

island or two islands, consisting of a sand and gravel

deposited by the ocean currents [61] (Fig. 1a) (Fig. 1b).

Waves strike a beach obliquely are creating longshore

currents that move sediment along the coast in a

direction determined by the prevailing wave

approach [63]. In most tombolo around the world, the

sediment material comes from coastal erosion, rivers,

underwater reefs and coastal glacial deposits [18].

Initially, so-called salients are formed. This flattening

of the coastal zone is created in response to the

erection of a hydrotechnical structure or barrier

parallel to the shoreline [39]. If the hydrodynamic

conditions do not change due to the long-term impact

of the structure, a tombolo is formed. The tombolo

phenomenon is a process involving the interaction

between an obstacle and hydrodynamic forces, debris

and the bottom’s bathymetric profile. These factors

after the movement of water, which in turn, affects the

transport of the debris, a change in the bottom

morphology, and the intensity of transport of the

bottom sediment. This is particularly visible when the

coastline is shifting towards a detached obstacle [11].

This phenomenon may have a natural or

anthropogenic origin [5]. Natural tombolo occur when

an obstacle is created by exogenous processes.

Anthropogenic tombolo occur when the obstacle has a

Impact of Hydrotechnical Structures on Forming the

Tombolo Oceanographic Phenomenon in Kołobrzeg and

Sopot

Specht, O. Lewicka, M. Specht & S. Zblewski

Gdynia Maritime University

, Gdynia, Poland

ABSTRACT: The process of global sea level rise is causing several significant changes in the coastal zone. Sea

level rise and the frequency, strength and duration of storms are also occurring on the Polish coast. As a result,

coastal protection measures, such as man-

made engineering structure, are necessary. These engineering

structures affect (among others) the marine ecosystem in different ways. Although the presence of such

engineering structures can cause changes in the bathymetry of waterbody and the transport of sediments along

the basin, it also slows down the erosion of the shoreline. For this reason, comprehensive knowledge of natural

conditions, including dynamic and variable factors, is essential in the construction of a hydro-engineering

structures. The correct determination of the environmental conditions helps to minimize environmental

damage. Prior to interventions on the coast, the issues addressed in the paper should be analysed and studied.

In this paper, the influence of shoreline structures on the main factors responsible for the development of

tombolo phenomenon is discussed. In addition, the lithological diversity of surface sediments on which the rate

of coastal erosion depends, is also discussed. An important element of the work is the descriptions of tombolo

in Poland. They contain information on the causes of the phenomenon, as well as about the negative

consequences of a disturbance of the hydrodynamic dynamics caused by the structure.

http://www.transnav.eu

the

International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 15

Number 3

September 2021

DOI: 10.12716/1001.15.03.25

688

form of man-made infrastructure. An improperly

constructed engineering structure may lead to local

abrasions, bottom deepening, the occurrence of strong

rip currents, and even deterioration of water purity in

the future. For this reason, it is very important to

determine the appropriate parameters for the

construction of a breakwater or other hydrotechnical

structures.

a) b)

Figure 1. Example of tombolo occurrence when the obstacle

is a breakwater (a) or an island (b)

Figure 2. Satellite photo provided by Google Earth Pro

showing the development of tombolo in Sea Palling, UK.

Sources and dates of the images: Google Erath, 28.09.2020

The considerable complexity of the hydrodynamic

processes occurring during tombolo formation leads

to general regularities regarding the influence of

waves and currents on sediment movement.

Computer modelling (currently the most effective tool

for complex analyses) are used to determine changes

in the sea and the coastal zone. Considering the threat

posed by the impact of a hydrotechnical structure on

the coastline, various studies, including bathymetric

analyses [37, 58], are being conducted on the tombolo

phenomenon. Bathymetry change modelling provides

information on the predicted change of a beach profile

[68]. The tombolo phenomenon is analysed from both

geological [53] and geomorphological aspects and

hydrodynamics [2, 45]. The Simulating WAves

Nearshore-SWAN model was used to determine the

influences of waves on the tombolo phenomenon [4].

However, scientists at South Korea designed a CST3D

model, which recreated the process of tombolo

formation in this region. The model confirmed the

hypothesis that the length of the hydrotechnical

structure have an important role in the tombolo

formation [25, 40]. In 1994, an algorithm was

developed in a numerical coastline model that could

calculate changes in the coastline position near the

breakwater [62]. Since 2018, bathymetric

measurements of this phenomenon carried out in

Sopot, completed by research using satellite

photogrammetry [60], UAV photogrammetry [59] and

TLS laser scanning [57].

This review paper presents elements of the marine

ecosystem that have changed due to the impact of the

engineering structures known as tombolo forms. The

paper is divided into eight sections. Section 1

introduces the tombolo phenomenon and reviews

studies dealing with it. Sections 2 to 5 describe the

factors related to the presence of a breakwater that

influences the formation of a tombolo. The paper ends

with general conclusions on dynamic processes

forming a tombolo, while sections 6 to 8 discuss two

examples of tombolos in Poland. The paper ends with

general conclusions on dynamic processes forming a

tombolo.

2 WAVE

Winds are the main energy-generated factor in the

coastal zone, causing the development of wind waves

[70]. The southern coast of the Baltic Sea is dominated

by wind sea and swell [38]. The process that cause

wind waves depend on the character of energy

exchange between the sea and the atmosphere [12].

The wind sea move and grow as long as the wind

blows and die out when the winds stops. The long-

term influence of wind from one direction forms a

swell [41], which is a hazard to navigation. A swell is

characterized by short, discontinuous crest lengths

that are closely spaced. Such a wind wave can reach a

considerable height and have an irregular shape [27].

The strongest waving occurs in areas of shallow water

depth (littoral zone). The largest waves (7-8 m) in the

southern Baltic Sea can be observed in the autumn-

winter period and are caused by long-lasting storms

[71]. In the coastal zone, obstacles in the form of reefs,

breakwaters, islands and rocks are quite common and

disturb the propagation of waves. Waves commonly

approach the beach obliquely–rarely at right angles to

the beach. However, when waves approaching the

breakwater change their characteristics due to the

influence of this structure and participate in the

formation of the coastline. One of the effects of

engineering structures on the wind waves is

diffraction [47]. Wave diffraction is a process in which

the energy of waves propagates perpendicularly to

the dominant direction of wave propagation. Fig. 3

shows the direction of the wave propagation with a

wave diffraction scheme near a vertical wall.

a) b)

Figure 3. Figure showing the direction of the wave

propagation at the ends of an obstacle (a) and the diffraction

scheme near the vertical wall (own work based on: [50])(b)

Wave diffraction is considered as the bending of

waves around an object. It is a kind of movement that

allows waves to move at barriers into harbors as

energy moves laterally along the crest of the wave

[64]. The other water movement is a reflection. This is

a phenomenon of bending of waves when they

689

approach the shore. It causes the crest to rotate into a

position parallel the depth contour of the bottom in

the shallow water near shore. During refraction, most

of the energy transported by the waves is dissipated

and the remaining energy is used to generate currents

that cause sediment transport, both along and across

the shore [36].

There are three types of wave refraction near the

shore (Fig. 4):

− Spilling with the height of the wave decreasing

gradually and foam-forming crest of the wave

− Plunging when part of the wave crest is rolled and

breaks after reaching the maximum wave height

− Surging when after refraction the wave creates a

surf stream that reaches the shore

Figure 4. The figure shows types of wave refraction (own

work based on [36])

The above-mentioned refractive-diffraction

processes form a so-called shadow zone, which

influences the transport and morphology of sediments

[29].

3 CURRENTS

In sea basins, currents are an essential element

responsible for water movement. The ocean current is

defined as the progressive movement of waters,

characterised by a distinctive direction of the resultant

movement and velocity speed equal to the average

velocity of elementary masses [12]. Currents can be

formed under the influence of wind, pressure

gradient, differences in temperature, salinity and

gravity [8]. In the short-term perspective, surface

currents [15] in the Baltic Sea are generated by wind

and their distribution in the coastal zone depends on

bottom topography [17], and coastal morphology.

Moreover, in the Baltic Sea, there is a local deep ocean

current, resulting from differences in depth associated

with salinity and temperature. In addition to local

currents, large-scale circulation is also observed in the

individual Baltic Sea basins, which takes the form of

cyclonic vortices [48], resulting from the interaction of

the Earth's rotation and changes in water depth.

In the area between the breakwater and the shore,

coastal currents play a special role. Characteristic

current circulation is occurs when wave propagation

direction is diagonal to the shore, generating a

parallel and perpendicular flow of water [56]. A

longshore current which occurs when the wave

approaches the shore at a certain angle (Fig. 5.a) is the

strongest in the littoral zone. Longshore currents are

caused by refractive waves, e.g. by the influence of

hydrotechnical structures.

a) b)

Figure 5. Figure showing the longshore current in the shore

zone, without the influence of the structure (a) (own work

based on: [56] and longshore current with the influence of

the engineering structure (breakwater) (b)

The main factor contributing to the formation of

longshore current is the so-called radiation stress [31].

The term radiation stress is defined as the excess of

the momentum stream in a sea area, which results

from the wave motion [48]. The currents generated in

this way flow parallel to the shore. The existence of

technical infrastructure in the coastal zone causes

weakening of the longshore current which, in

consequence, leads to excessive accumulation of

sediments along the coastline. This is visible in a

coastal zone developed with structures (groins, piers,

breakwaters) where a shore bulge is created (Fig. 5b).

Another important current responsible for

transporting thicker sediment fractions from the

coastal zone into deeper sea areas and creating

peculiar sedimentary structures is the rip current [28].

Rip currents are usually generated by waves hitting

the shore perpendicularly and are the effect of the

convergence of water masses in the hitting zone as a

result of the so-called process of water pumping by

the breaking waves [52]. The figure below exemplifies

how rip currents (Fig. 6.a) are created near a

breakwater (Fig. 6.b).

a) b)

Figure 6. Figure showing an example of rip currents (a) and

an example of the formation of rip current near the

breakwater together with the direction of the wave (b) (own

work based on: [48])

In addition to affecting the movement of sediments

in the coastal zone and the formation of coastal

landforms. These currents contribute to many

drownings [55], especially near breakwaters and other

man-made coastal structures. This is due the fact that

the water masses refracting the obstacles and

changing their direction towards the sea. This creates

a so-called corridor where rip currents can reach

speeds of up to about 0.2-0.5 m/s during storms in the

South Baltic [16]. A particularly high risk for a change

in direction of the wave exists where the water meets

the shore at a certain angle due to obstacles or flows

parallel to the shore. In addition, the occurrence of rip

currents leads to stagnation and deterioration of water

cleanliness, which is conducive to the development of

cyanobacteria in summer [14].

690

4 COASTAL ZONE STRUCTURE

The sedimentary material [10], which is largely

involved in the formation of new relief in the coastal

zone, comes mainly from the lithological structure of

subsoil, the geomorphological processes taking place

and, to a small extent, from leaching of Pleistocene

sediments in the seabed and transport of material by

the rivers [10]. The deposition of individual sediments

on the seabed occurred under the influence of the

great Atlantic transgression, which initiated the

processes of abrasion, redeposition and deposition

[42]. The destruction of cliffs in the process of

abrasion brings Pleistocene clay and sand sediments

to the sea [20]. Rivers, in turn, are a source of fluvial

sediments, which include: sands and gravels that were

previously eroded. Another factor in the supply of

material to the coastal zone is eolith. Material carried

by the wind from the land to the sea. Typically, the

beaches are enriched with sediment from beach

nourishment supplied to the shore and from deeper

sea areas. Fig. 7 shows the sources of sediment in the

coastal zone in the southern part of the Baltic Sea.

Figure 7. Figure showing the sources of sediment in the

Baltic Sea

Due to the need to distinguish sediment types in

the Baltic Sea region, Shepard's sediment classification

and Wentworth's grain size classification were used

[42]. The lithological differentiation of the surface

sediment is characteristic for the geological structure

of the ice sheet and results from material selection

during transport under the influence of wave and

bottom currents [26]. In the littoral area, the southern

Baltic Sea is dominated by fine-grained sands [67].

Quartz predominates in the sands, and lamina are

found, often enriched in heavy minerals and shells

with abrasion marks. Other, coarser, sediment types

occur locally, especially near the cliffs. Outside the

coastal zone, there are medium-and coarse-grained

sands, as well as sands of varying grain, sandy

gravels, gravelly sands and gravels. The sediment

distribution is indicative of the highly dynamic nature

of processes occurring in the seabed [43]. In the area

of the tombolo formation in Sopot and Kołobrzeg (Fig.

8), there are three granulometric types of sediments:

fine-grained, coarse-grained, and medium-grained

sands. However, near the coastline fine-grained sand

predominates. Medium and fine-grained sands in the

coastal zone are the group of sediments that are most

easily moved because sediments in the coastal zone

are affected by storm waves that impact the seabed

and cause in the erosion of sandy sediments.

Nevertheless, beaches with fine-grained sand are

frequently found in areas with little influence from

waves or tidal currents.

a) b)

Figure 8. Map showing the lithological diversity of surface

sediments on the area adjacent to the pier in Sopot (a) and

Kołobrzeg (b) according to the classification of F.P. Shepard

(own work based on layers available from: [32])

Unfortunately, the natural distribution of

sediments is increasingly disturbed by anthropogenic

factors [51]. The construction of breakwaters,

hydrotechnical development of the shore and

dredging lead to local changes in hydrodynamic

conditions and associated lithodynamic processes.

Hydrotechnical structures lead to the accumulation of

sandy sediments in the shoreline zone, while

dredging leads to a deterioration of the living

conditions for benthos [22]. The change in the

direction in which waves approach the leeward side

of the breakwater is responsible for sediment

accumulation. Groynes are another example as they

restrict the flow of water along the shore, resulting in

accumulation of material on the beach [22]. The beach

cannot rebuild after a storm surge due to disruption

of circulation along the shore. Most engineering

structure cause the formation of new

geomorphological forms and interrupt of the natural

sediment circulation.

5 SEDIMENT TRANSPORT

Sediment transport is one of the most important

lithodynamic processes, because it determines the

formation of shoreline forms and the formation of

erosion and accumulation zones near hydrotechnical

structures. In the coastal zone, sediments are

transported within the depth range with a noticeable

effect of surface waves on the seabed [35]. Sediment

transport begins with sluggish movement,

accumulation of individual grains in the bottom zone,

or in close proximity. As the waves gradually become

more powerful (for example, during storms), the

wave motion becomes more intense and more grains

are detached from the bottom. This begins the process

of moving the sedimentary material. The magnitude

of transport is rather random depending on factors

such as the diameter of the sediment grains, their

weight (taking into account the buoyancy force),

roughly defined structural features, the roughness of

the bottom and the viscosity at the water-sediment

interface. Two directions of sediment transport can be

distinguished: longshore [3] and cross-shore

(onshore/offshore). The main factors determining the

691

intensity of longshore and cross-shore sediment

transport are the wave parameters at the external

boundary of the coastal zone, themorphology of the

coastal platform and the sediment composition [1].

Longshore transport consists of the accumulated

parallel movement of beach and coastal sand towards

the shore [54]. It is caused by waves approaching the

shore obliquely causing movement of materials along

the beach by a process called drifting. This is formed

by tides, wind, and wave action, and creates a

peculiar system of ocean currents along the coast. The

strongest is the longshore current [1]. Longshore

movement of sediments is particularly noticeable in

the shore zone, which has been developed with

structures perpendicular to the shore. Strong

downstream erosion or upstream accumulation in the

shoreline area is then clearly visible [48]. A negative

sediment balance develops behind the obstacle and a

positive sediment balance appears in front of it, which

may contribute to the formation of new

geomorphological reliefs (Fig. 9.b). As breakwater

blocks sediment runoff, it causes losses in the adjacent

areas that were previously supplied by it [51]. When

there are no obstructions, the sediment material is

transported along the shore and deposited in more

distant sections on the shore or beach (Fig. 9.a). The

sediment participates in the natural exchange of

sedimentation, which demonstrates itself in processes

that lead to leveling to the coast or erosion of the part

of land extending towards the sea [24].

a) b)

Figure 9. Drawing presenting the beaches with natural

sedimentation exchange (a) with the influence of a

breakwater (b)

Cross-shore sediment transport consists of the

movement of sediment perpendicular to the shore

under the influence of waves and balancing wave

currents. It includes both transport towards the sea

during storms and transport towards land, which

predominant during calmer periods [1]. Sediment

transport in the transverse direction is associated with

local changes of the seabed aimed at rebalancing [44].

It is important to observe transport to predict the

seasonal shoreline variability and pollution. Excessive

sedimentation contributes to the increase in pollution

by accumulating harmful substances that can a

negative impact on both the marine ecosystem and

humans [30]. This is why it is so important for cities to

intervene and clean beaches when soil material is

deposited near the beach.

6 TOMBOLO IN KOŁOBRZEG

The Polish coast is largely sandy and vulnerable to

climate change threats [22]. Previous observations and

analyses show a rise in sea level [69, 71], an increasing

frequency of extreme meteorological phenomena and

storms, which leading to an increasing area affected

by coastal erosion. This is particularly noticeable east

of Kołobrzeg and affects up to 334 km of the Polish

coastline (according to the coastline of Maritime

Office) [33]. The sea level fluctuation in this area

reaches 3.4 m and the sea is retreating at an estimated

rate of 0.9 m/year [24]. Taking into account the

changes in coastal morphology, several coastal

protection measures have been implemented in

Kołobrzeg.

Beach nourishment measures have been regularly

carried out in this region, every two to three years

since 1993 [22]. Unfortunately, the reclaimed beach is

destroyed not only by storms, but also by erosion of

the coastal bottom, where continuous deepening is

observed. Investments in the construction of maritime

infrastructure play a significant role in here. In 2010,

as part of the Coastal Protection program (2003) [13],

the old breakwater was demolished and a new 450m-

long rubble-mound western breakwater was

constructed and the eastern breakwater was extended

by 150m [19]. In 2012, a 3-km-long tailrace was

constructed and 35 groynes were installed [22].

However, due to the large bottom depth in this area

and the negative debris balance, increased wash-off of

beach sediments occurs during major storm surges.

Despite the presence of factors threatening beach

reclamation, the formation of tombolo can be

observed on the beach in Kołobrzeg after a storm,

which contributes to the accumulation of pollutants,

algae and bacteria (Fig. 10a) (Fig. 10b).

a) b)

Figure 10. Kołobrzeg beach-view of the eastern breakwater

(a) and groins (b)

Unlike many other tombolos, these occur mainly

seasonally. In Kołobrzeg, the extent of this

phenomenon in every year, which is due to

meteorological factors affecting the hydrodynamics of

the area. It is most noticeable when an anticyclone

forms over the Polish coast, which is characterised by

calm atmospheric conditions. The measures taken to

protect and reclaim the beach and coast did not meet

the initial expectations, and even worsened the

condition of the beach. Groynes and an extended

breakwater stop the flow of water along the coast,

causing fine sand and silt to settle on the beach. In

addition, the protective structures hinder the natural

reclamation of the beach during major water surges

[23]. In the future, tombolo formation in Kołobrzeg

may prevent bathing due to the formation of a muddy

sea bottom near the beach and the occurrence of rip

currents. However, there is no doubt that it was

necessary to build a hydrotechnical structure to

mitigate the effects of coastal erosion.

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7 TOMBOLO IN SOPOT

Thanks to the number and complexity of the

measurements [58], the tombolo in Sopot remains the

best studied phenomenon of this kind. It is located on

the breakwater in the town of Sopot, in the southern

part of the Baltic Sea, on the Gulf of Gdańsk. The Gulf

of Gdansk is a special area because there is a very

peculiar long peninsula in the vicinity, which strongly

influences propagation and thus the energy of the

waves [9]. In this region, storm surges have an uneven

distribution throughout the year, with a maximum

number of surges between September and February.

The strongest surface wind ripples is generated from

north to east, causing extreme waves. However, it

must be taken into account that wind undulation in

the coastal zone, is subject to significant changes

associated with decreasing depth and increasing

bottom friction, which may additionally contribute to

increased movements of bottom sediments along the

coast [7]. On 13-14 October 2009, a severe storm took

place along the eastern part of the Polish coast,

destroying many hydrotechnical facilities and

structures, including the breakwater in Sopot [34].

This was the argument for the construction of a

marina to protect the coast and development the

maritime infrastructure in the area. The orgin of the

tombolo phenomenon dates back to 2011 when the

marina became operational. The analysis of the

coastline [60] clearly confirms the positional changes

due to the influence of the hydrotechnical structure

built in 2011. The structure, which is an extension to

the pier, started to cause changes in the coastline in

the form of emerging tombolo. Bathymetric

measurements carried out by the Maritime Office in

Gdynia since 2010 both the needs of the construction

of the facility and due to changes in the coastline. In

addition, a survey and hydrographic team has been

conducting bathymetric measurements since 2018

unmanned surveying vessel (USV) [59]

photogrammetric measurements from unmanned

aerial vehicles (UAV) [5], terrestrial laser scanning

(TLS) [57] and precision receivers of global navigation

satellite systems (GNSS). Based on analysis and

research, it was determined that the developing

tombolo phenomenon constitutes a threat to tourism

in Sopot, and failure to intervene in the future may

lead to significant changes in the beach structure.

Increased blooms of cyanobacteria and algae,

especially green algae [65] are increasingly observed

in the resort, mostly on the south-western side of the

marina. Blooming causes water turbidity and reduces

water transparency. Eutrophication of the sea is one of

the major threats to the correct functioning of the

marine ecosystem. In addition, mud is deposited on

the boundary between the coast and the beach, where

it lingers in the summer and creates a peculiar smell.

Another visible effect is that the water near the

marina is getting shallower. Failure to update

hydrographic maps in the eastern part of the pier and

the southern part of the marina on an ongoing basis

may cause damage to vessels mooring in the marina.

The negative consequences of the erected structure

also include the occurrence of rip currents, which are

responsible in many cases for drowning [21]. In terms

of the dynamics of changes in the morphological

parameters of the waterfront, the shore section

between the cliff in Gdynia Orłowo and the pier in

Sopot is very distinctive because it is supposed that

material from the cliff is deposited at the pier in

Sopot. Due to the uneven distribution of sediments,

the Maritime Office in Gdynia undertook to reclaim

the beach in Gdynia using sand obtained from the

vicinity of the Sopot pier. In 2020, 58 thousand cubic

meters of sand from the Sopot pier area was used to

reclaim the seashore [66].

a) b)

Figure 11. Beach in Sopot Orthophotomap (a) [5] and a

photo showing blooming algae (b) of the area adjacent to

the pier in Sopot [65]

As a result of the development of the marina, a

process was set in motion, the effects of which are felt

mainly in tourism. Sopot is a resort town that can be

severely affected by an ecological disaster if the

appropriate response is not taken. Although the Sopot

authorities occasionally extract black sand and dredge

the bottom, this is not a solution to the tombolo

problem.

8 CONCLUSIONS

Tombolo is a complex oceanographic phenomenon

that is strongly conditioned by the interactions

between an obstacle and hydrodynamic forces, debris

and the bathymetric profile. In this paper the

components that contribute to the formation of this

phenomenon are discussed. These components are

presented along with their impact on hydrotechnical

structures [49]. In addition, the lithological diversity

of surface sediments is described, on which the rate of

sediment exchange depends. The influence of these

factors was discussed with reference to the

hydrometeorological conditions prevailing in the

south Baltic Sea using the two tombolo examples

mentioned in this article. The two examples are

among the most common phenomena of this type in

Poland, but they differ in their dynamics and

frequency. The tombolo in Sopot has been the subject

of research and analysis for many years. It is also

worth emphasising the need to take environmental

changes into account before planning construction,

since any intervention in the environment causes a

disturbance in the naturally occurring processes. For

this reason, using the examples given, this paper uses

examples to highlight the negative effects of such

phenomena as: algal blooms, cyanobacteria, rip

currents, changes in bathymetry, shoreline

displacement and changes in the structure of the

coastal zone. These phenomena should be a subject of

more comprehensive studies in the future to better

understand the tombolo phenomenon and reduce the

factors that contribute to its formation. A complete

analysis of these factors will help to reduce the effects

and better understand the impact of the different

components of this oceanographic phenomenon.

693

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