COASTAL EROSION MANAGEMENT IN

THE MEDITERRANEAN:

AN OVERVIEW

by

Prof. Dr. Erdal Özhan

PAP-4/CE/02/PP.1

Priority Actions Programme

Regional Activity Centre

Ankara/Split, May 2002

COASTAL EROSION MANAGEMENT IN THE MEDITERRANEAN:

AN OVERVIEW

Prof.Dr. Erdal Özhan

Chairman of MEDCOAST, Professor of Coastal Engineering and Management

Middle East Technical University, 06531 Ankara, Turkey

1. Introduction

Coastal erosion is one of the most important socio-economical problems that challenge the capabilities of states and local authorities. Whether it is due to natural or anthropogenic reasons, coastal erosion causes significant economical losses, social problems, and ecological damages. The problem of erosion may extend its influence hundreds of kilometres alongshore in the case of large deltaic areas, and may have transboundary implications. In the case of pocket beaches on the other hand, it could be a very local phenomenon affecting only the residents of a nearby town and/or the tourism industry.

The Mediterranean Action Plan (MAP), an action-oriented co-operative effort among the riparian countries and the European Union, has the general objective of creating a healthier Mediterranean environment, resting on the principles of sustainable development. MAP is a part of the United Nations Environmental Programme (UNEP), and was its first Regional Seas Programme, set-up in 1975. The Priority Actions Programme (PAP), implemented by the Regional Activity Centre in Split, Croatia is a part of MAP, and deals with implementation of priority actions, the most important being the Integrated Coastal Area Management (ICAM). PAP wishes to contribute to rational management of the coastal erosion in the Mediterranean. An Expert Meeting was organised by PAP/RAC in Split during 10-11 January 2002, with the aim of discussing the coastal erosion issues in the Mediterranean, and the concrete actions that could be taken to combat it. This paper was prepared for the purpose of setting the stage for discussions in the Expert Meeting.

  1. The problem of coastal erosion

Coastal erosion is defined as the long term loss of the shore material (volume) relative to fixed reference line (baseline) and initial reference volume seaward of this line above some, arbitrary vertical datum (Basco, 1999). Coastal erosion is always accompanied with the shoreward recession of the shoreline and the loss of land area.

Coastal erosion is usually judged as “critical “ when it presents a serious problem wherever the rate of erosion, considered in conjunction with economic, industrial, recreational, agricultural, navigational, demographic, ecological and other relevant factors, indicates that action to remedy (stop or slow down) erosion may be “justified”

In the Mediterranean, coastal erosion has been a longstanding, large-scale issue around the deltaic areas, such as the deltas of the Nile and Po Rivers, and the smaller deltas like those of the Albanian rivers. It has also been a major issue at smaller scales, especially in the municipal or tourist resort beaches along the relatively more densely developed northern coast, following the flux of people from inland areas to the coast and the boom of the tourism industry. More than 40 % of beaches in France, Italy and Spain was found to be confronting erosion in the EU project CORINE completed in 1990. Two graphs showing the findings of this study for four EU countries of the Mediterranean are given in Appendix A. These graphs were reconstructed from the figures given in Blue Plan (1996). One of the graphs provides the lengths of various coast types, and the second gives the lengths of beaches that are eroding, accreting or in equilibrium. According to the Atlas of the Italian Beaches (Fierro and Ivaldi, 2001), 27 % of the Italian beaches which constitute 61 % of the total Italian coastline are retreating, 70 % in equilibrium, and only 3 % prograding. (The information provided in Fig. 2 in Appendix A somewhat differs from these percentages).

Damages of coastal erosion may depict itself in the followings (van der Weide et al, 2001)

  1. Life of people: This is a serious issue for people living in low-lying lands (below the sea level) adjacent to seas and oceans such as the Netherlands. It is generally not a problem in the Mediterranean.
  2. Property and associated economical values: Loss of extremely valuable land (recreational and tourist beaches) and damages to urban infrastructure are important for the Mediterranean countries.
  3. Ecological values: Good examples to this type of damage are turtle nesting beaches like the Kazanli Beach (Turkey) and several beaches in the Zakynthos Island (Greece) that have been loosing critical turtle nesting grounds due to anthropogenic erosion.
  4. Cultural values: There exist numerous examples to this type of damage all along the Mediterranean coast in areas where people have settled since ancient times, such as the Phoenician ruins in Tyre, Lebanon (Özhan, 1993).
  1. Causes of coastal erosion

Factors that cause changes (erosion / accretion) in the seabed topography and the time scales of these changes are summarised in Table 1. Some of these factors (waves, alongshore currents, rip currents, undertow, overwash) usually combine together to reshape the sea bottom during a storm, causing an aerial pattern of erosion and accretion and resulting an overall gain or loss for the volume of beach sand. On the other hand, variations in the wave conditions (large, short period, small steepness) have seasonal (cyclic) effects. The type of change due to these factors does not necessarily indicate the long-term nature of the change, but they contribute to it. The factors that are associated

Table 1. Factors that cause changes in the sea bed topography and the time scales

FACTOR / EFFECT / TIME SCALE / COMMENTS
Sediment supply (source and sinks) / Accretion/erosion / Decades to millennia / Natural supply from inland or shoreface and inner shelf sources can contribute to shoreline stability or accretion
Sea Level Rise / Erosion / Centuries to millennia / Relative sea level rise
Phenomenal storm, storm surge / Erosion / Hours to days / Very critical to erosion magnitude
Large wave height / Erosion / Hours to months / Individual storms or seasonal effects
Short wave period / Erosion / Hours to months / Individual storms or seasonal effects
Waves of small steepness / Accretion / Hours to months / Summer conditions
Alongshore currents / Accretion, no change,
or erosion / Hours to millennia / Discontinuities (updrift//downdrift) and nodal points
Rip currents / Erosion / Hours to months / Narrow seaward-flowing, near-bottom currents may transport significant quantities of sediment during coastal storms.
Underflow / Erosion / Hours to days / Seaward-flowing, near-bottom currents may transport significant quantities of sediment during coastal storms.
Inlet presence / Net erosion;high
instability / Years to centuries / Inlet-adjacent shorelines tend to be unstable because of fluctuations or migrations inlet position; net effect of inlets is erosional owing to sand storage in tidal shoals.
Overwash / Erosional / Hours to days / High tides and waves cause sand transport over barrier beaches
Wind / Erosional / Hours to centuries / Sand blown inland from beach
Subsidence, compaction / Erosion / Years to millennia / Natural or human-induced withdrawal of subsurface fluids
Tectonic events / Erosion / Accretion / Instantaneous, centuries to millennia / Earthquakes; Elevation or subsidence of plates

with effects having time scales in the order of years or more (eg. changes in sediment supply, sea level rise, coastal subsidence and tectonic events) are directly responsible for the long-term behaviour of the coast.

The natural and anthropogenic causes of coastal erosion in the Mediterranean are not different from the causes elsewhere. In the followings, the natural events and the anthropogenic activities that cause coastal erosion are briefly discusses. The references that are provided indicate the part of the Mediterranean where the mentioned factor is a primary reason for erosion.

The natural causes of long term coastal erosion are the followings:

  1. Sea level rise (Pranzini & Rossi, 1995; Khalil, 1997);
  2. Coastal subsidence due to tectonic events (Khalil, 1997);
  3. Climatic changes (changing of the storm intensities, shift of the dominant storm directions affecting the approach angle of waves; variation of precipitation and the river regimes and discharges) (Medina & Lopez, 1997);
  4. Increased vegetation cover over the river watersheds due to climatic changes (causing decreased soil erosion and sediment supplied to the coast);
  5. Sediment sinks (presence of offshore canyons, movement to great depths at steep slopes, wind transport of sand to inland areas) (Golik & Rosen,1999);
  6. Changing of river courses and mouths in deltas (PAP/RAC, 2000; Berriolo, Fierro & Gamboni, 2001)

Anthropogenic causes of long term coastal erosion are more or less parallel to the natural causes. These are:

  1. Decreasing sediment supply by rivers to the coastal physiographic unit (cutting of the sediment transport by damming the rivers, sand and gravel mining along the river beds, decreasing the sediment transport efficiency by lowering water discharges due to increased fresh water use or due to river works such as bank and bed erosion control) (Simeoni et al., 1997; Eronat, 1999; Loizidou & Iacovou, 1999; PAP/RAC, 2000);
  2. Erosion control works and afforestation in coastal and riverine watersheds (Eronat, 1999);
  3. Decreasing the volume of sand in the physiographic unit (sand mining from the beach and dunes, offshore sand mining) (Özhan, 1993; Loizidou & Iacovou, 1999);
  4. Alteration of the usual pattern of coastal currents and the associated sediment transport along and across the shoreline, due to man-made coastal structures and urban development too close to the shoreline (Silva et al., 1993; Loizidou & Iacovou, 1999; Fatallah & Gueddari, M., 2001, Rakha & Abul-Azm, 2001);
  5. Anthropogenic changes made to river courses and mouths in deltas (PAP/RAC, 2000; Berriolo, Fierro & Gamboni, 2001);
  6. Maintenance dredging of approach channels and estuarine inlets;
  7. Land subsidence due to anthropogenic effects (Preti, Carboni & Albertazzi, 1997; Fierro & Ivaldi, 2001).

In Cyprus for example, dam construction, sand mining, coastal structures and urban development too close to the shoreline are cited as the factors that have triggered and accelerated coastal erosion (Loizidou & Iacovou, 1999). Eronat (1999) lists dam construction, erosion control in the watershed, sand and gravel mining along the river bed, illegal sand mining from the beach, and construction of coastal structures by property owners as the main reasons for significant erosion rates that has been observed in the vicinity of the mouth of the Madra Creek, the northern Aegean coast of Turkey. Along the coast of the Emilia-Ramagna Region (Adriatic Sea, Italy), the main causes of coastal erosion are sand and gravel extraction from the river beds, building of river impoundment works, subsidence caused by ground water exploitation and the coastal structures (Preti, Carboni & Albertazzi, 1997). Along the Venetian coast of the Adriatic Sea, the main causes of the erosion by far are said to be the reduction of the sediment inputs from the rivers and the construction of the jetties at three inlets of the Venice Lagoon (Silva et al., 1993).

Anthropogenic causes (a), (b), (e) and (f) result in the net erosion of the physiographic unit; e.g. there appears an overall shoreline recession. Causes (c) and (d) on the other hand induce shoreline erosion at local scales. These are usually associated with accretion elsewhere. In the case of cause (c) for example, the shore around the old river mouth swiftly erodes, whereas the location of the new river mouth accretes. For (d), shoreline erosion takes place down-coast of the coastal structures (at the side of the net longshore sediment transport), but a similar rate of accretion occurs up-coast of the coastal structure.

There are several reasons that decrease the sediment load brought by rivers to the coast. The most important of these is the construction of dams and engineering works along the rivers. Dams intercept almost all the sediment brought from upstream sections. Furthermore, by modifying the water discharge (making it more uniform), the sediment transport downstream of a dam is also altered and the sediment transport efficiency is decreased.

In addition to damming of rivers, alterations of the flow regimes by diversions and engineering works, and modification of the discharge rates and patters are also responsible for the decreased sediment loads brought to the coast, and thus for coastal erosion. This is also a significant cause for the Albanian coastal erosion (Simeoni at al, 1997). Darci River was observed to have lost a great part of its discharge rate due to diversions and upstream water use. This is the main reason behind the recession of the shoreline next to the river mouth at an annual rate of 1-2 meters. This erosion will continue until the shoreline and the nearshore topography is changed to have their equilibrium shapes, which do not yield net sediment transport rates in the alongshore direction.

Another major cause for decreased sediment input to the coast and the consequent erosion, is sand and gravel quarries located along the active riverbeds. This activity is usually not allowed from the beaches, but significance of mining from riverbeds is often overlooked. For example, sand and gravel extraction from the riverbeds has been a significant economical activity in Albania (PAP/RAC, 2000), and it is still a legitimate undertaking in Turkey.

Removal of sand and gravel directly from the coast by illegal quarrying could be a significant factor triggering coastal erosion and shoreline recession. The shoreline south of the Town of Tyre in Southern Lebanon receded more than 100 meters in about 20 years due to commercial sand extraction from inshore and the beach face (Özhan, 1993).

Topological changes in the seabed slope resulting from offshore sand mining for beach nourishment have also been criticised for the accelerated shore erosion when the borrow area is not sufficiently far away from the coast and not at a large enough water depth.

Changing of the river course and location of the mouth in a deltaic coast can be a natural phenomenon. Simeoni et al. (1997) mentions that several Albanian rivers (e.g. Ishmi, Shkumbini, Semani, and Vjose) have undergone such changes. Such alterations are accompanied with local erosion around the old mouths and accretion around the new ones as mentioned earlier.

For small pocket beaches, sand transported by wind into inland locations (which does not return back to the beach) may constitute a major sink. Also, mechanical cleaning of dead Mediterranean seagrass (Posidonia ocenica) for aesthetic purposes from the surface of recreational beaches could cause significant sand losses from the system. The material removed by special-duty machines was found to contain sand up to 30 percent in some Italian beaches (Pranzini, 2002).

  1. Management of coastal erosion ( Shoreline management)

4.1 Detecting coastal erosion

The locations of the historical coastal erosion and the erosion rates can be estimated by using the followings:

  1. The historical aerial photographs and coastal topographic maps (Preti, Carboni & Albertazzi, 1997, Golik and Rosen, 1999; Suzen & Özhan, 2000; Berriolo, Fierro & Gamboni, 2001; Bowman & Pranzini, 2001, Fatallah & Gueddari, M., 2001). These sources provide information on the past shoreline positions and the rate of shoreline recession.
  2. The old bathymetric maps. Comparison of the successive bathymetric maps provides information on the regions of erosion and accretion, and their average rates) (Golik and Rosen, 1999).
  3. A numerical model to calculate the sand transport rates from the historical time series of wave data, and the resulting morphological changes (Golik and Rosen, 1999; Rakha, & Abul-Azm 2001)

Golik and Rosen (1999) uses all three approaches above in their study on management of Israeli sand resources.

The present and the future trends of coastal erosion can be monitored through the following schemes:

  1. Visual observations on erosion indicators (location of erosion and in some cases the rate of shoreline retreat) (PAP/RAC, 2000);
  2. Shoreline position surveys (location and the rate of shoreline retreat/advance) (Micallef, 2001);
  3. Topographic and bathymetric surveys (zones of erosion and accretion, and their rates) (Bowman & Pranzini, 2001; Certain, et al., 2001; Delbono et. Al. 2001);
  4. Use of aerial digital photography and satellite images (location and the rate of shoreline retreat/advance) (Eronat, 1999; Suzen & Özhan, 2000).

Cultural features present along the coast (such as the bunkers along the Albanian coast) serve as useful landmarks for assessing erosion areas, and in some cases the erosion rates (PAP/RAC, 2000). Although the resolution of the satellite images has become finer in the recent years and one can obtain shoreline images with an accuracy of 50 cm., the use of this data in coastal erosion studies still suffers from the inaccuracies inherent in the analysis. The most important problem encountered is the detection of the exact location of the shoreline in the image, especially during the stormy periods (when the shore face is wet due to wave run up) (Golik & Rosen, 1999). Aerial digital photography provides a fast and reasonably accurate tool for monitoring the shoreline locations along lengthy stretches of the coast (Edwards et al., 1996; Curr et al., 1997; Eronat, 1999)

.2Predicting future erosion

Numerical modelling has developed to be a powerful tool for predicting past, present and future changes in the sea bed topography and shoreline position. The prediction of the historical changes by numerical models helps to understand the scale and composition of the factors that contribute to coastal erosion. Knowledge on the present and future erosion patterns and rates that would occur under different scenarios and strategies is a very important information that contribute significantly to rational coastal development plans and management practices.

A coastal morphodynamic model has four components:

  1. Wave prediction and transformation;
  2. Wave breaking and breaker zone hydrodynamics;
  3. Coastal sediment transport;
  4. Morphological changes of the sea bed.

Among these four components, (b) and (c) are the most complicated ones. Modelling of highly irregular, turbulent water motion in the breaker zone due to wave breaking and broken waves, and associated sediment transport has been challenging subjects for many researchers for years (Özhan, 1982, 1983, 1987). Several sediment transport models of various complexity levels have been developed. The greatest difficulty in using these models arises from the inadequacy of available information on the parameters that are required by the models (especially by the comprehensive models).