Sustainable Measures of Shore Protection Against Erosion and Flooding ABSTRACT

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1 Sustainable Measures of Shore Protection Against Erosion and Flooding Rafal Ostrowski Marek Szmytkiewicz ABSTRACT Protection of sea coasts often refers to the manipulation of environmental processes on the level of coast within spatial and time scale It should be aimed at coastline preservation together with saving of the natural environment Within the worldwide trend in shore protection, activities friendly to environment have been lately very much preferred In many countries, recommendations existing in the framework of various legal regulation imply use of soft and ecologically friendly measures (eg artificial beach nourishment) while hard solutions (like seawalls, revetments and groins) are most often declined In most European Union countries, due to various requirements and restrictions, a limited number of shore protection methods can be applied An ultimate choice of the optimal option depends on site-specific issues The sustainable (environmentally friendly) available solutions, aside from artificial nourishment (either by dumping from a hopper dredger/barge or by pumping of the water-soil mixture using a hydro-monitor or by beach fills made directly on the emerged part of a shore), comprise bio-technical shore protection by use of sand-trapping fences, brushwood and special types of grass, bushes and trees Besides, protective measures based on geo-textiles are widely used, eg in the form of Geotubes and Geocontainers constituting cores of such massive structures like detached breakwaters (both emerged and submerged) 1 INTRODUCTION Many coastal segments in the world are subject to erosion and flooding These problems particularly concern non-rocky shores, namely the ones built of loose sediments, either non-cohesive (sandy) or cohesive (eg clayey or silty) Erosive processes can affect regions of occurrence of both beach/dune and cliff shores If a hinterland is a low lying area, after some time, progressing shore abrasion can be followed by flooding phenomena Identification, description and mitigation of the above processes plays an important role in the maritime and coastal economy Recently, in management of coastal regions a great emphasis is put on mutual integrity and sustainability of all the elements of the managing process, called ICZM (Integrated Coastal Zone Management) In the ICZM context, severity and local importance of erosive and flooding problems depend not only on intensity of hydrodynamic and lithodynamic processes but also on features of a coastal land under consideration, eg the population density, existing infrastructure and functions, as well as environmental and cultural values Consequently, any measures aimed at shore protection against flooding and erosion ought to satisfy various requirements of ICZM Coastal erosion in consequence leads to the following risks: lowlands flooding by sea waters, Institute of Hydro-Engineering of the Polish Academy of Sciences (IBW PAN), Poland rafalo@ibwpangdapl (Rafal Ostrowski) D-1

2 infrastructural damage, environmental losses, dune and cliffs damage as well as beach disappearance, spit breaches possibility In case of erosion there are basically three ways of action to be taken: do not prevent, accept losses, postpone solving the problem for the future, relocate endangered facilities, initiate coastal protection in order to stop or at least delay erosion processes Protection of sea coasts refers to the manipulation of environmental processes on the level of coast within spatial and time scale, which aims at coastline preservation together with saving natural environment Selection of coastal protection measures depends on: protection goal eg flooding prevention of infrastructure situated on lowlands, cliff protection from being washed away, coastline preservation, reduction of waves approaching coast etc, required intervention level whether to reduce coastal erosion rate or to stop erosion completely, safety level required by a planned protection From the environmental point of view, leaving sea coasts to its own activity would make ideal solution However, this solution is unacceptable Coastal regions around the world belong to the most urbanised areas, which are inhabited by a significant part of world population Therefore, activities against coastal retreat are in most cases necessary Within the worldwide trend in shore protection, activities friendly to environment have been lately preferred In many countries, recommendations existing in the framework of various legal regulation imply use of soft and ecologically friendly measures (eg artificial nourishment) while hard solutions (like seawalls, revetments and groins) are most often declined Variety of shore protection measures is shown in Fig 1, while the exemplary drawback of hard structure is depicted in Fig 2 Bearing in mind the requirements, aims and restrictions discussed above, a limited number of shore protection methods can be applied An ultimate choice of the optimal option depends on site-specific issues The sustainable (environmentally friendly) available solutions are presented in the next sections artificial dunes seawalls & revetments groins beach fills emerged offshore breakwaters submerged D-2

3 Fig1 Hard and soft coastal protection measures Fig2 Beach disappearance due to hard structures, Ustronie Morskie, Poland 2 ARTIFICIAL NOURISHMENT Artificial nourishment with sandy sediment is aimed at reconstruction of optimum shape of nearshore sea bottom, as well as beach and dune The nourished material may be placed on the dune, beach or in the foreshore (see Fig 3) The sandy material used in the nourishment activities is taken from a sea or land site, transported it to the nourished coastal segment and placed it in the coastal zone to make the foreshore shallower, to widen the beach and increase the dune height The effectiveness of nourishment depends mainly on the grain size of supplied sand The supplied sand should not be finer than the native sediment of the area subject to nourishment Nourishment is frequently applied in combination with other coastal defence methods Besides restoring eroded coastal segments, nourishment is sometimes used to form new beaches for recreational purposes With respect to the other coastal protection measures, artificial nourishment of sea shores has the following advantages: natural appearance the coast (no solid structures); flexibility, ability to match natural processes; quick implementation Artificial nourishment of sea shores has the following drawbacks: necessity of cyclic repetitions; need of sufficient sources of proper nourishing material at relatively small distances; D-3

4 problems with environmental protection at sand mining sites (possible destruction of habitats) The nourishment of the foreshore, either by dumping from a hopper dredger/barge (Fig 3 b) or by pumping of the water-soil mixture using a hydro-monitor (Fig 3 c), is less efficient than beach fills made directly on the emerged part of a beach (Fig 3 a) The latter requires a pipeline transporting the water-soil mixture along the shore and supplying it to an appropriate location (see Fig 4), as well as the equipment to distribute and shape the nourished sediment adequately on a cross-shore profile (see Fig 5) Dunes are a very important element of coastal resistance to erosion and flooding In the process of shore abrasion, disappearance of the beach is followed by degradation of the dune system This takes place particularly during storm surges, when the wave impact touches the dune toe directly Hence, artificial nourishment should also concern the dune system, if necessary Therefore, within profiling of the cross-shore transect, it is worthwhile to form an artificial dune The exemplary cross-shore profile with artificial beach and dune satisfying minimal requirements for the south Baltic Sea is drawn in Fig 6 Numerous examples from European sites show that coastal protection by artificial nourishment coexist very well with maintenance of harbours Frequently, operation of a harbour requires regular dredging works, particularly at the harbour entrance and along the approach channel Particularly, this takes place if the harbour is an obstacle for the intensive longshore sediment transport and the sediment is trapped in the navigable channels in front of the harbour In such cases, the sediment dredged from waterways is most often successfully used for formation of artificial beaches and dunes, especially at the lee side of the harbour In some other circumstances, the sediment is taken from the up-drift side of a harbour and pumped as the water-soil mixture to the other side Thus, a kind of sand bypassing is carried out, which restores natural longshore flux of sediment The example of such solution is shown in Fig 7 The joint application of two shore protection measures presented in Fig 7 (groins supplemented by artificial beach fills) in most cases results from a situation in which previously constructed (old) groins have lost their efficiency either due to structural damage or intensification of erosive processes In such a case, sand is placed into the fields between groins which help to retain the nourished material During extreme and long-lasting storms, the artificial dune and beach at the most vulnerable shore segments can be washed away Therefore, within contemporary implementation of entirely new ventures in protection of important shore segments, as a second line of coastal defence, revetments are proposed, built-in the artificial dune, eg in the form of so-called gabion structures or heavier mildly sloped structures, made of prefabricated elements Each of the above revetments has advantages and disadvantages The gabion revetments are more vulnerable to damage, particularly if gravels are present in the beach sand These gravels can bomb the revetment within the process of wave run-up and can break the wires of the boxes On the other hand, the maintenance and repair/reconstruction of damaged gabions is relatively easy and inexpensive The heavier revetment are more resistant to hydrodynamic impacts but are more costly The economical analysis should yield the optimal choice, but expensive D-4

5 solutions as the second line of defence (ultimate protection with a low probability of being active) seem to be out of sense In any case, if such a revetment is found uncovered after a storm, the artificial beach and dune should be rebuilt Possible damages of the revetments ought to be repaired before The exemplary gabion structure built-in the artificial dune and beach is shown in Fig 8 Fig3 Options of artificial beach nourishment Fig4 Direct beach nourishment D-5

6 Fig5 Beach profiling by use of bulldozers + 40 m 1 : m 1 : m recommended profile of beach and dune + 15 m existing cross-shore profile 1 : Fig6 Profile of artificial dune and beach recommended for the south Baltic coast D-6

7 Fig7 Wladyslawowo harbour (Poland): accretion on west side (bottom), lee side protected by groins and beach fills (top left hand side) Fig8 Gabion revetment built into artificial dune D-7

8 3 BIO-TECHNICAL PROTECTION The extension and strengthening of dunes is carried out by covering of the dune slope with brushwood and/or building of dune-generating fences, made of natural materials, mainly live willow twigs, brushwood or fascine These fences are usually located parallel to the shore at the foot and on the slope of the existing or formed dune They catch the sand transported by wind, and in effect the dune gradually builds up Stabilisation and further development of the dunes is achieved by planting special species of grass, which can grow in mobile, salty sand In the next stages (after several years) first bushes (mainly on the landward slope of the dune), and then trees (further inland, behind the dune) are planted The necessary condition for dune development is the existence of a wide beach On cliffs, grass, bushes and trees protect the cliff slope against surface erosion by rain- and melt-water They also intercept and remove (by evaporation) the groundwater In effect the probability and reach of landslides on the cliff slope are reduced The bio-technical shore protection is widely used on the south and south-east Baltic Sea shores The idea of such solutions probably comes from Germany where fascine fences at dune toes and brushwood on dune slopes have been used for many decades, at least since the beginning of 20th century The examples from Poland, Russia (Kaliningrad district) and Lithuania are shown in Figures 9-14 Fig9 Brushwood on dune slope and fascine fence at dune toe in Lubiatowo, Poland D-8

9 Fig10 Sand accumulation at dune toe caused by the fascine fence Fig11 Accumulative frames at the south-east Baltic shore D-9

10 Fig12 Sand accumulative measures at Curonian Spit, Russia Fig13 Accumulative effects of fences at Curonian Spit, Russia D-10

11 Fig14 Sand accumulative measures at Curonian Spit in Lithuania 4 NOVEL SOLUTIONS According to the present ecological attitude, optimal solution in shore protection requires the applied measure to be friendly for the environment and invisible to coastal users (eg tourists) Such criteria can be satisfied, aside from the artificial beach nourishment, by the submerged offshore breakwaters These structures (see Fig 1) cause attenuation of wave energy in the nearshore zone The wave energy dissipation which takes place while the wave is moving over the submerged breakwater (most often due to wave breaking), together with wave-induced specific circulations of water, are favourable to accumulation of sand between the structure and the shoreline Recent investigations show that an efficiency of the submerged breakwater in wave energy dissipation increases with the width of the structure Further, this structure must be solid as it is subject to intensive hydrodynamic loading The submerged breakwaters are most often built as rip-rap structures Satisfaction of all the above mentioned structural requirements needs huge volumes of rubble material of good quality (stones or concrete blocks) which is always costly (sometimes unavailable near the considered site) Therefore, in many cases, this material is used for building only the cover layer of a rubble-mound structure, while its core is constructed of cheaper (easy available) material The material, being mostly sand or clayey sand, constituting the core part of the structure should be placed inside a container preserving the soil grains from washing out by water flowing through the structure To this end, so-called geo-textiles are used to form the containers The geo-textiles are highly resistant to tensile stresses and tearing Applied in marine and coastal engineering, they are built of plastic nets with holes D-11

12 having a diameter of 100 microns, thus being permeable for water but impermeable for sediments The geo-textiles, presently most commonly used in coastal engineering, have tubular forms Here, one can mention popular American and Dutch prefabricated materials, called Geotubes and Geocontainers, respectively Geotubes have diameters of 2-4 m and lengths of hundreds of metres They are filled in with a local sandy soils by use of hydraulic methods Geotubes have been successfully used in USA as cores of the offshore breakwaters, both submerged and emerged (see Fig 15) Geocontainers are initially open forms, fastened or sewn after having been filled in mechanically by the soil material and finally placed at a desired location on the seabed Their dimensions, particularly lengths, are limited by an equipment used in the process of their preparation, namely hopper-dumping barges or barges with hulls specially constructed for Geocontainer operations (see Fig 16) Geocontainers have capacities of m3 and can be filled in by many types of non-cohesive or cohesive soils, eg by material dredged from navigable channels They have already been applied in the Netherlands as cores of the offshore submerged breakwaters In Poland, although the geo-textile materials are widely used in civil engineering (eg in the reinforced soil structures), there are few applications of this technology in coastal protection, eg as reinforced soil structures stabilising cliff shores or small-scale geo-textile tubes filled by sand and built-in the artificial dunes as the second line of defence Till now, they have not been implemented as the cores of large-scale coastal protection structures, like offshore breakwaters or dikes This situation is because of lack of appropriate highly resistant materials and specialised equipment, as well as enterprises experienced in such ventures 100 rocky blocks 6-30 kn geo-textile HD 430 Fig15 Exemplary application of geo-textile prefabricated elements: Geotube as core of offshore breakwater D-12

13 Side view 700 m 2400 m 675 m A Top view capacity: 240 m m A 575 geo-textile cross-section A-A sand 180 m 3 Fig16 Filling up of Geocontainer on hopper barge and placement on seabed REFERENCES [1] Douglass, SL 2002 Saving America s Beaches, Advanced Series on Ocean Engineering 19, World Scientific Publishing Co [2] Marcinkowski, T and Szmytkiewicz, M, 2006 Contemporary trends in coastal protection Zeszyty naukowe PG, Gdańsk (in Polish) [3] Pilarczyk, K and Zeidler, RB, 1996 Offshore Breakwaters and Shore Evolution Control, AA Balkema, Rotterdam D-13