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Preventive Methods for Georgian Sea Coast Erosion Nodar Tsivtsivadze.
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Preventive Methods for Georgian Sea Coast ErosionNodarTsivtsivadze
Ocean shore is a special natural zone of the lithosphere, hydrosphere, atmosphere and biosphere interaction. Perhaps, there is no other such “attractive” object on Earth. About 40% of the world population lives on the marine and ocean shore zones. Numerous industrial, communication infrastructure and other facilities are built along the sea coasts. Unfortunately, quite often, construction of these objects is followed by loosing of the coastal zone’ (mainly the beach) vast quantities of sand, pebble –gravel and shell material. Fig. Highway along Batumi coast
It is well known that beaches are the main defender of coasts from destruction. In the coastal zone wave permanently influences on earth, while the later is protected by the sediment layer- beach. • A beach profile is in dynamic equilibrium with the forces to which it is exposed. It is constantly in motion, changing its configuration in response to superimposed forces. Thus, where a beach profile is not stable and well defended, beach may disappear in a very short time and erosion process can be activated. • Beach erosion depends not only on wave impact, but availability of beach material and human activity as well that influences increase in time. Fig. Breaking wave
Applied engineering practice – (conducted coast protection measures) - has shown that their effect is only localized and temporary. On some coasts majority of them not only prevents coastal erosion, but provokes negative consequences of their influence on shore environment. • Fig. Wave influence on beach protected by Seawall
The anthropogenic factor, supporting destruction of coasts, is added by natural phenomena– sea level rise due to glaciers melting, caused by climate change.The mentioned phenomena, as the main problem of coastal land degradation, will strengthen, cause storms frequency and wave power increase. • Human impact on Georgian sea coast has started since the construction of Batumi port and natural replacement of riv. Chorokhi channel to Southern side of the coast, close to canyon. In the result- beach erosion of Adjara shore has begun. • Previously Chorokhi sediments at amount of 5, 5mln.cubic m./year -(2,0 mln c.m. of sand, 450 000 c.m. of gravel, 3,0 mln. c.m. of silt) was distributed as follows: beach forming sediments 90% was lost in canyon, only 50-60 th.c.m was transported by longshore current to Northern direction and rest-20-25 th.c.m to Southern one. • Nowadays degradation processes is accelerated due to construction of dam cascade on riv. Chorokhi, consisting of 24 reservoirs, prevented sediment transport to Adjara coast for 500 years.
Nowadays, in our country, special attention is paid to development and installation of new and inexpensive structures, intended to preserve Georgian coastal zone from wave and surge destructive impact. • From our point of view the most rational way to achieve this aim is to create new structures and building technologies using low-cost construction materials. • In recent decades, in the World, cheaper and more efficient method of coast protection via artificial beach creation was offered. • This method is as follows: in coastal zone beach-building material is downloaded at certain points and then the storm waves themselves distributed the load and built the beach along the coast.
Fig. Georgian beaches before and after the artificial nourishment
But sometimes, the sediments that are transported through alongshore currents accumulate on places close to canyons, that provokes landslide into the canyons, sediment loose, canyon activation and beach retreat. • For example such phenomena took place in 1999 on Batumi coast, when earthquake provoked the movement of accumulated underwater sediments and huge quantity of beach materials to Batumi canyon, caused the disappearance of 11 200 sq.m coastal land- beach of 200 m length and 50-60 m. width has been lost. • Adlia-Batumi airport shoreline is retreated for 500 m. during a century, 1998-2001 the beach of 4 km length was washed out (appr. 2,5 ha.).
Fig. Bathimetric map of Chorokhi -Batumi canyons coastal section
Protection of Adjara marine coasts was required, as the existed beaches were insufficient to absorb storm wave causing shoreline retreat for several meters annually. • That is why for the beach artificial replenishment of Adjara coast, during 1982-1992-s approximately 6 198 thousand cubic meters of inert material, by the special ships and vehicles, was brought and unloaded on eroded beaches. Unfortunately the measures carried out were appeared not enough for coastline stability and beach erosion was begun very soon. • It should mentioned, thatpreservation of its valuable coastal lands and their development for recreational purposes remains an important problem for the national economy of Georgia.
Fig. Artificial Beach with protection structures along the railway.
Our approach to protect mentioned coast is new and based on idea to create artificial beach in complex with submerged breakwater of an extended profile. Fig. Artificial beach with submerged breakwater of an extended profile
In spite of considerable size, the cost of this type breakwater is low, owing to the use of unsorted stones (100-500kg.), used tires and geo/textile tubes only.
An extended-submerged breakwater has been designed, built and researched in TSU Fluid Mechanics and Oceanography Laboratory wave tank. It is characterized by effective reduction in beach erosion; it has been shown to be more favorable for ecological conditions of the coastal zone. • Usage of submerged breakwater is intended to reduce the volume of alongshore sediment transport, to retain the sediments brought by alongshore current or artificially loaded from outside and reconstruct the wave transformation zone. • In principle, this method differs from traditional methods and can be widely used as a beach forming material-retaining construction. • The effectiveness of this type of breakwater can be calculated using the following formula: e = 1 - G/G0 where G0 is the volume of alongshore sediment transport for the case of a shoreline without a structure, G is volume that transported for the shore with the breakwater.
Analysis of experimental results has made it possible to understand the characteristics of the breakwater. If the breakwater is located farther the natural zone of wave breaking, it will force waves transformation on its seaward side, causing them to roll onto its surface; • This, in turn, reduces the wave height, hence its energy, also mean wave level at beach line. Thus, wave heights behind the breakwater are lower than under normal conditions, when they tend to break on the free, undefended, slope. • This type of breakwater also causes supplementary refraction of waves, since finally they approach beach more normally than originally. • The combined action of these two mechanisms (wave height reduction and reconstruction of their velocity fields) produces, essentially, a reduction of alongshore sediment transport volume.
The derived results for the efficiency of different varieties of breakwater, and for different depths of installation, are shown in Fig. The analysis of these curves shows that for increases of a relative depth h/b from 0 to 4-5, efficiency of a one-sided breakwater quickly reaches 70%; when h/b = 4-5, an increase in depth does not influence the efficiency (h/b) Fig. Breakwater efficiency (e) dependence upon relative depth (h/b) and water depth (h).
Analysis of obtained data has also shown that, under the same conditions, the use of the two-sided breakwater decreases down drift alongshore transport by a factor of 1,4-2,7 times in comparison with the one-sided breakwater. • Approximate calculations proved that the use of artificial beaches in combination with submerged breakwater of an extended profile has decreased considerably the volume and the cost of construction material; • It also accelerates the coast-reinforcement construction; • It considers evasion of coastline washing-out that occurs during construction of some other traditional, beach protection structures and stabilizes adjoining parts of the coastline by the way of alongshore current reduction.
On the coastal sections of shore with abundant natural sediment flows, could be applied the a.m. submerged breakwater with one traverse. Such structures were not widely used, since their construction with existed technology was difficult and practically impossible. • Fig. An extended submerged breakwater with traverse: • 1-Seawall; 2 - artificial beach; 3 - submerged breakwater; 4 – traverse
On the sections of natural sediment flow absence of pebble beach, two traverses have to be built at both ends of submerged breakwater, elevation of which at the root parts must be adopted in accordance with projected artificial beach’ level mark there. • On the coast of the pebbly beaches, the distance between the breakwater edge to be built and shoreline has be at least 100 m and the depth of its placement -not less than 8 m. • In case of systematic replenishment of beach, the average width of the artificial one’ surface space behind the breakwater has to be not less than 1/3-rd of the distance between the breakwater and the shoreline. The shoreline in a confined space may be approximately parallel to the front of the design wave.
On Adjara shore, the building of the submerged breakwater is recommended with shape of a prism with a width of 10-15m. of the crest and side edges slope 1: 2 • Breakwater has to be built from the sea using barge, equipped with pumping system for filling with sediments of breakwater slopes apron and traverse, made by geo textile tubes. • The artificial beach has to be built from the coast. The same method for its further replenishment can be used. • It should be mentioned that presence in a coastal water of submerged breakwater impairs water exchange. That is why the submerged breakwater of an extended profile proposed, has to be built with deepened crest (2m. below the NSL-of 50% probability).
To avoid water quality worsening due to eutrophication, caused by development of blue-green algae (cyan bacteria) bloom, provoking different kind of diseases, we recommend creation of one-two layer artificial “reef ” behind and in front of the breakwater, by the sorted porous stones of 0,5-1,0 m. diameter and their occupation with sea filtration organism like double swing mussels are. • Existence of the “reefs” also will reduce bed erosion and scour formation at the bottom of breakwater. Artificial “reefs” at the sea bottom perform the same role as natural ones. For the same purpose fully depreciated, small ships or artificial “reef balls” can be used. Their presence creates habitats for underwater inhabitants, increases biodiversity which also will attract holidaymakers’ attention to resort. • To carry out such measure is neccessary, because while construction of a new beach and during the first years of its operation temporary reductions of biomass, buried by structures, might occur. However, after 2-3 years the phyto and zoo/benthos will be completely restored and the conditions of bio-efficiency of coastal water - increased.
Fig. Artificial reef (stones and broken ship) • Creation of beach protecting -recreational complexes in shallow waters on artificial grounds ensures effective retaining of unique coastal landscapes and valuable land resources, as well as existing beaches or newly built ones. In addition, the construction of such facilities contributes to creation of additional territories in scarce resort areas and provides a spatial dispersal of holidaymakers, reducing recreational load on beach line and improving sanitary/hygienic conditions in the coastal waters of the sea.
