Preventive Methods for Georgian See Coast Erosion

Report
 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
 Fig. Seawall with groynes and breakwaters
Fig. Eroded beach with broken groyne made by stones
Fig. Seawall with groyn of full profile
Fig. Broken seawall protected with shaped structures.
 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.
Fig. Map of Adjara coastal zone current state.
Fig. Scheme of alongshore current at Adjara coastline
 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 19821992-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, that preservation 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.
Fig. Scheme of an extended submerged breakwater
 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.

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