An Integrated Sediment Management Scheme for the Coastal Area of Batumi (Georgia)

Transcription

1 An Integrated Sediment Management Scheme for the Coastal Area of Batumi (Georgia) Alessio Giardino (1), Maria di Leo (2), Giulia Bragantini (2), Hans de Vroeg (1), Pieter-Koen Tonnon (1), Bas Huisman (1), Mark de Bel (1) (1) Deltares, Unit Marine and Coastal Systems, Rotterdamseweg 185, P.O. Box 177, 2600 MH Delft, The Netherlands Tel: + (31) Fax: +31 (0) alessio.giardino@deltares.nl; Hans.deVroeg@deltares.nl; Pieterkoen.tonnon@deltares.nl; bas.huisman@deltares.nl; mark.debel@deltares.nl (2) Technital S.p.A, Verona, Italy Tel: maria.dileo@technital.it Giulia.bragantini@technital.it Abstract Batumi is the capital of the Autonomous Republic of Adjara (Georgia) and one of the major cities on the Georgian Black Sea coast. For centuries, the Chorokhi River (just south of Batumi) transported sediment (both sand and pebbles) towards the coast. Due to this, a delta was created. The City of Batumi is situated on the delta of the Chorokhi River. For decades now, part of the coast where Batumi is located is eroding. The erosion is caused by: autonomous development, sediment mining from the Chorokhi River mouth, construction of power dams along the Chorokhi River and underwater landslides in submarine canyons close to the coastline. Due to these very complex physical and land-use settings, the development of an appropriate solution to the erosion problems requires an in-depth knowledge of the sediment budget and historical development of the coastline. In this study, an integrated sediment management study for the coastal area of Batumi will be presented, supported by numerical modelling calculations of waves and sediment tranport, and integrated by available measurements. The sediment budget will include information on alongshore

2 sediment transport and coastal changes, sediment input from the river and sediment loss in deep water due to submarine landslides. Based on this information, a number of solutions are proposed at conceptual level and evaluated from a technical point of view, with support of numerical modelling and a cost-benefit analysis. Introduction Coastal erosion is a world-wide phenomenon which affects all type of coastlines: muddy coasts, sandy coasts, gravel coasts and cliffs. Within Europe, the area lost or seriously impacted by erosion is estimated to be about 15 km 2 per year with an average yearly cost of 5,400 million euro (EUROSION study, 2004). Different types of erosion exist, depending on the spatial and temporal scale of the problem: small scale (meters, days/months), medium scale (kilometres; years), large scale (tens/hundreds of kilometres; from decades to centuries) (Giardino et al., 2013). Different approaches and solutions have been developed to tackle the erosion problems at each of the above mentioned scales. Nevertheless, all different spatial and temporal scales should be always taken into account when implementing a solution for one specific problem, in order to provide the boundary conditions if possible to solve, at the same time, other possible problems. For this reason, sustainable solutions to coastal erosion issues can only be implemented when an in-depth and integrated understanding of the sediment system exist. In this study, an integrated sediment management scheme for the coastline of Batumi is proposed, accounting for sediment transport processes alongshore the coastline, but also changes in sediment input from the river and sediment losses in the underwater canyons in front of the coastline. The study aims to find a suitable and sustainable solution to the erosion problems of the coastline of Batumi already described in several previous studies (Zenkovich and Schwartz, 1987; Alkyon et al., 2000; Alkyon et al., 2009; ARCADIS, 2012; di Leo et al., 2015; Deltares, 2015a; Technital, 2015). The coastal erosion is mainly the result of: Autonomous development, influenced by historical changes in river mouth position of the Chorocki river mouth (see section Study Area ). Sediment mining from the Chorocki river mouth, which has taken place during the last decades. Nowadays it has been prohibited in the last 20 km from the river mouth. Regulation of the river flows by power dams. According to the Coruh multiple dams project, a total of 27 dams are planned along the whole river catchment on the territory of Turkey (Gamma Consulting LtD, 2011). As per information provided by the representatives of the Adjara Department for Environmental Protection and Natural Resources and of the Batumi Municipality, a new dam is now being constructed about 15 km from the river mouth, in Georgian territory. This dam might become operational in about 5-year time. The effect of the power dams on sediment transport is two-fold: on one side sediment is retained behind the dams. On the other side, peaks in river discharges are smoothed out. Those peaks are responsible to move the gravel towards the river mouth, which is the main building material of the beaches in front of Batumi.

3 The study is supported by a detailed analysis of existing data (i.e. bathymetry, coastline changes, sediment characteristics, wind, waves, river flows, canyons stability) and numerical modelling calculations to predict the sediment transport rates and assess the efficiency of different solutions. The morphological computations have been coupled to a cost-benefit analysis which supports the choice between the different solutions. The study was carried out by a consortium led by Technital S.p.A. together with the sub-consultants Saunders Group (Georgia) and Deltares (The Netherlands) for the Municipal Development Fund of Georgia (the client). Study area The city of Batumi is located in Georgia, in the eastern part of the Black Sea (Fig. 1). Batumi is the capital of the Autonomous Republic of Adjara (Georgia) and one of the major cities on the Georgian Black Sea coast, playing a significant role in economic, cultural and tourist development of the country. The part of coastline subject to the study has a length of approximately 9 km and stretches between the Chorocki river, in the south, and the Batumi harbour, in the north. The coastline and nearshore is characterised by a central part, with a quite uniform alongshore bathymetry, fringed by two submarine canyons in the north and in the south (Fig. 2). Measured water depths reach -240 m for the Batumi canyon and -150 m for the Chorockhi canyon. In the central part of the littoral, the beach width ranges between 20 m (in the southernmost portion) to 90 m (in the northernmost section). The emerged beach is mainly characterized by gravel type of material moving naturally due to wave action in alongshore direction, from south to north. Beach slopes are about 1/5 on the emerged part of the beach, decreasing to 1/10 1/15 between the shoreline and the -5 m depth and then further to about 1/50 seaward. The change in slope is accompanied by a change in sediment size from gravel to fine sand, moving from the beach to the foreshore. In the south, the Chorocki river discharges water and sediments to the coastal system. The Chorokhi river originates in the mountainous region of Anatolia, Turkey. It has a catchment area of 22,100 km 2 of which approximately 9% lies in Georgia. The Chorokhi river is the most important sediment source for the beaches on the Georgian shore next to Batumi. Historically, the main branch of the Chorockhi river reached the Black Sea near Magine, some 3-4 km north of the present river mouth. A smaller southern branch reached the coast approximately 2 km south of the present river mouth (Alkyon, et al., 2009). In 19th century, the main northern branch was abandoned and the river mouth shifted towards its present position, at the head of the underwater canyon. In the second half of the 20th century the location of the mouth has been eventually fixed by dikes and revetments. The new location of the mouth causes loss of a large part of the sediment from the Chorokhi to the canyon: nowadays, it is estimated that approximately 90% of the river sediment load is lost in the depths of the canyon (Russo and Bilashvili, 2004).

4 Fig. 1: Geographical location of the Batumi coastal stretch with respect to the Black Sea. Material and methods Data collection and analysis Different data have been collected and analysed to describe the baseline situation (without adaptation option) and to be used as input for the numerical modelling calculations (see Section Numerical Models ). Bathymetry data. A combination of different bathymetric datasets were used in order to cover an area sufficiently large for the setting up of a wave model for the propagation of the wave conditions from offshore to nearshore, and with sufficient resolution close to the Batumi coastline. In particular the following datasets were used: Global GEBCO08 dataset ( GE neral B athymetric C hart of the O ceans). Bathymetry data from the year 2007 (Alkyon/ARCADIS 2009) Bathymetry data collected within the scope of this contract in November/December 2014 (Technital, 2015) (Fig. 2). The advantage of this last data set is that they extend further offshore in correspondence of the two submarine canyons. This is likely to improve the accuracy of the wave modelling at the coastline close to those features. The nearshore bathymetry is shown in Fig.2.

5 Batumi Canyon Fig. 2: Plan view of the nearshore bathymetry, showing the location of the two canyons (Deltares, 2015a). Sediment characteristics. Sediment samples have been collected on several cross-shore transects on the beach and at water depths of respectively 0 m, -5 m, -10 m and -20 m. The measurements have shown that the beach mainly consist of gravel with small amount of course sand and some cobbles (D mm). The sediment type change from gravel to sand at a depth of approximately -1 / -2 m. The sediment size at - 10 m is fine sand with D50 approximately of 0.1 mm (ARCADIS, 2012). Wind and waves. ECMWF (European Centre for Medium-Range Weather Forecasts) wind data were used in this study. In particular, the most recent reanalysis of these data was used in this study (Dee et al., 2011). The data are 6 hourly, from 1979 until 2013 and available on a global grid with a resolution of about 0.75 x The data contains information on wind speed (U10), wind direction as well as on wave conditions (wave height, period and direction). The offshore wind climate is mainly characterized by winds coming from the SE direction and from W-NW direction. The wave climate study was carried out following two different approaches for cross-validation: 1) Using the ECMWF dataset; 2) based on two nested WAVEWATCH III simulations which were run over the same period of the ERA-INTERIM dataset and forced by the same wind field. Wave information derived from 1) and 2) was intercompared at one location close to Batumi. Given the minor differences between the two datasets, it was decided to use the ECMWF dataset to derive the wave climate to carry out the numerical computations of waves and sediment transport (Fig. 3). More details are given in Deltares 2015a.

6 Fig. 3: Wave roses derived from the ERA-Interim dataset at the location longitude = 40.5 E; latitude = 42 N (Deltares, 2015a). Hydrology and sediment input from the Chorocki river. The annual average discharge at Erge is about 275 m 3 /s. Previous studies have estimated that in natural conditions the river Chorokhi was carrying annually approximately 5 million m 3 alluvial sediments to the sea from which pebble amounted approximately to million m 3 in a year and the rest of the volume consisted of sand (Gamma Consulting LtD, 2011). Although this estimate seems quite high, we do not have other estimate at this stage to verify this value. Nowadays, it is estimated that approximately 90% of the river sediment load is lost in the underwater canyon. Moreover, due to sediment mining and the construction of the planned dams along the river, we have assumed that the sediment input from the river will tend to zero in the coming years, which is the worst case scenario for this stretch of coast. Canyons stability. Most important for this study is the stability of the Batumi canyon. Instabilities of this canyon due to submarine landslides can lead in fact to large losses of volumes of material, including part of the beach. For example in 1999 a landslide triggered by an earthquake occurred, leading to the loss of m of beach located just in front of the canyon (di Leo et al., 2015, Bilashvili et al, 2007) (Fig. 4). Fig. 4: Deep and shallow instability processes in the main and secondary canyon area - Plan view (di Leo et al., 2015).

7 Numerical models Numerical modelling was carried out to simulate alongshore transport rates and coastline changes in the reference situation and in case of interventions. In particular, simulations were carried out using a chain of wave models and alongshore sediment transport models. Wave modelling. Wave modelling was carried out to transform the offshore wave climate to nearshore. In particular, 116 model runs were carried out to transform the 116 offshore wave conditions describing the wave climate to nearshore, using the Delft3D-WAVE model (SWAN). The new wave climate extracted at a depth of -6 m is shown in Fig.5. Fig. 5: Modelled nearshore wave climate along the Batumi coastline at a water depth of -6 m (Deltares, 2015a). Sediment transport and coastline evolution modelling. The alongshore sediment transport rates and the expected coastline evolution were carried out by means of the UNIBEST-CL+ (Deltares, 2011) modelling suite. The coastline has been schematized in the model based on 37 cross-shore transects, corresponding to the position of the transects which were measured within the project. Sediment transport rates were computed for each one of the 116 schematized wave conditions and then added up to derive a yearly alongshore transport rate. As most of the available alongshore transport formulas have been derived for sandy situation, the recent formulation of Van Rijn (2014) has been implemented into the model, which is especially suitable for the modelling of coarser fractions (gravel and shingle), as typical for the Batumi coastline. According to Van Rijn (2014), the total alongshore transport (Q t,mass ) is computed as: Q K (tan ) ( d ) ( H ) V t, mass swell s 50 s, br wave

8 with: K swell : swell factor [-]. Provides the percentage of time with swell conditions s : density of sediment [kg/m 3 ] tan : tangent of bed slope [-] computed between the wave breaking point and the water line d 50 : median grain diameter [m] H s,br : wave height at breaking [m] V wave : wave-induced longshore current velocity (m/s) averaged over the cross-section of the surf zone [m/s] and computed according to: V gh 0.5 wave 0.3( s, br ) sin(2 br ) br : wave angle at breaking [ w.r.t. shore normal] A d 50 of 15 mm was chosen as representative of the sediments along the coastline of Batumi in the active depth. This is slightly finer than what is found directly on the beach, to account for the fact that the average sediment size changes to a finer fraction below water. This value also provides the best agreement in terms of computed and observed morphological changes. Results and discussions Coastal evolution study for the reference situation The computed net alongshore sediment transport rates in the reference situation (in case of no intervention) for the next 30 years are shown in Fig. 6. The lines show an increase in alongshore transport rate moving from south to north, approximately until km 6.5. From this point to the north the transport rate starts decreasing. A net increase in alongshore transport, in terms of coastline changes, corresponds to an eroding coastline. On the other hand, a decrease in alongshore transport corresponds to an accreting coastline. A comparison between computed and observed coastline changes between year 2004 and year 2014 used for model validation is shown in Fig. 7. The figure indicates an excellent agreement between computed and observed historical coastline changes. Fig. 8 shows the effects on predicted coastline evolution for the next 30 years. This trend confirms the observations of an eroding coastline in the southern part, also characterized by very narrow beaches, and an accreting coastline in the northern part, characterized by much wider beaches.

9 Fig. 6: Computed alongshore transport rates along the Batumi coastline excluding sediment input from the river for the reference situation for the next 30 years (Deltares 2015b). Fig. 7: Average coastline changes along the Batumi coastline between year 2004 and year 2014, derived from observation (left) and model calculations (right) (Deltares 2015a). Fig. 8: Bar plot showing the predicted erosion (red), accretion (green) along the Batumi coastline for the next 30 years for the reference situation. The length of the bar is proportional to the erosion/accretion rate (Deltares 2015b).

10 Alternatives for optimal sediment management Several alternatives to address the erosion problems have been investigated in details in Deltares, 2015b and Technital, In this paper, only the best solution is described, as outcome from the technical and cost-benefit analysis. The proposed scheme consists of extracting material from the northern stretch of the coast close to Batumi cape, where it accumulates naturally and before it falls into the Batumi canyon. The material will be transported towards the south, where beach nourishments will be implemented ( sand recirculation scheme) (Fig. 9, left panel). This will also prevent submarine landslides at Batumi cape. We estimate that a minimum of 30,000 m 3 /year of material are necessary to compensate the alongshore gradients shown in Fig. 6. However, this value should also be confirmed by the monitoring campaign that we advise to set-up to monitor the morphological changes induced by the dredging and nourishment operations. Monitoring should also be extended to the river, to assess possible morphological changes induced by the construction of the dams. The predicted erosion-accretion for the next 30 years after implementation of the sand recirculation scheme is shown in Fig. 9 (right panel). The Figure shows how the erosion will be almost absent after the implementation of this scheme. Different schemes for the recirculation of the material have been proposed and are now under discussion. In combination to the sand recirculation scheme, possible interventions aiming at directing and increasing the amount of sediment brought by the river towards the north might be considered. This in order to diminish the large sediment losses into the deep canyon located in front of the river mouth. A cost-benefit analysis has also been set-up to support the findings from the technical study, accounting for a technical life span of the project of 30 years. In particular, benefits from the proposed intervention will include: a reduction in land loss estimated by numerical modelling calculations up to 4.2 ha for the next 30 years, a reduction in damages to building and public infrastructures (4.5 MGEL in the next 30 years), a relative increase in turnover from the tourism sector in case of no erosion (30% of the total turnover from the touristic sector). Those values were used to assess the investment capacity of the project, and support the financing request to the ADB. Fig. 9: Left figure: location of the proposed dredging and dumping locations. Right figure: predicted erosion (red), accretion (green) along the Batumi coastline for the next 30 years (Deltares 2015b).

11 Conclusions In this study, an integrated sediment management scheme for the coastline of Batumi (Georgia) has been presented. The implementation of this scheme should aim at solving the structural erosion problems affecting part of the coastline in front of Batumi. The scheme has been derived based on data analysis and modelling of the coastal area, including information from the river and the submarine canyons located in front of the coastline. The proposed solution is based on a sediment recirculation system which will prevent the erosion of the coastline and also landslides on the Batumi canyon. The technical study has also been supported by a cost-benefit analysis, which was used in the selection among different alternatives against erosion and to show its investment capacity for project financing. The study showed how data and numerical models can be combined using an integrated approach with an economical study with the scope of deriving an optimal sediment management strategy. Acknowledgments The authors wish to gratefully thank Municipal Development Fund of Georgia and Asian Development Bank for allowing use of the data, models and study results, developed under contract SUTIP2/C/QCBS/ We also thank Professor Leo Van Rijn for providing his support and Mr Alberto Scotti of Technital for his knowledgeable guidance and advice. Finally, we would also like to acknowledge the competent work that colleagues Francesco Carnevale and Pierpaolo Smorgon at Technital have carried out on interpretation of soil stratigraphy and canyon stability. References Alkyon/ARCADIS/HKV, IMWM Project in Georgia, Coastal Protection study for Batumi. Project Report, The Netherlands. Alkyon/HKV/ARCADIS, 2009, River and coastal protection Adjara, Georgia, Feasibility study, PVW07078, December Project Report. The Netherlands. Arcadis Nederland B.V., Alternative Feasibility Study for Batumi Coastal Protection. Bilashvili K., Russo G., Megreli N. and Savaneli Z. (2007). Dynamics of the Deltaic Canyon Area of the Rv. Chorokhi, Georgia, Submarine Mass Movements and Their Consequences, 3rd International Symposium. Dee, D. P. and co-authors, The ERA-Interim reanalysis: configuration and performance of the data assimilation system, Q. J. R. Meteorol. Soc., 137 (656), , doi: /gj.828.

12 Deltares, UNIBEST-CL+ manual. Manual for version 7.1 of the shoreline model UNIBEST-CL+. Delft, The Netherlands. Deltares, 2015a. Batumi Coastline Protection. Costline evolution modelling study of the baseline situation. Project report, project num Delft, The Netherlands. Deltares, 2015b. Batumi Coastline Protection. Assessment of technital alternatives for coastal erosion protection based on numerical modelling. Project report, project num Delft, The Netherlands. di Leo, M., Giardino, A., Bragantini, G., De Vroeg, H., Tonnon, P.-K., Batumi Coastal Protection: Facing Decreasing River Discharges of Gravel and Canyons Traps. Proceedings of the 36 th IAHR World Congress, Den Haag, The Netherlands. EUROSION (2004), A Guide to Coastal Erosion Management Practices in Europe. Contract B4e3301/2001/329175/MAR/B3. Prepared by Rijkswaterstaat/RIKZ. The Hague, The Netherlands. Gamma Consulting LtD (2011). Project on Construction and Operation of HPP Cascades on the river Chorokhi. Project report. Tbilisi, Georgia. Giardino, A., de Boer, W., den Heijer, K., Huisman, B., Mulder, J., Walstra, D.- J., Innovative approaches and Tools for Erosion Control and Coastline Management., Proceedings of the MEDCOAST2013 Conference, Marmaris, Turkey. Russo G., Bilashvili K. (2004). Terrigenous Mass Dynamics in the Deltaic Canyon of the Rv. Chorokhi, Georgia. Poster Presentation, 32, Intern.Geological Congress, Florence, Italy. Technital, ME032I-FS-TR-0001-C1_Review of Feasibility Study-Rev part 1 and part 2. Technital Report, Verona, Italy. Van Rijn, L.C., A simple general expression for longshore transport of sand, gravel and shingle. Journal of Coastal Engineering, 90, Zenkovich, V.P. and Schwartz, M.L., Protecting the Black Sea Georgia S.S.R. Gravel Coast. Journal of Coastal Research. 3, 2,