The Geotechnical Properties of the Sediment Body in the Estuary of the Raša River, Croatia

The geotechnical properties of the sediment body in the estuary of the Raša River, Croatia

Ž. Arbanas1, 2, Č. Benac2 & J. Rubinić2 1Civil Engineering Institute of Croatia, Department of Rijeka, Croatia 2Faculty of Civil Engineering, University of Rijeka, Croatia

Abstract

The Raša River valley and its estuary are located on the eastern part of Istrian peninsula (north-eastern coast of the Adriatic Sea). Intensive sedimentation in microtidal estuary environment started due to a lowering of sea level during the Holocene. Most of the sediments were brought over by erosion of Paleogene flysch rock mass on the uperstream catchments area and Quaternary deposits from the ancient Čepić Lake. Slow sea level rise during the last 6.000 years created the conditions for filling the earlier formed karstic estuary. Because of intensive sedimentation the mouth of the Raša River was shifted for approximately 4 km in the last 240 years. The sediment body has uniform granulometric composition, where silt and clay material prevails. The process of consolidation by gravitational compaction is not finished yet. Liquid consistency at the surface of all profiles is a result of high water contents. Substitution of fresh-water by salt seawater in pores causes new changes of cation substitution and changes in clay plasticity properties. The Raša River estuary is a typical model of river-dominated disequilibrium estuary. The sedimentation occurs under micro tidal conditions and low wave energy. An average sedimentation rate is approximately 78.000 t/yr of terrigenous matter, and average deposition rate is 0.15 m/yr. Today the Raša River valley is one of the best locations for the harbour and the industrial development in the North Adriatic area. But, unfavourable geotechnical properties of the sediment deposit will be the main problem for the construction of the future frames. In this work are presented results of executed geotechnical investigations in the area of the Raša River estuary and indicated properties of soils in the sediment body. Keywords: karstic estuary, sea level, plasticity limits, consolidation, Adriatic Sea.

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 210 Coastal Environment V, incorporating Oil Spill Studies

1 Introduction

The Raša River valley and its karstic estuary are located on the eastern part of Istrian peninsula (northeastern coast of the Adriatic Sea).

Figure 1: Generalized geological and hydrographical map of Istria peninsula: 1. Upper Cretaceous and Paleogene limestone; 2. Paleogene flysch; 3. Holocene alluvial, lacustrine and estuarine sediments; 4. Limits of topographic drainage area; 5. Karstic underground flow.

Istria peninsula is situated in the northwest of the Adriatic Sea. The Adriatic is the deepest penetration into the European continent from the south. The sea passages for various cargoes destined for the Middle Europe end on the Istrian

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 Coastal Environment V, incorporating Oil Spill Studies 211 northwestern part. Already in the last century, two large harbors were built in the northwest of the Adriatic: Rijeka (Croatia) and Trieste (Italy). The third large harbor Koper (Slovenia) is growing at present time. The Rijeka harbor extends its capacities to the adjacent suitable locations and new equipment is built also on the mouth of River Raša in Istria, Fig. 1, 2.

Figure 2: Geological map and morphogenesis of studied area: 1. Upper Cretaceous limestone–bedrock; 2. Holocene alluvial and estuarine sediments (a/red clay, b/yellow clay, c/grey clay); 3. Age of shoreline position; 4. Isobaths of sea bottom (in 1975.); 5. Traces of geological profile; 6. Finding place of salt water snail; 7. Boring holes (a/single boring hole, b/group of boring holes).

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 212 Coastal Environment V, incorporating Oil Spill Studies

Three larger rivers flow through the Istrian peninsula: Dragonja, Mirna and Raša. According to the historic data, there were large and deep bays on the mouths of the two former rivers (Dragonja and Mirna) as marked on the old geographical maps originating from the year 1744. and 1799. Today these areas are covered with large quantities of material brought along by the rivers. Entirely contrary, the Raša river is an estuary still today, i.e. it is a long inlet penetrating into the continent, Fig. 2. The bay region and part of the valley around the mouth of the Raša river is the most valuable area on the northern Adriatic Sea as far as traffic and industry are concerned. It represents an ideal location for reloading operations for certain groups of cargoes. The harbor terminals for wood and live stock are already built here. There is also a great interest for development of the water culture. These capacities are built on very suitable geographic positions. Good road and railway connections are already available-remote from the big cities. As anticipated for the future, it will be necessary to enlarge reloading capacities as well as open and covered warehouses. Larger areas have not been developed so far due to a series of circumstances among which some very unfavorable geotechnical properties are to be emphasized. The existing harbor equipment is built on the most suitable geotechnical locations were bedrock has shallow position. The data so far published about the morphogenesis of the valley and the bay of the Raša River as well as the data about the form, geological fabric and geotechnical properties of the sediment body are very scanty. More systematic and extensive data are to be found in later works of the authors. The form of the sediment body is evident here and the relation between the Quaternary oscillations of the sea level and the intensity of the river erosion is explained [1, 2].

2 Geological description of studied area

In the catchments area of Raša River Upper Cretaceous and Paleogene sedimentary rocks are presents. Paleogene foraminiferal limestone and flysch rocky complex (marls, sandstones and breccias in alternation) dominate in upstream part of catchments area. Downstream part of Raša River valley and its estuary are situated on eastern part of Istrian karstic plateau which is composed of well-bedded and slightly undulating Upper Cretaceous rudist limestone [3]. Red soil (terra rossa) is often found on limestone, whereas on flysch a cover of weathered material and slope deposits is found. The bottom of Čepić polje depression is filled by Pleistocene and Holocene alluvial and lacustrine sediments. The bottom of the Raša River valley and its estuary is covered by Holocene alluvial and estuarine sediments. Intensive morphogenetic processes caused by tectonic movements and rapid sea level changes, as well as climatic changes, provoked the present shape of the studied area. It assumed that the valley of the Raša River and its estuary was formed by cutting during Upper Pleistocene when the sea level was lower than present [2]. The sea began to flood the northern Adriatic Sea area, and in this respect the Kvarner area some 18.000 years ago. The rapid rise was between

17.000 and 6.000 years BP [4]. Hydrographic and sedimentary conditions similar to the present ones appeared only after decrease in sea level rise some 6.000 years ago [5]. The sea covered the karst relief and penetrated deep into the ancient river valley. Slow sea level rise during the last 6.000 years created the conditions for filling of the earlier formed estuary. Transport and deposition of sediments at river mouths caused the seaward shift of the coastline. The recent history of the filling processes clear from geographical maps printed since the eighteenth century. By reason of intensive sedimentation, the mouth of Raša River has been translated for 4 km approximately during the last 250 years, Fig. 2. Marine sediments were found upstream of the present river mouths [1].

3 Hydrographical conditions and recent sedimentation

The direct topographic drainage area of the Raša River is 205 km2, but the estimated hydrogeological catchments area is 450 km2. The hydrological regime of the Raša River is typically karstic and characterized by short-term high- discharge events. Mean annual discharge in the upstream part is 2.3 m3/s. Karstic groundwater flows in the downstream part of river valley increase the flux and 75% of water quantity is originated by permanent and intermittent karst springs. For this reason, estimated mean annual discharge is 8.0 m3/s at the Raša River mouth, maximal discharge is 185 m3/s and minimal is only 0.3 m3/s [6]. In the upper part of drainage area Paleogene flysch prevail, Fig 1. This area has a well developed surface drainage network, with numerous torrent flows. Intensive process of erosion is present and large quantities of fine-grained particle are produced. During intense short-term events high concentration of suspended matter is present. The central part of drainage area is dominated by river flow resulted the confluence of several streams, Fig. 1. This part of catchments area characterized by karst springs which intermittently contribute additional quantities of predominately fine-grained particles. An average sedimentation rate is approximately 78.000 t/yr of terrigenous matter. The estimated quantity of sediments is 1.450.000 m3 in the last 30 years [8]. The lake situated on the location of today Čepić polje is eliminated after excavation of hydrotechnical tunnel in year 1932. The Raša River estuary is a typical model of river-dominated disequilibrium estuary. The sedimentation occurs under micro tidal condition and low wave energy. More than 90 % of the total terrigenous load is brought into the estuary as fine grained suspended matter. Great part of sedimentation occurs at the salt wedge in 3 km long delta and prodelta zone [7]. The river and estuary sediments have mean size between 3.0 µm and 8.1 µm. Carbonate share is between 19 and 46 %. The mineral composition of the clastic source rocks is predominately carbonate, but quartz and clay minerals are present in significant portion. Suspended matter has an increased share of clay minerals. The mineral composition in sediment body is similar to the suspension in estuary.

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 214 Coastal Environment V, incorporating Oil Spill Studies

4 Investigation methods

Geotechnical researches were carried out in the bay and the valley of the river Raša between years 1971 and 1986 with regard to projecting and constructing the harbour with its accompanying objects such as warehouses and railway junctions. All elaborated works showing the results of the geotechnical researches in this area were carefully examined. There were surveyed the results of geological charting, research boring, "in situ" in hole investigations, geophysical measurements and laboratory experiments. The data from 47 investigated boring holes (Fig. 2) as well as form 120 different laboratory analyses were carefully worked out. For better survey and easier comparison, the results were grouped into 4 profiles. Some of the physical and mechanical properties inside the sediment body were examined by investigation.

Figure 3: Geological profiles [1]: 1. Upper Cretaceous limestone–bedrock; 2. Holocene alluvial and estuarine sediments; 3. Boring hole.

This work tries to find out the relation "cause-consequence" between arising the bay and the valley of the Raša river as well as the properties of the sediment body according to the available data.

5 Geotechnical properties of the sediment body

The physical and mechanical properties of the material from the sediment body are shown grouped according to characteristic cross-sections, Fig. 3. It is obvious that the composition of all tested samples is pretty uniform as far as particle size is concerned. It results in very close range of grading curves regardless of the place where samples were drawn, Fig. 4. Even 95 % of all samples contain 40 to 50 % of clay particles and less than 10 % contain sand fractions.

Figure 4: Range of grading curves in the sediment body [1]: 1. Limit of all grading curves; 2. Limit and range of grading curves for 95 % samples.

Great uniformity of granulometric composition of the sediment body shows- as horizontally on 4 cross-sections so also vertically into the depth-that sedimentation was formed from suspension at low speed but at emphasized influence of interaction of fresh and sea waters. It is characteristic for the estuary conditions of sedimentations [8]. Larger, predominantly sandy particles were sedimented upstream.

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 216 Coastal Environment V, incorporating Oil Spill Studies

Contrary to the uniformity of granulometrical composition, the characteristics of plasticity of single samples are more dependent on the position of drawing samples along the river flow. The scattering of points on the plasticity chart is diminished with profiles downstream. The material according to its characteristics of plasticity in the area of the profile A - A' range from the organic clay of low plasticity up to the clay of high plasticity while the clay of high plasticity is predominantly spread on profiles C - C' and D - D'. Such appearance is characteristic for the type of sedimentation in the estuary where the river flow is dominant and the influence of waves and the tide is limited. Therefore, the clay particles remain relatively long floating on the surface of the fresh-water after reaching the sea. Only after getting in touch with ions from the sea water, it comes to joining of clay mineral particles, flocculation and sedimentation [9].

Figure 5: Geotechnical properties of the sediment body [1]: 1. Plasticity limit (w ), liquid limit (w ) and natural water content (w ) (in %); 2. p l 0 Cohesion (in kN/m2); 3. Angle of internal friction (in 0); 4. Constrained modulus (in MN/m2).

Explanation of the previous characteristics is to be found in the more uniform conditions of sedimentation on profiles C - C' and D - D'. As the sedimentation in this area followed under greater marine influence than on the positions of the profiles A - A' and B - B', it can be supposed that cation substitution on the clay mineral particles was carried out more uniform, Fig. 5. By depth of the single profile you can see Atterberg's limits and natural moisture content, shear strength parameters and compression modules. Strength parameters for material inside the sediment body are mostly defined through the direct shear tests. Values of the strength parameters vary

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 Coastal Environment V, incorporating Oil Spill Studies 217 from 150 to 300 for the angle of internal friction and 5 to 35 kN/m2 for cohesion. These magnitudes are spread widely around their average value. As it can be ascertained that the angle of internal friction does not essentially depend on the depth internal geotechnical profile and accordingly on increase of geostatical stress, but for cohesion an evident increase of value is noticed with parallel increase of depth. Constrained modulus from consolidation test as an indicator of compressed sediments increases parallel with increase of geostatic stress to which the material is exposed. As the permeability of the sediment clay material is bad (varying from 10-7 to 10-8 cm/s), the consolidation time will be exceptionally long under an additional loading on the surface. The process of consolidation by gravitational compaction is still going on due to fast sedimentation of the bay (some 4-5 m in the last thirty years), due to great thickness of the sediment body and due to bad permeability of the material. Further, a liquid consistency of clay-silty material is noticed at the surface of all profiles. It is a result of high water contents and of still present process of consolidation by gravitational compaction [10] with the downstream profiles but the explanation for the upstream profiles can be looked for in reaction of fresh- water from the Raša river respectively in leaching process. Substitution of seawater by fresh-water in pores causes new changes of cation substitution on mineral clay particles, a real leacing of Na cations. The result of this reaction is a reduced plasticity index and possible transition of plastic consistency clay material into liquid consistency clay material at the same water content, the known process in normally consolidated marine clays [11].

6 Conclusion

The Raša River estuary is a typical model of river-dominated disequilibrium estuary. The sedimentation occurs under micro tidal conditions and low wave energy. An average sedimentation rate is approximately 78.000 t/yr of terrigenous matter. More than 90 % of total load is fine grained suspended matter where prevail silt and clay. This fine grained matter is flocculated and preferentially accumulates at the head of estuary. This deposition causes a progradation of the estuarine and obviously shoreline changes. The sediment body in Raša River valley was formed during the Holocene sea level rice. The sea penetrated deep into the ancient river valley and covered the karstic relief. The great part of sediment matter has origin from upper part of drainage area where Paleogene flysch prevail and intensive process of erosion has produced large quantity of fine-grained particles. The mineral composition in sediment body is similar to the suspension in estuary. By reason of intensive sedimentation, the mouth of Raša River has been translated for 4 km approximately during last 250 years. Similar or the same conditions for sedimentation are reflected in physical and mechanical properties of the material. Distribution of the results by depth of sediment body shows that the process of consolidation by gravitational compaction is still going on. Every construction in the valley of the river Raša

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8 218 Coastal Environment V, incorporating Oil Spill Studies must take into consideration the subsiding of the ground, which cannot be prevented within reasonable interventions. It refers to subsiding of the remaining part of consolidation by gravitational compaction. Because of the high deposition rate a dredging will be necessary in small Bršica port.

References

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