A Study on Relationships Between Forcing Factors and Morphological Parameters of Outlet Areas of Some Rivers Flowing Into the Estonian Coastal Sea

River Basin Management III 225

A study on relationships between forcing factors and morphological parameters of outlet areas of some rivers flowing into the Estonian coastal sea

T. Koppel & J. Laanearu Department of Mechanics, Tallinn University of Technology, Estonia

Abstract

The river mouths in the Estonian coastal areas consist predominantly of sediments. The main problem on these coastal sites is unanticipated beach changes. In some cases this is due to natural processes and sometimes due to the effects of past anthropogenic activity which have modified sediment transport dynamics. An important factor affecting the morphological evolution of outlet areas of some rivers flowing into the coastal sea is constructions. Comparatively large sediment deposition at the river mouth can affect the local hydrological system. As a result of settling of sediments partial blocking of the river mouth due to intensive seaside transport can occur. Such situations can be related to relatively high sea levels and a natural reduction of river flux, the joint effect of which results in the increase of sediment motion along the shore. In this study the significance of forcing factors relevant for bathymetrical changes at the outlet areas of rivers flowing into the Estonian coastal sea are discussed. Keywords: river mouth, coastal bathymetry, river discharge, wind properties, sea level, sediment transport.

1 Introduction

Estonia is located entirely within the Baltic Sea catchment area, whereas rivers are divided between four watersheds: the Peipsi basin, the Gulf of Finland basin, the Gulf of Riga basin and the island’s watersheds (cf. Figure 1). The long-term mean run-off into the coastal sea exceeds 10 billion m3 per year. Of these rivers only 7 have catchment areas exceeding 400 km2, and 3 rivers have an annual

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 226 River Basin Management III mean run-off exceeding 10 m3 s-1. The Pärnu and Kasari rivers follow the Narva River in terms of water abundance. From the total outflow from the mainland rivers, around 80% runs to the Gulf of Finland and the rest into the Gulf of Riga and connected archipelagic area of the western Estonia – the Väinameri.

Gulf of Finland Kunda N Tallinn Bay Bay Narva Bay Matsalu Pirita Kunda Narva Bay

Hiiumaa Baltic proper Väinameri Kasari ESTONIA Lake Peipsi Saaremaa Pärnu

Pärnu Emajõgi Bay

Irbe Strait

Gulf of Riga 60 km

Figure 1: Coastal areas and some parent-streams of Estonia.

The Baltic Sea, surrounding the mainland of Estonia, is comparatively shallow and has variable bathymetric as well as archipelagic conditions. The northern coast is strongly meandering, including smaller and larger bays. For instance, the easternmost Narva Bay is well opened to the gulf interior, but the westernmost Tallinn Bay is rather closed because of the island Naissaar. In contrast, the western coast of Estonia is well blocked by large islands: Saaremaa and Hiiumaa. The largest semi-closed basin nearby the western side of Estonian mainland is the Gulf of Riga. This semi-closed sea area has rather complicated archipelagic pathways to the Baltic proper and the Gulf of Finland, and a relatively broad connection to the Baltic proper through the Irbe Strait (Laanearu, et al [5]). The northern as well as the western coasts of Estonia are parts of sedimentary rock, the cliffs of which rise along the shores of the island Saaremaa and southern coast of the Gulf of Finland, towering occasionally up over 50 metres over the sea level. Coastal modelling requires accurate description of the seabed bathymetry. In the areas of complex topography, like soft-bed river mouths, there can be substantial differences between the predictions of sediment transport from different models. This is particularly the case where in addition to the river flow there are considerable effects due to residual currents and beaching waves. The water circulation in the river outlet area is controlled by a number of

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 227 characteristics of the river flow itself, coastal processes (such as currents, waves etc.), and the structure, position and materials of the outlet (Carter, 2002). The combined influence of the above mentioned factors can form an underwater barrier, which can gradually move in the prevailing direction of the littoral drift. Such a movement of sediments forces the stream to bend in the same direction and, at times, creates inconveniences in the hydrologic system or complicates navigation. To reduce economic pressures in harbour management, some river mouths are protected by the construction of sea walls. These appear to trap the sediments necessary to maintain the coastline against littoral drift. Relatively large water-level changes and long-lasting winds can cause considerable changes in the bathymetric depth of the soft-bed river mouth. However, the key problem in modelling of coastal processes is how important the river discharge, wind waves and residual currents to the bathymetric changes on the particular coastal site are? There are a number of relationships which must be carefully studied. The main idea of this study is to estimate relevance of forcing parameters which are related to the bathymetric changes in the outlet areas of rivers flowing into the Estonian coastal sea.

2 Bathymetric conditions

The Estonian coastal sea has rich variations of bathymetric as well as archipelagic conditions (see Figure 1). Some parts of the coastal areas are well opened to sea interior and other parts are made up of smaller or larger semi- closed bays. Under the influence of the forces of nature beaches and peninsulas are in constant motion, advancing seaward in some places and receding landward in others. Variable forcing factors, affecting sediment motion at river mouths, can be important for different parts of the coast. Whether to decide how important the wind-induced beaching waves and coastal currents, related to the large water-masses motions, for sediment motion, are, the coastal shelf and geographical orientation of river outlet in respect of the entrance-basin morphologic characteristics like coastline and shape can be analysed. In this study rather simple parameters are introduced to characterize morphological conditions at the mouth area of rivers. The slope of coastal shelf indicates how important the different scale motions can be in particular area. On a mildly sloping shelf the main force causing beach processes is due to short- period surface waves, which can move without breaking relatively long distances. In contrast, a relatively steep ascent shelf can be very sensitive to currents, which are usually created due to boundary effects of large water-masses motions. However, for particular entrance basins of rivers the mildly and deeply sloping coasts can be distinguished due to depth changes, which are perpendicular to coastline. For this purpose, the available hydrographical maps of coastal areas can be used. It is also important that together with costal slope the geographical orientation of beach is fixed. The structure of coastline indicates how effective different scale motions for beach processes are. Both coastal slope and coastline orientation are necessary to know for analysis of effects due to different motions in coastal region of river-water intrusion.

Morphological characteristics of a number of rivers flowing into the Estonian coastal sea are presented in Table 1. From this general analysis it is evident that Estonian rivers have very different outflow conditions. For instance, the Narva River flows into the Narva Bay, a widely open sub-basin in the eastern part of the Gulf of Finland (for details see Laanearu and Lips [4]). The depth of water in the sea reaches 10 metres at the distance of around half a nautical-mile offshore. Whereas the geographical orientation of shore is almost SW-NE directional (indicated by angle π/4 in Table 1). The Pärnu and Kasari rivers, which follow the Narva River in terms of water abundance, are main fresh water suppliers into relatively closed sea areas at the western coast of Estonia. The Pärnu River flows into the Pärnu Bay, an inner sea area of the Gulf of Riga. The surface area of this small bay can be estimated to be around 411 km2 and the mean depth 4.7 m (maximum 8 m). In this oval-shape bay the pattern with a cyclonic and an anticyclonic circulation cells is favoured due to the river intrusion (cf. Suursaar et al. [11]). In its delta region the Kasari River has extensive flood plain and eutrophic ends in the Matsalu Bay. The entrance basin of the Pirita River is the Tallinn Bay, which has surface area less then 200 km2 and the depth maximum can reach to 40 metres. In the Tallinn Bay both the wind waves and local currents can cause near-bottom velocities, which affect the beach processes (cf. Soomere et al. [10]). The Kunda Bay, which is the entrance basin of the Kunda River, is a 7×4-km semi-closed sea area, the maximum depth of which is around 15 metres. This shallow bay has the deepest opening from the north and since the waves originating from this direction can only be effective for the beach processes. However, the wave activity can be considered to be weak due to its shallowness and short fetch, and because currents can have considerable effects on sediments deposition near the Kunda River mouth and import sand from the nearby regions.

Table 1: Morphological characteristics of some Estonian river-mouths.

Name of river Basin of entrance Coastal slope Orientation Pärnu Pärnu Bay 5m/1800m 7π/4 Kasari Matsalu Bay 1m/2340m 2π Pirita Tallinn Bay 10m/450m π/4 Kunda Kunda Bay 2m/400m π/2 Narva Narva Bay 10/900m π/4

The coastal erosion can be considered comparatively rapid near the river mouth which consists of relatively soft bed, whereas nearby areas the beach processes vary between stability and accretion (cf. Bonora et al. [1]). However, the coastal motions at river-mouth areas are complex and difficult to model, and engineering judgment is always required to decide on the best mathematical representation of certain model components, because there will always be site criteria that cannot be accounted in sediments transport models. There is still therefore an important role for specialists to decide which process in particular study area is important, or, conversely, which process does not have a significant

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 229 effect and can therefore be ignored. Modelling the beach processes which are strongly related either to waves or currents the river mouth and its entrance basin’s morphologic parameters can be analyzed by a methodology introduced above.

3 River discharges

River run-off into the Baltic Sea is one of the main characteristics describing the available water resources. River discharge from the Estonian watershed regions is characterised by a very high temporal and spatial variability. Hydrologic measurements have been performed in Estonia for a long period. The first information on river run-off dates back to 1902 from Vasknarva (the Narva River). In order to decide how important the discharge on sediment motion at river mouth is, the annual run-offs from rivers into the sea can be analysed.

Figure 2: Long-term mean of runoffs of some Estonian rivers.

There are 420 rivers of length over 10 km. The longest is the Pärnu River the length of which is around 144 km. Only two rivers discharging into the Baltic Sea have an average run-off around 50 m3.s-1 and more: the Pärnu River and Narva River. (The third river in terms of water abundance is the river Emajõgi, which flows into the Lake of Peipsi, see Figure 1.) The mean annual flow from the mainland of Estonia into the Baltic Sea is around 475 m3.s-1, of which waste water makes up 17 m3.s-1 – around 3.5% (Tarand and Kallaste [12]).

Hydrological characteristics of a number of rivers flowing into the Baltic Sea are presented in Figure 2. It is clearly evident that among the rivers of Estonia the Narva River discharge is the largest, the long-term mean discharge of which is around 400 m3.s-1 (Protasjeva and Eipre [6]). The monthly flux can be two times higher during spring months. The hydrograph of the Narva River at its mouth area is characterized by a spring maximum and a smaller flux during the rest of the year. For instance, the daily maximum and minimum discharge of the year 2002 were around 700 m3 s-1 and 150 m3 s-1, respectively. (Estimating the average velocity the cross-sectional area at the top of the river can be taken around 700 m2). If the Narva River discharge can exceed the mean run-off around two times, in comparison the Pärnu River discharge during spring months can exceed the mean runoff around five times. River run-off in Estonia is characterized mainly by seasonal variability. Longer time-scale variations in the river run-off are apparently caused by climatic changes (Tarand and Kallaste [12]). Analysis of the run-off of rivers allows us to draw some conclusions. The fresh-water forcing on the coastal sites of Estonia is very unequally distributed. In terms of water flow only three rivers: the Narva, Pärnu and Kasari, can be important in the sense of affecting considerably the beach processes in particular coastal area. Only the mouths of Narva and Pärnu rivers are protected by constructions. However, over half of Estonian rivers are still unregulated or only moderately modified; therefore many floodplains have been preserved in their semi-natural state, like the Kasari River delta.

4 Forcing factors

The Estonian mainland is surrounded by the Gulf of Finland in the north, and by the Gulf of Riga and archipelagic area – Väinameri in the west (see Figure 1). Due to the variable morphologic conditions also different forcing factors, affecting local sediment depositions at river mouths, can be important in different parts of the coast. In order to decide how important beaching waves and coastal currents are for sediment motion, the wind properties and sea-level variations can be analysed for different coastal areas. The Baltic Sea is located within the global west-wind zone and because of that mostly cyclones coming from the west and SW dominate in the weather pattern. However, the local winds in different areas of the coast are closely related to the atmospheric pressure gradients and coastal "roughness". Latter explains why the open-sea winds are usually stronger than those observed on the coast. The wind regime of the Gulf of Finland consists of SW and north winds, which dominate in the whole Baltic Sea, but also local east and west winds blowing along the axis of the gulf (Soomere and Keevallik [9]). The strongest winds blow during autumn and winter months and very rarely occur during the rest of the year. However, the effect due to waves, on the beach are also dependent on water level situations. In the Estonian coastal sea the water level changes reveal annual as well as shorter time-scale fluctuations. On the basis of long-term sea level recordings it was found that a distinct seasonal cycle with

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) River Basin Management III 231 low water level in March-May and high level in October-November exists (Raudsepp et al. [7]). However, the sea level time-series reveal also shorter fluctuations. For instance, the sea level in the mouth of the Narva River in 2002 dropped by around 1 metre during a spring month. An important issue in coastal engineering is prediction of the beach changes related to extreme wind and sea level events. However, morphological changes at the river-mouth areas are complex, this already because of the forces, due to wind and sea level variations, can affect differently sediment motion. As an example of the directional distribution of the moderate and strong winds on the coast of the Gulf of Finland, the annual winds in Kunda are represented in Figure 3. As an expected feature, evident from the plot, the northern and south-western sector winds dominate. The situation would change drastically if one included weak winds in this plot. In respect of wind energy content, weak winds are less important, and, however, winds with speeds below 6m.s-1 are likely to have importance for motions in the sea. If only directional distribution of strong winds is analysed for this year, the winds within the northern sectors dominate. The situation for the western coast is different in some aspects because usually strong winds in Pärnu originate from the SW (cf. Soomere and Keevallik [9]). The measurements of winds on the coast of Estonia allow one to investigate the forcing conditions. However, a problem that pertains to the available winds is associated with the observation place and in some cases open-sea winds are necessary to be used. Long lasting moderate winds can also have strong effect on the beach processes, because under suitable topographic and meteorological conditions they can lead to large vertical displacements of coastal waters (Laanearu [3]).

Figure 3: Distribution of strong-moderate winds in the northern Estonia.

The rather complicated temporal and spatial pattern of forcing conditions in the Estonian coastal areas suggests that winds and sea level can be important for coastal processes in different ways. In some areas the currents dominate and in others the waves are important. However, using wind wave or coastal current models it is possible to show how a specified sequence of motions affects sediment balance in a particular river mouth. The modelling approaches are important also because in many instances it is not possible to gain access to a sufficiently long, continuous record or spatial distribution of measured wave and current data, and it is necessary to reconstruct the motions on the basis of forcing data.

5 Concluding remarks

As the result of this study it is found that the bathymetrical depths of the Estonian coastal sea can be affected by cross-shore currents most drastically in the vicinity of the Narva, Pärnu and Kasari river mouths. To prevent choking effects of large deposition of sediments, only the mouths of Narva and Pärnu rivers are protected by huge breakwaters. In the mouth of the Narva River the sea wall is approximately 300 metres long. It was built in the 1980s to facilitate navigation between the harbour and the sea, and to prevent extensive beach erosion, which was observed earlier. This béton armé structure and the eastern coastal bank form together outlet conditions for the river. In the mouth of the Pärnu River the two stone breakwaters with the total length of 5 km, each reaching about 2 km into the sea were built during the 19th century. Without these breakwaters the river mouth was almost blocked by sand, which restricted navigation of large cargo vessels. The Kasari River, which flows into the euthrophic Matsalu Bay, transports water directly into the open area of the Väinameri. However, the Pirita and Kunda river mouths are protected by harbour constructions, but the main obstacles for land-water streams are dams located around 14 km and 2 km upstream, respectively. The bathymetric height of a soft-bed river mouth is in constant change, which depends on cross- and long-shore motions above the bed. The bottom changes due to erosion processes may prevail in the river mouth due to comparatively low sea level and large river discharge. Of course, the river sediment deposition can also contribute to bottom changes at the river mouth. Controversially, the deposition of sediments into the mouth, which may cause partial blocking of the river stream, may occur in comparatively high sea level and reduced river discharge situations. As an illustrative example we examine the effects of erosion in the sand composed coast. In the energetic environment, like the river mouth, the cross- shore transport of sediments can be estimated on the basis of an equation similar to Bagnold’s Equation: 5.8 ρ ue 3 (1) f * , qcs = ()s ερρ g tan Φ−

where e f is the efficiency factor which is for water around 0.1, tan Φ is the coefficient of internal friction, which for naturally shaped sediments is around 0.6, ε is the coefficient of sediment volumetric concentration, which for sand is around 0.65, and ()s − ρρ g is the submerged weight of sediments which for sand is around 16 kN.m-3. For one-dimensional flow, the frictional velocity can be computed from: * 8 ⋅= AQfu , where f is Darcy-Weisbach friction factor, Q is the volume flux and A is the cross-sectional area of stream. According to the 2002 data for the Narva River case the specific flux during spring months is around 1.29×10-5 m2.s-1 and for autumn months around 1.26×10-7 m2.s-1. Notwithstanding of a large difference in the mean velocities of the both calculation examples, the river flow can be classified as rough turbulent. The Narva River mouth is an interaction zone between the wave- and current- induced sand motions (indicated by dashed arrows in Figure 4), and, however, the erosion of the soft bed river-mouth must be compensated by the sediment deposition, which mainly can be related to the along-shore transport. Apparently the wave-induced sediment motion in the vicinity of the Narva River mouth is essential. This is due to the fact that the mildly sloping coast in question is totally open to the gulf and the wind storms in this area originate predominantly from northern sectors.

Narva Bay

300m

NARVA- JÕESUU Narva River

Figure 4: Hydrological system of the Narva River mouth.

According to our general analysis it can be concluded that blocking of river mouths in the Estonian coastal sea is favoured during periods when the windstorms are frequent, and therefore beaching waves and currents can have strong impact on along-shore sediment motion. The dominating effects from waves or currents can be distinguished by the geographical location of the river mouth and its entrance basin. However, the discussed temporal and spatial

WIT Transactions on Ecology and the Environment, Vol 83, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 234 River Basin Management III pattern of forcing factors suggests that in some areas currents dominate and in others waves are important. The hydraulic models are important tools for integrating circulation and morphological changes in a river mouth. They allow bridging the gap between the scales which we can observe in the field, and the generally large scales for which forecast is necessary. On the other hand, models are derived on the basis of simplified assumptions which place necessary limitations to their applicability. It is therefore important to develop models which can integrate the different scales of hydrological and coastal processes with necessary accuracy. At present, the Estonian rivers and the coasts operate as two independent systems. However, beach and hydrological processes are complex and difficult to model, and engineering judgment is always required to decide on the best mathematical representation of certain model components.

Acknowledgements

Financial support by the Estonian Science Foundation Grant 5879 and Targeted Financing No 0142514s03 is greatly appreciated. We thank Anatoli Vassiljev who made most of hydrological data used in this study available, and Tanel Tuisk who helped to determine the parameters of sediments.

References

[1] Bonora, N., Immordino, F., Schiavi, C., Simeoni, U. & Valpreda, E. Interaction between Catchment Basin Management and Coastal Evolution (Southern Italy). J. Coast. Res., 36, pp. 81-88, 2002. [2] Carter, R.W.G. Coastal Environments (8th ed.), Academic Press, London. 2002. [3] Laanearu, J. On coastal waves and related upwellings in the Narva Bay. Coastal Engineering VI, Computer Modelling and Experimental Measurements of Seas and Coastal Regions. pp. 53-61, 2003. Edited by C.A. Brebbia, D. Almorza, F. Lopez-Aguayo. WIT Press, Southampton. [4] Laanearu, J. & Lips, U. Observed thermohaline fields and low-frequency currents in Narva Bay, Proc. Estonian Acad. Sci. Engng., 9(2), pp. 99– 106. 2003. [5] Laanearu, J., Lips, U. & Lundberg, P. On the application of the hydraulic theory to the deep–water flow through the Irbe Strait. J. Marine Syst., 25(3-4), 323–332, 2000. [6] Protasjeva M. & Eipre T. Resources of the surface water of the UUSR. Baltic region. Estonia. Leningrad, 1972. (In Russian) [7] Raudsepp, U., Toompuu, A. & Kõuts, T. A stohastic model for the sea level in the Estonian coastal area. J. Mar. Syst., 22, pp. 69-87, 1999. [8] Soomere, T. & Keevallik, S. Directional and extreme wind properties in the Gulf of Finland. Proc. Est. Acad. Sci. Eng., 9(2), pp. 73-90, 2003. [9] Soomere, T. & Keevallik, S. Anisotrophy of moderate and strong winds in the Baltic Proper. Proc. Est. Acad. Sci. Eng., 7(1), pp. 35-49, 2001.

[10] Soomere, T., Rannat, K., Elken, J. & Myrberg, K. Natural and anthropogenic wave forcing in the Tallinn Bay, Baltic Sea. Coastal Engineering VI, Computer Modelling and Experimental Measurements of Seas and Coastal Regions. pp. 273-282. 2003. Edited by C.A. Brebbia, D. Almorza, F. Lopez-Aguayo. WIT Press, Southampton. [11] Suursaar, Ü., Kullas, T. & Otsmann, M. Modelling of flows, sea level variations and bottom stresses in the coastal zone of West Estonia. Coastal Engineering VI, Computer Modelling and Experimental Measurements of Seas and Coastal Regions. pp. 53-61, 2003. Edited by C.A. Brebbia, D. Almorza, F. Lopez-Aguayo. WIT Press, Southampton. [12] Tarand, A. & Kallaste, T., (eds). Country case study on climate change impacts and adaptation assessments in the Republic of Estonia. Report to the UNEP/GEF. – SEI, Tallinn. 146 pp., 1988.