NUMERICAL MODELLING IN ENVIRONMENTAL IMPACT ASSESSMENT OF CONSTRUCTION WORKS WITHIN RIVER BAYS - A CASE STUDY

Transcription

1 NUMERICAL MODELLING IN ENVIRONMENTAL IMPACT ASSESSMENT OF CONSTRUCTION WORKS WITHIN RIVER BAYS - A CASE STUDY M. Jovanović 1, R. Kapor, B. Zindović University of Belgrade Faculty of Civil Engineering mjovanov@grf.bg.ac.rs 1 ABSTRACT Construction of the new cable stayed bridge on the Sava River in Belgrade - the "Ada Bridge", required extensive foundation works at the entrance of the Ada Bay, with two environmentally unfavourable consequences - increased siltation of the Bay entrance, and dislodgement of protected bird species (the pygmy cormorant). These consequences are unacceptable, as this stretch of the Sava River's right-bank is the city's main recreational resort, and a small boat harbour. This paper deals with actions undertaken in order to cope with consequences of bridge construction in the Bay. Numerical flow simulations were done in order to design a reconstructed Bay entrance with reduced siltation, and to shape the existing sand bars to become, after backfilling and planting, a renewed habitat for the protected bird species. A finite-element 2D model was used to calculate the flow patterns at the Bay entrance, and the siltation potential was related to the size and strength of the horizontal recirculation in the Bay (primary gyre). As a solution, a low stone sill across the entrance was proposed, its inclination, length, and height optimized in respect to the quantity of entrained sediment, and aesthetic requirements. This solution, together with the cultivated sand bars, was also advantageous from the point of ambient quality restoration. Keywords environmental restoration, bays, siltation, recirculating flows, numerical modelling.

2 1. INTRODUCTION The Ada peninsula (once a river island that has artificially been turned into a peninsula), is located on the Sava River's right bank in central Belgrade, the capital of Serbia (Fig 1). The name Ada also refers to the adjoining artificial lake with beaches, and the Ada Bay, with rowing clubs and a small boat harbour. This is an immensely popular recreational zone, which, during summer seasons, can receive over visitors daily, and up to visitors over weekends. Fig. 1. Location of the Ada Bridge over the Sava River in Belgrade. The new cable stayed bridge, called The Ada Bridge (Fig. 2), was inaugurated on the New Year s Eve, This, architecturally and structurally unique bridge [1], passes over the tip of the Ada peninsula, carrying six traffic lanes, a double track rail line, and two pedestrian/cycling lanes. The bridge is a 6 span, continuous superstructure with an overall length of 929 m and 45 m wide deck. The main support structure consists of a single 200 m high pylon with an asymmetric system of cables (Fig. 2). Fig. 2. The Ada Bridge under construction (May 2011). 2

3 Among a number of controversies following design and construction of this bridge (location, cost, etc.), two environmental issues were raised: (i) how to restore the entrance of the Ada Bay after extensive foundation works for the pylon, and (ii) how to provide new habitat for a protected bird species - the pygmy cormorant, in exchange for their dwelling space destroyed in the course of bridge construction. This paper describes how these two environmental problems were solved. 2. ENVIRONMENTAL ISSUES Figure 3 depicts the bridge layout, and structures at the entrance of the Ada Bay. The pylon required very strong pile foundation, and the natural shape of the Bay entrance was considerably altered by a platform under the pylon, made of concrete and stone (Fig. 3, on the left). Temporary piers were also erected in the Bay (Fig. 3, on the right). Since the morphodynamics of the Bay entrance is determined by the local flow and sediment regimes, which in turn, largely depend on the entrance shape, it was necessary to carry out hydraulic studies and design the Bay entrance in such a way, not only to restore previous conditions, but also to reduce siltation as much as possible for environmental and navigational reasons. Fig. 3. Structures at the entrance of the Ada Bay. Fig. 4. The Pygmy Cormorant The second environmental issue deals with restoration of the pygmy cormorant habitat at this location. The Pygmy Cormorant (Phalacrocorax pygmeus, Fig. 4) has the,,near threatened (NT) status on the IUCN (International Union for Conservation of Nature) Red List of Threatened Species [6]. This migratory bird falls into the first category of species requiring protection, and is considered as a globally endangered species with inadequate protection in Europe [7, 8]. In Serbia, the pygmy cormorant is protected by law. It is estimated that around such birds are present in Europe - from Italy, over the Balkans, to the Caucasus Mountains - and that 5%, or some 3000 birds dwell on the Danube and Sava Rivers in Serbia [5] (Fig. 5). Fig. 5. Habitat of the pygmy cormorant in the vicinity of the new bridge; locations 1 and 2 were radically disrupted by extensive works during bridge construction. 3

4 3. ENVIRONMENTAL IMPACT ASSESSMENT In order to cope with the two environmental problems, it was necessary to justify any considered solution with appropriate hydraulic calculations. For this reason, a mathematical model of the Sava River and the Ada Bay was created, and numerous geometric shapes of the Bay entrance were investigated, under various initial and boundary flow conditions. Mathematical model and its calibration A finite element model was applied, implemented in the hydrodynamic software system Telemac [2]. The computational domain and a detail of the mesh (16356 elements, 8732 nodes) are shown in Fig. 6 [3]. By solving equations of mass and momentum conservation in two dimensions, water depth and two depth-averaged velocity components are obtained in each node at any time instant. Thus, temporal and spatial distributions of various hydraulic and sedimentologic parameters, as well as the local water surface and river channel bottom elevations, can be determined for a chosen set of initial and boundary conditions. Fig. 6. Computational domain and finite element mesh [3]. In this case, steady-state numerical simulations were done for a range of discharges covering the Sava River flow regime, from the low flow conditions (500 m 3 /s), mean flow (1350 m 3 /s), to the flood with 100 year return period (6510 m 3 /s). As is known, 2D depth-averaged models require calibration of two parameters controlling flow resistance and turbulence characteristics - the roughness coefficient and the eddy viscosity coefficient. The value of the first coefficient was determined by adjusting the calculated water surface to the measured one, while the value of the second coefficient was determined from measurements of the velocity flow field, performed with the Acoustic Doppler Current Profiler (ADCP) device [3, 4] (Fig. 7). The diagram on top of this Figure represents the velocity magnitude, measured over a cross-section of the Sava River at the Ada Bay entrance; diagram below shows the calculated velocity distribution, fitted to the measured velocity distribution; the calibrated eddy viscosity value is the one yielding the best fit, and this value was used in all 2D flow calculations. 4

5 Fig. 7. Calibration of the 2D numerical model by the ADCP technique [3]. Results of numerical simulations and the proposed solution In this paper, only a few results are presented, purely for illustration purposes. Some results pertaining to the 100 year flood are chosen because, due to extremely high velocities, they best depict the trends of physical processes. Firstly, hydraulic calculations were done for the "present state" - the Ada Bay entrance geometry as it was at the end of the bridge construction works. This case was termed Case "0". Then, calculations were done with the same initial and boundary conditions for a number of altered Bay entrance geometries, and the results were compared with the ones of the Case "0". The criterion for choosing the optimal solution was the least net sediment inflow into the Bay, or the least siltation potential. This criterion reflects not only the financial requirements for navigation maintenance, but also specific aesthetic and environmental (ichthyologic and ornithologic) requirements, as is later shown. Only results of the final solution, Case "R", are presented here. Case "R" refers to construction of a 50 m long, 1-2 m high bottom sill, elongating the tip of the Ada peninsula [3] (Fig. 8). The sill, made of large stone, is low enough to be submerged during mean and high discharges, thus not affecting the architecturally designed scenery around the pylon. Fig. 8. The proposed bottom sill for best hydraulic effects on the Bay entrance [3]; the sill is visible only during low discharges, and is completely submerged during mean and high flows. 5

6 Some results of hydraulic calculations are presented in Fig. 9. Comparing the vector velocity fields under present conditions (Case "0") and with the bottom sill (Case "R"), it can be remarked that in the latter case, the horizontal recirculation at the Bay entrance (primary gyre) is weaker in intensity and reduced in diameter. The same conclusion can be drawn by comparing the unit discharge distribution and intensity for the two cases. Quantitative comparison is given in Fig. 10. Fig. 9. Results of hydraulic calculations for the Sava River flood discharge of 6510 m 3 /s; vector velocity fields are shown on the left, for the "present state" - Case "0" (above), and for the case with the bottom sill - Case "R" (below); spatial distribution of the unit discharge is shown on the right, for the Case "0" (above), and the Case "R" (below) [3]. Fig. 10. Distribution of unit discharge across the Bay entrance during high flow rate of 6510 m 3 /s in the Sava River; notice that unit discharge values abruptly drop in the center of the recirculation zone (primary gyre); by integrating, the inflow into the Bay equals 40 m 3 /s in Case "0", and 25 m 3 /s in Case "R", which means that the sill has reduced inflow into the Bay for almost 40%, and consequently the amount of sediments entrained from the river into the Bay [3]. (Weakening of the primary gyre by making the entrance narrower is a principle well established in literature [9].) 6

7 Results of other calculations dealing with distribution of concentration of suspended sediment at the Bay entrance (not presented here) confirm that the proposed sill has favourable effects on reduction of sediment entrainment into the Bay and sediment settling on the entrance. After completion of hydraulic analysis, the next step was to trace hydraulically optimal shore lines taking into consideration imposed restoration of bird habitats 1 and 2 in Fig. 5. The finally proposed solution for the Bay entrance is shown in Fig. 11. Fig. 11. The proposed solution for the Ada Bay entrance considering hydraulic, sedimentologic, and ecologic requirements [3]. The main features of this solution, as shown in Fig. 11, can be summarized as follows: The platform built during construction of the pylon is to be reduced and hydraulically shaped; The existing levees on the Ada peninsula are to be extended around the pylon to protect it from extremely high flows of the Sava River; A 50 m long, 1-2 m high stone sill is to be built on the bottom of the Bay entrance, elongating the tip of the peninsula, with the aim to reduce sediment entrainment into the Bay; Two existing bird habitats (marked 1 and 2 in Fig. 5) are to be restored and enlarged by the shoreline design; habitat 1 is situated on the right bank of the Sava River immediately upstream from the platform with the pylon, while the habitat 2 is located opposite the pylon, immediately downstream from the Bay entrance. 7

8 Habitat 1 is enlarged by moving the shoreline into the river, thus providing more space (yellow area on the left in Fig. 11) for riparian forest. Habitat 2 is renewed by shaping, backfilling and planting the existing sand bar at the Bay entrance (yellow area on the right in Fig. 11). The presented solution was proposed with conviction that the ornithologic requirements are technically satisfied. Yet, it remains to be seen whether the pygmy cormorans will continue to dwell at this location after the traffic over the bridge reaches its maximum. 5. CONCLUSIONS 1) Solution of some environment restoration projects require thorough knowledge of river hydraulics and application of appropriate numerical modelling tools. 2) Numerical simulations, performed by a well calibrated numerical model, for the entire range of initial and boundary conditions, provide sound basis for investigation of various design solutions within an environment restoration project, and for choosing the optimal solution which simultaneously satisfies a number of requirements. 3) The particular case study shows that: (i) siltation at a bay entrance can be reduced by construction of a bottom stone sill of adequate inclination, length, and height, and (ii) sand bars, developing in the course of river channel morphodynamic processes, can be effectively adapted to provide habitat for rare bird species. ACKNOWLEDGEMENT Some research presented in this paper has been carried out within the scientific project "Monitoring and Modeling of Rivers and Reservoirs (MORE) - Physical, Chemical, Biological and Morphodynamic Parameters" (Project TR37009), financed by the Serbian Ministry of Education and Science. REFERENCES 1. Ada Bridge, 2. Hervouet, J-M. (2007) 'Hydrodynamics of Free Surface Flows - modelling with the finite element method', John Wiley & Sons. 3. Jovanović., M., Kapor, R., Zindović, B. (2011) 'Hydraulic study of the Ada Bay during construction of the new bridge on the Sava River', University of Belgrade, Faculty of Civil Engineering, Belgrade (in Serbian). 4. Jovanović, M., Kapor, R., Prodanović, D., Zindović. B. (2006) 'Upgrading Environmental Projects by CFD Modelling', 23. Conference of the Danube Countries on the Hydrologic Forecasting and Hydrological Bases of Water Management, Belgrade, Serbia, Monitoring of Pygmy Cormorants in Belgrade, 6. The IUCN Red List of Threatened Species, 7. Pygmy Cormorant (Phalacrocorax pygmeus), 8. Pygmy Cormorant (Phalacrocorax pygmeus), 9. Van Schijndel, S., Kranenburg, C. (1998) 'Reducing the siltation of a river harbour', Journal of Hydraulic Research, Vol. 36, No. 5, pp