Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study

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1 448 Hydrology of die Mediterranean and Semiarid Regions (Proceedings ofan international symposium ileid id Montpellier. April 2003). IAHS Publ. no Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study MAURIZIO BONARDI, LUIGI TOSI, ANDREA CUCCO, GEORG UMGIESSER & FEDERICA RIZZETTO Istilulo per Io Studio della Dinamica délie Grandi Masse - CNR, San Polo 1364, Venezia, Italy mbonardi@isdam.ve.cnr.it Abstract Detailed studies of the sedimentary layers underlying the Venetian basin have identified the physical and geomorphic processes that took place in the past. In addition, our study was intended to evaluate the current geomorphic changes, to determine the erosion-deposition processes and to identify short- and medium-term trends, both naturally occurring and those induced by anthropogenic activities, in the Scanello salt marsh area of the northern lagoon basin. A mathematical model based on the finite element method was used to investigate the hydrodynamic water circulation inside the Scanello marshy area. The results obtained have been compared with the empirical data collected during field work. This has allowed the investigation of the relationship between the hydrodynamics and the morphological characteristics of the area. Key words geomorphology; hydrodynamic model; hydrodynamics; mathematical model; sedimentary processes; Venice Lagoon INTRODUCTION The Venice Lagoon geomorphology, characterized by a complex system of mud-flats, salt marshes, shallows and brackish ponds, and a network of channels and tidal creeks, has undergone a continuous modification since its formation about 6000 years BP, mainly due to mean sea level (m.s.l.) variations that followed paleoclimatic changes (Gatto & Previatello, 1974). However, it is in recent times, with anthropogenic impact on the lagoon, that complex morphodynamic changes have occurred, caused by natural processes and by the direct or indirect effects of human activities. The available historical information has contributed to a better understanding of the evolutionary processes that took place in the Venice Lagoon since Roman times to the present, and of how Man, during the Early and Late Medieval Epochs, at first adapted to and then modified the natural morphodynamic evolution of the lagoon by digging new canals and diverting the major rivers outside the Venetian tidal embayment. In more recent times, natural and man-induced subsidence, eustacy and impact of human activities, have noticeably accelerated the geomorphodynamic processes to the point that urgent measures had to be taken in order to reduce the negative effects, mainly erosion and surface reduction of the salt marshes, which play an important role in regulating the lagoon hydrodynamics.

2 Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study 449 RAUIAA NOHI1 IJVI «I I HUN) \ 1! Mil N 4V>( (>\'.» section 3 sedion 2. '-i \\i I I D i II \V-:i I HMU.V.WWi Ml\ section 1 Fig. 1 The study area of Scanello in the Venice Lagoon. STUDY AREA The Scanello area (Fig. 1), chosen for this study, located near the Burano Island in the northern basin of the Venice Lagoon, is characterized by a system of mud flats and salt marshes, channels and tidal creeks, is known to have witnessed intense human activity since Roman Times, and offers datable geoarcheological findings. The salt marsh (Fig. 2), here indicated as Barena Vecchia (BV) and the mud flat, indicated as Barena Nuova (BN) in the Scanello area, because of evident erosion (BV) and deposition (BN), may be considered representative of the geomorphic and hydrodynamic processes presently affecting the entire Venice Lagoon (Bonardi et al., 1997; Bonardi & Tosi, 2000). METHODS The comparison of topographic and bathymétrie maps dated 1931, of more recent aerial photographs, and of all the available information on the morphology and bathymetry of the area, has allowed the identification and definition of the morphological boundary variations, on a decadal scale, of the salt marsh and the soft inaccessible mud flat. Detailed topographic surveys were carried out in July 1996, April 1997 and December 1997 in order to evaluate and quantify the erosion/deposition rate and tend on an annual and seasonal scale. The results are reported in Fig. 2 where points of more intense erosion (Fig. 2(a)) and deposition (Fig. 2(c)) processes are indicated along with those that are fairly stable. In particular, erosion at the border of the southwest tip of the BV facing the Burano Channel, is quite intense and that along the Scanello Channel was quite steady from 1931 to 1997.

3 450 Mawizio Bonardi et al m Erosion Accretion Stable edge S Fig. 2 Barena Vecchia (BV): evolution of salt marsh edges ( ). (a) BV: erosion in the southern section facing the Burano Canal; the escarpment is 1 m high. (b) BV: morphological restoration of the southern section using containment piling. (c) BN: accretion, transition from mud flat to salt marsh. The BN salt marsh area, was not shown on the 1931 map, the comparison, although quite approximate, of the 1931 map with 1961 and 1968 aerial photographs, indicates the formation of the mud flat as a transitional morphologic structure to the present salt marsh (Fig. 2(c)). At present, although the subsidence and eustacy rates in the lagoon of Venice are almost negligible (Tosi et al., 2002), important areal and topographic variations of the geomorphic landscape of the BV and BN have been reported and monitored. Our study has indicated that, while marshland channels and creeks have been morphologically quite stable over several decades, in fact they have been used for the direct overlay of aerial photographs. The borders of the salt marshes, in particular those of the BV, are subject to strong erosion due to the waves generated by the combined action of tidal currents, strong seasonal winds and high speed motor boats. The rapidly increasing

4 Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study 451 loss of marsh terrain in the southwest tip of the BV has prompted the Magistrato allé Acque (Water Authority), responsible for the management of the Venetian Lagoon, through the Consorzio Venezia Nuova, to intervene with containment pilings in order to stop erosion and restore morphological structures (Fig. 2(b)). A bathymétrie survey of the Scanello Canal bordering the BV and, with one of its branches, separating BV from BN, indicates evident bottom morphology iitegulariti.es, such as an 11 m depression, due to the strong tidal currents. The comparison of the newly obtained bathymétrie data with those given in the historic map of 1931 indicates that the Scanello Canal has undergone intense erosion on the lateral slopes and less intense on the bottom, where only few small depositional episodes are known to have occurred. In order to understand the link between erosion in BV processes and deposition processes in BN, and the sediment re-suspension and transport and the hydrodynamics in the study area, a high-resolution hydrodynamics mathematical model has been developed and applied. HYDRODYNAMIC MODELLING Tidal and wind currents, especially for the marshy area of Scanello, are investigated by mathematical modelling. A two-dimensional (2-D) hydrodynamic finite element model, developed at CNR-ISDGM in Venice, was used to study the circulation pattern of this area. The hydrodynamic results are used to interpret sediment transport processes. The distribution pattern of the water circulation inside the Scanello area was investigated to detect the main directions of the suspended matter transport. The results obtained from the analysis of the instantaneous currents are used to evaluate the influence of the meteorological forcing on erosion and deposition processes. Fig. 3 Finite element grid of the Venice Lagoon.

5 452 Maui'izio Bonardi et al. The model The hydrodynamic model used is a 2-D finite element model. The finite element method gives the possibility of following the morphology and the bathymetry of the area. The numerical computation has been carried out on a spatial domain that represents the entire Venice lagoon through a finite element grid (Fig. 3). The grid contains 8072 nodes and triangular elements. A higher grid resolution (Fig. 4) has been imposed inside the Scanello area in order to obtain more information about the water circulation. The model considers as open boundaries the three inlets of Lido, Malamocco and Chioggia. The model resolves the vertically integrated shallow water equations in their formulations with levels and transports: -jt + gh^- +RU+X=0 dt dx 3Ç du dv n + + = 0 dt dx dy ~ dt + jv + gh^ J & dy + RV + Y = 0 where Ç is the water level, U and V the vertically-integrated velocities (total or barotropic transports), g is the gravitational acceleration, H = h + Ç the total water depth, h the undisturbed water depth, t the time and R the friction coefficient. The terms X and Y contain all other terms like the wind stress, the nonlinear terms and those that do not need to be treated implicitly in the time discretization (Umgiesser & Bergamasco, 1993, 1995). In the following section the simulations and results are described. Fig. 4 Zoom of the Scanello marshy area.

6 Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study 453 Simulations The model has been calibrated using the sea level data measured by 14 tide gauges located inside the lagoon. No current velocity measurements have been used to corroborate the model results. The parameter to be varied was the bottom friction (Strickler coefficient) (Umgiesser & Bergamasco, 1993, 1995). Different values of bottom friction were assigned to channels and shallow water zones, because of the different morphology and bottom vegetation. The calibrated model reproduces quite faithfully the tidal oscillation in most parts of the lagoon. A 300-s time step was used to simulate the three idealized cases studied. First, we consider a simulation with only tidal forcing assigned at the open boundary, then, in the last two cases, we also consider forcing the wind on the sea surface. For the wind forcing, two typical wind regimes have been taken into account. The first is the bora, a strong wind from the northeast, the other is the sirocco, a southeasterly wind. These two winds were assumed to be spatially constant over the lagoon, with a wind speed of 10 m s" 1 for the bora and 5 m s" 1 for the sirocco. A spin up time of one day was always used for the simulations. Spring and neap tide events were simulated. Results discussed concern the main hydrodynamic features of the Scanello tidal marsh area. Results are considered after the system has reached a dynamic steady state. RESULTS AND DISCUSSION No wind case Only tide forces the model and wind is assigned. Tidal circulation in this area is completely driven by the inflowing and outflowing of water through the three main channels of Gaggian, Burano and Delia Dolce. The channel of Scanello plays a marginal role in the hydrodynamics of the system because of its smaller section. The magnitude of the current inside this channel reaches the maximum value at the beginning and decreases towards the end. In the present simulation, the peak values of current have been calculated when spring tide forced the system. During ebb flow the velocity increases to a maximum value of 0.36 m s" 1 However, during the flood flow, the maximum current velocity reaches a lower intensity of 0.32 m s" 1. Discharge through different transects displaced inside the studied area has been computed (the sections are defined in Fig. 1). The discharge intensity is maximum inside the Burano Channel (section 1) during ebb flow, with a peak value of 775 m 3 s" 1. The Burano Channel, because of its greater flow and water velocity, drives mostly the suspended matter coming from surrounding areas. Bora wind case In a second simulation the entire lagoon was forced with tide and a bora wind of 10 m s"'. Due to this northeasterly wind the water is driven southwest and the water level in Scanello area is lower. The wind direction coincides closely with the major axes of the Venice lagoon and increases the total flow. In Figs 5 and 6 the inflow and outflow circulation pattern in the Scanello area is plotted. Inflowing velocities of up to 0.8 m s"' have been calculated inside the Burano Channel.

7 454 Maurizio Bonardi et al. Fig. 5 Circulation pattern of the Scanello area forced by bora wind observed during the ebb tidal cycle. Fig. 6 Circulation pattern of the Scanello area forced by bora wind observed during the flood tidal cycle. The wind drives the water in a south-westerly direction and causes a sea level set up of about 0.09 m above the m.s.l. in the southern part of the lagoon. On the other hand, in the northern areas the mean water level is about 0.08 m below the m.s.l. by the wind action. The increased water velocity and the lower mean water level induce a typical erosion condition in the adjacent areas to the largest channels (Burano, Gaggian and Della Dolce Channel). As shown in Figs 5 and 6 the shallow water flats in the inner

8 Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study 455 salt marsh of Scanello are dry, even during flood and ebb. In this area no sediment erosion or deposition is possible during the entire tidal cycle. Sirocco wind case Finally, the effects of the sirocco wind on tidal circulation have been taken into account. Sirocco wind drives water in a north-northwest direction, inside the northern lagoon areas. Maximum values of water velocity (0.7 m s" 1 ) were Fig. 7 Circulation pattern of the Scanello area forced by sirocco wind observed during the ebb tidal cycle.

9 456 Maurizio Bonardi et al. increasing the current mainly in the central and southern part of the lagoon. Because of its direction and intensity, wind influences to a lesser extent the water circulation calculated inside the Burano Channel. During sirocco wind events, the mean water level computed at the Scanello Channel shows a set up of about 0.10 m a.m.s.l. The higher mean water level and the lower current velocity values observed induce typical sediment deposition dynamics in areas surrounding the biggest channels. On the other hand, inner areas that are flooded during the entire tidal cycle (shown in Figs 7 and 8), are subjected to both erosional and depositional dynamics. CONCLUSIONS Geomorphic changes, that occurred naturally in the Lagoon of Venice since its formation, have been recognized. However, it is in recent times that the complex morphology and hydrodynamic of the lagoon have undergone extensive and increasing changes due to the direct or indirect impact of mans activities. High-resolution hydrodynamic model has shown the close interaction between the erosion-transportdeposition processes presently affecting the study case area of Scanello, the tidal currents and the north-northwest Scirocco and northeast Bora winds. Wind forcing induces different dynamic conditions of the water circulation in the area. During bora wind events the lower sea level and the higher current velocity, induced by the action of the wind, lead to enhance the intensity of the erosion processes inside the main channels of the Scanello marshy area. Scoured sediment is removed from the northern areas as it flows out through the Lido inlet into the Adriatic Sea. Due to the effects of the scirocco wind forcing, sediments in this area are subjected to both erosion and deposition processes. Nevertheless, owing to the direction and the intensity of the wind, the suspended matter coming from the whole lagoon is transported to the northern basin (Scanello area) where, after a wind event, it is probably subjected to deposition processes. Acknowledgements This work was partially carried out within the framework and with the financial support of the CO.RI.LA. Project Targeted subproject 3.2 (Work packages Hydrodynamics and Morphology). The authors thank Jane Frankenfield Zanin, CNR-ISDGM, Venice, for her effective editing of the paper. REFERENCES Gatto, P. & Previalello, P. (1974) Significato stratigrafico, comportamento meccanico e distribuzione nella Laguna di Venezia di una argilla sovraconsolidata nota come "caranto". Tech. Rep. 70, Venezia, Italy. Bonardi, M., Canal, E., Cavazzoni, S., Serandrei Barbero, R., Tosi L. & Enzi, S. (1999) Impact of paleoclimatic fluctuations on depositional environments and human habitats in the Lagoon of Venice (Italy). World Resource Review 11(2), Bonardi, M., Canal, E., Cavazzoni, S., Serandrei Barbero, R., Tosi, L., Galgaro, A. & Giada, M. (1997). Sedimenlological, archeological and historical evidences of paleoclimatic changes during the Holocène in the Lagoon of Venice, Italy. World Resource Review 9(4), Bonardi, M. & Tosi, L. (2000) Geomorphological survey for lagoon and coastal pond management projects: the Lagoon of Venice and the Orbetello coastal pond study cases. Geologia Tecnica & Amhientale 1,

10 Geomorphic processes and hydrodynamics in the Venice Lagoon, Italy: a case study 457 Tosi, L., Carbognin, L., Teatini, P., Strozzi, T. & Wegmtiller, U. (2002) Evidence of present relative land stability of Venice, Italy, from land, sea, and space observations. Geophysical Research hellers 29(0), l^t. Umgiesser, G. & Bergamasco, A. (1993) A staggered finite element mode) of the Venice Lagoon. In: Finite Elements in Fluid (ed. by K. Morgan, E. Ofiate, J. Periaux & O. C. Zienkiewicz). Pineridge Press, Swansea, UK. Umgiesser, G. & Bergamasco, A. (1995) Outline of a primitive equation finite element model. Rapporta e Studi,Venice, Italy: Istituto Veneto di Scienze. Lettere edarti XII, Umgiesser, G. (2000) Modelling residual currents in the Venice Lagoon. In: Interactions between Estuaries, Coastal Seas and Shelf Seas (ed. by T. Yanagi), Terra Scientific Publishing Company (TERRAPUB), Tokyo, Japan.