Long-term variations of the water flow through three Venice lagoon inlets are analysed seeking for the possible impact of structures under construction for the mobile gates for defending Venice against flooding. Time-series of monthly

mean flow and standard deviations show long-term variations, mainly on the seasonal time-scale. An important increase in the water flow variance due to an increase

in amplitudes of major tidal constituents in the Chioggia inlet can probably be explained as a consequence of the structures constructed recently that narrowed the channel

Gačić, M.

Mancero Mosquera, I.

Mazzoldi, A.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2007-10004-xIL NUOVO CIMENTO Vol. 30 C, N. 2 Marzo-Aprile 2007Long-term changes in the kinematics of inlets of the VenetianLagoon (NE Italy)I. Mancero Mosquera(1), M. Gacˇic´(1) and A. Mazzoldi(2)(1) OGS, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - Trieste, Italy(2) ISMAR, CNR - Venice, Italy(ricevuto il 7 Marzo 2007; revisionato il 6 Luglio 2007; approvato il 6 Luglio 2007; pubblicatoonline il 27 Luglio 2007)Summary. — Long-term variations of the water flow through three Venice lagooninlets are analysed seeking for the possible impact of structures under constructionfor the mobile gates for defending Venice against flooding. Time-series of monthlymean flow and standard deviations show long-term variations, mainly on the seasonaltime-scale. An important increase in the water flow variance due to an increasein amplitudes of major tidal constituents in the Chioggia inlet can probably beexplained as a consequence of the structures constructed recently that narrowed thechannel.PACS 92.10.A- – Circulation and currents.PACS 91.50.Cw – Beach and coastal processes.PACS 92.10.Sx – Coastal, estuarine, and near shore processes.1. – IntroductionSince February 2001 bottom-mounted Acoustic Doppler Current Profiler (ADCP)measurements have been carried out continuously in the three inlets of the Lagoon ofVenice (Lido, Malamocco and Chioggia; see fig. 1). The long-term records have revealedspatial and temporal characteristics of the flow field and water exchange between thelagoon and the open sea, which have been presented in a series of papers publishedrecently [1-3]. One of the prominent features of inlet flows is a very high contributionof tidal currents to the total flow variance that in average reaches even more than 95%.Another important property of the water flow is a strong polarization approximatelyalong the geometrical axis of inlets due to the channel walls constraint. Cross-inlet currentcomponents are negligible. In fact, more than 99% of the flow variance is associated withthe variability along axes of maximum variance that on its turn coincide with the inletaxes. The long-term current records enable us to evidence weekly time-scale, seasonal oreven year-to-year variations of the inlets’ flow properties [3].c© Societa` Italiana di Fisica 149150 I. MANCERO MOSQUERA, M. GACˇIC´ and A. MAZZOLDIFig. 1. – Bathymetric map of the Venice Lagoon.2. – Brief description of carried out civil worksExternal wave-breaking barriers have been completed in front of Malamocco (June,2003-November 2004) and Chioggia (April 2004-April 2005) inlets within the frameworkof a project for the Venice lagoon defense against the “acqua alta” events. In addition, thewidth of the Chioggia inlet was, in one part of the channel, reduced within the frameworkof building of the haven for fishery vessels (see fig. 2 showing aerial photographs ofthe Chioggia inlet before and during the civil works on the haven for fishery vessels).Presumably the width of the Malamocco inlet has not been changed. The width of theFig. 2. – Aerial photograph of Chioggia inlet before (left) and during the construction works(right). Note different horizontal scales. The construction of fishery vessels’ haven is evidentalong the northern shore of the inlet in a more recent photograph. The position of the ADCPis presented by a dot.KINEMATICS OF THE VENETIAN LAGOON INLETS 151Fig. 3. – Time-series of the monthly mean vertically averaged inflowing current component inthe three lagoon inlets. Negative values stay for the inflow.inlet of Lido was not changed either and the wave-breaking barrier has not yet beenbuilt there. All these construction works may have changed characteristics of the flow ininlets. As an example, local fishermen have claimed that an increase in the velocity ofcurrents in Chioggia channel have taken place recently.A detailed study was carried out in order to seek for possible changes in the kinematicsof the inlets. Moreover, the aim of this paper is to investigate long-term variations in theflow characteristics and to assess whether some changes that took place can be associatedwith the construction works.3. – Analysis of the long-term flow variationsDetailed description of the experimental set-up of current measurements in the lagooninlets and ADCP data available were presented in [2]. In order to analyze long-termvariability, a number of parameters describing the flow characteristics were estimated ona monthly basis for each inlet. Following the principal component decomposition [4],time-series of the first two statistical moments of the flow (figs. 3 and 4) have beencomputed for the along-axes vertically averaged current component. Results have shownthat prominent long-term variations in time-series of both average flow and its standarddeviation are present. There is a good agreement with previous studies that indicated aprevalent long-term mean inflow in Lido and an outflow in Chioggia [3]. Also, a ratherprominent seasonal variability in the water exchange is evident in Lido and Chioggia with152 I. MANCERO MOSQUERA, M. GACˇIC´ and A. MAZZOLDIFig. 4. – Time-series of monthly standard deviations of the inflowing vertically averaged currentcomponent. Numbers denote different phases of the completed construction works in the area ofthe Chioggia inlet (1—External breakwater completed, 2—Southern wharf of the refuge havencompleted, 3—Lock and navigation chambers done).a maximum inflow and outflow, respectively, in late autumn/early winter. Conversely,monthly mean water flow in the inlet of Malamocco is rather weak and does not showany significant seasonal signal. On the other hand, time-series of the flow standarddeviations show in Malamocco the maximum value of all three inlets with the mostprominent seasonal signal. Lido shows slightly weaker seasonal signal in the standarddeviations and the values smaller than in Malamocco. In Chioggia the seasonal signalcan hardly be distinguished from the background noise and the flow standard deviationsare the smallest of all three inlets. The flow kinetic energy (variance time-series) showsobviously the same features as standard deviations. In addition, time-series of eithervariance or standard deviations show coherent temporal variations in Malamocco andLido, while Chioggia appears uncorrelated with other two inlets.Landmarks of the works carried out in the inlets and surroundings are shown in fig. 4.Standard deviations and variance show a clear trend of increase in Chioggia startingfrom the last quarter of the year 2004 that continues until the end of the time-series.The point of change in the variance was found to be significant applying the cumulativesum of square statistic test [5,6]. Other tests aimed to find changes in the mean yieldedno change detected.The trend in the current field in Chioggia inlet was sought also in the time-seriesof maximums of the inflow (negative current values) and the outflow (positive currentKINEMATICS OF THE VENETIAN LAGOON INLETS 153Fig. 5. – Time-series of maximum monthly inflows, dotted curve (negative values) and outflows,continuous line (positive values) for the Chioggia inlet.values). Maximums were extracted from the monthly sub-sets and their time-series forthe entire study period are presented in fig. 5. Both inflow and outflow show clearincreasing trend as observed in the average current and the flow kinetic energy. Themaximum values of outflow are generally larger than inflow as they are superimposed onthe long-term average outflow in the Chioggia inlet. Moreover, the outflow shows largertemporal variations than the inflow. This can be explained in terms of the influence ofbora episodes that reinforce the outflow as shown in previous studies [2]. Extremely largeoutflows that occurred in March 2003 and in November 2004 coincide in fact with strongbora events with the wind speed of about 20m/s taking place immediately prior andduring the maximum tidal outflow occurrence. In addition, the two strong bora episodeswere taking place in concomitance with the spring tide.Harmonic analysis [7] was applied in order to determine whether any variability intidal parameters occurred as a consequence of construction works. This technique allowsextracting variability due to astronomical forcing by fitting currents time-series by a sumof well-known constituents via the least-square method. Explained variance associatedwith the tidal oscillations in the Venice lagoon inlets reaches about 95% of total variabilityas already reported in previous papers (see, e.g. [1]). The most important group of con-stituents is the semidiurnal one with M2 and S2 as the most energetic ones. For currentvector time-series from harmonic analysis, the following estimates of ellipse parameterscan be obtained: major-axis amplitude, minor-axis amplitude, phase, orientation angle,and errors of each one.154 I. MANCERO MOSQUERA, M. GACˇIC´ and A. MAZZOLDIFig. 6. – M2 semi-major tidal constituent as a function of time calculated from the 13-monthmoving time series of the vertically averaged currents.As expected, tidal variability is strongly polarized along channel axes, hence onlyestimates of major-axis amplitudes were considered. The three most prominent con-stituents, namely M2, S2 and K1, have been analyzed, and they account for about 87%of total currents variability. In order to analyze continuous temporal variations in am-plitudes of the three most important tidal constituents, harmonic analysis was appliedto a 13-month moving period with one-month time-step for the entire time-series, andmajor ellipse axes are presented in figs. 6, 7 and 8 as a function of time. Low-frequencyvariability is the most prominent in the M2 constituent and occurs on the six-monthtime-scale. S2 shows the most important variability on a biennial time-scale. However,the length of the time-series does not allow us to give a definite answer on the time-scaleof the variability of that constituent. The diurnal K1 tidal constituent changes ratherweakly in time. Also, temporal variations of major ellipse axis values for inlets of Lidoand Malamocco result rather highly correlated for all three tidal constituents as it wasshown for the mean flow and its standard deviation. On the other hand, major ellipseaxes for all three constituents in the Chioggia inlet show an increase starting from thesecond half of 2004 suggesting that the noticed trend in the flow variance is connectedto the augmentation in the tidal currents magnitudes.This increase in the flow intensity can possibly be associated with the reduction ofthe inlet cross-section after it has been narrowed down as evident from the chronologyof the construction works carried out.KINEMATICS OF THE VENETIAN LAGOON INLETS 155Fig. 7. – The same as in fig. 5 except for the S2 semi-major tidal constituent.Fig. 8. – The same as in fig. 5 except for the K1 semi-major tidal constituent.156 I. MANCERO MOSQUERA, M. GACˇIC´ and A. MAZZOLDI4. – ConclusionsLong-term variability of the water flow characteristics in the three inlets of the Lagoonof Venice is analyzed. Time-series of the monthly mean flow and its standard deviationshow prominent seasonal signal as well as the overall mean flow compensation betweenLido and Chioggia. Long-term temporal variations in amplitudes of the three mostimportant tidal constituents (M2, S2 and K1) are analyzed from time-series obtained byharmonic analysis applied to a 13-month moving period. In Lido and Malamocco inletssemi-diurnal M2 constituent shows coherent six-month temporal variations, while the S2is mainly varying on the biennial time-scale. The diurnal K1 constituent in the two inletsshows very weak but coherent temporal low-frequency variability. On the other hand, thethree tidal constituents in Chioggia inlet display a trend of increase in amplitude startingfrom the second half of the year 2004, feature that may suggest a possible impact of theinlet narrowing down on the water flow. Thus, the long-term kinetic energy increase ofthe flow through the Chioggia inlet is mainly due to the increase in tidal energy. Alsofrom the available time-series it can be observed that the steady state has not yet beenreached.∗ ∗ ∗This research was supported by CORILA (Consortium for Promoting and Coordinat-ing the Scientific Research on Venice and its Lagoon). The following institutions makepart of the consortium: the University of Venice, the University of Padua, the Architec-tural Institute of Venice, Istituto Nazionale di Oceanografia e di Geofisica Sperimentaleand the National Research Council of Italy. For current measurements in the inlet ofChioggia financial support was obtained from “Centro Previsioni e Segnalazioni Maree”of the Municipality of Venice. IMM participated in the work with the partial supportof the “Programme for Training and Research in Italian Laboratories (TRIL)” of theAbdus Salam International Centre for Theoretical Physics, Trieste, Italy.REFERENCES[1] Gacˇic´ M., Kovacˇevic´ V., Mazzoldi A., Paduan J., Arena F., Mancero MosqueraI., Gelsi G. and Arcari G., Eos, Trans. Am. Geophys. Union, 83 (2002) 217.[2] Gacˇic´ M., Mancero Mosquera I., Kovacˇevic´ V., Mazzoldi A., Cardin V., ArenaF. and Gelsi G., J. Mar. Syst., 51 (2004) 33.[3] Gacˇic´ M., Kovacˇevic´ V., Mancero Mosquera I., Mazzoldi A. and Cosoli S., inFlooding and Environmental Problems of Venice and Venice Lagoon: State of Knowledge(Cambridge University Press, London) 2005.[4] Preisendorfer R. W., Principal Component Analysis in Meteorology and Oceanography,edited by Mobley C. D. (Elsevier Science Publisher, Amsterdam, Oxford, New York,Tokyo) 1988.[5] Tanizaki H., Commun. Stat., B, 24 (1995) 1019.[6] Inclan C. and Tiao G. C., J. Am. Stat. Assoc., 427 (1994) 913.[7] Foreman M. G. G., Manual for tidal currents analysis and prediction (revised edition).Pacific Marine Science Report 78-6, Institute of Ocean Sciences, Patricia Bay, Sidney, BritishColumbia (1996).

Long-term changes in the kinematics of inlets of the Venetian Lagoon (NE Italy)

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Long-term changes in the kinematics of inlets of the Venetian Lagoon (NE Italy)

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