Presentation on theme: "Tidal prism variation in the Venice lagoon G. Umgiesser 1, R. Helsby 2, C. L. Amos 2, C. Ferrarin 1 1) ISMAR-CNR, Venezia, Italy 2) NOC, Southampton, UK."— Presentation transcript:
Tidal prism variation in the Venice lagoon G. Umgiesser 1, R. Helsby 2, C. L. Amos 2, C. Ferrarin 1 1) ISMAR-CNR, Venezia, Italy 2) NOC, Southampton, UK curriculum in natural environmental science, vol. 2, 2010
The Venice Lagoon 50 km long, 10 km wide Average depth 1 m Narrow deep channels Connection to Adriatic Sea through 3 inlets 15% tidal marshes Tidal range +/- 50 cm
Exchanges with the Adriatic Sea Exchanges with the Adriatic Sea is regulated by the three inlets The inlets are between 500 and 1000 m wide Depth has adapted to the flow conditions during the last century
Data collected 2004
Objectives Are the tidal channels in the northern Venice lagoon stable? Did the tidal prism of the inlets change significantly in the last 70 years? Is the tidal prism related to the cross sectional area of the channels?
Theory The tidal prism and the cross sectional area of an inlet can be related to each other through a simple formula: This formula has been calibrated by various authors (O’Brien, Jarret), taking into account different sites and types of inlets
Formulae relating cross section area to tidal prism AuthorNotesxn O’Brien (1969)Pacific, natural inlets O’Brien (1969)Pacific, jettied inlets Jarrett (1976)Atlantic
Methodology Tidal prism and cross-section area were calculated for a number of profiles of 1990 The hydrodynamic model SHYFEM, already applied and callibrated for the Venice Lagoon, has been used to compute the tidal prism data The process was repeated using the bathymetry from 1930, 1970 and 2000 The data has been compared with the theoretical relationships of O’Brien and Jarret
Hydrodynamic model finite elements primitive equations semi-implicit time stepping scheme z or sigma coordinates in the vertical calibrated and validated for the Venice Lagoon
Hydrodynamic model: grid and bathymetry
Location of sections Sections are located along the channel system Lido – Treporti – Burano Sections are taken close to the nodes in the numerical model
Computation of tidal prism: three formulas can be used Original formula of O’Brien Modified formula of O’Brien Simulated discharges
Computation of tidal prism: comparison of the formulas
Cross section versus tidal prism This plot shows the relation between O’Brien’s formula and the data from the Venice lagoon. As can be seen, O’Brien’s relationship is not followed strictly.
Cross section versus tidal prism This plot shows various theoretical formulas compared to data from Atlantic, Pacific and Venice lagoon.
Changes of cross section and tidal prism Changes through the years 1930 – 2000 in the three datasets
Changes of cross section and tidal prism Detailed changes of the three datasets Treporti data changes strongly over the years Most data follows Jarrett’s relationship
Cross section vs. Tidal prism This plot shows the three theoretical formulas compared to data collected in the Venice lagoon. Most data is best represented by Jarret’s relationship, except maybe Burano data.
O’Brien relationship for inlets The same as before, but for the inlet data. It seems that Malamocco is too small, and Lido too wide.
Conclusions (1) The tidal channels in the northern Venice lagoon are basically stable The tidal prism of the inlets did not change significantly in the last 70 years Morphological changes in the central lagoon must be due to other processes
Conclusions (2) The tidal prism can be best related to the cross sectional area through the formula of Jarret The Lido inlet is too big for its tidal prism. In fact, it has to be dredged continuously. The Malamocco inlet is a little too small but is not evolving.
Further Reading General reading: Matthias Tomczak, Shelf and Coastal Oceanography (especially chapter 15) Books and articles: Lakhan,V. C Advances in Coastal Modeling, Elsevier Oceanography Series, 67, Elsevier, Amsterdam, The Netherlands (2003) 595 pp. D. Luketina, Simple Tidal Prism Models Revisited, Estuarine, Coastal and Shelf Science, Volume 46, Issue 1, January 1998, Pages Original reports: Ralph R. Clark, Sebastian Inlet – Tidal Hydraulic Characteristics J. T. Jarret,Tidal Prism – Inlet Area Relationship, 1976 (original report) Other material on the web: days/2008/presentations/session4/NCK08_Ali_Dastgheib.ppt ftp://ftp.soc.soton.ac.uk/pub/cla8/Kuwait/tidal%20inlets.ppt
Acknowledgements We thank CORILA for the kind permission to use figures from their recent publication: Helsby, R., C. L. Amos and G. Umgiesser Tidal prism variation and associated channel stability in N. Venice lagoon. In: Scientific research and safeguarding of Venice, Proceedings of Corila Research programme Vol. VI. P. Campostrini (ed). Corila, Venezia