Presentation on theme: "curriculum in natural environmental science, vol. 2, 2010"— Presentation transcript:
1curriculum in natural environmental science, vol. 2, 2010 Tidal prism variation in the Venice lagoon G. Umgiesser1, R. Helsby2, C. L. Amos2, C. Ferrarin1 1) ISMAR-CNR, Venezia, Italy 2) NOC, Southampton, UK
2The Venice Lagoon 50 km long, 10 km wide Average depth 1 m Narrow deep channelsConnection to Adriatic Sea through 3 inlets15% tidal marshesTidal range +/- 50 cm
3Exchanges with the Adriatic Sea Exchanges with the Adriatic Sea is regulated by the three inletsThe inlets are between 500 and 1000 m wideDepth has adapted to the flow conditions during the last century
5ObjectivesAre 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?
6TheoryThe 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
7Formulae relating cross section area to tidal prism AuthorNotesxnO’Brien (1969)Pacific, natural inlets1.00Pacific, jettied inlets0.86Jarrett (1976)Atlantic1.05
8MethodologyTidal prism and cross-section area were calculated for a number of profiles of 1990The hydrodynamic model SHYFEM, already applied and callibrated for the Venice Lagoon, has been used to compute the tidal prism dataThe process was repeated using the bathymetry from 1930, 1970 and 2000The data has been compared with the theoretical relationships of O’Brien and Jarret
9Hydrodynamic model finite elements primitive equations semi-implicit time stepping schemez or sigma coordinates in the verticalcalibrated and validated for the Venice Lagoon
11Location of sectionsSections are located along the channel system Lido – Treporti – BuranoSections are taken close to the nodes in the numerical model
12Computation of tidal prism: three formulas can be used Original formula of O’BrienModified formula of O’BrienSimulated discharges
13Computation of tidal prism: comparison of the formulas
14Cross 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.
15Cross section versus tidal prism This plot shows various theoretical formulas compared to data from Atlantic, Pacific and Venice lagoon.
16Changes of cross section and tidal prism Changes through the years 1930 – 2000 in the three datasets
17Changes of cross section and tidal prism Detailed changes of the three datasetsTreporti data changes strongly over the yearsMost data follows Jarrett’s relationship
18Cross 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.
19O’Brien relationship for inlets The same as before, but for the inlet data. It seems that Malamocco is too small, and Lido too wide.
20Conclusions (1)The tidal channels in the northern Venice lagoon are basically stableThe tidal prism of the inlets did not change significantly in the last 70 yearsMorphological changes in the central lagoon must be due to other processes
21Conclusions (2)The tidal prism can be best related to the cross sectional area through the formula of JarretThe 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.
22Further 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 77-84Original reports:Ralph R. Clark, Sebastian Inlet – Tidal Hydraulic CharacteristicsJ. T. Jarret,Tidal Prism – Inlet Area Relationship, 1976 (original report)Other material on the web:ftp://ftp.soc.soton.ac.uk/pub/cla8/Kuwait/tidal%20inlets.ppt
23AcknowledgementsWe 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