Tidal prism variation in the Venice lagoon

G. Umgiesser1, R. Helsby2, C. L. Amos2, C. Ferrarin1

1) ISMAR-CNR, Venezia, Italy2) NOC, Southampton, UK

curriculum in natural environmental science, vol. 2, 2010NEAR

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

nc xPA

Formulae relating cross section area to tidal prism

Author Notes x n

O’Brien (1969) Pacific, natural inlets 7.607 10-3 1.00

O’Brien (1969) Pacific, jettied inlets 7.489 10-4 0.86

Jarrett (1976) Atlantic 3.039 10-5 1.05

nc xPA

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

TVA

P c max

TQ

P max

ttQP )(

Original formula of O’Brien

Modified formula of O’Brien

Simulated discharges

Computation of tidal prism:comparison of the formulas

Cross section versus tidal prismThis 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 prismThis 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

prismDetailed changes of the three datasets

• Treporti data changes strongly over the years

• Most data follows Jarrett’s relationship

Cross section vs. Tidal prismThis 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 inletsThe 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 ReadingGeneral reading:Matthias Tomczak, Shelf and Coastal Oceanography (especially chapter 15)http://gyre.umeoce.maine.edu/physicalocean/Tomczak/ShelfCoast

Books and articles:Lakhan,V. C. 2003 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 77-84

Original reports:Ralph R. Clark, Sebastian Inlet – Tidal Hydraulic Characteristicshttp://bcs.dep.state.fl.us/reports/sitidhyd.pdfJ. T. Jarret,Tidal Prism – Inlet Area Relationship, 1976 (original report)http://chl.erdc.usace.army.mil/Media/6/6/3/GITI-Report_Number_3.pdf

Other material on the web:http://www.nck-web.org/pages/NCK/NCK-days/2008/presentations/session4/NCK08_Ali_Dastgheib.ppthttp://www.coastal.udel.edu/faculty/jpuleo/CIEG680/slide_24.pptftp://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 2008. Tidal prism variation and associated channel stability in N. Venice lagoon. In: Scientific research and safeguarding of Venice, Proceedings of Corila Research programme

2004-2006. Vol. VI. P. Campostrini (ed). Corila, Venezia. 453-466.