1. curriculum 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

2. 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

3. 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

4. Data collected 2004

5. 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?

6. 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

7. Formulae relating cross section area to tidal prism
Author
Notes x n
O’Brien (1969)
Pacific, natural inlets
1.00
Pacific, jettied inlets
0.86
Jarrett (1976)
Atlantic
1.05

8. 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

9. 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

10. Hydrodynamic model: grid and bathymetry

11. Location of sections
Sections are located along the channel system Lido – Treporti – Burano
Sections are taken close to the nodes in the numerical model

12. Computation of tidal prism: three formulas can be used
Original formula of O’Brien
Modified formula of O’Brien
Simulated discharges

13. Computation of tidal prism: comparison of the formulas

14. 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.

15. Cross section versus tidal prism
This plot shows various theoretical formulas compared to data from Atlantic, Pacific and Venice lagoon.

16. Changes of cross section and tidal prism
Changes through the years 1930 – 2000 in the three datasets

17. 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

18. 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.

19. 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.

20. 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

21. 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.

22. 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 77-84
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:
ftp://ftp.soc.soton.ac.uk/pub/cla8/Kuwait/tidal%20inlets.ppt

23. 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