Estuary Processes and Managment - River Dynamics and Integrated River Management - page 474

The Venice Water Authority realizes the system of activities aimed at the physical and environmental
safeguarding of Venice and the lagoon. The activities refer to a “General Plan of Interventions” that ties
the defence of Venice and the lagoon ecosystem from high tides together with the environmental problems
that for some time have been attacking the territory. The plan also outlines a series of activities for the
resolution of the elements of crisis according to distinct but reciprocal and systemic lines of action:
defence from high tides, defence from sea storms, and environmental defence.
As shown in Fig. 8.13 the high water defence system consists mainly of mobile barriers at the three
outlets, which is combined with other measures have being underway for many years in the lagoon,
tackling the various aspects of safeguarding Venice by means of a systematic approach. The mobile
barriers automatically close the three outlets if the tidal water stage is higher than a critical level and
automatically open if the tidal water stage reduces below the critical level.
Fig. 8.13 High water defence system consists mainly of mobile barriers at the three outlets of the Venice Lagoon
(Source: http://www.salve.it/)
The coastal strip, in the meantime, is protected by construction of new artificial beaches and the
widening of eroded ones with sand. In the past, the coasts were protected by the construction of jetties,
seawalls, groynes and offshore breakwaters, while today the most modern coastal engineering techniques
attach ever greater importance to the role played by beaches for their ability to oppose and counteract the
action of sea storms. Specific rock structures have been designed to protect the new beaches from erosion
by “retaining” and stabilizing the sand spread or actually capturing part of the sand in transit suspended in
the sea water. Depending on the area concerned, these consist in groynes (rock structures perpendicular
to the coast) or “containment cells” (consisting of groynes and a submerged breakwater parallel with the
beach) designed to take account of local coastal drift and current conditions, the characteristics of the
section of coast concerned and the configuration of the sea bed. It is, however, impossible to completely
eliminate erosive forces and the new beaches will therefore be subject to periodic maintenance. This will
involve “reinforcement” with at least ten per cent every ten years.
Like Venice many cities on large river deltas and by the coast have the same problem of subsidence
and increasing flooding risk, such as Shanghai, Tianjin, Los Angeles, Mexico City, New Orleans, and Osaka.