Vízügyi Közlemények, 1999 (81. évfolyam)

2. füzet - Nagy László: Velence megóvása az árvíztől

Velence tnegóvása az árvíztói 307 The direct reason of flooding is the high sea water level induced by winds. This is further aggravated by the wave action and the seiches, wind set-ups, of the sea water level. Between 1900 and 1975 the sea water level was increased by 0.07-0,08 m everywhere in the Italian sea shore. The increasing number of flooding is also due to the subsidence of the terrain level. Venice was built onto the less consolidated sediments of the delta of the River Po. As a consequence of this the level of the terrain of the city has subsided by 0.03-0.04 m. Water supply for the city was solved by abstracting water from the aquifer below the sediment of the Delta area. The suppression of this water bearing layer was estimated as 0.12 m between 1900 and 1975. The relative loss of elevation of the city is estimated as 0.23 m during 70-75 years. Flood damages are further aggravated by the subsidence of the buildings which are on pile foundation. Flooding in Venice causes further problems: — The traditional structure of the houses and palaces of the city is a pile-foundation the head beam of which is a hard marble of the Istria type, which latter is highly water resistant. At higher water levels the softer and more porous building materials (bricks) are also exposed to water and thus can be damaged by the water of high salt content; — The water of the Laguna is highly polluted because the historic houses of Venice were built over the so called "black-wells" which were to receive the effluent sewage waters. The centre of the city and the other settlements of the Laguna are not provided by modern sewer system and sewage treatment plants. Flooding related problems of Venice are complex ones and the results are wide spreading. The solution is not a simple engineering task. The remedial works arc based in an integrated planning of the river catchment and the shore-line zone, based on the concept of sustainable development and considering several time scales. Development of flood defence in Venice is based on three independent approaches, which are closely interrelated: integration of the economic life of the city, prevention of pollution and flood defence. The economy of the declining city must be upgraded. The industry which supports tourism should be developed along with the other services, as these are the main economic activities in the city; Improvement of the water quality of the Laguna demands the reduction of the loads from the sources of pollution in the catchment area by about 80%. Simultaneously the communal sewage- and industrial waste water discharges within the Laguna should be eliminated. In order to improve sediment load conditions the flood flows into the Laguna should be curtailed. Runoff control and the construction of flood storage reservoirs provide the solution. Navigation in the Laguna should be radically decreased, thus eliminating the need for wide and deep waterways in the canals of the Laguna, which induce erosion in the neighbouring areas. Several solutions to the flooding problem of Venice have been developed. The most well known is the construction of three flood-gates of variable height in the three entrances to the city. When the three gates are closed water can not enter the Laguna from the Adriatic Sea. The moveable gates seem to be the long-term solution. Nevertheless, they are extremely expensive and would solve only some of the rarely occurring (but severe) flooding problems, a small fraction of all the flooding hazards. Smaller works are efficiently keeping Venice dry for the 90% of the floods, but would become inefficient in the case of the larger floods. The options of the complete solution are complex ones. A three-stage plan has also been made on the basis of technical and economic considerations, which provides solution partly without considering the rise of the sea level and partly by considering a 0.3 m rise (during 50-60 years). * * *