7. Alternatives to the mobile gates project
7.1. Alternative strategies for high water protection
Beside the mobile gates, other measures can be considered to protect Venice and the other urban centers from high waters. Some of these measures are not strictly alternative to the mobile gates project. Some diffuse measures, for instance, are included in the special laws for Venice (EIS, Planning Reference Framework, Section B) and are meant, in the first place, to restore the morphology of the lagoon and to impede further degradation. However, since these measures may serve to reduce the tidal peak in the lagoon, together with insulae protection they may be considered as an alternative package which may provide sufficient protection against high water.
In broad terms, two main strategies for protecting Venice emerge:
This chapter addresses the second strategy. Two alternative solutions are analyzed in detail. The first is based on insulae protection up to 100 cm complemented with the full package of diffuse measures that have been studied (EIS, Annex 6) and which could reduce the tidal peak. The second considers only insulae protection, but enhanced to protect the city up to 120 cm.
Diffuse measures are studied in detail in the EIS. The augmented insulae project, on the contrary, is not included in the EIS. It has been considered by the Collegio in response to the claim that local protection measures could be developed to a sufficient level of protection, so that the effects of remaining floods are acceptable, and the mobile gates unnecessary.
7.2. Description and effectiveness of alternative measures
The EIS considers a set of diffuse measures proposed by the Municipality of Venice (1995), with the objective of reducing the peak of high water events and hence the negative impact of the high waters on the historical centers of the lagoon (EIS, Annex 6, Topic 6). This set of measures includes:
1. opening the fish farm enclosures;
2. reducing the cross-section of the lagoon inlets and changing the end sections of the breakwaters;
3. closing the Malamocco-Marghera navigation channel (the Canale dei Petroli);
4. other diffuse measures, such as reconstructing salt marshes, sand-banks, etc. and opening up reclaimed land.
Opening up the fish farms. The idea is to increase storage for storm water by creating new openings to the 23 fish farms (surface of 88 km2) along their embankments. The EIS analysis indicates that the measures will not be effective: the mean tidal peaks at Punta della Salute would reduce by about 1.3 cm only. The reasons for the ineffectiveness are that the total area of the fish farms is small compared to the area of the lagoon, and that they are located far away to the north and south of Venice.
Reducing the cross section of the lagoon inlets and
modifying the end section of the breakwaters. The possibility of reducing
water level in the lagoon by reducing the dimensions of the inlets has been
evaluated. By reducing the depth to 8 m at Chioggia, to 12 m at Malamocco and
to 10 m at Lido inlets, the cross-sectional areas of the inlets can be reduced
by 12 %, 25 % and 15% respectively. It was found however that the water level
at Punta della Salute would only be reduced by 4.1 cm.
Changing the end section of the breakwaters increases the flow resistance and
thus provides some additional decrease in the lagoon peak levels. The most effective
solution in terms of high water level requires changing the final part of the
breakwaters and the bending of the entrance channel parallel to the coastline.
The effect will be a reduction of the tidal peak of 1.9 cm. This solution represents
a serious hindrance for navigation. Other solutions which would minimize the
effects on navigation would have negligible effects on the water level.
Closing Canale dei Petroli connecting Malamocco and Marghera. To alleviate the problem of morphological variations caused by Canale dei Petroli between Malamocco and Marghera, it has been suggested that the Canale be closed and shipping traffic be rerouted through Canale Fisolo which will be deepened up to 10 m. The effect of such a change on the tide level at Punta della Salute is negligible (a few mm).
Other diffuse measures. Other wide-area diffuse measures that have been considered in EIS include opening up reclaimed land, dredging channels connecting Val di Brenta, reconstructing sandbanks, shoals, and sand bars, etc. Detailed analyses estimated that the total effect of all these would be negligible (one or few millimeters).
Effectiveness of diffuse measures. After a thorough analysis of the studies on the effects of diffuse measures, it can be concluded that diffuse measures do not have a significant effect on the tidal level inside the lagoon. The effect depends on the shape of the tide and on the duration of the high water. Only for steep tides, which at the current sea level occur 4 times a year on average, could the peak level in Venice be reduced by about 12 cm (EIS, Annex 6, Summary of the topics, p.40). In particular, high tides of relatively long duration that cause the greatest damage are not affected by the diffuse measures. The only diffuse measure that may have some effect is the partial closure of the entrances by raising a sill and bending the breakwaters in a direction parallel to the coast. This type of intervention will increase the resistance for the tidal flow entering the lagoon. As a result it may dampen the high water level by a few centimeters. The limited effectiveness of diffuse measures is also demonstrated by empirical analysis of the record of high water events since the beginning of this century by Cecconi, Canestrelli, Corte and Di Donato (1998). Their study shows that the total effect on high water levels of the local changes in the channel cross-section, lagoon morphology, fish farms etc. has been only 2 cm for Venice. However, the effect of diffuse measures may be greater around the periphery of the lagoon, but this does not help reducing damage to Venice.
7.2.2. Local defense (insulae)
Local defense consists of plans for:
1. elevating the paved surfaces outside the buildings, and if necessary inside (including the "vasche di contenimento");
2. raising the height of the seawalls surrounding the islands and along canal banks;
3. lifting the structures directly.
If only local defense is adopted, the effectiveness of protection against high water depends on the level of the insulae protection, while the feasibility of implementation depends on the extent of simultaneous construction, which determines the duration of the construction phase and the cost of construction.
While the insulae protection has proven successful in Malamocco, and it is likely to be feasible for parts of the city, a full protection to 120 cm or more will be extremely difficult to achieve, will take a very long time (60 years or more are estimated), will be extremely expensive and is likely to have irreversible consequences for some historical buildings.
The effectiveness of insulae protection will be further diminished in case of sea-level rise. Even if it were feasible to protect all buildings and historic sites from tidal levels up to 120 centimeters, there would still be one serious flooding event per year on average, under the current sea level, and the number would increase with sea-level rise. Moreover the risk of exceptional high tide would remain unchanged.
7.3. Environmental and morphological effects of alternative solutions
According to the EIS (Topic 6, Vol. 6a), the opening of the fish farm enclosures would increase the flow rate of tidal water in the channels, the effect being more marked toward the lagoon margins, especially near the fish farms. This would cause a reduction of sedimentation in these channels and increased deposition on the sandbanks and shoals. There would be a favorable global effect with regard to eutrophication (in a context of reappearance of Ulva proliferation). These positive effects are to a certain extent counterbalanced by increased risk of contamination of the fish ponds from lagoon water, causing possible economic damage.
The opening of the reclaimed lands ("casse di colmata") would have a similar and more limited effect, but it would undoubtedly create more biodiversity for aquatic life.
The opening of the Val di Brenta would have very little effect since gaps and channels have already been established under the Romea road to give access to lagoon water. The impact would also be very local, albeit positive.
Raising the seabed at the lagoon inlets, resulting in a reduction of the flow rate and of the mean volume of water exchange, would enhance sedimentation in the channels and the deepening of the shallows, a slightly negative impact. Changes in breakwater configuration at the inlets would reduce tidal volume, which would have little or no effect on the ecology of the lagoon, but would increase sedimentation in the channels and deepen the shallows. There might also be a lower transfer of sediments to the Lido beaches.
The reconstruction of sandbanks (barene), shoals (velme) and sandbars would have the positive effect of increasing the depth of the channels, locally, with consequently more deposition in the shallows and a reduced loss of sediments to the sea, estimated at 400,000 m3, if the dredged material is used to build up the sandbanks. However, on a more general scale, away from the reconstructed elements, there would be an increase in erosion of the shallows and of sedimentation in the channels. The material from the additional dredging required would be used to build the structures. The barene in particular would increase biodiversity, have a cleansing function for contaminants and nutrients through absorption by their macrophytes, and improve landscape values.
Raising the shallows around the historical center of Venice is claimed to reduce the build-up of macroalgae. An indirect advantage of the reconstruction of sandbanks, shoals and ridges would be the covering of contaminated sediment by cleaner material.
In conclusion there would be no or little negative impact from the diffuse measures. Some, especially the reconstruction of structures, would be very beneficial and should be carried out in any case to restore the lagoon ecosystem.
There will be no or extremely local, temporary and negligible effects of the insulae project, limited to the construction phase.
7.4. Economic valuation of alternative solutions
7.4.1. Diffuse measures and insulae protection up to 100 cm
According to the EIS, the cost of implementing the diffuse measures is 3600 billion lire. Since it will take ten years to complete them, the present value of the cost of diffuse measures and local defenses up to 100 cm (insulae) is 3200 billion lire (5% discount rate).
The EIS gives estimates of high water costs avoided by the diffuse measures. It assumes that local defenses and diffuse measures will protect Venice up to 107 cm (taking 7 cm as the estimate of the effects of the combined diffuse measures). However, to take the most favorable position toward the diffuse measures, the EIS estimates of the avoided high water costs have been increased in the following cost-benefit analysis. Moreover, negative impacts of some diffuse measures on navigation (bending the breakwaters, using the Fisolo canal instead of the Canale dei Petroli, raising the sea bed at the inlets) have been ignored. The overview of costs and benefits of diffuse measures is shown in Table 7.
Table7. Costs and benefits of diffuse measures in billion lire (discount rate 5%).
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Costs of the project |
3200 |
3200 |
3200 |
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Benefits (avoided high water costs) |
1090 |
1270 |
1510 |
|
Short-term |
130 |
220 |
360 |
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Long-term |
960 |
1050 |
1150 |
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Lagoon shoreline |
- |
- |
- |
|
Low water after flooding |
170 |
170 |
200 |
|
Salt aggression |
620 |
650 |
660 |
|
Drainage system |
120 |
150 |
190 |
|
Others |
50 |
80 |
100 |
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Net Present Value of the project |
-2110 |
-1930 |
-1690 |
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Benefit/cost ratio of the project |
0.35 |
0.40 |
0.47 |
|
High water costs without the project |
3530 |
4170 |
5200 |
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Percentage of high water costs avoided by the project |
31% |
30% |
29% |
Table 7 clearly shows that the benefits of the package of insulae and diffuse measures will always be much lower than their construction costs. In addition, this project would be able to avoid a maximum of 31% of the costs required to repair the high water damage. The Collegio concludes that, even from a purely economic point of view, the set of diffuse measures cannot be considered as a suitable alternative to the mobile gates project.
7.4.2. Local defenses (insulae) up to 120 cm
Protecting Venice from high waters up to 120 cm will imply actions covering 40% of the city. Serious doubts have been raised by many interviewed experts on the technical feasibility of this project. According to the proponent of the mobile gates project, protecting the historical center of Venice up to 120 cm would be feasible, but it would require 90 years and would cost approximately 4200 billion lire. According to other interviewed experts, the costs would be considerably higher. In the cost-benefit analysis presented below, the time horizon to complete the augmented local defenses is reduced to 60 years in order to make possible comparisons with the mobile gates project.
Using information provided by the proponent, it is assumed that the project when completed will be able to avoid 45% of the short term costs under scenario A, 40% under scenario B and 35% under scenario C. During the implementation period, the share of these avoided costs is assumed to increase linearly to the final percentage.
The augmented local defenses will not be able to avoid long term-high water costs due to the damage to the lagoon shoreline, damage to the drainage system, and other minor long-term damage. According to information provided by the proponent of the gates project, the augmented insulae project will totally avoid costs related to low waters following the flooding under scenarios A and B, while not providing any protection against this damage under scenario C. Salt aggression damage will be avoided for 56% under scenario A, 53% under scenario B, 50% under scenario C.
The result of the cost-benefit analysis is summarized in Table 5. Since the time span to complete the augmented insulae will be a multiple of the construction time of the mobile gates, the present value of the cost of the augmented insulae is lower than that of the mobile gates project. Thus, the augmented insulae seem to make economic sense even with a lower present value of benefits. Nevertheless, the mobile gates project clearly dominates in scenario C with a 5% discount rate, and in any case with a discount rate of 3%.
Table 8. Overview of costs and benefits of the augmented local defenses (insulae up to 120 cm) in billion lire
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Costs of the project |
1330 |
1330 |
1330 |
1940 |
1940 |
1940 |
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Benefits (avoided high water costs) |
1410 |
1490 |
1320 |
1850 |
2030 |
1970 |
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Short term |
80 |
190 |
370 |
160 |
360 |
690 |
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Long term |
1330 |
1300 |
950 |
1690 |
1670 |
1280 |
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Lagoon shoreline |
- |
- |
- |
- |
- |
- |
|
Low water after flooding |
330 |
330 |
- |
400 |
400 |
- |
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Salt aggression |
1000 |
970 |
950 |
1290 |
1270 |
1280 |
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Drainage system |
- |
- |
- |
- |
- |
- |
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Others |
- |
- |
- |
- |
- |
- |
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Net Present Value |
80 |
160 |
-10 |
-90 |
90 |
30 |
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Benefit/cost ratio of the project |
1.06 |
1.12 |
0.99 |
0.95 |
1.12 |
1.02 |
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High water costs without the project |
3530 |
4170 |
5200 |
4660 |
5680 |
7450 |
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Percentage of high water costs avoided by the project |
40% |
36% |
25% |
40% |
36% |
26% |
The Collegio considers these results favorable to the mobile gates project for two reasons. First, the cost-benefit analysis does not consider the social and psychological costs of a set of interventions lasting decades on the very delicate physical structure of the historical centers. Second, cost-benefit parameters are only one indicator of the economic and social value of a project, which is useful to compare the monetary investment with the economic returns of a project. Cost-benefit criteria cannot be used to appraise other social objectives, such as, in this case, the degree of protection from high water. It is clear that even if in some case the augmented insulae project performs better than the mobile gates project from a cost-benefit point of view, it always performs worse from the point of view of the degree of protection from high water damage. The augmented local defenses are not able to avoid more than 40% of the high water costs.
7.4.3. Efficiency and effectiveness of the mobile gates and of the alternative measures
Table 9 shows a combined index which takes into account the objective of maximizing the economic return of investment and the objective of maximizing the degree of protection from the high water. For each alternative considered here, the index is the ratio between the benefits produced (avoided damage of high waters) and the sum of costs of the project and of the remaining costs (those not avoided) of high waters. As can be seen from Table 9, by combining economic efficiency and high water effectiveness, the mobile gates project dominates both the augmented insulae project and the diffuse measures project.
Table 9. A combined index for economic efficiency and effectiveness for high-water protection (5% discount rate)
Mobile gates project Insulae 120 cm Diffuse measures
Effectiveness for high water protection; The
basic premise of controlling the flood level by increasing storage capacity
inside the lagoon is an untenable one. Without temporary closure of the
inlets, significant reduction of water level over an area of 550 km2
cannot be achieved by altering the dimensions of parts of
the lagoon. This is also demonstrated by empirical analysis of the record
of high water events since the beginning of this century which shows that
the total effect on high water levels of the local changes in the channel
cross-section, lagoon morphology, fish ponds, etc., has been equivalent
to a change in sea level of 2 cm only for Venice. However, the effect
of diffuse measures may be greater around the periphery of the lagoon.
The other premise of reducing the water level by increasing the resistance
to water propagation in the lagoon is also weak. The reduction does happen
for steep tides and may be significant for short events. For a more gradual
rising tide, with a high tide that lasts several hours, the effect of
slowing water propagation on the maximum water level in the lagoon will
be negligible. The augmented insulae protection (120 cm) will reduce the
frequency of floods to about one per year at the current sea level but
will not protect Venice against extreme events. In addition, this project
raises feasibility concerns and will take a long time to complete. In
the case of climate-change induced sea-level rise the effectiveness of
the augmented insulae will be almost totally cancelled. The long construction
time also implies that the city will remain unprotected for many decades
in the future Environmental effects. Diffuse measures will
have some positive and some negative environmental impacts. Some measures,
however, are very beneficial for the lagoon and should be implemented
in any case to restore the lagoon ecosystem. Economic considerations. The benefit-cost
analysis shows that the package of insulae up to 100 cm combined with
diffuse measures has a very poor economic performance under any scenario.
In addition, this solution prevents only about 30% of the costs of high
water. The same analysis carried out on the augmented insulae project
(protection up to 120 cm) shows that the net present value of the project
is positive under some scenarios. In any case, the percentage of avoided
costs is at best 40%. By combining the indications of benefit-cost analysis
and the degree to which a solution is capable of reducing the costs of
high waters, it appears that the mobile gates project is always a superior
solution under any scenario assumption, as well as for different discount
rates.