Design problems often involve multiple, and sometimes conflicting, objectives. Examples of these objectives are costs, reliability, and sustainability. As a result, identifying the optimal design cannot rely on a single criterion alone. A Multi-Criteria Decision Analysis (MCDA) provides a structured approach to evaluate design alternatives by considering several objectives simultaneously. It also incorporates the perspectives of various stakeholders, each of whom may assign different weights to the criteria and score the alternatives differently on these criteria.
Tetra is a software tool that simplifies the MCDA process by allowing users to assign weights and scores to both stakeholders and criteria. It then performs calculations to determine the final scores of all alternatives, supporting transparent and balanced decision-making.
In the case of the protection of Venice against the rising water level inside the lagoon, five design alternatives were taken into consideration.
This design alternative suggests building permanent barriers at the three lagoon inlets to fully isolate the Venetian Lagoon from the Adriatic Sea. Although this solution would offer constant defence against high tides, it would interfere with natural tidal currents, resulting in ecological harm, sediment buildup, and decreased water quality. Additionally, it would obstruct navigation and economic operations and sluices would have to be built.
The super levee idea is to raise and strengthen the current coastal protections surrounding Venice to endure severe water levels. While this design would have been fairly simple to build and upkeep, it would not safeguard regions situated below sea level or stop flooding caused by groundwater and rain. Moreover, it would significantly transform the city's historical environment and offer only minimal long-term resilience to rising sea levels.
This alternative involves the installation of vertical lift gates at the lagoon inlets that could be elevated during high tides (you can think of the Oosterscheldekering as an example). Although these gates are technically less complex than the MOSE design, they would need considerable supporting infrastructure and ongoing maintenance to avoid corrosion and guarantee functionality. The visual and environmental effects are notable as the gates would still be visible above the water level even when inactive.
This creative plan seeks to address Venice's slow sinking by pumping seawater into deep geological layers under the city. The approach includes pumping seawater into a brackish aquifer situated around 600-850 meters underground to increase the city's height by approximately 25-30 cm over ten years. The objective is to reduce the impact of acqua alta (high water) by raising the whole city instead of preventing the tides. Nevertheless, there are uncertainties about long-term geological stability, irregular uplift, and possible effects on current structures.
The selected solution, MOSE, is a set of movable barriers set up at the three lagoon entrances (Lido, Malamocco, and Chioggia) that can be lifted temporarily during high tides to separate the lagoon from the ocean. This design provides adaptability and the ability to reverse, facilitating normal tidal flow and navigation in ordinary circumstances, all while safeguarding Venice during storm surges. It tries to balance technical effectiveness, environmental conservation, and long-term viability, although this makes it an expensive option and it requires much maintenance.
The Tetra model takes the following criteria per stakeholder into account.
For the residents, the most crucial criteria are those that affect their everyday routines and personal security. They have a deep concern for water quality, as it influences health and living conditions, and the effectiveness of flood protection, since robust defences against flooding are crucial for their homes and infrastructure. The visual effect is significant for them, as substantial flood defence structures can change the look of the city and its environment. Ultimately, the duration of implementation matters since residents tend to favour solutions that can be executed rapidly and reduce disturbances.
The tourism industry has certain common concerns but highlights factors that influence the city's appeal and accessibility. The visual effect holds significant importance for them since the aesthetic charm of Venice directly affects tourism. The quality of water impacts the visitor experience, particularly due to the city's strong connection with its waterways. Furthermore, the dependability of flood protection is essential to prevent harm and disruptions to tourist activities, while shipping durations are connected to the efficient functioning of transport and services that aid tourism.
For environmental organizations, the primary emphasis is on ecological integrity. Their primary focus is on water quality and biodiversity, since these are direct indicators of the environmental sustainability of every option. Their assessment emphasizes options that reduce ecological disruption and uphold the natural equilibrium in the lagoon and adjacent environments. Therefore, alternatives that keep the lagoon open to natural tides as much as possible are preferred.
Ultimately, the government usually focuses on economic viability and lasting dependability. Their primary factors are expenses, both upfront construction and upkeep, and dependability of the flood defence. These indicate the necessity for financial accountability, efficient utilization of public assets, and the enduring operation of the selected system. These costs can also only be justified if the defence is highly reliable.
Local · Global · Weighting values
| Criterion | Local weights | Global weights | Weighting values |
|---|---|---|---|
| Criteria | |||
| Environmental Groups | 0.150 | 0.150 | 15.000 |
| Bio Diversity Env. | 0.600 | 0.090 | 60.000 |
| Water Quality Env. | 0.400 | 0.060 | 40.000 |
| Government | 0.500 | 0.500 | 50.000 |
| Costs | 0.200 | 0.100 | 20.000 |
| Initial Costs | 0.700 | 0.070 | 70.000 |
| Maintenance Costs | 0.300 | 0.030 | 30.000 |
| Reliability Flood Protection Gov. | 0.800 | 0.400 | 80.000 |
| Inhabitants | 0.200 | 0.200 | 20.000 |
| Implementation Time Inh. | 0.087 | 0.017 | 20.000 |
| Reliability Flood Protection Inh. | 0.348 | 0.070 | 80.000 |
| Visual Impact Inh. | 0.174 | 0.035 | 40.000 |
| Water Quality Inh. | 0.391 | 0.078 | 90.000 |
| Tourism Business | 0.150 | 0.150 | 15.000 |
| Reliability Flood Protection | 0.333 | 0.050 | 30.000 |
| Shipping times | 0.167 | 0.025 | 15.000 |
| Visual Impact | 0.167 | 0.025 | 15.000 |
| Water Quality | 0.333 | 0.050 | 30.000 |
In this stakeholder analysis for the MOSE barrier, weights reflect each group's influence in decision-making: government leads with 0.5 thanks to its control over financing, regulation, and long-term reliability; residents follow at 0.2 because they live with the flood risk; environmental organisations hold 0.15, focusing on biodiversity and water quality; and the tourism industry also carries 0.15 due to its economic dependence on lagoon stability. Together these weights balance authority with lived impact before the alternatives are scored.
These varying opinions emphasize how each group of stakeholders perceives the value of the design uniquely. Some emphasize social or environmental effects, while others prioritize economic or aesthetic issues; the model combines these viewpoints for a comprehensive and clear assessment of all options, by assigning weights per stakeholder to each criterion and scoring the design alternatives.
When solving the Tetra model, we get the following result. More details of the solution report can be found in the appendix.
| Rank | Design alternative | Rating | Interpretation |
|---|---|---|---|
| 1 | MOSEPreferred Preferred option | 100.000 | Balances robust flood protection with acceptable ecological and visual trade-offs while keeping the lagoon open during normal tides. |
| 2 | Closed Dams Trade-off heavy | 61.303 | Delivers constant defence but permanently blocks tidal exchange, harms water quality, and disrupts navigation and economic activity. |
| 3 | Lifting Gate Maintenance intensive | 60.039 | Simpler than MOSE on paper but requires large supporting works, ongoing maintenance to fight corrosion, and remains visible above the waterline. |
| 4 | Super Levee Limited coverage | 41.042 | Strengthens existing embankments yet leaves areas below sea level exposed and significantly alters the historical cityscape. |
| 5 | Water Injection High uncertainty | 0.000 | Raises the city gradually without intercepting tides, but the long-term geotechnical stability and uniform uplift remain unclear. |
It can be interesting to perform a sensitivity-analysis, where you look at the preferred alternative of every stakeholder by setting the weight of the other stakeholders at 0. This gives us the following results.
| Rank | Alternative | Rating | Rationale |
|---|---|---|---|
| 1 | Water Injection Preferred | 100.000 | Lifts the city without blocking tidal exchange, directly supporting water quality and biodiversity goals. |
| 2 | MOSE Strong performer | 90.000 | Temporary closures preserve most tidal flow while still delivering strong flood protection when surges threaten the lagoon. |
| 3 | Lifting Gate Mixed | 83.421 | Vertical gates need frequent operation and remain visible above the water, reducing ecological and scenic quality compared with MOSE. |
| 4 | Super Levee Low preference | 52.632 | Shoreline fortifications alter habitats and do not address the lagoon water exchange environmental groups value. |
| 5 | Closed Dams Low preference | 0.000 | Permanent separation from the Adriatic would interrupt tidal currents and degrade lagoon ecology. |
As the water injection option does not influence the tide inside the lagoon and thus does not negatively influence the water quality and biodiversity, this is logically the best alternative for the environmental groups.
| Rank | Alternative | Rating | Rationale |
|---|---|---|---|
| 1 | Closed Dams Preferred | 100.000 | Offers the highest reliability, giving confidence in long-term protection despite heavy capital costs. |
| 2 | MOSE Strong performer | 76.574 | Delivers high reliability with the flexibility to reopen the lagoon, justifying the investment through dependable operation. |
| 3 | Super Levee Viable backup | 73.238 | Relatively straightforward to build and maintain, though it leaves some areas vulnerable and alters the cityscape. |
| 4 | Lifting Gate Mixed | 58.222 | Requires significant supporting infrastructure and maintenance, introducing lifecycle cost and reliability risks. |
| 5 | Water Injection High uncertainty | 0.000 | Limited evidence of long-term effectiveness makes it difficult to justify as a primary flood defence. |
The government is most interested in reliability and costs. Since the closed dams are the most reliable, these score best, but the MOSE barrier and super levee are good options as well. The reliability of water injection is not high, so this is the least favourable option.
| Rank | Alternative | Rating | Rationale |
|---|---|---|---|
| 1 | MOSE Preferred | 100.000 | Provides strong flood protection while keeping the lagoon open between surge events, protecting quality of life. |
| 2 | Lifting Gate Viable backup | 61.583 | Improves reliability but introduces visual intrusion and long closure windows that disrupt daily routines. |
| 3 | Water Injection High uncertainty | 43.788 | Long implementation time and uncertain uplift make day-to-day safety gains unpredictable for residents. |
| 4 | Closed Dams Low preference | 36.544 | Would degrade water quality and restrict movement, even if the barriers guarantee protection. |
| 5 | Super Levee Low preference | 0.000 | Fails to protect low-lying districts and cannot address rainfall or groundwater flooding that residents experience. |
The inhabitants of Venice care by far the most about reliability, since they want to live safely. Also the water quality matters to them, as this also impacts their daily life. MOSE therefore scores best for them, as it combines both of these.
| Rank | Alternative | Rating | Rationale |
|---|---|---|---|
| 1 | MOSE Preferred | 100.000 | Protects the city while staying hidden during normal conditions, keeping Venice attractive and accessible. |
| 2 | Lifting Gate Mixed | 48.633 | Maintains access during low tides but leaves visible structures and demands closures that may disrupt services. |
| 3 | Water Injection High uncertainty | 43.247 | Lengthy implementation and unpredictable results make it difficult for tourism operators to plan with confidence. |
| 4 | Closed Dams Low preference | 11.983 | Blocks navigation and diminishes the maritime character that underpins visitor experiences. |
| 5 | Super Levee Low preference | 0.000 | Alters the historic skyline and still leaves some tourist areas exposed during extreme events. |
The tourism business has a similar view as the inhabitants and also has MOSE as the best option.
Although the preferred design alternative differs per stakeholder, the total combination of the stakeholders, as calculated by the Tetra model, is the MOSE barrier. The next step is to optimise the design of the MOSE barrier, as will be done in the next chapter Design optimization.