Earth Science: Geology, the Environment, and the Universe

Unit 4: The Atmosphere and the Oceans

WebQuest Projects 2

Venice: At the Mercy of the Sea

Introduction

In this WebQuest, students use Internet resources to learn how a combination of a rise in sea level and subsidence are causing the city of Venice, Italy, to be inundated by water from the Adriatic Sea more than 100 times per year. Students will answer questions based on their research that will lead them through an investigation of why this city is susceptible to this type of flooding, as well as the effects of flooding on the population, art, and landmarks of the city. Students will analyze and evaluate the effectiveness of several proposed plans to stabilize the “sinking” city, and to prevent flood water from entering the city during high water periods.

Task

While students are conducting research using the Internet resources provided, they will answer a set of nine questions. Each Web site has some of the answers, but none of the Web sites alone will address all of the questions. Students should be able to compile information to answer the questions as they read through each Web site.

As students research the answers to the questions, they will focus on proposed plans for protecting Venice from high water. The plans include building mobile barriers between the Venetian Lagoon and the Adriatic Sea, and raising parts of the city or even the entire city through various methods. Based on their research, which will include the opinions of scientists and others, students will formulate an opinion about the best possible approach to saving this important city.

Objectives

  • Describe the unique geographic location of Venice
  • Define the term subsidence, and explain how this process is affecting Venice
  • Describe changes in sea level worldwide, and explain how these changes are affecting Venice
  • Use numerical values to describe the rate of change of land level relative to sea level within the past 100 years, as well as the yearly frequency of flooding in Venice
  • Explain the artistic, cultural, and architectural significance of Venice within the world community
  • Describe and compare several proposed plans for keeping water from entering Venice during high water events
  • Analyze the possible advantages and disadvantages of proposed plans for keeping water from entering Venice during high water events
  • Form an opinion, based on research and evaluation of expert opinions, about the best approach to take to keep water from entering Venice during high water events


Resources

Students will use the Internet links provided to learn how flooding is, and for much of its history, has been, a problem for residents of Venice, Italy. They will identify the reasons behind these high water events, and will be able to describe the role of global sea level rise in the process. Students will examine several proposed plans for keeping flood waters out of the city. They will also identify some of the landmarks, art, and architecture that make this city unique.

Further research will lead students to analyze the proposed plans for saving the city from the effects of rising water. Students will identify key components of the plans, compare and analyze their effectiveness based on expert opinion, and form their own opinions about the most effective measures to take to keep flooding from destroying Venice.


Time

Three to four class periods to answer the set of questions, develop a comparison of the proposed plans, and form an opinion about their effectiveness.


Process

As students conduct research by studying the Web sites provided, help them focus on the information in the questions. This will provide students a sense of direction when approaching each site. If time allows, consider allowing students to further explore the problems other world cities are experiencing due to subsidence, global sea level rise, or a combination of both factors. Students may want to examine some of the plans underway to protect these areas, and compare them to those being suggested for Venice.

Students will ultimately focus their research on the proposed plans for keeping flood waters from invading the city. Through animation, descriptions, diagrams, and photographs, students will learn how the mobile barrier system will work. They will study various expert opinions from those who favor this method, as well as those which believe they system will not work. Through comparing this system with proposals that aim to “raise” all or parts of the city, students will develop and create a written opinion about the best possible approach to take to protect this important city.

Have students review the rubric to understand how their final product will be evaluated.

If time permits, allow students to share their opinions with other members of the class. Students could be arranged in groups with all members of a group sharing a similar opinion. Given time to discuss rationales, groups could present their arguments in classroom debate. Or, students could be arranged in groups with members of each group having different opinions. Students could then engage in debate within small groups. Debates should be structured, monitored, and well led for optimum effectiveness.


Evaluation

You may assign ten points to each of the ten questions for a total of 100 possible points. The answers to the questions are given below. Rate the answer to each question using the following scale: Excellent – 9-10 points; Very Good – 7-8 points; Good – 5-6 points; Satisfactory – 3-4 points; Poor – 1-2 points; and Unsatisfactory – 0 points.

Answers to Questions About Natural Threats to Venice and Possible Solutions

  1. Venice is located on an island which is in the Venetian Lagoon. The lagoon
    borders the Adriatic Sea. This low-lying island is not very far above sea level. During periods of high tide, ocean water enters the lagoon, naturally raising the overall water level. If tides are unusually high, or are compounded by wind, runoff from heavy rain, or storm surge, flooding can easily occur.
  2. Venice is sinking at a rate of one-half inch per century. The relative land level has dropped by more than 23 inches with respect to sea level in the last 100 years.
  3. Sirens alert Venice residents of impending high water, alerting shop owners and those who live and work in businesses, restaurants, and museums in this area to be prepared for flooding. St. Mark’s Square is only two inches above normal high tide level, so any abnormally high tide easily affects it. The square can be covered by four inches of water during a high water event, forcing residents and visitors to wade through water to get from place to place, or converge on relatively narrow wooden walkways elevated above the square.
  4. Venice is on a low-lying island in a lagoon. The lagoon adjoins the Adriatic Sea. Therefore, changes in sea level affect the water height in the lagoon. Globally, sea level is rising, making water levels surrounding Venice slightly higher each year. The second factor which contributes to flooding in Venice is known as subsidence. Venice is built on ground that is a composite of poorly consolidated sand, silt, and clay. The weight of the city, combined with the fact that groundwater was removed from aquifers at an enormous rate in the 1900s, leads to compaction of this underlying material. As this sand, silt, and clay compacts, the city “sinks.”
  5. As businesses, industries, and farming grew in and around Venice in the 1900s, groundwater was pumped from aquifers below the city for industrial and agricultural use at an increasing rate. The removal of groundwater took place at a higher rate than it was naturally recharged, which increased the compaction of materials on which the city was built. While large-scale removal of groundwater ceased in the 1970s, much damage was already done, which set the stage for subsidence that continues today.
  6. Answers will vary widely. Students may mention the Marco Polo arch, which dates around 1300 and stands in the neighborhood where Marco Polo is said to have lived, the sarcophagus of Giovanni Priuli, a famous Venetian senator who died in 1375, or the “Bridge of Sighs,” built in 1600 to connect an inquisition room with a prison across one of the cities many canals.
  7. This system is called the MOSE system, which stands for Modulo Sperimentale Elettromeccanico, or Electromechanical Experimental Module. The system consists of 79 gates which will lie flat on the sea bed, out of sight and filled with water, when not in use. When a high water event is expected, the gates will be filled with air, which will force the seawater out and cause them to rise. The gates will block all entrances between the Adriatic Sea and the Venetian Lagoon, preventing flood water from entering the lagoon and causing flooding in the city.
  8. Critics of the MOSE system say that the gates will not be able to block all water entering from the Adriatic, and that increased rates of sea level rise over the next century will soon make these very expensive gates obsolete. Some environmentalists argue that closing the gates for long periods of time will damage the lagoon ecosystem, as they will prevent the natural flow of water between the sea and the lagoon necessary to flush pollutants from the lagoon. Proponents say the gates are a good solution because they are only activated when needed, can remain in use for short periods of time rather than being a permanent feature, and that they can adjust easily to changes in sea level because of their mobility.
  9. Currently, as has been the case in much of the history of Venice, the city has simply been “built up” to accommodate increasingly higher water levels. Sidewalks are raised, lower levels of buildings abandoned and higher levels added when possible, railings and curbs are raised. Another plan involves injecting either seawater or carbon dioxide into aquifers below the city to “pump up” the entire city.
  10. Critics of the plan to build up parts of the city on a continual basis contend that this can only go on for so long before structural stability of some buildings becomes an issue, the proportion of raised areas to existing structures becomes visually unappealing, and, in some cases, doorways simply become too small. Proponents say this method has worked for much of the history of Venice, because construction and changes can occur at a pace which stays ahead of changes in water levels. While advocates of the plan to inject seawater or carbon dioxide into underground aquifers say this is a low-cost solution which is relatively easy to implement, critics say it would be impossible to uniformly raise all parts of the city, which would lead to widespread structural instability.

Evaluating the Written Opinion

Use the evaluation rubric or other means to assess each students analysis of the proposed plans to prevent further flooding in Venice. Evaluation of the written opinion should include self-assessment and teacher assessment. Students may want to add a third column to the rubric to be completed after debating their opinions with class members.

Conclusion

Based on Internet research, students should be able to explain the factors that contribute to the flooding that affects Venice on a weekly basis. They should be able to describe the unique nature of this city among world cities based on its history, art, and architecture, and recognize the importance of preserving Venice. Students should be able to describe and compare several proposed plans for preventing flood waters from entering the city. Based on their research, as well as the opinions of experts, students should be able to prepare a written opinion about the best approach to take to save this important city for future generations.


Written Opinion Rubric

Written Opinion RubricPoints PossibleSelf-AssessmentTeacher Assessment
The statement clearly describes the opinion of the writer.
10
  
The statement provides evidence, based on research, which supports the opinion of the writer.
10
  
The statement clearly outlines the approach the writer believes should be taken.
10
  
The statement is well-written and its content clear and understandable.
10
  

Rate each category according to the following scale:
Excellent – 9-10 points; Very Good – 7-8 points; Good – 5-6 points; Satisfactory – 3-4 points; Poor – 1-2 points; and Unsatisfactory – 0 points.

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