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The Venetian canal network performs a double duty for the city of Venice: it is simultaneously both a transportation grid and a sewage disposal network. Boats ply the city’s canals, transporting all manner of goods and people, as trucks and buses do in “normal” cities. Everything that moves on wheels on the mainland (terraferma) moves on water in Venice. Taxis are boats, as are buses, ambulances, police cruisers and fire engines.
The uniqueness of Venice is that the transportation grid never intersects the pedestrian routes at grade. LeCorbusier (among others) considered this an ideal system for urban mobility. Similarly, up until the late 1800’s, most people saw Venetian canals as ideal sewage removal conduits. Before the advent of modern sewer systems, large mainland cities were infested by untreated sewage and the accompanying disease. Human and animal excrement piled up along streets and in back lots until rains mercifully washed away the noxious refuse.
Not so in Venice, thanks to its canals. Unfortunately, to this day a majority of the city’s residential sewage still ends up in the canals, raw and untreated, to be flushed out to sea by the daily tides. What was once a marvel in the eyes of a nineteenth-century visitor, has since become an eyesore to contemporary travelers and a source of embarrassment and occasional discomfort to today’s Venetians.
Given the tremendous importance of the dual roles of the city’s canals, one would imagine that, over the centuries, Venetian authorities would have committed significant resources to the study of these bodies of water. Surprisingly, this has not been the case. For example, the hydrodynamic behavior of the canals, which is of considerable importance in the dispersal of the sewage outflows and which can also affect boat traffic, was never quantified until WPI students carried out the first scientific measurements in the 1990’s. The earliest record of a qualitative investigation dates back to 1900, and the only limited quantitative campaign was carried out in 1966. In all, even counting studies of dubious scientific value and studies that have been only vaguely referenced in literature, only about nine publications existed, as of 1990, on the topic of the hydrodynamics of the inner canals. Of these, not one was truly scientific, systematic or representative of the canals’ typical properties.
Introduction
Urban maintenance is the work that is required to preserve a city. It is a necessary measure to be taken in any large area to assure a high quality of life for its residents and visitors. Essential areas that specifically require maintenance are anything that allows a means for transportation and provides a way to get necessary utilities. For example, roads serve as a method of transportation that connects one place to another which makes all destinations within a city accessible. People drive automobiles on paved roads to transport goods, services, and people to their destinations quickly and efficiently. They also provide a means for public transportation, which is a necessity to all largely populated areas. Typical forms of public transportation include buses, subways, and automobile taxis. Like all other aspects of a large city, roads require systematic maintenance to ensure that they remain in full working condition. Another entity all cities require is the use of utilities, which are normally an underground arrangement of pipes and wires that include intricate waste removal systems. Like all other utilities, city sewer systems have modernized to adapt to the evolving technology of today’s world.
In Venice, canals are more than just waterways between buildings; they are the roads of the city. These “roads” function similarly to ones typically found in other large cities because they perform the same tasks as traditionally paved streets. The exception in Venice is that the “roads” are actually filled with water; therefore boats must be used for transportation in place of cars and trucks. Canals serve the same function as modern sewer systems under paved roads except for, instead of flowing into a complex underground network, the sewage empties directly into the canals. Venice’s current system of waste removal is not up to date with the rest of the world, meaning it requires a different kind of maintenance process to ensure it remains functional.
Since canals are highly essential to the everyday life of a Venetian, it is important to understand the significance of maintaining them. Similar to how city maintenance departments clear roads after snowfall for automobiles to safely drive on them, it is just as imperative for canals to be maintained so they remain navigable. Not only do people travel through canals to get from one place to another, but cargo boats utilize them to transport goods to different stores and shops throughout the entire island. This increases the importance of maintaining canals because if they can’t be used to move merchandise then it can have a serious negative effect on the economy. Canals can also be hard to travel through because of the buildup of sediment, including the accumulation of sewage. Often the significance of canal maintenance cannot be put into perspective unless one can experience being in Venice, which allows them to appreciate how essential canals are for the city to fully function.
Canal maintenance is a very involved process which requires major organization. A routine maintenance plan for Venice has been continuously neglected in the past. After years of inattention, the city has realized the effects of canal damage caused by the lack of maintenance. To preserve their historical city, the Comune di Venezia created a company, Insula S.p.A., to facilitate the maintenance throughout Venice. Insula has organized various projects on all aspects of infrastructure, including canal maintenance. Venetians who live in the city and deal with the issues associated to canal maintenance experience the effects of damage first-hand. Since they are familiar with the maintenance work and can directly relate to it, many publications have already been written in Italian. In order to broaden the knowledge about Venetian canal maintenance it is just as important to make the same information available to outsiders, including the English speaking community.
In the past three decades serious measures to preserve the condition of canals has been taken. As of 2007, only ten years after they were created, Insula has completed approximately two thirds of their maintenance plan. The Integrated Canals Project is one of their large scale projects that detail the scheduled progress of canal maintenance. Their work on this project includes the dredging of canals and the restoration of canal wall linings that become damaged by excessive boat traffic. Insula’s work maintaining the canals is projected to be completed in a 30 year plan. There is currently no plan by Insula to repeat the maintenance cycle after The Integrated Canals Project is completed. The success of Insula’s work to this point indicates their ability to perform future maintenance measures which is the next step in actively preventing serious canal damage.
Formation of Canals
Canals in Venice are more unique than other canals because they were initially created when masses of islands closed in on each other to create hundreds of thin waterways that flow between the small islands. The city was planned around the natural creation of these canals. It is comprised 367 artificially improved waterway segments that channel through the entire city. These segments were then broken down into 182 canals which were given names and codes by students from the earliest projects in the 1990’s as part of an Interactive Qualifying Project (IQP). As an early city, the canals of Venice proved vital to the survival of the city’s economy. Over time the city expanded and advanced and modifications to the canals were made as they evolved artificially to meet the needs of the city.
Functions of Canals
Venice’s most distinguishing characteristic is that it is built entirely on water and therefore has to use its canals as streets. This forces its residents to live a very different lifestyle than other people who live in highly traveled destinations similar to Venice. Venetians must find a way to adapt to a way of life that forces them to use the resources available. Although other cities don’t have to deal with some of the problems canals create, they are so important to life in Venice that it is beneficial for them to take notice in maintaining them. To outsiders it may seem like canals are more of a problem than they are worth but the truth is that if you compare the functions of canals to the functions of roads and sewer systems in typical cities, they are identical. Besides the difference of
Canal Widths
When the islands of the historical district of Venice formed, the canals developed various sizes. Of the 182, the average minimum width is 7.2 meters (23.4 feet) . When one factors The average width of a boat as 2.1 meters and boat parking occurring In every canal, this leave boat drivers with an average space of 3 to 5.1 meters to navigate through Venice. This leads to boat drivers having to look for alternate routes to get to their destinations, and poses a threat to public service boats such as taxis and ambulances, which rely on being time efficient. With hundreds of boats traveling through the canals each day, the small canal widths lead to several boats bumping into each other, but with the low speed limits of the canals, there are only a handful serious accidents reported each year.
being on water versus being on land canals require the same attention in terms of maintenance as both roads and sewer systems. One difference is that because it may be more difficult to perform maintenance on water as opposed to land, canals may be more expensive to maintain than paved roads or sewer systems.
Unlike most developed cities, Venice does not have paved roads for motor vehicle transportation. Instead it is comprised of narrow streets and alleys on each isole, or small island, that makes the use of motor vehicles impractical. This makes Venice the only major city in the world where vehicular and pedestrian traffic never intersect. Although it lacks streets, Venice has the same characteristics of any large city in that it still needs a way provide transportation to its residents and visitors. Because there are no motor vehicles in Venice, the canals serve as the roads. Boats travel through the canals on a daily bases performing the same tasks as cars or trucks would in a typical city. The intricate network of canals that flow throughout Venice is the city’s trademark and a staple of Venetian culture that attracts the attention of over 20 million tourists who visit the city every year.
Besides transportation, the canals also serve another major function in the city. Venice does not have a modern sewer system so canals are used as the outlet for the city’s sewage. The sewer system is very basic in that the sludge from the sewers simply travels through the canals until it eventually washes out into the lagoon. It has been said that, “When you pay a gondolier to row you on the canals, he’s rowing you through the sludge of Venice”. Although Venice was the first ancient city to develop a type of sewage disposal system, they have not modernized their technology to meet the standards of today’s more recent systems.
The Evolution of a City
As the “roads” of the city, canals have always been used as a means of travel from one island of Venice to another. Before what is now described as modern Venice was formed, people commonly used rowboats, ferries and even wooden planks in the earliest days to cross the canals. Often when people needed to cross certain canals to get to different small islands they would pay someone who lived on the land a small fee to cross on a wooden plank. The only other option was to walk between the islands by crossing bridges which wasn’t always an option. Soon enough, tradesmen started building wooden boats, which quickly became the primary mode of transportation in Venice. This provided an effective means of traveling until newer technology was invented and replaced the outdated methods. It wasn’t until after WWII, in the 1950’s that this newer technology was introduced to Venice and motorboats replaced rowboats to perform everyday tasks because they could provide quicker and more efficient services.
As the city became more populated, walls were eventually put up on both sides of the canals to stop them from closing in on each other. This ensured that passageways wide enough for suitable water travel remained between the islands. The walls served as a foundation for building by increasing the required stability needed for construction. After these walls were added, it stabilized the land enough so that bridges that linked the small islands together could be built. As talked about in greater detail in the bridges chapter, bridges were built in Venice as early as the 13th century. They continued constructing bridges over the canals until al islands were connected, now making it possible to travel across all of Venice, either by boat or foot. Before the addition of bridges nothing connected the islands, so each of them was considered their own separate community, meaning Venice was actually comprised of 125 small separate pieces. These manmade changes have helped shape Venice into the unique and historic city it has evolved into today.
The Venetian Island – A Revealed Truth There is controversy regarding the number of island that Venice occupies. If one were to look only at the historical district of Venice including the Guidecca, they would be looking at 125 islands (refer to figure 8). Branching out from the historical district are the several islands that run at the above the historical district. These islands, which include the famous glass factories of Murano, the very colorful island of Burano, and the Lido, are not islands to be part of Venice.
Methods of Canal Transportation
When in Venice it is easy to notice that there are many different types of boats that travel through the canals. If studied more closely you can see that some boats are utilized more often than others. These boats were divided into five different classes and categorized by type including; service, sport, taxi, merchant, and gondola. Although each of these boats serves different purposes they all contribute to the regular traffic flow of Venetian canals. Based on a study done in 2002 on the makeup of canal traffic in Venice, cargo boats and public taxis make up the majority of canal traffic. Being the most prominently used boats, they also contribute the most to canal wall damage, which will be discussed in further detail later in the chapter. As determined during the study, taxis account for 46% of the traffic and cargo boats represent 36% of the canal traffic .
A Quest for a Parking Space in Venice
With all the boats that maneuver around Venice, there is a need to park all those boats. WPI Students in October 2006 investigated the use of canal parking spaces in Venice. This project gathered data about the interaction between boat traffic and parking in Venice in order to identify criteria for the allocation of permanent and temporary parking permits. They found that there were 1200 permanent canal parking spaces and 3000 temporary parking spaces. To monitor and implement these changes, an electronic parking management system was designed to assist the city in reducing the amount of time needed to process permit applications. The implementation of this system would thus benefit both the citizens and the city, and would help decrease the cost of traffic congestion. This team of students recommended that the city charge a 10 euro fee to all Venetians applying for a temporary parking space. This would help maintain the system while forcing Venetian residents to reconsider applying for temporary parking spaces continuously, and not applying for a permanent space.
Although canals provide for a unique form of public transportation in Venice, their primary
function is just as important as in any other populated city. Vaporetti, or motorized waterbuses, are the most commonly used form of public transportation. Like subways or public buses in other major cities, Vaporetti provide transportation to large amounts of people. It makes different boat stops along the Grand Canal and also travels to outlaying islands in the lagoon. Both Venetians and tourists regularly rely on this form of public transportation for their daily traveling needs. Unlike land based cities where people often use their own cars for transportation, it is not as easy to have immediate access to a boat in Venice. There are no driveways leading into a residence where boats can be parked like there would be outside of houses normally. This creates space problems and contributes to the lack of parking available in the canals as determined by a student research group who studied the optimization of canal parking space in 1996. For this reason many residents do not even own a personal boat. Water taxis are another popular form of public transportation in Venice. Taxis are mainly utilized as an alternative to Vaporetti by tourists who are not as familiar with the city and prefer to be brought to their exact destination in a shorter amount of time. Considering it is estimated that over 20 million tourists visit the city each year, it is not surprising that taxi’s are in such high demand. Venetians don’t regularly use taxis because the Vaporetti is a cheaper method of transportation, comparable to how a public bus or subway would be to an automobile taxi on a land based city. Because there is such a high demand for efficient public transportation in large cities, it makes the most sense that the types of boats that best support that need in Venice are the most widely used.
Assessing the Problems of Venice’s Canals: The Need for a Routine Maintenance Plan
Venice is an extremely overpopulated city relative to its size in total area, 6.3 square kilometers, or 2.4 square miles in solely the historical area. The amount of actual residents in Venice is has been declining over the past few years and is currently approximately 62,000. Causes of the decrease in population are suggested to be related to the historical face of the city. Based on the fact that Venice is full of original structures which date back to over 1000 years ago, proposals to restore or maintain the infrastructure have serious limitations or are often even completely rejected by the superintendant of the city. Being that a large amount of Venice’s population is considered elderly, this poses problems with them being able to live and function in a city that cannot always be modified to meet their needs, which is the reason why so many of them are forced to leave the city.
Over 20 million tourists visit Venice every year leading to the estimation that tourism brings in approximately $1.5 billion in revenue annually, an extremely imperative facet to Venice’s economy. Because Venice is heavily occupied by tourists it generates additional job opportunities for non native Venetians. Since there aren’t enough Venetians to fulfill all employment positions, more than 47,000 people commute from the mainland into Venice on a daily basis for available work. On an average day the amount of nonresidents in Venice outnumbers the amount of residents two to one. Like all largely populated cities, it is crucial to provide all the people who live, work and vacation in Venice with public transportation. As the amount of actual residents in Venice continues to decrease, the amount of people coming into the outside, therefore needing public transportation increases. Every year the amount of canal traffic increases by approximately 2.5% due to the constant rise in people coming into Venice. Peak visiting periods occur between the summer months of June-August, around the holiday season and throughout Carnival, a traditional Venetian event that takes place in the beginning of February. The abundance of tourists contributes to the factors that draw attention to the need for routine maintenance. Other than the aforementioned increase in of boat traffic; clogged sewer outlets, boat collisions, and biological and chemical agents also increase as more people occupy the city, all factors which significantly contribute to the causes of canal damage.
Construction of Canal Walls
Canal walls of Venice were traditionally constructed of two different materials. The lower parts of some canal walls, which are submerged in water, are made up of Istria stone. This type of stone is nonporous, which prohibits water from penetrating the surface, making corrosion almost nonexistent. Brick is generally used for construction on top of the stone. Fondamente, or sidewalk platforms, that run along the canals are made entirely of brick. Brick is a popular building material because it is readily available and inexpensive. Its major setback is that it is extremely porous, allowing it to be easily corroded by the salt water of the lagoon. Consequently, this makes brick a poor choice of material for building along the canals. Concrete is used when repairing canal walls and is installed at the very base of the walls to allow for additional reinforcement.
When the use of stone in construction of the city of Venice began, the sea level was at a much lower height than today. Due to the rising sea levels around the world, the water level in the Venetian lagoon has risen approximately 23 centimeters since 1897. When the city was first built, only the Istria stone sections of the canal walls were exposed to the water. Today, since sea level has risen substantially, the extremely porous brick used in the building construction is also exposed to the salt water of the sea.
Canal walls serve as the foundation for most buildings in the city so once they become damaged the buildings built on them are subjected to problems with structural instability. As the walls begin to deteriorate from the bottom of the building upward, it reduces the amount of forces it can withstand and greatly risks caving in. Residencies are not the only buildings that border the canals; local businesses and shops are also affected by the damage. Docks, which are considered part of canal walls, are similarly subjected to this same damage. Docks are built on the canal walls and either extends inward or outward depending on their design, allowing docks to be simultaneously maintained as canal walls are repaired.
The Effects of Boat Traffic & Wake Damage on Canal Walls
Traffic congestion is also damaging to canal walls because of the water turbulence caused by maneuvering. When breaking, boat drivers must put their engines in reverse to slow themselves down. Boat drivers alternate between the reverse and drive gears to remain in a constant position. This motion causes large amounts of turbulence underwater and is more evident in taxi and cargo boats.
Collisions between boats and canal walls that mainly occur because of traffic congestion also make canal walls vulnerable to erosion. Collisions most commonly take place at canal intersections and at docks. Once the holes caused by the collisions are created, then the pollution and sewage in the canal water start to corrode and decompose the mortar and brick. After a small amount of damage has been done to the wall surface, the damage will only continue and become exponentially more severe. Imperfections in the wall surface, such as missing or chipped bricks, make the canal walls more vulnerable to erosion. Sometimes a wall will appear to be almost flawless, but due to a missing brick, water has been entering and eroding behind the surface. Water currents carry away the eroded pieces which weakens the foundation. Over time the wall surface may still be intact, but the structure that lies behind the surface has been damaged and the entire wall may eventually collapse.
Additionally, as a boat moves through a canal it creates underwater turbulence caused by the spinning of propellers, called wake. Wake is visible on the surface of the water and can be seen from the back of the boat. The intensity of the wake changes based on factors such as: the speed of a boat, the size of a boat’s propeller, the power of a boat’s engine, the shape of a boat’s hull, the depth of the canal being traveled, and the boat’s load.
The damage caused to canal walls by the wake is called Moto Ondoso. Moto Ondoso is not usually problematic in large bodies of water like oceans or lagoons because there is a lot of area for the turbulence to disperse into. This is not the case in Venetian canals where there is a limited depth and width. The energy created by boat motors reflects back up to surface of the narrow waterways, causing them to crash into the canal walls. The deterioration of the canal walls occurs when their physical conditions become altered. They then slowly erode the mortar that acts as a seal between the bricks and stones in the wall. Erosion, the most destructive force to the canal walls, occurs when water crashes against the surface of the canal wall, causing friction. When the mortar erodes, the bricks and stone become disjointed and are much more susceptible to the destructive stresses and forces of boat wakes.
One reason why wake damage is such an extreme contributing factor to canal wall deterioration is because boat speed limits are not strictly enforced. The one exception is the Grand Canal where the current 7 km/hr speed limit is more closely monitored. A boat speed limit for Venice was first established in 2001 making the maximum boat speed in the canals 5 km/hr and the Giudecca lagoon 20km/hr.
Based on data from the Moto Ondoso Index, a study done by students at Worcester Polytechnic Institute, which includes the energy value of each boat that passes through the canal segment each day, small cargo boats release by far the most amount of energy into the canals at 66%. This conclusion is based on the assumption that the speed limits in the canals are loosely enforced and rarely followed. These calculations are relevant to canal wall damage because if all boats were to obey the posted speed limit of 5 km/hr it was determined that taxi boats would be responsible for majority, 53% of the energy released in the canals. The data also revealed that very few boats travel within the posted speed limits. Based on testing it was noted that 100% sampled taxis exceeded current speed limit of 5 km/hr, averaging a speed of 11.7 km/hr. The average speed in all boats in all canals was over 12 km/hr, which is more than 7 km/hr over the legal maximum speed. Based on the index that was created it was determined that if boats were to travel within the speed limits, there would be a drastic decrease in the heights of boat wakes and therefore much less erosive water motion. After the study it was concluded that only 3% of the total types of boats observed obey the speed limits. When the range was extended to traveling within 2 km/hr of the speed limit, still only 13% followed the speed limit. Furthermore according to their study they calculated that if speed limits were enforced, Moto Ondoso would be reduced to 1/10 of current levels.
Figure 19: This chart shows the speed limit then the average speed limit divided by types of boat, as part of the Moto Ondoso Index study
Another factor that also strengthens the effects of Moto Ondoso is the different hull shapes of boats. Observations again taken during the study of the Moto Ondoso Index noted that boats with narrow, streamlined hulls produced far less wake than those with boxy, square hulls. It was concluded that regardless of boat type; boats with boxy hulls would therefore cause more damage to the canal walls than their sleek counterparts.
The continuous wake impact on the canal walls has resulted in the need for constant repairs. When damaged walls go unnoticed, the result can be dangerous. Over time some walls can become damaged so severely that they develop large holes and eventually collapse. Since canal walls are essentially the foundations that support the buildings constructed on them, excessive damage can cause them to cave in to the canals they border. Once a wall has been weakened or damaged it is more susceptible to the erosive powers of the water, and is therefore destroyed at a much faster rate.
Before motorboats were introduced into the canals of Venice, the canal walls were essentially only subjected to the forces of water as it flowed in and out of the lagoon with the tides. Canal walls were built long before motor boats were invented, and therefore were not designed to be able to withstand the force produced by boat wakes today. The amount of boat traffic that travels through the canals, as it increases, has the potential to destroy the city.
As long as Venice continues to use motorboats to travel through the canals they will always have to deal with the damage caused by Moto Ondoso. Although there will never be a solution to completely eliminate canal wall damage, measures can be taken to actively reduce its effects. One simple action could include beginning to strictly enforce speed limits in the canals. This would not entirely resolve canal wall damage but it would greatly reduce it so that such extensive measures would not have to be taken as regularly. This is also referred to as a form of preventative maintenance. As more powerful boats are being created due to advanced technology, there lays a greater risk of more damage caused to the canal walls. Even though routine maintenance can be costly, in the case of Venice where it was seriously neglected for so long, it is essential in order to return to a state where the optimal quality of life can be observed. Long-term maintenance in Venice was necessary before it became too late and the city suffered the consequences and literally fell apart.
Additional Factors That Contribute to Canal Wall Damage
Chemical materials like sulfuric acid create the pollution that weakens canal walls by expediting the disintegration of wall materials. Sulfuric acid damages stone when it interacts with the calcium carbonate, chemically changing it to calcium sulfate. In the case of the canal walls, the change in composition of the stone causes the interior structure to crumble. The source of the sulfuric acid comes from the Mestre-Marghera industrial zones where chemicals were released into the canals. These chemical plants were extremely active up until the end of the 1970’s.
Biological materials which live and grow naturally in the canals such as bacteria, fungi, seaweed, algae, and lichens also cause the deterioration of the canal walls. These organisms eat away at the inner supports of the canal walls as well as the stone. The humid climate of Venice exacerbates this process since the warm, moist air promotes the growth of these organisms.
Venice’s Sewage Removal System
Canals are a major component of the sewer system. Sewage flows from houses, into conduits located under the streets, and eventually enters the canals through outlets below the waterline where it contributes to the sediment buildup. Sewer outlets on the canal walls sometimes become blocked by silt that has built up over the years on floor of the canal. Sewage then gets backed up in the pipes behind the walls and eventually must find a different way to exit. Alternatively, the sewage is forced out through the mortar that binds the building materials, weakening the structure and making the wall more susceptible to the erosive powers of the water. Structural damage is caused by the sewer system when the silt levels get high enough to block the sewer holes in the canal walls. Since the sewage cannot flow out of the pipes and into the canal, tremendous pressure builds up in the pipes housed within the canal walls. When enough pressure has accumulated, the pipes rupture and the sewage is rerouted to other areas within the foundations of the canal walls. Finally it seeps out through the mortar, weakening the structure considerably. After the sewage starts to affect the structure causing it to crumble, the debris falls into the water and settles at the bottom of the canal.
Analysis of Structural Damage to the Canal Walls present
The deterioration of the canal walls occurs when their physical and mechanical conditions become distorted. WPI groups in the early 1990’s came to Venice to analyze the causes of damage to the canal walls. This study presented an analysis and quantification of the different types of damage on the canal walls of Venice. They focused their studies around the San Marco Sestiere and the boundary between this area and the San Castello Sestiere. Photometric techniques, coupled with an intricate archival system, were used to evaluate the extent of damage within the canals. Building materials, traffic levels and mud build-up were analyzed along with structural damage, in an effort to relate these causes to the existing damage. The research developed in this project provided the information necessary to evaluate the extent of damage, the causes for this damage, and the social ramifications resulting from this damage.
An alarming trend was revealed in the study of the structural damage and sewage outlets during this project. The instances of illegal sewage outlets are becoming greater every day, as are the occurrences of canal damage. From their analysis, the groups recommended that regulations be enforced more strictly in an attempt to alleviate all cases of exposed sewage outlets.
Buildup of Sediment on Canal Floors
A combination of sewage, masonry from buildings and particles brought in by tides all contribute to the high amounts of sediment buildup found on the bottom of the canals. Based on the results of their research and testing, a study by students from Worcester Polytechnic Institute concluded that 83% of sediment in Venetian canals is unknown, 11% is due to sewer discharge and 6% accumulates from masonry debris. Sediment builds up on the floor of the canals at a rate of up to two centimeters per year. As sediment builds up, it decreases the depth of the canals, which is why they need to be dredged routinely. Because of the buildup of sediment, the water level becomes higher, which can lead to problems with bridge clearance for boats, especially during periods of high tide.
The Plan to Preserve Venetian Infrastructure: The History of Canal Maintenance
Canal maintenance has been performed for much longer than most people realize, although the degree of the measures have not always been as extensive as now. There is evidence of the earliest stages of maintenance from before the fall of the Venetian Republic in 1797. Maintenance continued during the first period of Austrian rule from 1799-1805. Canals were seriously neglected during the period of the French reign. It wasn’t until the Austrians returned to power in 1818 that an extensive maintenance program was set up to improve the poor state of sanitation created by the sewer system. Serious maintenance measures were again taken from 1869-1875 when Venice was annexed by the Kingdom of Italy. After this, in 1892, large scale dredging resumed. The conditions of the canals were so bad after WWI that this forced the State to perform emergency, non-routine work in the areas in need of the most immediate work. The creation of an ad hoc law in the Forties granted additional funding for maintenance and allowed large scale work to continue. Venice experienced a shortage of funding in the Seventies and Eighties which caused maintenance work to become less frequent than previous years. This seriously delayed routine maintenance and made the sanitary conditions of the canals throughout the city an alarming issue.
Beginnings of a New Maintenance Plan for Venice
Over the past 30 years as damage created by the amount of boat traffic and increase of sediment accumulation has increased, it has become imperative to make more extensive and regular canal repairs in order to maintain a high quality of life within the city. One hundred percent of the money budgeted by the Venetian government for urban maintenance is handed over to an agency whose mission is to provide sufficient maintenance to the city. They are responsible for organizing and carrying out methods to control and reduce the effects of damage on all types of infrastructure, including canals. The city spends millions of Euro annually on urban maintenance, although it is not necessarily distributed equally every year. For example, in 2006 the government set aside 35 million euro to continue the scheduled canal maintenance, but the following year they were only able to budget 5 million euro. Inconsistency in the amount of money available to spend on maintenance can cause setbacks to the projected date of completion of the maintenance cycle.
Venice’s Leader in Operating Canal Maintenance
Over the past few decades extensive work by independent groups has been done in the area of urban maintenance in Venice. Examples of work researched by students in Venice includes extensive testing, data counts, and project proposals all related to the topic of urban maintenance. In association with companies such as Forma Urbis, all of this extensive research has been organized into databases and handed over to established agencies that have the available resources such as funding and manpower to further investigate and implement maintenance plans. Worcester Polytechnic Institute’s Global Studies Project Center, with the assistance of Forma Urbis continues to explore areas related to urban maintenance in Venice to this day. They remain in close contact with many sponsors that they have collaborated with in the past including The City of Venice, UNESCO, and Insula to provide assistance collecting and organizing the data needed to complete proposed maintenance projects throughout the city. The leading agency dedicated to fighting to maintain canals is Insula s.P.a. Insula was created by the City Council in 1997 in order to accelerate the maintenance process. They have quickly become the operative branch of urban and infrastructure maintenance in Venice. Prior to Insula’s creation, minimal action in maintenance preservation was taken. Insula’s involvement is broken up into two different stages units, work done on land and work done on water, which includes the maintenance of canals. The primary works associated with water maintenance as it relates to canals are the dredging of silt and sludge from the bottom of canals, the readjustment and rationalization of the subsoil dredged from the canal, the static restoration of the banks and foundation of canals and the improvement of the city’s hygienic and environmental conditions through the renewal of the sewer system. Another measure they are responsible for concerning water maintenance which is not directly related to canals is the raising of pavements to protect the city from acqua alta, or high tide as referred to in The Acqua Alta sidebar.
Acqua Alta and the MOSE Project
Acqua Alta, or “High Water” occurs when the waters in the canals from the high tide rise above flood levels, pouring onto the streets forcing areas of Venice to rely on elevated walkways and water boots for transportation. Acque Alta occurs on a yearly basis mainly during the months of November through January . Venetians are informed of an Acqua Alta approaching when warning sirens are sounded. The level for an Acqua Alta to be official is 90 mm above normal high tide. The biggest factor of Acqua Alta is considered to be astronomical tidal flow of the Adriatic sea . This tidal flow is strongest at the beginning and end of the full moon cycle. The winds that blow from the Balkans and the Sahara also greatly contribute to Acqua Alta . Humidity and the air temperature can also play a major role with the flooding of Venice especially if it is humid and unseasonably warm. The flooding of Venice, when it does happen, Acqua Alta lasts around 1 to 2 hours depending on the severity of the tide . The level of the tide does not mean that the city will be flooded by that number. For instance, when "+100 cm Acqua Alta is mentioned, that meter only represents the increase in comparison with the average sea level(the average sea level conventionally accepted is the 1897 one measured at the Punta della Salute). At this level, only a very small number of low-lying areas in the city points get flooded .
The worst Acqua Alta on record was on November 4, 1966, a day when tides rose 1.94 meters, nearly all of the city, inculding Piazza San Marco (St. Mark’s Square). This Acqua Alta led many to believe that Venice was sinking. However, a rare event occurred during a cold November night in 2005, in what could be called the exact opposite of Acqua Alta. Temperatures that night dipped below zero and there was zero humidity. This combination led to the water levels dropping down to as low as minus 0.70 meters at low tide. The tide was so low that Venetian residents could almost walk around Venice across the exposed sand banks and marshes . The lowest record height for this type of occurrence is dated back to February 14, 1934 when the waters dropped 1.21 meters below sea level.
Level of Tide Percentage of Venice Flooded Up to 80 cm Normal Tide 100 cm 4% 110 cm 12% 120 cm 35% 130 cm 70% 140 cm 90%
As the mean level of the land has lowered, the sea levels have risen. Although the city did sink about 10 cm in the 20th Century because of industrial groundwater extraction, the sinking largely stopped when artesian wells on the mainland were capped in the 1960s. Today, subsidence is estimated at 0.5 to 1 mm per year, mostly due to geological factors and compression of the land beneath the city's millions of wooden pilings.
In May 2004, In order to stop this flooding, construction of the controversial MOSE Project started. This was a project to design 79 flood barriers fixed to the lagoon bed. At normal times the barriers will be full of water and lie flat but when there is a flood warning they will be pumped full of air and raised, therefore creating a dam and saving Venice from being flooded. The project has made progress, but as the intentions are good from the city, many Venetians feel that this project is an inappropriate use of city money.
Insula’s work encompasses the entire historic center of Venice as well as the inhabited areas of the islands of Murano, Burano, Pellestrina, and sections of the urban areas of the Lido. One of their main water maintenance projects is called “The Integrated Canals Project”. This coordinated program of urban renewal and protection is one of their first projects, dating back to their establishment. Before they started the project each canal was classified into different priority levels based on their level of sediment, circulation of water, and importance of use and navigation. The main focus of the program is to tackle the largest and most urgent maintenance works, which include dredging the canals and restoring the walls that line them. The cycle of special maintenance determined by the Integrated Canals Project is organized and divided into two phases. The first phase is to complete the hydraulic, structural, and sanitary functionality of the canals and their walls. It is expected to be completed by 2014. The second phase was put into action in 2005 and deals with the radical restoration and upgrade of the entire sewage collection and disposal system. This stage of the project is projected to be completed by 2025. At this point 71% of canals have been dry dredged. The advancement of their scheduled plan relays on the continuity and proper disbursement of the maintenance funds by the government. In order to complete the entire project Insula has calculated a 1,213 million Euro budget.
One issue that slows down the progress of Insula’s work is the fact that the majority of infrastructure in Venice is considered historical because the original structures remain or have had minimal changes made to them over the years. There are limitations in Venice that prevent work from being done to change these historical structures or require special permission from the city’s superintendant before any work can be considered. These restrictions create issues when repairing and restoring infrastructure, since previous occasions have shown that it is unlikely the superintendant of Venice will approve major modifications to original structures. Depending on the specific situation, maintenance work is sometimes even rejected by Venetians. Even though Insula is working to preserve and protect the city, some of their work requires changes to be made to historical structures which are not easily accepted by many Venetians who take pride in the historical value of their city. This obstacle hinders Insula’s progress because often there is a lot of behind the scenes action that must be taken care of and approved before they can begin their maintenance work.
Today, only 10 years after they were formed, Insula has made significant progress in carrying out their exhaustive maintenance plans. Insula continues to set their standards high to carry on a rapid pace of repairs for the future. This can be measured by the progress of their entire maintenance plan which was grouped into three categories based on severity; first, second, and third priority. As of now Insula has completed all the first and second priority level work. They have currently moved to the third priority level maintenance and are approximately 71% completed with the entire project.
Figure 25: A canal being dredged as Figure 26: The same canal after Insula
part of Insula’s scheduled maintenance completed their work
plan
Canal Maintenance Preparation
Part of Insula’s work dealing with the restoration of canal wall linings and dredging of sediment buildup includes the systematic process they have created which enables maintenance to precede at a quicker rate. As part of the first stage of the process, wooden piles are driven into the bottom of the canal along the outer edge of the foundation on each side. These are used to support a kerb, or a continuous metal sheet piling that is extends approximately 2.5 meters deep.
Next, before the canal can be dredged the section will then go through the process of water removal by setting up barriers at the ends of the section, similar to a dam. Then, water is then pumped out of the section until workers are able to walk along the edges of the canal. Wooden planks are added along the sides of the walls and evenly placed across the bottom of the canal to allow for easy movement of the workers. Once the canal is drained and the planks are laid, the canal is ready equipment necessary for the dredging to be delivered and the workers can begin their maintenance.
There are however, exceptions to the maintenance preparation routine in Venice. One of these exceptions includes areas where the widths between walls are too spread out to perform traditional maintenance tasks. For example, maintenance in the Grand Canal is more unique than the rest of the canals because of its extreme difference in depth and width. Since the depth of the Grand Canal is approximately 4 meters, there is no need to dredge the entire canal, nor is it feasible. Crews can not actually block the canal because when maintenance work has to be done on the sides of the Grand Canal it would pose a threat to the city’s commercial transportation. In order to prepare for the Grand Canal maintenance, a section is chosen on either side of the canal. That section is blocked off along the side of the canal. Unlike the inner canals of the city, a barrier is placed parallel to the section of the canal in need of repair and extended outward enough for maintenance crews to work. With the section blocked off, the water is drained out and the crews are ready to work, using a method that does not affect boat traffic. The average cost for this maintenance procedure is approximately €8000 / meter which is a massive difference from the normal average of €129 / meter - €3264 / meter. Although, the lagoon is not a canal, maintenance to the wall of the lagoon must be performed similar to the maintenance of the Grand Canal.
The Grand Canal
The Canal Grande is Venice’s most famous canal. It’s the longest and widest canal, measuring 3 kilometers (2 miles) in length, and a width averaging 175 feet with a peak width of 350 feet. Its’ unique S-shape figure, as shown in Figure S1, starts from the lagoon, passes through the central districts of Venice and ends at the Basilica di Santa Maria della Salute. The buildings that run along the Grand Canal are masterfully decorated, which adds to the beauty of the canal. Of these buildings, there are over 100 palaces, including Ca d'Oro, Palazzi Barbaro, Ca' Rezzonico, Ca' Foscari, and Palazzo Barbarigo. There is also the famous Basilica di Santa Maria della Salute. With the train station and the bus station along the canal, the Grand Canal is the main access point for public transportation in the city. It is the only canal in Venice that is actually a river-be. Its signature characteristics make the Grand Canal an icon of Venetian beauty.
Cleaning the Canals
Canals must be dredged to guarantee the flow of water in the canals and ensure they remain navigable. There are two different techniques that can be used for dredging, wet dredging or dry dredging. The most common method used to dredge is dry dredging. During the dry dredging process the area of the canal that needs treatment is cut off from the rest of the canal by damming the area at both ends. A large steel sheet piling is driven into the bottom of the canal. A hydrodynamic vibrating head replaces the ancient wooden caissons that are filled with clay. Once the area is secure, pumps are used to drain the stretch of canal. The canal must maintain a sufficient water level to keep the pontoons afloat where the grab-bucket dredges are mounted.
Wet dredging is often used as a preliminary phase before canals are drained and tends to be more economical. This process is usually limited to the central strip of the canal so it does not disturb or damage building facings. During this technique hydraulically driven grab-bucket dredges are mounted on the bow of the barges. Once the canal has been drained, it remains dry for an average of six weeks while various maintenance work is done. In both cases of wet and dry dredging, the sediment that is dredged out is then transferred onto barges and taken away to designated treatment sites.
After Insula completes the dredging process, they are responsible for managing the removal of the subsoil. When the sludge at the bottom of the canal is removed it has to be classified by type according to Insula’s predetermined standards. There are four classification levels, A-D which determines the state of the subsoil. This process is important because the different types of subsoil are relocated based on this determination. Type A subsoil contains the least amount of polluted material and Type D is classified the worst. It contains radioactive and other heavy materials which are the cause of the excessive amounts of pollution found in the silt. Type D, the most common of the four types has to be pretreated before it is brought to a special outlaying lagoon island which was created for the sole purpose of the removal of this material. Type D is so abundant because for over 20 years Margetta, the largest industrial chemical complex in Europe at the time was located in Maestre on the mainland and was releasing hazardous chemicals into the water. These hazardous materials included chemicals used in the manufacturing of items such as PVC, plastic and soda.
The Restoration of Canal Walls
Insula’s method of constructing and reinforcing canal walls make the walls much more durable and resistant to erosion. This method of canal wall construction incorporates a slanted wall rather than a vertical wall, and uses more concrete than the traditional walls. Although a more modern canal wall construction would help preserve the city structurally, it would also ruin the aesthetic value of the city’s appearance and therefore people are resistant to this change. They want to preserve the authenticity of the city by maintaining the original construction of the canal walls, even though this method is not as practical anymore.
After the canals are fully dredged, the first step in restoring the canal wall linings can be performed. During maintenance the mortar joints are first restored by making injections into them which reinforces the surface of the wall. Walls are resealed by a technique known as "binder sealing". During this process the missing part of the outer face is reconstructed and the body of the wall must be restored to its former compactness which allows physical continuity. The ground behind the walls is treated by a sealing and waterproofing process. This prevents water from resuming its deteriorating action and stops the building fabric from being undermined. Corrective action, construction, or partial rebuilding, is performed in order to compensate for any swelling that alters alignment or shifts the original geometry of the wall. As they continue their work around the city, Insula consistently monitors the conditions of the walls and foundations to ensure they are being maintained effectively.
In less than three years after the start of their newly revised schedule over 22 kilometers of canals has been dredged. The success of their actions to this point shows the potential to develop a permanent, systematic routine.
The Future of Canal Maintenance in Venice: Effectively Communicating About Maintenance Measures
Some of Insula’s work is dependent on other maintenance work done by different companies involved in urban maintenance. It is important for them to be able to effectively communicate with all parties to keep them regularly informed of their maintenance activities. Insula also strives to keep the public informed of all their work. They have an easily accessible, interactive website where all of their information is posted. The website, www.insula.it is in Italian but includes an option to allow you to automatically translate it into English. The website includes information regarding what they do, their maintenance schedule including projected completion dates, and information breaking down their financial expenditures. Other informative measures they have used include the organization of public meetings to advise the public of project updates, the time and money needed to finish them and to hear any appeals regarding their work. Insula produces and distributes informative leaflets in the area they are working in and sets up their worksites with street signs and hoardings to advise people of the constructions zones and detours. In the past they published quarterly reviews entitled Insula Informa: Quaderni. Today they continue to send out press releases in local newspapers to inform the public about works in progress or ones scheduled for the near future. Insula also organizes and participates in various conventions, seminars and press conferences. It has hosted local, national, and foreign television crews on its work sites and participated in radio and television transmissions in the past. Insula continues to advance communication measures by working on creating a complete record of their works with a full photographic archive and the production of cd-rom and video documentation.
The Need for Understanding Canal Maintenance
Canal maintenance an essential part of the urban maintenance system as it is related to the two primary functions canals serve; the transportation of people, goods, and services, and the removal of the city’s sewage. For the maintenance of the city to be fulfilled, the City of Venice must be sure to provide a way to preserve the quality of life of everyone who occupies the city. For this reason it is extremely important to make information available to people other than the Venetians who are most familiar with the cities maintenance plan.
References
"Acqua Alta" & the Floods in Venice.” http://tours italy.com/venice/floods%20in%20venice.htm. (Accessed November 29, 2007). Bakerejian, Martha. “Record Number of Tourists Travel to Venice, Italy.” Italy Travel. (2007) http://goitaly.about.com/b/2007/11/21/record-number-of-tourists-travel-to-venice-italy.htm (Accessed December 6, 2007). Berendt, John. The City of Falling Angels. New York: The Penguin Press, 2005. Binkerd, Chad R., Ralph A. Maselli II, and Scott H. Stoddard. Analysis of Structural Damage to the Canal Walls of the Sestiere Castello di Venezia. Worcester, MA: Worcester Polytechnic Institute, 1992. Bon, Enzo. “Acqua Alta (High Tide).” http://www.comune.venezia.it/flex/cm/pages/ServeBLOB.php/L/EN/IDPagina/1066. (Accessed November 28,2007) Borrelli, Alexander, Matthew Crawford, James Horstick, and Izzettin Ozbas. Quantification of Sediment Sources in the City of Venice, Italy. Worcester, MA: Worcester Polytechnic Institute, 1999. Bravo, Victor, Jose Lopez, and Zung Nguyen. A Documentation and Analysis of Canal Boat Parking within Santa Maria Formosa Insula and Santa Maria dei Frari Insula. Worcester, MA: Worcester Polytechnic Institute, 1996. Bukowski, Gregory, Briana Dougherty, Russell Morin, and Patrick Renaud. Optimizing the Use of Canal Parking Space in Venice. Worcester, MA: Worcester Polytechnic Institute, 2006. Carrera, Fabio and Caniato, Giovanni. Venezia la Citta Dei Rii. Ceriana, Stefano, Dan Nashold, Joan Olender, and Matthew Poisson. Monitoring and Analysis of Cargo Delivery System in Venice, Italy. Worcester, MA: Worcester Polytechnic Institute, 1998. Chiu, David, Anand Jagannath, and Emily Nodine. Moto Ondoso Index: Accessing the Effects of Boat Traffic in the Canals of Venice. Worcester, MA: Worcester Polytechnic Institute, 2002. Cioffi, Carlo, Vicky Dulac, José Marsano, and Robert Reguero. Development of a Computerized Decision Support System for the Scheduled Maintenance of the Inner Canals of Venice. Worcester, MA: Worcester Polytechnic Institute, 1997. Comune di Venezia. http://www.comune.venezia.it. Durant Imboden's Venice for Visitors. “Acqua Alta: High Tides and Flooding in Venice.” http://europeforvisitors.com/venice/articles/acqua-alta.htm. (Accessed November 29, 2007). GIS Layer. Isole. Forma Urbis. (Venice, Italy) Last Updated: November 3, 2006. GIS Layer, Rii, Forma Urbis. (Venice, Italy) GIS Layer, Segmenti Perímetro, Forma Urbis. (Venice, Italy) Last Updated: October 13, 2006. GIS Layer, Segmenti Totale, Forma Urbis. (Venice, Italy) “History of Venice Rilato Bridge.” Destination 360, 2007. http://www.destination360.com/europe/italy/ponte-di-rilato.php (Accessed December 6, 2007) Insula S.p.A. www.insula.it. (Accessed October 2007 – December 2007). Richards, Genevieve. “Facts About Venice.” www.travellady.com/Issues/November04/1071FastFactsAboutVenice. (Accessed November 4, 2007) UNESCO. http://www.unesco.org/culture/heritage/tangible/venice/html_eng/menacemon.shtml.
Fabio's Paper
The Venetian canal network performs a double duty for the City of Venice. It is at the same time both a transportation and a sewage disposal network. Boats ply the network to transport all manner of goods and people, as trucks and buses do in “normal” cities. Everything that moves on wheels on the mainland moves on water in Venice. Taxis are boats, as are buses, ambulances, police cruisers and fire engines. The uniqueness of Venice is that the transportation grid never crosses the pedestrian routes at grade. LeCorbusier (among others) considered this an ideal system for urban mobility. Similarly, up until the late 1800’s, most people looked at the Venetian canals as the ideal sewage removal conduits. Before the advent of modern sewer systems,158 large cities were infested by untreated sewage. Human and animal159 excrements piled up along streets and in back lots until rains mercifully washed off the noxious refuse. Not so in Venice, thanks to its canals. Unfortunately though, to this day (2004) a majority of the residential sewage still ends up in the canals, to be flushed out to sea by the daily tides. What was once a marvel in the eyes of XIX century visitors, has become an eyesore to contemporary travelers and a source of embarrassment and occasional discomfort to today’s Venetians. Given the importance of these dual roles, one would think that, over the centuries, Venetian authorities would have committed significant resources to the study of these bodies of water. Surprisingly, such was not the case. For example, the hydrodynamic behavior of the canals, which is of considerable importance in the dispersal of the sewage outflows and can also affect boat traffic160, was never really quantified until WPI students carried out their first measurements in the 1990’s. The earliest record of a qualitative investigation dated back to 1900161 and the only limited quantitative campaign was carried out in 1966162. In all, even counting studies of dubious scientific value and studies that were only vaguely referenced in the literature, as of 1990, only about nine publications existed about the topic of the hydrodynamics of the inner canals and none of these were truly scientific, systematic or representative of typical behavior. Even when previous data existed, it was nearly impossible to compare them to what we were measuring, since the location of the past measurements was identified just by a canal name, without any specification of where, along its length, the measurement was taken. This issue was nontrivial since traditional canal names often referred to waterways that didn’t change name through several intersections, just like many roads in American cities don’t change name every time a side street intersects with them. Since water currents are affected by the flows of intersecting canals, not knowing exactly where the measurement was taken vis à vis these intersections meant that there was no reliable way to make an apple-to-apple comparison with later measurements. The problem was that, whereas we knew exactly where we took our measurements, the reference systems of past studies did not allow for the pinpointing of their measurement locations with the same level of accuracy, rendering such comparisons impossible. In order to change this state of affairs and to make possible future comparisons with our data, we devised a system for the unequivocal identification of each tract of canal, through a process of segmentation. The next section explains how such a system was developed.
In order to define the indivisible, fundamental components of the canal network that would make hydrodynamic comparisons feasible, the discriminant was the presence of an intersection. The “atom” was defined as a tract of canal between two intersections and was labeled a “canal segment”. This simple concept was complicated a tad by the fact that in Venice there exist several filled-in canals (called rii terà) that were turned into pedestrian streets. In many cases, these rii terà maintain a subterranean conduit that is overlaid with pavement, thus preserving the hydrodynamic function of the former canals. Therefore, for our segmentation, we had to also consider as intersections the points where rii terà joined regular canals. In fact, since nobody had ever mapped out the canal network on a Geographical Information System (GIS), we were faced with the task of determining the shapes of the canal segments as we identified their boundary intersections. Of course, we could have created “stick canals” or “stick segments” by simply representing the water network as one-dimensional centerlines, as is still frequently done in many GIS applications when dealing with roads163. Back in 1990, working on Mapinfo® for DOS, lines were certainly more appealing than full two-dimensional shapes or regions to represent the canal segments. However, the linear representation was deemed inadequate early on for a variety of reasons. For example, it didn’t allow to tap into the geospatial dimensioning features of GIS, such as the ability to automatically determine surface areas which are needed to calculate water and sediment volumes. Also, the arc representation of canals prevented the measurement of widths, which are crucial for canal navigation. Conversely, shapes do not directly provide canal length information, which lines would provide instantly. What aided our decision was the consideration that, once regions are defined, it would be somewhat easier to derive the respective centerlines than to transform lines into shapes in the opposite direction. But, before we even began the creation of two-dimensional canal regions, we needed to first define the most fundamental units of space that gave shape to the canals, i.e. the islands. Once we had defined the perimeters of all the islands, everything between these islands would belong to the canals layer164. In 1990, when we had started this project, G.I.S. maps were still treated as rare commodities and access to the few existing layers was essentially barred to anyone but a few insiders. Thus, we began to make our own maps by tracing on a digitizing tablet the printed outlines of islands that were published in a marvelous publication that had just been issued at that time, called the Atlante di Venezia165. A few years later, once we obtained electronic maps from the City, we repeated the process and produced a final “official” version of the layers for islands, rii terà, intersections, canals and canal segments, all of which have been adopted in day-to-day use by all departments of the City of Venice and also by Insula S.p.A. and other governmental and non-governmental organizations166. The other fundamental layer needed before segmentation could be applied to the whole network was an intersections layer, representing the connections between two canal segments and also between a canal segment and a rio terà. Thus the second layer that we developed was a rii terà layer. This layer was extracted primarily from existing publications on the topic of rii terà167. These former canals were classified as vaulted or filled, based on the available information, but both types were equally used to determine valid intersections for the intersection layer. Once all of the 125 islands and the 50 rii terà had been defined, it was possible to identify and map out all of the intersections that bracketed all of the segments in the city. Intersections could come in two flavors: one type of intersection occupies physical space and has a definite surface area; another type is a symbolic line that represents a potential location for an intersection, but does not physically occupy space. The latter are generally connections between real canals and filled-in rii terà, but can also represent junctions between small canals and very large ones like the Grand Canal. “Real” intersections, which occupy physical space, occur at intersections between segments and include tracts of water that do not belong to any of the intersecting segments, which were terminated flush with the end of the two islands that formed the sides of the segment. After all 307 intersections were identified and represented, we could define all of the 367 segments in the network. By definition, each segment could only connect to exactly two intersections. Concatenations of two or more segments could then be used to reconstruct as completely as possible the traditional canals that are still the “unit” that is understood and referred to in common parlance by today’s Venetians. Even these traditional canals have been nonetheless “formalized” and must begin and end on an intersection, since they are “molecules” composed of the “atoms” of canal segments. In all, we defined 182 neo-traditional canals with official names, and distinct beginnings and ends. The main lesson we learnt from trying to deal with an unstructured framework of traditional canal names was to divide the territory into the smallest units (or atoms) that made sense at that time. The guiding principles in determining when to stop the breakdown were technical and logical. First of all, we would only be able to keep track of elements that were adequately representable and measurable using current technology. Moreover, we would only divide reality into units that “made sense” in terms of what our purposes were for the practical utilization of our territorial datasets168. The units of analysis that we foresaw using for urban maintenance, management or planning dictated the units of measurement that we set up. I guess the generalizable lesson is that one ought to make the units of measurement as small as technologically feasible and as big as practically appropriate in terms of transaction cost vs. analytical benefit.
In order to make all references to these atomic segments uniquely distinguishable, each segment was assigned an alphanumeric code as the primary identifier. Each of the 182 newly created canals was assigned a fourletter code representing the most significant letters of the prevailing name of the traditional canal whose course most closely approximates the course of the new concatenated one. For example, the canal commonly known as Rio de San Salvador was assigned the code SALV. Once canal codes were assigned, based on the prevailing traditional name, the latter became the “official” name of the canal and was permanently associated with that canal. Many Venetian canals however are known by more than one name, so an arbitrary decision had to be made about which name to assign to the canal as the official appellation. To allow multiple names to be connected to a canal, we created an “alias” list in a database table, so that each canal code could be associated to a variable number of names. Any segment that is part of this newly defined canal would subsequently be assigned a unique identifier according to the syntax XXXXnn, where XXXX is the code of the longer, neo-traditional canal that encompasses that segment, and “nn” is a two-digit consecutive numeric index that sequentially numbers the segments from north to south, starting at 1. So, for instance, if a traditional canal is called Rio dei Scoacamini, the code for the neo-traditional canal that most closely approximates the course of that ancient waterway would be assigned a canal code of SCOA, and the two segments that make up the new canal would be labeled, from north to south, SCOA1 and SCOA2. If a new canal was made up of only one segment, then such segment would not have a numeric suffix, thus the canal code and segment code would be identical, as in the case of the aforementioned SALV. Islands had already been numbered, though not completely, by the City of Venice, so we adopted the existing enumeration, and numerically labeled the remaining islands. In addition, being a fervent admirer of alpha codes, because of their inherently higher information content, I also insisted on a four-letter code for each island. The four letters would represent the most significant letters of the most prominent landmark on that island, be it a church, a palace or even a famous street or square. Thus, each island would have a dual link to outside data, through the pre-existing numeric code and through the new alpha code. For example, the island where Saint Mark’s square is located is doubly labeled with the mnemonic character label MARC, as well as with the more compact, but less identifiable number 92. In spite of my personal bias toward the more explicit character labels, intersections were simply numbered sequentially from northernmost to southernmost, according to the Y coordinate (latitude) of the centroid. The increment was set at 5 units instead of 1 to allow for potential insertions of additional intersections in future years. Thus, the northernmost intersection was numbered 5, the next one to the south 10, then 15, until the 309th intersection that became number 1,545169. This north-to-south ordering implied that the two intersections that bracketed a specific segment would frequently have widely differing intersection numbers, but the northernmost node would always have a lower number than the southernmost one170. Rii terà were not assigned text codes, but were simply numbered sequentially. After their important initial role in the determination of the intersections to use for the segmentation of the network, filled-in canals were not associated with specific datasets until a later date, when I wrote a small article on rii terà for the book on the canals that I co-authored in the year 2000171. On that occasion, the date of the conversion of a canal to a pedestrian street and the government172 that commissioned the work were recorded and a preliminary analysis was conducted to determine what public administration was most culpable for these ante litteram “urban renewal” activities. Aside from the standard codes that uniquely identify each element, the fundamental layers discussed so far – namely the islands, rii terà, intersections, segments and canals – each had additional pieces of information that were permanently embedded into the structure of the layer. First of all, individual objects in most layers are frequently associated with a number of additional reference codes and IDs that are part of the heritage that these objects carry with them. Pre-existing IDs should always be included when possible173, to allow for some retroactive linking to legacy databases that may have been in use up to the present. My personal approach is to limit the amount of data embedded in the layer itself and instead keep all related data in separate external databases. Nevertheless, a modicum of information is permanently affixed to each element in each layer, in addition to all of the necessary identifying codes and labels discussed above. The permanent parameters that may be typically embedded within each element of a layer are: • immutable physical features of the geographical object, such as its surface area or its perimeter174, and/or • topological aspects, such as codes of objects that are connected to the element in question, and/or • categorical classifications, such as typologies (e.g. whether a rio terà is vaulted or not), and/or • information about the object’s creation history (such as what government filled-in the canal, to stay with the above example) or other metadata, where appropriate. The information attached to each object in a layer should be homogeneous and, to be truly useful, it should be as complete as possible across all objects in the layer. The unique coding of each element of each of the fundamental layers had to be done manually, but once this was done, any piece of data subsequently collected on any aspect or phenomenon related to any of these objects could be automatically linked to its location in space, thanks to these spatial units of reference. This spatial framework made it possible for all of the data collected in later years by dozens of WPI teams to be forever referenceable and hence re-usable ad infinitum, as will be shown in detail in later chapters. Once defined and labeled, a typical segment (like the aforementioned SCOA1), could very easily be linked at any later time to additional information such as: physical dimensions, boat traffic counts, hydrodynamic current velocities, the extent of canal wall damage, the quality of the water, estimates of sewage discharge and many other pieces of city knowledge. This is exactly what we did in the rest of the 1990’s. The next several sections describe in some detail many of the varied pieces of knowledge that we collected following the creation of the reference system, showing how we were able to instantly link several datasets to the corresponding segments via the aforementioned standard codes. Alongside the atomization of the spatial objects that compose the city, it is important to assign unique codes to each of these atoms, in order to make further references possible. Once each object has a clear nametag, many disparate sets of attributes can be connected to it by a variety of different organizations or individuals to suit different needs. This simple concept, which has been around for years in the Relational DataBase Management System (RDBMS) community, is not often applied wholesale to all mappable objects that make up the municipal territory. Later on, in Part IV, I suggest ways in which these identifiers can be assigned uniquely and reliably by specific public agencies who have jurisdiction over the “birth” and “death” of these physical objects in reality.
The first pieces of city knowledge that were attached to the newly defined and identified canal segments were related to physical aspects of the segments, specifically length, width, surface area, depth and volumes of both water and sediment in each segment. All of the dimensional parameters except for the depth (and consequently the volumes) were obtained directly from the GIS maps. The measurement of canal depth (also known as bathymetry) took several years from 1990 until 1994, since students were only in Venice for a few months every year. The easiest dimension to calculate for each canal segment was its surface area, since this measure was automatically implicit in the segment’s geographical shape. GIS systems will instantly provide the surface area of an object once it has been spatially defined. Now that segments had a clear beginning and end at an intersection, it was possible, for the first time ever, to truly determine the length of canal segments and of neo-traditional canals in a definitive way. It may seem patently obvious, but one cannot measure the length of a stretch of transportation network – be it a road segment or a canal segment – if there is no clear definition of the extremities of the stretch being measured. Knowing the length of a transportation arc is useful for a myriad of mundane maintenance and management tasks. Contracts and payments are often based, for example, on lengths of road plowed, or miles paved, or – in Venice – on the meters of canal dredged. It is amazing that Venetian authorities had operated for so long without an official listing of canal lengths. Once again, we were the first to produce a fundamental piece of city knowledge – one that will conceivably remain unaltered for ever and will be used for years to come. Determining the length of a canal segment entailed first and foremost the definition of centerlines. These were constructed manually for all canal segments, by drawing lines along the segment’s midpoint from one extremity of the segment to the other, keeping the centerline equidistant from both sides at all times. Intersections received a similar treatment. The centerline of a neo-traditional canal was simply formed by the concatenation of the centerlines of all of its component segments together with the centerlines of all the intervening intersections. Once the centerlines were drawn, segment and canal lengths were computed instantly through standard GIS functions175. The width of a canal is an elusive measure, due to the irregular shape of the Venetian waterways. Even though typical city roads are more regularly shaped, any single measure for the width of a transportation link is often misleading. One can use an average or resort to other statistics, such as the width at the widest and narrowest points, but it will be rarely possible to define a single width as easily as one defines a single length or single surface area. Measuring the narrowest and widest points is also non-trivial since this determination implies the laborious ranking of several measurements. With highly irregular segment shapes, it is even difficult to determine exactly what point on the opposite side to measure to.
Fortunately, the average width of a segment can be computed by simply dividing the surface area by the segment length. Thus, we were able to determine this dimension for all canal segments in Venice. Average widths were very useful for the creation of the geometry of the hydrodynamic model discussed in later sections. In modeling applications, the complex network is reduced to an abstract chain of parallelepipeds characterized by the length, average width and average depth of the segments that they represent. For practical purposes, the longer and the more irregular the network arc, the less useful a single measure like the average width will be. For instance, a cargo boat driver looking at the thematic map of the average canal widths (above) could detect some potentially troublesome canals that one should probably avoid traversing with a wide boat (the red canals in the above map), but there is no guarantee that there are no additional choke-points in the other canals that would still impede navigation. The narrowest width of each segment is extremely useful for practical purposes, and the author’s company (Forma Urbis) has just completed all such measurements for the development of a boat traffic model for the City176.
Measuring the depth of the canal segments was a non-trivial endeavor. First of all, depth is a three-dimensional quantity that is hard to represent succinctly. Ideally, depth could be determined with some sort of a sonar or radar device that would produce a topographical map of the bottom of a canal. Unfortunately, since canals are only about 1-2 meter deep on average – depending on the tide – the error that many commercial electronic sounding devices produce is often greater that the entity being measured, thus making such an automatic approach not feasible. Instead, we resorted to manual soundings of the canal bottom using a weighted tape measure that was reeled down from the surface to the bottom of the canal at intervals of one meter across the canal’s width, starting with a measurement along the northernmost wall and ending with a measurement along the southernmost wall177. To take into account tide fluctuations, we had to normalize all our soundings to the so-called “mareographic zero” of 1897 – a standard reference level for all tide-related measurements in Venice. To do this, we had to monitor tide levels in parallel with our bathymetries, in order to measure the difference of each of our depth soundings from the standard zero. These parallel tide measurements were conducted near the location of the bathymetric sounding by a team member who measured the height of the water from a sidewalk of know altimetry (or elevation). Cross-sectional soundings were repeated at intervals of 25 meters along the length of the segment. In some ways, this could be interpreted as a “sub-atomic” segmentation of the atomic segments. Each section was uniquely identified by the code of the segment in which it occurred plus the distance of the section, in meters, from the northernmost end of the segment along the centerline. This type of sub-segmentation is similar to the so-called “dynamic segmentation” used in “linear referencing systems” to, for instance, pinpoint accident locations on road segments178. The Venetian bathymetric cross-sections prove that whatever unit of space is chosen as the “fundamental atomic particle”, there will invariably be cases in which subatomic divisions will be necessary, therefore all we can really do is to settle for a “reasonable” happy medium and adopt the breakdown that seems most appropriate as the unit of analysis for the majority of users. Whereas we could determine the definitive lengths, surface areas and average widths for all segments, we only obtained reliable depth measurements for 130 of the 367 segments179.
In the end, the bathymetric dataset was represented by 7,768 individual measurements along 850 section lines. We chose not to represent each individual measurement graphically as a dot in its exact location along the section180, though this approach may have been useful to determine navigation paths, as explained below. The tedious complexity of such a task made us opt instead for a sort of recursive dynamic sub-segmentation, achieved by including the section’s identifiers (i.e. the segment code and the distance of the section from the northern end of the segment) with each bathymetric sounding in the database. In the linked database, each depth measurement was labeled by the code of the segment where the measurement was taken (e.g. SALV), plus the distance of the sounding from the northern end (e.g. 25) and the distance of the sounding from the northern wall of the segment (e.g. 4). Similar to widths, depths are also difficult to express uniquely for each segment. Moreover, unlike widths, average depths cannot be easily computed from other GIS-derived dimensions, but require instead extensive field campaigns. As mentioned, almost 8,000 measurements181 had to be conducted – all at night to avoid interference from boat traffic – to quantify canal depths. On average, 55 soundings were performed in each canal segment. The average of all the depth measurements in one segment was used as the overall average depth of the entire segment182. Average depths, like average widths, proved useful for modeling purposes, but not too practical for everyday life. However, there is another useful utilization of the average depth – in conjunction with the surface area – namely for the calculation of sediment volumes, which are used, among other things, for estimating dredging costs. Being the first to systematically determine these dimensions, we were also the first to be able to produce reliable estimates of the volumes of sediment that needed to be removed from each canal segment. Such volume calculations obviously depend on a definition of where the bottom of a segment lies.
Being semi-natural watercourses, canal segments do not have an easily identifiable bottom. For practical purposes, the city of Venice has arbitrarily defined the so-called fondo di progetto (project bottom) of each segment as the desired depth that the segment should be maintained at to allow smooth navigation of emergency boats even during low tides. Such a bottom is generally set between 1.8 and 2 meters below mean sea level183. Once the target bottom depth is defined, sediment thickness is simply derived by subtraction. Complementary to the notion of sediment volume is that of water volume. In a perfectly dredged canal, all of the volume, dictated by the surface area and the desired depth, would be occupied by water. As sediment begins to accumulate, the total capacity of the canal segment is shared by a combination of water and sediment. Water volume is indirectly useful in modeling applications, both for hydrodynamics and for transport of suspended matter in water, as will be discussed later. Depending on the draft of one’s boat and on the tide conditions, one would probably tend to avoid canals with low average depths (red in map above) no matter what. But, once again, there would be no guarantee of being able to navigate the segments with medium average depths (yellow above) or paradoxically even the ones with the highest average depths (green). One shallow point in an otherwise deeper channel would be sufficient to completely impede passage, even though that canal segment may be very deep on average as a whole. Looking at just the data, one may be tempted to identify the shallowest data point as the bottleneck. This would be fallacious, since the absolute shallowest points are almost always along one of the canal walls. Knowing that such points are the shallowest would be useless for all intents and purposes. No boat could possibly travel along the canal walls anyway. In fact, a boater would always attempt to tread a route through the deepest part of the canal, so that, at any moment, the boat would be passing over the deeper point of a particular cross-section, staying well clear of the shallowest. Therefore, what would be truly useful, is to know where these deeper points are across any section. We did precisely that, with a two-step manipulation of the fundamental soundings dataset. First of all, we identified the deepest points of each section, and then we selected the shallowest of these to truly determine the navigability bottlenecks. In our research, we defined what we called a navigability axis as the line connecting the deepest points of each of the bathymetric sections. Thus, even though we could not vouch for the depth of any water between sections, we could determine the point along this navigation axis where the water was shallowest, hence providing a more useful indicator of navigability to local boaters. This “shallowest of the deepest” concept is one demonstration about the difference between data and information. As a further example of the usefulness of dynamic segmentation, one could imagine an intelligent application that could determine, for a boat with a given draft traveling during a given tide phase, the impassable sections in all canal segments. Depending on the exact destination of the boat’s travel, one could even imagine that a system could allow travel part of the way down a segment with a known shallow obstruction, as long as the boat did not have to go through such a point, but was going to dock before reaching the impasse. Although we didn’t use such level of sophistication, the ambulance dispatching application discussed earlier184 could potentially benefit from such dynamic segmentation of the canal network. Unlike the other physical dimensions, the determination of a canal depth is ephemeral in nature. With the passage of time, additional sediment will accumulate and canals will get shallower, rendering our measurements obsolete. Also, as Insula proceeds with its dredging program, canal segments will be deepened all the way to their fondo di progetto and sedimentation will start over from a tabula rasa. The action of human beings and nature combine to make these measurements short-lived, but not necessarily futile. In fact, our data collection jump started the operations of Insula when it was a fledgling company in 1997. Since then, several canals have been dredged and Insula has maintained the information up-to-date by resetting the bathymetries to the fondo di progetto for those segments. More complex is the issue of maintenance of the information for segments subjected to natural sedimentation. One way to monitor the silting process is to conduct periodic measurement campaigns similar to the ones we pioneered. However, this would be a rather expensive proposition. An alternative that we have proposed to Insula, through UNESCO, is the creation of a sedimentation model to simulate the natural processes and estimate sediment levels over time, to plan preventive maintenance on a regular basis. This model is discussed in more detail later. Our data, even though it is obsolete by now, is still useful as a baseline to compare with subsequent measurements. The “growth” of sediment over repeated measurements, using our initial efforts as a baseline, will help determine sedimentation rates and thus inform the development of the sedimentation model. Having set up the geographical framework made up of coded spatial objects, this aspect of our canal studies, despite its simplicity, taught us a few additional lessons. Lesson one was that many of the physical characteristics of realworld objects are easily managed by GIS as long as the representation of the real objects is “literal”, i.e. unfiltered and un-simplified. If we represent a canal segment as a “region”, i.e. a polygon whose shape reflects as accurately as possible the shape of the actual segment, then we can rely on the GIS representation to tell us the basic physical dimensions (like area and perimeter) automatically and for free. Simplifying the canal network into a series of centerline segments may bring some efficiency savings in some arenas (like calculating the segment length and enabling transportation modeling of links), but it also produces a net loss of usefulness in many other areas. The fact is that these two representations are not mutually exclusive. What worked for us was to use the geometric representation that best suited our analytical needs. In general, this lesson suggests to create GIS objects that reflect the true shape of the objects in real life, which means to use “regions” (or polygons, a.k.a. “shapes”) whenever an object with a real surface area is being represented. More specifically, it seems appropriate to represent roadlike elements (like our canal segments) using both a region/shape geometry as well as linear objects (centerlines). Lesson two was that most permanent physical traits can be measured and archived once and for all, as soon as we have settled on an appropriate atomization of the objects. GIS provides us with basic measurements for free, but we first have to define the exact extent of each object using the atomization that best suits our needs185. The choice of how much detail to include and where to draw the line while divvying up reality is not so simple186. In fact, just when we thought we had identified the “indivisible” atoms of the canal network – namely the segments – we immediately ran into “sub-atomic particles” when we had to deal with depth measurements at cross-sections. The bottom line is that we will use whatever segmentation makes sense at the time. Some of these partition decisions may have staying power and prove useful for an overwhelming majority of uses and some may not. But we should not refrain from making these choices lest no progress will ever be made in representing our urban features187. The natural process of sedimentation immediately emphasized the shortcomings of a 2-D GIS platform. Depth is an inherently 3-D measure, so we couldn’t quite visualize it in our Mapinfo system188. This process is also dynamic, so our measurements are ephemeral and bound to become quickly obsolete. Yet, the sedimentation process is gradual and somewhat predictable, so our third lesson was that it makes sense to develop a sedimentation model to predict the silting up that will happen over time. The cost of such a model, if successful, would certainly be cheaper, in the long run, than repeating the bathymetric measurements periodically for ever and ever. 187