Comparing Collected Data to 1990s Data

The current velocities collected were compared to current velocities collected in the same canal segments in 1999. All of the data for the 1999 versus 2012 comparison can be found in Appendix F. This comparison is shown below in Figure 50.

The data collected in 2012 overall shows an increase in current velocity compared to 1999. For most canal segments measured, this increase was slight and can be explained by a variety of different reasons. The propeller device used to collect measurements, on average is approximately 7 cm/s faster than the float method used in 1999. This along with human error, are possible explanations for the difference in current velocity and the presence of change cannot be confirmed or denied. However, two canal segments showed a very large current velocity increase, Rio di San Felice (FELI2) and Rio della Sensa (SENS6). These locations are shown as red arrows in Figure 51 below.

0   5   10   15   20   25   30   35   40   45   50  

ALVI   FELI1   FELI2   GIRO   SENS1   SENS6  

Current  Velocity  (cm/s)  

Segment  

Cannaregio  -­‐  Outgoing  Tide  

1999   2012  

Figure 50. Current velocity comparison between 1999 and 2012 in Cannaregio outgoing tide

59 The increase in velocity of these canal segments is too large to be explained by the previously mentioned factors. Segment FELI2, had an increase in current velocity of 17 cm/s. This leads to the conclusion that the segment has increased in velocity. A possible explanation for this change could be a bathymetry change. Although this canal segment has not been dredged between 1999 and 2012, between 1999 and 2005, when the most recent bathymetry data was collected, the canal became 12 cm shallower.

This difference in bathymetry would cause an increase in current velocity but the magnitude of the change is unknown. If the rate of sediment accumulation continued at the same rate, by 2012 the canal segment could be approximately 25 cm shallower. The change in canal depth explains an increase in current velocity; a canal with a higher current velocity would carry more sedimentation and debris and dredge itself. While this change in bathymetry is a possible cause of the current velocity increase, it cannot be identified as the confirmed cause and other factors are likely to have contributed. Another possible cause for this increase involves canals feeding into the segment FELI2. In comparing 1999 to 2012, there was a slight decrease in current velocity in the segment feeding directly into FELI2. The flow of a small network of canals that add to FELI2 from the east are also a possible factor in the velocity increase of FELI2. Further study is needed in this area to gather more information about these canals. See Appendix G for segment identification.

The canal segment SENS6 also had a notable current velocity increase between 1999 and 2012.

Between 1999 and 2005, the depth of this canal increased by 20 cm, which alone would indicate a decrease in current velocity. While the bathymetry change in this canal is not a cause of the current velocity increase, it may be a result of increased velocity, as a faster moving canal is able to displace more sedimentation.

Although the cause is mainly unknown, it appears that change is occurring in some canals in Cannaregio during outgoing tide. Future studies are necessary to draw accurate conclusions.

Cannaregio – 9/11/12

OUTGOING TIDE Speed Increase Speed Decrease

Velocity Comparison: 1999 vs. 2012

SENS6

FELI2

Figure 51. Change in current velocity from 1999 to 2012 in Cannaregio outgoing tide

60 The current velocities collected were also compared to current velocities collected in the same canal segments in 1999 in San Marco. This comparison is shown below in Figure 52.

The overall trend of change in San Marco is a small decrease in current velocity, although the change in most canals is not significant and could be explained by error due to the use of different devices, human error and other factors such as weather. This general trend is more pronounced further south in the San Marco region, as shown below in Figure 53.

OUTGOING TIDE

Speed Increase Speed Decrease Velocity Comparison:

1999 vs. 2012 LUCA1

MAUR 0  

10   20   30   40   50  

ANZO1   BARC2   GARZ   LUCA1   MAUR   MOIS  

Current  Velocity  (cm/s)  

San  Marco  Outgoing  Tides    

1999   2012  

Figure 52. Current velocity comparison between 1999 and 2012 in San Marco outgoing tide

Figure 53. Change in current velocity between 1999 and 2012 in San Marco outgoing tide

61 One canal segment, LUCA1 did not seem to follow the trend of all of the other canals. Rio de San Luca (LUCA1) increased in current velocity by 8.5 cm/s. Between 1999 and 2005, the canal depth increased by 32 cm, which would predict a decrease in velocity. The increase in velocity may then be due to some cause other than bathymetric change, and the increase in depth may have been a result of the increased velocity. Because of the segment LUCA1’s location, the effect of the Grand Canal on its velocity should be studied.

The canal with the largest decrease in speed was MAUR. This canal segment had a depth of 1.3 meters in 2005 compared to 0.98 meters in 1999. This increase in depth is a possible cause for the current velocity decrease.

The current velocities measured in San Marco during incoming tide were compared against incoming tide data collected in 1999 for the same segments (see Figure 54).

The comparison shows both increases and decreases in velocity since 1999. All but one of the measured segments showed a small amount of change (less than 5 cm/s) and should not be considered indicators of overall change in those segments since 1999 due to possible measurement error (see Figure 55). However, the LUCA1 segment (as in the outgoing tide data) showed a more noticeable increase of 8.40 cm/s. This increase during incoming tide is very similar to the increase of 8.49 cm/s found for LUCA1 during outgoing tide. This consistent change since 1999 in both outgoing and incoming tide for LUCA1 further confirms that an increase in speed has increased in this canal segment directly connected to the Grand Canal.

0   10   20   30   40  

ANZO1   GARZ   LUCA1   MAUR   MOIS  

Current  Velocity  (cm/s)  

San  Marco  -­‐  Incoming  Tides  

1999   2012  

Figure 54. Current velocity comparison between 1999 and 202 San Marco incoming tide