The evidence
speaks for itself. The 1870 survey followed by an overlay showing this
survey on today's map. Also shown in the overlay is a measurement line
showing the distance to the bridge from the head of the canal in the 1867 profile. The last image is the 1867 Excavation profile
with landmarks (like the Broad
River Bridge)
noted with their distance from the canal head.
Click here for a large full scan of the entire
canal survey.
Below
is the overlay of the 1870 survey onto today’s map. I lined up the overlay
images with the canal locks of 1870 at the same position of today’s locks
(from 1890). This is not an assumption. The 1870 survey shows Smith’s
Branch to be exactly 400 feet above the 1870 lock. Today, the newer lock is
exactly 400 feet below Smith’s branch. Also, the 1867 survey shows the exact same
distance from the lock to the bridge as the 1870 survey and the 1867 survey
shows a creek about ½ a mile below the bridge. That creek still exists
today and it lines up exactly with the 1867 position thus proving that the
locks of 1867 and 1870 are in the same position as today’s locks. A third
supporting fact is that the overlay of the 1870 survey and 2008 aerial
photo shows that the new canal embankment would cut-off a portion of the
old Bull Sluice canal. Today, when you hike down the canal embankment wood
trail from the diversion dam, a little way down from the steps on the left,
you can see the obvious continuation of the now dry Bull Sluice canal. I
took a GPS reading from this spot and it lines up exactly with where the
1890’s embankment cuts off the old canal on the overlay.
Below
is an overlay of the entire canal. Notice how the Columbia-Greenville rail
(on the east side of the Broad River,
lines up exactly. Same with the Charlotte-Augusta rail on the east side of
the Congaree
River. All the
streets in Columbia
also line up.
Click here for a much larger scale overlay of
the entire canal.
Here is another
very important thing to note about the abutment location on the west side
of the river: I took GPS readings from the top of this abutment and then
plotted this location on an aerial photo using Google Earth. My handheld
GPS is very accurate with Longitude/Latitude (within 5 feet) but not so
good with elevation. That’s where a combination of Google Earth and the Richland County GIS
system is useful. I matched the Google Earth aerial position with the
corresponding position on the GIS system and then took advantage of the
elevation data in the GIS system. The result is that this abutment is at an
elevation of 166’ or 28’ above the normal level of the river. This is 2’
above the height of the abutment at 154 Castle Road but also 28’ above
the normal level of the river at 154
Castle Road (because of the natural fall that
occurs when heading south). From research, I found that during the bridge
building of the 1790’s through the 1800’s, that it was standard to build a
bridge 5 feet above the greatest recorded flood level. This was accepted to
be 24 feet (worst flood + 5’) above the normal level of the river. Below is
an image of the 1825 Design and Requirements document for what would be the
first bridge built in the area of today’s Broad River Bridge.
Again, this specifies a bridge height of 24’ above the normal level of the
water. So why are these abutments 4’ above this specification. You have to
consider the fact that a canal did not exist across from these sites until
the late 1830s when Bull Sluice Canal
was extended down to the Columbia
Canal. Initially, this canal was only for
navigation and it did not carry a significant amount of water compared to
the river. In 1890, however, the canal’s width was increased but a factor
of 10 and the depth was more than doubled. Today, under normal river
conditions, the canal carries twice as much water as the river. The affect
this has on the river height can be seen by viewing the hourly historic
data maintained at multiple sites along the river. This data includes the
water’s height as well as the amount of water flow. Since we really want
quantity of water, the best we can do is look at the river data when a
large amount of water is flowing and then find a time when half as much of
this amount is flowing. Interestingly, the numbers are the same for several
locations on the Broad River as well as a
location on the upper Congaree. Twice as much water translates into an
increase in water height of 4 feet! This new location for the Confederate
bridge and Compty’s 1796 bridge, exactly meet the 1825 height
specification.
Below
are the GPS plot of the abutment remains and the 1825 Design/Requirements
document.
Below
is the 1867 Canal Excavation Survey. Notice the Broad River Bridge
landmark noted at 37 chains.
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