Hydraulic engineering is very much applied commonsense. Failure modes are obvious and their control just as obvious. This is not bridge building were you are fighting gravity and natural leverage .
Locking water is a natural response to a steepening gradient and awareness of river arcs a proper response to flood control None of this has changed a bit since the Bronze Age and our first irrigation super states.
This happens to be an an excellent example of a sophisticated response to a real flood risk. We have since dammed the river but that may well be taken down in order to restore the silt inundations and the general fishery. Yet it is also an excellent argument for flood control. Recall that the Three Gorges in China and the Aswan in Egypt and the Hoover were both compelled first by the real need for reliable flood control. Power was a bonus that often hep sell the project..
1450 AD . . . A clever civil engineering project at Kusa
Even today, vast torrents of water rush down the gorges of the Coosawattee River and Talking Rock Creek. These waters drop over 700 feet in a few miles of horizontal flow. Prior to the construction of Carters Dam, floods annually dumped very fertile alluvial soil on Carters Bottom. This is what attracted the establishment of a Kanza Village there around 1325 AD and the establishment of the Kawshe Capital, nearby, around 1375 AD. However, these same floods also regularly did damage to the town. It soon became clear that unless a permanent solution to the flood problem was found, it would be necessary to relocate the Capital.
As the Elite Precinct grew from a few houses around an oval plaza to a town laid out in a gridiron pattern with streets and blocks with courtyards, drainage from even minor rainstorms became an increasing problem. The Elite Precinct sloped upward from the flood plain to a rocky hill . . . where the conquistadors of Hernando de Soto would camp in the summer of 1540. Human waste drained down into the plaza, from where it was applied to courtyard gardens.
Then around 1450 AD, a major flood devastated the housing of the Elite Precinct and portion of the Kanza Village near the river. About all that was left of the royal compound were the eroded hulks of mounds. With the town swept clean of occupied buildings, it was time for a major civil engineering project, which would:
- Raise the ceremonial center of the town above all flood waters.
- Channel the waters of Talking Rock Creek away from the town.
- Channel water, draining down from the residential blocks, away from the ceremonial centers.
The base, created by the dyke, was filled with sand so that any rain waters would be absorbed by the sand then drained away to the opposite ends of the dyke. The top elevation of the dyke was above that of the lowest elevation of the residential neighborhood, so that most storm drainage would also flow to the outer edges of the dyke and down into either the creek or the river. Sand covered the lower portions of the original mounds. The new mounds were built on top of the old ones.
Near the confluence of Talking Rock Creek and the Coosawattee River, the dyke bowed northward in order to encourage the creek’s floodwaters to flow into the cultivation fields on the north side of the river. It then turned southward to provide more cross sectional area for flood waters downstream from the confluence. This showed a great deal of sophistication toward the science of hydrodynamics. Known as the Bernouli Effect, the acceleration of flood waters on a curve would reduce the lateral pressure of deep water on the dyke, while centrifugal force would cause the water to flow northward into the cultivation fields.
The dyke was very soundly constructed. On August 22, 2006, when the waters of the Carters Lake Lower R-regulation Reservoir were drained down to expose the mounds, after 47 years of being submerged, the dyke was still solid. It was about the only place exposed by the drainage on which I could stand without sinking into deep muck!