Most shocking Vicksburg research discovery

[tony_gunter]

The river really wasn't that high in 1863. Probably the lowest peak river stage in all five years of the war.

When Grant says he's waiting for the river to fall another foot before marching south of the city, he's probably worried about the tops of the transports passing below the level to which the guns can be depressed.

 

[Josh The Lighthouse Guy]

The river really wasn't that high in 1863. Probably the lowest peak river stage in all five years of the war.

Which might have been the biggest reason for the failure of the canal.

 

[tony_gunter]

Which might have been the biggest reason for the failure of the canal.

I think the canal failed because the ground was super-saturated and the earthen dam keeping the river out began piping water.

Failure of the dam ended work on the canal by everyone except the dredge operators.

 

[gary]

Didn't rising water level caused the levee to break?

 

[tony_gunter]

Didn't rising water level caused the levee to break?

That's the story.

 

[tony_gunter]

Didn't rising water level caused the levee to break?

Long story short, it's an Edenville Dam (Michigan, 2020) situation but a little more complicated. At Edemville, the sandy soil liquefied, with the Mississippi River clays I think the soil began to experience piping.

Heavy rains filled the area behind the dam and super-saturated the soil. Pore water pressure buildup probably caused piping which led to internal erosion and eventual catastrophic failure.

Feb 15, 16, 17 rain for three days straight, Feb 26th "it rained as hard all day as I have ever seen." Some workers reported working in water waist deep even before the dam failure.

Meanwhile, the river rose barely a foot and a half and peaked 13.5 feet below the level reported in 1862.

 

[Josh The Lighthouse Guy]

Feb 15, 16, 17 rain for three days straight, Feb 26th "it rained as hard all day as I have ever seen." Some workers reported working in water waist deep even before the dam failure.

Meanwhile, the river rose barely a foot and a half and peaked 13.5 feet below the level reported in 1862.

So locally heavy rain, but the larger river system remained in the dry season.

 

[tony_gunter]

So locally heavy rain, but the larger river system remained in the dry season.

Extreme rain, levees are designed to hold water in. Holding water out, not so much. That's why newspapers are full of stories of crevasses in 1863, but only one newsworthy crevasse during the great flood of 1862 at Bonnet Carre.

I wouldn't say they were in a dry season. The entire Mississippi watershed was unseasonably warm that winter, so you had a long winter / spring / summer of rather average peak water when normally you have a very pronounced peak from spring snowmelt.

 

[tony_gunter]

AI explains:

Key Mechanisms in Rainfall-Induced Clay Dam Failures
Based on analysis of similar failures (like the 2018 Xe Nammoy dam in Laos or the 2020-2022 dam studied in the Mekong basin), the process often involves:

1. Saturated Clay Core: Heavy, sustained rain saturates the clay embankment, reducing its permeability and increasing pore pressure.
2. Effective Stress Reduction: As pore pressure rises, the "effective stress" between soil particles drops. When the pore pressure equals the load, the soil loses all shear strength.
3. Tension Cracks: Wetting and drying cycles cause tension cracks in clay dams, which then fill with water during heavy rain, creating localized high pressure that initiates piping or slopes sliding.
4. Slope Instability: Increased weight and reduced strength lead to rotational failures (slides) on the downstream slope.