GC4GQ8P Hickman Bluff Project (Earthcache) in Kentucky, United States created by trailhound1

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A cache by trailhound1 Message this owner

Hidden : 7/20/2013

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Geocache Description:

After a century of instability, erosion of Hickman Bluff caused the closing of Magnolia Street and reached the point of threatening several public structures, including the town's water supply. When initial efforts to stabilize the bluff proved unsuccessful, city officials requested that the federal government intervene. Between 1996 and 2000, the U.S. Army Corps of Engineers spent more than $17 million in federal funding to stabilize the bluffs at which you stand.

This Earthcache will educate you on the geology of the Hickman Bluff and surrounding area, types of bluff erosion, and different methods to stabilize landslides.

Figure 1 - Hickman Bluff Marker Figure 2 - Bayou de Chien and old river channel

TASKS
A. Select the "trailhound1" link at the top of the page and send the answers to the following questions. Please do not include the answers in your post log.
1) Based on your observations, whattype of Mass Movement occurred at this location? What evidence did you use to make your decision?
2) Based on your observations (and not only from the marker (there are others present), what methods were used to stabilize the bluff?
3) Estimate the height of the Hickman Bluff at this location.

B. Optional (but greatly appreciated): Take a photo of you (and your group) at the railing overlooking the bluff.

GEOLOGY
This site lies in the central Mississippi Valley, primarily in Kentucky and Tennessee, and includes 192 mi of bluffs that form the eastern edge of the Mississippi alluvial plain from Barlow, Kentucky, 7 mi east of Cairo, Illinois, to Walls, Miss., 12 mi south of Memphis, Tennessee. The area lies in a northern coastal plain of a more extensive Cretaceous and Tertiary Gulf of Mexico (or Mississippi) Embayment (145 to 33.9 million years ago), and roughly parallels the Mississippi River.

Figure 3 - Area of Bluff and Mississippi Embayment

The average height of the bluffs in the study area is 120 ft, though at some localities they are as high as 225 ft. The slope angle of the bluff face ranges from only a few degrees to vertical in areas where the Mississippi River has undercut the bluffs. The slope angle throughout most of the area is 15-25°. The bluffs are composed of eroded loess over glacial gravel that is slide prone.

The top strata of Pleistocene (aabout 2,588,000 to 11,700 years ago) loess is the result of three forces: (1) the grinding action of glacers pulverizing rocks intoflour-like silt, (2) glacial melt water moving the silt onto floodplans, and (3) wind carrying the loess to the bluff area and beyond. The loess strata ranges from 10 to 150 feet thick. The Lafayette Formation is composed of as much as 65 ft of Pliocene (5.332 million to 2.588million years ago) terrace gravels and sands that are easily saturated with ground water. Beneath lies the Eocene (56 to 33.9 million years ago) Jackson Formation, which generally consists of discontinuous layers of shallow-marine embayment deposits of clay and silt, that range in thickness from a few inches to tens of feet. The two lower layers are mostly uncemeted, hence, easily erode.

BLUFF EROSION or MASS MOVEMENT
Loss of a slope's surface soil layers can be caused by wind, water, or ice. Erosion can occur in the form of rills and gullies, in the case of flowing water, seepage and/or frost wedging from groundwater flow, and wave cutting. When a downward movement of a relatively intact mass of slope material occurs, it is called mass movement (instead of erosion). Mass movement, or Landslides can take the form of slides, earth/debris flows and slumps, and rock falls/earth topples.

Rock Fall
Falls are abrupt, downward movements of rock or earth, or both, that detach from steep slopes or cliffs.

Figure 5 - Rock Fall

Topple
A topple is recognized as the forward rotation out of a slope of a mass of soil or rock sometimes driven by gravity exerted by the weight of material upslope from the displaced mass. Sometimes toppling is due to water or ice in cracks in the mass.

Figure 6 - Topple

Slide
A slide is a downslope movement of a soil or rock mass occurring on surfaces of rupture or on relatively thin zones of intense shear strain. Movement does not initially occur simultaneously over the whole of what eventually becomes the surface of rupture; the volume of displacing material enlarges from an area of local failure.

Rotational Landslide - A landslide on which the surface of rupture is curved upward (spoon-shaped) and the slide movement is more or less rotational about an axis that is parallel to the contour of the slope.

Figure 7 - Rotational Slide

Translational (Sheet) Landslide - The mass moves out, or down and outward, along a relatively planar surface with little rotational movement or backward tilting. This type of slide may progress over considerable distances if the surface of rupture is sufficiently inclined.

Figure 8 - Translational Slide

Spreads
Spreads usually occur on very gentle slopes or essentially flat terrain, especially where a stronger upper layer of rock or soil undergoes extension and moves above an underlying softer, weaker layer.

Figure 9 - Spreads

Flows
We will only focus on flows common to this area. Lahars (volcanic) and Permafrost flows will not be discussed for obvious reasons.

Debris Flows - A form of rapid mass movement in which loose soil, rock and sometimes organic matter combine with water to form a slurry that flows downslope. They have been informally and inappropriately called “mudslides.”

Figure 10 - Debris Flow

Debris Avalanche - Debris avalanches are essentially large, extremely rapid, often open-slope flows formed when an unstable slope collapses and the resulting fragmented debris is rapidly transported away from the slope.

Figure 11- Debris Avalanche

Earthflow- Earthflows can occur on gentle to moderate slopes, generally in fine-grained soil, commonly clay or silt, but also in very weathered, clay-bearing bedrock. The mass in an earthflow moves as a plastic or viscous flow with strong internal deformation.

Figure 12 - Earthflow

Creep - Creep is the informal name for a slow earthflow and consists of the imperceptibly slow, steady downward movement of slope-forming soil or rock. Movement is caused by internal shear stress.

Figure 13- Creep

METHODS OF STABILIZATION
Planting Vegertation can help stabilize slightly or moderately eroding bluffs. Vegetation tends to remove groundwater, strengthen soil woth roots, and lessens impact of heavy rain on bluff face.

Backfilling with lightweight material - related to height reduction is to excavate the upper soil and replace it with a lightweight backfill material such as woodchips or logging slash.

Benching - a series of “steps” cut into a deep soil or rock face for the purpose of reducing the driving forces.

Slope Strengthening
Reinforcement mesh of plastic polymer stretched to form a ightweight, high-tensile-strength grid. The grid acts similarly to reinforcing mesh in concrete, adding strength to the shear strength of the soil.
A berm or buttress of earthfill can be easily dumped onto the toe (bottom) of a slope.
Check dams are small, sediment-storage dams built in the channels of steep gullies to stabilize the channel bed.

Improve Drainage
Horizontal drainpipe are used to elevate pressures caused by saturating groundwater.
Site-leveling or smoothing the topography of the slide surface can prevent surface water from ponding or connecting with the ground water.
Surface drainage can be through either surface ditches or shallow subsurface drains to redirect run-off water and to prevent the build up of groundwater.

Retaining walls to prevent further erosion of upper layers. Includes timber cribs, steel or reinforced earthen walls, gabion walls, or large diameter piling.

Shotcrete and Gunite are types of concrete that are applied by air jet directly onto the surface of an unstable rock face.

Anchors, Bolts, and Dowels are tools composed of steel rods or cables that reinforce and tie together a rock face to improve its stability.

Figure 14 - Methods of Stabilization

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Hickman Bluff Project EarthCache

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