On a warm summer evening, the earth gave way and half of a beautiful resort home fell into the lake:
Was it caused by a geological fault? Were other homeowners in danger?
The event also caused a large, deep crack at the surface, north of the house, close to another residence.
The crack continues to the left and is visible downward (i.e. left) face of the cliff. At the top right, some of house's foundation remains. The remains of the house were set on fire. What was left after that was pushed into the lake. Note the debris accumulated at the water's edge, on an erosion-resistant limestone shelf.
We acquired, processed, and analyzed high-resolution satellite data, historical air photos, maps, government data, peer-reviewed articles, and conducted field work and interviews.
Our analysis revealed:
- Thin, unstable topsoil at the surface.
- A thick layer of clay below the topsoil. It is rated low-strength, it swells up when it gets wet—and slick—and shrinks when it dries out again.
- Below that, shale.
- Then a massive, dense, strong and thick limestone.
- Under that, an interbedded limestone-shale member.
- Below that are more limestone strata interbedded with thin shales.
- ...these limestone/shale members are very prone to erosion.
The River's Role:
- We found an old airphoto of the river (below) before the lake was impounded: in previous eras, the river was much stronger, larger and higher, carrying water volumes five to nine times greater than it does today.
- This action created the steep, hundred-foot high cliffs.
- The river makes an extreme (324º) turn, buttressed at the cliffs it created in earlier times.
These days, the river continues its active, strong, and persistent flow whether the lake level is high or low: its turbidity and wave action nudges away the cementing properties that holds together the horizontally-oriented rocks—in particular the interbedded limestone and shale—that make up the cliffs. In time, larger and larger rocks are shed. Support for the rocks and the soil above is undermined.
This is how the river looked a few years before it was covered by the lake. The eventual location of the house is shown in red, on the cliff (which is the prominent white, linear feature that extends from the lower right edge of the photo to the top middle).
U.S. Geological Survey map shows the river channel (dashed line), today's normal lake level (blue), the "full" level (light blue) and the house location (in red) on the cliff face.
Lake water levels fluctuated 61' over the years. The water level shown in the above picture is 521' ASL. The highest lake level has been 570.25’ ASL.
The record of lake level changes.
What The Lake Did:
- Over decades, high lake water levels saturated the soils, rocks, clays, and shales. When the waters receded, the cliff face would dry out. This cycle repeated many times (see above). When this happened, the clay strata swelled up when wet and shrunk when dry. Between rock strata, natural cementation weakened and resulted in more pronounced erosional effects.
- At every water level, natural turbidity of the river channel, wind, and wave action from boats operated to further erode the cliff.
- At low water levels, the west-facing cliff was exposed to hot sun and eroded by prevailing south and southwesterly winds.
- The "layer cake" geologic composition of the cliff was eroded to the point that the cliff became cantilevered and therefore weakened.
- Using medium and higher-resolution satellite data and airphotos, we found no evidence of geologic faults or fractures. Neither was there evidence of Earth shifting nearby, e.g. cracks in street/driveway pavements, in the foundations of above-ground structures, cracks in brick and mortar, pool leaks, visible on the surface nor in outcropping rock units below.
- Downward and toward-the-lake pressure on the bluffs was brought on by several heavy elements: the weight of the new house, concrete patio and steps, topsoil added, a very substantial concrete garden retaining wall (which was long, heavy, 5–10 feet tall, and near the cliff face) into which more soil was added and irrigated.
- Normal rainfall there is 36 inches per year: lawn and garden practices can add another 20 inches.
- Meteoric groundwater runoff increases in volume and force as it travels downhill, toward the house and the cliff.
- Impervious pavements and rooftops from houses at higher elevations, to the east, increased runoff volumes and speed.
- These manmade, impervious materials occupy areas which used to be water-absorbing vegetation and soils.
- These factors do not indicate a pre-existing natural fault so much as a "made-made" tension crack.
- Nearby homes exhibited no tension crack/fracture problems.
- Houses should be located back away from any cantilevered limestone strata and placed on piers to solid rock.
The remaining part of the house in place above the cantilevered cliff face. The tension crack is visible in the extreme lower right.
Four years after the initial cliff failure, the northern part of the cliff fell during a five inch rain event with wind speeds of 47 mph.
"Before" shows the cliff before it failed. The house, retaining wall and garden are intact. "During" shows the south half of the cliff gone and the tension crack as it trends NNW; the "After" image is four years later, after the northern half of the cliff fell off into the water, unattended by a seismic event.
David G. Koger
is a Remote Sensing Image Analysis expert with over 40 years of experience in the field. A Vietnam Era Marine Corps veteran, he configured and operates a state-of-the-art remote sensing image analysis and geographic information system and consults on various remote sensing applications. Mr. Koger offers Forensic Digital Image Analysis of remotely sensed, aerial photo (digital or film), and underwater video. His services include forensics, time-series studies, and documentation of surface damages, wildfires, material movements. Expert witness strategies and research methodologies are available to attorneys representing plaintiff and defendant.
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