Earth Sciences 80A - Lecture 23

Human Induced "Geological" Catastrophes

So, we are destroying the ozone layer in order to keep our beer cold, burning up the oxygen-providing forests of the world to make room for new babies, and prompting extinctions at a rate rivaling a bolide impact: what else are humans doing to add to the catastrophe story?

It is important to note that human activity has achieved the capability for doing things on a larger scale and faster than many prior natural processes. The outcome of this is very hard to gauge, but as we have seen throughout this class there are delicate balances in the Earth system that are self-regulating under most circumstances, but which may be overwhelmed if driven past a natural threshold. Let us consider a few of the ways in which human activity accelerates natural phenomena:

1. Landslides. Recall that hillslopes and vegetation play a critical role in defining slope stability. Human activity can remove trees and ground cover in very short time, reducing the soil cohesion and friction associated with roots. Humans also love to control the flow of water, and change the balance of infiltration and runoff both intentionally and unintentionally with surface paving, water channeling and other activities. We also love to modify hillslope geometry both at the crest of hills (where the views are best) and at the toe of hills, where water is nearby. This can activate slope failures directly.
2. Land Subsidence. Our craving for resources drives massive extraction of fluids from underground at orders of magnitude faster rates than it can be replenished. This includes pumping of oil and water, as well as excavating of salt, coal, minerals. Extraction of oil has caused the San Diego harbor to drop, forcing the construction of large dikes. There is extensive subsidence in Los Angeles as well due to oil extraction. The Great Valley of California has dropped as much as 50 feet in some locations due to water extraction. This has undermined the foundations of many structures in the valley, but even worse the process is not reversible. The ground water pores tend to collapse when so much water was present and is removed, compacting the ground so that it can't be renewed. Similar water extraction has collapsed much of the Okalala aquifer in Kansas, Oklahoma and the Texas panhandle. Water pumping is causing salt water intrusion on many coastal margins, including along the Monterey Bay. It is not known how long it takes to reverse such trends.
3. Sedimentation/Erosion Patterns. These are extensively modified by human construction, land terracing, road building, deforestation. Paving greatly increases runoff power, leading to much greater erosion/sedimentation than was typical in the environment before hand. The infiltration rate is also modified, diminishing the ground water recharge process.
4. Dam Failures. Human control of surface water has led to the creation of lakes that are even more fragile than natural lakes (and we have seen the floods that can occur for natural dam failures). There is great exposure to threat of flooding in many regions around the world due to human dams which may fail either due to engineering failure (erosion, destabilization) or as a result of unusual rainfall, earthquakes, etc..

   Let us consider the worst flood ever to strike the U.S., in terms of death toll. This was the Johnstown Flood which happened in Pennsylvania on May 31, 1889. This involved a dam that was built in 1853, as part of the Pennsylvania canal. It was an earthen dam 70 feet high. Due to financial difficulties it was sold in 1857 to the South Fork Hunting and Fishing Club, which installed a grating to control fish population. Predictably, the grating began to clog with logs and leaves, leading to a rise in the lake. This prompted repeated warnings of the potential for a flood if the dam were to give way. Several decades later, in 1889 there were very heavy Spring rains over a two month period, compounding a high snow melt from a 14 inch snowfall in April. From May 30-31 there was a huge rainstorm. At 3:15 a.m. on May 31 the lake overtopped the dam, and began rapid erosion of the earthfill. The erosion was rapid, leading to a 30-40 foot high wave that flooded down the valley 15 miles to the town of Johnstown. There were 2200 people drowned in the flashflood, and the event has been recounted in many folk songs.

5. Agricultural Disasters. Human land-use activities can prompt erosion, dust storms, and changes in microclimate which can then in-turn prompt crop failures and famine. We'll talk about the greatest of such disasters later, The Dust Bowl.
6. Earthquake Triggering. We have even learned how to cause some earthquakes, by processes such as building dams, pumping fluids out of or into the ground, and by detonating large explosions.

In almost all of these cases there is a cumulative effect, where relatively small changes add up and compound each other. Individual human activities are seldom the driver, it is the cumulative effects of entire communities (housing developments, well systems, massive pumping efforts, etc.).

Granted, in most instances, the human intent was not to induce the associated geological catastrophe. So, what are the underlying causes of most of the negative effects?

1. Poor construction. This is a major factor in dam failures for example. The societal response is largely to blame the contractor, and thus liability drives improved practices. However, in many countries there are not many legal constraints on government projects, and in many the resources are so scarce that building compromises are made of perceived necessity.
2. Poor design. Errors in dam and road design are often responsible for erosional failures. The societal response is to hold the engineer liable.
3. Poor Understanding. Flawed understanding of ground water and hydrology effects, meteorology and potential for periodic high water levels from natural phenomena, ecological factors, and geological effects often lie at the heart of poor decision making. This goes back over centuries, but even when there has been improved understanding of some effects, the information has not been exploited. An example is the frustration of John Wesley Powell, a geologist active in the 1800's, who became the first director of the U.S. Geological Survey. He made many recommendations on how best to settle and develop the West, including land partitioning according to natural watersheds and drainage basins, but he was regularly ignored as square patches of land were surveyed out, often leading to great difficulties in accessing water. There is increasing emphasis on multi-disciplinary assessment of hazards due to human activities (EPA Environmental Impact Statements for example).
4. Poor Education of Decision Makers. Even if technical understanding is attained at the developer level, there is often poor, or even rash decision making by politicians and the voting public. The lack of understanding of probabilistic effects of geological phenomena is a major factor, and there is usually inadequate quantification of cost/benefit criteria for major decisions.

Let's consider a bleak case history or two:

The Dust Bowl. We often hear of bad crop harvest years and concerns about global warming during the past decade, but land-use practices caused a far more dramatic effect in the 1930's (and again in the 1950's) that leads to the designation: The Dirty Thirties.

First, the ecological story. The Great Plains of the central U.S. were extensive regions of short grass prairie, grazed by buffalo herds, and plowed by billions of prairie dogs in extensive 'cities'. The grasses tended to be spotty, making clumps of fragile contained sands and soil (sod). Removal of the grass allows rapid wind erosion. The region was arid, with vast sand dunes in Nebraska and downwind loess deposits which make up much of the rich soil of the mid-west, and both the buffalo and the dependent Indians were nomads, which kept providing self-regenerating opportunities for the ground.

In the mid- to late- 1800's there was a massive influx of white settlers. In 1846 the Mormons moved through to found Salt Lake City. In the 1860s-1870s the Okies followed the example of the Mormons, operating under the mistaken mythology that "rain follows the plow", and they began to occupy the prairie lands, attempting to grow long grass crops such as corn. Corn proved non-viable, and the short grass wheat began to be developed over Colorado, Kansas, Nebraska, Texas Panhandle, Oklahoma, North and South Dakota. This settlement and agricultural challenge is immortalized in the works of Willa Cather, Ole Rolvaag (Giants of the Earth) and others. New technologies (tractors/plows) allowed extensive cultivation of what was a rather arid region, with irrigation allowing large areas to be used repeatedly. Plowing broke up the sod of previous short-grasses.

In the post-Great War agricultural depression of the 1919-1921 interval the agricultural practices were stressed, and crop rotation was not practices. In the 1930's the region underwent a natural fluctuation in rainfall, resulting in a drought that persisted for about a decade. Only Maine and Vermont were spared the most intense drought from 1930-1936. Dust began to blow as the arid soil eroded, and diseases were born in the dust, including spores found in creosote (Valley Fever). This became the period of great dust storms and severe topsoil erosion that devastated the agricultural belt of the U.S.. This was the time of the Grapes of Wrath.

Much of the wind erosion was concentrated just east of the Rockies, where jet stream winds drive cold fronts down into the plains. The most severe areas of wind erosion were in eastern Colorado, western Kansas, the Oklahoma and Texas panhandles, and northeastern New Mexico. Intense topsoil erosion occurred from 1935-1940. The wind storms entrained dust 1-4 km high, into storms that were called Black Rollers, Dusters and Black Blizzards.

The number of storms during which visibility dropped to less than a mile was from 20-60 each year from 1933-1941. The blackest year was 1935, when the Dust Bowl became full blown. The town of Amarillo endured 908 hours of such low visibility in 1935, with zero visibility occurring 7 times during January to March. The most intense interval was April 14, 1935, called Black Sunday, where vast areas were opaque.

The effects were extensive:

Soil Erosion - Deep grooves and gullies were excavated, and the busted-sod areas were denuded.

Crop Failures - Both the drought and the dust storms caused wheat shortages.

Animal Deaths - Poultry and cattle died in dust and sand storms and from disease.

Tumbleweeds (Russian Thistle and others) were extensive and piled high on structures.

Disease - Dust Fever was prevalent, with many respiratory ailments. Some people died by suffocation in the dust.

Insect 'Plagues'- Mormon Crickets ran amuck

Dunes - Mobilized sands created local dunes in many areas that had been cultivated. Trains derailed by the sand piles.

Exodus: The Okies suffered, with crop loans failing again and again, and many became Exodusters, moving to California to survive.

In fact, similar wind erosion damage occurred in the mid-1950's, and again in the mid-1970s. This indicates that intermittent climatic fluctuations are to be expected, and we must mitigate against them. One approach was the establishment of a shelterbelt plan, with extensive plantings along the 100th meridian from North Dakota to Texas. The idea was to have treelines that would break up winds, inhibit dust and sand transport, and would help to retain topsoil. The Soil Conservation Service was founded to help regulate land-use. Government subsidies were established for farmers to lay fallow lands on alternating basis. This was too offset economic incentives to fully cultivate every year. New contour plowing methods were developed (circular fields) which would inhibit wind erosion. National grassland areas were designated to preserve sod and stabilize sands. The prognosis is still uncertain given that there are periodic fluctuations in the rainfall that will strike the area with drought again and again. Groundwater use is drawing down the watertable throughout the region. And there is always global warming to compound things...

The U.S. problem is not unique, and vast areas of drought prone lands exist elsewhere that are being influenced by human activity. Among these the most severe source of human misery has been the Sahel region south of the Sahara in Africa. This region fluctuates in size due to climatic cycles, but straddles the continent from west to east, cutting through the Sudan, Ethiopia, and Somalia, all countries stricken by horrible droughts, famine, and dust storms in the past decade.

We'll finish up with a few landslides induced by human activity, both of which should have been preventable. One was the 1966 Aberfan, Wales disaster. In this coal-mining region, piles of excavated debris from the coal mine were piled on a hillside above the coal seam. The piles, or tips, had been there for more than 20 years, on top of the Brithdir Sandstone, a highly porous rock layer. The sandstone has many springs coming out of it, and several of the tips were placed above springs, which had led to destabilization of the piles in 1944 and 1963, with non-damaging slides. In almost the same area as the 1963 minor slide, there was a large landslide in 1966 which swept down the slope, destroying a farm and the Pantglas Junior School, and sweeping into the Aberfan Village. The callous disregard for the geological conditions (springs feeding out of the sandstone) and the past experience (previous small slides) were major factors in the loss of life.

The largest dam disaster in the world was also a case of ignorance and inattention to geology. This was the Vaiont Reservoir, Italy disaster of October 9, 1963. In this case the dam itself did not fail, but 2600 lives were lost. The dam was built on the Piave river in 1959-1960, with this being the worlds second highest dam. An elongate reservoir formed behind the dam. At 10:40 at night, a 1.8 x 1.6 km area of the slope on the south side of the reservoir slipped down the hillside at large speed (25-30 m/s), filling the western end of the reservoir with 150 m of rock over a section 1.8 km long with 30-60 s. This created a huge wave, which overtopped the dam by 100m, and swept down the valley causing the fatalities. The slip surface of the slide was along weak bedding planes in the dipping limestone valley (a syncline, or U-shaped limestone layer with clayey interbeds. The upper 100-150 m of the limestone were unstable due to active unloading of the surface due to the river cutting down into the region to begin with. Ancient slide deposits were present in the area, but the massive slide was in part the result of weakening of the rock with time by the ground water recharge prompted by the impounding of the reservoir. The limestone was very porous, with caverns in it (Karst), which allowed the groundwater to penetrate. The lake levels had been swollen by heavy rains in August and September as well. Simply having done geological studies would have exposed the danger of developing a reservoir in this location.

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