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Benjamin A. Morgan

Publications and source records attributed to Benjamin A. Morgan.

9 recordsLinked to original sources
USGS2009

An Examination of Selected Historical Rainfall-Induced Debris-Flow Events within the Central and Southern Appalachian Mountains of the Eastern United States

Generally, every several years, heavy amounts of rainfall trigger a large number of debris flows within the central and southern Appalachian Mountains of the Eastern United States. These types of landslides damage buildings, disrupt infrastructure, and occasionally injure and kill people. One of the first large debris flows was described in Pennsylvania in August 1779. The most destructive event occurred during August 19-20, 1969, in Nelson County, Va. During a period of 8 hours, 710 to 800 milimeters of rain triggered more than 3,000 landslides, killing more than 150 people. As the population increases in this region, future storms will likely increase the risks of property damage and loss of life. We provide a general overview of debris flows in the Appalachians, using a compilation of 19 storm events for which rainfall, duration of the storm, and descriptions of the resulting landslides have been substantially documented.

Open-File Report↗
USGS2008

Debris-Flow Hazards within the Appalachian Mountains of the Eastern United States

Tropical storms, including hurricanes, often inflict major damage to property and disrupt the lives of people living in coastal areas of the Eastern United States. These storms also are capable of generating catastrophic landslides within the steep slopes of the Appalachian Mountains. Heavy rainfall from hurricanes, cloudbursts, and thunderstorms can generate rapidly moving debris flows that are among the most dangerous and damaging type of landslides. This fact sheet explores the nature and occurrence of debris flows in the central and southern Appalachian Mountains, which extend from central Pennsylvania to northern Alabama.

Fact Sheet↗
USGS2003

Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains.

Virginia↗
USGS2002

Natural hazards on alluvial fans: The Venezuela debris flow and flash flood disaster

In December 1999, rainstorms induced thousands of landslides along the Cordillera de la Costa, Vargas, northern Venezuela. Rainfall on December 2-3 totaled 200 millimeters (8 inches) and was followed by a major storm (911 millimeters, or 36 inches) on December 14 through 16. Debris flows and flash floods on alluvial fans inundated coastal communities, caused severe property destruction, and resulted in a death toll estimated at 19,000 people. Because most of the coastal zone in Vargas consists of steep mountain fronts that rise abruptly from the Caribbean Sea, the alluvial fans are the only areas where slopes are not too steep to build. Rebuilding and reoccupation of these areas requires careful determination of potential hazard zones to avoid future loss of life and property.

Fact Sheet↗
USGS2002

Debris-flow and flooding deposits in coastal Venezuela associated with the storm of December 14-16, 1999

Heavy rainfall from the storm of December 14?16, 1999, triggered thousands of shallow landslides on steep slopes of the Sierra de Avila north of Caracas, Venezuela, and caused flooding and massive debris flows in the channels of major drainages that severely damaged coastal communities along the Caribbean Sea. Within this region we characterized geologic conditions where landslides initiated on hillsides and examined the texture of debris-flow deposits in the channels of nine drainages. In one of the most severely damaged areas on a highly developed alluvial fan at Caraballeda, we measured debris-flow deposits that ranged up to 5 meters (m) in thickness, inundating structures and roads over a large portion of the fan. Boulders up to 5 m long were carried along by the flows, impacted structures causing serious damage, and were deposited on the fan. Using field measurements and comparing pre-event and post-event topography from aerial photographs, we determined the volume of debris-flow and flood deposition on the fan to be about 2 million cubic meters. The total volume of material transported and deposited by landslides throughout the Vargas region ranks this as one of the most severe historical erosional events worldwide.

IMAP↗
USGS2001

A comparative analysis of hazard models for predicting debris flows in Madison County, Virginia

During the rainstorm of June 27, 1995, roughly 330-750 mm of rain fell within a sixteen-hour period, initiating floods and over 600 debris flows in a small area (130 km2) of Madison County, Virginia. Field studies showed that the majority (70%) of these debris flows initiated with a thickness of 0.5 to 3.0 m in colluvium on slopes from 17 o to 41 o (Wieczorek et al., 2000). This paper evaluated and compared the approaches of SINMAP, LISA, and Iverson's (2000) transient response model for slope stability analysis by applying each model to the landslide data from Madison County. Of these three stability models, only Iverson's transient response model evaluated stability conditions as a function of time and depth. Iverson?s model would be the preferred method of the three models to evaluate landslide hazards on a regional scale in areas prone to rain-induced landslides as it considers both the transient and spatial response of pore pressure in its calculation of slope stability. The stability calculation used in SINMAP and LISA is similar and utilizes probability distribution functions for certain parameters. Unlike SINMAP that only considers soil cohesion, internal friction angle and rainfall-rate distributions, LISA allows the use of distributed data for all parameters, so it is the preferred model to evaluate slope stability over SINMAP. Results from all three models suggested similar soil and hydrologic properties for triggering the landslides that occurred during the 1995 storm in Madison County, Virginia. The colluvium probably had cohesion of less than 2KPa. The root-soil system is above the failure plane and consequently root strength and tree surcharge had negligible effect on slope stability. The result that the final location of the water table was near the ground surface is supported by the water budget analysis of the rainstorm conducted by Smith et al. (1996).

Virginia↗
USGS1998

Surficial geology of Shaver Hollow, Shenandoah National Park

At the request of Shenandoah National Park and the Department of Environmental Sciences at the University of Virginia, the US Geological Survey has completed an examination and map of the surficial deposits in Shaver Hollow. The work was carried out as part of the US Geological Survey - National Park Service cooperative agreement implemented in 1994. Shaver Hollow is a small, well defined drainage basin on the west slope of the Blue Ridge about 6.5 miles south of Thornton Gap and can be reached by trail from mile 37.9 on the Skyline Drive. The hollow is drained by the North Fork of Dry Run, and the watershed within the Shenandoah National park is only 2 square miles in area. The area has been the site of extensive investigations by faculty and students at the University of Virginia and by NPS scientists and investigators studying the interaction of atmosphere chemistry, water composition, and the biota of the hollow (Furman and others, written communication, 1997). Modeling of the chemistry of Dry Run surface water, based on atmospheric, biologic, and geologic data, has been attempted with limited success. Better understanding of the surficial deposits and the interaction of streams and springs with near surface materials is needed before more sophisticated models can be devised. Although the bedrock lithology was mapped at a small scale (1:62,000-scale; Gathright, 1976) no examination of the surficial deposits of the hollow was made. The description of deposits contained herein is based on field observations carried out in September - November, 1996. Also included with this report is a 1/12,000-scale map of the surficial geology of Shaver Hollow (figure 1).

Virginia↗
USGS1975

Rutile and sphene in blueschist and related high-pressure facies rocks

Sphene and rutile are characteristic accessory minerals of blueschist facies metamorphic rocks. However, only sphene is present in the lowerr grade blueschist assemblages. In many areas of the world these blueschists pass into, or are in fault contact with, higher grade glaucophane schists, rutile-bearing amphibolites, and eclogites. The origin of the rutile may be related to a prograde metamorphic reaction that involves the alteration of sphene to rutile, but few data are available to support this hypothesis. It seems more probable that the rutile crystallized directly from ilmenite or titanium-rich clinopyroxene and magnetite in basalts and gabbros during the conversion of these rocks to amphibolites and eclogites containing omphacite, amphibolite, and garnet. Rutile in the higher grade blueschists, regardless of its origin, is a potential titanium resource and is currently being mined from eclogite and glaucophane-eclogite rocks in the Soviet Union. Resources are likely to be up to millions of tons contained Ti0 2 .

Open-File Report↗