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Geology topics

R. B. Jacobson

Publications and source records attributed to R. B. Jacobson.

29 records · Page 2Linked to original sources
USGS1995

Woody vegetation and channel morphogenesis in low-gradient, gravel-bed streams in the Ozarks Plateaus, Missouri and Arkansas

Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM 2 , Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes. Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities. Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.

Geomorphology↗
USGS1994

Miller City levee break and incipient meander cutoff

On July 15, 1993, the flooding Mississippi River broke through a levee near Miller City, Ill., at the head of the Mississippi Embayment, approximately 55 km upstream from Cairo, Ill. Flow through the break crossed a high-amplitude meander bend and reentered the main channel approximately 24 km upstream from Cairo, bypassing 31 km of the river channel. The incipient meander cutoff is one of the more dramatic examples of geomorphic change accompanying the 1993 flood. Discharge and bathymetry data were collected in the incipient cutoff channel every other day during the 2 weeks before and after the flood peak. During the peak on August 7, as much as 8,100 m3/s, or approximately 25 percent of the Mississippi flood discharge, was bypassing the meander bend. The flow excavated an irregular channel in the flood plain up to 25 m deep. This irregular channel extends as far as 2 km downstream from the levee break. By August 25, as much as 2,900 m3/s was still flowing through the levee break; with recession of the flood, extensive sand deposits were exposed on the margins and downstream from the scoured areas. Preliminary data indicate that local relief, such as relict channels and preexisting county roads, affected the extent of new channel formation.

Conference Paper↗
USGS1990

Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid seismic zone

Many types of liquefaction-related features (sand blows, fissures, lateral spreads, dikes, and sills) have been induced by earthquakes in coastal South Carolina and in the New Madrid seismic zone in the Central United States. In addition, abundant features of unknown and nonseismic origin are present. Geologic criteria for interpreting an earthquake origin in these areas are illustrated in practical applications; these criteria can be used to determine the origin of liquefaction features in many other geographic and geologic settings. In both coastal South Carolina and the New Madrid seismic zone, the earthquake-induced liquefaction features generally originated in clean sand deposits that contain no or few intercalated silt or clay-rich strata. The local geologic setting is a major influence on both development and surface expression of sand blows. Major factors controlling sand-blow formation include the thickness and physical properties of the deposits above the source sands, and these relationships are illustrated by comparing sand blows found in coastal South Carolina (in marine deposits) with sand blows found in the New Madrid seismic zone (in fluvial deposits). In coastal South Carolina, the surface stratum is typically a thin (about 1 m) soil that is weakly cemented with humate, and the sand blows are expressed as craters surrounded by a thin sheet of sand; in the New Madrid seismic zone the surface stratum generally is a clay-rich deposit ranging in thickness from 2 to 10 m, in which case sand blows characteristically are expressed as sand mounded above the original ground surface. Recognition of the various features described in this paper, and identification of the most probable origin for each, provides a set of important tools for understanding paleoseismicity in areas such as the Central and Eastern United States where faults are not exposed for study and strong seismic activity is infrequent.

Professional Paper↗
USGS1989

The role of catastrophic geomorphic events in central Appalachian landscape evolution

Catastrophic geomorphic events are taken as those that are large, sudden, and rare on human timescales. In the nonglaciated, low-seismicity central Appalachians, these are dominantly floods and landslides. Evaluation of the role of catastrophic events in landscape evolution includes assessment of their contributions to denudation and formation of prominent landscape features, and how they vary through space and time. Tropical storm paths and topographic barriers at the Blue Ridge and Allegheny Front create significant climatic variability across the Appalachians. For moderate floods, the influence of basin geology is apparent in modifying severity of flooding, but for the most extreme events, flood discharges relate mainly to rainfall characteristics such as intensity, duration, storm size, and location. Landslide susceptibility relates more directly to geologic controls that determine what intensity and duration of rainfall will trigger slope instability. Large floods and landslides are not necessarily effective in producing prominent geomorphic features. Large historic floods in the Piedmont have been minimally effective in producing prominent and persistent geomorphic features. In contrast, smaller floods in the Valley and Ridge produced erosional and depositional features that probably will require thousands of years to efface. Scars and deposits of debris slide-avalanches triggered on sandstone ridges recover slowly and persist much longer than scars and deposits of smaller landslides triggered on finer-grained regolith, even though the smaller landslides may have eroded greater aggregate volume. The surficial stratigraphic record can be used to extend the spatial and temporal limits of our knowledge of catastrophic events. Many prominent alluvial and colluvial landforms in the central Appalachians are composed of sediments that were deposited by processes similar to those observed in historic catastrophic events. Available stratigraphic evidence shows two scales of temporal variation: one related to Quaternary climate changes and a more-recent, higher-frequency variation due to rare events during the Holocene. In much of the central Appalachians, landforms related to Quaternary climate changes persist as the most prominent features, despite the modifying effects of late-Holocene catastrophic events.

Geomorphology↗
USGS1986

HOLOCENE AND LATE PLEISTOCENE(? ) EARTHQUAKE-INDUCED SAND BLOWS IN COASTAL SOUTH CAROLINA.

Multiple generations of prehistoric sand blows, interpreted as earthquake induced, have been discovered throughout coastal South Carolina. These sand blows extend far beyond 1886 earthquake induced sand blows, in sediments having approximately the same liquefaction susceptibility. The seismic source zone for the prehistoric sand blows is unknown. The different distributions of prehistoric and 1886 sand blows have two possible explanations: (1) moderate to strong earthquakes originated in different seismic source locations through time or (2) at least one earthquake much stronger than the 1886 event also originated from the same seismic source as the 1886 earthquake.

Conference Paper↗