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Research about Virginia

Source-linked reports with geographic coverage including Virginia.

At least 541 records · Page 30Linked to original sources
USGS1978

Hydrogeology of the observation well site at the United States Geological Survey National Center, Reston, Virginia

The U. S. Geological Survey's National Center is on a 105-acre tract straddling rocks of two distinct types. These are pelitic schists of late Precambrian and early Paleozoic age; and sandstone, shale, siltstone, and conglomerate of Triassic age. Within the Triassic part is a diabase dike and an associated contact metamorphic zone. Two observation wells and two core holes were drilled on the part of the property underlain by Triassic rocks. The wells are 10-inch and 8-inch diameter and were drilled to depths of 165 feet and 205 feet. The wells were drilled to monitor water levels, for equipment testing, and to determine the hydraulic properties of the Triassic rocks. Geophysical logs were run, and good correlations were found between the electric and caliper logs and the lithologic logs prepared from drill cuttings and cores. An aquifer test was made and was analyzed by the Theis method which considers nonleaky artesian conditions and the Streltsova method which considers flow in fractured rocks. The test results suggest that transmissivity of the sandstone aquifer is about 40 ft2/day. Flowmeter surveys showed that the water comes from two thin zones parallel to the bedding. The flowmeter surveys, core samples, and geophysical logs suggest that the Triassic sandstone is a fractured-rock aquifer. Water is probably derived from partings and joints parallel to bedding which in turn are fed by vertical joints and fractures. A water sample taken at the close of the aquifer test indicates that the water has a low dissolved solids content, is soft, and is generally of excellent quality.

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USGS1978

Flood-plain delineation for Cub Run basin, Fairfax County, Virginia

Flood-plain delineation for Cub Run basin water-surface profiles of the 25-, 50-, and 100-year recurrence interval discharges have been computed for all streams and reaches of channels in Fairfax County, Virginia having a drainage area greater than 1 square mile except for Dogue Creek, Little Hunting Creek, and that part of the Cameron Run above Lake Barcroft. Maps having a 2-foot contour interval and a horizontal scale of 1 inch equals 100 feet have been used for base on which flood boundaries were delineated for 25-, 50-, and 100-year floods to be expected in each basin under ultimate development conditions. This report is one of a series and presents a discussion of techniques employed in computing discharges and profiles as well as the flood profiles and maps on which flood boundaries have been delineated for the Cub Run basin in Fairfax County.

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USGS1978

Risk analyses for a water-supply system; Occoquan Reservoir, Fairfax and Prince William counties, Virginia

This report demonstrates two techniques for evaluating risks in the operation of a water-supply system. Both rely on reconstructed historical streamflow data to develop estimates of the probabilities of certain specific events occurring in the future. These techniques are applied to the Occoquan Reservoir which was experiencing an unprecedented low level of storage in the autumn of 1977. The two techniques are used respectively to evaluate the overall adequacy of the existing reservoir and to evaluate the risks in the 1977 crisis. In the first technique, the general risk analysis model (GRAM), simulations of the reservoir's contents are carried out under a set of assumptions about withdrawal rates and emergency procedures. The results of the GRAM simulation for the Occoquan Reservoir are in the form of estimates of the probabilities that in any year certain emergency procedures will have to be invoked. These estimates are given for a range of rates of withdrawals and for four different stages of emergency actions. Also given are the estimated probabilities of entering emergency conditions in one year given that an emergency had occurred in the previous year. Due to the year-to-year persistence of low flows, these latter (conditional) probabilities are higher than the former (marginal) probabilities. The second technique used is position analysis. In this procedure probability distributions of future storages are estimated under existing storage conditions and an assumed rate of withdrawal from the reservoir. The position analysis which was initialized at the October 1, 1977, conditions indicates that the probability of entering a Stage III emergency (the prohibition of all uses of water nonessential to life, health, and safety) in the autumn of 1977 or winter of 1978 was 10 percent at that time. However, with a reduction in water use by 8 million gallons per day, this probability would have fallen to 4 percent. If a long reconstructed historical streamflow record is available to a water supply agency, then the agency will have the capability to undertake its own risk analyses. It can carry out comparisons of alternative operating policies by using techniques such as GRAM. It can also evaluate short-term risk of different plans of operation during crisis situations by using position analysis.

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USGS1978

Relationship of landslides to fractures in Potomac Group deposits, Fairfax County, Virginia

Landsliding is a common problem in eastern Fairfax County, an area underlain by Potomac Group (Lower Cretaceous) Coastal Plain deposits of silt and clay interbedded and interfingered with sand and gravel. The slides commonly are present in clay and silt that, on the basis of laboratory tests, appear to be much too strong to have failed. However, the very plastic silt and clay deposits are commonly cut by long continuous to short discontinuous high-angle and subhorizontal joints, shears, and faults. These fractures can contribute to sliding. Failure along faults and shears takes place because the relative movement has greatly weakened and softened the clay and silt. The mechanism for failure along the joints is less obvious but may be related to infilling along joints, slight movement due to erosional unloading, swelling of clays, and softening along joints. Field investigations suggest that the three best developed high-angle joint sets in eastern Fairfax County have trends in the northeast quadrant. Lineaments plotted on a lineament map prepared from Landsat imagery also trend northeast parallel to the regional joint orientation. This regional orientation suggests that some of the fractures are tectonic in origin. Therefore, they should be anticipated in a broad zone many miles wide in the vicinity of the Fall Line. In addition, fractures of unknown origin contributing to landsliding, have been observed in massive Potomac Group clay and silt deposits. Irrespective of origin, these fractures require careful investigation and engineering to prevent slope failure at critical locations during and after major excavation.

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USGS1978

A lithologic log of the Oak Grove core, northeastern Virginia

The Oak Grove core was taken as part of a multifaceted project to develop a stratigraphic and tectonic framework for the Rappahannock drainage basin, northeastern Virginia. This core is at present the only continuously cored Cretaceous and Tertiary section on the Northern Neck of Virginia and is one of the few deep cores in the entire Salisbury embayment. The core includes most of the Chesapeake Group sediments (Miocene part) in the Oak Grove area, the Pamunkey Group (Paleocene and Eocene) and nearly all of the Potomac Group (Lower Cretaceous part) (see Table 1).

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USGS1978

Flood-plain delineation for Occoquan River, Wolf Run, Sandy Run, Elk Horn Run, Giles Run, Kanes Creek, Racoon Creek, and Thompson Creek, Fairfax County, Virginia

Water-surface profiles of the 25-year and 100-year floods and maps on which the 25-, 50-, and 100-year flood boundaries are delineated for streams in southwestern Fairfax County tributary to the Occoquan River, that part of the Occoquan River within Fairfax County and those streams on Mason Neck tributary to the Potomac River are presented in this report. The techniques used in the computation of the flood profiles and delineation of flood boundaries are presented, and specific hydraulic problems encountered within the study area are also included.

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USGS1978

Technique for estimating magnitude and frequency of floods in Virginia

A technique is presented for estimating the magnitude and frequency of floods in Virginia. For 299 gaged sites, the magnitudes of floods having recurrence intervals of 2 to 100 years are provided in tables. For ungaged sites, regression relations are presented that allow the estimation of flood magnitudes based on upstream drainage area, average main-channel slope, and a factor to account for regional variations in basin terrain characteristics that would bias the results. A summary of maximum known peak discharges are presented for 363 sites together with the relationship of drainage-area size to flow in cubic feet per second per square mile. Also given is table of 10 basin characteristics for 403 gaging stations.

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USGS1977

Heavy-mineral variability in bottom sediments of the lower Chesapeake Bay, Virginia

Heavy minerals in bottom-sediment samples of the lower Chesapeake Bay show distribution patterns and interrelationships that denote characteristic mineral suites associated with defined geographic provinces. The Baymouth province has a garnet-hornblende-pyroxene suite, which is largely attributed to the influx of littoral and shelf sediments; the Eastern Shore province has a similar suite, derived largely from coastal erosion of the Eastern Shore peninsula. The Northern and Combined River provinces have a zircon-tourmaline-staurolite assemblage, which reflects derivation from an Appalachian Piedmont-Atlantic Coastal Plain sourceland. The Western Shore province is associated with a zircon-epidote-staurolite assemblage, apparently derived jointly from tributary influx and coastal erosion of the western shore. Factor analysis identified two major factors that account for 63% of the total variation in the relative amounts of the seven most common heavy minerals. The dominant factor (44%) is based on a zircon-hornblende-staurolite-pyroxene relationship, which indicates that mineral stability, as influenced by sediment maturity, is a major contributing factor. The second factor (19%) based on a tourmaline-epidote-staurolite-garnet relationship indicates that provenance is another major cause of heavy-mineral variability within the lower bay.

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USGS1977

Flood-plain delineation for Accotink Creek Basin, Fairfax County, Virginia

Water-surface profiles of the 25-year and 100-year floods maps on which the 25-, 50-, and 100-year flood limits are delineated for streams in the Accotink Creek basin are presented in this report. Excluded are segments of Accotink Creek within the Fort Belvoir Military Reservation. The techniques used in the computation of the flood profiles and delineation of flood limits are presented, and specific hydraulic problems encountered within the study area are also included.

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USGS1977

Flood-plain delineation for Bull Run, Little Rocky Run, Johnny Moore Creek, and Popes Head Creek basins, Fairfax County, Virginia

Water-surface profiles of the 25-year and 100-year floods and maps on which the 25-, 50-, and 100-year flood boundaries are delineated for Little Rocky Run Basin, Johnny Moore Creek Basin, Popes Head Creek Basin and that part of Bull Run basin within Fairfax County are presented in this report. The techniques used in the computation of the flood profiles and delineation of flood boundaries are presented, and specific hydraulic problems encountered within the study area are also included.

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USGS1977

Bedrock map of Fairfax County, Virginia

The generalized rock-type of lithologic map separates the firm consolidated bedrock that underlies most of Fairfax County into thirteen (13) major rock types; each type groups rock units with grossly similar physical characteristics. Although crystalline bedrock underlies the Coastal Plain deposits of Eastern Fairfax County, it was not subdivided on this map. The only available information is based on interpretations from a few widely scattered deep drill holes and aeromagnetic and gravity surveys. Three sedimentary and ten (10) crystalline rocks on the Explanation are arranged in order from softest, best bedded, most foliated and readily splittable - to hardest, most massive, and least readily splittable.

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USGS1977

Earthquake history of Virginia

Virginia is a State of considerable seismic activity, although the earthquakes are rarely strong. Thirty-five shocks, intensity MM V or greater (Modified Mercalli Scale), are listed with epicenters within its borders. The locations of several of the older events are not precise; thus, the above count i subject to alteration. A detailed study of Virginia earthquakes by G. A. Bollinger and M. G. Hopper of the Virginia Polytechnic Institute and State University listed 137 shocks (71 from 1774 to 1899, 66 from 1900 to 1970). Many of these were felt with intensities below MM V.

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