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21.
22.
Comparison of flood hazard assessments on desert piedmonts and playas: A case study in Ivanpah Valley, Nevada 总被引:1,自引:0,他引:1
Colin R. Robins Brenda J. Buck Amanda J. Williams Janice L. Morton P. Kyle House Michael S. Howell Maureen L. Yonovitz 《Geomorphology》2009,103(4):520-532
Accurate and realistic characterizations of flood hazards on desert piedmonts and playas are increasingly important given the rapid urbanization of arid regions. Flood behavior in arid fluvial systems differs greatly from that of the perennial rivers upon which most conventional flood hazard assessment methods are based. Additionally, hazard assessments may vary widely between studies or even contradict other maps. This study's chief objective was to compare and evaluate landscape interpretation and hazard assessment between types of maps depicting assessments of flood risk in Ivanpah Valley, NV, as a case study. As a secondary goal, we explain likely causes of discrepancy between data sets to ameliorate confusion for map users. Four maps, including three different flood hazard assessments of Ivanpah Valley, NV, were compared: (i) a regulatory map prepared by FEMA, (ii) a soil survey map prepared by NRCS, (iii) a surficial geologic map, and (iv) a flood hazard map derived from the surficial geologic map, both of which were prepared by NBMG. GIS comparisons revealed that only 3.4% (33.9 km2) of Ivanpah Valley was found to lie within a FEMA floodplain, while the geologic flood hazard map indicated that ~ 44% of Ivanpah Valley runs some risk of flooding (Fig. 2D). Due to differences in mapping methodology and scale, NRCS data could not be quantitatively compared, and other comparisons were complicated by differences in flood hazard class criteria and terminology between maps. Owing to its scale and scope of attribute data, the surficial geologic map provides the most useful information on flood hazards for land-use planning. This research has implications for future soil geomorphic mapping and flood risk mitigation on desert piedmonts and playas. The Ivanpah Valley study area also includes the location of a planned new international airport, thus this study has immediate implications for urban development and land-use planning near Las Vegas, NV. 相似文献
23.
Varved sediments of Lake Yoa (Ounianga Kebir,Chad) reveal progressive drying of the Sahara during the last 6100 years 总被引:1,自引:0,他引:1
Pierre Francus Hans von Suchodoletz Michael Dietze Reik V. Donner Frédéric Bouchard Ann‐Julie Roy Maureen Fagot Stefan Kröpelin 《Sedimentology》2013,60(4):911-934
The sedimentological and geochemical properties of a 7·47 m long laminated sequence from hypersaline Lake Yoa in northern Chad have been investigated, representing a unique, continuous 6100 year long continental record of climate and environmental change in the eastern Central Sahara. These data were used to reconstruct the Mid to Late Holocene history of this currently hyper‐arid region, in order to address the question of whether the Mid Holocene environmental transition from a humid to a dry Sahara was progressive or abrupt. This study involved a suite of analyses, including petrographic and scanning electron microscope examination of thin sections, X‐ray diffraction, X‐radiography, granulometry, loss on ignition and magnetic susceptibility. The potential of micro‐X‐ray fluorescence core scanning was tested at very high resolution. Detailed microscopic investigation revealed the sedimentary processes responsible for the formation of the fine laminations, identified the season during which they were formed, and confirmed their annually rhythmic nature. High‐resolution X‐ray fluorescence core scanning allowed the distinction of each individual lamination over the entire record, opening new perspectives for the study of finely laminated sediment sequences. Geochemical and mineralogical data reveal that, due to decreasing monsoon rainfall combined with continuous and strong evaporation, the hydrologically open and fresh Mid Holocene Lake Yoa slowly evolved into the present‐day hypersaline brine depleted in calcium, which has existed for about the past 1050 years. During the oldest part of the investigated period, Lake Yoa probably contained a permanently stratified lower water column that was nevertheless disrupted relatively frequently by mixing events. Deep‐water anoxia became more stable because of increased salinity‐driven density stratification. In parallel, the sediment grain‐size proxies record a progressive increase of aeolian input in the course of the last 6100 years. Altogether, all geochemical and sedimentological indicators point to a progressive drying of the eastern Central Sahara, strengthening previous conclusions based on palaeoecological indicators. 相似文献
24.
Maureen B. Steiner 《Earth and Planetary Science Letters》1977,35(2):205-214
Measurement of the remanent magnetization of samples of Jurassic oceanic red sediments recovered in the western Atlantic on Leg 11, site 105 of the Deep Sea Drilling Project yields quite different results, depending on the demagnetization processes used. Both the Jurassic section and the Berriasian-Valanginian part of the Lower Cretaceous were measured, but with less satisfactory results for the Lower Cretaceous. The natural remanent magnetization of the Jurassic section is almost entirely normal, with 44.6° inclinations (standard deviation = 13.9°) and is not changed by 1000 Oe alternating field (AF) demagnetization. Thermal demagnetization to temperatures of 630°C brings the inclination and polarity sequence in line with that expected for Oxfordian through Tithonian time at this site. The average inclination after thermal demagnetization is 22.1°, standard deviation = 12.1°, and the polarity pattern is one of frequently alternating polarity, much more similar to published reversal patterns for this time than the all normal results of AF demagnetization. The polarity pattern is not identical to the published ones as a result of insufficiently detailed sampling. Thermomagnetic and X-ray analyses were ambiguous, but suggest the presence of titanomagnetite, hematite, and possibly titanomaghemite and pyrrhotite. The primary remanence is carried by hematite. 相似文献
25.
MacKenzie K.; Longshaw M.; Begg G. S.; McVicar A. H. 《ICES Journal of Marine Science》1998,55(2):151-162
26.
Maureen B. Steiner 《Earth and Planetary Science Letters》1978,38(2):331-345
Three-quarters of the Summerville Formation and a small portion of the Curtis Formation of central Utah have been sampled at close stratigraphic spacing. The two formations constitute an intertonguing marine-marginal marine, tidal flat sequence representing middle Callovian time. The magnetic polarity sequence observed is dominantly reversed with a few relatively short normal intervals. The age of these formations corresponds to some portion of time within the Jurassic oceanic quiet zone. The fact that normal polarity was not prominent in these formations casts some doubt upon the hypothesis that a long period of normal polarity was the cause of the quiet zone. Furthermore, a literature compilation indicates that normal polarity did not dominate the Jurassic as had been thought. The pole position determined from the Middle Jurassic Summerville Formation slightly revises earlier results and suggests a tiny amount of apparent polar motion relative to North American between Late Triassic and Middle Jurassic time. 相似文献
27.
AbstractScholarship on collaboration in natural resource management is restricted by a lack of large-N assessments and mixed methods approach to examine relationships between collaborative structures and processes, and management actions and outcomes. This paper examines the relationship between perceived levels of collaboration, collaborative process indicators, and benefits of USDA Forest Service stewardship contracts at a broad spatial and temporal scale using mixed methodology. This study found higher levels of collaboration were strongly associated with jointly initiated processes that included a breadth of interests and utilized a broad range of outreach mechanisms and opportunities for engagement. Highly collaborative processes were closely linked with attaining project objectives and social and economic benefits. Findings contribute to defining collaboration by uncovering the relationship between perceived levels of collaboration, the role of process characteristics, and perceived benefits of collaborative stewardship contracting processes through a large-N dataset (n?=?1,064) and case study (n?=?61) findings. 相似文献
28.
29.
This is the first focused study of non-Eos K asteroids. We have observed a total of 30 K-complex objects (12 K-2 Sk- and 13 Xk-type asteroids (from the Bus taxonomy), plus 3 K-candidates from previous work) and we present an analysis of their spectral properties from 0.4 to 2.5 μm. We targeted these asteroids because their previous observations are spectrally similar enough to suggest a possible compositional relationship. All objects have exhibited spectral redness in the visible wavelengths and minor absorptions near 1 micron. If, as suggested, K-complex asteroids (including K, Xk, and Sk) are the parent bodies of carbonaceous meteorites, knowledge of K-asteroid properties and distribution is essential to our understanding of the cosmochemical importance of some of the most primitive meteorite materials in our collection. This paper presents initial results of our analysis of telescopic data, with supporting analysis of laboratory measurements of meteorite analogs. Our results indicate that K-complex asteroids are distinct from other main belt asteroid types (S, B, C, F, and G). They do not appear to be a subset of these other types. K asteroids nearly span the range of band center positions and geometric albedos exhibited by the carbonaceous chondrites (CO, CM, CV, CH, CK, CR, and CI). We find that B-, C-, F- and G-type asteroids tend to be darker than meteorites, and can have band centers longer than any of the chondrites measured here. This could indicate that K-complex asteroids are better spectral analogues for the majority of our carbonaceous meteorites than the traditional B-, C-, F- and G-matches suggested in the literature. This paper present first results of our ongoing survey to determine K-type mineralogy, meteorite linkages, and significance to the geology of the asteroid regions. 相似文献
30.
Delineation of capture zones for municipal wells in fractured dolomite, Sturgeon Bay, Wisconsin, USA 总被引:1,自引:1,他引:1
A wellhead protection study for the city of Sturgeon Bay, Wisconsin, USA, demonstrates the necessity of combining detailed
hydrostratigraphic analysis with groundwater modeling to delineate zones of contribution for municipal wells in a fractured
dolomite aquifer. A numerical model (MODFLOW) was combined with a particle tracking code (MODPATH) to simulate the regional
groundwater system and to delineate capture zones for municipal wells. The hydrostratigraphic model included vertical and
horizontal fractures and high-permeability zones. Correlating stratigraphic interpretations with field data such as geophysical
logs, packer tests, and fracture mapping resulted in the construction of a numerical model with five high-permeability zones
related to bedding planes or facies changes. These zones serve as major conduits for horizontal groundwater flow. Dipping
fracture zones were simulated as thin high-permeability layers. The locations of exposed bedrock and surficial karst features
were used to identify areas of enhanced recharge. Model results show the vulnerability of the municipal wells to pollution.
Capture zones for the wells extend several kilometers north and south from the city. Travel times from recharge areas to all
wells were generally less than one year. The high seasonal variability of recharge in the study area made the use of a transient
model necessary.
Electronic Publication 相似文献