Evaluating the effects of urbanization age on the morphology of low-order urban streams in the U.S. southern Piedmont     

Philip Johnson 《自然地理学》2019,40(1):1-27

We used space-for-time substitution, correlation, and regression analyses to study the effects of urbanization age on stream channel variables from 19 low-order urban watersheds in the southern Piedmont of the United States. The results are used to address questions regarding which variables are most responsive to urbanization age, the duration of response relative to system size, and changes in adjustment rate over time. Few morphological variables were found to be well correlated with metrics of urbanization age. The maximum-width/maximum-depth ratio (Wmax/Dmax) exhibited the best correlations to multiple metrics of urbanization age, with all correlations positive. A dearth of young urbanization ages in our dataset makes mathematically derived response times and end states for Wmax/Dmax highly uncertain. Inferences from the age range that is better represented suggest that the period of major response has occurred within the first three decades following urbanization. Subsequently, gradual increase in Wmax/Dmax appears to continue over 40 or more years. Response of channel form to urbanization and time elapsed in these low-order Piedmont streams is constrained by the presence of resistant bedrock below streambeds, a circumstance addressed by recent channel evolution models.  相似文献   

RadioAstron Science Program Five Years after Launch: Main Science Results     

N. S. Kardashev  A. V. Alakoz  A. S. Andrianov  M. I. Artyukhov  W. Baan  V. E. Babyshkin  N. Bartel  O. S. Bayandina  I. E. Val’tts  P. A. Voitsik  A. Z. Vorobyov  C. Gwinn  J. L. Gomez  G. Giovannini  D. Jauncey  M. Johnson  H. Imai  Y. Y. Kovalev  S. E. Kurtz  M. M. Lisakov  A. P. Lobanov  V. A. Molodtsov  B. S. Novikov  A. V. Pogodin  M. V. Popov  A. S. Privesenzev  A. G. Rudnitski  G. M. Rudnitski  T. Savolainen  T. V. Smirnova  A. M. Sobolev  V. A. Soglasnov  K. V. Sokolovsky  E. N. Filippova  V. V. Khartov  M. E. Churikova  A. E. Shirshakov  V. I. Shishov  P. Edwards 《Solar System Research》2017,51(7):535-554

The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on baselines of 100–200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure testifying to recollimation shocks on scales of 100–250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer baselines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which considerably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20–60 μas correspond to linear sizes of about 5–10 million km (several solar diameters).  相似文献   

The clay mineralogy of particulate material from some surface seawaters of the eastern Atlantic Ocean     

A.K Behairy  R Chester  A.J Griffiths  L.R Johnson  J.H Stoner 《Marine Geology》1975,18(1):M45-M56

The concentrations of the clay minerals montmorillonite and illite have been determined in 28 samples of particulate material from surface (0–～5 m) waters along the eastern margins of the Atlantic Ocean, and the concentrations of chlorite and kaolinite have been determined in nineteen of the samples.The average concentration of the four clays in the particulate material decreases in the following order: illite (～42%) > kaolinite (～25%) > chlorite (～19%) > montmorillonite (～14%). There are specific trends in the latitudinal distributions of illite and kaolinite in the particulate material; illite decreases and kaolinite increases towards low latitudes. These trends are also found in eolian dusts and deep-sea sediments from the same region, and these clays are considered to have a detrital origin.The concentrations of montmorillonite in the particulate material and deep-sea sediments are similar in the North Atlantic. However, in the South Atlantic the sediments contain about twice as much montmorillonite as does the particulate material. It is suggested that whereas the montmorillonite in North Atlantic deep-sea sediments is largely detrital, that in South Atlantic sediments is both detrital and authigenic.  相似文献   

Sediment transport due to tree root throw: integrating tree population dynamics,wildfire and geomorphic response     

J. M. Gallaway  Y. E. Martin  E. A. Johnson 《地球表面变化过程与地形》2009,34(9):1255-1269

A field study was conducted to analyze root throw and associated sediment transport in Hawk Creek Watershed, Canadian Rockies. A large crown fire in 2003 allowed the opportunity to study pre‐fire and post‐fire root throw. Based on field data, a significant relation was found between gradient and root plate volume, as well as individual root plate dimensions. Given that tree diameters increase as trees age and that a relation in the field data was found between tree diameter and root plate volumes, sediment transport due to root throw is expected to change in response to forest disturbance and stand age. Sediment disturbance, which is the amount of sediment upheaved during tree topple and does not take into account transport distance, shows higher values on steeper gradients. Sediment transport was notable for the steepest plots, with pre‐fire values of 0·016 cm³ cm^–1 a^–1 and post‐fire values of 0·18 cm³ cm^–1 a^–1. A tree population dynamics model is then integrated with a root throw transport model calibrated for the Canadian Rockies to examine the temporal dynamics of sediment transport. Fire is incorporated as a disturbance that initiates development of a new forest, with the model cycling through generations of forest. Trees fall according to an exponential rate that is based on time since death, resulting in a time lag between tree mortality and sediment transport. When values of time‐since‐previous‐fire are short, trees are generally <13 cm, and minimal sediment is upheaved during toppling. If trees reach a critical diameter at breast height (dbh) at time of fire, a pulse of sediment occurs in the immediate post‐fire years due to falling of killed trees, with tree fall rates decreasing exponentially with time‐since‐fire. A second pulse of root throw begins at about 50 years after the previous fire, once new recruits reach a critical dbh and with initiation of competition‐induced mortality. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Stabilization of fine gravels by net‐spinning caddisfly larvae     

Matthew F. Johnson  Ian Reid  Stephen P. Rice  Paul J. Wood 《地球表面变化过程与地形》2009,34(3):413-423

We examined the impact of Hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine‐gravel (4–6 and 6–8 mm). This impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine‐gravels. Trays of gravel were placed in the River Soar, Leicestershire, UK, where they were colonized to natural densities by caddisfly larvae. Identical trays that were surrounded by a 250 µm mesh were also deployed, preventing colonization but allowing field conditioning of sediments, including minor reworking of grains and biofilm development. After 21 days in the river, trays were removed to a laboratory flume where grain entrainment stresses were established. In addition to the colonized and conditioned treatments, critical shear stresses were measured for identical sediments that were not placed in the river (laboratory gravels). Gravels that were colonized by Hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays (p ≤ 0·002), however, there was no significant difference between conditioned and laboratory gravels. This implies that the presence of caddisfly can be a more important influence on fine‐gravel stability than some conditioning processes. Shields parameter was compared across treatments and across the two gravel size‐fractions using two‐way ANOVA. No significant differences or interactions were observed, indicating that 4–6 mm gravel was stabilized to a similar degree as 6–8 mm gravel by conditioning and colonization processes. Our results extend earlier studies in two important ways: (1) entrainment stresses were established for fine gravels that were colonized at natural densities, under natural stream conditions; and (2) the caddisfly effect was measured relative to both field‐conditioned and unconditioned laboratory controls. The temporal and spatial distribution of silk‐spinning caddisfly larvae suggests that they have the potential to influence fine‐sediment mobility in many rivers, worldwide. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Impacts of stormwater runoff in the Southern California Bight: Relationships among plume constituents     

Kristen M. Reifel  Scott C. Johnson  Paul M. DiGiacomo  Michael J. Mengel  Nikolay P. Nezlin  Jonathan A. Warrick  Burton H. Jones 《Continental Shelf Research》2009,29(15):1821-1835

The effects from two winter rain storms on the coastal ocean of the Southern California Bight were examined as part of the Bight ‘03 program during February 2004 and February–March 2005. The impacts of stormwater from fecal indicator bacteria, water column toxicity, and nutrients were evaluated for five major river discharges: the Santa Clara River, Ballona Creek, the San Pedro Shelf (including the Los Angeles, San Gabriel, and Santa Ana Rivers), the San Diego River, and the Tijuana River. Exceedances of bacterial standards were observed in most of the systems. However, the areas of impact were generally spatially limited, and contaminant concentrations decreased below California Ocean Plan standards typically within 2–3 days. The largest bacterial concentrations occurred in the Tijuana River system where exceedances of fecal indicator bacteria were noted well away from the river mouth. Maximum nitrate concentrations (~40 μM) occurred in the San Pedro Shelf region near the mouth of the Los Angeles River. Based on the results of general linear models, individual sources of stormwater differ in both nutrient concentrations and the concentration and composition of fecal indicator bacteria. While nutrients appeared to decrease in plume waters due to simple mixing and dilution, the concentration of fecal indicator bacteria in plumes depends on more than loading and dilution rates. The relationships between contaminants (nutrients and fecal indicator bacteria) and plume indicators (salinity and total suspended solids) were not strong indicating the presence of other potentially important sources and/or sinks of both nutrients and fecal indicator bacteria. California Ocean Plan standards were often exceeded in waters containing greater than 10% stormwater (<28–30 salinity range). The median concentration dropped below the standard in the 32–33 salinity range (1–4% stormwater) for total coliforms and Enterococcus spp. and in the 28–30 salinity range (10–16% stormwater) for fecal coliforms. Nutrients showed a similar pattern with the highest median concentrations in water with greater than 10% stormwater. Relationships between colored dissolved organic matter (CDOM) and salinity and between total suspended solids and beam attenuation indicate that readily measurable, optically active variables can be used as proxies to provide at least a qualitative, if not quantitative, evaluation of the distribution of the dissolved, as well as the particulate, components of stormwater plumes. In this context, both CDOM absorption and the beam attenuation coefficient can be derived from satellite ocean color measurements of inherent optical properties suggesting that remote sensing of ocean color should be useful in mapping the spatial areas and durations of impacts from these contaminants.  相似文献   

Quantitative analysis of net-transgressive shoreline trajectories and stratigraphic architectures: mid-to-late Jurassic of the North Sea rift basin     

Gary J. Hampson  Peter J. Sixsmith  Rachel L. Kieft  Christopher A. -L. Jackson  Howard D. Johnson 《Basin Research》2009,21(5):528-558

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A Practical Approach to the Design, Monitoring, and Optimization of In Situ MTBE Aerobic Biobarriers     

Paul C. Johnson  Cristin L. Bruce  Karen D. Miller 《Ground Water Monitoring & Remediation》2010,30(1):58-66

A paradigm for the design, monitoring, and optimization of in situ methyl tert -butyl ether (MTBE) aerobic biobarriers is presented. In this technology, an oxygen-rich biologically reactive treatment zone (the "biobarrier") is established in situ and downgradient of the source of dissolved MTBE contamination in groundwater, typically gasoline-impacted soils resulting from leaks and spills at service station sites or other fuel storage and distribution facilities. The system is designed so that groundwater containing dissolved MTBE flows to, and through, the biobarrier treatment zone, ideally under natural gradient conditions so that no pumping is necessary. As the groundwater passes through the biobarrier, the MTBE is converted by microorganisms to innocuous by-products. The system also reduces concentrations of other aerobically degradable chemicals dissolved in the groundwater, such as benzene, toluene, xylenes, and tert -butyl alcohol. This design paradigm is based on experience gained while designing, monitoring, and optimizing pilot-scale and full-scale MTBE biobarrier systems. It is largely empirically based, although the design approach does rely on simple engineering calculations. The paradigm emphasizes gas injection–based oxygen delivery schemes, although many of the steps would be common to other methods of delivering oxygen to aquifers.  相似文献   

Geographically localised shelf waves on curved coasts     

G. Kaoullas  E.R. Johnson 《Continental Shelf Research》2010

This paper presents a combined analytical and numerical extension to recent theoretical work on trapped continental shelf waves over smoothly curved coastlines. The geometry considered consists of an annular region joining two semi-infinite straight sections and, even though the isobaths remain evenly spaced, trapped modes appear in the annular region due entirely to the longitudinal curvature of the shelf there. Expanding in alongshore modes and requiring that the error is orthogonal to the transverse modes gives a linear algebraic system of equations for the frequency of trapping. Solutions for barotropic, non-divergent shelf waves show that trapped modes always exist if the coastline curves to form a headland.  相似文献   

Global Earth structure: inference and assessment   总被引：3，自引：0，他引：3  

D. W. Vasco  Lane R. Johnson  & Osni Marques 《Geophysical Journal International》1999,137(2):381-407

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