Event-scale floodplain accretion revealed through tree-ring analysis of buried plains cottonwoods,Powder River,MT, USA

Six plains cottonwoods along the axis of a meander were excavated to determine if dendrochronology could identify the year and location of germination and date past overbank sedimentation events. Samples from all excavated trees showed clear anatomical changes associated with burial, including increased vessel size, decreased definition of annual ring boundaries, and decreased ring widths. Some of these burial signatures were created by deposition of only a few centimeters of sediment, and most burial events were detected by multiple samples from the same tree. Four of the trees germinated at or near the upper surfaces of bar deposits, while two germinated within thin overbank deposits draped over bar deposits, indicating that germination is closely associated with bars. Dates and inferred thicknesses of overbank sedimentation events are consistent with repeated topographic surveys and data obtained from cesium-137 (¹³⁷Cs) analyses. However, the record of overbank sedimentation extracted from the trees does not entirely reflect the history of past peak discharges documented by stream gaging, largely because individual trees are progressively less likely to be flooded through time as the river migrates farther away. Germination dates and locations closely track past positions of the river channel. Germination elevations and the elevations of the tops of point bars appear to be decreasing with time as the bend migrates, implying vertical incision by Powder River at a rate of 7.1 ± 4.3 mm/yr. The rate of floodplain growth determined by elevation changes decreases progressively through time, ultimately reaching an apparent plateau after 0.8–1.3 m of vertical accretion. While similar patterns of vertical accretion have previously been interpreted as resulting from decreasing flood probability with increasing floodplain elevation, distance from the channel is also a first-order control on vertical floodplain growth. © 2019 John Wiley & Sons, Ltd.     

Comparing different methods of bed shear stress estimates in simple and complex flow fields

Bed shear stress is a fundamental variable in river studies to link ?ow conditions to sediment transport. It is, however, dif?cult to estimate this variable accurately, particularly in complex ?ow ?elds. This study compares shear stress estimated from the log pro?le, drag, Reynolds and turbulent kinetic energy (TKE) approaches in a laboratory ?ume in a simple boundary layer, over plexiglas and over sand, and in a complex ?ow ?eld around de?ectors. Results show that in a simple boundary layer, the log pro?le estimate is always the highest. Over plexiglas, the TKE estimate was the second largest with a value 30 per cent less than the log estimate. However, over sand, the TKE estimate did not show the expected increase in shear stress. In a simple boundary layer, the Reynolds shear stress seems the most appropriate method, particularly the extrapolated value at the bed obtained from a turbulent pro?le. In a complex ?ow ?eld around de?ectors, the TKE method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent ?uctuations close to the de?ectors. It is suggested that when single‐point measurements are used to estimate shear stress, the instrument should be positioned close to 0·1 of the ?ow depth, which corresponds to the peak value height in pro?les of Reynolds and TKE shear stress. Copyright © 2004 John Wiley & Sons, Ltd.     

Electromagnetic‐Induction Logging to Monitor Changing Chloride Concentrations

Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple‐well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride‐cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality—possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high‐chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high‐chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.     

Sensitivity of bed shear stress estimated from vertical velocity profiles: the problem of sampling resolution

Bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile. However, most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed. The lack of near-bed measurements is a critical loss of information which may affect bed shear stress estimates. Detailed velocity profiles obtained from a field acoustic Doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near-bed points is critical for the estimation of bed shear stress in a shallow river environment. Moreover, the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform. © 1998 John Wiley & Sons, Ltd.     

Mixing efficiency from microstructure measurements in the Sicily Channel

Ocean Dynamics - The dissipation flux coefficient, a measure of the mixing efficiency of a turbulent flow, was computed from microstructure measurements collected with a vertical microstructure...     

Relationships between water flow rate and geophysical measurements in an alluvial aquifer

Our study focuses on the potential usefulness of surface geophysical data to constrain the water content within an alluvial aquifer. On a study area where two wells have been drilled, we have performed several geophysical measurements, including ground penetrating radar, DC resistivity prospecting, seismic refraction survey and magnetic resonance soundings. From these data, we estimated several parameters, namely, the water height in the deposits, the effective porosity, the water content, the permeability, and the transmissivity of alluvial deposits. These physical parameters allow us to characterize the alluvial deposits in order to constrain the estimation of the potential water flow. The lithology and water flow rate known from the wells enabled us to compare geophysical results obtained in a high water flow rate zone to those in a low water flow rate zone. Correlation has been found between the water flow rate observed in both wells and the geophysical data obtained in the vicinity of these wells.     

Neural networks and landslide susceptibility: a case study of the urban area of Potenza

For those working in the field of landslide prevention, the estimation of hazard levels and the consequent production of thematic maps are principal objectives. They are achieved through careful analytical studies of the characteristics of landslide prone areas, thus, providing useful information regarding possible future phenomena. Such maps represent a fundamental step in the drawing up of adequate measures of landslide hazard mitigation. However, for a complete estimation of landslide hazard, meant as the degree of probability that a landslide occurs in a given area, within a given space of time, detailed and uniformly distributed data regarding their incidence and causes are required. This information, while obtainable through laborious historical research, is usually partial, incomplete and uneven, and hence, unsatisfactory for zoning on a regional scale. In order to carry this out effectively, the utilization of spatial estimation of the relative levels of landslide hazard in the various areas was considered opportune. These areas were classified according to their levels of proneness to landslide activity without taking recurrence periods into account. Various techniques were developed in order to obtain upheaval numerical estimates. The method used in this study, which was applied in the area of Potenza, is based on techniques derived from artificial intelligence (Artificial Neural Network—ANN). This method requires the definition of appropriate thematic layers, which parameterize the area under study. These are recognized by means of specific analyses in a functional relationship to the event itself. The parameters adopted are: slope gradient, slope aspect, topographical index, topographical shape, elevation, land use and lithology.     

Assessment and mapping of the intrinsic vulnerability to pollution: an example from Keritis River Basin (Northwestern crete, Greece)

Due to the geological and hydrogeological characteristics, the carbonate aquifers are frequently vulnerable to natural and/or anthropogenic polluting sources. The aim of the present study is to evaluate the intrinsic vulnerability to pollution of the Keritis River Basin (northwestern Crete, Greece). The huge amount of the flowing groundwater represents a strategic water resource to be allocated to drinking water purpose and to agricultural activities, too. The studied groundwater are characterized by quality degradation processes represented by the anthropogenic and natural pollution such as olive oil farming, agriculture activities, industrial waste and salt water intrusion. For a better management and protection of the water resources, the assessment and mapping of groundwater vulnerability to pollution are very important. From this investigation, the Keritis Basin showed an intrinsic vulnerability degree from extremely high to very low. This study allows to define the strategies which will permit a proper safeguard criteria, against the pollution sources of the Keritis Basin groundwater resources.