摘要:

Large amplitude river meanders have been observed to contain multiple bars within a single loop and to exhibit planforms that are asymmetric or have subsidiary bends. Here we report experiments conducted in symmetric sine‐generated meanders of large amplitude which document the bed topography, flow patterns, and sediment transport fields in such bends. Multiple pools spaced at a distance of 3–4 widths develop successively along the outer concave bank. Downstream of each pool a distinct bar lies along the inside convex bank. The series of bars overlap and appear shingled downstream; the leading edge of each bar is oblique to the channel trace and curves toward the inside bank. The first and most pronounced pool, opposite the first inner bank bar, is consistently positioned where the projection of the inner bank tangent at the upstream crossing intersects the outer bank. Successive shingled bars in the bend migrate downstream if bars arriving at the persistent first pool deliver large enough pulses of sediment to the pool to form new bars. In experiments with an erodible bank, the banks near pools are sites of accelerated retreat. This suggests that the shear stress fields engendered by these multiple features may cause the planimetric distortion seen in larger riv

ISSN:0043-1397    

DOI:10.1029/93WR01755

年代:1993

数据来源: WILEY

摘要:

Large‐amplitude river meanders develop multiple scour holes within a single bend that are observed to be part of overlapping shingle bars (Whiting and Dietrich, this issue). These bars, having wavelengths near 4 channel widths, consist of a pool along the outer concave bank and a depositional lobe along the inner convex bank. The development of multiple pools and lobes within a bend has been shown to cause localized bank erosion and channel trace distortion expressed as planform asymmetry and compound heads. In this second paper we describe experiments designed specifically to test hypotheses that propose multiple pools to be the result of an overshoot phenomenon, to be a consequence of an alternate bar‐like instability, or to develop from a flow/planform instability. Although our experiments favor the hypothesis that multiple bars in bends of large amplitude arise from an alternate bar‐like instability, experiments conducted over an immobile flat bed also reveal longitudinal oscillations in flow that may arise in these long bends. This secondary effect interacts with shingle bars to amplify pool expression at particular points along the ch

ISSN:0043-1397    

DOI:10.1029/93WR01756

年代:1993

数据来源: WILEY

摘要:

The response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand‐gravel mixture. After a series of quasi‐stationary alternate bars were formed under equilibrium sediment transport, feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant. The primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased. After the first feed rate reduction, the channel incised by twice the mean water depth, on average, and caused distal bar surfaces to emerge as terracelike features. Bar roughness decreased, and mean boundary shear stress exerted on bed surface particles increased. Little incision occurred after the second feed rate reduct

ISSN:0043-1397    

DOI:10.1029/93WR01673

年代:1993

数据来源: WILEY

摘要:

The grain roughness of stable armored beds that formed in a laboratory flume under a range of steady flow conditions on rounded, flat and angular gravel is analyzed. Gravel roughness geometry is determined from bed surface profiles and vertical photographs. These techniques have been employed in field situations. Thus the methodology is potentially applicable to the analysis of grain roughness in natural gravel bed channels. The description of representative roughness geometry is also analogous to that used to characterize artificial roughness arrays. Armor roughness increases with increasing flow. Armored surfaces composed of angular gravel are roughest, and surfaces formed of flat gravel offer least resistance to the flow. Stable armored beds may exhibit a tendency to maximize the ratio of the shear due to drag on representative roughness elements to total shear. Roughness concentration is strongly correlated with the energy slope, and there is a linear increase in equivalent roughness height with increasing roughness concentration. The friction factor for an armored surface varies in a linear manner with representative roughness geometry. The equation defining this relation is probably similar to that used to characterize variations in the friction factor with artificial roughness geometry at low roughness concentrations. However, to reconcile the relations for artificial and natural roughness completely, it may be necessary to explicitly consider the contribution to flow resistance made by roughness shape, background roughness, and blocking in shallow flows.

ISSN:0043-1397    

DOI:10.1029/93WR01490

年代:1993

数据来源: WILEY

摘要:

The hydrogeology and redox geochemistry of CH4in two hydrogeological settings within a late Wisconsinan till and loess sequence in central Iowa were investigated using CH4, Eh, dissolved O2, NO3‐N, NH4‐N, dissolved Fe, SO4, H2S, dissolved organic carbon (DOC), particulate organic carbon (POC), δ13C‐CH4, δ2H‐CH4, δ13C of dissolved inorganic carbon (DIC), and14C−CH4. Depleted δ13C‐CH4and δ2H‐CH4values, enriched δ13C‐DIC values, and the lack of higher hydrocarbons suggest that the CH4is microbial and is formed by CO2reduction. Carbon 14 dates of CH4indicate that 14,000‐ to 17,000‐year‐old POC and DOC in the 1oess and till compose the substrate for methanogenesis. Production of CH4probably began after deglaciation at about 12,000 years B.P. and has continued to the present. Groundwater recharge, particularly in the upland topographic position at the site, has decreased CH4concentrations in

ISSN:0043-1397    

DOI:10.1029/93WR01687

年代:1993

数据来源: WILEY

摘要:

Much controversy centers on whether groundwater flow velocities in basin‐scale circulation are large (tens of centimeters per year or more) or small (a few millimeters per year or less). Resolution of the controversy is particularly important in sedimentary basins in which salt beds are unknown but interstitial brines are pervasive. An example of one such basin is the Illinois Basin. The total dissolved solids (TDS) content of interstitial waters above the Upper Devonian New Albany Shale in the Illinois Basin increases with depth at an average rate of 15 wt% km−1. Mass transport calculations show that if no salt dissolved above the New Albany, upward diffusion alone should have lowered the salinity gradient over a 200 m.y. time span (i.e., since the Mid‐Triassic) to between 10 and 30% of the present value. The high salinity gradient found today requires either that halite dissolved slowly in rocks above the New Albany Shale over the past 200 m.y. or that the paleosalinity gradient, say, 200 m.y.B.P., was much greater than the value today. The flushing out of brines by meteoric water was simulated in a rectangular cross section of a generic basin 500 km wide and 1.5 km deep with a freshwater head gradient of 0.06% across the top. Simulations were run using the computer code PORFLOW (Runchal, 1987) which models coupled transport of fluid, heat, and dissolved salt. The simulations of brine flushing suggest that if topography‐driven flow occurred for time spans of several million years, then the preservation of brines in rocks above the New Albany shale could only have been possible if macroscopic flow rates (flow rates averaged over large rock volumes) were very small, on the order of 10−3m yr−1or less. When the horizontal permeability was 10−13m2(10−1darcy) and the vertical permeability 10−14m2, brines that initially filled the basin were flushed out and replaced by fresh water in less than 5 m.y., despite the fact that the initial concentration gradient assumed was almost twice the present day value. For such high permeabilities, even very high salt dissolution rates equivalent to the removal of 4.7 m of halite column per million years were insufficient to maintain high concentrations. When the permeabilities were lower, 10−15m2along the horizontal and 10−16m2along the vertical, high dissolved salt concentrations were maintained for longer times. Even so, about half of the dissolved salt initially in the pore waters was

ISSN:0043-1397    

DOI:10.1029/93WR01812

年代:1993

数据来源: WILEY

摘要:

Annual natural discharge (Q) of the River Murray and its most extensive tributary, the Darling River system, is often inversely related to sea surface temperature (SST) anomalies in the eastern tropical Pacific Ocean. These SST variations are components of a planetary‐scale phenomenon referred to as El Nino‐Southern Oscillation (ENSO). Darling and Murray river historical values ofQindicate that annual surface runoff from regions dominated by subtropical summer monsoon precipitation and annual surface runoff primarily responding to temperate winter storms are both strongly influenced by ENSO cycles. Forecasting, approximately 1 year in advance, of ENSO‐related SST from geophysical model calculations thus provides a mechanism for estimating probabilities of annual river discharge amount. Contingency tables relating annualQto SST, based on combining observed data for 95 years and forecast SST over a period of 15 years, provide probabilities of expected annualQas a function of forecast SST. The SST of the eastern tropical Pacific was successfully forecast to be appreciably warmer than long‐term mean conditions for much of the year beginning in mid 1991. Precipitation data through 1991 indicated that annual naturalQfor the Darling River was probably substantially below the mean. However, winter precipitation in higher‐runoff portions of the Murray Basin was above average during this El Nino episode, contrary to the trend for most such events over the pas

ISSN:0043-1397    

DOI:10.1029/93WR01492

年代:1993

数据来源: WILEY

摘要:

The evaporative fraction is a ratio of latent heat flux to the sum of latent and sensible heat fluxes. It has been used to characterize the energy partition over land surfaces and has potential for inferring daily energy balance information based on midday remote sensing measurements. The HAPEX‐MOBILHY program SAMER system provided surface energy balance data over a range of agricultural crops and soil types. Data from this large‐scale field experiment was analyzed to study the behavior and daylight stability of the evaporative fraction in both ideal and general meteorological conditions. Strong linear relations were found to exist between the midday evaporative fraction and the daylight evaporative fraction. Statistical tests, however, rejected the hypothesis that the two quantities were equal. Relations between the evaporative fraction and surface soil moisture as well as soil moisture over the complete root zone were explored, but no correlation was identif

ISSN:0043-1397    

DOI:10.1029/93WR01958

年代:1993

数据来源: WILEY

摘要:

Installation of surface levees and subsurface slurry walls around a 1600‐m by 950‐m hazardous waste landfill in southeastern Louisiana has inadvertently converted the site into a large‐scale permeameter. Differences in water levels in wells screened above and below a 15‐m‐thick clay “confining layer” define a vertical hydraulic gradient of +0.1. Basic climatological data permit calculation of a complete water budget for the site, including vertical rechargeqzdown through the clay. Cumulative precipitation over a 44‐month period was 5.5 m, and recharge was over 1.0 m. The calculated vertical hydraulic conductivity of the clay sequence is approximately 10−5cm s−1, up to 4 orders of magnitude higher than laboratory values for the same sediment. Intercalated sands and zones of pedogenic secondary porosity and fracturing developed during periods of subaerial weathering are apparently the dominant controls on vertical permeability, not the matrix pro

ISSN:0043-1397    

DOI:10.1029/93WR01913

年代:1993

数据来源: WILEY

摘要:

Two‐phase (air‐water) flow experiments were conducted in horizontal artificial fractures. The fractures were between glass plates that were either smooth or artificially roughened by gluing a layer of glass beads to them. One smooth fracture with an aperture of 1 mm and three rough fractures, one with the two surfaces in contact and two without contact, were studied. For both types of fractures, the flow structures are similar to those observed in two‐phase flow in a pipe, with structures (bubbles, fingering bubbles, films, and drops) depending on the gas and liquid flow rates. The pressure gradients measured for different liquid and gas velocities were interpreted by three models. First, using Darcy's law leads to relative permeability curves similar to conventional ones for porous media. However, these curves depend not only on saturation but also on flow rates. This effect is caused by inertial forces which are not included in this approach. Second, the standard approach for two‐phase flow in pipes (Lockhart and Martinelli's equation) agrees with experimental results, at least for small pressure gradients. Finally, the best fit was obtained by treating the two phases as one homogeneous phase. All the properties are averaged, and the pressure drop is deduced from an empirical correlation between the two‐phase Reynolds number and the fricti

ISSN:0043-1397    

DOI:10.1029/93WR01529

年代:1993

数据来源: WILEY