Peter J. Whiting

Sediment-transporting flows in headwater streams

The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 km 2 , stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.

Alluvial architecture in headwater streams with special emphasis on step–pool topography

Alluvial mountain streams exhibit a range of channel forms: pool–riffle, plane bed, step–pool and cascades. Previous work suggested that these forms exist within discrete, and progressively steeper slope classes. Measurements conducted at over 100 sites in west-central and central Idaho confirm that slope steepens progressively as one moves from pool–riffle, to plane bed, to step–pool, and finally to cascades. Median slope for pool–riffle topography is 0·0060, for plane beds 0·013, for step–pools 0·044, and for cascades 0·068. There is substantial overlap in the slopes associated with these channel forms. Pool–riffle topography was found at slopes between 0·0010 and 0·015, plane beds between 0·0010 and 0·035, step–pools between 0·015 and 0·134, and cascades between 0·050 and 0·12. Step–pools are particularly striking features in headwater streams. They are characterized by alternating steep and gentle channel segments. The steep segments (step risers) are transverse accumulations of boulder and cobbles, while the gentle segments (pools) contain finer material. Step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers. This result differs from past studies that have reported channel slope to be the dominant control on step wavelength. The presumed geometry and Froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step–pools are formed by antidunes. Copyright

ANNUAL HYSTERESIS IN BED LOAD RATING CURVES

The relationship between flow and bed load transport measured for 10 years in six gravel-bed streams in Idaho exhibits annual hysteresis. At a given flow rate, more bed load is carried by discharges preceding the first annual occurrence of a “threshold” rate, which is characteristic of each stream. Incorporating the effect of hysteresis leads to a small improvement in the fit of the bed load–flow regression. As the turning point for hysteresis, a constant threshold discharge is found to work better than the annual peak discharge. This bimodal hysteresis model is also found to out perform one with a more gradual transition, based on cumulative discharge. These results are interpreted to reflect a buildup of readily moved sediment supplies during the low-flow periods from late summer to early spring, supplies which are then exhausted by rising springtime discharges up to the threshold. The threshold is greater than mean annual discharge and about one-half bank-full discharge.

Suspended sediment sources and transport distances in the Yellowstone River basin

The activity of fallout radionuclides ( 7 Be, 1 3 7 Cs, and 2 1 0 Pb) was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the Yellowstone River basin. Samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of Soda Butte Creek to Billings, Montana, a 423-km-long reach of channel. The inventory of radionuclides in soil increases with precipitation and is highest in the headwaters. The activity of radionuclides in suspended sediment decreases downstream, and more activity is observed earlier than later in the flood hydrograph. The radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion. Fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment. At sites high in the drainage, soils contribute 50% to the suspended load and this value decreases to 11%-26% downstream. Fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source. Transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream. These estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure. This study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size.

A numerical study of bank storage and its contribution to streamflow

Abstract Floodplains mitigate against extreme annual hydrologic phenomena by storing substantial volumes of water which would otherwise increase flood volumes. Later floodplains gradually release this water which serves to maintain baseflows. This phenomenon, called bank storage, has important physical and ecological ramifications which in addition to reducing flood peaks, include sustaining riparian vegetation and improving water quality. We developed a model of bank storage based upon the earlier work of Neuman and Witherspoon (Wat. Resour. Res., 6 (1990) 889, 1376; 7 (1971) 611). This model (WaTab2D) treats the flow of water through floodplain soils for a general two-dimensional case: non-symmetrical valleys, non-symmetrical channel banks, non-uniform hydraulic geometry and non-zero boundary fluxes. The total volume that can potentially be released from bank storage is nearly proportional to the width of the floodplain, the height of the bank, and the specific yield; the duration over which water is released from bank storage increases with increasing floodplain width and decreases with hydraulic conductivity. Drainage of water from the floodplain with a drop in channel water level occurs over a period of days in gravel, weeks to a few years in sand, years in silt, and decades in clay. The rate of drainage decreases in an exponential-like manner.

Depth and areal extent of sheet and rill erosion based on radionuclides in soils and suspended sediment

Sheetwash and rilling are two important mechanisms of soil erosion by runoff. The relative contribution of each mechanism has been a vexing question because measuring thin sheet erosion is difficult. Fortuitously, various fallout radionuclides have distinct distributions in the soil column; thus, different depths of erosion produce suspended sediment with unique radionuclide signatures. Those signatures can be used to estimate the depth and areal extent of sheet and rill erosion. We developed a model to execute multiple mass balances on soil and radionuclides to quantify these erosion mechanisms. Radionuclide activities (7Be, 137Cs, 210Pb) in the soil of a 6.03 ha agricultural field near Treynor, Iowa, and in suspended sediment washed off the field during thunderstorm runoff were determined by gamma spectroscopy. Using the model, we examined 15.5 million possible combinations of the depth and areal extent of rill and sheet erosion. The best solution to the mass balances corresponded to rills eroding 0.38% of the basin to a depth of 35 mm and sheetwash eroding 37% of the basin to a depth of 0.012 mm. Rill erosion produced 29 times more sediment than sheet erosion.

The hydrology and form of spring-dominated channels

Abstract The form of channels that receive the bulk of their flow from springs differs in a number of ways from channels receiving direct runoff from rain and snowmelt. Spring-dominated channels tend to have poorly developed bars, and logs and aquatic vegetation may clog the channel. The organic-rich floodplain soils of spring-dominated channels are very moist. Channel beds lack a cover of fines indicating that sediment transport occurs frequently enough to flush fine sediment even though the drainage area of channels is very small. These differences in form reflect the distinctive hydrology. The range of discharge in spring-dominated channels is narrow. Spring-dominated channels often flow at bankfull or above 20 percent of the time while the typical value for runoff-dominated channels is 2–4 percent. At Browns Creek, an example of spring-dominated channels, the ratio of baseflow to bankfull flows is 0.65, whereas the respective value for runoff-dominated channels is 0.10. Peak flows often occur in late summer or fall whereas peak flows in runoff-dominated channels in the study areas occur with the spring snowmelt. Bankfull flows occur with a recurrence interval of about 1.67 years which is roughly equivalent to values for snowmelt- and runoff-dominated channels.

Measuring Soil Erosion Rates Using Natural (7Be, 210Pb) and Anthropogenic (137Cs, 239,240Pu) Radionuclides

This chapter examines the application of natural (7Be and 210Pb) and anthropogenic fallout radionuclides (134Cs, 137Cs, 239,240Pu) to determine soil erosion rates. Particular attention is given to 137Cs because it has been most widely used in geomorphic studies of wind and water erosion. The chapter is organized to cover the formation and sources of these radionuclides; how they are distributed in precipitation and around the globe: their fate and transport in undisturbed and tilled soils; and their time scales of utility. Also discussed are methods for soil collection, sample preparation for 137Cs analysis by gamma spectroscopy, and the selection of standards and instrument calibration. Details are presented on methods for calculating soil erosion, including empirical methods that are related to the Universal Soil Loss Equation (USLE), box models that compare 137Cs activities in a study site to a reference site, and time dependent methods that account for the temporal inputs of 137Cs and precipitation induced erosion. Several examples of recent applications, including the combination of radionuclides with other techniques or measurements, are presented. The chapter concludes with suggestions for future work: the value of new methods and instrumentation to allow for greater spatial resolution of rates and/or greater accuracy; the need to incorporate migration of radionuclides in the time-dependent models; the opportunities to concurrently use the global and Chernobyl signals to better understand temporal variation soil erosion processes and rates; and the importance of the use of these tracers to characterize C storage and cycling.

The Effect of Stage on Flow and Components of the Local Force Balance

Flow fields and water and bed surface topography were measured at two different stages as flow shoals over a submerged mid-channel bar in a straight reach downstream of a bend in Solfatara Creek, Wyoming. The data allow calculation and comparison of the magnitude of the component terms in the downstream and cross-stream force balance at the different stages. At the lower stage, corresponding to a discharge that is 30 per cent of the bankfull discharge, the convective acceleration terms in the equations describing the force balance are important, particularly the terms associated with the cross-stream transport of momentum. These terms are large because of the large accelerations and cross-stream flow forced by the shallow flow over the bar. At the higher stage, corresponding to a discharge that is 45 per cent of the bankfull discharge, flow is more directly downstream and cross-stream velocity is generally less in most of the channel. Downstream flow velocities at the higher stage are larger, but the acceleration is more gradual. Consequently, the convective accelerations at the higher stage tend to be less important than at the lower stage. Results from the two different stages suggest that some of the difference in conclusions reached by various workers on the significance of the various terms in the governing equations may be associated with the relative depth of flow.

Reconstruction of subgrid-scale topographic variability and its effect upon the spatial structure of three-dimensional river flow

A new approach to describing the associated topography at different scales in computational fluid dynamic applications to gravel bed rivers was developed. Surveyed topographic data were interpolated, using geostatistical methods, into different spatial discretizations, and grain-size data were used with fractal methods to reconstruct the microtopography at scales finer than the measurement (subgrid) scale. The combination of both scales of topography was then used to construct the spatial discretization of a three-dimensional finite volume Computational Fluid Dynamics (CFD) scheme where the topography was included using a mass flux scaling approach. The method was applied and tested on a 15 m stretch of Solfatara Creek, Wyoming, United States, using spatially distributed elevation and grain-size data. Model runs were undertaken for each topography using a steady state solution. This paper evaluates the impact of the model spatial discretization and additional reconstructed-variability upon the spatial structure of predicted three-dimensional flow. The paper shows how microtopography modifies the spatial structure of predicted flow at scales finer than measurement scale in terms of variability whereas the characteristic scale of predicted flow is determined by the CFD scale. Changes in microtopography modify the predicted mean velocity value by 3.6% for a mesh resolution of 5 cm whereas a change in the computational scale modifies model results by 60%. The paper also points out how the spatial variability of predicted velocities is determined by the topographic complexity at different scales of the input topographic model.