Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. The original document contains color images. ABSTRACTIn this paper we quantitatively tested the hypothesis that soil freeze-thaw(FT) processes significantly increase the potential for upland hillslope erosion during runoff events that follow thaw. We selected a frost-susceptible silt to obtain an upper bound on FT effects, and completed three series of six experiments each to quantify differences in soil erodibility and rill development for bare soil following a single FT cycle. Each series represented a specific soil moisture range: 16-18%, 27-30%, and 37-40% by volume, with nominal flow rates of 0.4, 1.2, and 2.4 L/min and slopes of 8º and 15º. Each experiment used two identical soil bins, one a control (C) to remain unfrozen, the other to be frozen and thawed. Standard soil characterization tests did not detect significant differences between the FT and C bins. Experimental results were closely related to conditions of the experiment, imposing a requirement for minimum differences in soil weight, bulk density, and soil moisture through each series. We measured cross-sectional geometry of an imposed straight rectangular rill before each experiment, sediment load during, and rill cross sections after. Changes in cross section provided detailed measures of erosion at specific locations along the rill, while sediment load from time series runoff samples integrated the rill erosion. Several parameters, including average maximum rill width, average maximum rill depth, rill cross-section depth measures, and sediment load all followed similar trends. Each was greater in the FT than in the C, with values that generally increased with slope and flow. However, soil moisture was the only parameter that affected the FT/C relationship. For example, average sediment load grouped by soil moisture provided FT/C ratios of 2.4, 3.0, and 5.0 for low, mid, and high moisture, respectively. In contrast, a "dry" experiment at 4.4% soil moisture had FT/C of 1.02 for sediment load. These results indicate a dramatic increase in the rate and quantity of bare soil eroded as a result of the FT cycle that is in direct proportion to soil moisture. ABSTRACTIn this paper we quantitatively tested the hypothesis that soil freeze-thaw (F...

Low-frequency (< 10 Hz) volcanic earthquakes originate at a wide range of depths and occur before, during, and after magmatic eruptions. The characteristics of these earthquakes suggest that they are not typical tectonic events. Physically analogous processes occur in hydraulic fracturing of rock formations, low-frequency icequakes in temperate glaciers, and autoresonance in hydroelectric power stations. We propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low-frequency volcanic earthquakes (tremor). The fluid dynamic source mechanism explains lowfrequency earthquakes of arbitrary duration, magnitude, and depth of origin, as unsteady flow is independent of physical properties of the fluid and conduit. Fluid transients occur in both lowviscosity gases and high-viscosity liquids. A fluid transient analysis can be formulated as generally asis warranted by knowledge of the composition and physical properties of the fluid, material properties, geometry and roughness of the conduit, and boundary conditions. To demonstrate the analytical potential of the fluid dynamic theory, we consider a single-phase fluid, a melt of Mount Hood andesite at 1250øC, in which significant pressure and velocity variations occur only in the longitudinal direction. Further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations. For a simple, constant pressure reservoir-conduit-orifice fluid system, a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour. Fluid kinematic viscosity (0.412 m2/s) is both included and neglected in the analysis to illustrate its effect upon system damping. Tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures. The entire volcanic fluid system does not generally experience a transient flow condition at a given time. Closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system. Total fluid movement may not be accurately monitored by tremor data alone, since steady state and slowly changing flows are aseismic.

In this paper we consider long‐period, shallow‐water waves in rivers that are a consequence of unsteady flow. River waves result from hydroelectric power generation or flow control at a dam, the breach of a dam, the formation or release of an ice jam, and rainfall‐runoff processes. The Saint‐Venant equations are generally used to describe river waves. Dynamic, gravity, diffusion, and kinematic river waves have been defined, each corresponding to different forms of the momentum equation and each applying to some subset of the overall range of river hydraulic properties and time scales of wave motion. However, the parameter ranges corresponding to each wave description are not well defined, and the transitions between wave types have not been explored. This paper is an investigation into these areas, which are fundamental to river wave modeling. The analysis is based on the concept that river wave behavior is determined by the balance between friction and inertia. The Saint‐Venant equations are combined to form a system equation that is written in dimensionless form. The dominant terms of the system equation change with the relative magnitudes of a group of dimensionless scaling parameters that quantify the friction‐inertia balance. These scaling parameters are continuous, indicating that the various river wave types and the transitions between them form a spectrum. Additional data describing the physical variability of a river and wave are incorporated into the analysis by interpreting the scaling parameters as random variables. This probabilistic interpretation provides an improved estimate of the friction‐inertia balance, further insight into the continuous nature of wave transitions, and a measure of the reliability of wave type assessments near a transition. Case studies are used to define the scaling parameter ranges representing each wave type and transition and to provide data with which to evaluate the usefulness of the analysis for general application.

An understanding of the downstream propagation of sharp-fronted, large-amplitude waves of relatively short period is important for describing rapidly varying flows in tailwaters of hydroelectric plants and following the breach of a dam. We developed a numerical model of these waves by first identifying the primary physical processes and then performing an analysis of the solution. A linear analysis of the dynamic open channel flow equations provides relationships describing flow wave advection, diffusion, and dispersion in rivers. A one-dimensional diffusion wave model modified for application to tailwaters simulates the important physical processes and is straightforward to apply. The "modified equation" and von Neumann analyses provide insight into the effects of numerical parameters 0, Ax, and At upon stability and dissipative and dispersive behavior of the solution, but the Hirt analysis is found to yield incorrect phase relationships. The capability and accuracy of the model are enhanced when physical diffusion of a river wave is balanced by numerical diffusion in the model. Field studies were conducted in two greatly different tailwaters to assess our understanding of large-scale, rapidly varying flow waves. The accurate simulation of waves having wide-ranging amplitudes, shapes, periods, and base flows attests to the soundness of both the physical basis of the model and the numerical solution technique. These studies reveal that diffusion of short-period waves in natural, free-flowing rivers is significant and that inertia is negligible. FERRICK ET AL.' TAILWATER FLOW ditions required for stability of many numerical schemes are known, and numerical instability is generally apparent. The von Neumann and Hirt analyses are frequently used to develop stability criteria I-Roache, 1976]. Numerical solutions of the unsteady open channel flow equations typically exhibit errors in both amplitude and phase that may not be apparent without further analysis. Numerical dissipation or diffusion causes the Fourier components of the solution and the errors to be damped. Numerical dispersion results when the modeled wave celerity of some wavelength components differ from those of the governing equation, improperly modifying the wave form as the computation proceeds. The effects of numerical dissipation and dispersion upon the solution are subtle but gradually destroy the correspondence between model and prototype. An improved understanding of the dissipative and dispersive behavior of the numerical model enables the analyst to minimize or exploit these effects to enhance model accuracy and to better interpret computed results. Though only strictly applicable to linear equations, the "modified equation" [Warming and Hyett, 1974], Hirt and von Neumann analyses are used to relate the dissipative and dispersive behavior of the model to parameters of the numerical solution. A set of linear routings is used to demonstrate the model behavior predicted with the analysis and to verify the adequacy of an expression for numerical dif...