Lawrence W. Gatto

Soil freeze–thaw-induced changes to a simulated rill: potential impacts on soil erosion

Abstract Flows in natural rills on hillslopes transport significantly more sediment down slope than overland flows, which makes rills geomorphically significant landscape features. The erosivity, e.g., sediment transport capacity, of the flows in rills is partially a function of the cross-sectional geometry of the rill which determines flow velocity and depth. Conversely, rill geometry is determined by flow erosivity and by soil processes that affect the erodibility and stability of soil along the rill. Thus, complex feedbacks exist in the mechanics of rill erosion. The objective of this study was to measure the effects of one of the soil processes that affect rill geometry, soil freeze–thaw (FT) cycling. An unvegetated rectangular rill was subjected to two FT cycles, each lasting several days, in a controlled laboratory setting. The FT cycling increased the water content and reduced the cohesion in the surface soil along the rill sufficiently to induce soil slumps and mud flows along the sidewalls of the rill. These mass failures changed the rectangular rill to a triangular one, which reduced the hydraulic radius of the rill by 32%. Using Mannings equation, it was estimated that this new geometry could reduce the velocity of the flow in this altered rill by about 25%. The persistence of the new rill shape and, thus, that of the slower flow would depend on the complex interactions between flow velocity and the resistance of the slumped sediment to those flows. That persistence was not investigated in this experiment. These results can be used in parameterizing models of rill evolution that incorporate widening of rills by mass failures along the rill sidewalls.

Alaskan thermokarst terrain and possible martian analog.

A first-order analog to martian fretted terrain has been recognized on enhanced, ERTS-1 (Earth Resources Technology Satellite) imagery of Alaskan Arctic thermokarst terrain. The Alaskan analog displays flat-floored valleys and intervalley uplands characteristic of fretted terrain. The thermokarst terrain has formed in a manner similar to one of the processes postulated for the development of the martian fretted terrain.

Soil compaction and over-winter changes to tracked-vehicle ruts, Yakima Training Center, Washington

Abstract We monitored two experimental areas at the Yakima Training Center (YTC) in central Washington to measure changes to M1A2 Abrams (M1) tank-rut surface geometry and in- and out-of-rut saturated hydraulic conductivity ( K fs ), soil penetration resistance (SPR) and soil bulk density (BD). Profile-meter data show that rut cross-sectional profiles smoothed significantly and that turning ruts did so more than straight ruts. Rut edges were zones of erosion and sidewall bases were zones of deposition. K fs values were similar in and out of ruts formed on soil with 0–5% moisture by volume, but were lower in ruts formed on soil with about 15% water. Mean SPR was similar in and out of ruts from 0- to 5-cm depth, increased to 2 MPa outside ruts and 4 MPa inside ruts at 10- to 15-cm depth, and decreased by 10–38% outside ruts and by 39–48% inside ruts at the 30-cm depth. Soil BD was similar in and out of ruts from 0- to 2.5-cm depth, and below 2.5 cm, it was generally higher in ruts formed on moist soil with highest values between 10- and 20-cm depth. Conversely, BD in ruts formed on dry soil was similar to out-of-rut BD at all depths. This information is important for determining impacts of tank ruts on water infiltration and soil erosion and for modifying the Revised Universal Soil Loss Equation (RUSLE) and the Water Erosion Prediction Project (WEPP) models to more accurately predict soil losses on army training lands.

Overwinter changes to near-surface bulk density, penetration resistance and infiltration rates in compacted soil

Abstract Previous studies at Yakima Training Center (YTC), in Washington State, suggest freeze-thaw (FT) cycles can ameliorate soil compacted by tracked military vehicles [J. Terramechanics 38 (2001) 133]. However, we know little about the short-term effects of soil freezing over a single winter. We measured bulk density (BD), soil penetration resistance (SPR), and steady-state runoff rates in soil newly tracked by an Abrams tank and in uncompacted soil, before and after a single winter at YTC. We similarly measured BD, SPR and saturated hydraulic conductivity (kfs) in simulated tank tracks at another site near Lind Washington. Average BD was significantly greater in tank ruts at YTC and in simulated tracks at the Lind site than in uncompacted soil soon after tracking and did not change significantly during the winter of 1997–1998. Measurements of SPR were strongly influenced by soil moisture. When soil was moist or tracks were newly formed, SPR was significantly higher in tank ruts than in uncompacted soil from the surface to a depth of about 10–15 cm. The greatest average SPR in compacted soil was observed between 4 and 6 cm depth. We observed less difference in SPR between tank ruts and uncompacted soil near-surface at YTC as the time after trafficking increased. We observed highest SPR ratios (compacted rut:undisturbed) in fresh tracks near the surface, with lower ratios associated with increasing track age or soil depth, indicating that some recovery had occurred at YTC near-surface. However, we did not observe a similar over-winter change in SPR profiles at the Lind site. Rainfall simulator data from YTC showed higher steady-state runoff rates in tank ruts than over uncompacted soil both before and after winter. However, more time was required to reach steady-state flow in tank ruts and the proportion of runoff was slightly lower in May 1998 than in August 1997. At the Lind site, kfs was lower in newly compacted soil than in one-year old compacted soil or uncompacted soil. Our data suggest that indices of water infiltration such as steady-state runoff rates or kfs, are more sensitive indicators of soil recovery after compaction than are BD or SPR.

Bank conditions and erosion along selected reservoirs

This analysis was done as the preliminary step in an ongoing study of reservoir bank erosion processes that are active in the northern U.S. The objectives of this analysis were to observe and document bank characteristics, conditions, and changes along reservoirs with eroding banks, to estimate the amounts of historical bank recession and to discuss its possible causes. Aerial photographs were used to observe the historical bank changes and estimate bank recession. Site reconnaissance, discussions with on-site personnel, and published reports were used to evaluate possible relationships between the local erosion and bank conditions. As part of this analysis linear regressions were done to determine if the estimated recession rates correlate with selected bank and climatic conditions and with physical characteristics of the reservoirs. The regression results, however, were generally not useful because they suggested relationships that are contrary to field observations and published results. Dominant bank erosion processes were wind-wave erosion, capillary wave erosion during high-water periods, groundwater-induced sliding, freeze-thaw processes, rain splash and rainwash, and boat waves. However, because of the complexity of the interrelationships of these and many other bank erosion processes and the variability of the processes at and between sites, it would be necessary to make site-specific measurements and observations year-round to evaluate the processes that are active along a particular bank.

Monitoring river ice with landsat images

Abstract In the northern United States, ice can delay or stop river navigation in the winter and cause unexpected problems and emergencies. As part of a program to develop a river ice forecasting model, photointerpretation techniques were used to map the areal distributions of four classes of river ice along the navigable reaches of the Allegheny, Monongahela, and Ohio Rivers and the Illinois Waterway each winter from 1972 to 1985 from Landsat Multispectral Scanner (MSS Band 2, 0.6–0.7 μm), Thematic Mapper (TM Band 3, 0.63–0.69 μm), and Return Beam Vidicon (RBV, 0.580–0.680 μm and 0.505–0.750 μm) images. The four classes, 1) ice-free, 2) partial gray ice, 3) complete gray ice, and 4) white ice, were usually readily apparent on the images due to differences in gray tones produced by the various ice types and conditions that make up the different classes. Landsat-derived ice observations compared favorably with available ground and aerial observations 64–80% of the time. During mild winters without extensive and long-lasting ice, or along rivers which normally do not have such ice, Landsat images are less-useful because of the time gaps between cloud-or haze-free images. Yet for many rivers in cold regions, Landsat images may be the only source of data on river ice.

Reservoir bank erosion caused by ice

Abstract The purpose of this study was to evaluate the documented and potential importance of ice erosion along reservoir banks. The evaluation is based on a literature review and on inferences drawn from field observations and experience. Very little is known about the amount of reservoir bank erosion caused by ice action, although considerable information exists on ice erosion processes along the shorelines and beaches of oceans, rivers and lakes. The importance of ice-related erosion along a reservoir bank would depend primarily on water level, but ice conditions and bank sediment characteristics would also be important. If the reservoir water level is at bank level, ice could directly erode a bank face. If the water is below the bank, ice would have no direct effect on it. However, ice could indirectly increase bank instability by disrupting and eroding nearshore and beach zones, which could lead to bank erosion.

Ice Distribution and Winter Surface Circulation Patterns, Kachemak Bay, Alaska,

Abstract Development of the hydropower potential of Bradley Lake, Alaska, would greatly increase winter freshwater discharge from the Bradley River into Kachemak Bay, which may result in increased ice formation and related ice-induced problems. The objectives of this investigation were to describe winter surface circulation in the bay and document ice distribution patterns for predicting where additional ice might be transported if it forms. Landsat MSS bands 5 and 7 and RBV imagery with 70% cloud cover or less, taken between 1 November and 30 April each year from 1972 to 1980, were analyzed. Surface circulation patterns inferred from suspended sediment patterns and ice distribution and movement were observed and mapped from the Landsat imagery. The generalized circulation patterns indicate that any additional ice formed due to future increased winter discharge from Bradley River would be likely to accumulate along Homer Spit and to be blown into the outer bay by the dominant northerly winter winds.