Darren B. Sjogren

Incipient tunnel channels

Abstract Glaciated terrains in east-central Alberta and south-central Michigan contain channels that have hummocks and transverse ridges separating depressions along their floors. This association imparts a linked pothole appearance. Similar channels are often interpreted as tunnel channels or subaerial channels, partly filled with sediment from a subsequent glacial advance, a stagnating ice roof, or slumped sediment from the channel margins. However, the truncation of sedimentary packages in the channel walls and intrachannel hummocks indicates that they are erosional landforms, cut into glacial sediments (till), bedrock, or gravel. Eskers overlie and are found within a few channels, indicating that these channels formed before the final stagnation that produced the eskers. These two characteristics, combined with the observation that many channels have convex-up long profiles, indicate that the channels were eroded by pressurized, subglacial water. Because the formative mechanisms for this type of channel are not clear, and modern environments that could produce this type of landform are inaccessible, we draw on several morphologic analogues to propose mechanisms for channel erosion. We conclude that the erosion of these linked pothole channels (incipient tunnel channels) was the product of the complex interaction between complex turbulent flow structures and various scales of roughness elements.

An evaluation of electrical resistivity imaging (ERI) in Quaternary sediments, southern Alberta, Canada

The ability to characterize the geometry and lithology of Quaternary sediments is important to scientists who investigate groundwater movement, geoarchaeology, materials prospecting (e.g., gravel), environmental contamination and remediation, and paleoenvironmental studies. Often these studies are restricted by the limited information attainable via traditional geomorphological techniques. While there are geophysical methods for gaining information about the near-subsurface, such as ground penetrating radar (GPR) or shallow seismic surveys, they only function well under select conditions. Electrical resistivity imaging (ERI) can quickly produce high-resolution images of the shallow subsurface under many field conditions. ERI measurements work well in both resistive sediments, such as gravels and sands, as well as conductive sediments like silt and clay. Resistivity is an inherent property of all materials, and it measures the degree to which a material resists the flow of electrical current. If a current is introduced into the ground, the resulting electrical field can be measured. Thus, a two-dimensional cross section can be produced showing the resistive properties of a sediment package several meters behind an exposure. This aids in the interpretation of the material and structural features that may be present but not exposed. This methodology is successful in imaging some subsurface architecture, but there are limitations to the resolution of the surveys. ERI, when integrated with detailed geomorphologic analysis, provides enhanced insight for inferring the processes of sediment emplacement and deformational processes.

Rates of planimetric change in a proglacial gravel-bed braided river: field measurement and physical modeling: Rates of planimetric change in a braided river

Planimetric change was measured on daily hydrographs over two meltwater seasons using time-lapse images of the proglacial, gravel, braided, Sunwapta River, Canada. Significant planimetric change occurred on 10–15 days per year. Area of planimetric change correlated with peak and total daily meltwater hydrograph discharge. A clear threshold discharge can be identified below which no planform activity occurs, an intermediate range over which change occurs conditionally, and a peak flow range at which significant change always occurs. Field conditions were reproduced in a physical model in a laboratory flume. Photogrammetric DEMs of bed morphology and measurements of bedload output were made for each hydrograph experimental run. The physical model results for planimetric change had a threshold discharge for change, and trend with discharge, similar to the field data. The model data also show that planimetric change correlates strongly with volumes of erosion/deposition measured from successive DEMs, and with bedload transport rate. The relation between planimetric change and topographic change is also apparent from previous cross-section surveys at the field site. The results highlight the planimetric dynamics of braiding rivers in relation to discharge forcing, and the relationship between planimetric change, morphological change, and bedload transport in braided rivers. This also points to the potential use of measurements of planimetric change from time-lapse imagery as a low-cost method for high-frequency monitoring for braiding dynamics and also a surrogate for bedload transport measurement.