Constraining bedrock erosion during extreme flood events

The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have a long lasting legacy on landscape morphology. Geomorphological mapping and topographic analysis document the evidence for, and impact of, extreme flood events within the Jökulsárgljúfur canyon (North-East Iceland). Surface exposure dating using cosmogenic 3He of fluvially sculpted bedrock surfaces determines the timing of the floods that eroded the canyon and helps constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three periods of rapid canyon cutting during erosive flood events about 9, 5 and 2 ka ago, when multiple active knickpoints retreated large distances (> 2 km), each leading to catastrophic landscape change within the canyon. A single flood event ~9 ka ago formed, and then abandoned, Ásbyrgi canyon, eroding 0.14 km3 of rock. Flood events ~5 and ~2 ka ago eroded the upper 5 km of the Jökulsárgljúfur canyon through the upstream migration of vertical knickpoints such as Selfoss, Dettifoss and Hafragilsfoss. Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1); there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor. The erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the discharge of the river, large knickpoints and associated plunge pools. The influence of these controls on the dynamics of knickpoint migration and morphology are explored using an experimental study. The retreat rate of knickpoints is independent of both mean discharge, and temporal variability in the hydrograph. The dominant control on knickpoint retreat is the knickpoint form which is set by the ratio of channel flow depth to knickpoint height. Where the knickpoint height is five times greater than the flow depth, the knickpoints developed undercutting plunge pools, accelerating the removal of material from the knickpoint base and the overall retreat rate. Smaller knickpoints relative to the flow depth were more likely to diffuse from a vertical step into a steepened reach or completely as the knickpoint retreated up the channel. These experiments challenge the established assumption in models of landscape evolution that a simple relationship exists between knickpoint retreat and discharge/drainage area. In order to fully understand how bedrock channels, and thus landscapes, respond and recover to transient forcing, further detailed study of the mechanics of erosion processes at knickpoints is required

Fill characteristics of abandoned channels and resulting stratigraphy of a mobile sand‐bed river floodplain

Floodplains are diverse sedimentary environments where infill processes of abandoned channels interact with overbank sedimentation and bank erosion. The result, particularly in river systems with high suspended load and rapid channel migration, is a complex three‐dimensional mosaic of deposits with spatial variability in terms of grain‐size, age, organic carbon content and resistance to erosion. Abandoned channels represent a significant deposition volume in fluvial systems that can accommodate large proportions of the equivalent material mobilised during their abandonment. However, time scales and fill processes vary between different kinds of abandoned channels and the sediment calibre involved and are not fully understood, particularly in respect to highly dynamic sand‐bed rivers. This study investigates time scales and spatio‐temporal patterns of infill of abandoned chute channels and abandoned channel segments left behind following neck cutoff of meander bends. The study focuses on the Rio Beni, a large, tropical, sand‐bed river in the Bolivian Amazon basin. Electrical resistivity ground imaging is used to elucidate the stratigraphy of floodplains and satellite imagery is employed to investigate contemporary fill processes and rates. Given suitable bend migration patterns, chute channels may remain stable for several years but are eventually abandoned and rapidly filled with bed material during a single flood season. Smaller scroll sloughs can convey coarse bedload across point bars and, when filled, present stratigraphic bodies similar to chute fills. Abandoned meander bends tend to develop plug bars at both ends immediately after cutoff. Of these bars, the downstream plug aggrades at a faster rate due to the often larger diversion angles with the main channel and efficiently seals off the bend. The subsequent infill of the channel is a function of hydraulic connectivity and distance to the active channel as well as rate of lake deposition. Considerable overbank deposition can increase the spatial sedimentological heterogeneity of these floodplains, which needs to be taken into account in floodplain evolution models

A Multidimensional Research Strategy for the Evaluation of Settlement Pits: 3D Electrical Resistivity Tomography, Magnetic Prospection and Soil Chemistry

Irrespective of the region and time period, settlement pits are common features at archaeological sites. Variations in shape, size and fill reflect great diversity in terms of primary and secondary functions. Careful analysis of these aspects and a pit's refilling history yield key information for the reconstruction of economic, social and chronological aspects of settlement history. At present, it is unusual for a settlement site to undergo complete excavation; in most cases, only small areas within a larger settlement are opened to archaeological inquiry and, accordingly, only a scarce few pits can be excavated. Therefore, the application of a combination of new prospection methods can help to classify and preselect representative pits for subsequent excavation in order to guarantee the highest information output. The interdisciplinary investigations at the Early Bronze Age of Fidvar in Slovakia enabled the development of just such a multidimensional research strategy for the evaluation of settlement pits. Used in concert, magnetic survey and electromagnetic prospection delivered 2D data which revealed the locations of pits at the site. This was the basis for the 3D electrical resistivity tomography measurements which followed, enabling the three-dimensional reconstruction of prehistoric pits' geometries. Supplementary sedimentological and geochemical investigations of the pits' contents revealed important information about the pits' former functions as well as the mechanisms of the processes through which they were filled in. Thus, the integrated approach provides us with a new means of obtaining an overview of pit assemblages at archaeological sites in general and demonstrates its potential for the acquisition of valuable information about the food acquisition and storage processes at Fidvar in specific. Copyright (C) 2015 John Wiley & Sons, Ltd