Abstract
Differences in deposit geometry and texture with depth along ephemeral gravel-bed streams strongly reflect fluctuations in bedload which are due to environmental changes at the basin scale and to morphological channel adjustments. This study combines electrical resistivity tomography (ERT) with datasets from borehole logs to analyse the internal geometry of channel cross-sections in a gravel-bed ephemeral stream (southeast Spain). The survey was performed through longitudinal and transverse profiles in the upper channel stretch, of 14 to 30 m in length and 3 to 6 m in depth, approximately. ERT values were correlated with data on sediment texture as grain size distribution, effective grain sizes, sorting, and particle shape (Zingg’s classification). The alluvial channel-fills showed the superposition of four layers with uneven thickness and arrangement: (1) the softer rocky substrate (<1000 Ω.m); (2) a thicker intermediate layer (1000 to 2000 Ω.m); and (3) an upper set composed of coarse gravel and supported matrix, ranging above 2000 Ω.m, and a narrow subsurface layer, which is the most resistive (>5000 Ω.m), corresponding to the most recent armoured deposits (gravel and pebbles). The ERT results coupled with borehole data allowed for determining the horizontal and vertical behaviour of the materials in a 3D model, facilitating the layer identification.
Highlights
Ephemeral streams are watercourses of arid and semi-arid environments with unstable morphology and high temporal variability of runoff
In the absence of direct porosity data, the objective of this research is to: (i) assess the grain shape, particle size distribution, and sorting in heterogeneous alluvial deposits; (ii) assess the vertical and lateral behaviour by creating a 3D model based on electrical resistivity tomography; and (iii) set a methodology to evaluate this type of ephemeral channels
In the case of gravels and pebbles, we have found a greater relationship between D20 and the electrical resistivity tomography (ERT) results, which can probably be maintained with the hydraulic conductivity (HC) values
Summary
Ephemeral streams are watercourses of arid and semi-arid environments with unstable morphology and high temporal variability of runoff. Extreme discharge events, that are isolated in time and alternate with long dry periods These types of streams are sensitive to short-term climatic changes, and human impacts may alter their degree of response, sometimes leading to large morphological adjustments during flash floods [1,2,3]. The ephemeral channels show a changing geometry, highly conditioned by differences in slope and textural variations in the bed materials and banks Often along their upper reaches and on alluvial fans, these channels have a steep slope which promotes a fast hydraulic regime. Under such conditions, and considering the abundant sediment stored within the channel, important transport rates contribute to most of the morphological changes in the channel. An uncertainty in bedload estimates for this type of streams is largely driven by the inability to characterise arrangement, orientation, and resultant forces of fluvial sediment in river beds
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
