Ground‐penetrating radar and coring used to study the large‐scale structure of point‐bar deposits in three dimensions

Abstract

ABSTRACTDetailed three‐dimensional (3‐D) observations of sandy point‐bar deposits from the River South Esk in Scotland were made using very closely spaced (metres) vibracores and ground‐penetrating radar (GPR) profiles. In order to explain the origin of the observed patterns of deposition, use was made of previous studies of channel geometry, flow and sediment transport. In addition, the mode and nature of channel migration and point‐bar accretion were determined using published maps, aerial photographs and detailed topographic surveys. Point‐bar deposits accumulated in response to channel‐bend expansion and downstream migration, resulting in preservation of sequences that fine upwards and downstream. Lower‐bar deposits are mainly very‐coarse to coarse sands with medium‐scale trough cross‐strata overlying basal gravels: associated radar facies are generally low‐amplitude, relatively discontinuous inclined reflectors. Upper‐bar deposits are mainly fine to medium sands with medium‐ and small‐scale cross‐strata and vegetation‐rich layers: associated radar facies are generally moderate‐ to high‐amplitude, laterally continuous, inclined reflectors. Large‐scale inclined stratasets seen in GPR profiles resulted from episodic point‐bar accretion. Abrupt lateral changes in inclination of these stratasets, and preservation of distinct unit bars (bar heads, scroll bars), lower‐bar platforms and inner‐bank swale fills, record discrete episodes of erosion and deposition associated with floods with recurrence intervals of decades to centuries. Such detailed 3‐D description and interpretation of these large‐scale features of point‐bar deposits was only possible through the use of GPR profiles tied closely to cores, and through the availability of much previously collected information on channel geometry, water flow, sediment transport, erosion and deposition.