Flood frequency and magnitude estimates based on valley flood morphology and floodplain sedimentary sequences :the Tyne Basin, N.E. England

Abstract

This study examines the sedimentary and morphological flood record of the River Tyne, northern England, over the last 400 years. Temporal trends in flooding and spatial variations in the timing and nature of river response to floods (flood "effectiveness") are investigated at 3 reaches representative of upland, piedmont and lowland locations in the Tyne catchment. Deposition of coarse flood sediments in headwater tributaries of the River Tyne appears to be strongly related to flood water:sediment ratios that is primarily controlled by sediment supply. Cobble - boulder bars, sheets and splays are deposited under Newtonian flow conditions from flows with relatively low sediment concentrations. Boulder berms and lobes result from floods with high sediment loads, associated with slope-channel coupling. Flood sedimentation in lower reaches of the Tyne comprises vertical accretion of fine-grained, well-bedded sediments at overbank and within-channel locations. Larger floods can be identified on the basis of textural discontinuities that reverse the overall fining-upward sequence. Estimation and interpretation of flow magnitude in each setting is complicated by progressive river bed incision over the period of investigation and significant changes in sediment supply which have affected the nature of flood sedimentation. Trace metal dating of fine-grained, and lichenometric dating of coarsegrained, flood units has enabled detailed and temporally well-defined records of channel and floodplain response to flooding to be established within the Tyne catchment. Since 1600 A.D. fluvial activity at all 3 study reaches has been characterised by alternating periods of river bed incision and stability. Episodic changes in vertical channel stability appear to coincide with non-random decadalscale fluctuatiions in flood frequency: 3 periods with distinct clustering of large floods (1740-1790, 1860-1899, 1940-1965) were associated with vertical instability (channel trenching) throughout the Tyne basin, while intervening periods with relatively few exceptional floods were associated with channel accretion and floodplain rebuilding in general, but with significant upstream/downstream variation. Distinct clustering in the timing of large floods correspond with shortterm hydroclimate changes associated with fluctuations in the configuration of the upper atmosphere. These findings have significant implications for longer-term fluvial development and interpretaion of Holocene alluvial sequences.