Extracting palaeoflood data from coarse‐grained Pleistocene river terrace archives: an example from SE Spain

Abstract

Field‐based palaeoflood event reconstruction has the potential to contribute to the development of our understanding of long‐term landscape evolution. However, the reconstruction of past flow event histories (magnitude and frequency) over long‐term (Quaternary) timescales is fraught with difficulties. Here we make a preliminary exploration of some of the practicalities of flood reconstruction from fluvial terrace archives using commonly available sedimentological and geomorphological observations from a field perspective. We utilize Manning and palaeostage indicators to reconstruct historic events that can be used as benchmarks for a lesser used competence based approach, which is applied to coarse‐grained strath terrace deposits. We evaluate the results against gauged records for extreme and catastrophic events that affected the same region in 1973 and 2012. The findings suggest that the competence approach is most effectively applied to terrace deposits if the channel geometry is taken into account when sampling both in cross‐section and in longitudinal section and calibrated against the sedimentology for palaeo‐flow depth. Problems can arise where constrictive channel geometries allow boulder jams to develop, acting as sediment traps for the coarsest material and leading to downstream ‘boulder starvation’. Useful sites to target for palaeoflood reconstruction, therefore, would be upstream of such constrictive reaches where the coarsest transportable bedload has been effectively trapped. Sites to avoid would be downflow, where the deposited material would poorly represent palaeoflood competence. Underestimation from maximum boulder preservation and limited section exposure issues would appear to outweigh possible overestimation concerns related to fluid density and unsteady flow characteristics such as instantaneous acceleration forces. Flood data derived from river terrace deposits suggests that basal terrace geometries and coarse boulder lags common to many terrace sequences are likely the result of extreme flow events which are subsequently filled by lesser magnitude flood events, in this environmental setting. Copyright © 2016 John Wiley & Sons, Ltd.

Bedrock structural control on catchment-scale connectivity and alluvial fan processes, High Atlas Mountains, Morocco

“…Surveys were terminated after the first significant tributary system to enter the catchment (typically >1km into each catchment). To facilitate analysis of potential peak discharges within the catchments the competence approach of Clarke (1996) was utilised (Stokes et al 2012;Mather & Stokes 2016). Historic flood estimates were derived from appropriate sites recognised within the lowermost portion of the backfilled catchment sediments and uppermost fan apex in an attempt to assess the peak discharge reaching the fan for the catchment.…”

The application of geomorphic indices in terrain analysis for ground engineering practice

“…The where up to 600 mm of net rainfall was recorded in close proximity to the study area triggering flash flood discharges of ~500 m 3 /sec in the Río Aguas at Sorbas (Capel-Molina, 1974;Thornes, 1974Thornes, , 1976Mather and Stokes, 2016). with stream definition verified against the drainage blue line 1:25,000 topographic map data.…”

River terraces and alluvial fans: The case for an integrated Quaternary fluvial archive

“…Thus, integrated river terrace and alluvial fan studies can add significant information to palaeoenvironmental reconstructions, providing potential information on upstream sediment source and flood magnitude (the terrace e.g. Stokes et al, 2012a,b;Mather and Stokes 2016) and hillslope connectivity and processes (the alluvial fan, e.g. Harvey, 2012).…”