Predicting reach‐specific properties of fluvial terraces to guide future fieldwork. A case study for the Late Quaternary River Allier (France) with the FLUVER2 model

Abstract

Numerical models have not yet systematically been used to predict properties of fluvial terrace records in order to guide fieldwork and sampling. This paper explores the potential of the longitudinal profile model FLUVER2 to predict testable field properties of the relatively well‐studied, Late Quaternary Allier system in France. For the Allier terraces an overlapping 14C and U‐series chronology as well as a record of 10Be erosion rates exist. The FLUVER2 modelling exercise is focused on the last 50 ka of the upper Allier reach because for this location and period the constraints of the available dating techniques are tightest. A systematic calibration based on terrace occurrence and thicknesses was done using three internal parameters related to (1) the sediment erodibility; (2) the sediment transport distance; and (3) the sediment supply derived from the surrounding landscape. As external model inputs, the best available, reconstructed, tectonic, climatic and base‐level data were used. Calibrated model outputs demonstrate a plausible match with the existing fluvial record. Validation of model output was done by comparing the modelled and measured timing of aggradation and incision phases for the three locations. The modelled range of landscape erosion rates showed a reasonably good match with existing erosion rate estimates derived from 10Be measurements of fluvial sands. The quasi‐validated model simulation was subsequently used to make new testable predictions about the timing and location of aggradation and erosion phases for three locations along the Allier river. The validated simulations predict that along the Allier, reach‐specific dynamics of incision and aggradation, related to the variations in sediment supply by major tributaries, cause relevant differences in the local fluvial terrace stratigraphy. Copyright © 2016 John Wiley & Sons, Ltd.

Preservation of the last aggradation phase in climate‐driven terraces: Evidence from Late Quaternary reach‐specific fluvial dynamics of the Allier River (France)

“…It might be that ongoing volcanic activity did not leave a downstream signature in the Allier sediments because there was limited hillslope-floodplain connectivity in the tributaries (Wainwright, 2006). The numerical modelling exercise using climate drivers (Veldkamp et al, 2016) yielded three different predictions that we will now evaluate based on the new dating results. A detailed comparison with predicted age estimates is difficult as the simulation was focused on the last 50ka while our new age estimates are only in the range 38-15ka.…”

“…[Colour figure can be viewed at wileyonlinelibrary.com] FIGURE 11. Timing of observed aggradational events compared with simulated landscape erosion rates in the western tributary basins (Veldkamp et al, 2016) and climate data from Guiot et al (1989Guiot et al ( , 1993 that were used for this simulation. Black arrows indicate basalt-rich and red arrows basalt-poor units.…”

“…In reality, this is not straightforward because the FLUVER2 model is only 1D and does not simulate preservation: it simulates alternating erosion/deposition events in time along the longitudinal profile. Despite this limitation, the following specific predictions were made using the FLUVER2 model (Veldkamp et al, 2016(Veldkamp et al, , p. 2266):…”

“…A numerical modelling exercise (using the 1D longitudinal profile FLUVER2 model; Veldkamp and van Dijke, 2000), using published and generally accepted external drivers (climate: Guiot et al ., 1989, 1993; base level: Bintanja et al ., 2005; crustal movement: Westaway, 2004) for the last 50 ka of the upper Allier basin in the Massif Central in France (Veldkamp et al ., 2016), displayed significant reach‐specific differences in fluvial dynamics and expression for the past 50 ka. When a major tributary joins the Allier, the local erosion/deposition dynamics pattern can change.…”

“…In reality, this is not straightforward because the FLUVER2 model is only 1D and does not simulate preservation: it simulates alternating erosion/deposition events in time along the longitudinal profile. Despite this limitation, the following specific predictions were made using the FLUVER2 model (Veldkamp et al ., 2016, p. 2266): Sediment fluxes in the Allier basin are climate‐controlled. The timing of aggradation events changes along the longitudinal profile in relationship with tributaries. The time lags (transitions) in the fluvial records are different along the longitudinal profile. …”