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A new method for fl ood reconstruction over the past millennia

Philip S.J. Minderhoud

1

, Kim Cohen

1,2

, Gilles Erkens

1,2

, Wim Z. Hoek

1

1

Department of Physical Geography, Faculty of Geosciences, Utrecht University, Heidelberglaan 2, 3508 TC Utrecht, The Netherlands, p.s.j.minderhoud@students.uu.nl;

2

Deltares, Princetonlaan 6, 3508 AL, Utrecht, The Netherlands

Introduction

Reconstruction of the fl ooding history of the river Rhine is important for the study of extreme fl ooding events.

Sedimentary sequences from palaeochannel fi lls and oxbow lakes can hold records that cover multiple millennia.

In this case, their sediments provide a good source of information on past river activity. In this study, the infi ll of a palaeochannel of the Rhine near Rheinberg, Nordrhein-Westfalen, Germany is used to develop a new method for fl ood reconstruction on decadal timescales.

Location of the palaeochannel near Rheinberg, Germany.

Channel abandonment datings by Erkens (2009).

Conclusions and recommendations

- High resolution loss-on-ignition analysis proves to be a valid and cheap method for fl ood reconstruction. Increasing the resolution by decreasing the sample size will enhance the results.

- Applying an age-depth model that incorporates LOI as a proxy for aggradation rate performs well and improves the age interpolation results.

- Information on former positions and shifts of the Rhine, which can no longer be derived from today’s morphology, since all corresponding river deposits are reworked, is stored in the palaeochannel record.

- The statistical labelling and characterizing of signifi cant LOI spikes worked well and created reproducible results.

- Applying more advance analyses like grain-size, Magnetic Susceptibility or XRF-core logging will add useful information.

By performing similar analyses on multiple palaeochannel cores and correlating the results, locally induced fl oods can be fi ltered out, leaving the real, extreme, deltaic wide fl oods that correspond with extreme river discharge. With these results we are one step closer in defi ning the corresponding discharge belonging to these extreme fl ooding events of the Rhine. The method will be applied in the research by Willem Toonen (see poster nearby), so watch this space!

References:

Bronk Ramsey, C., 2008. Deposition models for chronological records. Quaternary Science Reviews, Volume 27, Issue 1, Pages 42-60. Oxcal v4.1.5 http://c14.arch.ox.ac.uk/oxcal.html

Erkens, G. 2009. Sediment dynamics of the Rhine catchment: Quantifi cation of fl uvial response to climate change and human impact. http://igitur-archive.library.uu.nl/dissertations/2009-1119-200148/UUindex.html

Results 1A) Loss-on-Ignition record

The loss-on-ignition record of the palaeochannel fi ll on a 2-cm-sample resolution. LOI has been determined after combustion for 4 hrs at 550ºC.

1B) Age-depth model

The developed age-depth model is based on calibrated radiocarbon dates and LOI measurements, where low LOI values mean fast aggra- dation. The

14

C datings are calibrated with Oxcal (deposition model:

P_Sequence, K=400m

-1

)(Bronk Ramsey, 2008).

2) Flood Frequency

Flood reconstruction of the Rhine near Rheinberg. Signifi cant, negative LOI spikes are plotted with their PCA-1 score, which is translated into fl ood frequency and magnitude. The distinct jumps in the LOI curve refl ect former channel shifts.

Photographs of core segments. The ten annual deposition cycles at the bottom mark the time after which the channel cut-off is completed.

A B

2 1

Methods

The principal method used is Loss-on-ignition (LOI), providing information on the organic matter content of the sequence. A high-resolution LOI analysis is preformed in order to:

1) apply a new age-depth model that translates LOI into aggradation rate for improved age interpolation.

2) investigate the registration and recurrence of extreme fl oods in the oxbow lake.

Because siliciclastic material has been deposited by fl owing water in the palaeochannel, negative spikes in the LOI

record, meaning increased clastic material inputs, represent fl ood events (1A). The infl ux of clastic material is used

to calculate changes in deposition rates and, in combination with a P-sequence age model using 4 AMS 14C dates,

construct an age-depth model (1B). Explorative statistical techniques are applied in order to distinguish and char-

acterize signifi cant LOI spikes, thereby defi ning suspected major fl ood events on a cal BP timescale (2). The LOI

record shows distinct breaks in mean LOI that might refl ect shifts in former positions of the river Rhine.

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