Middle-Holocene palaeoflood extremes of the Lower Rhine
W.H.J. Toonen, M.M. de Molenaar, F.P.M. Bunnik, H. Middelkoop
w.h.j.toonen@uu.nl, tel: 030-2534014
Dept. Physical Geography, P.O. box 80.115, 3508 TC Utrecht
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HG
SB
Ice pushed ridge Late Glacial terrace
YD-Early Holocene terrace Boreal-Atlantic floodplain Subboreal floodplain
Palaeochannels on terrace Abandoned meander bend Present Rhine course
Subboreal Rhine (remnant) Subboreal Rhine (projected)
0 1 2 3 4 5 6 7
8 100 200 300 400 500 600
SB HG
Palaeochannel geometries
Width (m) Depth (m)
VK
Rees
Wardt
VK
500m Sample core
SW NE
PSI’s
Lange Renne
5m
Borehole at location of hydraulic model
Projected borehole (Vossekuhl-transect; Fig. 1) Projected borehole (Erkens et al., 2011)
Middle Holocene Rhine
Meandering channel deposits*
Braided channel deposits*
Overbank deposits*
Eolian deposits*
Organic channel fill
1 km
*Colours as in upper frame
6 4 8 10 12 14 16 18 20
5 20 50 100 200 1000
Lower Rhine discharge (m sec ) (*10 )3-13
Recurrence time (yrs)
Q1250
2500
Current extrapola
tion of Q12 50
(with 95% c
onfidence in
terval)
BGE
estimated recurrence interval +6%
+16%
3.0m 2.5m 2.0m 1.5m
Clastic channel fillPeat channel fill
LH Over banks
0% 20 40 60 80 100 Pollen assemblage Age
1.40m:
AP: Pinus+++, Alnus+++, Quercus+, Corylus+, Fagus+, Tilia+, Ulmus+, Acer+, Betula+, Hedera+, Prunus+, Picea+, Abies+
NAP: e.g., Cerealia+, Hordeum+, Chenopodiaceae+, Anthemis+, Plantago+, Calluna+, Ranunculus+
CP+++, NAP+++, AP+
Fagus+, no Carpinus Early Subatlantic
1.50m:
AP: Pinus+++, Alnus+++, Quercus+++, Corylus+++, Picea+++, Fagus-, Ulmus-, Acer-
NAP: Calluna+, Liguliflorae+, Poaceae+, Polygonum+, Pteridium+, Cerealia-, Hordeum-
CP+, NAP+, AP++
Fagus-, Cerealia-, Ulmus- Subboreal 1.80m:
AP: Corylus+++, Alnus+++, Quercus+, Ulmus+
NAP: Bryophyta+++, Poacaeae+, Dryopteris+, Calluna-
2.00m:
AP: Pinus+++, Corylus+++, Alnus+, Quercus+, Ulmus+, Salix-, Hedera-
NAP: Bryophyta+++, Nuphar+, Dryopteris+, Cyperaceae+, Poaceae+, Viscum-
2.75m:
AP: Corylus+++, Quercus+, Pinus+, Ulmus-, Tilia-, Betula- Acer-, Hedera-
NAP: Poaceae+++, Dryopteris+++, Sparganium+++, Tyhpa latifolia-, Cyperaceae-
no CP, NAP-, AP+++
Alnus+++, Ulmus+
Atlantic
no CP, NAP-, AP+++
Alnus+
Atlantic
no CP, NAP-, AP+++
Corylus+++, Quercus+
Ulmus-, Tilia- Late Boreal 2.25m:
AP: Corylus+++, Alnus+, Quercus+, Ulmus+, Salix- NAP: Bryophyta+++, Nuphar+, Dryopteris+, Cyperaceae+, Poaceae+
2.50m:
AP: Corylus+++, Alnus+, Quercus+, Ulmus+
NAP: Bryophyta+++, Dryopteris+, Poaceae+, Calluna-
no CP, NAP-, AP+++
Alnus+
Late Boreal - Atlantic
no CP, NAP-, AP+++
Alnus+,
Late Boreal - Atlantic
Organic content Loss on Ignition (1cm sample res.)
4,135 + 30 yrs BP
7,880 + 30 yrs BP
Elevation m +O.D 5 . 25
9 13 17 21
SW NE
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Cumulative distance (m) 18.4m
A B C D E D C B A
19.5m VK
SW Salix-Alnus Riparian forest vs pioneer vegetation Quercus-Alnus
Chezy = 38 6 10 46
INTRODUCTION
Assesment of design discharges for dikes requires knowledge on the frequency and magnitude of extreme flooding events. Current estimates for the magnitude of the design flood are based on the extrapolation of ~100 year of instrumental data, which lacks the registration of rarely occurring extreme events. The reconstruction of palaeoflood magnitudes and frequencies from sedimentary records provides insight in the potential maximum size of Rhine floods. This helps to improve the estimation of the magnitude of the design discharge with a reduced uncertainty.
The magnitude of a Middle-Holocene flood was reconstructed along a transect (II) in the Lower Rhine valley (Germany) based on the highest slackwater deposits found in a palaeochannel fill located on an elevated terrace level, which flanks the Middle Holocene floodplain (I).
METHODS
A Chézy-based hydraulic model was used to calculate the palaeoflood discharge out of carefully selected geological input data (palaeochannel dimensions (III), reconstructed palaeofloodplain topography, surface and river bed roughness, and palaeostage indicators). To account for uncertainty, we considered 10 sets of input variables, which represent a realistic range of model inputs and results.
RESULTS
A best guess estimate of 13,250 cumecs was calculated for the minimum magnitude of floods that left the highest slackwater deposits in our cross section.
The associated recurrence time was estimated to 1,250-2,500 years, based on AMS dating and palynological analysis of the organic palaeochannel fill (IV).
At present, discharge waves are steeper than millennia ago, due to deforestation and river management. Correcting for these human impacts (which add ~6-16%), the investigated palaeoflood would correspond to ~15,000 cumecs, which is slightly lower than the present design discharge (V).
CONCLUSIONS
Geological-based discharge reconstructions provide unique information to bracket magnitudes of extreme flooding events.
Reconstructed Middle-Holocene palaeoflood magnitudes correspond well with previously estimated design discharges for Dutch dikes (V).
II
IV
III
V
I) Digital elevation model of the research area with the location of the model transect, boreholes, the Vossekuhl sample core (VK), and the Middle Holocene palaeochannels
‘Schloss Bellinghoven’ (SB) and ‘Haus Groin’.
II) Modelled valley cross-section with classication per floodplain compartment with general vegetation composition, the projected location of the Vossekuhl (VK) sample core, upper and lower estimated flood water levels, and zone-averaged Chézy roughness values for the best guess estimate scenario.
IV) The Vossekuhl sample core with general lithology, organic content, pollen assemblages and estimated ages (CP = culture crop pollen, NAP = non-arboreal pollen, AP = arboreal pollen). Arrows indicate the location of extreme floods (clay laminations in peaty matrix), squares represent AMS dates in radiocarbon ages, and black circles mark the location of samples for palynological analyses.
III) Reconstruction of the Middle Holocene Lower Rhine Valley with Subboreal Rhine courses and older morphology (YD = Younger Dryas). The inset shows palaeochannel dimensions of the SB and HG palaeochannels, which have been used as variables in the model scenarios. A schematized geological cross section is shown in the lower frame.
V) Current flood-frequency extrapolation (redrawn after Chbab et al., 2006) with our calculated range in recurrence interval and the best guess estimate (BGE) for the lower limit of Middle Holocene palaeoflood extremes. The BGE is corrected with +6-16% ‘deforestation and engineering’.
I
References:
Chbab et al., 2006. Estimating exceedance frequencies of extreme river discharge using statistical methods and physically based approach. Osterreichische Wasser- und Abfallwirtschaft 58, p 35-43.
Erkens et al., 2011. Complex fluvial response to Lateglacial and Holocene allogenic forcing in the Lower Rhine Valley (Germany). Quaternary Science Reviews 30:5-6, p 611-627. doi: 10.1016/j.quascirev.2010.11.019.
