2. Natural delta: millennial time scale
The Holocene Rhine delta contains a well preserved geological record of sediment deposition.
Method: Using our coring database we determined the amounts of sediment trapped over the past millennia using 8 detailed cross
sections. Age control was based on >200 14 C-datings.
Results: Sediment accumulation increased after 2000 BP.
The Rhine Delta – A record of fluvial sediment trapping over various time scales
Hans Middelkoop, Gilles Erkens, Marcel van der Perk
Department of Physical Geography, Utrecht University, P.O. Box 80.115 3508 TC Utrecht – NL, h.middelkoop@geo.uu.nl
1. The delta as sediment trap
The Rhine delta forms the last trap for fine sediments from the Rhine basin before the river reaches the estuary.
In addition to sea level rise, human impacts in the upper basin as well as channel modifications in the delta have greatly affected the amounts of sediment trapped in the delta.
We quantified the amounts of fine overbank sediment trapped in different compartments of the delta over different time slices since 6000 BP.
N
0 1000 m
>4.0 (kg/m
2)/yr 3.5
3.0 2.5 2.0 1.5 1.0 0.5 0.0
4. Embanked rivers: century time scale
After embankment (1200 and 1350 AD), sediment deposition was limited to narrow zones along the main channels.
Method: Using coring data we calculated the amount of overbank
deposits. OSL dating and historic river maps were used to reconstruct lateral accretion and post-depositional erosion of floodplains.
Results: After embankment, the floodplains have trapped 640 Mton overbank fines. About 70-80% of the floodplain deposits were re- eroded by lateral channel migration.
8. Conclusions
Deforestation in the river basin increased deposition in the delta after 2000 BP. The bulk of these deposits have been well preserved.
Embankment of the river channels enhanced deposition, but preservation of deposits was low.
Channel fixation resulted in aggraded floodplains, which reduced the trapping efficiency, but preserved sediment.
Landscaping measures will rehabilitate floodplain deposition.
Rhine delta Rhine delta
87.125 6.255.375 4.53.625 2.751.875 1
>12.8 kg/m
211.2
9.6 8.0 6.4 4.8 3.2 1.6 0.0
l l
l l
l l
l ll
ll
l l
l
A B C
D
E F
0 500 m
N
heavy met
A
lBemmel floodplain, measured deposition
Simulated annual floodplain deposition
Bemmel floodplain
< 0.25
0.25 – 0.5 0.5 – 1.0 1.0 – 2.0
> 2.0
Simulated present-day annual sediment deposition (kg m -2 y -1 )
6. Embanked floodplains: Present-day sedimentation
The present total embanked floodplain area is 168 km 2 .
Method: We measured contemporary overbank sedimentation after several flood events using sediment traps.
Using 2D models for overbank flow and sedimentation, we calculated average annual sedimentation.
Results: During the high-magnitude 1993 flood, total
deposition along the Waal (main branch) was 0.24 Mton.
To date, about 39% of the annual suspended sediment load (3.1 Mton) is trapped by all lower Rhine floodplains together.
7000 – 6000 cal BP
6000 – 5000 cal BP 5000 – 4000 cal BP 4000 – 3000 cal BP
3000 – 2000 cal BP 2000 – 1000 cal BP
Inland Waal Inland Waal
delta delta
C C’
C
C’
ZG WU TU
0 500 1000 1500
metal content (mg/kg)
1970 1960 1950 1940 1930 1920 1910 1900 1890 1880 1870 1860 1980
1850
Zn
N
S AM-1 AM-2 AM-3
6 4 2 8
0 250 m
5. Normalized rivers: decennial time scale
After 1850 the channels were normalized by groynes, preventing lateral bank erosion.
Method: Post-1850 sediments were traced by enhanced heavy metal concentrations. The pollution history was reconstructed from floodplain lake sediments.
Results: The total sediment deposition after 1850 AD amounts to 173 Mton (10 9 kg).
Due to the channel fixation, re-mobilisation of older sediments has not occurred.
3. Biesbosch: inland Waal delta
In an inland lake formed by the 1421 AD storm surge the Waal river built a delta.
Method: Using coring data and historic maps we reconstructed the delta growth from 1421 until its embankment in 1850.
Results: The inland delta trapped about 200 Mton sand and 220 Mton mud
1693
AM-1 0 cm
20
40
60
AM-2
AM-3
