fluvial response to climate change
TRANSCRIPT
Faculteit Geowetenschappen
Aardwetenschappen
Time
Yiel
d (%
of i
nput
)
100 %
0 %instantaneous base-level drop
Quanti�cation
Results
Elev
atio
n
Distance
Elev
atio
n
Distance
T 1
T 6T 5T 4T 3T 2
Begin et al. (1981)
Our �ume
Modeling �ux
Qw / Qs
Yield at the delta apex
125 %
100 %
0 5 10 15 20 25 30
Disc
harg
e / s
ea le
vel
sea levelscenario 1
scenario 2
Time (h)
scenario 1 scenario 2constant supply
0%
50%
100%
150%
200%
0 5 10 15 20 25 30
Yiel
d (%
of i
nput
)
Time (h)
Timedischarge
yield at �uvial valley outlet
Yiel
d (%
of i
nput
)
100 %
0 %
Flume
horizontal position
elev
atio
n
Numerical model
∙=xh
kxt
h∂∂
∂∂
∂∂
∙( )
horizontal position
elev
atio
n
T 1
T 4T 3T 2
river shelf
No discharge pulse
5 h
10 h
15 h
30 h
25 h
20 h
erosion surface
Discharge pulse (scenario 1)
river shelf
additional deposition
additional erosion
additional erosion
additional deposition
additional erosion
rivershelf
basin
river shelf
Discharge pulse (scenario 2)
highstand
early regression
late regression
early lowstand
late lowstand
early transgression
late transgression
Fluvial response to climate changeAart-Peter van den Berg van Saparoea, George Postma, Paul Meijer & Joris EggenhuisenUtrecht University, Department of Earth Sciences, Budapestlaan 4, 3584 CD Utrecht, The Netherlands phone: +31 (0)30 253 5119, fax: +31 (0)30 253 5030, e-mail: [email protected]
Base-level drop
Objectives We wish to investigate conceptually the impact of 1) frequency, amplitude and duration of climate change (discharge) and 2) timing of the change relative to the sea-level curve on �uvial stratigraphy and sediment �ux at the delta apex (outlet of the �uvial valley).
Stratigraphic architectureShort changes (scenario 2) appear to enhance deposition (and preservation) in the downstream part of the valley relative to the
long change (scenario 1).Both scenarios cause erosion in the upstream part of the valley and a reduction of headward erosion rates caused by increased
sediment delivery to the shelf. In the long pulse scenerio (2), however, the �nal
erosion surface (sequence boundary) is much better
developed.
Three basically di�erent climate (discharge) scenarios superimposed on late Quaternary sea-level change are shown: constant supply (black); sea level related change (scenario 1); pulse during rise (scenario 2). A characteristic pattern emerges: increased yield at higher discharge is followed by decreased yield before a return to the ‘normal’ level. This is caused by the �lling of accommodation space created durig high discharge. Timing of the pulse relative to sea-level �uctuation appears to control yield. The lower amplitude, shorter pulse of scenario 2 has a stronger impact than that of scenario 1.
We are now testing if di�usion can also be used to accurately describe time-averaged sediment preservation under conditions of varying discharge and sea-level rise. First comparisons of our �ume results with our numerical model based on linear di�usion are shown. In this experiment discharge was increased, then decreased, while
base-level was �xed.
Quanti�cation of time-averaged sediment transport induced by base level lowering can be described reasonably well by linear di�usion in both experiment and real world (e.g. Begin et al. 1981; Begin, 1988) as a process of headward erosion. The topographical development in our experiments (shown here as
longitudinal pro�les at 1 hour intervals) shows good agreement with similar experiments of Begin. The
response in terms of yield at the �uvial valley outlet is shown below.
Problem Climate is, next to sea level, one of the major external forcing mechanisms acting on �uvial systems. It �uctuates on frequencies ranging from days to millions of years. Its e�ect in terms of time-averaged sediment transport is di�cult to measure in real world systems.
Approach A series of �ume experiments on the scale of an entire river-valley-delta-shelf landscape have been conducted. The diagram shows the sea-level curve, which is a common Quaternary scenario for the Gulf of Mexico, with a two climate (discharge) scenarios.
Increased discharge