fluvial response to climate change

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: apvdbvs@geo.uu.nl

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