RIVER RESPONSE TO POST-GLACIAL SEA LEVEL RISE: THE FLY-STRICKLAND RIVER SYSTEM, PAPUA NEW GUINEA...

RIVER RESPONSE TO POST-GLACIAL SEA LEVEL RISE: THE FLY-STRICKLAND RIVER

SYSTEM, PAPUA NEW GUINEA

Gary Parker, Tetsuji Muto, Yoshihisa Akamatsu, Bill Dietrich,

Wes Lauer

RIVER MOUTHS, LIKE NAVELS, HAVE TWO BASIC TYPES:INNIES AND OUTIES

The delta of the Mississippi River protrudes into the Gulf of Mexico

THE EAST COAST OF THE UNITED STATES,

HOWEVER, IS DOMINATED BY

DROWNED RIVER MOUTHS

Chesapeake Bay

Delaware Bay

Susquehanna River

Potomac River

Delaware River

SO WHY THE DIFFERENCE??

InnieOutie

SEA LEVEL HAS RISEN ABOUT 120 METERS SINCE THE END OF THE LAST ICE AGE

How does a river mouth respond to sea level rise?• Does a delta continue to prograde into the ocean?• Or does the sea drown the delta and invade the river valley (transgression)?

Years before present

EXPERIMENTS OF MUTO: RISING BASE LEVEL, SHORELINE STARVATION AND AUTORETREAT!

VIDEO CLIP

topset

foreset

autoretreatautobreak

shoreline trajectory

PHOTOGRAPH AND INTERPRETATION OF ONE OF THE EXPERIMENTS OF MUTO

THE ESSENTIAL RESULTS OF MUTO’S EXPERIMENTS

• When constant sea level is maintained the shoreline and delta prograde outward (shoreline regresses).• If sea level rises at a constant rate, the shoreline first progrades outward, but the progradation rate is suppressed.• If sea level continues to rise, progradation is eventually reversed and the shoreline is pushed landward.• If sea level still continues to rise, sediment transport at the shoreline drops to zero, the delta is drowned and the shoreline rapidly moves landward (transgresses).

Whether or not a delta continues to prograde, or instead is drowned depends on a) the rate and duration of sea level rise (higher values favor drowning) and sediment supply at the bedrock-alluvial transition (a higher value favors continued progradation).

MORPHODYNAMIC MODELING OF DELTA RESPONSE TO SEA LEVEL RISE

Modeling of Muto’s highly simplified 1D laboratory deltas is a first step toward modeling the response of 2D field river mouths to sea level rise.

THE FUN PART IS THE PRESENCE OF THREE MOVING BOUNDARIES!!!

sediment feed

topset-foreset break(shoreline)

bedrock-alluvial transition

foreset-basement break

here!here!

and here!

SOME SAMPLE RESULTS

14.6

-0.1

-0.05

0

0.05

0.1

0.15

-1 -0.5 0 0.5

x m

eta

m

0 sec

35.9 sec

71.7 sec

107.6 sec

143.4 sec

179.2 sec

215.1 sec

250.9 sec

286.8 sec

322.7 sec

358.5 sec

394.4 sec

430.2 sec

466.1 sec

501.9 sec

537.8 sec

573.6 sec

609.5 sec

645.3 sec

681.2 sec

717 sec

APPLICATION TO LARGE, LOW-SLOPE SAND-BED RIVERS:HOW DID THEY RESPOND TO SEA LEVEL RISE?

All such rivers flowing into the sea were subject to ~ 120 m of eustatic sea level rise since the end of the last glaciation.

DELTA PROGRADATION

Even when the body of water in question (lake or the ocean) maintains constant base level, progradation of a delta into standing water forces long-term aggradation and an upward-concave profile. Both the channel and the floodplain must prograde into the water.

Missouri River prograding into Lake

Sakakawea, North Dakota.

Image from NASA website:

https://zulu.ssc.nasa.gov/mrsid/mrsid.pl

Wash load cannot be neglected: it is needed to form the floodplain as the river aggrades.

Missouri River progradinginto Lake Sakakawea,

North Dakota.I mage f rom NASA

website:https://zulu.ssc.nasa.gov/mrsid/mrsid.pl

FORMULATION OF THE PROBLEM: EXNER

Sediment is carried in channel but deposited across the floodplain due to aggradation forced by sea level rise. Adapting the formulation of Chapter 15, where qtbf denotes the bankfull (flood) value of volume bed material load per unit width qt, qwbf denotes the bankfull (flood) value of volume wash load per unit width and denotes channel sinuosity,

x

Q

x

Q

B

I

t)1( wbftbf

f

fp

xB

Bf

xv

xv

xv+xv

xxwbftbfbffsxwbftbfbffsvpfs )qq(BI)qq(BIx)1(B

t

wbfbfwbftbfbftbfv

qBQ,qBQ,x

x

FORMULATION OF THE PROBLEM: EXNER contd.

It is assumed that for every one unit of bed material load deposited units of wash load are deposited to construct the channel/floodplain complex;

Thus the final form of Exner becomes

x

Q

B

)1(I

t)1( tbf

f

fp

xB

Bf

xv

xv

xv+xv

x

Q

x

Q tbfwbf

River channels are self-formed! For example, channel width must be a computed rather than specified parameter.

25050

bf

50

bf50bf

DgD

QQ̂,

DR

SH

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02 1.E+04 1.E+06 1.E+08 1.E+10 1.E+12 1.E+14

GravelGravel AverageSandSand Average

0487.0:bedgravel

86.1:bedsand

50bf

50bf

Q̂

50bf

Closure using constant Chezy resistance coefficient, set channel-forming Shields number form* and Engelund-Hansen relation for total

bed material load

SQR

CzQ bf

formEHtbf

5.2n,05.0 tEH

tbf

bf2formEH

bf

tbf

formEH

2tbf

5.2

formEH2

Q

Q)(Cz

D

H

Q

Q

Cz

RS

DgD

Q

RCz

1

D

B

A RIVER SYSTEM AFFECTED BY RISING SEA LEVEL

The Fly-Strickland River System in Papua New Guinea has been profoundly influenced by Holocene sea level rise.

Fly River

Strickland River

Fly RiverImage from NASA website:

https://zulu.ssc.nasa.gov/mrsid/mrsid.pl

SOME CALCULATIONS APPLIED TO THE FLY-STRICKLAND RIVER SYSTEM, PAPUA NEW GUINEA

Gravel-sand transition is approximated as bedrock-sand transition.

CASE OF CONSTANT SEA LEVEL

Bed Profiles

0

20

40

60

80

100

120

140

160

180

200

-200000 0 200000 400000 600000 800000

Downvalley distance m

Ele

vat

ion

m

0 yr2000 yr4000 yr6000 yr8000 yr10000 yr12000 yrfinal w.s.

CASE OF 1 MM/YEAR RISE AFTER YEAR 2000

Bed Profiles

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-200000 0 200000 400000 600000 800000


Ele

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Bed Profiles

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-200000 0 200000 400000 600000 800000


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Bed Profiles

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-200000 0 200000 400000 600000


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Bed Profiles

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-200000 0 200000 400000 600000


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INNIE!

autoretreat!!!


Bed Profiles

0

50

100

150

200

250

300

350

400

450

-200000

-100000

0 100000 200000 300000 400000 500000 600000 700000


Ele

vat

ion

m


SEDIMENT SUPPLY INCREASED BY FACTOR

OF 2.17 OUTIE!

Recovery from autoretreat?

CONCLUSIONS

Autoretreat can be successfully reproduced in

a moving-boundary morphodynamic model.

The field-scale response of rivers to

rising sea level can be modeled by:

• including wash load and floodplain

processes,

• adding backwater effects, and

• using field-scale transport relations.

Morphodynamics is fun.

RIVER RESPONSE TO POST-GLACIAL SEA LEVEL RISE: THE FLY-STRICKLAND RIVER SYSTEM, PAPUA NEW GUINEA...

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