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

1
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
2
RIVER MOUTHS, LIKE NAVELS, HAVE TWO BASIC
TYPES INNIES AND OUTIES
The delta of the Mississippi River protrudes into
the Gulf of Mexico
3
THE EAST COAST OF THE UNITED STATES, HOWEVER, IS
DOMINATED BY DROWNED RIVER MOUTHS
Delaware River
Susquehanna River
Potomac River
Delaware Bay
Chesapeake Bay
4
SO WHY THE DIFFERENCE??
Outie
Innie
5
SEA LEVEL HAS RISEN ABOUT 120 METERS SINCE THE
END OF THE LAST ICE AGE
Years before present

• 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)?

6
EXPERIMENTS OF MUTO RISING BASE LEVEL, SHORELINE
STARVATION AND AUTORETREAT! VIDEO CLIP
7
PHOTOGRAPH AND INTERPRETATION OF ONE OF THE
EXPERIMENTS OF MUTO
8

• THE ESSENTIAL RESULTS OF MUTOS 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).

9
MORPHODYNAMIC MODELING OF DELTA RESPONSE TO SEA
LEVEL RISE
Modeling of Mutos 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!!!
here!
here!
and here!
10
SOME SAMPLE RESULTS
11
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.
12
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//zul
u.ssc.nasa.gov/mrsid/mrsid.pl
13
Wash load cannot be neglected it is needed to
form the floodplain as the river aggrades.
14
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,
15
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
16
River channels are self-formed! For example,
channel width must be a computed rather than
specified parameter.
17
Closure using constant Chezy resistance
coefficient, set channel-forming Shields number
?form and Engelund-Hansen relation for total bed
material load
18
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 River
Image from NASA website https//zulu.ssc.nasa.gov
/mrsid/mrsid.pl
19
SOME CALCULATIONS APPLIED TO THE FLY-STRICKLAND
RIVER SYSTEM, PAPUA NEW GUINEA
Gravel-sand transition is approximated as
bedrock-sand transition.
20
CASE OF CONSTANT SEA LEVEL
21
CASE OF 1 MM/YEAR RISE AFTER YEAR 2000
22
CASE OF 2 MM/YEAR RISE AFTER YEAR 2000
23
CASE OF 5 MM/YEAR RISE AFTER YEAR 2000
24
CASE OF 10 MM/YEAR RISE AFTER YEAR 2000
INNIE!
autoretreat!!!
25
CASE OF 10 MM/YEAR RISE AFTER YEAR 2000
SEDIMENT SUPPLY INCREASED BY FACTOR OF 2.17
OUTIE!
26
Recovery from autoretreat?
27
CONCLUSIONS
Morphodynamics is fun.
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.