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?? Innie Outie

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 autoretreat autobreak 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!!! here! and here!

SOME SAMPLE RESULTS

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:

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

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 q tbf denotes the bankfull (flood) value of volume bed material load per unit width q t, q wbf denotes the bankfull (flood) value of volume wash load per unit width and  denotes channel sinuosity,

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

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

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

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:

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

CASE OF 1 MM/YEAR RISE AFTER YEAR 2000

CASE OF 2 MM/YEAR RISE AFTER YEAR 2000

CASE OF 5 MM/YEAR RISE AFTER YEAR 2000

CASE OF 10 MM/YEAR RISE AFTER YEAR 2000 INNIE! autoretreat!!!

CASE OF 10 MM/YEAR RISE AFTER YEAR 2000 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.