Transcript Slide 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 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 Delaware River Susquehanna River Potomac River Delaware Bay Chesapeake Bay SO WHY THE DIFFERENCE?? Outie Innie 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)? EXPERIMENTS OF MUTO: RISING BASE LEVEL, SHORELINE STARVATION AND AUTORETREAT! VIDEO CLIP PHOTOGRAPH AND INTERPRETATION OF ONE OF THE EXPERIMENTS OF MUTO autoretreat autobreak shoreline trajectory topset foreset 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) here! here! bedrock-alluvial transition foreset-basement break SOME SAMPLE RESULTS 14.6 0.15 eta m 0.1 0.05 0 -0.05 -0.1 -1 -0.5 0 xm 0.5 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 prograding into Lake Sakakawea, North Dakota. Image from 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, sB f (1 p )x v sIf Bbf (qtbf qwbf ) x sIf Bbf (qtbf qwbf ) x x t x , Qtbf Bbf qtbf x v xv x B xv (1 p ) Bf xv+xv , Qwbf Bbf qwbf Qwbf I Q f tbf t Bf 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; Q wbf Q tbf x x Thus the final form of Exner becomes xv x B Bf xv+xv xv If (1 ) Qtbf (1 p ) t Bf x River channels are self-formed! For example, channel width must be a computed rather than specified parameter. bf 50 Hbf S RD50 Qbf 2 gD50 D50 ˆ , Q sand bed : bf 50 1.86 gravel bed : bf 50 0.0487 1.E+01 1.E+00 bf 50 1.E-01 Gravel Gravel Average Sand Sand Average 1.E-02 1.E-03 1.E+02 1.E+04 1.E+06 1.E+08 ˆ Q 1.E+10 1.E+12 1.E+14 Closure using constant Chezy resistance coefficient, set channelforming Shields number form* and Engelund-Hansen relation for total bed material load B 1 D Cz 2 R EH form 2.5 R Qtbf S Cz EHform Qbf H 2 Qbf Cz EH ( form ) D Qtbf EH 0.05 , nt 2.5 Qtbf Cz EH form Qbf S R Qtbf gD D2 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 Image from NASA website: https://zulu.ssc.nasa.gov/mrsid/mrsid.pl Fly River SOME CALCULATIONS APPLIED TO THE FLY-STRICKLAND RIVER SYSTEM, PAPUA NEW GUINEA Gravel-sand transition is approximated as bedrocksand transition. CASE OF CONSTANT SEA LEVEL Bed Profiles 200 180 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. Elevation m 160 140 120 100 80 60 40 20 0 -200000 0 200000 400000 Downvalley distance m 600000 800000 CASE OF 1 MM/YEAR RISE AFTER YEAR 2000 Bed Profiles 200 180 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. Elevation m 160 140 120 100 80 60 40 20 0 -200000 0 200000 400000 Downvalley distance m 600000 800000 CASE OF 2 MM/YEAR RISE AFTER YEAR 2000 Bed Profiles 200 180 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. Elevation m 160 140 120 100 80 60 40 20 0 -200000 0 200000 400000 Downvalley distance m 600000 800000 CASE OF 5 MM/YEAR RISE AFTER YEAR 2000 Bed Profiles 200 180 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. Elevation m 160 140 120 100 80 60 40 20 0 -200000 0 200000 400000 Downvalley distance m 600000 CASE OF 10 MM/YEAR RISE AFTER YEAR 2000 Bed Profiles 200 180 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. Elevation m 160 140 120 autoretreat!!! 100 80 60 40 20 0 -200000 0 200000 400000 Downvalley distance m 600000 CASE OF 10 MM/YEAR RISE AFTER YEAR 2000 SEDIMENT SUPPLY INCREASED BY FACTOR 450 OF 2.17 Bed Profiles 0 yr 2000 yr 4000 yr 6000 yr 8000 yr 10000 yr 12000 yr final w.s. 400 Elevation m 350 300 250 200 150 100 50 0 -20000 -10000 0 0 0 100000 200000 300000 400000 500000 600000 700000 Downvalley distance m Recovery from autoretreat? CONCLUSIONS Morphodynamics is fun. Autoretreat can be successfully reproduced in a moving-boundary morphodynamic model.