1. hinterlands sea alluvial plain alluvial river bedrock river QUESTION: What is the role of relative sea level in controlling alluvial aggradation and degradation? (first addressed by G.K.Gilbert 1877) shoreline Sea Level Control on Alluvial Aggradation and Degradation

2. RCEM 2005 @ University of Illinois Urbana–Champaign Tetsuji M UTO 1 & John B. S WENSON 2 1 Nagasaki University Japan 2 University of Minnesota Duluth USA

3. subaerial accommodation Graded profile Profile of an existing river During stationary sea level, rivers approach to grade (equilibrium state) and finally attain it. The Conventional Notion of Sea Level Control on Alluvial Aggradation and Degradation Rivers aggrade with sea level rise Rivers degrade with sea level fall Sea Level Control on Alluvial Aggradation and Degradation

4. How does sea level control alluvial aggradation and degradation? GRADE Relative Sea Level Rise Time AGGRADATION DEGRADATION Conventional View ? Time Relative Sea Level Rise DEGRADATION GRADE AGGRADATION Our View Sea Level Control on Alluvial Aggradation and Degradation

5. What is grade? hinterlands sea q so q ss q ss = q so at grade alluvial plain alluvial river bedrock river shoreline Sea Level Control on Alluvial Aggradation and Degradation No net deposition No net erosion No change in height No change in slope Perfect sediment bypass system

6. Run 1: Grade attained with a downstream weir weir Landward Basinward Constant water discharge Constant sediment supply 200 times faster than real

7. ungraded alluvial slopes (q OUT < q IN ) graded alluvial slope (q OUT = q IN ) sediment influx downstream end fixed with weir sediment outflux basement floor q OUT q IN River easily attains grade with downstream weir No weir available in natural alluvial rivers! delta toe shoreline Alluvial-bedrock boundary moving boundaries Run 1 Sea Level Control on Alluvial Aggradation and Degradation

8. basinward  height       s qsqs u r Z sl slope   slope   Moving-Boundary Morphodynamic Model of Alluvial Grade (by John.B.Swenson) Exner’s continuity equation Linear diffusivity Basics Alluvial-bedrock boundary shoreline Deposition above delta’s subaqueous slope Alluvial bed Delta toe

9. For an alluvial river to attain grade: Model of Alluvial Grade Relative SEA LEVEL MUST FALL!!! rate of sea level change time positive coefficient High-gradient, uniformly-sloping shelf Run 2 sea delta alluvial river Sea Level Control on Alluvial Aggradation and Degradation

10. Run 2: Decelerating fall of sea level NO WEIR!Landward Basinward Run 2 (except the beginning of run ) Constant water discharge Constant sediment supply 100 times faster than real

11. Run 3: What if sea level falls at a constant rate? Time Relative Sea Level Fall DEGRADATION GRADE AGGRADATION INSTANT OCCURRENCE OF GRADE There inevitably occurs aggradation in the early stage, which is followed by degradation within a finite time. [in a high-gradient, uniformly-sloping shelf] Sea Level Control on Alluvial Aggradation and Degradation

12. Run 3: Constant fall of sea level 100 times faster than real Run 3: Constant fall of sea level (in a high-gradient, uniformly-sloping shelf)

13. Run 4: Constant fall of sea level with a wide flume 100 times faster than real Run 4: Constant fall of sea level with a wide flume (in a high-gradient, uniformly-sloping shelf)

14. For alluvial grade to be attained and sustained: Model of Alluvial Grade relative SEA LEVEL MUST FALL!!! High-gradient, uniformly-sloping shelf Run 2 Drowned, antecedent alluvial plain Run 5 sea delta alluvial river any positive value rate of sea level change time Sea Level Control on Alluvial Aggradation and Degradation sea delta alluvial river

15. Run 5: Delta progrades in a drowned, antecedent alluvial plain, with constant fall of sea level (1.33 mm/min) alluvial slope quasi-graded during constant sea-level fall (0.89 mm/min) of a previous cycle Run 5 alluvial slope graded with downstream weir Constant water discharge Constant sediment supply 100 times faster than real

16. graded with downstream weir 5b: graded with constant sea level fall (1.33 mm/min) 5a: quasi-graded with constant sea level fall (0.89 mm/min) 5c: graded with constant sea level fall (2.00 mm/min) 5d: graded with constant sea-level fall (3.00 mm/min) 5a 5b 5c 5d

17. Our View Time Relative Sea Level Rise DEGRADATION GRADE AGGRADATION GRADE Relative Sea Level Rise Time AGGRADATION DEGRADATION Conventional View? The critical condition to discriminate between aggradation and degradation is fall of relative sea level. Alluvial rivers can continue to aggrade even during sea level fall, as long as sea level does not fall below the grade curve. Basin slope configuration affects geometry of grade curve.

18. R.S.L. Fall ALLOGENICGRADE Time Time AUTOGENICGRADE Run 2 Run 5 AGGRADATION DEGRADATION Drowned, antecedent alluvial plain sea delta alluvial river High-gradient, uniformly-sloping shelf sea delta alluvial river UNSTEADY FORCING STEADY FORCING Sea Level Control on Alluvial Aggradation and Degradation