Earth-surface Dynamics Modeling & Model Coupling

Transcription

1 Earth-surface Dynamcs Modelng & Model Couplng A short course James PM Syvtsk & Erc WH Hutton, CSDMS, CU-Boulder Wth specal thanks to Pat Wberg, Carl Fredrchs, Courtney Harrs, Chrs Reed, Rocky Geyer, Alan Nedoroda, Rch Sgnell, Chrs Sherwood

2 Module 5: Shelf Sedment Transport ref: Syvtsk, J.P.M. et al., Predcton of margn stratgraphy. In: C.A. Nttrouer, et al. (Eds.) Contnental-Margn Sedmentaton: From Sedment Transport to Sequence Stratgraphy. IAS Spec. Publ. No. 37: Shelf dffusvty (3) Gravty-drven slope equlbrum (4) Event-based models (7) Coastal Ocean Models (4) Summary (1)

3 Shelf dffusvty Local transport occurs f the probablty of wave resuspenson s exceeded at a partcle s water depth. Sedment dffusvty (m 2 /hr) Eel across shelf along shelf Exceedance Probablty NDBC Buoy 46022, u bs >50 cm/s u bs >35 cm/s u bs >25 cm/s u bs >15 cm/s Depth (m) Water Depth (m)

4 Shelf dffusvty Resuspenson and Advecton by Bottom Boundary Energy h t = x k(t, x) h x h t = k h t k(t,) vares over tme t (pdf of storms), and water depth. Followng Ary wave theory, k falls off exponentally wth water depth. k s an ndex between 0 and 1 that reflects the ablty of gran se to be resuspended and advected. k(t,) k for sedment transport

5 Plumes Only Plumes & Wave Dffuson

6 Gravty-drven slope equlbrum Gradent Rchardson Number (R) = (a) densty stratfcaton velocty shear (b) Shear nstabltes occur for R < R cr suppressed for R > R cr Heght above bed R = R cr R > R cr Heght above bed R < R cr R = R cr Sedment concentraton Sedment concentraton (a) If excess sedment enters BBL & R ncreases beyond R cr, then turbulence s dampened, sedment s deposted, stratfcaton s reduced and R returns to R cr (b) If excess sedment settles out of boundary layer, or bottom stress ncreases & R decreases beyond R cr then turbulence ntensfes. Sedment re-enters base of boundary layer. Stratfcaton s ncreased n boundary layer and R returns to R cr.

7 Gravty-drven slope equlbrum = h = 0 () Momentum balance: U w θ v c Down-slope pressure gradent u grav c' (Wrght et al., Mar.Geol. 2001) U max = (U w v c u grav ) 1/2 y x = Bottom frcton total velocty= (U 2 w v 2 c u 2 grav ) 1/2 x-shelf bed slope depth-ntegrated buoyancy anomaly α B = c d < u u > = c d U max u grav bottom drag coeffcent wave-averaged, x-shelf component of quadratc velocty x-shelf gravty flow velocty () Maxmum turbulent sedment load: (c.f. Trowbrdge & Kneke, JGR 1994) Rchardson Number Buoyancy = = Crtcal value Shear

8 COMPARISON OF MODEL PREDICTIONS TO OBSERVED DEPOSITION Applcaton to Eel Rver Flood at 60-meter Ste Deposton Rate = R cr 2 (1-P) c d g s d dx (α U max 3 ) Porosty (ρ sed /ρ water -1) Bed slope Rchardson # Drag coeff. P = 0.9 s = 1.6 α = R cr = 0.25 c d = (Observatons from Traykovsk et al., CSR 2000) Wave orbtal velocty Suspended sedment Predcted bed elevaton Observed bed elevaton

9 Top: Plumes & Wave Dffuson; Bottom: Plumes, Waves & Flud Muds

10 Event-based transport model Calculate suspended sedment flux (by gran se) usng a 1-D shelf sedment transport model at a cross-shelf grd of nodes of specfed depth and sedment characterstcs. For each event (set of wave & current condtons), the net flux s calculated at each node. The dvergence of the flux gves the change n bed elevaton. q s Mxed layer Surface actve layer q x = D cx c x η t = 1 c b x D cx q y = D cy c y c x y D cy c y η Exchange layer (after Parker) F t = 1 c b L a η t = N =1 q x η t η F e t σ

11 Bed elevaton (cm) 5 Example of 5 repettons of a transport event on Eel Margn Slt fracton Sand fracton Cross-shelf dstance (km) <45 µ m µ m µ m µ m

12 SLICE Descrpton SLICE Descrpton waves wnd "des Nedoroda & Reed grd

13 x s c s c D x c D x )c w w ( x uc t c λ = 0 c D )c w w ( s = s F c D )c w w ( = ),, ( F j b c α τ τ = Μ 2 3 / wc k k x x k 2 Cu P G P k K x k K x wk x uk t k = σ σ h h κ α κ α = T v = k u C T x K constant K =ν ν = SLICE Descrpton Nedoroda & Reed

14 Nedoroda & Reed -20 Depth ( m) Depth ( m) Dstance (m) Depth ( m) Dstance (m) Depth ( m) Dstance (m) SLICE Densty Flows Dstance (m) Hegth ( m) Mudflow Profle Mudflow Depost Dstance Offshore (m)

15 Nedoroda & Reed

16 Nedoroda & Reed

17 Nested Modelng Average Sedment and Currents. Sept, 2002 May, 2003 C.Harrs, VIMS C.Harrs, VIMS Regonal Hydrologcal Model (HydroTrend) (atm-landsurface model) to Regonal Ocean Model (ROMS) for Sedment Supply, Buoyancy, Sedment Plumes Global Ocean Model (NOGAPS) (coupled ocean-atm model) to Regonal Ocean Model (ROMS) (coupled ocean-atm model) for Regonal Crculaton and Current Shear Global Met. Model (NOGAPS) (coupled ocean-atm model) to Regonal Met. Model (COAMPS) (coupled ocean-atm model) to Wave Model (SWAN) for Sedment Resuspenson (ROMS)

18 Crculaton and Sedment-Transport Modelng ROMS: Regonal Ocean Modelng System RANS for heat & momentum fluxes 3-8 km grd, 21 vertcal S levels Intaled wth shp data Zero-gradent b.c. near Otronto, seven tdal components LAMI forcng every 3 hours, SWAN waves, Po Rver dscharge k-ω turbulence model, Styles & Glenn wave-current boundary layer Resuspenson & transport of sngle gran se, w s = 0.1 mm/s, τ c =0.08Pa Wnds; Wave heght Bottom currents; Sus. Seds. Salnty; Depth-mean currents

19 C. Sherwood, USGS

20 VIMS-NCOM 3D Transport Model 2 km Sedment Model Currents, Bed Shear Inputs: sedment sources, ses, crtcal shear stress, settlng velocty. Calculates: flux, concentraton, eroson / deposton. NCOM Vertcal Grd Sea Floor Grd

21 Conclusons: Shelf dffusvty Advantages: uses daly pdf of regonal ocean energy, smple and robust; compatble wth landscape evoluton models Dsadvantages: depends wave energy pdf -- how varable s dffuson n response to decadal and longer term varablty? Gravty-drven slope equlbrum Advantages: uses daly pdf of local total velocty, smple and robust; can be tested aganst feld data Dsadvantages: Needs pdf for wave energy and sedment dscharge from rvers, to calculate Rchardson number Event-based Approach Advantages: uses wave, current, and sedment nformaton avalable for a ste, preservng all correlatons, can be tested aganst feld data Dsadvantages: tme scales short, data needs ntensve for long-term smulatons, nshore boundary condton dffcult to specfy Coastal Ocean Model Advantages: Can get t rght f all terms are ncluded & approprate resoluton s used. Dsadvantage: Computatonally ntensve: data needs ntensve