Tidal Flat Morphodynamics: A Synthesis 1) On tidal flats, sediment (especially mud) moves toward areas of weaker energy. 2) Tides usually move sediment landward; waves usually move sediment seaward. 3) Tides and/or deposition favor a convex upward profile; waves and/or erosion favor a concave upward profile. 4) South San Francisco Bay provides a case study supporting these trends. Carl Friedrichs Virginia Institute of Marine Science, College of William and Mary Main Points Photo of Jade Bay tidal flats, Germany (spring tide range 3.8 m) by D. Schwen,

(b) Estuarine or back- barrier tidal flat (a) Open coast tidal flat Tidal flat = low relief, unvegetated, unlithified region between highest and lowest astronomical tide. Tidal Flat Definition and General Properties (Sketches from Pethick, 1984) e.g., Yangtze mouth e.g., Dutch Wadden Sea 1/17

(b) Estuarine or back- barrier tidal flat (a) Open coast tidal flat Tidal flat = low relief, unvegetated, unlithified region between highest and lowest astronomical tide. Tidal Flat Definition and General Properties (Sketches from Pethick, 1984) Note there are no complex creeks or bedforms on these simplistic flats. e.g., Yangtze mouth e.g., Dutch Wadden Sea 1/17

Where do tidal flats occur? Mixed Energy (Wave-Dominated) Mixed Energy (Tide-Dominated) Tide-Dominated (Low) Tide-Dominated (High) According to Hayes (1979), flats are likely in “tide-dominated” conditions, i.e., Tidal range > ~ 2 to 3 times wave height. Mean wave height (cm) Mean tidal range (cm) Wave-Dominated 2/17

Higher sediment concentration Tidal advection High energy waves and/or tides Low energy waves and/or tides What moves sediment across flats? 3/17

High energy waves and/or tides Higher sediment concentration Lower sediment concentration Tidal advection High energy waves and/or tides Low energy waves and/or tides Low energy waves and/or tides What moves sediment across flats? Ans: Tides plus energy-driven concentration gradients 3/17

High energy waves and/or tides Higher sediment concentration Lower sediment concentration Tidal advection High energy waves and/or tides Low energy waves and/or tides Low energy waves and/or tides What moves sediment across flats? Ans: Tides plus energy-driven concentration gradients < or supply-driven 3/17

Typical sediment grain size and tidal velocity pattern across tidal flats: Mud is concentrated near high water line where tidal velocities are lowest. Ex. Jade Bay, German Bight, mean tide range 3.7 m; Spring tide range 3.9 m. 5 km Fine sand Sandy mud Mud Photo location U max (m/s) 1 m/s (Reineck 1982) (Grabemann et al. 2004) 4/17

Typical sediment grain size and tidal velocity pattern across tidal flats: Mud is concentrated near high water line where tidal velocities are lowest. Ex. Jade Bay, German Bight, mean tide range 3.7 m; Spring tide range 3.9 m. 5 km Fine sand Sandy mud Mud Photo location U max (m/s) 1 m/s (Reineck 1982) (Grabemann et al. 2004) 4/17

Tidal Flat Morphodynamics: A Synthesis 1) On tidal flats, sediment (especially mud) moves toward areas of weaker energy. 2) Tides usually move sediment landward; waves usually move it seaward. 3) Tides and/or deposition favor a convex upward profile; waves and/or erosion favor a concave upward profile. 4) South San Francisco Bay provides a case study supporting these trends. Carl Friedrichs and Josh Bearman Virginia Institute of Marine Science, College of William and Mary Main Points Photo of Jade Bay tidal flats, Germany (spring tide range 3.8 m) by D. Schwen,

1 km Storm-Induced High Water (+5 m) Spring High Tide (+4 m) Spring Low Tide (0 m) (a) Response to Storms(b) Response to Tides Study Site 0 20 km (Yang, Friedrichs et al. 2003) Following energy gradients: Storms move sediment from flat to sub-tidal channel; Tides move sediment from sub-tidal channel to flat Ex. Conceptual model for flats at Yangtze River mouth (mean range 2.7 m; spring 4.0 m) 5/17

xx = x f (t) Z(x) z = R/2 z = - R/2 x = 0 h(x,t)  (t) = (R/2) sin  t x = L z = x/L U T90 /U T90 (L/2) Spatial variation in tidal current magnitude Landward Tide- Induced Sediment Transport Maximum tide and wave orbital velocity distribution across a linearly sloping flat: (Friedrichs, in press) 6/17

xx = x f (t) Z(x) z = R/2 z = - R/2 x = 0 h(x,t)  (t) = (R/2) sin  t x = L z = x/L U T90 /U T90 (L/2) U W90 /U W90 (L/2) Spatial variation in tidal current magnitude Landward Tide- Induced Sediment Transport Seaward Wave-Induced Sediment Transport Spatial variation in wave orbital velocity Maximum tide and wave orbital velocity distribution across a linearly sloping flat: (Friedrichs, in press) 6/17

Ems-Dollard estuary, The Netherlands, mean tidal range 3.2 m, spring range 3.4 m Day of 1996 Wind events cause concentrations on flat to be higher than channel (Ridderinkof et al. 2000) Wind Speed (meters/sec) Sediment Conc. (grams/liter) (Hartsuiker et al. 2009) Germany Netherlands 10 km Flat Channel Flat site Channel site 7/17

Ems-Dollard estuary, The Netherlands, mean tidal range 3.2 m, spring range 3.4 m Day of 1996 Wind events cause concentrations on flat to be higher than channel (Ridderinkof et al. 2000) Wind Speed (meters/sec) Sediment Conc. (grams/liter) (Hartsuiker et al. 2009) Germany Netherlands 10 km Flat Channel Flat site Channel site 7/17

Elevation change (mm) Wave power supply (10 9 W s m -1 ) ACCRETION EROSION Significant wave height (m) Sediment flux ( mV m 2 s -1 ) LANDWARD SEAWARD Wadden Sea Flats, Netherlands (mean range 2.4 m, spring 2.6 m) Severn Estuary Flats, UK (mean range 7.8 m, spring 8.5 m) Larger waves tend to cause sediment export and tidal flat erosion Flat sites 20 km (Xia et al. 2010) Depth (m) below LW Sampling location (Janssen-Stelder 2000) (Allen & Duffy 1998) 5 km 8/17

Tidal Flat Morphodynamics: A Synthesis 1) On tidal flats, sediment (especially mud) moves toward areas of weaker energy. 2) Tides usually move sediment landward; waves usually move sediment seaward. 3) Tides and/or deposition favor a convex upward profile; waves and/or erosion favor a concave upward profile. 4) South San Francisco Bay provides a case study supporting these trends. Carl Friedrichs and Josh Bearman Virginia Institute of Marine Science, College of William and Mary Main Points Photo of Jade Bay tidal flats, Germany (spring tide range 3.8 m) by D. Schwen,

Accreting flats are convex upwards; Eroding flats are concave upwards (Ren 1992 in Mehta 2002) (Lee & Mehta 1997 in Woodroffe 2000) (Kirby 1992) 9/17

As tidal range increases (or decreases), flats become more convex (or concave) upward. Wetted area / High water area Elevation (m) Mean Tide Level Wetted area / High water area Mean Tide Level MTR = 1.8 m MTR = 2.5 m MTR = 3.3 m Elevation (m) German Bight tidal flats (Dieckmann et al. 1987) U.K. tidal flats (Kirby 2000) Convex Concave Convex Concave 10/17

Models incorporating erosion, deposition & advection by tides produce convex upwards profiles At accretionary equilibrium without waves, maximum tidal velocity is nearly uniform across tidal flat. 1.5 hours Envelope of max velocity Evolution of flat over 40 years Initial profile Last profile High water Low water Ex. Pritchard (2002): 6-m range, no waves, 100 mg/liter offshore, w s = 1 mm/s,  e = 0.2 Pa,  d = 0.1 Pa (Flood +) Convex 11/17

Model incorporating erosion, deposition & advection by tides plus waves favors concave upwards profile concave upwards profile Tidal tendency to move sediment landward is balanced by wave tendency to move sediment seaward. 4-m range, 100 mg/liter offshore, w s = 1 mm/s,  e = 0.2 Pa,  d = 0.1 Pa, H b = h/2 Across-shore distance Elevation Equilibrium flat profiles (Roberts et al. 2000) Concave Convex 12/17

Tidal Flat Morphodynamics: A Synthesis 1) On tidal flats, sediment (especially mud) moves toward areas of weaker energy. 2) Tides usually move sediment landward; waves usually move sediment seaward. 3) Tides and/or deposition favor a convex upward profile; waves and/or erosion favor a concave upward profile. 4) South San Francisco Bay provides a case study supporting these trends. Carl Friedrichs and Josh Bearman Virginia Institute of Marine Science, College of William and Mary Main Points Photo of Jade Bay tidal flats, Germany (spring tide range 3.8 m) by D. Schwen,

South San Francisco Bay case study: km (Bearman, Friedrichs et al. 2010) 766 tidal flat profiles in 12 regions, separated by headlands and creek mouths. Data from 2005 and 1983 USGS surveys. Semi-diurnal tidal range up to 2.5 m San Mateo Bridge Dumbarton Bridge South San Francisco Bay MHW to MLLW MLLW to m 13/17

San Mateo Bridge Dumbarton Bridge South San Francisco Bay MHW to MLLW MLLW to m Dominant mode of profile shape variability determined through eigenfunction analysis: Mean concave-up profile (scores < 0) Amplitude (meters) Across-shore structure of first eigenfunction Normalized seaward distance across flat Height above MLLW (m) Mean profile shapes Normalized seaward distance across flat First eigenfunction (deviation from mean profile) 90% of variability explained Mean + positive eigenfunction score = convex-up Mean + negative eigenfunction score = concave-up Mean tidal flat profile Mean convex-up profile (scores > 0) Profile regions 4 km 9 (Bearman, Friedrichs et al. 2010) 14/17

Profile regions 4 km 10-point running average of profile first eigenfunction score Convex Concave Convex Concave Eigenfunction score Tidal flat profiles Regionally-averaged score of first eigenfunction 9 Significant spatial variation is seen in convex (+) vs. concave (-) eigenfunction scores: (Bearman, Friedrichs et al. 2010) 15/17

Average fetch length (m) Profile region Eigenfunction score Mean grain size (  m) Eigenfunction score Profile region Mean tidal range (m) Eigenfunction score Profile region Tide Range Net 22-year deposition (m) Eigenfunction score Profile region Deposition r = -.82 r = -.61 Fetch Length Grain Size -- Deposition & tide range are positively correlated to eigenvalue score (favoring convexity). -- Fetch & grain size are negatively correlated to eigenvalue score (favoring convexity). (Bearman, Friedrichs et al. 2010) r = +.92 r = Profile regions 4 km Convex Concave Convex Concave Convex Concave Convex Concave 16/17

Tide + Deposition Fetch Explains 89% of Variance in Convexity/Concavity Tide + Deposition – Fetch Explains 89% of Variance in Convexity/Concavity Profile regions r = +.94 r 2 =.89 Profile region (Bearman, Friedrichs et al. 2010) Observed Score Modeled Score Eigenfunction score Modeled Score = C 1 + C 2 x (Deposition) + C 3 x (Tide Range) – C 4 x (Fetch) Convex Concave San Mateo Bridge Dumbarton Bridge South San Francisco Bay MHW to MLLW MLLW to m 17/17

Tide + Deposition Fetch Explains 89% of Variance in Convexity/Concavity Tide + Deposition – Fetch Explains 89% of Variance in Convexity/Concavity Profile regions r = +.94 r 2 =.89 Profile region (Bearman, Friedrichs et al. 2010) Observed Score Modeled Score Eigenfunction score Modeled Score = C 1 + C 2 x (Deposition) + C 3 x (Tide Range) – C 4 x (Fetch) Convex Concave Increased tide range Increased deposition Increased fetch Increased grain size Convex-upwards Concave-upwards Seaward distance across flat Flat elevation San Mateo Bridge Dumbarton Bridge South San Francisco Bay MHW to MLLW MLLW to m 17/17

Tidal Flat Morphodynamics: A Synthesis 1) On tidal flats, sediment (especially mud) moves toward areas of weaker energy. 2) Tides usually move sediment landward; waves usually move sediment seaward. 3) Tides and/or deposition favor a convex upward profile; waves and/or erosion favor a concave upward profile. 4) South San Francisco Bay provides a case study supporting these trends. Carl Friedrichs and Josh Bearman Virginia Institute of Marine Science, College of William and Mary Main Points Photo of Jade Bay tidal flats, Germany (spring tide range 3.8 m) by D. Schwen,