Wind waves drive multiple mechanisms of erosion of the marsh scarp in Barataria Bay, LA

Transcription

1 Wind waves drive multiple mechanisms of erosion of the marsh scarp in Barataria Bay, LA Kendall Valentine Giulio Mariotti Department of Oceanography and Coastal ciences, Louisiana tate University

2 ite A - Erodes at ~0.5 m/yr ite B - Erodes at ~1.3 m/yr

3 tudy ite: Barataria Bay, LA Low Erosion High Erosion 1 km

4 Large Fetch Bigger Waves More Erosion But what if fetch is equal?

5 Land lost Current land Water

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7 Barataria Bay, LA Low Erosion N Wind Rose N x 10 5 Jm -1 s -1 3 WW E 0 E Wind peeds in m/s W W < W < W High Erosion 5 W < 7.5 < W < 5 0 W < x 10 5 Jm -1 s km imple model wave power (30 yr) Data wave power (1 yr)

8 What other process is causing asymmetry in erosion?

9 What do we know about LA Bays? Microtidal Water level controlled by wind

10 Water level matters Data: Tonelli et al Figure: Fagherazzi et al Tonelli et al a.k.a. wave thrust

11 Wind Water Level Relationship outherly winds Higher water level in the bay wind N N Barataria Bay Barataria Bay Gulf of Mexico Gulf of Mexico wind Northerly winds Lower water level in the bay

12 Wind Water level: tation Data (Northerly) (outherly) (Northerly)

13 Wind Water level: Field 5 m/s 5 m/s 7 7 m/s m/s 9 9 m/s m/s m/s m/s 13 m/s 13 m/s 15 m/s 15 m/s Wind Direction (degree) (Northerly) (outherly) (Northerly)

14 Idea 1: Overshooting at high water Overshooting waves wave thrust (Tonelli et al. 2010) Little to no edge erosion

15 Evidence for overshooting North ite outh ite 36% 18% What about when water is low?

16 Idea 2: Increased erosion at low water Case 1: Less erosion Case 1: Low Erosion Case 2: More erosion Case 2: High Erosion Case 1: Winds blow from (Impacts North) Water level Waves hit roots or overshoot Low erodibility Case 2: Winds blow from N (Impacts outh) Water level Waves hit below roots High erodibility

17 hear strength decreases below roots Turner 2011 Howes et al. 2010

18 hear strength decreases below roots 0 hear trength (kpa) Depth (m) North outh 0.5

19 Where are the waves hitting? North outh hear trength (kpa) 0 Elevation (m) Depth (m) Percent of Total Wave Power

20 Where are the waves hitting? North ite outh ite 36% 18% 20 cm 34% 20 cm 19% 55 cm 30% 50 cm 63%

21 hear strength differences 0 hear trength (kpa) Depth (m) North outh 0.5

22 Idea 3: Decreased deposition during Northerly winds Case 1: C deposits Case 1: Low Erosion Case 2: C doesn t deposit Case 2: High Erosion Case 1: Winds blow from (Impacts North) Water level ediment deposits Higher elevation, stronger sediments Case 2: Winds blow from N (Impacts outh) Water level ediment unable to deposit Lower elevation, weaker sediments

23 Marsh Elevation Elevation (m) cm North outh Elevation related to - Plant zonation - pecies root strength - Decomposition - Mineral vs. organic X Distance (m)

24 ediment Characteristics 0 0 hear trength (kpa) North 0.15 Depth (cm) outh Depth (m) Bulk Density (g/cm 3 )

25 Water Level Matters Where waves hit on marsh edge Marsh Elevation Asymmetry in erosion Water Level 3 Marsh strength 1 Overshoot 2 Below roots Water Level 0 hear trength (kpa) Depth (m)

26 Model Framework Uniform Erodibility Model Rand(wind) Calculate Wave Power Wave Power α Erosion Probabilistic Erosion Young and Verhagen 1996 Leonardi 2016 Mariotti and Canestrelli 2017 Wind Rose 90 N W E d peeds in m/s 15 W < 15 W < W < 10 W < W < W < 2.5

27 Example Model Run Land lost Current land Water

28 Model Results Land lost Current land Water Reality Uniform Erodibility Model

29 Model Framework Uniform Erodibility Model Rand(wind) Calculate Wave Power Wave Power α Erosion Probabilistic Erosion Young and Verhagen 1996 Leonardi 2016 Mariotti and Canestrelli 2017 Variable Erodibility Model Rand(wind) Calculate Wave Power Wave Power α Erosion Probabilistic Erosion Depends on the direction of the wind

30 Model Results Land lost Current land Water Reality Uniform Erodibility Model Variable Erodibility Model

31 Where the model fails Areas of faulting Barrier islands Ponding Changes in salinity Boundary of Brackish and aline Marsh * These processes aren t in the model Land lost (Model accurately eroded) Current land Water Model eroded and shouldn t have Model did not erode and should have CRM data

32 Larger Implications Inform Coastal Master Plan Protect marsh base at north-facing shores Protect marsh top at south-facing shores

33 Conclusions Wind and water level are correlated Overshooting at N and waves hitting below roots at North erodes less Elevation differences erodibility differences Depth (m) hear trength (kpa) Empirical correction to better predict erosion kvalen7@lsu.edu