Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Exploring the morphodynamic response of coastal barriers to sea‐level rise along the Texas Gulf Coast
Swanson, T.1, Lorenzo‐Trueba, J.2, Anderson, J.1, Nittrouer, J.11Rice University 2Montclair State University
Image: Google Earth160 km 1
John Fest!Saturday April 21st 2018
You are here (approximately)
México Texas
Texas
Louisiana
Texas coastal communities benefit from:
Over 80% coverage from coastal barriersforming a coastline which spans 590 km!
Upper Coast
Lower Coast
BarrierNon‐barrier
160 km
2
You are here (approximately)
México Texas
Texas
Louisiana
Texas coastal communities benefit from:
Over 80% coverage from coastal barriersforming a coastline which spans 590 km!
Upper Coast
Lower Coast
BarrierNon‐barrier
160 km
3
Big question: What is the long‐term (102 yr) trajectory of Texas’ coastal barrier
system?
Reasonable response:Apply a suitability complexmodel to estimate barrier response to relative sea‐level rise (RSLR).
Upper Coast
Lower Coast
160 km
1Ortiz and Ashton (2016), 2Paine et al (2016)
BrazosDelta
Colorado River
Río Grande
Texas Coast: VITAL STATISTICSTidal range < 1m
Sig. Wave 0.9 to 2.1 m @ SE direction
Depth of Closure 1 4.2 to 11 m @ 500 yr
Shoreline disp.2 p10, p50, p90: ‐4.6, ‐0.9, 1.6 m yr‐1
RSLR 2 to 6 mm yr‐1
Shelf slope 10‐3 to 10‐4.2
Barrier Height <1 m to >5 m
Barrier Width <300 m to >8000 m
4
Upper Coast
Lower Coast
Follets Island
Mustang Island
North Padre Island
Three key regions of interest:1) Follets Island 2) Mustang Island 3) North Padre Island
160 km
Paine et al, 20165
Upper Coast
Lower Coast
Follets Island
Mustang Island
North Padre Island
160 km
Paine et al, 20166
4 km
Three key regions of interest:Follets Island (sensitive canary)
• Flanks a major deltaic headland• Retreating landward > 2m yr‐1• Low, narrow
Mustang Island (stable and stout)• updrift of “longshore convergence zone”• Retreating landward < 1m yr‐1• Tall, wide
North Padre Island (modestly prograding)• Within “longshore convergence zone”• Slightly prograding 0.03 m yr‐1• Very tall and wide
Image: Google Earth
Follets Island
Upper Coast
Lower Coast
Follets Island
Mustang Island
North Padre Island
160 km
Paine et al, 20167
7.5 km
Three key regions of interest:Follets Island (sensitive canary)
• Flanks a major deltaic headland• Retreating landward > 2m yr‐1• Low, narrow
Mustang Island (stable and stout)• updrift of “longshore convergence zone”• Retreating landward < 1m yr‐1• Tall, wide
North Padre Island (modestly prograding)• Within “longshore convergence zone”• Slightly prograding 0.03 m yr‐1• Very tall and wide
Image: Google Earth
Mustang Island
Upper Coast
Lower Coast
Follets Island
Mustang Island
North Padre Island
160 km
Paine et al, 20168
7.5 km
Three key regions of interest:Follets Island (sensitive canary)
• Flanks a major deltaic headland• Retreating landward > 2m yr‐1• Low, narrow
Mustang Island (stable and stout)• updrift of “longshore convergence zone”• Retreating landward < 1m yr‐1• Tall, wide
North Padre Island (modestly prograding)• Within “longshore convergence zone”• Slightly prograding 0.03 m yr‐1• Very tall and wide
Image: Google Earth
North Padre Island
Reduced complexity modeling:
Barrier morphology
Surface Processes
External forcing
Task: simplify Texas’ coastal barrier morphology into characteristic scales and morphodynamic processes into parameterized expressions which resolve fundamental geomorphic responses to external forcing
Morphodynamicinteractions
9
Reducing a barrier to characteristic scales
Lorenzo‐Trubea and Ashton (2014)
Coastal plain
Conceptual figure: highly vertically exaggerated!10
Lorenzo‐Trubea and Ashton (2014), Ashton and Lorenzo‐Trubea (2015)
Reduced complexity process representation in Barrier Sections:
1) Passive flooding during RLSR:
11
Passive flooding drives:
1) Inundation of barrier (↓ H)2) Depth of closure (Dc) translates3) Barrier shoreface steepens (↑ )
a) Perturbs shoreface from equilibrium
Lorenzo‐Trubea and Ashton (2014), Ashton and Lorenzo‐Trubea (2015)
Reduced complexity process representation in Barrier Sections:
12
2) Shoreface sediment fluxes: Local wave climate drives shoreface toward equilibrium slope:
Under−steepended ( < )Onshore directed sediment flux
Over‐steepended ( > )Offshore directed sediment flux
Example: under‐steepened shoreface = onshore flux
Lorenzo‐Trubea and Ashton (2014), Ashton and Lorenzo‐Trubea (2015)
Reduced complexity process representation in Barrier Sections:
3) Net longshore sediment flux
13
Longshore sediment flux is assumed:1) Driven by:
a) shoreline curvatureb) local significant wave class
2) remove subaerial barrier3) linear with shoreface depth4) vary significantly along the TX coast
Lorenzo‐Trubea and Ashton (2014), Ashton and Lorenzo‐Trubea (2015)
Reduced complexity process representation in Barrier Sections:
Critical width (Wc) = 300 mCritical height (Hc) = 2 m
14
4) Overwash sediment transport
1 kmFollets Is.
1 kmS. Padre Is.
Within any barrier section:if
Overwash aggrades barrierif
Overwash widens barrier
Modern transgressive barriers:
Active shoreface
Shoreline
Individual barrier sections have local morphodynamic parameters.
Barrier sections communicate via longshore sediment flux.
Ashton and Lorenzo‐Trubea (2015),Ashton and Lorenzo‐Trubea (2018)
Boundary conditions set by long‐term rates.
Connecting coastal barrier sections:
15
Shelf
Active shoreface
Shoreline
Individual barrier sections have local morphodynamic parameters.
Barrier sections communicate via longshore sediment flux.
Boundary conditions set by long‐term rates.
Connecting coastal barrier sections:
16
Shelf
Big assumptions:1. Barrier sediment composition is sand2. Inlets/jetties allow longshore bypass3. Seawalls prevent shoreline displacement4. Wave climate is unchanging5. Fluvial contributions are steady6. No further human modification7. No geomorphic clearing events
Simplified regional barrier morphology:Barrier height and width (NOAA survey)Shoreline shape (Paine et al. 2016)Shoreface, shelf slope (NOAA survey)
Data sources & methods
Local relative sea‐level rise(NOAA tide gauges)
Parameterizedmorphodynamic processes:local morphodynamically significant wave class (ACoE, Ortiz and Ashton, 2015)longshore sediment transport (theoretical + calibration, inhomogeneous diffusion)
cross‐shore sediment transport (theoretical, Lorenzo‐Trubea and Ashton, 2014)Morphodynamic depth of closure (theoretical, Ortiz and Ashton, 2015)
17
Simplified regional barrier morphology:Barrier height and width (NOAA survey)Shoreline shape (Paine et al. 2016)Shoreface, shelf slope (NOAA survey)
Data sources & methods
Local relative sea‐level rise(NOAA tide gauges)
Parameterizedmorphodynamic processes:local morphodynamically significant wave class (ACoE, Ortiz and Ashton, 2015)longshore sediment transport (theoretical + calibration, inhomogeneous diffusion)
cross‐shore sediment transport (theoretical, Lorenzo‐Trubea and Ashton, 2014)Morphodynamic depth of closure (theoretical, Ortiz and Ashton, 2015)
18
Model execution:1. Initial condition: modern coast2. Timestep: 1 day3. Run time: 500 yr4. Model output
1. Moving boundaries2. Barrier scales3. Sediment fluxes
A sensitive canary of the coastUpdrift of major deltaic headland
Elevation (M
HHW)
Offshore Distance km
Sea‐level
mean profile
Follets Island
Dc
H = 1.3 mW = 412 m
A stout, stable barrier Updrift of “longshore convergence zone”
Offshore Distance km
Sea‐level
Mustang Island
Closure depth
H = 3.0 mW = 751 m
Modestly progradingWithin “longshore convergence zone”
Offshore Distance km
Sea‐level
North Padre Island
Closure depth
H = 3.4 mW = 1736 m
Post‐processing: Monitor three key regions
19
mean
max
min
seaward
landward BarrierheightCriticalheightBarrierWidthCriticalWidth
overwash
no overwash
Follets Island: A sensitive canary of the coast
20
shrinking
growing
seaward
landward
Longshore flux
Overwash: Height deficit
Overwash: Width deficit
Cross‐shore flux
mean
max
min
seaward
landward
BarrierheightCriticalheightBarrierWidthCriticalWidth
overwash
no overwash
Mustang Island: A Stout, stable barrier
21
shrinkinggrowing
seaward
landward
Longshore flux
Overwash: Height deficit
Overwash: Width deficit
Cross‐shore flux
mean
max
min
seaward
landward
BarrierheightCriticalheightBarrierWidthCriticalWidth
overwash
no overwash
N. Padre Island: Modestly prograding
22
shrinking
growing
seaward
landward
Longshore flux
Overwash: Height deficit
Overwash: Width deficit
Cross‐shore flux
Conceptual summary – retreating barriers:3 Overwash is sourced from shoreface
1) retreat rates increase2) progradation rates decrease3) barrier volume stabilizes
Sea‐level increasesBarrier scales diminish
1
Overwash becomes frequent2
23
Longshore transport system is modified4
Map viewx
y
Retreating barriers:
Conceptual summary – retreating barriers:3 Overwash is sourced from shoreface
1) retreat rates increase2) progradation rates decrease3) barrier volume stabilizes
Sea‐level increasesBarrier scales diminish
1
Overwash becomes frequent2
24
Longshore transport system is modified4
Map viewx
y
Prograding barriers:
Conclusions• Texas’ long‐term barrier trajectory is set by initial scalesand location within Texas’ longshore transport system
• Barrier responses to sea‐level rise modify longshore transport patterns
• Dependence on initial barrier scales highlights needs:• Barrier topography• Barrier bayline motion• Barrier sediment composition
25
Download this presentation @ www.tswanson.net
Thanks John! Thanks ya’ll!A special thanks to Dr. Hongbo Ma! (Rice U.)
26
Download this presentation @ www.tswanson.net
27
Modeling strategy:
28
Texas coastal system is segmented into large regionalCHUNKS based on subaerial barrier morphology.Specifically,
1) wide barriers2) narrow barriers3) barriers fused/welded to coastal plain4) coastal headlands5) deltaic headlands
These regions partially inform the initial condition for 1000 nodes alongshore (∆ = 5.1 km).
200 km
Model region
Specifically, cross‐shore morphology and process:1) barrier height2) barrier width3) Depth of closure (Dc)4) shoreface slope ( )5) shelf slope ( )3) shoreface response rate (K)
Afterward,Each node adopts its own trajectorywith further timesteps.
Image: Google Earth
What is closure depth, Dc?
29
ACoE hindcast http://wis.usace.army.mil/
Goal: find the spatial variability of a single wave classthat describes long‐term morphological change. Use this wave to describe Dc, and other long‐term processes.
1) upper and lower 2% are removed2) Histogram weighted by H5, a first order controlon sediment flux → ∝ ∝
‐sediment flux‐ Wave orbital velocity
‐ significant wave heightOrtiz and Ashton, JGR Earth Surface, 2016
Sabine
Rio Grand
e
Alongshore distance km
Breaker zone
swash
Beach
Gulf
Sea‐level