Exploring the morphodynamic response of coastal …...Stutz and Pilkey, 2001, J. Coastal. Res. Upper...

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

Colorado School of MinesThursday September 20th 2018

México Texas

Texas

Louisiana

Texas coastal communities benefit from:

Over 80% coverage from coastal barriersacross a coastline which spans 590 km!

Upper Coast

Lower Coast2

México Texas

Texas

Louisiana


Over 80% coverage from coastal barriersacross a coastline which spans 590 km!

Upper Coast

Lower Coast

BarrierNon‐barrier

160 km

3

The global coast has ~7% coverage too!Stutz and Pilkey, 2001, J. Coastal. Res.

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

Shoreline disp.2 p10, p50, p90: ‐4.6, ‐0.9, 1.6 m yr‐1

RSLR 2 to 6 mm yr‐1

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





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





Image: Google Earth

North Padre Island

México Texas

Texas

Louisiana


Over 80% coverage from coastal barriersforming a coastline which spans 590 km!

Upper Coast

Lower Coast


160 km

9

Big question: What is the long‐term trajectory of Texas’ coastal barriers?

México Texas

Texas

Louisiana



Upper Coast

Lower Coast


160 km

10

https://coast.noaa.gov/slr/

+1.8 m

MHHW

+0.9 m

Example: “bath tub” model of Follets Is., TX

Communicating coastal change:

Extrapolation from historical rates andInundation (bath tub) models:

are dangerously inadequate!

Coastal barriers are dynamic landforms:→ rarely drown in place!

barriers respond to SLR→extrapolation!

SLR is accelerating

Benefit: Very easy to understand and communicate

México Texas

Texas

Louisiana



Upper Coast

Lower Coast


160 km

11

Big question: What is the long‐term trajectory of Texas’ coastal barriers?

Reasonable response:Use a process‐basedmodel to estimate barrier response to anticipated sea‐level rise along the Texas coast.

Reduced / intermediate complexity modeling:

Barrier morphology

Surface Processes

Sea‐level rise

Task: simplify Texas’ coastal barrier morphology into characteristic scales and physical processes into parameterized expressions which resolve fundamental barrier responses to sea‐level rise

Dynamicinteractions!

12

4 km

Follets Island

Reducing a barrier to characteristic scales

Gulf of Mexico

Bay


Lorenzo‐Trubea and Ashton (2014)

Coastal plain

Conceptual figure: high vertical exaggeration!14


Lorenzo‐Trubea and Ashton (2014)

Coastal plain

Conceptual figure: high vertical exaggeration!15

What is closure depth, Dc?

Deep water waves

𝐷

swashBeach

Cross‐shore distance

Elevation

Waves “feel bottom”, steepen & break

Wave energy decreases with depth.

As waves shoal (enter shallow water), beyond 𝐷 , waves move sediment

16

Galveston: fair weather waves

Galveston: not‐so‐fair weather waves

Hurricane Ike, 2008

Closure depth, Dc@ 500 yr timescale

Upper coast Lower coast

18

Local significant wave class

*morphologically significant

Significant* wave class:

Ortiz and Ashton, JGR Earth Surface, 2016


Lorenzo‐Trueba and Ashton (2014), Ashton and Lorenzo‐Trueba (2015)

Reduced complexity process representation in Barrier Sections:

19

2) Shoreface sediment fluxes:1) Passive flooding during RLSR:

3) Net longshore sediment flux 4) Overwash sediment transport

Conceptual figure: high vertical exaggeration!

Note: every single process terminates at 𝐷 !

Lorenzo‐Trueba and Ashton (2014), Ashton and Lorenzo‐Trueba (2015), Swanson et al., (in prep.)


1) Passive flooding during relative sea‐level rise:

20

Passive flooding drives:

1) Inundation of barrier (↓ H)2) Depth of closure (Dc) translates3) Barrier shoreface steepens (↑ 𝛼 )


Relative sea‐level rise (RSLR) is spatially variable, and includes local subsidence.



21

2) Shoreface sediment fluxes:

Example: under‐steepened shoreface = onshore fluxUnder‐steepened (𝛼 < 𝛼 )

Onshore directed sediment flux

Over‐steepened (𝛼 > 𝛼 )Offshore directed sediment flux

Local wave climate drives shoreface toward equilibrium slope:

𝑄 𝑘 𝛼 𝑎

𝐾 ~ wave energy

grainsize

If wave climate is unchanging, shorefaces trend toward“equilibrium profiles”, which have slopes that dependon wave energy and grainsize.

Cross‐shore fluxes attempt to maintain an equilibrium slope!

Equilibrium slope

Shoreface response rate

Upp

er coa

st

Lower coa

st

22

~ 120 𝜇𝑚

Central

~ 150 𝜇𝑚

Lower coast

~ 80 𝜇𝑚

Upper coast

Grainsize: always fine sand or finer!

Shoreface response rate

Swanson et al., in prep.

Abrupt wave climateIncrease, fine grain size

Significantly coarserbroader shorefacesimilar wave climate

Tranquil wave climateVery fine grained


Local wave class



3) Net longshore sediment flux

23

Longshore sediment flux is assumed:1) Driven by:

a) shoreline curvatureb) wave height

2) remove subaerial barrier3) linear with shoreface depth4) vary significantly along the TX coast




3) Net longshore sediment flux

24Conceptual figure: high vertical exaggeration!

Net accumulation:Barrier shoreface steepens (↑ 𝛼 )→ Helps to prograde barrier

Net erosion:Barrier shoreface relaxes (↓ 𝛼 )→ Helps barrier retreat



Equilibrium width (We) = 300 mEquilibrium height (He) = 2 m

25

4) Overwash sediment transport

1 kmFollets Is.

1 kmS. Padre Is.

Overwash: a threshold process:if 𝐇 𝐇𝒆

Overwash aggrades barrierif 𝐖 𝐖𝒆

Overwash widens barrier

Modern transgressive barriers:


Overwash attempts to maintain an equilibrium subaerial barrier volume!

𝑄 ~ 𝐻 𝐻 𝑊 𝑊 𝑊 𝐻



Equilibrium width (We) = 300 mEquilibrium height (He) = 2 m

26

4) Overwash sediment transport

1 kmFollets Is.

1 kmS. Padre Is.

Overwash: a threshold process:if 𝐇 𝐇𝒆

Overwash aggrades barrierif 𝐖 𝐖𝒆

Overwash widens barrier

Modern transgressive barriers:


Overwash attempts to maintain an equilibrium subaerial barrier volume!

𝑄 ~ 𝐻 𝐻 𝑊 𝑊 𝑊 𝐻

Overwash is limited to be less than a maximum value: 𝑄 .

𝑄 is a very important GLOBAL parameter, so we will explore sensitivity in upcoming results. Reasonable values span 0 to 100 m2 yr‐1

Initial width: 2 kmInitial height: 3 mRSLR: 10 mm yr‐1

Height drowning:Gulf Bay

BarrierGulf / Bay

Antecedent

Gulf Bay

1) SLR increases 𝛼2) Drives offshore flux3) Height↓ and Width↓4) Overwash threshold broken

27

Initial width: 350 mInitial height: 3 mRSLR: 2 mm yr‐1

Width drowning:

Barrier failure during sea‐level rise: Width drowning:shoreface response rate is too low to keep all sediment!

Height drowning:Maximum overwash 𝑄is too low to keep up with SLR!

4 km

Follets Island

Gulf of Mexico

Bay

Connecting coastal barrier sections:

Active shorefaceShoreline

Individual barrier sections have local morphodynamic parameters.

Barrier sections communicate via longshore sediment flux.

Swanson et al., in prep., Ashton and Lorenzo‐Trueba (2015), Ashton and Lorenzo‐Trueba (2018)

Boundary conditions set by historical rates.


29

Shelf

Coastal barrier (3D!)


Active shoreface

Shoreline



Ashton and Lorenzo‐Trueba (2015)

Boundary conditions set by historicalrates.


30

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

Active shoreface

Shoreline



Ashton and Lorenzo‐Trueba (2015)

Boundary conditions set by historicalrates.


31

Shelf

Model execution:1. Initial condition: modern coast2. Timestep: 1 day3. Run time: 500 yr (or until barrier failure)4. EXPLORE!

1. Sea‐level rise (0 to 10 mm yr‐1)2. Maximum overwash (0 to 100 m2 yr‐1)

A sensitive canary of the coastUpdrift of major deltaic headland

Elevation (M

HHW)

Offshore Distance km

Sea‐level

Regionalmean profile

Follets Island

DcH = 1.7 mW = 610 m

A stout, stable barrier Updrift of “longshore convergence zone”


Sea‐level

Mustang Island

Dc

H = 3.4 mW = 1700 m

Modestly progradingIn “longshore convergence zone”


Sea‐level

North Padre Island

Dc

H = 3.9 mW = 2100 m

Post‐processing: Monitor three key regions

32Swanson et al., in prep.

Low, med., high cases:Low: SLR 3 mm yr‐1 , 𝑸𝒐𝒘𝐦𝐚𝐱 30 m2 yr‐1

→ Recent rates of SLR (eustatic) → Reasonable overwash given multi‐century record (Odezulu et al., 2018)

Medium: SLR 5 mm yr‐1 , 𝑸𝒐𝒘𝐦𝐚𝐱 50 m2 yr‐1→Upper limit of IPCC “500‐700 ppm” (~5 mm yr‐1 over 500 yr)→Overwash: reasonable given radiometrically derived overwash rates

(Odezulu, Anderson, and Swanson, In Prep)High: SLR 10 mm yr‐1 , 𝑸𝒐𝒘𝐦𝐚𝐱 = 100 m2 yr‐1

→ Upper‐estimate of IPCC “>700 ppm” (~13 mm yr‐1 over 500 yr)→ Unprecedented rates of overwash

Concept: higher rates of SLR correlate to higher rates of overwash

A sensitive canary of the coastUpdrift of major deltaic headland

Elevation (M

HHW)


Sea‐level

Regionalmean profile

Follets Island

DcH = 1.7 mW = 610 m

A stout, stable barrier Updrift of “longshore convergence zone”


Sea‐level

Mustang Island

Dc

H = 3.4 mW = 1700 m

Modestly progradingIn “longshore convergence zone”


Sea‐level

North Padre Island

Dc

H = 3.9 mW = 2100 m

Post‐processing: Monitor three key regions


34

Follets Island: Dip sections


Modified from Odezulu et al., 2018

Simplified interpreted core transect

Simulation stats:Low: 470 yr / ~ 1.3 m SLRMed. : 380 yr / ~1.9 m SLR High: 250 yr / ~2.5 m SLR

Low

Med.

High

Follets Island exhibits strong landward retreat (always!)

Gulf Bay

Follets Island: Low case

No overwash

retreatoverwash

landward

growing

shrinking


WidthEq. Width

HeightEq. Height

Longshorecum. Sed. Flux

Total overwashcum. Sed. Flux

1) Slow retreat, then accelerated retreat!Notable barrier responses:

2) Subaerial barrier shrinks, then stabilizes3) Constant overwash by height

Overwash by width by ~180 yrs.4) Strong retreat reduces longshore flux

Follets Island: Low and mid. cases


No overwash

retreatoverwash

landward

growing

shrinking

WidthEq. Width

HeightEq. Height

Low

Med.

1) Overwash by width at ~ 170 yrs2) Faster and further retreat

Notable barrier responses:



Follets Island: Low, mid., and high cases

No overwash

retreat

overwash

landward

growing

shrinking


WidthEq. Width

HeightEq. Height

Low

Med.High

1) Largest and fastest retreat2) Overwash by width at 100 yrs3) Strong retreat redirects longshore

transport.4) Increases in SLR and 𝑄

vastly accelerate barrier behavior!




Follets Island: Mean annual rates of changeMean annual shoreline disp.


Extrapolation from historical records may severely under‐estimate responses!

p10 ~ ‐3.3 m yr‐1p50 ~‐2.5 m yr‐1p90 ~‐0.4 m yr‐1

Historical shoreline disp. rates:

Paine et al., JCR, 2016

39

Mustang Island: Dip sections

Forward modeled dip section


Modified from Rodriguez et al., 2001, Sedimentology

Simplified interpreted OFFSHORE core transectLow

Med.

High

Mustang exhibits significant progradation on marine deposits→ History of retreat recreated by high SLR and overwash rates

Gulf Bay


No overwash

overwash

landward

Mustang Island: Low case


progradation

shrinking

WidthEq. Width

HeightEq. Height

1) Fast progradation, then slower!Notable barrier responses:

2) Subaerial barrier shrinks, then stabilizes3) Overwash by height by ~ 300 yrs4) Overwash slows progradation +

stabilizes barrier vol.



No overwash

overwash

landward

Mustang Island: Low and mid. cases

retreat

shrinking

WidthEq. Width

HeightEq. Height

Low

Med.

1) Overwash by height at 220 yrsNearly halts barrier progradation




No overwash

retreat overwash

landward

shrinking

Mustang Island: Low, mid., and high cases


WidthEq. Width

HeightEq. Height

Low

Med.

High1) Overwash by height at 110 yrs

and high SLR causes a progradationalbarrier to retreat!




Mustang Island: Mean annual rates of changeMean annual shoreline disp.


Although progradation has occurred for ~ 500 yrs within the central Texas region, this isn’t reflected in shoreline surveys

p10 ~ ‐1.2 m yr‐1p50 ~‐0.5 m yr‐1p90 ~ 0.5 m yr‐1



44

N. Padre Island: dip sections


Modified from Rodriguez et al., 2001, Sedimentology

Simplified interpreted OFFSHORE core transectLow

Med.

High

Like Mustang, N. Padre exhibits significant progradation→ History of retreat recreated by high SLR and overwash rates

Gulf Bay


No overwash

overwash

landward

N. Padre Island: Low caseProg

rada

tion

shrinking


WidthEq. Width

HeightEq. Height

1) Fast progradation, then slower!Notable barrier responses:

2) Subaerial barrier shrinks, then stabilizes3) Overwash by height by ~ 425 yrs4) Overwash slows progradation +

stabilizes barrier vol.



No overwash

overwash

landward

N. Padre Island: Low and mid. casesProg

rada

tion

shrinking


WidthEq. Width

HeightEq. Height

Low

Med.

1) Overwash by height at 310 yrsNearly halts barrier progradation




No overwash

overwash

landward

N. Padre Island: Low, mid., and high casesProg

rada

tion

shrinking


WidthEq. Width

HeightEq. Height

Low

Med.

High 1) Overwash by height at 175 yrsand high SLR causes a transition:progradation to retreat!




N. Padre Island: Mean annual rates of changeMean annual shoreline disp.


Although progradation has occurred for ~ 500 yr within the central Texas region, this isn’t reflected in shoreline surveys

p10 ~ ‐0.75 m yr‐1p50 ~‐0.03 m yr‐1p90 ~ 0.5 m yr‐1



𝑄 m2 yr‐1

SLR mm yr‐1

Failure mode

49

+’s indicate: min, med., and max scenarios

When, how, and where do barriers fail?


𝑄 m2 yr‐1

SLR mm yr‐1

Time to failure yr

𝑄 m2 yr‐1

SLR mm yr‐1

Failure location

No failure

Height drowning

Width drowning No failure

Upper Galveston

Matagorda Peninsula

LowerGalveston

Follets Island

Time to failure*


𝑄 m2 yr‐1

SLR mm yr‐1

No failure

Upper Galveston

Matagorda Peninsula

LowerGalveston

Follets Island

Where do barriers fail?

Clustering of failures: The upper Texas coast Low barriers of various widthLow response rate (K)


𝑄 m2 yr‐1

SLR mm yr‐1

Matagorda Peninsula

Matagord Peninsula: Failure by width‐drowning1) minimal input from Colorado/Brazos2) Within low 𝑄 :

Width reduction by:1)Longshore transport2)Overwash due to low H

Why do barriers fail?


𝑄 m2 yr‐1

SLR mm yr‐1Upper

Galveston


Upper Galveston: Failure by height‐drowning1) No fluvial source2) Wide (~2.2 km), Low (~2.2 m)3) low overwash rates cannot keep up with:

RSLR (subsidence = 3.4 mm yr‐1)

𝑄 m2 yr‐1

SLR mm yr‐1

53



Follets Island – Height drowning

Failure by width‐drowning1) low H (~1.7 m), modest width (~ 0.6 km)2) low shoreface response rate (K)

Sediment loss to the shelf during SLR!

Follets Island – Width drowning

Follets Is.: Failure by height‐drowning1) low H (~1.7 m), modest width (~ 0.6 km)2) low overwash rates cannot keep up with:

RSLR (subsidence = 1.2 mm yr‐1)

𝑄 m2 yr‐1

SLR mm yr‐1

54



LowerGalveston

Lower Galveston: Failure by width‐drowning1) Follets retreats into Christmas Bay2) too wide (~1.2 km) to retreat3) relative change in shoreline position

→ drives longshore fluxes to Follets

Lower Galveston

Conceptual summary3 Overwash is sourced from shoreface

1) retreat rates increase2) progradation rates decrease3) barrier volume stabilizes

Sea‐level increasesBarrier scales diminish

1

Overwash becomes frequent2

55

Longshore transport system is modified4

Map viewx

y

Retreating barriers:


Conceptual summary3 Overwash is sourced from shoreface

1) retreat rates increase2) progradation rates decrease3) barrier volume stabilizes

Sea‐level increasesBarrier scales diminish

1

Overwash becomes frequent2

56

Longshore transport system is modified4

Map viewx

y

Prograding barriers:


Conclusions:• Texas’ long‐term barrier trajectory is set by initial scales and location within Texas’ longshore transport system

• Range of forecasts capture:• Historical behavior• late Holocene architecture

• Barrier responses to sea‐level rise modify longshore transport• Future work:• Use coastal barrier model to forward model: • stratigraphic architecture (constrain shoreline controls)

• Open‐source‐it! Open framework for coastal barriers

57

Conclusions:• Texas’ long‐term barrier trajectory is set by initial scales and location within Texas’ longshore transport system

• Range of forecasts capture:• historical behavior• late Holocene architecture

• Barrier responses to sea‐level rise modify longshore transport• Future work:• Use coastal barrier model to forward model • stratigraphic architecture (constrain shoreline controls)

• Open‐source‐it! Open framework for coastal barrier modeling

58

Thank you!A special thanks to Dr. Hongbo Ma! (Rice U.)

59

Thank you!

60

Download this presentation @www.tswanson.net

A special thanks to Dr. Hongbo Ma! (Rice U.)

Exploring the morphodynamic response of coastal …...Stutz and Pilkey, 2001, J. Coastal. Res. Upper...

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