Bulkheads on Urban Estuarine Shorelines on the Gulf Coast€¦ · 02/01/2018 · Mar 17, 2010 -...

Douglass - Living Shorelines

Mar 17, 2010- Moss Point, MS 1

Introduction to Coastal Processes

Scott L. DouglassUniversity of South AlabamaCivil Engineering Department

presentation toLiving Shorelines Workshop

M arch 17, 2010Pelican Landing Conference Center

M oss Point, M ississippi

Copyright S. Douglass ( 2010)



Alabama:60 miles of Gulf

beaches and600 miles of bay

and bayou shorelines!



Pelican Point

Daphne Bayfront Park

“ Natural” shorelines of M obile Bay



Alabama Port County Park

“ hardened” shorelines of M obile Bay

Eastern Shore – Mullet Point area



Progression of the Typical Response to Bay Erosiontime = t1

At time = t1, the home is located on a receding shoreline.



At time = t2, the homeowner constructs a bulkhead to protect the upland property.




At time = t3, the homeowner’s bulkhead begins to interfere with the nearshore coastal processes.




At time = t4, profile lowering occurs in front of the bulkhead leading to the loss of intertidal habitat.

Progression of the Typical Response to Bay ErosionTime = t4



Loss of intertidal area due to “passive erosion”

Progression of the Typical Response to Bay Erosion



Typical shoreline on eastern shore of Mobile Bay (circa 1990)



Example shoreline on eastern shore of Mobile Bay (circa 1996)

6 feet

4 feet

2 feet

Note barnacles on

wooden bulkhead

Tide range here is 1-2

feet.



Example shoreline on eastern shore of Mobile Bay

(circa 2006) Note barnacles on

wooden bulkhead

Tide range here is 1-2

feet.



Questions:

• What is the fate of these shorelines?

• What are the natural processes causing the erosion?

• What can we do about it?



M ethodology:

• historic air photo analysis –1955–1974–1985

• video-taping from air ( 1997)



Location of Armoring in 1955

8% of bayarmored




14% of bayarmored




26% of bayarmored




30% of bayarmored



Armored and Unarmored Shorelines

70%

30%In 1997, 30% of Mobile Bay’s shoreline was armored.

not armored

armored



Rate of increase in armoring similar to increase in population

0

5

10

15

20

25

30

35

1940 1950 1960 1970 1980 1990 2000250,000

300,000

350,000

400,000

450,000

500,000

% o

f sh

orel

ine

arm

ored

popu

lati

on



71%

29%

In 1997, the most common type of shoreline armoring was a vertical bulkhead

vertical bulkhead

other*



My guess as to what has happened since 1997,



• Loss of intertidal area ( 10-20 ac)

• Loss of intertidal shoreline ( 4-8 mi)

* from Douglass and Pickel ( 1999)

IM PLICATIONS?



IM PLICATIONS?

“The tide don’t go out no mo’!”walkingplayingoysters“jubilees”

Is this the fate of our urban estuaries?



Questions:

• What are the natural processes causing the erosion?



Big Sur, California

Dauphin Island,Alabama

Coastal Geology



Laguna Beach, California

beach sand comes from bluff erosion and creek floods



Mobile Bay, Alabama

beach sand comes from bluff erosion and creek floods



•Waves at an angle to the shore transport sand

•Conveyor belt analogy

•“river of sand”

Longshore Sand Transport



Breaker Line

Wave Crests

α = wave angle

Dry Beach

Longshore Sand Transport



Questions:

• What can we do about it?



1. Seawalls, revetments, and bulkheads:

Shore-parallel structures to protect upland property from waves

Shoreline Stabilization



2. Groins:

shore-perpendicular structures designed to reduce longshore sand transport


Groin


34Mar 17, 2010- Moss Point, MS

Douglass 35

Shoreline response to structures:

Multiple Groins



Groins were once a primary tool for engineers to stabilize shorelines




New groins are restricted or prohibited in many states due to their obvious negative downdrift impacts

Negative Groin Impacts

Aug 17, 2007

Old MapQuest image shows new sand fillet adjacent to south timber jetty



3. Breakwaters:

shore-parallel structures offshore designed to reduce wave energy and reduce longshore sand transport


• An example of shoreline position changes through time

Offshore or nearshore breakwater



Shoreline StabilizationNOTE: There are other functions for

“ Breakwaters”








Nourishment with Structures

Louisiana Highway 87, Holly Beach



Equilibrium Planform

Indentation, M, is a function of gap width, G:

32

31

<<GM



Breakwater



4. Hybrid structures:

Some combination of aspects of groins and breakwaters including “t-head groins” and “headland breakwaters”




Beach Nourishment

… is the most popular “shoreline stabilization” approach on America’s open-coast beaches today.

…no structure…just sand




Headland breakwaters and beach nourishment form permanent “pocket beaches” that replicate natural pocket beaches between rocky headlands



The shape of “pocket beaches” is controlled by the direction of wave approach and the location of the rock headlands

Half Moon Bay between Pillar Point headland and Three Rocks



Diffraction Point – Headland

( Seal Rock at Pillar Point)

Control Point Headland

( Three Rocks)

Dominant direction of wave approach



Diffraction Point – Headland

( Seal Rock at Pillar Point)

Control Point Headland

( Three Rocks)

Dominant direction of wave approach

Equilibrium shoreline

shape

Shoreline StabilizationOriginally, investigators modeled this with a

“ logarithm-spiral” shape



http://upload.wikimedia.org/wikipedia/en/3/3f/Logarithmic_spiral.svg�

http://upload.wikimedia.org/wikipedia/commons/0/08/NautilusCutawayLogarithmicSpiral.jpg�



Equilibrium shape downdrift of artificial headland



Equilibrium Shoreline Planform

2210

0

)()(θβ

θβ CCC

RR

++=

(Hsu & Silvester, 1989)

Wave angle (degrees)

CC1

C2

Co

β

θ


Mar 17, 2010- Moss Point, MS 57Children’s Beach, LaJolla, CA

Artificial pocket beaches



Yorktown, VA




Headland Breakwater Examples

Potomac River, Chesapeake Bay (from Hardaway and Gunn, 1999)

Shoreline StabilizationBodge’s “ 1/ 3rd” Rule for Headland Breakwaters:

Indentation, M , is a

function of gap width, G: 3

2GM

31

<<



Shoreline StabilizationRule for Headland Breakwaters:

Beware: Hardaway says that this ratio can be much higher in sand-starved situations typical of

the Chesapeake Bay ( as high as M / G= 1.7)





a sandy beach

Can we emulate these natural shorelines in constructed

alternatives to bulkheads?

MORE QUESTIONS:



Demonstration project of an alternative to bulkheads on bay shorelines

Brookley headland beach project - 2000

•built Aug 1998

•two low elevation rock headland breakwaters

•3000 m3 sand fill

•survived Hurricane Georges Sept. 1998



(an alternative to bulkheads on bay shorelines)

•Short structures can stabilize longer stretches of shoreline

•more natural shoreline than a bulkhead

Brookley headland beach project

September 2003

Pocket beaches and headland breakwaters



Today, this would be a vertical bulkhead with rip-rap at the base of it if not for this alternative

pocket beach.



a sandy beach as an alternative

to a bulkhead

Marriott’s Grand Hotel Resort, Mobile Bay, Point Clear, Alabama



a sandy beach as an alternative

to a bulkhead




Beach nourishment - Grand Hotel, Point Clear

(three weeks after initial 2001 construction)

•an engineered “pocket beach”

•built 2001

•3 rock headland breakwaters

•6000 m3 sand fill

•extended in 2003•lengthened breakwaters •added sand



construction2001

one year later2002

after extension2003



“ Tuning” process for Grand beach & breakwater configuration:

• Wind data ( 27 years of data from M obile)

• Wave model ( US Army 1984)

• Longshore sand transport model ( CERC Equation)

• Dominant & secondary wave directions

• M odified form of Silvester’s method for predicting equilibrium shoreline



Dominant wave directionsecondary wave direction

Predicted shoreline



Pocket beach constructed in front of bulkhead/seawall


1998

2003



Pocket “feeder “beach and artificial headland concepts in Fairhope

Oak tree and bluff south of Pier St. boat ramp were

threatened by erosion in

2004

2007



2004




Fall 2004:

Roots exposed by erosion

8-foot eroding bluff

Existing bulkhead was functioning as the headland2004




Project: Beach nourishment as “feeder

beach”+

Timber artificial headland moved the diffraction point headland 40 feet to the southwest to stabilize shoreline farther from tree roots 2006

2006



a “fringe” wetland

Can we emulate these natural shorelines in constructed

alternatives to bulkheads?

MORE QUESTIONS:



Can we protect our eroding

natural wetlands?

MORE QUESTIONS:

Mississippi Sound



How much wave energy can S. alterniflora tolerate?

One answered question:



Wave climate limit for wetlands

after Roland and Douglass ( 2005)

e



from Roland, R.M. and S.L. Douglass 2005 “An estimate of the upper limit of wave level tolerance for Spartina alterniflora in coastal Alabama” Journal of Coastal Research, vol. 21, no. 3, pp. 453-463.

The upper limit of wave energy for “ fringe” salt marsh existence:

- a median ( H50) H = 0.15 m

- a corresponding H80 = 0.25 m

Sites with less wave energy had vegetation along the shoreline.



• Can breakwaters be used to reduce wave energy to levels that allow wetland creation/ establishment?

• I f so, then…

“ how can we optimize/ minimize the

breakwater”

“ How low can you go?”

MORE QUESTIONS:



And…“ can segmented

offshore or headland breakwaters concepts

be used?”

MORE QUESTIONS:

Photo from Aspelin ( 2007)

Photo from Hauske ( 2007)

Texas examples



Site

Constructed wetland behind a “ wave fence”

boat wakes and

wind waves

Dog River, M obile, Alabama



Constructed wetland behind a

“ wave fence”

Wave transmission coefficienttechnology dates to D-Day!





Submerged or Emerged Breakwaters



( )bt

i i

b

b b eKH H

d B 3

41 2 1 ξ = + −

d’Angremond’s Equation

Predictive Data and Formulas for Wave Transmission



( )bt

i i

b

b b eKH H

d B 3

41 2 1 ξ = + −

d’Angremond’s Equation

Predictive Data and Formulas for Wave TransmissionSubmerged Breakwaters knock down less

than 50% of wave height



Do “Wave Attenuation Devices” really attenuate waves?

And how much?



What do we do about our eroding wetlands?

Little Bay, Alabama

ADCNR photo



Laboratory Tests of Wave Transmission though “Wave

Attenuation Devices”



Laboratory Tests of Wave Transmission though “Wave

Attenuation Devices”

•ADCNR funded at Univ. of South Alabama (with Volkert, Inc)

•1:5 scale models

•Results: 40% < Kt < 90%•emergence•configuration (e.g. 2-rows)

•Increased height needed




Little Bay, Alabama

ADCNR photo




Little Bay, Alabama

ADCNR photo




Little Bay, Alabama

ADCNR photo



Holt’s Landing State Park, DE

•Too much wave energy

•Fetch>5mi

•H>2-3 ft




•Offshore segmented breakwaters used

•Emergent breakwaters, little transmission

•Diffraction of wave energy through gaps controlled design (gap, length, etc.)

•Diffraction diagrams from SPM and Goda















Pocket beach constructed in front of bulkhead/seawall

Marriott’s Grand Hotel Resort, Mobile Bay, Point Clear, Alabama 2005

This solution cannot be built along most of America’s bay shorelines!



…this was permitted and built in 2008!

EXISTING POLICIES NEED TO BE CHANGED:

…now what should this property owner do?



…why can this owner (rather easily) get a

permit for this…

…but not this?

MORE QUESTIONS:



EXISTING POLICIES NEED TO BE CHANGED:



Existing policies ( 2) are

contributing to the destruction of

inter-tidal beaches along our bay

shorelines

Why can this owner easily get a permit to build another bulkhead

but cannot get a permit to build a pocket beach?



Conclusions•We are turning our urban estuaries into bathtubs with no intertidal zones

•We need to modify existing policies if we are have “ living shorelines”

• We need more applications of coastal engineering principles into ecosystem engineering



Contact Info

Scott L. DouglassCivil Engineering DepartmentUniversity of South Alabama

[email protected]

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Bulkheads on Urban Estuarine Shorelines on the Gulf Coast€¦ · 02/01/2018 · Mar 17, 2010 -...

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