Coastal Erosion Responses
for Alaska
Orson Smith, P.E., Ph.D.
Mikal Hendee, P.E.
Photo of Cook Inlet by Orson Smith
CONTENTS
Part 1
• Shoreline Processes
• Alaska Coastal Settings
• Evaluate Before You Buy
• Consider Erosion Before Building
Part 2
• Coastal Erosion Response Alternatives
– Adaptation
– Non-structural Responses
– Shore Protection Structures
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
2
Alaska Shoreline Hazards
•
•
•
•
•
•
coastal flooding
high winds
erosion
earthquakes
tsunamis
sea ice
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
3
Whittier photo provided by PND Engineers, Inc.
Shoreline Processes
• Tides
– Independent of
weather
• Storm surge
– Wind-induced water
level rise
Time (calendar days)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
4
Shoreline Processes
(used with permission)
• Wave action
– Waves are created by wind over water
– Height varies with wind speed, fetch, and duration
– Waves breaking obliquely against the shore cause
longshore sediment transport
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
5
Shoreline Processes
• Wave-induced “littoral” sediment transport
–
–
–
–
Occurs mainly in surf (littoral) zone
Longshore transport induced by oblique waves
Offshore transport induced by storm waves
Onshore transport induced by mild waves
(used with permission)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
6
Arctic Coastal Processes
•
•
•
Undercutting of glacially deposited bluffs
Thermal erosion of permafrost bluffs
Thaw subsidence of permafrost shores
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Photo at Wainwright by Mike Hendee
Coastal Erosion Responses for Alaska
7
Coastal Erosion
US Army Corps of Engineers photo
Coastal erosion is a natural dynamic process,
mainly caused by
•
•
Storm waves and surges (rapid and dramatic)
Natural changes to tidal inlets, river mouths, and entrances
to bays
–
–
•
migration or fluctuation of channels and shoals
interruption of longshore sediment transport
Manmade structures and human activities
–
–
–
–
–
shore protection structures
damming of rivers (trapping sediment)
dredging; mining sand from beaches and dunes
alteration of vegetation, surface or groundwater drainage at
coastal bluffs
local scour around structural elements
•
•
such as pilings and abutments
Long-term erosion occurs over decades from cumulative
effects due to:
–
–
–
sea level rise
depletion of sediment supply
factors mentioned above
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
8
Alaska Coastal Settings
Fjords and rock promontories
• Primarily in Southeast and Southcentral
Alaska
• Steep pocket beaches of sand and pebbles
• Subject to high tidal range
• Respond mainly to storms
• Comparatively slow erosion rates
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
9
Photo on Elrington Island, Prince William Sound, by Riley Smith
Alaska Coastal Settings
Bluff shorelines
•
•
•
•
•
Mostly unconsolidated glacially deposited sediments
Sand and gravel beaches at base
Typical of Cook Inlet, Kachemak Bay and Bristol Bay
Bluff erosion feeds sediment to beaches
Isolated catastrophic retreat due to wave-induced
undercutting
• Weakened by bluff-edge development, runoff concentration,
and foot or vehicle traffic
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
10
Photo at Kenai by Orson Smith
Bluff Erosion
Figure by Orson Smith
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
11
Alaska Coastal Settings
Permafrost shorelines
• Northwestern and Arctic Alaska
• Low tide range
• Narrow sand and gravel beaches
at base of low bluffs
• Permafrost of ice, organics, and
fine-grained sediments
• Subject to thermal erosion
• Most eroded matter lost with little
contribution to beaches
• Dramatic erosion during late
summer and early fall storms
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Photo on Elson Lagoon near Barrow by Orson Smith; map by Bill Lee
Coastal Erosion Responses for Alaska
Extract of Circum-Arctic Map of Permafrost
and
Ground Ice Conditions
1997
Permafrost
Chf
Cmf
Clf
Source Data
U.S. Geological Survey
International Permafrost Association
Chr
Continuous
(90 - 100%)
Clr
Dhf
Dmf
Dlf
Dhr
Discontinuous
(50 - 90%)
Dlr
Shf
Smf
Slf
Shr
Sporadic
(10 - 50%)
Slr
Ihf
Imf
Ilf
Ihr
Isolated
Patches
(0 - 10%)
Ilr
glacier
ocean/Inland seas
land
Subsea
sea-ice edge limit
subsea permafrost limit
500
0
500
treeline
Kilometers
12
Alaska Coastal Settings
• Deltaic shorelines composed of finegrained sediments covered with tundra
– Primarily located in western Alaska
– Wide beaches comprised of fine sand and silt
– Low coastal plains of discontinuous
permafrost subject to thermal erosion and
thaw subsidence
Photo near Hooper Bay on Bering Sea by James Hoelscher
© Alaska
Community
Database, ADCED
Orson Smith
& Mikal
Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
13
High Winds on the Alaska Coast
• Values are nominal design
3-second gust wind speeds
in miles per hour
• Mountainous terrain, gorges,
ocean promontories, and
special wind regions may
have unusual localized wind
conditions
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
From Minimum Design Loads for Buildings
and Other Structures, American Society of
Civil Engineers, (ASCE 1998)
Coastal Erosion Responses for Alaska
14
Tsunamis on the Alaska Coast
Alaska Tsunami Elevations
From FEMA Coastal Construction Manual (2000)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
15
Sea Ice
1 - 15 February
149°
#
Fi re Island
# Anchorage
154°
153°
152°
150°
149°
Fi re Island
#
# Anchorage
Kalgin Island
Kalgin Island
# Kenai
# Kenai
#
#
Chisik Island
60°
60°
#
60°
Chisik Island
60°
151°
25% Probabilities for
Ice of 5/10's or Greater Concentration
61°
61°
25% Probabilities for
Ice of any Concentration
62°
150°
61°
62°
151°
62°
152°
61°
153°
62°
154°
#
#Nini lchi k
# Nini lchi k
Homer
#
Homer
#
#
153°
152°
151°
150°
149°
0
153°
152°
151°
150°
149°
E
S
50
154°
August ine Island
Probability Range of
Selected Ice Concentration
N
W
59°
August ine Island
59°
154°
59°
59°
#
50
100
Kilometers
Scele 1:5,000,000
0%
> 0% to 25%
> 25% to 50%
> 50% to 75%
>75% to 100%
From Cook Inlet Marine Ice Atlas
•Variable conditions from year to year
•Long-term reduction in duration,
extent, and thickness
•Seldom causes coastal erosion
Photo of Anchorage by Orson Smith
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
16
Coastal Climate Change in Alaska
• Little or No Sea Level Rise
– Most of Southeast and Southcentral Alaska shores have net drop in
sea level due to ongoing glacial rebound
• Changes in frequency, intensity, & tracking of storms
– No Alaska trends apparent to date
• Increased winter rain and runoff on unfrozen ground
– Changes in coastal sediment budgets
• Reduced winter sea ice cover
– More wave energy reaching the shore
– Confirmed on Beaufort Sea coast of Alaska
• Melting permafrost and coastal thaw subsidence
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
17
Prudhoe Bay photo by Orson Smith
Kenai photo by Orson Smith
Avoid losing property Evaluate before you buy
• Property location
– City, borough jurisdictions
– Special zoning, set-backs,
building or land use restrictions
– Hazard area designations
– Natural resource protection areas
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
18
Evaluate before you buy
• Geometry of property
– Property shape and acreage
•
•
•
•
Shoreline frontage distance
Seaward boundary (MHW or other datum)
Depth of property perpendicular to shoreline
Useable acreage outside of natural or regulatory
restrictions
– Geometry of adjacent property
(after FEMA Coastal Construction Manual)
Orson Smith & Mikal Hendee
Coastal Erosion Responses for Alaska
PhotoUAA
near- Hooper
Bay Grant
on Bering
Sea by James Hoelscher
Alaska Sea
8/9/06
© Alaska Community Database, ADCED
19
Evaluate before you buy
• Physical and natural characteristics
– Geology, soils, and vegetation
– Topography (elevations and slopes of beach, dunes,
bluff, uplands)
– Site drainage (surface and ground water)
– Littoral sediment source and transport rate
– Storm flooding and erosion history
– Existing erosion control structures (type, age,
condition, history)
– Proximity to inlets or river mouths
– Sea level change (net, including seismic uplift or
settlement)
(after FEMA Coastal Construction Manual)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
US Army Corps of Engineers photo
Coastal Erosion Responses for Alaska
20
Evaluate before you buy
• Applicable regulations
–
–
–
–
–
–
–
–
Building codes and related restrictions
Flood hazard elevation and construction requirements
Erosion-related construction setbacks and restrictions
Hazard disclosure requirements for property transfer
Subdivision covenants, easements and rights-of-way
Local, State, and federal permitting requirements
Riparian (stream bank or seashore) rights
Habitat for endangered, depleted or sensitive species
(after FEMA Coastal Construction Manual)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
21
Photo by Orson Smith
Consider Erosion before Building
Proper planning, siting, and design of coastal
residential buildings require:
1. basic awareness of erosion processes,
2. erosion rate information,
3. appreciation for uncertainty in prediction of
future shoreline positions, and
4. knowing buildings landward of a setback
line are not guaranteed safe from erosion.22
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
Kenai photo by Orson Smith
Evaluating Erosion Risk
N
uff Line
W
1976
1. Define landward
shoreline location
expected during
life of building
Kenai Land Parcels
E
S
1999
300
0
300
600
Feet
2. Define lowest
expected ground
elevation during
life of building
3. Define highest
expected Base
Flood Elevation
during life of
building
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
•Erosion expressed as rate, in terms of:
•shoreline recession (feet per year), or
•Volumetric loss (e.g., cubic yards eroded sediment
per ft of shoreline frontage per yr)
•Surveys and aerial photographs are useful to determine
these parameters
Coastal Erosion Responses for Alaska
23
FEMA V-zone requirements
In Coastal High Hazard Areas
(V-zones), buildings must be
elevated on an open foundation
(e.g., pilings, posts, piers, or
columns) so that the bottom of
the lowest horizontal structural
member is at or above the Base
Flood Elevation (BFE).
From FEMA Coastal Construction Manual (2000)
“…A registered engineer or architect must develop or review the structural
design, construction specifications, and plans for construction and must
certify that the design and methods of construction to be used are in
accordance with accepted standards of practice for meeting the building
elevation and foundation design standards described above…”
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
24
Codes and Standards
•
In Alaska
–
•
International Building Code (IBC), published by the International Code Council (ICC) (ICC
2000a)
• Minimum Design Loads for Buildings and Other Structures, American Society of Civil
Engineers, (ASCE 1998, required by IBC)
Other Codes and Standards
–
–
–
–
–
–
–
Uniform Building Code (UBC), published by the International Conference of Building Officials
(ICBO) (ICBO 1997)
International Residential Code for One- and Two-Family Dwellings (ICC), published by the
International Code Council (ICC) (ICC 2000b)
The BOCA National Building Code, published by Building Officials & Code Administrators
International (BOCA) (BOCA 1996)
Standard Building Code (SBC), published by the Southern Building Code Congress
International (SBCCI) (SBCCI 1997)
International One-and Two-Family Dwelling Code, published by the Council of American
Building Officials (CABO) (CABO 1998)
SBCCI Standard for Hurricane Resistant Residential Construction, SSTD 10-99 (SBCCI
1999)
American Forest & Paper Association (AFPA 1996) High Wind Edition of the Wood Frame
Construction Manual for One- and Two-Family Dwellings
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
25
End of Part I
• Questions?
• Comments?
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
26
Part II - Coastal Erosion
Response Alternatives
• Adaptation
• Non-structural Responses
• Shore Protection Structures
Orson Smith & Mikal Hendee
8/9/06
- Alaska
Sea Grant
Shishmaref UAA
photo by
Curtis Nayokpuk
© Alaska Community Database, ADCED
Coastal Erosion Responses for Alaska
27
Adaptation
•
•
Allow erosion to
continue at natural rate
Loss prevention (coastal
zone management)
–
Setback lines and
development limitations
• Flood-proofing
–
–
•
National Park Service photo
National Park Service photo
Elevate buildings
Modify infrastructure to
survive intermittent inundation
Retreat
–
Demolish or relocate
threatened infrastructure
(buildings, utility lines)
National Park Service photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
28
Set-back lines
• Zoning
controls new
construction
near shore
• Difficult legal
issues
regarding
existing
structures
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
29
Flood-proofing
• Design structural features
to
– minimize damage from
intermittent inundation
– survive extreme events with
repairable damage
• Pro’s
– Build closer to shore
• Con’s
– Flood levels, extreme
waves, and erosion rates
difficult to predict
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
National Flood Insurance Program
“A-zone” requirements (from FEMA
Coastal Construction Manual; ref.
“V-zone” requirements shown earlier)
Coastal Erosion Responses for Alaska
30
Retreat
• Move (or demolish and
rebuild) inland
• Pros
– Favors natural coastal
environment
– Prevents future
economic loss
• Cons
– Difficult to predict future
erosion rates
– Some communities
have nowhere to retreat
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Kotzebue photo © Alaska Community Database, ADCED
Coastal Erosion Responses for Alaska
31
Reasons for Shore Protection
• Prevent damage to
property from
– Coastal floods and salt-water
intrusion
– Waves and wave runup
– Coastal erosion
• Prevent retreat of shoreline
• Preserve beaches
• Preserve coastal habitat
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
32
Shishmaref photo by Mike Hendee
Non-Structural Responses
Beach nourishment
•
Place fill material typical of
natural shore
–
–
–
•
Dune, beach, and submerged
profiles
US Army Corps of Engineers photo
US Army Corps of Engineers photo
Offshore mounds
Up-drift feeder beaches
Most popular solution in lower 48
Graph provided by Dr. Steve Hughes, US Army Corps of Engineers
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
33
Beach Nourishment
US Army Corps of Engineers photo
Grading initial trapezoidal
construction template
Engineering guidance:
•USACE, 2003. Coastal Engineering
Manual
•National Research Council, 1995.
Beach Nourishment and Protection
•Various publications of PIANC
(International Navigation Assn.)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
From USACE Coastal Engineering Manual
Coastal Erosion Responses for Alaska
34
Beach Nourishment
Pro’s
• Beach fill protects
onshore structures
from waves, runup,
and erosion
• Beach uses are
restored
• Appears natural
• Rapid rebound of
beach organisms
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
US Army Corps of Engineers photo
35
Beach Nourishment
Con’s
US Army Corps of Engineers photo
•Requires source of fill
•Dredging offshore material
may accelerate erosion
•Requires maintenance
•Expensive at remote sites
•Mobilize specialized
equipment
US Army Corps of Engineers photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
36
Non-structural Responses
Vegetation
•
•
•
US Army Corps of Engineers photo
Effective against wind and
runoff erosion on mild slopes
May survive brief inundation
and modest currents
Long lead time for vegetation
to become established
– Vegetation grows slowly in
Alaska’s climate
•
Usually ineffective against
direct ocean wave exposure
– Best as supplement to
seaward shore protection
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
37
Shore Protection Structures
• Revetments
– Slope stabilization
– Good for dissipating wave energy
• Seawalls
– Massive monolithic vertical
structures comprised of concrete or
stone masonry
– Need solid foundation
• Bulkheads (often also called
“seawalls”)
– Most commonly are steel sheetpile
retaining walls
– Increase reflective wave energy
– Require toe protection to prevent
scour
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Photo by Orson Smith
Coastal Erosion Responses for Alaska
38
Shore Protection Structures
• Offshore Breakwaters
– Constructed offshore to
dissipate wave energy.
– Require heavy durable
material (rock or concrete)
– May accelerate erosion at
margins
US Army Corps of Engineers photo
• Beach Groins
– Constructed perpendicular
to shore to trap sediment
moving longshore
– Affects material transport
• Accretes sediment on updrift side
• Starves areas immediately
down-drift
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
39
Revetments
• Constructed parallel to the shoreline
• Sloped to mimic natural coast
• Gravel or geotechnical fabric filter holds native
material in place
• Armor units holding filter dissipate wave energy
Homer Spit photo by US Army Corps of Engineers
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
40
Revetments
Materials
•
•
•
•
•
•
Quarry stone (riprap)
Concrete armor
Sand bags or gabions (mild conditions)
Gravel (filter stone)
Geosynthetic (filter) fabric
Prefabricated marine mattresses
Design Considerations
•
Functional
– Wave dissipation and reflection
– Wave runup and overtopping
•
Structural
–
–
–
–
•
Shore Protection Manual (1984)
Armor stability (weight)
Toe scour
Drainage
Foundation
Potential impacts to
– Nearshore habitat
– Adjacent shoreline
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
41
Revetments
Photo by US Army Corps of Engineers
Photo by Mike Hendee
Technical guidance:
• Coastal Engineering Manual (USACE)
• Revetments, HEC-11 (Federal Highway Admin.)
• Various publications of PIANC (International Navigation Assn.)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
42
Endicott Revetments
Prudhoe Bay, Alaska
Photo by Orson Smith
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
43
Articulating Concrete Mats
(Shishmaref)
© Alaska Community Database, ADCED
© Alaska Community Database, ADCED
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
44
Prefabricated concrete units
•
Pro’s
–
–
–
–
–
–
•
Transportable by barge
An alternative to larger rock or concrete elements
Requires modest construction skill
Conventional construction equipment is sufficient
Can be interlocked to form a wall or revetment
Moderately resistant to ice abrasion and pressure
Con’s
– Salt-water corrodes interlocking hardware
– Fine material can escape through gaps between modules
(without a gravel or fabric filter below)
– Once interlocking fails, the matrix comes apart
– Impermeability causes wave reflection
– Smooth surface allows maximum runup
Photos from International Navigation Assn. (PIANC), 2005. Catalog of Prefabricated Elements, Working Group 36 Report
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
45
Sand, Gravel, Rock,
or Concrete-filled Fabric
Pro’
Pro’s
•Transportable
•An alternative to larger rock or concrete elements
•Requires minimal construction skill
•No specialized equipment required
•Common emergency measure
Con’
Con’s
•Limited resistance to wave and ice forces
•An impermeable layer can fail from wave-induced pressures
•Sunlight degrades fabric, eventually spilling contents
•Synthetic (plastic) material of failed bags is hazardous to birds
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
46
Gabions
Wire-mesh baskets filled with
cobbles or sand-bags
– Pro’s
• Transportable
• An alternative to larger rock or
concrete elements
• Requires modest construction skill
• No specialized equipment
required
• Can be wired together as an
interlocked retaining wall
– Con’s
• Salt-water corrodes wire
• Ice distorts shape and causes
wires to fail
• Sunlight degrades sand bag fabric
• Wave forces distort baskets
• Wire from failed gabions is
hazardous
Homer photos by Orson Smith
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
47
Failed Gabion Revetment
Shishmaref, Alaska
Photo by Curtis Nayokpuk
© Alaska Community Database, ADCED
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
48
Seawall and Bulkheads
Seawalls
• vertical structures
• parallel to shoreline
• designed to prevent
erosion, wave damage,
and flooding
Bulkheads (also called
“seawalls”)
• vertical structures parallel
to shore
• often steel sheetpile
• retain fill behind
• protect upland areas from
wave damage and flooding
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
Homer photos by Orson Smith
49
Seawalls and Bulkheads
Materials
•Timber
•Steel or aluminum sheetpile
•Vinyl and other synthetics
•Concrete
Barrow photo by Mike Hendee
Design Considerations
•Functional
•Wave reflection
•Wave overtopping
•Structural
•Toe scour
•Drainage
•Tie-backs
•Fronting beach
•Adjacent shoreline
Potential impacts
•Beach offshore of wall
•Adjacent shore
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Barrow photo by Mike Hendee
Coastal Erosion Responses for Alaska
50
Bulkhead Toe Protection
(from USACE Shore Protection Manual 1984)
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
51
Seawall Impact on Beach
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
52
Concerns about Seawalls
Nikiski photo by Orson Smith
•
•
•
•
Toe scour (increased depth at
wall)
Decreased beach width
fronting the seawall
End effects (flanking)
Blockage of littoral drift when
projecting into surf zone
Homer photo by Orson Smith
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
53
Levees and Dikes
US Army Corps of Engineers photo
• Mounded structures
parallel to shore
• Usually made of
natural fill material
• Built primarily to
prevent flooding
• Seaward toe and
lower slope often
include a revetment
• Vegetated upper
slope and crest
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
US Army Corps of Engineers photo
54
Beach Groins
Ninilchik photo by Orson Smith
US Army Corps of Engineers photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
• Constructed
perpendicular to
the shoreline
• Create or widen
beaches by
capturing sand
moving
longshore
Coastal Erosion Responses for Alaska
55
Beach Groins
US Army Corps of Engineers photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
From USACE Coastal Engineering Manual
Coastal Erosion Responses for Alaska
56
Beach Groins
•
Materials
–
–
–
–
–
•
Stone
Concrete
Timber
Steel sheetpile
Geotextile tubes
Design Considerations
– Functional
• Crest elevation, groin length,
groin spacing, orientation
• Wave diffraction,overtopping,
and transmission
– Structural
•
•
•
•
Armor stability
Foundation
Toe scour
Flanking
– Potential impacts
• Nearshore habitat
• Adjacent shoreline
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
US Army Corps of Engineers photo
57
Beach Groin Design
From USACE Coastal Engineering Manual
•
Beach nourishment should be incorporated
–
•
•
•
Plan for renourishment
Choose dry beach width for effective shore protection
Taper offshore ends to reduce impacts to adjacent shores
Monitor performance
–
Periodic inspections, photographs, and surveys
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
58
Offshore Breakwaters
• constructed
seaward of
and usually
parallel to the
shoreline
• break
incoming
waves before
they reach
the shore
US Army Corps of Engineers photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
59
Offshore Breakwaters
US Army Corps of Engineers photo
Materials
•
•
•
Stone
Concrete
Geotextile tubes
Design Considerations
•
Functional
–
–
US Army Corps of Engineers photo
•
Structural
–
–
–
•
Armor stability
Foundation
Toe scour
Potential impacts
–
–
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Crest elevation, gap width,
distance offshore, orientation
Wave diffraction,overtopping, and
transmission
Coastal Erosion Responses for Alaska
Nearshore habitat
Adjacent shoreline
60
Reef Breakwaters
• Offshore breakwaters with submerged
crests
• Same considerations apply
US Army Corps of Engineers photo
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
61
Arctic Considerations
Ice Ride-up and Pile-up
•Generally thin ice on a
shallow slope leads to ride-up
•Ice fails in bending
•Greater slope angles result in
pile-up formation or grounded
rubble fields
From ASCE TCCRE Monograph “Arctic Coastal Processes and Slope Protection Design”
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
62
Arctic Considerations
Beach profile to
reduce ride-up
•Changes in bank slope
induce pile-up
•Ice rubble piles provide
shore protection
•Arctic artificial islands
often include a convex
bank area to encourage
ice rubble pile-up
development
From TCCRE Monograph “Arctic Coastal Processes and Slope Protection Design”
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
63
Arctic Considerations
Photos provided by USACE Cold Regions Research &
Engineering Lab
Freeze/Thaw Processes
•Rock, concrete and other
materials can fail from freezing
of internal moisture
•Winter construction can
introduce ice in fill that will
thaw and subside in summer
Photo provided by Mat-Su Borough
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
64
Questions or Comments?
Orson Smith:
[email protected]
907-786-1910
Mike Hendee:
[email protected]
907-244-3807
Orson Smith & Mikal Hendee
UAA - Alaska Sea Grant 8/9/06
Coastal Erosion Responses for Alaska
US Army Corps of Engineers photo
65