coastal dunes rda
TRANSCRIPT
8/23/2019 Coastal Dunes RDA
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Robin Davidson‐ArnottDepartment of Geography, University of Guelph
Coastal Sand DunesCoastal Sand Dunesand Beach/Dune Interactionand Beach/Dune Interaction
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Outline• Introduction
• Definition and examples
• Vegetation and coastal dunes
• Sand supply - controls, rates, seasonal patterns
• Storms, coastal dune erosion - erosion, overwash
• Dune recovery
• Dunes and lake levels - decadal, isostatic• Coastal dune management
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Coastal Dunes• Form along coast - sand primarily derived from
beach• Distinguish between free dunes and impeded
dunes formed largely by trapping and
stabilisation of sand by vegetation• Active dune zone consisting of foredune andembryo dune - interaction with waves and beach
• Dune field landward of foredune - consists of stabilised transverse dunes and variety of formsresulting from blowout and migration.
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Beach
Embryo
dune
ForeduneDune toe
Crest
Stossslope
Foreshore
Backshore
Beach and Foredune Definitions
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Coastal Dune Profiles
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Foredunes• Vegetated ridges that form just landward of the
backshore and parallel to the shoreline• Sediment supply from beach trapped by
vegetation
• Fordunes grow in height and width - vegetationgrows upward with dune
• Seaward margin determined by limit of waveaction
• Landward margin determined by height of dune,and transport over crest onto lee slope
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Embryo Dunes (incipient dunes)• Form at base of existing foredune or on backshore
seaward of foredune• May grow from coalescence initially around isolatedplants - shadow dunes - hummocky form
• Creeping form of vegetation - vines, rhizomes - produces
continuous coverage - sand platforms• On prograding shoreline embryo dunes may coalesce
and grow vertically - new foredune cuts off sedimentsupply to older foredune
• On stable and eroding coasts they are removed everyfew years by large storms
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Foredunes
Simple foredune ridge andephemeral embryo dune –
deposition on lee slope
Large foredune ridge, westcoast Scotland – no embryo
dune present
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Dunes and Dunefields
Unvegetated transgressivedunefield, Namibia
Vegetated, progradationaldunefield, Long Point Lake
Erie
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Blowouts and Parabolic Dunes• Disturbance to protective vegetation may result
in erosion of foredune or dunes in dunefield• Simplest form is roughly circular bowl blowout
• Rapid erosion leads to elongation and landward
migration of slip face which buries vegetationahead of it – base becomes stabilised whenreaches water table and sides may be stabilised
by vegetation to produce trailing arms• Disturbance may be natural – wave action,
animals, drought – or human actions such as
trampling, ATVs, construction
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Blowouts and Parabolic Dunes
Parabolic Dunes, NorthIsland, New Zealand
Bowl blowout, GreenwichDunes, PEI
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Blowouts and Parabolic Dunes
Burial by sand kills existing vegetation while sand mobility prevents
establishment of new vegetation
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Controls on Foredune Formand Evolution
• Coastal morphology - barrier,
headland/bay, straight mainland coast
• Vegetation species - especially pioneering
plants• Beach and dune sediment budget
• Wave and wind climate; temp and precip
• Natural and human disturbances
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Vegetation and Coastal Dunes• Vegetation acts to trap sediment blown off the
beach or through blowouts and parabolic dunes• Vegetation also acts to shelter the sand surface,
preventing subsequent erosion by winds and
stabilising the dune• Unlike sand fences or litter vegetation can grow
up through the sand resulting in foredunes thatmay be more than 10 m in height
• Roots of vegetation act to increase resistance toerosion by waves
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Vegetation and Coastal Dunessand trapping and protection
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Pioneer Vegetation• Backshore and foredune harsh environment
• Plants must be able to tolerate high surfacetemperatures, dessication, low fertility, instabilityand burial by sand, occasional inundation by
waves/water; and on marine coasts salt spray;• Annuals survive burial as seeds; perennials have
rhizomes or stolons that spread out and which can
emerge from burial depths of up to a metre in thecase of marram grass.
• Plant species may differ around the world but
there are only a few primary forms
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Pioneer Vegetation• Embryo dune and foredune development may
reflect the form of the colonising vegetation• Plant succession largely response to gradient of
salt spray and especially burial by sand – time is
not a major factor• Great Lakes dominant pioneer species is marram
( Amophila breviligulata) as well as other grasses,
annuals (including Cakile edentula); sea grapevine and cottonwood (Populus deltoides)
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Pioneer vegetation
St. Kitts, West Indies
Beach Morning Glory; Black Torch, SeaGra e wood bush
Long Point, Lake Erie
Marram; Little Bluestem (grass),Dogwood, Cottonwood, Grape (vine)
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“water
only”“water
and wind”
“wind
only”
Beach and Dune Sediment Budget
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Sand Transport to Foredune• Transport rate function of U3
• Allow for threshold – about 5 m/sec for dry sand
• Can predict gross sand transport from hourlymean wind speed collected at nearby climatestation
• Net deposition function of cosine of wind angleto beach
• Actual measured deposition generally much less
(5-20% of predicted) because of a range of potential transport-limiting factors
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Transport-Limiting Factors
• Moisture content of beach surface
• Presence of gravel lag deposits, flotsam,vegetation
• Presence of snow, ice, frozen beach
• Fetch distance (beach width and wind angle) –
controlled by beach sediment budget, waterlevels and wave run-up
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Measurement of Seasonal Patterns
of Deposition using a Bedframe
Original version 2m X 2m - Dexion on 4rebar corner posts. Used at Long Point1988-92
Mark 3 version 1m X 1m - aluminium oncentral wood post. Used at GreenwichDunes, PEI 2002-2009
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Measurement of Seasonal Patterns
of Deposition using a Bedframe
Line profile and measured depositionMay-December, 1988.
Site location, distal end Long Point
0 20 40 60 80
0
1
2
0
10
20
30
40
September
December
July
d e p o s i t i o n
( c m )
e l e v a t i o n ( m )
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Measurement of Seasonal Patterns
of Deposition using a Bedframe
November
September October
December
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Seasonal Patterns of Deposition
After Law and Davidson-Arnott, 1990
Winter
Late winter
Late spring/summer
Late summer/early fall
Late fall Spring
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2D sonic
north
east
west
time
machine
solar
panels
Box: USB terminal,
hobo, etc.
Intro Fetch effect RS methods Nature Events Model Conclusions
DelgadoFernandez et al.,
2009, Journal of Coastal
Research v25, 1160-1167
M
o n i t o r i
n g S t a t
i o n
M
o n i t o r i
n g S t a t
i o n
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I m
a g e P r o c e s s i n g
I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Rectification
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I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Vegetation
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I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Beach Extraction
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I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Surface Moisture
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I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Beach Surface
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I m a g e P
r o c e s s i n g
I m a g e P
r o c e s s i n g
Intro Fetch effect RS methods Nature Events Model Conclusions
Fetch Length
Incident wind direction
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e m b r y
o d u n e
Short fetch
and
moisture
Oblique‐onshore event, December 1
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Oblique‐onshore event, December 15
Snow‐ice
cover
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2.5% (15)
17% (25)
39% (19)33% (6)
8% (1)
n u m b e r o f e v e n t s
66 transport events
0tracelow
mediumstrongvery strong
Tansportintensity
very smallsmallmediumlargevery large
Wind eventmagnitude
184 wind events
n u m b e r o f e v e n t s 27% (11)
25% (4)7,6% (109)
11% (33)
30% (27)
Actual versus predicted transport
27 onshore transport events
n u m b e r o f e v e n t s
11.9% (19)
12.5% (5)
46%
(2)
29% (1)
0tracelowmediumstrongvery strong
Transportintensity
23 onshore transport events
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Short-term Field Experiment – April 2010
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Storms and Dune Erosion• Storms with large waves and storm surge
can erode embryo dunes and foredune,returning sand to the beach – integral partof beach/dune interaction
• Foredune thus acts as a source of sediment for the beach and a physicalbuffer against wave attack inland
• In Great Lakes storm impact is enhancedduring periods of high lake level andreduced during periods of low lake level
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Storms and Dune Erosion• Small storms erode embryo dune and cliff
base of foredune• Larger storms cliff whole of foredune and
may lead to breaching and overwash
• Dune recovery from smaller storms maytake only a few years
• Large overwash events may remove
pioneer vegetation from long stretch of coast – recovery make take a decade ormore
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Storms and Dune ErosionForedune erosion Long Point,
December 1985
Overwash Long Point,
December 1985
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Line 6 J une 2004 and May 2005
RampEmbryoDune Slumped Blocks
Before storm After storm
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Line 6 2002 -2009
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Dunes and Lake
Level Cycles
After Olson, 1958
I t ti L k L l Eff t
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Isostatic Lake Level Effects
Isostatic upliftcm/century
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Isostatic Lake Level Effects• Lake level change at a point on the
shoreline function of uplift rate minus upliftrate at entrance to connecting channel
• Lake Huron uplift at Sarnia about 7cm/C
• Relative differences: Grand Bend 3cm/C
• Sauble/Wasaga Beach 16cm/C
• Manitoulin 30cm/C
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Dynamic Beach Regulation
REGULATORY FLOOD STANDARD
Regulatory flood standard
100-year
Flood Level
(Not to scale)
Flood allowance for the wave uprushand other water related hazards
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Dynamic Beach Regulation
REGULATORY DYNAMIC BEACH STANDARD(non-eroding beach)
Regulatory dynamic beach standard
Dynamic beachallowance
Regulatoryflood standard
(Not to scale)
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Dynamic Beach Regulation
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Dynamic Beach RegulationGrand Bend/Oakwood,
1993
Regulatory FloodStandard
Regulatory DynamicBeach Standard
1972; 1986 duneerosion limit
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Dune Conservation
Simple posts and ropes keeppeople off the embryo dune andstoss slope of the foredune system
in shots from new Zealand and Australia (top and bottom left) incontrast to the sand fences andhighly artificial beach in NewJersey which is nourished and
raked in order to maintain anunnaturally wide beach.
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Dune Conservation• Primarily about managing people
• Conserve deposition close to the beach inembryo dune and on foredune slope –preserve vegetation adapted to these
areas especially at top of beach• Prevents sand inundation landward of
dune, provides protection from storms –
especially during high lake level –replenishes beach