coastal dunes rda

8/23/2019 Coastal Dunes RDA

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Robin Davidson‐ArnottDepartment of Geography, University of Guelph

rdarnott@uoguelph.ca

Coastal Sand DunesCoastal Sand Dunesand Beach/Dune Interactionand Beach/Dune Interaction

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

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.

Embryo

ForeduneDune toe

Stossslope

Foreshore

Backshore

Beach and Foredune Definitions

Coastal Dune Profiles

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

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

Foredunes

Simple foredune ridge andephemeral embryo dune –

deposition on lee slope

Large foredune ridge, westcoast Scotland – no embryo

dune present

Dunes and Dunefields

Unvegetated transgressivedunefield, Namibia

Vegetated, progradationaldunefield, Long Point Lake

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

Blowouts and Parabolic Dunes

Parabolic Dunes, NorthIsland, New Zealand

Bowl blowout, GreenwichDunes, PEI

Blowouts and Parabolic Dunes

Burial by sand kills existing vegetation while sand mobility prevents

establishment of new vegetation

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

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

Vegetation and Coastal Dunessand trapping and protection

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

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)

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)

“water

only”“water

and wind”

“wind

only”

Beach and Dune Sediment Budget

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

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

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

Line profile and measured depositionMay-December, 1988.

Site location, distal end Long Point

0 20 40 60 80

September

December

d e p o s i t i o n

( c m )

e l e v a t i o n ( m )

November

September October

December

Seasonal Patterns of Deposition

After Law and Davidson-Arnott, 1990

Winter

Late winter

Late spring/summer

Late summer/early fall

Late fall Spring

2D sonic

machine

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

o n i t o r i

n g S t a t

o n i t o r i

n g S t a t

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

Rectification

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

Vegetation

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

Beach Extraction

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

Surface Moisture

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

Beach Surface

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

Fetch Length

Incident wind direction

e m b r y

o d u n e

Short fetch

moisture

Oblique‐onshore event, December 1

Oblique‐onshore event, December 15

Snow‐ice

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)

29% (1)

0tracelowmediumstrongvery strong

Transportintensity

23 onshore transport events

Short-term Field Experiment – April 2010

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

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

Storms and Dune ErosionForedune erosion Long Point,

December 1985

Overwash Long Point,

December 1985

Line 6 J une 2004 and May 2005

RampEmbryoDune Slumped Blocks

Before storm After storm

Line 6 2002 -2009

Dunes and Lake

Level Cycles

After Olson, 1958

I t ti L k L l Eff t

Isostatic Lake Level Effects

Isostatic upliftcm/century

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

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

REGULATORY DYNAMIC BEACH STANDARD(non-eroding beach)

Regulatory dynamic beach standard

Dynamic beachallowance

Regulatoryflood standard

(Not to scale)

Dynamic Beach RegulationGrand Bend/Oakwood,

Regulatory FloodStandard

Regulatory DynamicBeach Standard

1972; 1986 duneerosion limit

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.

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

coastal dunes rda

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