Q1. What are the primary causes/contributors to coastal erosion atWestshore and the concept of longshore / littoral drift.

In order of (timing related) contribution to present problem

1. Beach is too far seaward for prevailing conditions, due to - earthquake uplift (2nd ebb delta collapse) - Ahuriri walls legacy (1st ebb delta collapse): subdivision of temp land

2. Beach nourishment paradox (artificially maintained misalignment)

3. Port breakwater & dredged channels: initial and continued... - cessation of gravel supply to upper beach - disturbed/deflected sand supply to sub tidal beach & nearshore causing deficit in wave drive alongshore transport of sediments (i.e., positive transport gradients alongshore)

Q1. continued...

5. Port breakwater wave sheltering: protective effect against severity of storm erosion at Westshore

?. Port breakwater alteration to wave refraction/diffraction patterns contributing to altered effects on beach alignment alongshore (no Unibest / Mike21 type modeling undertaken)

X. Storm erosion

Q1. further continued...

n. Legacy effects - detritus and culture: early (pre-moles Ahuriri) erosion due to minor dredging of harbour, ship ballasting, natural inlet instability, event-related bursts in littoral sediment supply (river flooding, cliff collapse)

b

-60

-40

-20

0

20

40

60

1920 1930 1940 1950 1960 1970

cha

ng

e in

bea

ch v

olu

me

ab

ove M

SL

(cub

ic m

etr

es p

er

me

tre

)

a

19

74

Ste

ven

sen

ero

sio

n

19

78

Sm

ith e

rosio

n

19

85 S

mith

ero

sio

n

1980 1990

20001920 1930 1940 1950 1960 1970

1980 1990

197

4 S

teven

sen

ero

sio

n

197

8 S

mith e

rosio

n

19

85 S

mith e

rosio

n

-20

-15

-10

-5

0

5

10

15

20

sho

relin

e d

isp

lace

men

t (m

etr

es)

2000

b

volume change shoreline displacement

between successive surveys

derived from data in Smith 1993, Gibb 1995

storm erosion is more cosmetic than significant

Q2. What role did the earthquake play both in the short term and longterm and are there still any lingering effects from that event?

• Dominant role in present problem – beach is in wrong place

• Bigger effect in vicinity of Ahuriri than further north due to shallowebb delta emergence causing ‘Westhore bulge’

• Re-addjustment due to littoral transfers take many decadesincluding 2nd ebb delta collapse and alongshore reworking)(US Pacific Columbia River coatal cell lesson)

• Ridge legacy: - prevents sediment loss inland due to overwash- but reduced effectiveness of beach as wave absorber’ trapping sediments in the high intensity littoral transport zone

Beca, 2007

from Worley (2002b

Q3. What influence the port breakwater, training moles and shippingchannel have on Westshore – particularly what Impact of deepening theport shipping channel on Coastal Erosion, both in the past and theproposed future deepening by a further 3+ metres... professional opinion

• Breakwater and Ahuriri moles- interruption of sediment supply / imbalance in littoral budget

• shipping channelssediment sharing system with sources and sinks

Q4. What is the influence of the predominant wave angle/direction inrelation to the coast and what this means in terms of erosion,specifically at Westshore

sediment

supply from

river

refracted wave crest

Komar

10 x wave power

5 x wave power

3 x wave power

fixed rate of sediment discharge

Headland attached spit

a

N

w

b

w

c

d

e

Q5. My analysis on the erosion / accretion phases at Westshore,particularly the point that erosion has been occurring from earlier thanthe mid 80’s.

-60

-40

-20

0

20

40

60

1910 1920 1930 1940 1950 1960 1970 1980 1990

beach

vo

lum

e:

cu

bic

metr

es p

er

metr

e o

f sh

ore

lin

e

near

shor

e sa

nd fe

ed: u

plift

res

pons

e?

depletion of uplift-induced

nearshore sand feed?

a

deple

tion o

f

Harb

our m

ole

-induce

d

ebb-d

elta

sand fe

ed?

El Niño dominated

1960

Gibb

La Niña dominated

1976

-IPO

sthn end accrete?

+IPO

sthn end erode?

+IPO

sthn end erode?

-IPO

accrete?

2000

Smith's data

Gibb's K10 data

Volume

Westshore

1920 1930 1940 1950 1960 1970 1980 1990

ho

rizo

nta

l m

sl sh

ore

lin

e d

isp

lac

em

en

t

(m)

near

shor

e sa

nd fe

ed: u

plift

res

pons

e?

nearshore sand feed depletion?

-25

-20

-15

-10

-5

0

5

10

15

20

2000

bSmith's data

Gibb's K10 data

Shoreline displacement

Westshore

1913 1921 1930 1938 1946 1954 1962 1971 1979 1987 1995 2004

cum

ula

tive v

olu

me c

hange p

er

m s

hore

line (m

3/m

)

a

-200

-150

-100

-50

0

50

100

150

200

post uplift response incl.

longshore feed from Westshore?

natural feed depleted from Westshore?

feed from Westshore nourishment?

El Niño dominated

1998

1960

Gibb La Niña

dominated

1976

-IPO

sthn end accrete?

+IPO

sthn end erode?

+IPO

sthn end erode?-IPO

accrete?-IPO

accrete?

? ?

post Ahuriri inlet wall.

depleting feed from

Westshore?

volume

survey data at HB15

HB15

1913 1921 1930 1938 1946 1954 1962 1971 1979 1987 1995 2004

cum

ula

tive M

WL-s

hore

line d

ispla

cem

ent (

metr

es)

b

-10

-8

-6

-4

-2

0

2

4

6

8

10

Wests

hore

no

urishm

ent b

egin

s

uplift r

espo

nse?

+57?

Ahuriri e

bb d

elta

colla

pse d

eclin

ing fe

ed

from

Wests

hore

?

Shoreline displacement

survey data at HB15

Q6. What is the near shore deficit and what effect that has had onWestshore

Q7. The importance of addressing the nearshore deficit and what willhappen if it is not. (and does it have to be sand from the shippingchannel). Once the nearshore deficit has been addressed discuss theneed to maintain sediment balance

• Without nourishment of submarine beachand with further reduction in sand supply as well as lossesdue to shipping channels… submarine beach will continue to lower and recede

• Like the upper beach, the submarine beach will require ongoingnourishment to offset losses alongshore and to sinks such as thenavigation channels

Q8. Different types of solutions, retreat, soft, hard engineering and theeffects they might have on the coastal environment – what provides thebest /bang for buck’ solution.

• greater stability and lower nourishment achieved the furtherlandward the beach is permitted to relocate - how far is the question

• low to moderate value assets near to the beach at present relocatedlandward… money saved on reduced nourishment probably wouldoffset this investment

• nourishment vs seawalls and groynes both viable if designed correctly

• seawall design standard challenging- expensive to build for say 100 year design life- wall failure and maintenance common because construction defects

• detailed costings and risk assessment required for optionsbut intuitively nourishment is best bang for back

Q9. Finally, is it possible for Westshore to become a sandy beach againas this is something people often say should happen.

• If nearshore was ‘flooded’ with sand...If volume of nearshore sand is sufficient, the possibility of a sandybeach exists

• More likely that the intertidal beach would experience periods of

sandier character during prolonged fair-weather conditions

gravel during periods dominated by storms