Q1. What are the primary causes/contributors to coastal erosion at - - PowerPoint PPT Presentation

• Q1. What are the primary causes/contributors to coastal erosion at

Westshore 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

20 40 60 1920 1930 1940 1950 1960 1970 change in beach volume above MSL (cubic metres per metre )

a

1974 Stevensen erosion 1978 Smith erosion 1985 Smith erosion 1980 1990 2000 1920 1930 1940 1950 1960 1970 1980 1990 1974 Stevensen erosion 1978 Smith erosion 1985 Smith erosion

• 20
• 15
• 10
• 5

5 10 15 20 shoreline displacement (metres) 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 long

term 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 shallow

ebb delta emergence causing ‘Westhore bulge’

• Re-addjustment due to littoral transfers take many decades

including 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 shipping

channel have on Westshore – particularly what Impact of deepening the port shipping channel on Coastal Erosion, both in the past and the proposed future deepening by a further 3+ metres... professional opinion

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

sediment sharing system with sources and sinks

• Q4. What is the influence of the predominant wave angle/direction in

relation to the coast and what this means in terms of erosion, specifically at Westshore

sediment supply from river r e f r a c t e d w a v e c r e s t

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 than the mid 80s.

• 60
• 40
• 20

20 40 60 1910 1920 1930 1940 1950 1960 1970 1980 1990 beach volume: cubic metres per metre of shoreline nearshore sand feed: uplift response? depletion of uplift-induced nearshore sand feed?

a

d e p l e t i

• n
• f

H a r b

• u

r m

• l

e

• i

n d u c e d e b b

• d

e l t a s a n d f e e d ? 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 horizontal msl shoreline displacement (m) nearshore sand feed: uplift response? n e a r s h

• r

e s a n d f e e d d e p l e t i

• n

?

• 25
• 20
• 15
• 10
• 5

5 10 15 20 2000

b

Smith's data Gibb's K10 data

Shoreline displacement Westshore

1913 1921 1930 1938 1946 1954 1962 1971 1979 1987 1995 2004 cumulative volume change per m shoreline (m3/m)

a

• 200
• 150
• 100
• 50

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 cumulative MWL-shoreline displacement (metres)

b

• 10
• 8
• 6
• 4
• 2

2 4 6 8 10 Westshore nourishment begins uplift response? +57? Ahuriri ebb delta collapse declining feed from Westshore?

Shoreline displacement survey data at HB15

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

Westshore

• Q7. The importance of addressing the nearshore deficit and what will

happen if it is not. (and does it have to be sand from the shipping channel). Once the nearshore deficit has been addressed discuss the need to maintain sediment balance

• Without nourishment of submarine beach

and with further reduction in sand supply as well as losses due to shipping channels … submarine beach will continue to lower and recede

• Like the upper beach, the submarine beach will require ongoing

nourishment to offset losses alongshore and to sinks such as the navigation channels

• Q8. Different types of solutions, retreat, soft, hard engineering and the

effects they might have on the coastal environment – what provides the best /bang for buck’ solution.

• greater stability and lower nourishment achieved the further

landward the beach is permitted to relocate - how far is the question

• low to moderate value assets near to the beach at present relocated

landward… money saved on reduced nourishment probably would

• ffset 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 options

but intuitively nourishment is best bang for back

• Q9. Finally, is it possible for Westshore to become a sandy beach again

as 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 sandy beach exists

• More likely that the intertidal beach would experience periods of

sandier character during prolonged fair-weather conditions gravel during periods dominated by storms