Q1. What are the primary causes/contributors to coastal ... · -20 1974 Stevensen erosion 1978...
Transcript of Q1. What are the primary causes/contributors to coastal ... · -20 1974 Stevensen erosion 1978...
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