GARY WILLIAMS TECHNICAL ASSESSMENT
HG PROJECT P401720-136527-0
G & E WILLIAMS CONSULTANTS LTD - Fax/phone (06) 362 6684
RD3, OTAKI
MEMORANDUM
To:
Tim Fergusson
Of:
Harrison Grierson
Date: 6 October 2009
Copy: Haydn Read - Whakatane District Council
SUBJECT:
WHAKATANE RIVER MOUTH— TRAINING WALL
N
PAGES 1 + 3
Introduction
The Whakatane District Council [WDCJ have applied for a resource consent for the
retention and maintenance of the rock training wall from Statue Rock back to the end
of the Piripi Spit at the Whakatane River mouth. In 2005 I carried out an
investigation of the river mouth and flood related issues about the spit and the
western training wall. This included a review of all the investigations carried out on
training walls at the mouth, since the physical modeling study of the late 1970s.
As part of an information request for the resource consent, I have re-visited my
assessments about the effects of a western training wall on both the enlargement of
the mouth at the spit during large flood events and navigation through the mouth.
In particular, this relates to the lower rock wall at 0.5 m Moturild datum, and the
substantial breakthrough of tidal flows at the spit in June 2009.
Flood break through at the Spit
My 2005 investigations were mostly focused on when and how the end of the spit at
the mouth is washed away in large flood events, and measures that could be used to
enhance this flood breakthrough. In the large flood events of 1998 and 2004, the end
of the spit was not washed away until around peak flood flows, with high flood
levels at the spit and upstream in the river estuary. The peak level recorded at the
spit (cross section Wi and 150 m seaward) in the July1998 event was around 2.15 m
RL, with a similar level recorded in the large flood event of 1970. The peak levels at
the spit in the July 2004 event were around 3 m RL.
In summary, my findings in regard to the washing away of the spit were that there
are a number of processes that can effect this mouth enlargement, with river levels,
tidal fluctuations, sea surge and wave conditions all having an influence. Severe sea
conditions with storm waves are likely to occur when there are large flood flows in
the river. The rock training wall is generally mostly covered by the sand material of
the spit (towards the landward end), with this material being continually moved off
the spit and into the river mouth, and this was the case in the 1998 and 2004 large
flood events.
The only failure processes that the rock training wall could inhibit is a progressive
erosion of the spit material from Statute Rock back towards the river estuary. In both
events a large proportion of the rock wall was naturally washed away during the
flood event and mouth enlargement. The presence of the rock training wall, at its
original height, was not, then, considered to have had a significant retarding
influence on the timing and extent of the mouth widening in these large flood events.
The length of wall that remained, after both events, was the outer part from Statue
Rock.
The removal of material off the spit to form a floodway was not considered an
effective measure for a number of reasons, and other possibilities were canvassed,
including fluidisation of the spit during flood events. Maintaining a lowered area at
the end of the spit is difficult and expensive, due to the constant working of material
onto and along the spit (and into the river channel) by wave action. Any lowering is
quickly re-built by the actions of the sea, necessitating continual effort (and expense)
to maintain it, while any benefit would only occur very rarely given the infrequency
of large flood events. Undertaking a mechanical lowering as a flood related response
(based on flood warnings) could be very hazardous given rough sea conditions,
would be dependent on the availability of suitable people and machinery at the time,
and may still have very little effect.
Recent Changes
The reconstruction of the training wall further back into the spit formation at a lower
level, of 0.5 m RL, reduces the crest level of the formed rock in the area of spit wash
out in flood events, and will increase the height of sand material above it. When a
wash out occurs in large flood events, the rock within the fast flowing area of the
flood break through will be removed again, as in the past events, but initially there
will be a lower hard layer of the rock.
The western training wall has only marginal effects on the flushing of the mouth
channel, as it has only extended seaward as far as Statue Rock. The wall does,
though, collect the spit material carried eastward to the river mouth, and is beneficial
in ensuring a connection between the spit and the rock platform at the river mouth
(which includes the rock pinnacles out to Centre Rock). The development of a
substantial tidal channel on the landward side of the rock platform with a retreated
spit at the mouth demonstrated this benefit, and directly led to the reconstruction of
the wall.
The wall, thus, has only marginal benefits in terms of the flushing through of the
sand material transported into the river mouth from the spit, but it is beneficial in the
formation of a substantial end to the spit and beach front on the seaward side. The
harbour surveys of the 1990s show a broadened end to the spit seaward to and along
the training wall, with the beach front curving out to Statue Rock.
A retreat of the spit end and development of significant tidal exchanges landward of
Statue Rock would adversely affect mouth navigation, increasing the direct input of
spit material to the mouth channel, and reducing the tidal exchange flows through
the mouth channel. The development of the tidal break through in. June was not the
2
N
direct result of sea storm wave action, and may have been an inevitable development
after a period of time of not re-instating the training wall after the 2004 flood events.
As conditions change the spit end would, though, have come and gone, closing off
the gap that formed, and then re-opening it again some (indeterminate) time later.
Comments
The principal reason for the reconstruction of the western training wall is to maintain
a larger end to the spit, and prevent significant tidal exchanges landward of Statue
Rock. There are navigation benefits in preventing the conditions that developed in
June, or minimising the risks of such occurrences. It is this development that has
highlighted benefits of the short training wall that were not fully appreciated at the
time of my 2005 investigations. The existing conditions for over a decade, since the
early 1990s (and during the large floods of recent years), was with the wall in place.
At the same time, any impacts of the rock wall on the washing out of the spit in large
flood events are likely to be minimal, if there are effects. The washing away of the
spit end depends on a range of influencing factors and failure processes unrelated to
the presence or absence of a western training wall from the spit out to Statue Rock.
The effect of the wall in retaining a larger end to the spit does, though, suggest that
the amount of sand at the spit end may, on average, be greater with the wall in place.
The proposed conditions of the consent include an alternative floodway to that of
past practice, with a lower level of 0.5 m RL. The lower level is within the normal
tidal range, and hence down to saturated sand conditions on the spit. This level will
greatly increase the difficulties of forming a lowered floodway area at the spit. The
previous practice was to cut down the floodway area to around 1.0 m RL when it
reached a height of 2 m RL.
The relevant level for the breaking through of the spit is the internal flood water
levels (within the estuary), and achieving a floodway level at the spit comparable to
the outer mouth levels or sea levels would be extremely difficult, even with the most
favourable of conditions. An alternative arrangement of the rock wall, as two
parallel and overlapping walls, with a gap between, which could be sluiced out, may
have been a better option.
There is undoubtedly a serious issue with the late washing out of the spit in large
flood events, and measures that will increase the likelihood of earlier breakthroughs
should be considered. The continual lowering of a 'floodway' area alongside the
training wall is not, however, recommended as an option, as it is ineffective and very
cost inefficient. it is also a hazardous exercise, and requires a machine access to the
spit to be formed and maintained, which has its own adverse impacts on the spit
environment.
A wide ranging and intensive study of the river mouth and harbour is being initiated
by the WDC, and this will provide the opportunity to assess a range of options and
measures concerning flood breakthrough as well as navigation improvements. The
undertaking of this study is another proposed condition.
The proposed consent is only for 5 years, given the investigations to be undertaken.
In view of this short term of the consent, my recommendation would be that any
artificial lowering of the spit be suspended and not be part of the conditions for this
consent. The issue of flood breakthrough of the spit can be addressed in the harbour
3
study, and the preferred method of facilitating breakthrough should be left until the
results of the study investigations are known.
Finally, it should be noted that consent conditions must be related to the mitigation
of the effects of the proposed measures. The presence of the training wall at the spit
certainly can not be held to be the sole or major reason for the late breakthrough of
flood waters in large flood events. if the proposed 'floodway' condition is to be
imposed, and the whole cost to fall on the WDC as consent holder, it must be related
to the adverse impact of the wall. In my opinion, continual artificial lowering of an
area of the spit is ineffective, unduly hazardous, impacting on the spit environment,
and not a mitigation measure in proportion to adverse effects.
Gary Williams
4