1. Te Puru south
Dr Willem de Lange

2. Te Puru geomorphology Coastal debris (alluvial) fan
Delta formed by episodic discharge of river sediment during floods
Accreting at 0.06 m.y-1 over last years
400 m ÷ 7200 y = 0.06 m.y-1
Dahm & Munro estimated m.y-1
Measured by Dravitzki (1988) as 0.06 m.y-1 following a flood event
Modified by low energy waves
Sort sands out of flood deposits & transport southwards
No significant sediment supply from north, or offshore
Extensively modified by human activities
River management
Reshaped to provide building platforms

3. Te Puru beach Classified as a mixed sand- gravel to composite beach
Amount of sand varies over time depending on frequency and magnitude of:
Flooding events that supply sediment (Accretion)
Storm and fair weather wave events that redistribute sediment (Erosion)
Storm events tend to move sand offshore
Fair-weather events tend to move sand onshore and southwards
Long term loss of sand is predominantly due to fair weather conditions
Jennings & Schulmeister, 2002.
Marine Geology 182:

4. Mixed sand-gravel beaches Storm response
Response of varies with
Sediment supply
Maximum wave run-up
Gravel moves onshore while sand moves offshore
Over time creates a sequence of storm ridges inland of beach

5. Historic shoreline fluctations
Due to storms and sea level rise
Maximum shoreline fluctuations occur around stream mouths
10-15 m
Much smaller fluctuations elsewhere
<2 m
At southern end of Te Puru, shoreline was 10 m landward of vegetation line in some places
Assumed to indicate 10 m of storm erosion?
Due to stream mouth at this location?
Since 1913 there has been little change in position of the high water line
Dahm and Gibberd (2009)
Interpretation of 10 m shoreline fluctuations at base of delta is not consistent with geomorphology and orientation of severe storm waves

6. Te Puru south
Corresponds to Figure 5 in Appendix 4
1950s
Unclear if walls were primarily built for coastal protection or as retaining walls
Figures in Appendix 4 suggest retaining walls
Walls are fronted by a wedge of predominantly sand, overlying coarse gravels
Sand is temporarily removed by episodic storms, but total volume of sand appears to be increasing over time
Walls are functioning as backstop seawalls
December 2013
407 Thames Coast Rd

7. Dahm (2014) Appendix 4 predicts future erosion if walls cease to be effective after of scour has occurred
Described as
“rare and severe erosion”
Scour depth is not a reliable indicator of profile response when seawall fails – McDougall et al (1996)
Dahm & Gibberd (2009) indicate  m of erosion, not m assumed by Dahm (2014)
Future erosion
Representative beach profile (from 411 Thames Coast Rd?) after severe scour and including a wall, translated  m landward to produce “likely” 1% AEP erosion profiles

8. CCEL Dahm & Gibberd (2009) based CCEL on storm cut & fill
Before bach rebuilt
Dahm & Gibberd (2009) based CCEL on storm cut & fill
However, for southern Te Puru appear to have used difference between cadastral survey & vegetation line survey
Long-term accretion combined with reshaped land & drainage
Doesn’t represent short-term storm induced fluctuations
Should be <2 m based on observed shoreline response to storms and sea level rise.
During bach rebuild 1982
After bach rebuilt