7.0 Geology and Geomorphology - Origin Energy

7.2.3 HDD Shore CrossingThe coastal cliffs are approximately 40m high, as shown in Figure 7-3. Also significant is thatthey are actively regressing at a rate assessed at up to 0.2m a year. Other sections of thecoast in the South Taranaki region are regressing up to 1m per year. The farmlandtopography continues to the cliff-edge, with no sand dunes to provide any form ofprotection.Figure 7-3: Coastal Cliffs Adjacent to Project AreaFigure 7-4 shows a cross section of the geology underlying the coastal area at the HDD ShoreCrossing location. The cliff is underlain by a sequence of Pliocene weak rock to an elevationof 0m to 9m. This material extends to at least 100m below sea level. The Pliocene rock isoverlain by Quaternary deposits comprising a thin marine sequence of sand, gravel andboulders, as well as a thicker, non-marine sequence of dense sand and thin coal seams. TheQuaternary layer is approximately 10m thick, and lies under lahar and debris flow materials(silt, sand and boulders up to 3m diameter). The upper layer is a mantle of andesitic ash 2mto 3m thick. Figure 7-4 also presents two cliff regression scenarios showing the position ofthe cliffs in 2054 under two different regression rates of 0.1m/year, and 0.2m/year.Section 7 – Geology Page 7-5