The role of deep-seated landsliding in the geomorphic evolution of the Esk Valley, Hawke's Bay: an innovative approach to hazard evaluation
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
An engineering geomorphological investigation of the Esk River catchment has been undertaken to quantify the relationships between the valley's geomorphic evolution, the many 1-10km2 deep-seated landslides present within the catchment, and a significant flood event that inundated the lower valley in April 1938. The identification of key geomorphic processes enabled the assessment of catchment's geomorphic stability, the development of generalised models for landsliding, and the delineation of a pre-disposed zone of instability. This information is then applied to assess key geomorphic controls on the flood event. The region lies within the forearc basin of the obliquely convergent Hikurangi Margin, and is underlain by soft, gently eastward dipping Pliocene marine strata. Structurally it is dominated by its close proximity to the Mohaka Fault, as well as two westward-dipping blind thrusts beneath the valley identified in this study: the Wakarara Fault – Trelinnoe Sector, and the Eastern Patoka Fault. Evidence from seismic reflection surveys indicates that these have both been active since the early Mangapanian (2.8 - 3.2 Ma), and an analysis of stream longitudinal profiles and plan form suggests limited displacement may have taken place within the last 10,000 years. A survey of rock mass defects within a representative sample area in the centre of the valley highlights four sub-vertical joint sets; conjugate sets strike 153° and 246°, and another sub-parallel to the folding strikes 033°. These defects correlate well with lineaments identified in aerial and satellite photographs and are attributed to extension of the sediments across the top of fault-propagated folds. The generally low power streams have exploited these defects and highly incised channels now run almost exclusively along them. Deep-seated landslides occur generally within the area of folding and their extents are defined by lineaments inferred to correspond to persistent joints in the rock mass. The slides are translational, and are facilitated by up to 80m of incision - ongoing since the abandonment of an extensive terrace inferred to be Ohakean (18-10ka) in age. Basal failure surfaces commonly dip at angles as low as 6°, and a combination of tectonically induced flexural shears sub-parallel to bedding and very low shear strength tuffaceous horizons are inferred to provide planes of sufficiently low shear strength to facilitate failure. While most deep-seated landslides appear active, there is no evidence to suggest they were substantially affected by recent major tectonic (e.g. 1931 Ms7.8 Hawke's Bay Earthquake) or climatic events (e.g. 1938 c.1 :1000yr Esk Valley Storm). The headwaters of the catchment are located on the Maungaharuru Range. This rises from 500m - l300m and provides baseflow to the Esk River. Extensive deep-seated landslides dominate the eastern face of the range. These are inferred to have been triggered by the removal of lateral support at the foot of the range following significant incision and denudation in the last interglacial c.125ka. A deep-seated gravitational slope deformation is proposed to extend to 1.2km below sea level, and provide a driving mechanism for the slides. While the 1938 Esk Valley flood was primarily a result of an exceptional three day storm event, suspended sediment load was also an important factor. This is inferred to have resulted primarily from channel erosion in soft colluvial sediments on the Maungaharuru Range. Combined with significant sediment load from shallow landsliding and possible tectonic subsidence preconditioning the lower reaches of the aggradational valley, this lead to c.1 m of silt being deposited in the lower reaches of the aggradational valley. Rapid stream incision in response to uplift in 1931 and aggradation in 1938 is returning the lower reach of the river to grade and decreasing the flood hazard.