Morphodynamics of an Armour-layered Fan Delta: Te Mata, Coromandel Peninsula

Abstract

A hydrodynamic and sedimentary analysis undertaken at Te Mata stream and fan delta is presented. Hydrodynamic investigations reveal that prevailing low incident wave energy conditions combine with strong tidal currents to form a surficial gravel armour layer on the fan delta that protects the subsurface from major geomorphic change during high-energy events; non-armoured sandy areas adjacent to the stream channel exhibit changes in width, depth and elevation caused by high stream flow. Wolman counts in the stream and on the fan delta show downstream fining, without notable change in gravel shapes between the stream and delta. Abrasion experiments show low rates of abrasion on the fan delta, with dacite abrading faster than andesite or greywacke. Mobile sand acts as a buffer against abrasion, reducing the effect of gravel-on-gravel impact. An RFID gravel tracer experiment shows gravels moving onshore, but at a maximum of 36m over 6 months. Compact elongate clast shapes are transported further than platey clasts; the opposite occurs in most coastal settings. Preferential transport of compact gravels occurs in both armoured gravel-bed rivers and armoured beaches. The armour layer preferentially imbricates flatter (platey) clasts while rounder (compact) shapes protrude from the delta surface and are subjected to the higher incident energy levels necessary for entrainment and transport. Channel reinforcement on six western Coromandel coast streams prevents natural stream mouth migration and may reduce sediment deposition on the fan delta below natural levels. Coupled with hypopycnal conditions, the delivery of sediments to the fan deltas may have reduced significantly since channel fixing. Reduced sediment supply to coasts, combined with rising sea levels, increases the likelihood of erosion during coastal storm events. Site- specific studies of hydrodynamics and geomorphology enhance our ability to undertake appropriate fluvial and coastal management and can enhance community resilience to high-energy events.