HR: 16:15h
AN: H34B-02    [Abstracts]
TI: Modelling the Holocene Evolution of a Drift-dominated Alluvial-fan Coast
AU: * Hicks, M
EM: m.hicks@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8011, New Zealand
AU: Dickson, M
EM: m.dickson@niwa.co.nz
AF: NIWA, PO Box 8602, Christchurch, 8011, New Zealand
AU: Coco, G
EM: g.coco@niwa.co.nz
AF: NIWA, PO Box 11-115, Hamilton, 3216, New Zealand
AB: A numerical model is being developed to simulate shore evolution along an alluvial fan coast over the Holocene. The alluvial fan of the Waitaki River, on the east coast of New Zealand's South Island, was built by Pleistocene glacial advances but has been eroded back by waves over recent millennia. The retreat has left a cliffed shore fronted by a narrow beach of mixed sand and gravel and a gently sloping seabed with only a thin, patchy sand cover over the Pleistocene substrate. The study motivation is to examine the sensitivity of shoreline movements in this setting to wave-climate change, sea-level rise, and river sediment supplies. The modelling couples a profile evolution model with a shoreline model. The profile evolution model is operational and is driven by a series of coupled process models which include seabed scour, berm construction during normal waves, berm overtopping and subsequent beach-stripping and scour of the exposed substrate and cliff-toe notching by storm waves, gravity failure of the cliffs and talus construction, and beach sediment abrasion. Negative feedback regulates the rate of cliff erosion through the protection provided by the new material added into the beach from the eroding cliffs and substrate. The model is forced by two wave conditions: a normal swell and a randomly-varying storm wave. These operate for proportions of the yearly time step. The model was begun on a sloping fan surface inundated by the last stages of post-glacial sea-level rise (8000 yr BP). The initial response is for rapid growth of a gravel beach ridge fed by wave-excavation of the nearshore. As the nearshore profile nears equilibrium with the wave climate, the onshore feed wanes below the abrasion rate and the beach ridge loses volume. As sea level rises the beach ridge moves upward and landward, but its capacity to do so is limited by the rate of sediment feed from the nearshore. When the beach size reduces to a threshold at which storm waves periodically overtop and strip the beach, substrate scour commences and a proto-cliff forms. The shore then settles into a mode of episodic retreat with gradually increasing cliff height. Without any net gravel gains or losses alongshore, the cliff retreat rate is simply related to the abrasion rate and the shore profile height. The equilibrium beach width is an emergent property that also relates to the abrasion rate. The modelled profile compares reasonably with the field profile at locations where the longshore transport divergence is small. A surprising result is the importance of nearshore substrate erosion to the beach sediment budget. Preliminary runs with a shoreline model show how the alongshore patterns of cliff height and historical shoreline movement on the Waitaki Fan shore reflect the net influence of abrasion and longshore transport divergence. They also suggest that the barrier shore north of the fan may have been arrested at the first stage of the profile evolution sequence because deposition due to a waning longshore drift potential just offsets gravel losses to abrasion.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 3020 Littoral processes
DE: 4546 Nearshore processes
SC: Hydrology [H]
MN: 2007 Fall Meeting