The sensitivity of sand barriers to the effects of sea level rise

Many harbours in New Zealand are fronted by barriers. Sand-dominated barriers are highly sensitive coastal systems which alter their morphology in response to rising sea-levels, undergoing extensive sediment reworking as wave activity reaches further inland. Farewell Spit is a typical sand-dominated barrier spit which extends 25kms eastward from the northwestern corner of the South Island, New Zealand. The spit provides protection for Golden Bay from the deep-water swells of the Tasman Sea. It is composed of barchan dunes over 20m high, sand sheets over 1km wide and vegetated linear dunes up to 14m high. During spring tide cycles a few isolated low-lying areas of the spit become completely inundated, the frequency of which is likely to increase with the projected increases in sea-level. The spit's vulnerability to the IPCC projected eustatic sea-level rise of 0.48m by 2100 was assessed through GIS analysis of aerial photographs and geomorphological investigations. It was found that the most notable geomorphic change was an increase in vegetation cover over the past 50 years which has led to stabilisation of foredunes and shoreline progradation of up to 170m over the tidal flats. This contrasts to the northern half of the spit where highly mobile barchan dunes still dominate the landscape. While deeper water levels and more frequent inundation is expected in the present across spit tidal channels, the large sediment supply and mobility of the dune systems means the landform is likely to have the capacity to buffer some of these processes, and thus unlikely to be drowned.