Storm fine sediment flux from catchment to estuary, Waitetuna-Raglan Harbour, New Zealand

Abstract

'Source to sea' storm sediment studies are rare. We constructed a fine sediment budget and examined the timing of sediment delivery from the Waitetuna catchment (167 km2) to its estuary (an arm of the Raglan Harbour, New Zealand) for a single large (10-year return period rainfall) storm. Storm flows were intensively sampled at five small (<3 km2) catchments, six sites on the main-stem of the stream network, and at two upper estuary sites. Silt (4-62 μm) was the dominant particle size transported during the flood event (80-90% of total suspended sediment), despite the catchment having high (c. 40%) clay content soils. During the storm event, about 3700 t of fine sediment (300 kg/ha) exited the catchment and about 25% of the sediment was retained within the upper 3 km of the estuary. Half of the fine sediment load was supplied by a 29-km2 sub-catchment, probably sourced from bank erosion. The bulk (70%) of the sediment load entered the estuary over a 12-h period, and the estuary water column quickly (four tidal cycles) reset to pre-flood conditions.