Contemporary Sediment Delivery Ratios for Small Catchments Subject to Shallow Rainfall Triggered Earthflows in the Waipaoa Catchment, North Island, New Zealand

The Waipaoa catchment is generally considered to have high hill slope channel coupling due to the large volumes of sediment output at the river mouth. Yet the percentage of sediment that is transported within the fluvial system is low when considered in terms of the total volume of sediment mobilised during episodic failure events. Clearly, there is a discrepancy between generation of sediment and its delivery to the fluvial network. Previous research has suggested there is a strong decrease in catchment connectivity as catchment size increases. However, little research has been undertaken to understand the changes in hillslope-channel coupling over time. This study focuses on the connectivity of shallow rainfall triggered earthflows located in small catchments located within three different land systems in the Waipaoa Catchment. A multiple regression model was developed to predict the sediment delivery ratio for individual earthflows based on an empirical dataset of earthflows which occurred during a storm event in 2002. The results from this modelling were applied to five larger sub-catchments where sequential aerial photograph analysis (1940s to 2004) was used to determine connectivity. From this, spatial and temporal patterns in the catchment sediment delivery ratios were identified. The expected decrease in sediment delivery ratios was observed as catchment size increased. However, the temporal pattern to sediment delivery is not so clear. It appears that catchment evolution, referring specially to the Terrain Event Resistance Model developed by Crozier and Preston (1999), does not have a significant influence on sediment delivery ratios within the six decades examined in this thesis. Furthermore, while earthflows are considered the ultimate source of sediment during storm events, they are not always the mechanism by which this sediment enters the fluvial network. It is also vital to consider rates of gullying, sheet erosion and riparian erosion.