Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the upper Waipaoa River, New Zealand

Although much is known about overall sediment delivery ratios for catchment s as components of sediment production and sediment yield, little is known about the component of temporary sediment storage. Sediment delivery ratios focused on the influence of storm-related sediment storage are measured at Matakonekone and Oil Springs tributaries of the Waipaoa River basin, east coast of New Zealand. The terrace deposits of both tributaries show abundan t evidence of storm-related sedimentation, especially sediment delivered fr om Cyclone Bola, a 50 year return rainfall event which occurred in 1988. Th e sediment delivery ratio is calculated by dividing the volume of sediment transported from a tributary to the main stream by the volume of sediment g enerated at erosion sites in the tributary catchment. Because the sediment delivery volume is unknown, it can be calculated as the difference between sediment generation volume and sediment storage volume in the channel reach of the tributary. The volume of sediment generated from erosion sites in e ach tributary catchment was calculated from measurements made on aerial pho tographs dating from 1960(1:44 000) and 1988 (1 :27 000). The volume of sed iment stored in the tributary can be calculated from measurements of cross- sections located along the tributary channel, which are accompanied by terr ace deposits dated by counting annual growth rings of trees on terrace surf aces. Sediment delivery ratios are 0.93 for both Matakonekone catchment and Oil S prings catchment. Results indicate that Oil Springs catchment has contribut ed more than twice the volume of sediment to the Waipaoa River than the Mat akonekone catchment (2.75 x 10(6) m(3) vs 1.22 x 10(6) m(3)). Although larg e volumes of sediment are initially deposited during floods, subsequent sma ller flows scour away much of these deposits. The sediment scouring rate fr om storage is 1.25 x 10(4) m(3) a(-1) for Matakonekone stream and 0.83 x 10 (4) m(3) a(-1) for Oil Springs stream. Matakonekone and Oil Springs channel s respond to extreme storms by instantaneously aggrading, then gradually ex cavating the temporarily stored sediment. Results from Matakonekone and Oil Springs streams suggest a mechanism by which event recurrence interval can strongly influence the magnitude of a geomorphic change. Matakonekone stre am with its higher stream power is expected to excavate sediment deposits m ore rapidly and allow more rapid re-establishment of storage capacity. Copy right (C) 1999 John Wiley & Sons, Ltd.