SEDIMENT BUDGET TO ASSESS THE GEOMORPHIC EFFECT OF A CYCLONIC STORM, NEW-ZEALAND

A short-term sediment budget was constructed to assess the erosion res ponse of a soft-rock hill country watershed to an intense rainstorm ev ent. The watershed is located in a landslide-prone area on the east co ast of the North Island, New Zealand. During March 1988, 753 mm of rai n was recorded over a four-day period with 320 mm and 329 mm on succes sive days. Known as Cyclone Bola, this was the largest rainstorm event in the Tutira watershed in the 93-year rainfall record. The budget qu antifies the total sediment generated during the storm, the relative c ontribution of erosion processes involved, the amount of sediment held in storage and the amount discharged into two lakes within the waters hed. A landform map of the watershed was constructed and the contribut ion of each landform type to the total budget was calculated. A total of 1.35 (+/- 0.13) million m3 of sediment was generated during the sto rm at an average of 420 m3/ha. This is equivalent to a denudation valu e of 42 mm for the total watershed (3208 ha) and 83 mm for the landfor ms severely affected by landsliding (1427 ha). Of the sediment generat ed, 21% remained on hillslopes, 22% was deposited on valley floors, 51 % was deposited on the lakebeds and the remaining 6% was discharged fr om the watershed via the lake outlet. Most of the sediment generated d uring the storm was from primary source areas on hill slopes, with sed iment in secondary storage providing only a small contribution. Landsl ide erosion was the main process, accounting for 89% of the sediment g enerated. Channel, tunnel gully and sheet erosion were only minor cont ributors to the budget. Six hillslope landforms, which occupy only 44% of the watershed generated 90% of the sediment. The results of this s ediment budget, when put in context with the storm magnitude-frequency history being analysed from lake cores, contribute to the identificat ion of sustainable land use and management of soft-rock hill country.