Sedimentation in Mangahao No. 1 reservoir and erosion within the Upper Mangahao catchment

Abstract

This study's focus is on sedimentation within Mangahao No. 1 Reservoir and erosion within the upper Mangahao Catchment. Sediment and volume surveys have enabled rates of sedimentation to be calculated, isopach maps to be created, and controls on sediment patterns to be identified. The calculation of sediment yields, the mapping of erosion scars and sediment source areas of erosion, the analysis of landslide parameters, plus the analysis of rainfall records and historical data, has enabled identification of those processes controlling the volume of sediment deposited within the reservoir. Sedimentation in the reservoir, since the construction of No. 1 Dam in 1928, has reduced the total volume of storage from 4.74x106m3 to less than 1.69x106m3. The remaining storage will take a minimum of fifty years to fill with sediment. The primary source for material deposited within the reservoir is from clastic sediment carried by the reservoir inflows. There are effectively no internal inputs of sediment from either biological or organic origins. The spatial distribution of sediment within the reservoir is controlled by highly variable lake levels, the changing morphology of the reservoir bed, and decreasing storage at the head of the reservoir. The rate that clastic sediment has been supplied to No. 1 Reservoir has been variable. Sediment yields from the upper Mangahao Catchment, measured at No. 1 Reservoir, have decreased from: 85000m3/yr during the period from 1933 to 1957, to 43000m3/yr during the period 1957 to 1992. The decreased sediment yields, and a decrease in the trap efficiency of No. 1 Dam, have been responsible for a decrease in the sedimentation rate within No. 1 Reservoir from 148mm/yr for the period 1933 to 1957, to 76mm/yr for the period 1957 to 1992. The probable cause of the substantial decrease in sediment yields is a decrease in the frequency of large magnitude rain storm events, since 1950, which are responsible for initiating episodes of landsliding. Between 1951 and 1981 the area of Harris Creek Catchment (a sub-catchment of the Mangahao) affected by landsliding has decreased by 41%, from 4.9% to 2.9% of the catchment area. Two extremely large storms in October 1935 and December 1936, with return periods of 205 and 922 years respectively, have been recognised as initiating an erosional episode within the upper Mangahao Catchment that lasted until the 1950's. A decade of extreme rainfalls and high deer numbers, retarding or preventing the recovery of erosion scars, are probably responsible for extending the duration and the impact of the landsliding episode.