Suspended sediment yields of New Zealand rivers and their relationship to precipitation characteristics : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Geography at Massey University

Abstract

Sediment ratings determined by plots of instantaneous suspended sediment concentrations and their associated discharges, were combined with the continuous flow record to give values of suspended sediment yields for 82 catchments throughout New Zealand. A discussion of the errors involved in this determination is undertaken and suggestions as to the best use of the current resources in order to lessen these errors is given. Multiple regression analysis including flow characteristics and rainfall characteristics (depth, intensity and variation) determined from the wide network of daily storage rain-gauges, resulted in a national equation of weak prediction identifying 24-hour 5-year return period rainfall as the best explanatory variable of variation in suspended sediment yield. The country was divided into eight regions and the regression rerun revealing mean annual rainfall as the principal predictor in all but one region. The high explanation exhibited in some areas must be rationalised against small sample numbers. A combination of four regions into one gave good prediction, supporting overseas workers', contentions that sediment yield is strongly influenced by rainfall depth and its variability. It is suggested that introduction of a bed material size component into the prediction equation may allow incorporation of at least two further regions into the combined region. The coefficients of the regional logarithmic equations relating sediment yield to mean annual precipitation are analysed. Important conclusions reached are that in relation to suspended sediment, catchment conditions are not necessarily reflected in rainfall depth or vegetation cover but primarily to the volume of transportable material in storage in the channel subsystem. The volume of sediment in storage in some catchments greatly exceeds annual export, stressing the limitations in determining erosion rates from sediment yield values. In Region 2 (East Cape) storage is so large that limits on yield are not controlled by the rate of supply but the competence of the rivers to carry it; some of these rivers may well be carrying limiting yields therefore. Evidence available suggests that a predictive equation derived herein can also be applied to streams draining largely glaciated areas.