A review of the effectiveness of sediment traps for New Zealand agriculture

Agricultural expansion, intensification and overgrazing are recognised as some of the key contributing factors to increasing sediment loss above natural levels.One strategy to mitigate such losses is the use of sediment traps, designed to slow overland flow velocity and trap sediment. . The objectives of this study were: (1) to determine if the available scientific literature justifies the use of such structures as viable options to mitigate sediment loss and (2) to identify sediment trap characteristics that determine their effectiveness. Reviewing the published literature on sediment traps identified a total of 21 publications, from which we could extract 16 annual average data points on sediment trapping efficiency in agricultural catchment contexts, 4 modelling data points and 6 data points for roadside runoff. Our review found the annual sediment trap effectiveness (STE) was highly variable, with results ranging from 10% to 98%. Average annual STE across the measured data from agricultural catchments was 59%. The key design metric presented in the literature was the storage ratio; volume of the sediment trap relative to the size of the catchment. Our analysis suggests a storage ratio of at least 120 m³ ha⁻¹ is required to deliver a STE of 55% or better.