Representative point-integrated suspended sediment sampling in rivers

Abstract

The vast majority of continental sediment delivered to the world's oceans moves by suspension in rivers. Depth-integrated or point-integrated bottle samplings are the traditional methods used to determine the mean concentration of suspended sediment in rivers. While there has been some investigation of the error associated with depth-integrated sampling, the representativeness of a point-integrated bottle sample has not been addressed in the literature. Here we analyze continuous hour-long measurements of suspended sediment and grain-size fractions collected using a LISST-SL in the sand-bed portion of the Fraser River, British Columbia, to determine an appropriate sampling time. The 2σ uncertainty range of individual 30 s samples varied from ±3% to ±33% about the observed mean, with a systematic increase toward the streambed. Mean concentrations for suspended sediment and grain-size fractions were computed over increasing time periods and compared with a long-duration mean concentration to determine when a sample becomes representative. A cumulative probability distribution was generated from multiple iterations of this process. All suspended sediment load and grain-size fractions bear a low probability of representing the mean concentration over standard bottle sample durations. A probability >90% of representing the mean concentration and grain size of various fractions requires ∼570 s (9.5 min) of sampling. Sampling for a shorter period of 264 s (4.4 min) can yield a sample with 73% probability of representing the mean concentration.