A COMPARISON OF SURFACE SAMPLING METHODS FOR COARSE FLUVIAL SEDIMENTS

In order to characterize variability associated with sampling coarse f luvial sediment, surface grain-size distributions were characterized a t eight sample sites within gravel- to boulder-bed channels. Four meth ods were used: (1) a random walk, (2) a sampling grid spaced at an int erval equal to the intermediate diameter of the largest clast in the s ampling area, (3) a sampling grid spaced at one half the interval of m ethod 2, and (4) a randomly chosen subsection within which a patch cou nt of most clasts at least partially exposed at the surface was conduc ted. At each site, six replicates of each method were performed, with each replicate consisting of 100 clasts. Sampling was performed by a s ingle operator at six of the sites and by multiple operators at two of the same sites and at two additional sites. The research objectives w ere to characterize variability among replicates of a method, among me thods, and among operators. Sample sites were divided into relatively well sorted (inclusive graphic standard deviation <1.4) and poorly sor ted (greater than or equal to 1.5). The first three methods produce st atistically indistinguishable values of D-50 and D-84 for both well-so rted and poorly sorted sites when performed by a single operator. Mult iple operators may produce statistically different population measures (D-50, D-84, sample distributions, and variance) using any of the fou r methods at either type of site. Because of operator-induced variabil ity, we caution against direct comparisons of grain-size distributions between channels or within a single channel through time if more than one operator performs the sampling.