# A Novel Suspended-Sediment Sampling Method: Depth-Integrated Grab (DIG)
## 1 Introduction  
Scientists, engineers, and natural resource managers have a need for fluvial suspended sediment data because they are not always available at locations of interest. Using suspended-sediment samplers has been the primary method for collecting fluvial suspended sediment data, but not all sampling and laboratory methods are representative of a river's suspended-sediment characteristics. Sediment may not be well mixed horizontally across the river cross section, especially when a tributary enters with different sediment characteristics [1]. Fluvial suspended sediment and its grain size distributions are rarely distributed uniformly in a river's cross section and can vary with depth [2], especially when sand-size particles (0.0625 to 2 millimeters [mm]) are present [3]. The occurrence and concentration of sand is often greater near the streambed, and it can be transported as bedload and/or suspended in the water column. Sand is more likely than fines to be deposited in the channel or floodplain when the river's velocities decrease. Fine-sized particles, which consist of silt and clay (less than 0.0625 mm), are more homogenously mixed in the river's cross section and can stay in suspension longer than sand. Grab sampling does not represent the horizontal and vertical distribution of sediment because it only incorporates the top of the water column (less than 0.5 meters [m]), and only one location in the cross section is sampled (Figure 0(c)). Additionally, the total suspended solids (TSS) laboratory method [4], which is often used to analyze grab samples, providesadditional errors [5; 6; 7; 8; 9]. The TSS laboratory method involves subsampling the original water sample, so the amount of suspended sediment in the subsample can be filtered and measured [4]. The TSS laboratory method is not representative of the whole water sample because sand can settle during the subsample extraction given Stokes's law. Several studies have shown that Grab-TSS is not as accurate as using specialized samplers and laboratory methods developed by the Federal Interagency Sedimentation Project (FISP) and the U.S. Geological Survey (USGS) because Grab-TSS fails to measure sand [5; 6; 7; 8; 9]. Even though this knowledge exists, Grab-TSS is widely used by water quality agencies and organizations as a water quality standard [10] for rivers and streams, such as in Minnesota where this study took place. There needs to be a less expensive and feasible alternative that is more representative of river conditions than Grab-TSS yet easier and cheaper than specialized samplers and methods. The US Federal agencies (USGS, U.S. Army Corps of Engineers, Bureau of Reclamation, and U.S. Department of Agriculture) that make up the FISP have specialized samplers, sampling methods, and laboratory methods to accurately represent fluvial suspended sediment in a river's cross section given that suspended sediment is not always uniformly distributed in fluvial systems due to the presence of sand. The primary suspended-sediment sampling and laboratory methods include equal-width increment (EWI; Figure 1a) or equal-discharge increment (EDI) sampling [11; 12; 13] and suspended-sediment concentration (SSC) laboratory methods [14; 15]. These standard methods (EWDI-SSC) provide the best measures of suspended sediment because they depth-integrate nearly the entire water depth (except the bottom \(\sim\) 10 centimeters (cm) if the sampler touches the river's bed), measure the river's cross section at multiple verticals, and sample isokinetically. Isokinetic sampling is when water and sediment particles enter the sampler's nozzle at the same velocity as the stream velocity outside the sampler's nozzle. The SSC laboratory method measures the entire water-sediment mixture without a subsample extraction used with the TSS method.