# A Novel Suspended-Sediment Sampling Method: Depth-Integrated Grab (DIG)
## 4 Discussion  
Since there was not a statistically significant difference between the EWDI-SSC and DIG-SSC sampling methods, the DIG-SSC shows promise as a more representative method than Grab-TSS. Furthermore, the DIG-SSC is easier and cheaper to use in the field than EWDI-SSC. However, the DIG-SSC was only tested at 9 sites within a specific range of river and suspended-sediment characteristics (Table 2). The DIG-SSC method would benefit from testing at other rivers with different streamflow and suspended-sediment characteristics that were not represented in this study. Additional results could help validate or challenge our study's results. The close agreement between EWDI and DIG methods may be explained by the rivers in this study being relatively well mixed horizontally. The main limitation with the DIG method occurs when a river is poorly mixed horizontally. If deploying the DIG method and the river is poorly mixed horizontally, possible solutions would be to go further downstream where the river is fully mixed horizontally or modify the DIG method to sample at multiple verticals by using EWDI methods with the DIG sampler. The DIG-SSC method would benefit from testing at sites with higher concentrations of sand than what was observed in this study. Sand caused greater variability when comparing EWDI and DIG sampling methods. The variability might be explained by the DIG method only sampling one vertical while the EWDI method sampled multiple verticals. Furthermore, the sample location of the DIG method was at the deepest and fastest part  
Figure 7: Box plots depict the minimum, first quartile, median, third quartile, and maximum, with measured values depicted as single points, and show the difference in the paired values between each sampling (equal-width or -discharge increment [EWDI], depth-integrated grab [DIG], and Grab) and laboratory method for a particular sample: (**a**) Suspended-sediment concentration (SSC) and total-suspended solids (TSS) laboratory methods; (**b**) Suspended-fines concentration (Fines) laboratory method; (**c**) Suspended-sands concentration (Sands) laboratory method. of the river, which probably had the highest concentrations of sand, and greater than the other EWDI sampling locations. This could be tested by comparing the same location and the same number of verticals with both the DIG and isokinetic samplers. This would make the sampling methods comparable to see if the variability seen in this study is from the different number of verticals used by DIG and EWDI methods. It would also be beneficial to test if the DIG sampler is isokinetic or not. The measure of isokinetic sampling is defined as the ratio of the velocity through the nozzle entrance (Vn) to the ambient stream velocity (V):  
\[\text{Isokinetic}=\text{Vn}/\text{V} \tag{3}\]  
where Vn and V are averaged over the sample time and depth for each specific sample. This could be another possible explanation of the variability between EWDI and DIG methods. If additional sampling data were to be collected in the future, it would be beneficial to determine the DIG sampler's intake efficiency (IE) which is a measure of isokinetic sampling [32]. Equation (4) contains K which is indexed to the nozzle opening, volume (Vol), duration, and stream velocity (SV). \[\text{IE}=\text{K}\times(\text{Vol}/\text{D})/\text{SV} \tag{4}\]  
The average IE should fall within 0.75 < IE < 1.25 [32]. Moreover, IE could be compared with temperature [33] and depth which would help determine the operational specifications of the DIG sampler. Furthermore, future research could entail testing the DIG sampler in a tow tank to see if it samples isokinetically and(or) by performing 3-dimensional (3D) modeling to see if the DIG samples isokinetically. Since there is no significant difference between the DIG-SSC and EWDI-SSC, the DIG-SSC could be isokinetic; however, the results in this study cannot confirm this without additional testing and evaluation.