3.1 | Rapid assessment analysis
The percentage of structures meeting the performance criteria for integrity and erosion was greater than the target objective of 90% from 2014 through 2018 (Figure 2). Both integrity and erosion dropped below the target threshold in 2020. Deposition rankings met the performance criteria (97%) for the first three years of monitoring (2014-2016), but dropped below the 90% target in 2017. Based on the maximum integrity ranking across all years, 74% of the structures met the performance criteria by the end of the study period. Similarly, 80% and 72% of the structures met the performance criteria for erosion and deposition, respectively, based on the maximum rankings across all years. As project construction was completed in 2014, the project met the goal that 90% of all habitat improvement structures be stable and functional three years after implementation, which corresponded to the 2017 survey. However, structural integrity and function diminished over time as rankings for integrity, erosion, and deposition all dropped below the target threshold by 2020.
Boulder toe structures exhibited the poorest performance among structure types, with 80% of those structures failing to meet the criteria for integrity and erosion based on the maximum ranking across all years (Figure 2). Log-vanes also failed to meet the performance target (<10% of structures) for integrity (47%), erosion (30%), and deposition (35%). Other structure types that failed to meet performance criteria for integrity included boulder vanes (27%), log toe (23%), and side-channel grade control (17%). In addition to boulder toe and log-vanes, boulder vanes (36%), side-channel grade control (25%), and log toe (18%) also failed to meet the performance criteria for erosion. No boulder cluster, cobble toe, or main channel grade-control structures were adversely affected by erosion. Boulder toe and main channel grade control were the only structure types that met the performance criteria for deposition. Deposition adversely affected the performance of more than 30% of the log toe, log vane, and side channel grade-control structures. Boulder clusters (25%), boulder vanes (18%), and cobble toe (13%) structures also exhibited some issues with deposition.
Results from the CLMM indicated that both year (p < 0.001) and structure type (p < 0.001) affected integrity rankings for instream structures, with significance differences occurring between all levels of response (p< 0.0001; Table 4). The odds of structures receiving higher integrity rankings, noting that higher rankings are indicative of diminished performance, increased in all subsequent years when they were compared to 2014 (p < 0.011), which represented as-built conditions following the completion of construction. Conversely, integrity rankings were more likely to be higher in 2020 (p < 0.0001) when compared to all preceding years. Higher integrity ranking were also observed in 2017 (p = 0.020) and 2018 (p = 0.015) when compared to 2015. Boulder clusters were more likely to receive lower integrity rankings than boulder-toe (p = 0.041) and log-vane (p = 0.007) structures. Cobble-toe structures were more likely to receive lower integrity rankings when compared to boulder-toe (p = 0.048) and log-vane (p = 0.084) structures. Log vanes received higher integrity rankings compared to side channel grade control (p = 0.070) and log-toe (p = 0.003) structures. Finally, boulder-toe structures were more likely to receive higher ranking when compared to log toe (p = 0.016).
Likelihood ratio tests indicated that both year (p < 0.0001) and structure type (p = 0.040) affected rankings for erosion, with differences in response occurring between moderate and severe rankings (p = 0.002; Table 4). Similar to the integrity rankings, pairwise comparisons for year showed that erosion rankings were more likely to be lower in 2014 compared to all subsequent years (p < 0.056) and higher in 2020 compared to all preceding years (p < 0.001). Differences between structure types were less evident, as boulder toe was the only structure type more likely to receive higher rankings when compared to log toe (p = 0.027) and cobble toe (p = 0.085). Year also affected deposition rankings (p < 0.0001), with differences observed between all levels of response (p < 0.0001; Table 4). Higher deposition rankings occurred in the years 2017, 2018, and 2020 when compared to 2014 (p < 0.0001), 2015 (p < 0.001), and 2016 (p < 0.002). Although the likelihood ratio test indicated that structure type had an effect on deposition rankings (p = 0.092), pairwise comparisons did not show any differences between structure types (p> 0.250). Complete results for the CLMMs and pairwise comparisons between structure types and years were included in the Supporting Information.