Statistical trend analysis of long-term nearshore monitoring data, Swinomish Indian Reservation

The Swinomish Indian Tribal Community has had long-standing concerns about the stability of Reservation shorelines. The Swinomish Indian Reservation, located in Northwest Washington about 40 miles south of the US/Canada Border, is about 90% bounded by saltwater and much of that boundary is modified by dikes, bulkheads, or other structures that affect nearshore processes. The Swinomish Water Resources Program began monitoring elevation, substrate character, and vegetation at seven transects perpendicular to the western Reservation shoreline in 2002, and continued monitoring with monthly to quarterly frequency through 2011, and periodically thereafter. Elevation is measured using laser level and surveyed benchmarks at regular intervals between a fixed upper origin and the lowest accessible point along each transect. Substrate sediment texture is characterized at each elevation point using visual estimation methods and a 0.25 m2 quadrat grid. Type and abundance of vegetation based on field identification in each quadrat was also cataloged during surveys from 2002 to 2011. Repeat elevation surveys at these transects showed little to no change over the period of record when visualized as standard profiles. It was similarly difficult to visualize temporal trends in substrate character or vegetation distribution using graphical methods. The 'noise' within the data set (seasonality, natural variability, etc.) overwhelmed any trend signal. Statistical trend testing, particularly Mann-Kendall and seasonal Mann-Kendall testing, provide more robust analysis of temporal variability in the data. Seasonal Mann-Kendall testing compensates for seasonal variability in the data to evaluate overall trends, while standard Mann-Kendall testing can be applied to the separate 'seasons' to identify trends within seasonal subsets. Application of these statistical tools has helped us identify specific locations at which change is more likely occurring, guides further work to link the changes to nearshore processes, and informs potential restoration or mitigation actions.