Natural Systems Design 12/16/2022

EVERETT PUBLIC WORKS DEPARTMENT  SMITH ISLAND ESTUARY RESTORATION YEARS 5 & 6 MONITORING Task 1: Verification of Area of Estuarine Habitat, Hydrology, and Salinity Per Tables 6-1 and 6-2 of the Advance Mitigation Plan, a verification of the Year 1 extent of estuarine habitat based on Mean Higher High Water (MHHW) is required in Year 5, as well as a verification that the site meets U.S. Army Corps of Engineers Regulatory standards under the Clean Water Act as an estuarine wetland and that a natural diurnal tidal regime with estuarine wetland salinity has been restored. Components of this determination as an estuarine wetland are verification of the hydroperiod of the site compared to Union Slough and verification that the salinity of the water in the site is comparable to that in Union Slough. Sample plots to verify conditions meet the Corps’ criteria as wetland are also required in the Advance Mitigation Plan. NSD will visit the project site in late May 2023 to deploy gages at low tide to measure the rise and fall of the water surface within the Project site and in the adjacent section Union Slough. Gages will be deployed in a protective PVC housing attached to T-posts or similar. Two gages will be deployed within the Project site, one within Union Slough, with a fourth installed above the high tide line for barometric compensation. The elevation of each hydrology gage sensor will be surveyed and recorded so recorded water depths can be reported on a consistent datum. Gages will be retrieved after four weeks of continuous (6 minute) data collection across the month of June. During the site visit, NSD will review the perimeter of the site and will ultimately review NSD’s 2023 drone images (Task 2) to identify any locations that have eroded or otherwise show evidence of alterations that would change the area of tidal inundation since construction. If available, NSD would also utilize the 2019 Snohomish estuary LIDAR to compare extent of site with 2023 conditions and with the as-built plans, and the low and high tide drone images ICF presented in the June 2020 Year 1 report. Recorded water level data will be analyzed and plotted to verify that water levels within the site rise and fall in conjunction with those in Union Slough. Verification that water level in the site rises to the same elevation as Union Slough at high tide across the month of June will demonstrate that the nature of the connection to Union Slough has not changed. NSD will record water column salinity near a daytime higher high tide, which in 2023 should occur in the month of July. Salinity will be monitored by deploying a small inflatable kayak or similar and a YSI or comparable meter to measure salinity in the site and in Union Slough. Salinity measures will be collected at near-surface, mid-depth, and near-bottom at locations with deeper water columns and at the surface and bottom of the water column in water depths less than 3 feet. Six sample locations within the Project site will be collected, with an even distribution of distances from the breach. Three locations will be sampled in Union Slough. All sample locations will be recorded with GPS equipment. Salinity measurements will be used to verify that the average concentration across all Project site samples meets a minimum salinity of 0.5 parts per thousand (the threshold used to characterize a wetland as ‘estuarine’). Since Union Slough and Smith Island typically experience oligohaline (low salinity) conditions, salinity consistent with that recorded in Union Slough will also verify that the site is an estuarine wetland. If NSD is able to verify before June 2023 that salinity was measured by Snohomish County and Tulalip Tribes during their fish sampling efforts in the site (see Task 3), then NSD will coordinate to obtain the water quality data and summarize the Year 5 salinity data collected. As required in the Mitigation Plan, NSD will verify that the site meets wetland criteria by collecting wetland determination data plots at representative wetland and upland locations. NSD will collect vegetation, soils, and hydrology indicator data, as well as photos from the data plot locations and DocuSign Envelope ID: B75A46E0-B4A7-4C6C-B91E-69B691B92514