2025/03/19 Council Agenda Packet

CITY OF EVERETT PUBLIC WORKS DEPARTMENT  SMITH ISLAND ESTUARY RESTORATION ADVANCE MITIGATION SITE: YEAR 7 MONITORING Page 3 | February 18, 2025 Union Slough. This is accomplished via recording the hydroperiod across the month of June and the salinity of the water column sampled at high tide in the site and in Union Slough during that same time period. NSD will visit the project site in late May 2025 to deploy water level 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. NSD will use an RTK unit to survey the elevation of each gage so recorded water depths can be reported on a consistent vertical datum. Gages will be retrieved after four weeks of continuous (6 minute) data collection across the month of June. 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 when the salt wedge can push furthest into the site, which in 2025 should occur the last full week of June. 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. Subtask 1.2 Estuarine Habitat Area Delineation 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 create a figure illustrating the sample plot locations. The sample plot data will be used to verify conditions meet the Corps’ criteria as wetland as required in the Advance Mitigation Plan. NSD will present the appropriate figures within the Task 5, Year 5 Monitoring Report, using the NSD drone aerial imagery collected as part of Task 2, Vegetation Monitoring to illustrate the water surface elevation gage locations, the salinity sampling locations, the wetland determination data plots, and the extent of estuarine habitat. During the delineation site visit, NSD will also review the perimeter of the site and will review NSD’s 2025 drone images (Task 2) to identify any locations that have eroded or otherwise show evidence of alterations like fill or dike expansion that would change the area of tidal inundation since construction. If necessary, NSD will also utilize the 2019 Snohomish estuary LIDAR to compare extent of site with 2025 conditions and with the as-built plans, and the low and high tide drone images ICF presented in the June 2020 Year 1 report.