Stream daylighting and changing hydrology in a Port of Seattle wetland mitigation site
Risa Askerooth, Professional and Continuing Education
Stream daylighting is the practice of uncovering streams that have been buried in order to restore hydrologic function. These actions have significant impacts on flood mitigation and water quality improvements for surrounding areas, although long-term effects on surrounding hydrology in urban environments are less well-studied. We examined how wetland hydrology in the Des Moines Nursery Mitigation Site in SeaTac, WA, owned by the Port of Seattle, may have changed due to the adjacent daylighting of Miller Creek and precipitation over time. This stream restoration project, spurred by the failure of a nearby culvert in 2017, involved relocating a piped stream segment to restore 450 ft of open channel, a portion of which occurred within an existing Port stream and wetland mitigation site. I analyzed baseline and post-construction data of shallow groundwater monitoring wells using GIS to validate that hydrologic functions within the wetland remained unchanged. Relative depth to groundwater from the surface was visualized through the Topo to Raster geoprocessing tool, from which the site's digital elevation model was subtracted. This output raster was generated for pre- and post-stream realignment conditions in wet and dry conditions and compared to projected depth to groundwater contours that were modeled prior to construction. Stream daylighting projects such as this one, with access to historical groundwater data, provide valuable insights into how wetland functions may be impacted both by stream restoration.