wetlands
Qwuloolt Wetland Restoration Project
The Qwuloolt project is a 370 acre wetland restoration project in Marysville, WA along the Ebey Slough, just outside of Everette, WA. NOAA purchased the property for the purpose of restoration, using available funding from litigation due to pollution of Ebey Slough from a landfill two miles west of the site twenty years ago. The property has been diked since the early 1900s. The intention is to remove the levees and flood the property, recreating wetland habitat and reinstating a functional marsh. However, the goal is to achieve this result without flooding the surrounding houses and commercial developments located on the periphery of the property. In order to accomplish this, significant modeling and evaluation has been conducted on this site and nearby reference sites. This is an interesting project because it shows the necessity of planning a marsh restoration site that is surrounded by developments, as well as illustrating how tidal data were incorporated into hydraulic models.
The U.S. Army Corps of Engineers published a comprehensive study of the project area in 2002, which includes a vegetation analysis, sediment analysis, hydraulic analysis, several options for levee breaches, and potential repercussions of each option. The most complicated aspect of breaching the levees is opening them enough so that water is able to move sufficiently in and out of the marsh to reestablish a functional habitat and natural hydrologic system without allowing flooding of the neighboring developments, which may occur if all flood prevention measures are completely removed, so several options were examined for the best way to breach the levees. The levees currently in place surrounding the project area range in height from 6.5 to 15.3' (NAVD88). Breach location must be chosen carefully because of the hydraulic elements present within the marsh, such as the two large tidal channels extending through the interior of the site. The ideal location for a breach in this marsh would utilize conveyance provided by these channels to achieve a maximum hydraulic connection to far reaches of the marsh surface.
A tide gauge was in place at the Qwuloolt site from November 2000 until March 2002, a total of 16 months. Tidal and geodetic datums were used to analyze this project. Using records from long-term NWLON station at Seattle, seasonal analysis and long-term sea level trends were performed. Local MHW and MHHW datums for Qwuloolt reveal that upon inundation, the marsh surface will become flooded above the 4 foot and 8 foot elevation contours at certain points in the normal tidal cycle. This puts the northwestern developments at risk of flooding. The estimated highest tide, based on a comparative study with Seattle, WA, is above the 12 foot elevation of the marsh surface, putting the developments on the eastern side of the property at risk of flood as well, in the case of an extreme tidal event. Frequency and duration of inundation analyses were performed for both the short-term station at the marsh and the long-term station at Seattle and it was found that it is not only a potential for flooding that exists, but a certainty. Nearly half of all high tides will flood the entire marsh surface, and it is likely to remain inundated for up to 18 hours at a time. If the levees are to be completely removed at this site, preventative measures will need to be put in place to protect outlying developed areas, as flooding is a certainty. Any future planning must take this into consideration, as well as accretion rates associated with opening the tidal sloughs, salinity dynamics throughout the marsh, long-term sea level rise, and long periods of inundation that may dictate the survival rate of potential plant species.