MORPHODYNAMIC CONDITIONS AT THE SONOMA
BAYLANDS RESTORATION SITE
Dingler,
Cacchione, & Hansen
USGS
Menlo Park CA
SYNOPSIS
The
conversion, since 1850, of 95% of San Francisco Bay's tidal wetlands to
other land types represents a huge loss of habitat for the Bay's wildlife
and those fish species that use the wetlands during breeding and maturing
stages. Furthermore, the remaining 125 sq km of tidal wetlands are threatened
by development, erosion, pollution, and sea-level rise. To preserve a viable
wetland habitat around the Bay, management agencies have begun to identify
the threats to existing wetlands and to determine viable strategies for
wetland restoration in areas that are now diked agricultural land or salt
evaporating ponds. Those agencies, in coordination with other appropriate
groups, have initiated the first major wetland restoration endeavor to use
dredge spoils, the Sonoma Baylands Restoration Project.
At the restoration site adjacent to the mouth of the Petaluma River, material dredged from the Oakland Estuary was pumped as a slurry into an impounded tract, which had been used as agricultural land for many years, until the sediment surface was approximately at mean bay level. After the sediment has consolidated for several months, part of the impounding levee will be breached (in October, 1996) to allow tidal circulation in the tract. That project, which is expected to be a model for future wetland restoration throughout the Bay, requires detailed monitoring to determine the morphodynamic processes that affect the project site and existing wetlands bayward of the site.
Monitoring the morphodynamic processes that can have an impact on that undertaking will provide valuable information for the proper design of future restoration sites. This study is designed to provide such information through measuring current, salinity, and suspended-sediment patterns; tidal-channel development; and meteorological conditions in the existing tidal wetland bayward of the site. Hydrologic measurements are made repeatedly at a site in the tidal creek closest to the expected levee breach, channel cross-section is measured at that same site, and wind shear and direction are measured nearby in the wetland.
Flow measurements in the tidal creek before and during the pumping of the sediment slurry into the impoundment show greatly increased flow during the latter period. Cross-sectional profiles taken before the impoundment was filled show a channel roughly 2.5 m wide and 0.8 m deep. After the pumping of the slurry ended, the creek was the same width but approximately 0.2 m deeper.
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