DEFICIT AND SURPLUS
Questions from DredgeFest California
San Francisco Bay is the largest estuary on the U.S. West Coast, and the 2nd largest in the United States; combined with the contiguous Sacramento-San Joaquin Delta it covers a total surface area of ~4100 km2 and a watershed area of ~162,000 km2. It contains several economically significant harbors ($20 billion worth of cargo annually) in one of the most developed regions of the United States, with a surrounding population of over [15 million] people. San Francisco Bay and the adjoining Delta are among the most human-altered estuaries and hydrologic systems, respectively, in the world. Major historical changes were driven by the extensive hydraulic mining influx of sediment in the late 19th century, massive alteration of the drainages entering San Francisco Bay in the 20th century, and the enormous amounts of sediment removed throughout the San Francisco Bay Coastal System from the early part of the 20th century to the present.
- Sediment transport in the San Francisco Bay Coastal System: An overview by Patrick L. Barnard, David H. Schoellhamer, Bruce E. Jaffe, Lester J. McKee. 2013
From June 13-19, 2016, teams of designers and experts met in a set of workshops at DredgeFest California to investigate sedimentary futures for California’s Bay-Delta. One of these workshops focused on the regional choreography of sediments -- the orchestration of relationships between sedimentary surpluses and deficits across the entire estuary watershed. Examining the hidden sediment reserve was a major component of that workshop. More information about the workshops and their outcomes can be found here.
A CHRONOLOGY OF SEDIMENT DEPOSITION
The past 150 years of human impacts on sediment flows in the Bay-Delta presents a fascinating and well-documented history. First, hydraulic gold mining and settlement-driven clearcutting sent an unprecedented pulse of sediment into the Bay-Delta. Then, multiple dams throughout the upland portions of the watershed began to capture sediment while a variety of other infrastructures, such as levees and storm channels redirected sediment away from where they are needed. Today, rising sea levels threaten to wash away vulnerable wetlands and breach critical levees.
ANNUAL SEDIMENT NEED
Scientists estimate that the Bay-Delta’s wetlands would require a total sediment input (i.e. organic matter and inorganic sediment) of up to 10.1 Mm3/year (~2.6cm/year) by 2100 to keep pace with the higher projections of sea level rise; presently only as much as 0.4 Mm3/year is actually being deposited.
Moving and utilizing this sediment for the Bay-Delta would face multiple challenges. The massive quantities of sediment would need to be physically transported downstream, through means such as dredging and trucking, dam retrofits, or deliberate flooding induced through dam management. Sediments vary greatly, and silts, sands, and gravels would all need to be matched to the specific locations where they could be used. Moreover, some sediment is contaminated by toxins and pollutants, including mercury from the hydraulic gold mining era.
But these challenges are not fixed. Though mobilizing this sediment may seem far-fetched today, the calculus will shift as sea level rises, sediment shortages manifest, and dams exceed their lifespans.
The following interactive visualization encourages the user to explore the sediment volumes trapped within California’s constellation of reservoirs. Sediment composes the material infrastructure that supports the Bay-Delta’s many ecologies and economies. As the 2015 State of the Estuary report by The San Francisco Estuary Partnership notes, "Like freshwater, sediment is a precious resource that is essential for keeping the Estuary healthy." Navigate the map, select reservoirs, and scrub the timeline to gain insight into the quantities and equivalent values of the San Francisco Bay-Delta’s displaced sediment.
THE HIDDEN SEDIMENT RESERVE
Quantifying Potential Sediment Trapped Within the San Francisco Bay-Delta Watershed’s Reservoirs
Visualization by Landscape Metrics, LLC, in collaboration with The Dredge Research Collaborative. Raster Tiles © Mapbox © OpenStreetMap
DFCA Regional Choreography Workshop Participants: Derek Hoeferlin, Kees Lokman, Matthew Seibert, Jackson Blalock, Johanna Hoffman, William Reinhardt, and Nick Shannon.
- Estimated Reservoir Sediment: Minear, J. T., and G. M. Kondolf (2009), Estimating reservoir sedimentation rates at large spatial and temporal scales: A case study of California, Water Resour. Res., 45, W12502, doi:10.1029/2007WR006703.
- Tributary Sediment: L.J. McKee, M. Lewicki, D.H. Schoellhamer, N.K. Ganju, Comparison of sediment supply to San Francisco Bay from watersheds draining the Bay Area and the Central Valley of California, Marine Geology, Volume 345, 1 November 2013, Pages 47–62.
- USACE San Francisco and Sacramento District dredge data compiled and georeferenced by John de Goede and Brett Milligan
Lead scientists on the study note that the above datasets represent rough estimates. More accurate datasets are not currently available.