Bay Sediment Budgets:Sediment Accounting 101
David SchoellhamerMegan LionbergerBruce JaffeNeil Ganju Scott WrightGreg Shellenbarger U.S. Geological Survey
NASA
A sediment budget describes the quantity of sediment
• Entering the Bay
• Leaving the Bay
• Depositing in or eroding from the Bay
Who cares?
• Dredgers: disposal of deposition in ports and channels
• Regulators and dischargers: sediment associated contaminants and TMDLs
• Restoration managers: deposition needed for restoration without eroding existing habitat
Accounting rules
• Where: SF Bay from Mallard Island to Golden Gate
• When: Water years 1955-1990, 1995-2002, and a normal water year 1995-2002
• Lifeline: Conservation of mass: inflow-outflow=change in storage
• Uncertainty: varies by term, “Not THE answer” – Steve Ritchie
Odgen Beeman and Krone 1955-1990
New information for 1955-1990
Term OBK New information
Supply from Delta No deposition in Delta
Delta sediment outflow 1/3 of inflow
Bathymetric change
Data from navigation charts
Data from actual surveys
Sand mining Assumed zero Rough estimate
Revised 1955-1990 annual budget
Units are mass in millions of metric tons per year (~1.5 Mcy)
Revised 1955-1990 annual budget
Assume Krone Central Bay deposition is ocean sand
Revised 1955-1990 annual budget
Suspended sediment outflow from conservation of mass (lifeline)
Revised 1955-1990 annual budget
Sand mining is a very rough estimate
1995-2002 annual budget
1995-2002 annual budget
Box model used to get change in storage and outflow
1995-2002 annual budget
Assume no change in supply of ocean sand
1995-2002 was all wet
0100200300400500600700800900
1000
Del
ta o
utf
low
, m
3/ s
1995-2002 1955-1990 1996,1997,1999-2002
Normal 1995-2002 annual budget
Remove water years 1995 and 1998 where possible
1995-2002
1955-1990
1955-1990
1995-2002
1955-1990
1995-2002
1955-1990
1995-2002
1955-1990
1995-2002
1955-1990
1995-2002
Implications
• Sediments disposed of in the ocean may be needed to maintain and restore Bay habitats
• The Bay bottom will continue to erode and supply legacy contaminants
• Restoration sites will be depositional and may increase erosion elsewhere
• Dredging, restoration, and water quality are connected by the sediment budget
Most important and most uncertain terms
• Golden Gate fine suspended sediment outflow (model because difficult to measure)
• Golden Gate sand supply, 1.2-2.9 MMT/yr (Central Bay bathymetric change)
• Eroded mass (new surveys, measure bed density)
• Sediment supply from watershed (±25%)
Acknowledgements
•U.S. Army Corps of Engineers•SF Regional Water Quality Control Board•California State Coastal Conservancy•USGS Priority Ecosystem Science Program•USGS Federal/State Cooperative Program
•Lester McKee•Patricia Chambers and Linda Wanczyk