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Humboldt Bay Sea Level Trend Analysis
Using Local Tide Gauge Data
29 August 2016
Jeff Anderson
Northern Hydrology and Engineering
Conducted for Cascadia Geosciences
Tectonic Land Level Changes and their Contribution to Sea Level Rise, Humboldt Bay Region, Northern California
Humboldt Bay Sea Level Trends
Tide gauge analysis based on: Methods/Approaches from peer
reviewed literature Tide gauges located at Crescent
City and 5 sites in Humboldt Bay NOAA tidal data, except for recent
observations at Mad River Slough and Hookton Slough by CG and NHE
All tidal analysis based on monthly mean sea levels (MSL)
MSL estimated from hourly tide data
Monthly MSL data have large oscillations (scatter) due to natural climate and ocean variability
Generally attempt to smooth monthly MSL data prior to assessing decadal trends
Humboldt Bay Sea Level Trends
General References:
Scientific Literature:
Mitchell, C. E., P. Vincent, R. J. Weldon, and M. A. Richards. 1994. Present-day vertical deformation of the Cascadia margin, Pacific northwest, United States: J. Geophys. Res. 99(B6), 12,257-12,277
Burgette, R. J., R. J. Weldon II, and D. A. Schmidt. 2009. Interseismicuplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone. J. Geophys. Res., 114, B01408, doi:10.1029/2008JB005679.
Zervas, C., S. 2009. Sea Level Variations of the United States 1854-2006. NOAA National Ocean Service Center for Operational Oceanographic Products and Services. NOAA Technical Report NOS CO-OPS 053. 78pp.
Komar, P. D., J.C. Allan, and P. Ruggiero. 2011. Sea level variations along the U.S. Pacific Northwest Coast: tectonic and climate controls. Journal of Coastal Research: Volume 27, Issue 5: pp. 808 – 823.
Zervas, C., S. Gill, and W. Sweet. 2013. Estimating Vertical Land Motion from Long-Term Tide Gauge Records. NOAA National Ocean Service Center for Operational Oceanographic Products and Services. NOAA Technical Report NOS CO-OPS 065. 78pp.
Local Reports/Studies:
Patton, J. R., T. B. Williams, J. K. Anderson, R. Burgette, and T. Leroy. 2015. Tectonic land level changes and their contribution to sea-level rise, Humboldt Bay region, Northern California: 2015 Status Update. Prepared for U.S. Fish and Wildlife Service Coastal Program. Cascadia GeoSciences, Bayside, CA.
Anderson, J. K. 2015. Humboldt Bay: Sea Level Rise, Hydrodynamic Modeling, and Inundation Vulnerability Mapping. Prepared for State Coastal Conservancy, and Coastal Ecosystems Institute of Northern California. Northern Hydrology and Engineering, McKinleyville, CA.
Regional Sea Level Rates on the U.S. West Coast
Regional Mean Sea Level (ReSL) Rates:
Average sea level change over a region of the Earth’s oceans (e.g. US West Coast)
Does not include vertical land motion
ReSL rates can be different than global sea level rates due to regional factors such as
• Ocean bathymetry
• Large scale climate and ocean patterns (e.g. ENSO and PDO)
ReSL Rates for Pacific Northwest:
ReSL rate for US West Coast: Long term tide station records on relatively geologically stable ground is ~1.9 to 2.1 mm/yr
Burgette et al. (2009) provides a detailed estimate of ReSL rate for Northernmost California and Western Oregon based on tidal and land level survey records (~1925 to 2006): ReSL rate = 2.28 mm/yr
Used ReSL = 2.28 mm/yr in our tide gauge analysis
Seattle, WA (POR: 1898-2013)
San Francisco, CA (POR: 1897-2013)
San Diego, CA (POR: 1906-2013)
Crescent City Sea Level Trends
Monthly Mean Sea Levels
Monthly Mean Sea Levels with Average Seasonal Cycle Removed
Average Seasonal Cycle of Mean Sea Level
Average seasonal cycle is caused by natural climate and ocean variability, such as
Ocean temperature and salinity
Winds
Atmospheric pressure
Ocean currents
El Nino, etc.
Crescent City tidal data downloaded from NOAA COOPS (POR analyzed: 1933-2013)
Crescent City Sea Level Trends
The monthly mean sea levels with average seasonal cycle removed is the approach used by NOAA to estimate sea level trends
Monthly Mean Sea Levels with Average Seasonal Cycle Removed
Crescent City Sea Level Trends
Monthly Mean Sea Levels with Average Seasonal Cycle Removed
3-Month Average Summer Mean Sea Levels
RSL and Vertical Land Motion (VLM) estimate at Crescent City Tide Gauge:
RSL rate is -0.97 mm/yr
Estimate of VLM:
• VLM = ReSL – RSL
• VLM = 2.28 – (-0.97) = 3.25 mm/yr (upward)
Land around tide gauge is uplifting faster than the ReSL rate of 2.28 mm/yr
RSL using average summer monthly mean sea levels:
Komar et al. (2011) showed that using the average summer MSL provided the statistically best RSL trends for Pacific Northwest tide gauges
• Primarily removes winter and El Nino extremes
• Further smooths MSL data
Average summer value is 3-month average centered on the unadjusted minimum monthly summer value (May through September)
Used average summer MSL approach in our final tide gauge analysis
Humboldt Bay Sea Level Trends
Overview of Humboldt Bay Analysis:
Humboldt Bay tide gauges (including North Spit) have less than 40 year continuous record
Records are considered too short to directly determine sea level trends from water level data
Used general approach of Mitchell et al. (1994) and Burgette et al. (2009) to determine RSL and VLM rates
• Relies on long-term Crescent City tide gauge RSL and VLM trend rates, and
• Determines relative VLM between tide gauges by differencing the short-term records in Humboldt Bay to the long-term Crescent City data
• Differencing the records generally removes the ocean water level and variability, further smoothing the data and leaves the VLM between the two stations
Show examples for North Spit and Mad River Slough, other tide gauge site analyzed in similar way
North Spit Sea Level Trends
Monthly Mean Sea Levels with Average Seasonal Cycle Removed
3-Month Average Summer Mean Sea Levels
North Spit tidal data downloaded from NOAA COOPS (POR analyzed: 1977-2013)
Relative VLM using Monthly Mean Sea Levels (NS – CC)
Relative VLM using Average Summer Mean Sea Levels (NS – CC)
North Spit Sea Level Trends
RSL and VLM estimate at North Spit Tide Gauge:
Uses average summer MSL estimates
North Spit subsides relative to Crescent City at approximately 5.58 mm/yr
Estimate of VLM:
• VLMNS = VLMCC – VLMRel
• VLMNS = 3.25 – 5.58 = -2.33 mm/yr (downward)
Estimate of RSL:
• RSL = ReSL – VLMCC
• RSL = 2.28 – (-2.33) = 4.61 mm/yr
Land around North Spit tide gauge is sinking faster than the ReSL rate of 2.28 mm/yr
Note: Estimated RSL of 4.61 mm/yr is close to average summer MSL trend of 4.70 mm/yr based on the short-term North Spit record
Relative VLM using Average Summer Mean Sea Levels (NS – CC)
3-Month Average Summer Mean Sea Levels
Mad River Slough Sea Level Trends
RSL and VLM estimate at Mad River Slough Tide Gauge: Mad River Slough subsides relative to Crescent City at
approximately 4.36 mm/yr Estimate of VLM:
• VLMMRS = VLMCC – VLMRel
• VLMMRS = 3.25 – 4.36 = -1.11 mm/yr (downward) Estimate of RSL:
• RSL = ReSL – VLMMRS
• RSL = 2.28 – (-1.11) = 3.39 mm/yr
Relative VLM using Monthly Mean Sea Levels (MRS – CC)
Preliminary Mad River Slough VLM Estimate Using 2016 Data
Mad River Slough Tide Gauge Analysis: NOAA data for 1977-78 (8 monthly MSL values) NHE collected tide data ~700 m upstream in
2008 (2 monthly MSL values) Analysis uses unadjusted monthly MSL values Not enough data to determine average
seasonal cycle or average summer MSL
Additional Mad River Slough Tide Data:
NHE is collecting additional tide data at Mad River Slough closer to historic NOAA site
Preliminary data show similar sea level trend as initial estimate
Humboldt Bay Sea Level Trends
Summary of Humboldt Bay Region Sea Level and Vertical Land Motion Trends
Tide Station (TS)
Trend Rates (mm/yr)
ReSL RSL TS - CC VLM
Crescent City (CC) 2.28 -0.97 3.25
North Spit (NS) 2.28 4.61 5.58 -2.33
Mad River Slough (MRS) 2.28 3.39 4.36 -1.11
Somoa (SO) 2.28 2.53 3.50 -0.25
Fields Landing (FL) 2.28 3.76 4.73 -1.48
Hookton Slough (HS) 2.28 5.84 6.81 -3.56
Trinidad (TR) [Preliminary] 2.28 3.15 4.12 -0.87
ReSL = regional sea level, RSL = relative sea level, and VLM = vertical land motion
Note: Sea level trends were determined for Somoa, Fields Landing, Hookton Slough, and Trinidad using same analysis approach as Mad River Slough