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